Risk-Based Classification of Radioactive and Hazardous Chemical Wastes (Ncrp Report, No. 139)

NCRP Report No. 139

Risk-Based Classification of Radioactive and Hazardous Chemical Wastes

Recommendations of the NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS

Issued December 31, 2002

National Council on Radiation Protection and Measurements 7910 Woodmont Avenue, Suite 400 / Bethesda, Maryland 20814

LEGAL NOTICE This Report was prepared by the National Council on Radiation Protection and Measurements (NCRP). The Council strives to provide accurate, complete and useful information in its documents. However, neither the NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this Report, nor any person acting on the behalf of any of these parties: (a) makes any warranty or representation, express or implied, with respect to the accuracy, completeness or usefulness of the information contained in this Report, or that the use of any information, method or process disclosed in this Report may not infringe on privately owned rights; or (b) assumes any liability with respect to the use of, or for damages resulting from the use of any information, method or process disclosed in this Report, under the Civil Rights Act of 1964, Section 701 et seq. as amended 42 U.S.C. Section 2000e et seq. (Title VII) or any other statutory or common law theory governing liability.

Library of Congress Cataloging-in-Publication Data Risk-based classification of radioactive and hazardous chemical wastes / National Council on Radiation Protection and Measurements. p. cm. — (NCRP report ; no. 139) Includes bibliographical references and index. ISBN 0-929600-72-X 1. Radioactive waste disposal—Risk assessment. 2. Hazardous wastes— Risk assessment. 3. Hazardous wastes—Classification. I. National Council on Radiation Protection and Measurements. II. Series. TD898.14.R57 R5725 2002 363.72⬘89—dc21 2002033766

Copyright © National Council on Radiation Protection and Measurements 2002 All rights reserved. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotation in critical articles or reviews.

[For detailed information on the availability of NCRP publications see page 410.]

Preface This Report presents recommendations of the National Council on Radiation Protection and Measurements (NCRP) on a hazardous waste classification system that would apply to any waste containing radionuclides or hazardous chemicals. This work faced two major challenges. The first was to justify the need to replace the separate classification systems for radioactive and hazardous chemical wastes in use at the present time. The second was to make the Report accessible to different audiences with different levels of expertise in areas of waste classification and health risk assessment. It was particularly apparent to the Committee that prepared this Report that the radioactive waste community was not generally familiar with laws and regulations governing chemical waste management and disposal, approaches to classification of chemical wastes, and methods of risk assessment for hazardous chemicals. As a consequence, this Report is lengthy and contains extensive discussions of existing waste classification systems and their deficiencies, methods of risk assessment, and approaches to risk management, not all of which may be of interest to particular audiences. Although the hazardous waste classification system proposed in this Report is simple in its concepts and principles, the technical issues and the history underlying waste classification are complex. NCRP believes that an appreciation of these complexities should be helpful in understanding the need of a new system and its benefits. To address the need of various audiences to understand this Report at different levels of detail, the Report consists of three essentially self-contained parts: a short Synopsis, an extended Technical Summary, and the main Report. The Synopsis presents a brief description of the proposed waste classification system, essentially in the form of an overview for legislators and other executive-level decision makers. The aim is to show that the system is simple in principle and concepts, and to illustrate its benefits. The Technical Summary (Section 1) presents an extended discussion of existing hazardous waste classification systems, difficulties with these systems, and the proposed classification system. The aim is to fully describe the proposed system and its rationale and benefits, but without much of the background information on technical and historical details that support the proposal. Many audiences may find that the Technical iii

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Summary meets their needs. The main Report (Sections 2 to 8) presents the complete record of this work, without assuming the presence of the Technical Summary. This Report was prepared by Scientific Committee 87-2 on Waste Classification Based on Risk. Serving on Scientific Committee 87-2 were: Allen G. Croff, Chairman Oak Ridge National Laboratory Oak Ridge, Tennessee Members Michael J. Bell International Atomic Energy Agency Vienna, Austria Yoram Cohen University of California Los Angeles, California Leonard C. Keifer U.S. Environmental Protection Agency Washington, D.C.

Dennis J. Paustenbach Exponent娃 Menlo Park, California Vern C. Rogers Rogers & Associates Engineering Corporation Salt Lake City, Utah Andrew Wallo, III U.S. Department of Energy Washington, D.C.

David C. Kocher SENES Oak Ridge, Inc. Oak Ridge, Tennessee NCRP Secretariat E. Ivan White, Senior Staff Scientist The Council wishes to express its appreciation to the Committee members for the time and effort devoted to the preparation of this Report.

Thomas S. Tenforde President

Contents Preface ........................................................................................

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Synopsis .....................................................................................

1

1. Technical Summary ............................................................ 1.1 Introduction ................................................................... 1.2 Purpose and Scope of Study ......................................... 1.3 Summary of Existing Waste Classification Systems .. 1.3.1 Radioactive Waste Classification in the United States ................................................................... 1.3.1.1 Classification of Fuel-Cycle Wastes ...... 1.3.1.2 Subclassifications of Fuel-Cycle Wastes... 1.3.1.3 Other Radioactive Wastes ..................... 1.3.1.4 Exempt Radioactive Wastes .................. 1.3.1.5 Deficiencies in the Radioactive Waste Classification System ............................. 1.3.2 Other Radioactive Waste Classification Systems ............................................................... 1.3.3 Classification of Hazardous Chemical Wastes . 1.3.4 Comparison of Classification Systems for Radioactive and Hazardous Chemical Wastes . 1.3.5 Mixed Radioactive and Hazardous Chemical Wastes ................................................................. 1.4 Approach to Development of a New Waste Classification System .................................................... 1.4.1 Basic Elements of Hazardous Waste Classification System ......................................... 1.4.2 Assumptions in Developing the Waste Classification System ......................................... 1.4.3 Challenges in Developing a Waste Classification System ......................................... 1.5 Development of the Recommended Waste Classification System .................................................... 1.5.1 Risk Index for Waste Classification .................. 1.5.2 Generic Exposure Scenarios for Waste Classification ....................................................... 1.5.3 Determination of Allowable Risk or Dose ......... 1.5.4 Recommended Framework for Risk-Based Waste Classification ...........................................

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1.5.4.1 Exempt Waste ........................................ 1.5.4.2 Low-Hazard Waste ................................ 1.5.4.3 High-Hazard Waste ............................... 1.5.5 Calculation of the Risk Index ............................ 1.5.5.1 Measure of Risk for Carcinogens .......... 1.5.5.2 Estimates of Probability Coefficients for Carcinogens ...................................... 1.5.5.3 Thresholds for Deterministic Effects .... 1.5.5.4 Risk Index for Mixtures of Hazardous Substances .............................................. 1.5.5.4.1 Risk Index for Mixtures of Substances That Cause Stochastic Effects (Carcinogens) .......................... 1.5.5.4.2 Risk Index for Mixtures of Substances That Cause Deterministic Effects (Noncarcinogens) .................... 1.5.5.4.3 Use of the Composite Risk Index in Classifying Waste .... 1.6 Implications of the Recommended Waste Classification System .................................................... 1.6.1 Classification of Existing Hazardous Wastes ... 1.6.2 Subclassification of Basic Waste Classes .......... 1.6.3 Legal and Regulatory Implications ................... 1.7 Further Development of the Recommended Waste Classification System .................................................... 2. Introduction ......................................................................... 2.1 Foundations and Directions ......................................... 2.1.1 Definition of Waste Classification ..................... 2.1.2 Purpose of Waste Classification ........................ 2.1.3 Bases for Waste Classification ........................... 2.1.4 Shortcomings of Current Waste Classification Systems ............................................................... 2.1.5 Focus on Classification of Waste ....................... 2.1.6 Classification of Waste for Purposes of Disposal ............................................................... 2.2 Limits and Relationships .............................................. 2.2.1 Regulatory Implications ..................................... 2.2.2 Risk Management ............................................... 2.2.3 Waste Classification in a Continuum of Waste Compositions ....................................................... 2.2.4 Subclassifications of Basic Waste Classes ........

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2.2.5 Site-Specific Risk ................................................ 2.2.6 Ecological and Other Potential Impacts ........... 2.3 Conceptual Framework of This Report ....................... 3. Technical Background on Risk Assessment and Risk Management ......................................................................... 3.1 Assessment of Risk ....................................................... 3.1.1 Definition of Risk ................................................ 3.1.2 Types of Responses from Exposure to Hazardous Substances ....................................... 3.1.3 Definition of Risk Assessment ........................... 3.1.4 Risk Assessment Process ................................... 3.1.4.1 Hazard Identification ............................. 3.1.4.1.1 Radiation Hazard Identification ........................... 3.1.4.1.2 Chemical Hazard Identification ........................... 3.1.4.2 Dose-Response Assessment ................... 3.1.4.3 Exposure Assessment ............................ 3.1.4.4 Risk Characterization ............................ 3.1.4.5 Risk Management .................................. 3.1.5 Use of Risk Assessment in Risk-Based Waste Classification ....................................................... 3.1.5.1 Risk Assessment of a Generic Site ....... 3.1.5.2 Dose-Response Relationships ................ 3.2 Assessment of Responses from Exposure to Hazardous Substances .................................................. 3.2.1 Assessment of Responses from Exposure to Hazardous Chemicals ......................................... 3.2.1.1 Basis for a Dose-Response Assessment ... 3.2.1.2 Dose-Response Assessment for Chemicals That Cause Deterministic Effects ........ 3.2.1.2.1 Dose-Response Concepts ........ 3.2.1.2.2 Safety Factor Approach for Chemicals That Cause Deterministic Effects ............. 3.2.1.2.3 Selection of the Database ........ 3.2.1.2.4 Determination of the Reference Dose ....................... 3.2.1.2.5 Selection of Uncertainty and Modifying Factors .................. 3.2.1.2.6 Assigning Confidence Levels .... 3.2.1.2.7 Mathematical Modeling and the Benchmark Dose Method

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3.2.1.3 Dose-Response Assessment for Chemicals That Cause Stochastic Effects .............. 3.2.1.3.1 Introduction to Mathematical Modeling for Chemicals That Cause Stochastic Effects ........ 3.2.1.3.2 Statistical Models ................... 3.2.1.3.3 Benchmark Dose Method ....... 3.2.1.3.4 Pharmacokinetic Models ........ 3.2.1.3.5 Biologically-Based Models of Cancer ..................................... 3.2.1.3.6 Use of Stochastic Modeling Results .................................... 3.2.1.4 Characterization of Dose-Response Estimates ................................................ 3.2.1.5 Uncertainties and Deficiencies in Dose-Response Assessment ................... 3.2.1.5.1 Uncertainties in DoseResponse Assessment ............. 3.2.1.5.2 Deficiencies in Dose-Response Assessment ............................. 3.2.2 Assessment of Responses from Radiation Exposure .............................................................. 3.2.2.1 Deterministic Responses from Radiation Exposure ............................... 3.2.2.2 Databases and Methods of DoseResponse Assessment for Stochostic Effects ...................................................... 3.2.2.3 Measures of Radiation-Induced Responses ................................................ 3.2.2.3.1 Measures of Deterministic Responses ................................. 3.2.2.3.2 Measures of Stochastic Responses ................................. 3.2.2.3.3 Effective Dose .......................... 3.2.3 Comparison of Dose-Response Assessments for Radionuclides and Chemicals .............................. 3.2.3.1 Deterministic Responses ......................... 3.2.3.2 Stochastic Responses ............................... 3.3 Approaches to Risk Management for Radionuclides and Hazardous Chemicals That Cause Stochastic Effects ............................................................................. 3.3.1 Radiation Paradigm for Risk Management of Stochastic Responses ........................................... 3.3.2 Chemical Paradigm for Risk Management of Stochastic Responses ...........................................

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3.3.3 Comparison of the Radiation and Chemical Paradigms ............................................................ 3.3.4 Reconciliation of the Radiation and Chemical Paradigms ............................................................ 3.3.5 Application of Risk Management Paradigms to Waste Classification ............................................. 3.4 Summary ........................................................................ 4. Existing Classification Systems for Hazardous Wastes 4.1 Classification and Disposal of Radioactive Waste ......... 4.1.1 Background .......................................................... 4.1.2 Radioactive Waste Classification in the United States .................................................................... 4.1.2.1 Introduction ............................................. 4.1.2.2 Early Descriptions of Radioactive Waste Categories ................................................ 4.1.2.2.1 Liquid Wastes .......................... 4.1.2.2.2 Solid Wastes ............................. 4.1.2.2.3 Summary of Bases for Early Descriptions of Radioactive Wastes ...................................... 4.1.2.3 Classification and Disposal of Wastes from the Nuclear Fuel Cycle ................... 4.1.2.3.1 High-Level Waste and Spent Fuel .......................................... 4.1.2.3.2 Transuranic Waste .................. 4.1.2.3.3 Low-Level Waste ...................... 4.1.2.3.4 Uranium or Thorium Mill Tailings .................................... 4.1.2.3.5 Characteristics of the System for Classification and Disposal of Fuel-Cycle Waste ................ 4.1.2.4 Naturally Occurring and AcceleratorProduced Radioactive Material ............. 4.1.2.5 Exempt Radioactive Waste ................... 4.1.2.5.1 Concepts and Definitions ....... 4.1.2.5.2 Exemption Levels for Radioactive Waste .................. 4.1.2.5.3 NCRP Recommendation on a Negligible Individual Dose ...... 4.1.2.5.4 Summary of Exemptions for Radioactive Waste in the United States ..........................

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175 175 176 182 187 191

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4.1.2.6 Proposals for Alternative Radioactive Waste Classification Systems ............... 4.1.2.6.1 NRC Discussion on Definition of High-Level Waste ............... 4.1.2.6.2 Generally Applicable Waste Classification System Proposed by Kocher and Croff ......................................... 4.1.2.6.3 Generally Applicable Waste Classification System Proposed by Smith and Cohen ....................................... 4.1.2.6.4 Waste Classification System Proposed by LeMone and Jacobi ...................................... 4.1.3 IAEA Recommendations on Radioactive Waste Classification and Exemption Principles .......... 4.1.3.1 Recommendations on Waste Classification .......................................... 4.1.3.2 Recommendations on Exemption Principles ................................................ 4.1.4 Comparison of the United States and IAEA Radioactive Waste Classification Systems ........ 4.2 Classification and Disposal of Hazardous Chemical Waste .............................................................................. 4.2.1 Classification System for Hazardous Chemical Waste Under the Resource Conservation and Recovery Act ......................................................... 4.2.1.1 Description of EPA’s Hazardous Waste Classification System ............................. 4.2.1.2 Discussion of EPA’s Hazardous Waste Classification System ............................. 4.2.1.3 State Programs ...................................... 4.2.2 Treatment and Disposition of Hazardous Chemical Waste .................................................. 4.3 Regulation of Mixed Radioactive and Hazardous Chemical Waste ............................................................. 4.3.1 Introduction ........................................................ 4.3.2 Establishing Dual Regulation of Mixed Waste 4.3.3 Facilitating Compliance with Dual Regulation of Mixed Low-Level Waste ................................. 4.3.4 Dual Regulation of Other Fuel-Cycle Wastes .. 4.3.4.1 High-Level Waste .................................. 4.3.4.2 Transuranic Waste ................................ 4.3.4.3 Uranium or Thorium Mill Tailings ......

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4.3.5 Dual Regulation of Naturally Occurring and Accelerator-Produced Radioactive Material Waste ................................................................... 4.3.6 Summary of Mixed Waste Issues ...................... 4.4 NCRP Recommendations Relevant to Waste Classification ................................................................. 4.4.1 Recommendations on Radiation Protection of the Public ............................................................ 4.4.1.1 Radiation Dose Limits ........................... 4.4.1.2 Negligible Individual Dose .................... 4.4.1.3 Application of NCRP Recommendations to Waste Classification .......................................... 4.4.2 Comparative Carcinogenicity of Ionizing Radiation and Chemicals ................................... 4.5 Summary ........................................................................

237 239

5. Desirable Attributes of a Waste Classification System .. 5.1 Risk-Based ..................................................................... 5.2 Exemption ...................................................................... 5.3 Comprehensive .............................................................. 5.4 Consistent ...................................................................... 5.5 Intrinsic .......................................................................... 5.6 Comprehensible ............................................................. 5.7 Quantitative ................................................................... 5.8 Compatible ..................................................................... 5.9 Flexible ...........................................................................

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6. Principles and Framework for a Comprehensive and Risk-Based Hazardous Waste Classification System .................................................................................... 6.1 Issues of Risk Assessment and Risk Management ..... 6.1.1 Measures of Response from Exposure to Hazardous Substances ....................................... 6.1.1.1 Measures of Response for Substances Causing Deterministic Responses ........ 6.1.1.2 Measures of Response for Substances Causing Stochastic Responses .............. 6.1.1.2.1 Incidence ................................. 6.1.1.2.2 Fatalities ................................. 6.1.1.2.3 ICRP’s Total Detriment ......... 6.1.1.3 Recommendations on Selection of a Measure of Response ............................. 6.1.2 Dose-Response Relationships .............................

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6.1.2.1 Deterministic Responses ....................... 6.1.2.2 Stochastic Responses ............................. 6.1.3 Exposure Scenarios for Waste Classification ... 6.1.4 Approaches to Risk Management ...................... 6.1.5 Legal and Regulatory Constraints .................... 6.2 Framework for Risk-Based Waste Classification ........ 6.2.1 Framework of the Proposed Waste Classification System ......................................... 6.2.2 Framework for Waste Classification ................. 6.2.2.1 Exempt Waste ........................................ 6.2.2.2 Nonexempt Waste .................................. 6.2.2.2.1 Low-Hazard Waste ................. 6.2.2.2.2 High-Hazard Waste ............... 6.3 Development of the Risk Index for Individual Hazardous Substances .................................................. 6.3.1 Establishing Allowable Risks or Doses of Individual Substances ........................................ 6.3.1.1 Establishing Allowable Doses of Substances That Cause Deterministic Responses ............................................... 6.3.1.1.1 Dose Corresponding to a Negligible Risk ....................... 6.3.1.1.2 Dose Corresponding to an Acceptable Risk ...................... 6.3.1.2 Establishing Allowable Risks or Doses of Substances That Cause Stochastic Responses ............................................... 6.3.1.2.1 Establishing a Negligible Risk or Dose ............................ 6.3.1.2.2 Establishing an Acceptable Risk or Dose ............................ 6.3.2 Developing Exposure Scenarios for Purposes of Waste Classification ....................................... 6.3.2.1 Exposure Scenarios for Classifying Exempt Waste ........................................ 6.3.2.2 Exposure Scenarios for Classifying Low-Hazard Waste ................................ 6.3.2.3 Classification as High-Hazard Waste ... 6.3.3 Application of the Modifying Factor in Risk Index ........................................................... 6.4 Development of the Composite Risk Index for Multiple Substances ...................................................... 6.4.1 Risk Indexes for Mixtures of Hazardous Substances ...........................................................

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6.6 6.7

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6.4.1.1 Risk Index for Multiple Substances That Cause Stochastic Responses ........ 6.4.1.2 Risk Index for Multiple Substances That Cause Deterministic Responses ... 6.4.2 Composite Risk Index for All Hazardous Substances ........................................................... 6.4.3 Implications of the Framework for Calculating the Risk Index ..................................................... 6.4.4 Example Calculations of the Risk Index .......... 6.4.5 Establishing a Waste Classification System Based on the Framework and Risk Index ........ 6.4.5.1 Process of Implementing the Waste Classification System ............................. 6.4.5.2 Time When Waste Should Be Classified ................................................ 6.4.5.3 Time Frame for Risk Assessment in Classifying Waste .................................. 6.4.5.4 Implementation of the Waste Classification System Over Time .......... 6.4.6 Shortcomings and Advantages of the Risk Index .................................................................... Expected Classification of Existing Wastes ................ 6.5.1 Wastes Expected to be Classified as Exempt ... 6.5.2 Wastes Expected to be Classified as Low-Hazard .. 6.5.3 Wastes Expected to be Classified as HighHazard ................................................................. Subclassification of Basic Waste Classes .................... Future Development Needs for Risk-Based Waste Classification ................................................................. 6.7.1 Standardization of Nomenclature ..................... 6.7.2 Approaches to Estimating Dose-Response Relationships for Radionuclides and Hazardous Chemicals ......................................... 6.7.2.1 Approaches to Estimating DoseResponse Relationships for Substances That Cause Stochastic Responses ........ 6.7.2.2 Approaches to Estimating DoseResponse Relationships for Substances That Cause Deterministic Responses ... 6.7.3 Allowable Risks from Exposure to Substances That Cause Stochastic or Deterministic Effects .................................................................. 6.7.4 Selection of Exposure Scenarios ........................ 6.7.5 Legal and Regulatory Development Needs ....... Summary of the Proposed Risk-Based Waste Classification System ....................................................

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7. Implications of the Recommended Risk-Based Waste Classification System ......................................................... 7.1 Example Applications of the Risk-Based Waste Classification System .................................................... 7.1.1 General Approach to the Example Applications .. 7.1.1.1 Exempt Waste ........................................ 7.1.1.2 Low-Hazard Waste ................................ 7.1.1.3 High-Hazard Waste ............................... 7.1.1.4 Development of Examples ..................... 7.1.2 Consideration of Exempt Wastes ...................... 7.1.2.1 Radioactive Wastes ................................ 7.1.2.2 Hazardous Chemical Wastes ................ 7.1.3 DOE Low-Level Radioactive Waste .................. 7.1.3.1 Classification by Calculation of Total Dose ......................................................... 7.1.3.2 Classification Using Pre-Established Limiting Concentrations ....................... 7.1.3.3 Classification Using Pre-Established Limiting Concentrations and Enhanced Access .................................... 7.1.3.4 Alternative Exposure Scenarios ............ 7.1.4 Average Commercial Low-Level Radioactive Waste ................................................................... 7.1.5 Typical Uranium Mill Tailings .......................... 7.1.6 Residues from Processing of High-Grade Uranium Ore ....................................................... 7.1.7 Mixed Waste: Electric Arc Furnace Dust ......... 7.1.7.1 Introduction to Analysis ........................ 7.1.7.2 Evaluation as Exempt Waste ................ 7.1.7.3 Approach to Example Analysis ............. 7.1.7.4 Deterministic Risk Index for Hazardous Chemical Constituents ....... 7.1.7.5 Stochastic Risk Index for Hazardous Chemical Constituents .......................... 7.1.7.6 Stochastic Risk Index for Radionuclides ......................................... 7.1.7.7 Calculation of the Composite Risk Index .............................................. 7.1.7.8 Consideration of Alternative Assumptions ........................................... 7.1.8 Hazardous Chemical Waste ............................... 7.1.9 Discussion of Example Analyses ....................... 7.2 Legal and Regulatory Ramifications ............................ 7.2.1 Establishment of an Exempt Waste Class .......

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7.2.2 Elimination of Source-Based Waste Classifications ..................................................... 7.2.3 Recognition of Permanent Disposal of Hazardous Chemical Wastes ............................. 7.2.4 Establishing the Potential for High-Hazard Chemical Wastes ................................................ 7.2.5 Elimination of the Mixed Waste Category ....... 7.2.6 Elimination of the Category of Waste Containing Naturally Occurring and Accelerator-Produced Radioactive Material .... 7.2.7 Impact on Subclassification of Waste Classes ..

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8. Conclusions and Recommendations .............................. 354 8.1 Conclusions .................................................................... 354 8.2 Recommendations .......................................................... 358 Glossary ...................................................................................... 360 Acronyms ................................................................................... 377 References ................................................................................. 379 The NCRP .................................................................................. 401 NCRP Publications .................................................................. 410 Index ........................................................................................... 420

Synopsis This Report presents recommendations of the National Council on Radiation Protection and Measurements (NCRP) on a new system for classifying waste that contains hazardous substances, either radionuclides or hazardous chemicals. NCRP’s recommendations incorporate three principles. First, the classification system is generally applicable to any waste that contains radionuclides, hazardous chemicals, or mixtures of the two. Over the last two decades, the separate and quite different systems for classifying and managing radioactive and chemical wastes have led to considerable difficulties in managing mixed wastes that contain both types of hazardous substances. For the most part, however, the development of different approaches to classifying and managing waste was not driven by differences in the properties of radionuclides and hazardous chemicals or their potential risks to human health. Thus, there is no evident need of separate systems for classifying and managing radioactive and chemical wastes. Second, waste that contains hazardous substances is classified based on considerations of health risks to the public that arise from waste disposal. The existing classification systems for radioactive and chemical wastes in the United States are not based primarily on considerations of health risks to the public. Rather, classification of hazardous wastes has been based primarily on the source of the waste or the presence of particular hazardous substances. The absence of risk-based waste classifications has had a number of undesirable ramifications: ●

Some wastes are managed more stringently and at higher cost than warranted by the health risks they pose. For example, lower-activity high-level radioactive wastes have been managed at high cost as if they require disposal in a geologic repository to protect public health, even though they are less hazardous than some low-level wastes that are managed at much less cost and are considered acceptable for near-surface disposal. Other wastes may be managed less stringently than warranted, such as wastes from mining and energy exploitation activities that contain elevated levels of hazardous chemicals, especially heavy metals, and naturally occurring radionuclides. 1

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Some wastes that pose similar health risks are placed in different classes (e.g., high-level, transuranic, and high-activity, longer-lived low-level radioactive wastes), whereas wastes that pose risks ranging from innocuous to highly hazardous may be placed in the same class (e.g., Class-A and greater-than-Class-C low-level radioactive wastes, diluted and highly concentrated listed hazardous chemical wastes). Such inconsistencies do not seem sensible to a nonexpert, and they may increase suspicion and mistrust of authorities by the public, which can only exacerbate the already difficult challenge of managing hazardous wastes.

Third, the hazardous waste classification system includes an exempt class of waste. Waste in this class would pose a sufficiently low risk that it could be managed in all respects as if it were nonhazardous material. Wastes that contain low levels of hazardous substances have been exempted on a case-by-case basis, but existing waste classification systems do not include generally applicable exemption principles. As a consequence, many slightly contaminated waste materials are managed at considerable cost as if they were hazardous. If an exempt class of waste were established, recycling and reuse of exempt materials, such as scrap metals at nuclear facilities, could be considered, thus saving valuable resources for other needs and reducing impacts on the environment. Based on these principles, the hazardous waste classification system recommended by NCRP includes three classes of waste: exempt, low-hazard, and high-hazard waste. Each waste class is defined in relation to the type of disposal system (technology) that is expected to be generally acceptable in protecting public health as follows: ●

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Exempt waste: any waste containing hazardous substances that is generally acceptable for disposition as nonhazardous material (e.g., disposal in a municipal/industrial landfill for nonhazardous waste). Low-hazard waste: any nonexempt waste that is generally acceptable for disposal in a dedicated near-surface facility for hazardous wastes. High-hazard waste: any nonexempt waste that generally requires a disposal system more isolating than a dedicated nearsurface facility for hazardous wastes (e.g., a geologic repository).

Given these conceptual definitions, NCRP recommends that the boundaries of waste classes should be quantified in terms of limits

SYNOPSIS

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on concentrations of hazardous substances based on the following principles: ●

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Waste would be classified as exempt if the concentrations of hazardous substances are sufficiently low that it poses no more than a negligible risk to a hypothetical inadvertent intruder at a municipal/industrial landfill for nonhazardous waste. A negligible risk, or the associated dose, is a value so low that further efforts at risk reduction generally are unwarranted (e.g., an excess lifetime cancer risk less than about 10ⳮ4 or doses of noncarcinogenic hazardous substances substantially less than nominal thresholds for induction of health effects in the general population). Waste that exceeds concentration limits for exempt waste would be classified as low-hazard if it poses no more than an acceptable (i.e., barely tolerable) risk to a hypothetical inadvertent intruder at a dedicated near-surface disposal facility for hazardous wastes, with the important condition that an acceptable risk or dose used to determine low-hazard waste should be substantially higher than a negligible risk or dose used to determine exempt waste. Waste that exceeds concentration limits for low-hazard waste would be classified as high-hazard.

NCRP recognizes that potential risks to residents near waste disposal sites are a primary consideration in determining acceptable disposal practices for hazardous wastes. Because such risks are highly site-specific, they do not provide a suitable basis for generally classifying waste. However, NCRP expects that nearly all exempt and low-hazard waste would be acceptable for disposal using the intended technology at well chosen sites when potential risks to nearby residents are considered in the process of licensing or permitting specific disposal facilities. Based on suitable precedents for defining negligible and acceptable (barely tolerable) risks to the public from exposure to hazardous substances, the proposed classification system should be largely consistent with current classifications of hazardous wastes and plans for their disposal. For example: most low-level radioactive waste and hazardous chemical waste would be classified as low-hazard; and most high-level, transuranic, and high-activity, longer-lived lowlevel radioactive waste would be classified as high-hazard. However, some hazardous chemical wastes, especially wastes that contain high concentrations of heavy metals, might not be generally acceptable for near-surface disposal and, thus, would be classified as highhazard. The existing classification system for hazardous chemical

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SYNOPSIS

waste does not include such a class, and there are no planned alternatives to near-surface disposal for highly hazardous chemical wastes. NCRP believes that the proposed hazardous waste classification system offers significant advantages compared with the classification systems currently used in the United States. In addition to addressing deficiencies in the existing classification systems for radioactive and hazardous chemical wastes and promoting more cost-effective management of all hazardous wastes, the proposed approach to classification of waste based on risk is simple and understandable. The clear association of hazardous waste classification with requirements for protection of public health, which is lacking in the existing classification systems, should serve to increase public confidence in waste management and disposal activities.

1. Technical Summary 1.1 Introduction Waste is any material that has insufficient value to justify further beneficial use, and thus must be managed at a cost. Wastes that contain hazardous substances, either radionuclides or toxic chemicals, are generated by many human activities. Management and disposal of these wastes must be conducted in ways that protect human health. Because hazardous wastes vary widely in their compositions and concentrations of hazardous substances and in their potential impacts on human health, the need to protect human health is met most efficiently by use of a variety of technological approaches to waste management and disposal, rather than a single approach for all wastes. Management and disposal of the wide variety of hazardous wastes has been aided by the development of waste classification systems. The term waste classification refers to broadly defined waste categories related, for example, to properties of waste materials, potential risks to human health that arise from waste management or disposal, or the source of the waste. Ideally, hazardous wastes in the same class should pose similar risks to human health and, thus, require similar approaches to safe management and disposal. The primary purpose of waste classification systems is to facilitate development of efficient strategies for waste management and disposal, such as planning for waste treatment and disposal capacity, by identifying wastes that could be managed and disposed of safely using essentially the same technologies. Waste classification also facilitates communication among organizations that generate, manage, or dispose of waste, regulatory authorities, and the public. NCRP emphasizes, however, that waste classification does not provide a substitute for establishing requirements on treatment and disposal of specific wastes at specific sites, requirements on remediation of contaminated sites, or decisions by regulatory authorities about the acceptability of any such activities. The acceptability of particular waste management or disposal activities must be based on site-specific assessments of risks posed by well characterized wastes. Waste classification, although useful, can only inform the process of 5

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selecting safe and cost-effective approaches to waste management and disposal. Over the last several decades, separate classification systems have been developed for radioactive and hazardous chemical wastes based on a variety of considerations, the most prevalent being the source of the waste. These classification systems have served their intended purpose of facilitating development of health-protective strategies for waste management and disposal reasonably well. However, they have exhibited a number of shortcomings and undesirable ramifications, which indicate that a new approach to classification of hazardous wastes would be beneficial.

1.2 Purpose and Scope of Study This Report is concerned with classification of hazardous wastes. Wastes are materials deemed to have no further beneficial use to their present custodian, although these materials may be useful to others. Unless otherwise indicated, the term ‘‘hazardous’’ as used in this Report refers to the presence of radionuclides, hazardous chemicals, or both. This term also may refer to certain characteristics of materials that pose a hazard, such as ignitability, corrosivity, or reactivity. The primary purpose of this Report is to present NCRP’s recommendations on classification of hazardous wastes. The Report is directed at a multidisciplinary audience with different levels of technical understanding in the fields of radiation and chemical risk assessment and radioactive and chemical waste management. A new hazardous waste classification system is proposed that differs from the existing classification systems for radioactive and hazardous chemical wastes in two fundamental respects. First, hazardous waste would be classified based on considerations of health risks to the public that arise from disposal of waste. Hazardous waste would not be classified based, for example, on its source. Second, the classification system would apply to any hazardous waste, and separate classification systems for radioactive and hazardous chemical wastes would not be retained. In the proposed system, waste would be classified based only on its properties, and the same rules would apply in classifying all hazardous wastes. The objective of the study presented in this Report was to address difficulties (elaborated, for example, in Sections 1.3.1.5 and 1.4) that have arisen from use of the existing classification systems for radioactive and hazardous chemical wastes. An important impetus for

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the proposed classification system is the difficulties that have been encountered in managing and disposing of so-called ‘‘mixed waste’’ that contains radionuclides and hazardous chemicals. The proposed system also addresses shortcomings of the existing classification systems, such as the lack of general principles for exempting wastes that contain small amounts of hazardous substances from regulatory control as hazardous material. This Report is concerned with classification of hazardous waste for purposes of disposal. However, the principles and concepts embodied in the waste classification system could be applied in classifying hazardous materials for any other purpose. The classification system is intended to be applied to hazardous waste prior to disposal. It is not intended to be applied to screening or ranking of contaminated sites, including existing hazardous waste disposal sites, because these activities involve site-specific considerations that cannot be included in a generally applicable waste classification system. However, any wastes exhumed from contaminated sites that then require disposal would be included in the waste classification system. This Report presents the foundations and technical principles for development of a generally applicable and risk-based hazardous waste classification system. Recommendations on suitable approaches to establishing boundaries of different waste classes are discussed; these boundaries could be expressed, for example, in terms of limits on concentrations of hazardous substances. However, a particular implementation of the proposed waste classification system in terms of quantifying the boundaries of different waste classes is not presented.

1.3 Summary of Existing Waste Classification Systems This Section summarizes the separate classification systems that have been developed for radioactive and hazardous chemical wastes. Impacts of the two classification systems on management and disposal of mixed wastes are also described.

1.3.1 Radioactive Waste Classification in the United States The existing classification system for radioactive waste in the United States is depicted in Figure 1.1. This classification system is in the form of a hierarchy of basic waste classifications and waste subclassifications.

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Fig. 1.1. Current radioactive waste classification system in the United States.

In the first level of the hierarchy, radioactive waste that arises from operations of the nuclear fuel cycle (i.e., from processing of uranium or thorium ores and production of nuclear fuel, any uses of nuclear reactors, and subsequent utilization of radioactive material used or produced in reactors) is distinguished from radioactive waste that arises from any other source or practice. The latter type of waste is referred to as NARM (naturally occurring and acceleratorproduced radioactive material), which includes any radioactive material produced in an accelerator and NORM [naturally occurring radioactive material not subject to regulation under the Atomic Energy Act (AEA)]. The distinction between nuclear fuel-cycle and NARM wastes is based on the definitions of source, special nuclear, and byproduct materials in AEA, which apply only to radioactive materials associated with operations of the nuclear fuel cycle, and the development of federal laws and regulations for management and disposal of radioactive waste that apply only to waste containing radioactive materials defined in AEA. This distinction originated in the national security and safeguards aspects of the nuclear weapons program established under AEA, but it is not based on differences in radiological properties of fuel-cycle and NARM wastes or on differences in requirements for their safe management and disposal. 1.3.1.1 Classification of Fuel-Cycle Wastes. As indicated in the second level of the hierarchy in Figure 1.1, radioactive waste that

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arises from operations of the nuclear fuel cycle in the United States is divided into five basic classes called spent fuel, high-level waste, transuranic waste, low-level waste, and uranium or thorium mill tailings. The current definitions of these waste classes and the intended type of disposal system (technology) for each class are summarized in Table 1.1. The key to the classification system for nuclear fuel-cycle wastes in the United States is the definition of high-level waste, because the definitions of transuranic waste and low-level waste depend on this definition. High-level waste is defined based on its source rather than its properties. The important properties of high-level waste include high concentrations of shorter-lived fission products, resulting in high levels of decay heat and external radiation, and high concentrations of long-lived, alpha-emitting transuranium radionuclides. At the present time, any waste that resembles high-level waste in its radiological properties but does not arise directly in reprocessing of spent fuel is not classified as high-level waste. In addition to being source-based, the definition of high-level waste is only qualitative, because minimum concentrations of fission products (or minimum levels of decay heat or external radiation) and minimum concentrations of long-lived, alpha-emitting transuranium radionuclides are not specified. Thus, the definition is ambiguous, as evidenced by several case-by-case decisions by regulatory authorities to exclude from high-level waste certain incidental wastes that arise in fuel reprocessing (e.g., fuel cladding and ion-exchange beds) or further processing of reprocessing waste (e.g., salts produced in decontamination of liquid wastes). Any ambiguity in the definition of high-level waste results in a similar ambiguity in the definitions of transuranic waste and low-level waste. Another important feature of the classification system for nuclear fuel-cycle wastes in the United States is the definition of low-level waste only by exclusion; there is no definition of what low-level waste is, only a definition of what it is not. As a result, in contrast to the earliest descriptions of low-level waste prior to the establishment of definitions in law, this class is not restricted to waste that contains relatively low concentrations of radionuclides compared with highlevel waste. Rather, low-level waste can range from virtually innocuous to highly hazardous over long time frames. The classification system for nuclear fuel-cycle wastes in the United States can be characterized in the following way. First, as a consequence of the definition of high-level waste as waste from fuel reprocessing, all waste classes, including mill tailings, are defined based essentially on the source of the waste, rather than its radiological properties, and most of the definitions are not explicit in regard

Geologic repository

Primary waste that arises from chemical reprocessing of spent nuclear fuelc Waste that contains more than 4 kBq gⳮ1 of alpha-emitting transuranium radionuclides with half-lives greater than 20 y, excluding high-level waste Any waste not classified as spent fuel, high-level waste, transuranic waste, or uranium or thorium mill tailings Residues from chemical processing of ores for their source material (i.e., uranium or thorium) content

High-level waste

Transuranic waste

Low-level waste

Mill tailings

Near-surface disposal in situ or at processing site;e small volumes may be managed as lowlevel waste

Near-surface disposal system or, for highactivity, longer-lived waste, geologic repository d

Geologic repository

Irradiated nuclear fuel that has not been chemically reprocessed

Geologic repository

Intended Disposal Systema

Spent fuel b

Fuel-cycle waste

Definition

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Class

TABLE 1.1—Summary of current definitions of radioactive waste classes and intended disposal systems for different waste classes in the United States.

10 1. TECHNICAL SUMMARY

No radiological restrictions

Determined on case-by-case basis g

Exempt waste

Disposal systems other than those listed, such as greater confinement disposal at depths intermediate between a nearsurface facility and a geologic repository, have been used for some wastes. b Spent fuel is not waste until it is so declared. In some laws and regulations, spent fuel is not distinguished from high-level waste for purposes of waste classification. c Certain incidental wastes that arise from fuel reprocessing or further processing of reprocessing wastes have been excluded from high-level waste on a case-by-case basis. d For commercial low-level waste, limits on concentrations of radionuclides that are generally acceptable for near-surface disposal are the Class-C limits specified by 10 CFR Part 61. e Large volumes of mill tailings may not be sent to a low-level waste disposal facility. f At DOE sites, large volumes of waste that contains NORM are managed as mill tailings, and small volumes of NORM and accelerator-produced waste are managed as low-level waste. Commercial NARM waste currently is regulated only by the states. States generally regulate accelerator-produced waste as low-level waste, but commercial NORM waste is subject to a variety of state regulations or, in some states, is unregulated as radioactive material. g Many specific wastes that contain small amounts of radionuclides have been exempted from regulatory control, but a general class of exempt waste has not been established in law or regulations. NRC currently is prohibited by law from establishing such a waste class.

a

No coordinated federal policy for disposal f

Any waste that does not arise from operations of nuclear fuel cycle

NARM waste

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to the primary constituents of the waste or its properties. Second, the definitions of all waste classes, including transuranic waste, are qualitative; i.e., the definitions are not expressed strictly in terms of limits on concentrations of radionuclides or other radiological properties. Third, the definitions are not based primarily on considerations of risks to human health, because waste in different classes can have similar radiological properties and pose similar health risks (e.g., spent fuel, high-level waste, transuranic waste, and highactivity, longer-lived low-level waste). As indicated in Table 1.1, the different classes of waste from the nuclear fuel cycle normally are intended for disposal either in a geologic repository or in a near-surface facility. The intention to dispose of spent fuel, high-level waste, and transuranic waste in a geologic repository is based primarily on assessments of long-term health risks to the public posed by the high concentrations of longlived, alpha-emitting radionuclides. Certain high-activity, longerlived low-level wastes, including greater-than-Class-C commercial low-level waste (see Figure 1.1), also are intended for disposal in a geologic repository, based in part on assessments which indicate that disposal in a near-surface facility would pose an unacceptable health risk to individuals who might inadvertently intrude onto a disposal site after loss of institutional control. Most low-level waste, except high-activity, longer-lived waste that is anticipated to be produced in small volumes, is intended for disposal in a near-surface facility. The acceptability of near-surface disposal for most low-level waste is based primarily on assessments of the long-term performance of such facilities, which indicate that the health risks to the public, including future inadvertent intruders, should be acceptable. Uranium or thorium mill tailings also are intended for nearsurface disposal. In contrast to low-level waste, however, this intention is based mainly on a judgment by regulatory authorities that disposal of the very large volumes of these wastes in underground facilities would be impractical. Most mill tailings are intended for disposal in situ at uranium or thorium processing sites where the wastes were generated. Because inadvertent intrusion into a tailings pile would result in unacceptable health risks to an intruder and possibly to individuals residing nearby, due to the high concentrations of radium and high emanation rates of radon, an intention to maintain perpetual institutional control over tailings piles to prevent intrusion is an important factor in protecting public health. Thus, assessments of long-term health risks to the public, including future inadvertent intruders at near-surface disposal sites, are important in selecting disposal technologies for radioactive wastes

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that arise from operations of the nuclear fuel cycle. However, as noted previously, the definitions of the different classes of fuel-cycle waste are not based on considerations of risks that arise from waste disposal but are based essentially on the source of the waste, with the result that waste in different classes can pose similar risks and require similar disposal technologies. 1.3.1.2 Subclassifications of Fuel-Cycle Wastes. As shown in the third level of the hierarchy in Figure 1.1, transuranic waste and low-level waste are further divided into different subclasses. The subclassification of transuranic waste as contact handled or remotely handled is based on the level of external radiation in contact with a waste package. This subclassification is related to requirements for protection of workers during waste operations, but it is not related to requirements for protection of the public following disposal. The subclassification of low-level waste applies mainly to commercial waste intended for disposal in facilities licensed by the U.S. Nuclear Regulatory Commission (NRC) or an Agreement State. Waste designated as Class A, B, or C is generally acceptable for nearsurface disposal in accordance with requirements for each subclass specified by NRC in Title 10, Part 61 of the Code of Federal Regulations (10 CFR Part 61) or compatible Agreement State requirements. Greater-than-Class-C waste, which contains the highest concentrations of radionuclides with half-lives of about 30 y or greater, requires disposal in a geologic repository, unless disposal elsewhere is approved on a case-by-case basis. The subclassification of commercial low-level waste is based on assessments of health risks to the public that arise from near-surface disposal, especially potential risks to inadvertent intruders at disposal sites after an assumed loss of institutional control. 1.3.1.3 Other Radioactive Wastes. As shown in the second level of the hierarchy in Figure 1.1, radioactive waste that does not arise from operations of the nuclear fuel cycle (NARM waste) is divided into waste that contains NORM and radioactive waste produced in an accelerator. This division is not formally defined in federal laws or regulations but is based mainly on the different sources and properties of the two types of waste. NORM waste often resembles uranium or thorium mill tailings in having large volumes but relatively low concentrations of radionuclides, although some radium wastes occur in small volumes and are highly concentrated. Acceleratorproduced waste resembles some forms of low-level waste in that it contains mainly short-lived radionuclides and relatively low concentrations of longer-lived radionuclides.

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Federal regulations governing NARM waste have not been established, and there is no coordinated federal policy for their disposal (see Table 1.1). Thus, commercial NARM waste currently is regulated only by the states. States generally regulate acceleratorproduced waste as if it were low-level waste. Several approaches have been taken in regulating commercial NORM waste, particularly wastes produced in mining, energy exploitation, and other industrial activities. Some states do not currently regulate these forms of NORM waste as radioactive waste. States that do regulate NORM waste generally specify concentrations of radium below which the materials are exempt from regulation as radioactive waste, but the concentrations of radium that distinguish regulated and exempt NORM waste vary from state to state. The distinction between regulated and unregulated (including exempt) NORM waste is indicated in the third level of the hierarchy in Figure 1.1. The U.S. Department of Energy (DOE) is responsible for management and disposal of NARM waste associated with any of its activities. Large volumes of NORM waste that contains relatively low concentrations of radionuclides generally are managed in the same way as uranium or thorium mill tailings. Accelerator-produced waste and small volumes of concentrated NORM waste generally are managed as low-level waste. 1.3.1.4 Exempt Radioactive Wastes. The radioactive waste classification system in the United States does not include a general class of exempt waste (see Table 1.1). Rather, many products and materials that contain small amounts of radionuclides (e.g., specified consumer products, liquid scintillation counters containing 3H and 14C) have been exempted from requirements for use or disposal as radioactive material on a case-by-case basis. The various exemption levels are intended to correspond to low doses to the public, especially compared with dose limits in radiation protection standards for the public or doses due to natural background radiation. However, the exemption levels are not based on a particular dose, and potential doses to the public resulting from use or disposal of the exempt products and materials vary widely. A general class of exempt radioactive waste would include any waste containing sufficiently small amounts of radionuclides that the materials could be managed and disposed of as if they were nonradioactive and still provide adequate protection of human health. An important benefit of establishing a general class of exempt radioactive waste would be a reduction in the resources required for waste treatment and disposal. Classification of waste as exempt also would allow consideration of beneficial uses of the materials.

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1.3.1.5 Deficiencies in the Radioactive Waste Classification System. The classification system for radioactive waste in the United States summarized in Table 1.1 is based primarily on the earliest descriptions of different classes of waste that arises from chemical reprocessing of spent nuclear fuel and subsequent processing of nuclear materials that were developed beginning in the late 1950s. These wastes were considered to be the most important in regard to potential radiological impacts on workers. This classification system has served its intended purpose of aiding development of strategies for management and disposal of nuclear fuel-cycle wastes reasonably well. Federal programs for spent fuel, high-level waste, and transuranic waste are well established, disposal of transuranic waste in a dedicated geologic repository has begun, and investigations of a recommended geologic repository site for disposal of spent fuel and high-level waste are underway. Lowlevel waste and uranium or thorium mill tailings have been managed and disposed of under federal and state programs. In addition, states and DOE have taken responsibility for accelerator-produced waste and some NORM wastes. As summarized below, however, the classification system that encompasses nuclear fuel-cycle and NARM waste also has exhibited a number of deficiencies that call into question its continued suitability. ●

●

●

●

The radioactive waste classification system is complex, it is not transparent to the public, who are increasingly involved in decisions about management and disposal of waste, and it is not understandable by anyone but a studied expert. The classification system lacks a set of principles for determining when a waste contains sufficiently small amounts of radionuclides that it can be exempted from regulatory control as radioactive material. The lack of a general class of exempt waste increases in importance as the resources required for management and disposal of radioactive waste increase compared with the resources required for management and disposal of these materials as nonradioactive waste, and it may foreclose possible beneficial uses of slightly contaminated materials. The distinction in law between nuclear fuel-cycle and NARM waste is completely artificial with respect to radiological properties of wastes and requirements for their safe management and disposal. This distinction cannot be defended on grounds of protection of human health. The classification system for fuel-cycle waste is increasingly unable to accommodate in a logical and defensible manner newer

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forms of waste that were not envisioned when the classification system was first developed. For example, some wastes resemble high-level waste or transuranic waste in their radiological properties and are intended for disposal in a geologic repository, but they must be classified as low-level waste because they are not produced directly in fuel reprocessing or they do not contain sufficient concentrations of long-lived, alpha-emitting transuranium radionuclides. Conversely, some reprocessing wastes, after decades of storage and further processing, now contain such low concentrations of radionuclides that they would be generally acceptable for near-surface disposal in accordance with NRC requirements for low-level waste, but these wastes must be classified as high-level waste and sent to a geologic repository unless NRC determines otherwise on a case-by-case basis. The classification system for fuel-cycle waste is essentially qualitative. As a result, there is substantial ambiguity about whether some wastes should be classified as high-level waste, transuranic waste, or low-level waste. This ambiguity has led to needless disputes about classification of specific wastes that are largely unrelated to important issues of protecting human health. The definition of low-level waste is particularly problematic. Contrary to the common meaning of ‘‘low-level’’ and the meaning of this term when this waste class was first defined, low-level waste can contain high concentrations of shorter-lived and longer-lived radionuclides similar to those in high-level waste, as well as relatively low concentrations of any radionuclide. Thus, the definition of low-level waste is not related to its radiological properties or to requirements for safe management and disposal. The definition only by exclusion also may foster mistrust by the public because the simple question of what lowlevel waste is cannot be given a direct answer.

The root cause of these deficiencies is a classification system for nuclear fuel-cycle waste that is based primarily on the source of the waste, rather than its radiological properties or assessments of risks to human health that arise from waste management or disposal, with the result that wastes in different classes can have similar radiological properties and require similar technologies for safe management and disposal. All of these deficiencies, except the lack of a general class of exempt waste, are overcome by basing decisions about management and disposal of specific wastes at specific sites on the radiological properties of waste, rather than its classification. However, the need to separate decisions about suitable approaches to waste management and disposal from considerations of waste

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classification makes the existing radioactive waste classification system in the United States difficult to defend on logical or technical grounds.

1.3.2 Other Radioactive Waste Classification Systems As part of this study, proposed radioactive waste classification systems that differ from the existing classification system in the United States were reviewed and evaluated. Of particular interest is the classification system currently recommended by the International Atomic Energy Agency (IAEA). This classification system and the disposal options for each waste class are summarized in Table 1.2. The basic waste classification system consists of exempt waste, lowand intermediate-level waste, and high-level waste. The radioactive waste classification system recommended by IAEA differs from the existing classification system in the United States in the following respects. ●

●

●

●

●

The basic waste classification system includes a general class of exempt waste, which is defined in terms of a dose to an individual member of the public, resulting from waste disposal, that is regarded as negligible. The basic waste classification system does not distinguish between radioactive waste associated with the nuclear fuel-cycle and other waste; i.e., fuel-cycle and NARM wastes are included in the same classification system. High-level waste is defined in terms of its radiological properties, rather than its source. Thus, this class includes waste from sources other than chemical reprocessing of spent nuclear fuel with radiological properties similar to those of reprocessing waste. Concentrations of shorter-lived radionuclides in low- and intermediate-level waste are limited by the criterion on thermal power density (decay heat). There is no such restriction on lowlevel waste as defined in the United States. The definitions of waste classes are linked to some degree with intended disposal technologies. This linkage is particularly apparent in the definitions of exempt waste and high-level waste. However, not all waste classes are defined in relation to intended disposal technologies, because low- and intermediate-level waste could be acceptable for near-surface disposal or require disposal in a geologic repository, depending on the radiological properties of the waste and requirements imposed by national authorities.

Concentrations of long-lived, alphaemitting radionuclides restricted to 4 kBq gⳮ1 in individual waste packages and average of 0.4 kBq gⳮ1 over all waste packages Concentrations of long-lived, alpha-emitting radionuclides that exceed restrictions for short-lived waste

Long-lived waste

Concentrations of radionuclides above exempt levels and thermal power density less than about 2 kW mⳮ3

Low- and intermediate-level waste

Short-lived wastea

Concentrations of radionuclides at or below levels corresponding to annual dose to members of the public from waste disposal of 10 ␮Sv

Exempt waste

Waste Characteristics

Geologic repository

Near-surface disposal system or geologic repository b

No radiological restrictions

Disposal Options

/

Class

TABLE 1.2—Summary of characteristics of radioactive wastes and disposal options in waste classification system currently recommended by IAEA.

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Contains uranium, thorium, or radium; generated in mining and milling of ores or similar activities, or decommissioning of nuclear facilities d

Waste that contains long-lived, naturally occurring radionuclides c

No radiological restrictions or systems similar to those for shortlived wastee

Geologic repository

b

Distinction between short- and long-lived radionuclides is half-life of about 30 y. Range of disposal options may be acceptable, due to variety of radionuclides and wide range of concentrations that may be present. c Waste is not part of basic waste classification system, but large volumes of waste that contains long-lived, naturally occurring radionuclides are given additional consideration. d Waste from decommissioning also may contain man-made radionuclides. e Disposal option would depend on results of safety assessments for particular wastes.

a

Thermal power density greater than about 2 kW mⳮ3 and concentrations of long-lived, alpha-emitting radionuclides that exceed restrictions for short-lived waste

High-level waste

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Waste that contains long-lived, naturally occurring radionuclides, including uranium and thorium mill tailings, is not part of the basic waste classification system, in contrast to the classification system for nuclear fuel-cycle waste in the United States. However, both classification systems recognize that management and disposal of these wastes require special considerations, due primarily to their very large volumes.

In considering whether waste would be exempt, the specified dose criterion normally would be applied only to waste that contains mainly man-made radionuclides. Since average annual doses to the public due to long-lived, naturally occurring radionuclides in their undisturbed state are considerably above 10 ␮Sv, this dose could not be used as the criterion to exempt large volumes of waste that contains these radionuclides. As indicated in Table 1.2, large volumes of waste that contains low levels of naturally occurring radionuclides also could be exempted from regulation as radioactive material, but the exemption levels normally would correspond to doses considerably higher than the criterion used to define the exempt waste class containing mainly man-made radionuclides.

1.3.3 Classification of Hazardous Chemical Wastes Management and disposal of many wastes that contain hazardous chemicals are regulated by the U.S. Environmental Protection Agency (EPA) under authority of the Resource Conservation and Recovery Act (RCRA). In the classification system for hazardous chemical wastes specified in 40 CFR Part 261, waste is classified as hazardous by its characteristics or by listing. ●

Waste is classified as hazardous by characteristics if it is ignitable, corrosive, reactive, or toxic, as defined by EPA. A waste is hazardous by characteristics based on its physical and chemical properties. Ignitable, corrosive, or reactive wastes must be treated to remove these characteristics prior to disposal. Properly treated ignitable, corrosive, and reactive wastes, if they are not otherwise hazardous, are no longer considered hazardous. The toxicity characteristic is defined in terms of the leachability of specified organic compounds and heavy metals. Toxic wastes also must be treated to remove this characteristic prior to disposal using such methods as destruction of organic materials by incineration or incorporation in an immobilizing waste form (e.g., cement). However, in contrast to ignitable, corrosive,

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or reactive wastes, a properly treated toxic waste may still be considered hazardous (e.g., if it contains heavy metals that are immobilized but not destroyed by treatment). ●

Waste is classified as hazardous by listing if it contains any amount of specified materials from nonspecific sources (the socalled ‘‘F’’ list), specified materials from specific sources (the ‘‘K’’ list), or specified chemicals from any source (the ‘‘P’’ and ‘‘U’’ lists). A listed hazardous waste cannot be rendered nonhazardous by treatment or by dilution or mixing with nonhazardous materials. EPA has issued proposals to establish exemption levels for listed wastes that contain small amounts of hazardous substances, but such exemption provisions are not yet established in regulations. Thus, a listed hazardous waste can be exempted from RCRA requirements only by the process of ‘‘delisting.’’ EPA also has exempted certain materials that contain hazardous substances to allow their beneficial use (e.g., ash and sludge from coalburning power plants, sewage sludge).

All wastes classified as hazardous under RCRA, including properly treated toxic waste that is still considered hazardous, are intended for disposal in near-surface facilities regulated under Subtitle C of RCRA. EPA has developed detailed technical requirements on waste treatment and the siting, design, operation, and closure of disposal facilities. Thus, when viewed in relation to intended disposal technologies, there is basically only one class of hazardous chemical waste, regardless of the amounts of hazardous substances present; i.e., a waste either is hazardous or it is not. Some states (e.g., California, Washington) have defined a category of extremely hazardous waste, and extremely hazardous substances are specified by EPA under the Emergency Response and Community Right-to-Know Act. Under RCRA and state regulations, however, requirements on waste treatment and disposal generally do not distinguish between extremely hazardous waste and any other hazardous chemical waste. Requirements on treatment and disposal of hazardous chemical waste under RCRA, especially the intention to limit contamination of groundwater, are based to some extent on considerations of risks to public health and the environment posed by waste. However, requirements on waste treatment and the siting, design, operation, and closure of disposal facilities are not based on long-term projections of the ability of disposal systems to limit releases of hazardous substances to the environment, nor is any consideration given to

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potential risks to inadvertent intruders at disposal sites after loss of institutional control. Rather, in addition to the detailed technical requirements on waste treatment and disposal that apply at any site, the approach to protection of public health and the environment under RCRA relies on monitoring of releases from disposal facilities, especially in groundwater, corrective actions should releases exceed specified limits, and an intention to maintain institutional control over disposal sites for as long as the waste remains hazardous. RCRA also governs disposal of nonhazardous waste in municipal/ industrial landfills. This type of waste includes household trash, various industrial wastes, and characteristically hazardous waste that has been properly treated and is no longer considered hazardous. In current EPA regulations implementing Subtitle D of RCRA, requirements on siting, design, operation, and closure of landfills for nonhazardous waste are similar to requirements that apply to hazardous waste disposal facilities regulated under Subtitle C. Thus, if general principles for exempting wastes that contain hazardous chemicals from RCRA requirements for hazardous waste were established, the primary benefits would likely include the reduction of resources expended on waste treatment, transportation, and storage and the reduced commitment to maintaining institutional control at disposal sites following closure. The system for classification and disposal of hazardous chemical waste developed by EPA under RCRA does not apply to all wastes that contain hazardous chemicals. For example, wastes that contain dioxins, polychlorinated biphenyls (PCBs), or asbestos are regulated under the Toxic Substances Control Act (TSCA). In addition, the current definition of hazardous waste in 40 CFR Part 261 specifically excludes many wastes that contain hazardous chemicals from regulation under RCRA, including certain wastes produced by extraction, beneficiation, and processing of various ores and minerals or exploration, development, and use of energy resources. Thus, the waste classification system is not comprehensive, because many potentially important wastes that contain hazardous chemicals are excluded, and it is not based primarily on considerations of risks posed by wastes, because the exclusions are based on the source of the waste rather than the potential risk.

1.3.4 Comparison of Classification Systems for Radioactive and Hazardous Chemical Wastes The existing classification systems for radioactive and hazardous chemical wastes in the United States and approaches to disposal of

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these wastes were developed largely independently. As a result, there are important differences in the two classification systems and in approaches to waste disposal, but there also are similarities. The similarities are of the following kinds. First, neither classification system includes a general class of exempt waste. Second, neither classification system is comprehensive, because the classification system for radioactive waste distinguishes between fuel-cycle and NARM waste and the classification system for hazardous chemical waste excludes many potentially important wastes that contain hazardous chemicals. Third, any waste must be managed and disposed of in a manner that is expected to protect public health and the environment. In addition, the approach to disposal of hazardous chemical waste under RCRA, which emphasizes monitoring of releases from disposal facilities and an intention to maintain institutional control over disposal sites for as long as the waste remains hazardous, is applied to disposal of uranium or thorium mill tailings under AEA. There also are two important differences. First, the classification system for radioactive waste from the nuclear fuel cycle includes different classes that are defined based essentially on the source of the waste. In addition, some classes of fuel-cycle waste (e.g., highlevel waste) often, but not always, contain higher concentrations of radionuclides than other classes (e.g., low-level waste) and, thus, pose a greater hazard in waste management and disposal. The classification system for hazardous chemical waste does not distinguish between hazardous wastes based on their source, with the exception of the ‘‘K’’ list of wastes from specific sources. Additionally, hazardous chemical wastes are not further classified based on their relative hazard (i.e., there is only one class of hazardous chemical waste). Second, different types of disposal systems are intended to be used for radioactive wastes (e.g., near-surface facilities or geologic repositories), whereas only a single type of disposal system (a nearsurface facility) is used for all hazardous chemical wastes, regardless of the potential risks posed by the waste. Furthermore, acceptable disposals of all radioactive wastes except mill tailings are determined based primarily on long-term projections of potential impacts on public health, including potential impacts on future inadvertent intruders in the case of near-surface facilities. In contrast, the approach to protecting public health in disposal of hazardous chemical waste emphasizes monitoring of releases, corrective actions in the event of unacceptable releases, and maintenance of institutional control over disposal sites, rather than long-term projections of potential impacts on public health.

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1.3.5 Mixed Radioactive and Hazardous Chemical Wastes The definition of solid waste in RCRA specifically excludes source, special nuclear, and byproduct materials as defined in AEA. Therefore, radioactive constituents of wastes that arise from operations of the nuclear fuel cycle are excluded from regulation as hazardous waste under RCRA. The term ‘‘mixed waste’’ refers mainly to waste that contains radionuclides regulated under AEA and hazardous chemical waste regulated under RCRA. Dual regulation of mixed waste has no effect on classification, management, and disposal of the hazardous chemical component or on classification of the radioactive component. The effects of dual regulation of mixed waste on management and disposal of the radioactive component are summarized as follows: ●

●

●

Technical requirements on treatment and disposal of spent fuel, high-level waste, and transuranic waste established under AEA should be largely unaffected by the presence of waste classified as hazardous under RCRA. Some of these wastes meet technologybased treatment standards for hazardous chemical waste established by EPA (e.g., vitrified high-level waste is an acceptable waste form under RCRA). Alternatively, a finding that disposal of the radioactive component of the waste complies with applicable environmental standards established by EPA under AEA can serve to exempt the disposal facility from prohibitions on disposal of restricted hazardous chemical wastes under RCRA [e.g., disposal of mixed transuranic waste at the Waste Isolation Pilot Plant (WIPP)]. Management and disposal of hazardous chemical waste under RCRA is based on detailed and prescriptive technical requirements that apply to any facility for waste treatment, storage, or disposal, whereas management and disposal of low-level radioactive waste is more flexible because AEA allows consideration of waste- and site-specific factors. As a consequence, acceptable approaches to management and disposal of mixed low-level waste probably will be determined primarily by RCRA requirements, unless exempt levels of hazardous chemicals are established that render the waste nonhazardous under RCRA. EPA regulations developed under AEA specify that operations and closure at uranium or thorium mill tailings sites must conform to RCRA requirements on hazardous waste. These requirements acknowledge the presence of hazardous chemicals in mill tailings, especially heavy metals.

1.4 DEVELOPMENT OF A NEW CLASSIFICATION SYSTEM

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Thus, dual regulation of mixed waste does not present insurmountable technical obstacles to waste disposal. The main technical impediment to successful management and disposal of mixed waste has been the difficulty in obtaining operating permits for treatment, storage, and disposal facilities under RCRA, especially facilities for mixed low-level waste at DOE sites. Similar considerations apply to waste that contains radionuclides regulated under AEA, especially low-level waste, and hazardous chemicals regulated under other environmental laws (e.g., TSCA). The exclusion of radioactive materials from regulation under RCRA does not apply to NARM. However, the current definition of hazardous waste in 40 CFR Part 261 does not include NARM waste, and the definition specifically excludes many wastes associated with production or use of energy and mineral resources that contain elevated levels of naturally occurring radionuclides compared with average background levels. These potentially important NARM wastes, which also may contain elevated levels of heavy metals, thus are not regulated under RCRA and issues of dual regulation may arise when NARM waste is mixed with waste regulated under RCRA. 1.4 Approach to Development of a New Waste Classification System NCRP’s recommendations on classification of hazardous wastes are intended to address deficiencies and inconsistencies in the separate systems for classification and disposal of radioactive and hazardous chemical wastes in the United States summarized previously. The most important of these include: ●

●

●

●

the lack of general principles for exempting wastes that contain small amounts of radionuclides or hazardous chemicals from regulatory control as hazardous material; the ambiguities and logical inconsistencies in the radioactive waste classification system, especially the difficulties with the source-based classifications of wastes that arise from operations of the nuclear fuel cycle and the artificial distinction between fuel-cycle and NARM wastes; the source-based exclusions of potentially important wastes that contain hazardous substances from regulation as hazardous waste; the potential problem that the classification system for hazardous chemical waste does not distinguish between wastes that pose a greater or lesser hazard and, thus, that disposal of all such wastes in near-surface facilities may not ensure long-term

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protection of public health in the absence of permanent institutional control over disposal sites; and the use of different disposal systems for radioactive wastes (e.g., near-surface facilities or geologic repositories) that are selected based in part on long-term projections of risks to public health posed by the waste, but the use of a single disposal system (i.e., a near-surface facility) for all hazardous chemical wastes without due consideration of the long-term health risks posed by the waste.

NCRP’s approach to addressing these difficulties is to develop a single hazardous waste classification system that is comprehensive and risk-based.

1.4.1 Basic Elements of Hazardous Waste Classification System NCRP’s recommendations on classification of hazardous wastes are based on two principles. First, a classification system should be generally applicable to any waste that contains radionuclides, hazardous chemicals, or mixtures of the two (i.e., the system should be comprehensive). Second, waste that contains hazardous substances should be classified based on considerations of health risks to the public that arise from waste disposal, because permanent disposal is the intended disposition of materials having no further use. Based on these principles, the essence of NCRP’s recommendations is that waste that contains radionuclides or hazardous chemicals should be classified in relation to the types of disposal systems (technologies) that are expected to be generally acceptable in protecting public health. Specifically, the classification system developed in this Report includes three classes of waste defined as follows: ●

●

●

Exempt waste: any waste containing hazardous substances that is generally acceptable for disposition as nonhazardous material (e.g., disposal in a municipal/industrial landfill for nonhazardous wastes). Low-hazard waste: any nonexempt waste that is generally acceptable for disposal in a dedicated near-surface facility for hazardous wastes. High-hazard waste: any nonexempt waste that generally requires a disposal system more isolating than a dedicated nearsurface facility for hazardous wastes (e.g., a geologic repository).

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Thus, the basic elements of the proposed classification system are, first, that there should be a general class of waste that contains sufficiently small concentrations of radionuclides or hazardous chemicals that it can be exempted from regulatory control as hazardous material and, second, that there should be two classes of nonexempt waste that contain increasing concentrations of hazardous substances and require dedicated disposal systems that provide increased waste isolation. The definition of exempt waste requires further elaboration. Although this Report is concerned with classification of waste for purposes of disposal, NCRP recognizes that some materials that contain only low concentrations of regulated hazardous substances may have beneficial uses if they could be exempted from regulatory control as hazardous material. Thus, NCRP intends that exempt waste could be used or disposed of in any manner allowed by laws and regulations addressing disposition of nonhazardous materials. However, waste that would be exempt for purposes of disposal would not necessarily be exempt for purposes of beneficial use as well. Exemption of materials that contain hazardous substances to allow beneficial use also should be based on considerations of health risks to the public. However, limits on the amounts of hazardous substances that could be present in exempt materials intended for a particular beneficial use could be substantially lower than the limits for disposal as exempt waste, due to differences in exposure scenarios for the two dispositions, and disposal may be the only allowable disposition of some exempt materials based on considerations of risk. In addition, exempt materials may consist of trash, rubble, and residues from industrial processes that have no beneficial uses and must be managed as waste. Based on these considerations and the purpose of this study, disposal is the only disposition discussed in developing recommendations on exemption of waste that contains small amounts of hazardous substances based on risk. Consideration of dispositions of exempt materials other than disposal as nonhazardous waste is beyond the scope of this study. However, the principles used in this Report to define exempt waste based on risk also could be used to define exempt material for any other purpose. 1.4.2 Assumptions in Developing the Waste Classification System Given the basic elements of a new waste classification system described in the previous section, NCRP proceeded with development of the system on the basis of several assumptions.

28 ●

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The recommendations on waste classification should focus on concepts, principles, and approaches to implementation. Recommendations on approaches to using assumed limits on risk or dose to establish quantitative boundaries of waste classes expressed as limits on concentrations of hazardous substances would be presented, and precedents that could be used to define the assumed limits on risk or dose and to assess risk or dose for purposes of waste classification would be discussed. However, specific recommendations on values of any such limits and many of the considerations involved in establishing them would not be given, because this is properly the role of policy makers and regulatory authorities. The waste classification system should be based on the distinct concepts of negligible and acceptable (i.e., barely tolerable) risks to the public that arise from waste disposal. Precedents for specifying negligible or acceptable risks that could be used in classifying waste, such as other NCRP recommendations, would be cited, but specific recommendations would not be presented in this Report. Legal impediments to development of a new waste classification system would be ignored. These include, for example, the distinction between radioactive waste that arises from operations of the nuclear fuel-cycle and NARM waste, which is based on provisions of AEA, the distinction between radioactive and hazardous chemical wastes, which is based on provisions of AEA and RCRA, and the provision in the National Energy Policy Act that prohibits NRC from establishing a general class of exempt radioactive waste.

Thus, NCRP’s recommendations focus on the technical foundations for a generally applicable and risk-based waste classification system.

1.4.3 Challenges in Developing a Waste Classification System Development of a generally applicable and risk-based waste classification system presents a number of technical challenges. ●

The classification system must apply to waste that contains carcinogenic and noncarcinogenic hazardous substances. Therefore, classification of waste based on risk must take into account the different forms of the assumed dose-response relationships for these two types of substances (response proportional to dose, without threshold, for carcinogens; threshold for noncarcinogens).

1.5 DEVELOPMENT OF THE RECOMMENDED SYSTEM ●

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The current approach to risk management (control of exposures) for hazardous chemicals differs from the approach for radionuclides under AEA. In particular, the two approaches to risk management attach different meanings to the terms ‘‘acceptable’’ and ‘‘unacceptable’’ commonly used to describe the significance of health risks. The term ‘‘dose’’ has different meanings for radionuclides and hazardous chemicals. In assessments of risk to human health, this term generally refers to energy imparted to tissue and its biological significance for radionuclides, but it usually refers to mass intakes for hazardous chemicals. The measure of risk (health-effect endpoint) calculated in risk assessments often differs for radionuclides and chemical carcinogens. Fatalities is the measure of risk most often used for radionuclides, but cancer incidence is generally used for chemical carcinogens. The approach to estimating health risks differs for radionuclides and hazardous chemicals in regard to the degree of conservatism incorporated in the assumed probabilities of an adverse health effect per unit dose and the number of organs at risk that are taken into account. Development of a waste classification system based on considerations of risks to the public requires assumptions about generic exposure scenarios (i.e., exposure scenarios that are generally applicable at any disposal site).

Each of these issues is addressed in presenting NCRP’s recommendations on classification of hazardous waste in the following section.

1.5 Development of the Recommended Waste Classification System

1.5.1 Risk Index for Waste Classification For the purpose of developing the waste classification system described in Section 1.4.1, a simple method of evaluating risks to the public posed by radionuclides and hazardous chemicals in waste is needed. The term ‘‘risk’’ generally refers to the probability of harm, combined with the potential severity of that harm. In the context of hazardous waste disposal, risk is the probability of a response in an individual or the frequency of a response in a population taking into

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account: (1) the probability of occurrence of processes and events that could result in release of hazardous substances to the environment and the magnitude of such releases, (2) the probability that individuals or populations would be exposed to the hazardous substances released to the environment and the magnitude of such exposures, and (3) the probability that an exposure would produce a response. In classifying waste based on risk, however, exposures are assumed to occur according to postulated scenarios, and the only component of the probability of harm considered in estimating risk is the probability of a response from a given exposure. NCRP recommends that risks to hypothetical individuals at waste disposal sites should be evaluated in classifying waste, as described in the following section, and that the risk to an individual that arises from disposal of any hazardous substance be expressed in the form of a dimensionless risk index (RI). The risk index for the ith hazardous substance (RIi) is defined in terms of the risk that arises from disposal of that substance relative to a specified allowable risk for an assumed type of disposal system (e.g., municipal/industrial landfill for disposal of exempt waste) as: RI i ⳱ Fi

(risk from disposal) i , (allowable risk) i

(1.1)

where F is a modifying factor (F ⬎ 0) that can depend on the particular hazardous substance and in each case the index i applies to the ith hazardous substance involved. The risk in the numerator is evaluated using generic exposure scenarios appropriate to the sumed type of disposal system for the particular waste class of concern. The modifying factor (F) in Equation 1.1 is intended to represent any considerations of importance to a decision about the general acceptability of waste disposal using an assumed technology, other than those directly incorporated in the calculated risk from disposal and the specified allowable risk. This factor can take into account, for example, the general design of a disposal facility, general requirements on waste packages and waste forms, the volume of waste, the intended emplacement of waste as it would affect credible exposure scenarios, the probability of occurrence of an assumed exposure scenario, and uncertainties in the assessment of risk that arises from disposal and in the data required to evaluate Equation 1.1. The modifying factor is exemplified by assumptions used by NRC in developing the concentration limits for near-surface disposal of the small volumes of Class-C low-level waste in 10 CFR Part 61. These limits incorporate assumptions about the probability that exposures to Class-C waste would occur according to a postulated scenario and

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the ability of waste emplacements and engineered barriers to delay exposures to Class-C waste that were not used in developing the concentration limits for the much larger volumes of Class-A lowlevel waste. The modifying factor also can incorporate considerations of risk management, such as the cost-benefit of different options for disposal of specific wastes, considerations of levels of naturally occurring hazardous substances (e.g., uranium, radium, arsenic) in surface soil and their associated health risk to the public, and any other judgments of importance to waste classification. Risk is not always a useful measure of health impact in evaluating the risk index, because risk is not proportional to dose when a hazardous substance is assumed to have a threshold dose-response relationship. For this type of substance, the risk is presumed to be zero at any dose below a nominal threshold. Since the allowable dose of such substances should always be less than the threshold in order to prevent the occurrence of adverse responses, expressing the risk index in terms of risk would result in an indeterminate value when the dose is below the threshold and, more importantly, a lack of distinction between doses near the nominal thresholds and lower doses of much less concern. For any hazardous substance, including carcinogens for which risk is assumed to be proportional to dose without threshold, a generally useful form of risk indexes (RI i) is in terms of dose: RI i ⳱ Fi

(dose from disposal) i , (allowable dose) i

(1.2)

where the index i applies to the ith hazardous substance involved. The difference in the meaning of ‘‘dose’’ for radionuclides and hazardous chemicals described in Section 1.4.3 is unimportant as long as the same meaning is used for a given hazardous substance in the numerator and denominator of Equation 1.2. Given the definition of risk indexes (RIi) in Equation 1.1 or 1.2 and assuming that risks from exposure to the different hazardous substances in waste are additive, waste classes are defined by the requirement on each waste class and associated disposal system that:

兺 RI

i

⬍ 1.

(1.3)

i

Adding risk indexes (RI i) for noncarcinogenic substances and combining risk indexes (RI i ) for carcinogenic and noncarcinogenic substances requires care, however, due to the assumed forms of the dose-response relationships. The evaluation of Equation 1.3 for mixtures of hazardous substances is described in Section 1.5.5.4.

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1.5.2 Generic Exposure Scenarios for Waste Classification The numerator in Equation 1.1 or 1.2 is calculated using generic scenarios for exposure of individual members of the public that arise from waste disposal. Two types of exposure scenarios can be considered: (1) scenarios involving release of hazardous substances from a disposal facility and exposure of individuals at locations beyond the boundary of the disposal site; or (2) scenarios involving exposure of individuals who inadvertently intrude onto a disposal site, including scenarios involving permanent residence on a disposal site or other unrestricted access after an assumed loss of institutional control. NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at disposal sites should be used in classifying waste. This recommendation is based on two considerations. First, scenarios for inadvertent intrusion can be applied to an assumed type of disposal system at any site, whereas scenarios for exposure of members of the public due to release and transport of hazardous substances to locations beyond the boundary of a disposal facility are highly site-specific and, thus, are not appropriate for use in generally classifying waste. Second, generic and site-specific assessments of near-surface disposal facilities for radioactive waste have shown that allowable doses to hypothetical inadvertent intruders usually are more restrictive in determining acceptable disposals than allowable doses to individuals beyond the boundary of the disposal site. This conclusion is based on predictions that concentrations of radionuclides in the environment (e.g., ground-water) at locations beyond the site boundary usually should be far less than the concentrations at the disposal site to which an inadvertent intruder could be exposed, owing to such factors as the limited solubility of some radionuclides, the partitioning of radionuclides between liquid and solid phases, and the dilution in transport of radionuclides in water or air beyond the site boundary. More people are likely to be exposed beyond the site boundary than on the disposal site, but acceptable disposals of radioactive waste in near-surface facilities have been based on assessments of dose to individuals, rather than populations. The recommendation that generic scenarios for exposure of hypothetical inadvertent intruders should be used in classifying waste is consistent with the approach used by NRC in 10 CFR Part 61 to establish different subclasses of low-level radioactive waste that are generally acceptable for near-surface disposal (Class-A, -B, and -C waste) or are generally unacceptable for near-surface disposal (greater-than-Class-C waste). Such scenarios have not been used to

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determine acceptable disposals of hazardous chemical waste in nearsurface facilities regulated under RCRA. However, scenarios similar to those developed by NRC have been used in risk assessments at sites contaminated with hazardous chemicals or radionuclides that are subject to remediation under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Generic scenarios for inadvertent intrusion to be used in classifying waste should be credible for an assumed type of disposal system at any site. Appropriate scenarios for inadvertent intrusion are discussed further in Section 1.5.4.

1.5.3 Determination of Allowable Risk or Dose Evaluation of the risk index (RI) in Equation 1.1 or 1.2 requires assumptions about allowable risks or doses from waste disposal to be used in defining the different waste classes (see Section 1.4.1). These assumptions should be based on an understanding of differences in the approaches to risk management for radionuclides and hazardous chemicals embodied in current laws and regulations, including the different meanings that have been attached to the terms ‘‘acceptable’’ and ‘‘unacceptable’’ commonly used to describe the significance of health risks. The approach to risk management for radionuclides, when they are regulated under AEA, incorporates a limit on acceptable dose (and therefore risk) and a requirement that doses be reduced below the limit as low as reasonably achievable (ALARA), economic and social factors being taken into account; this approach conforms to NCRP’s recommendations on radiation protection. In this approach, risks to individuals are divided into three categories of significance, which are commonly termed ‘‘unacceptable,’’ ‘‘acceptable,’’ and ‘‘negligible.’’ ●

●

The term ‘‘unacceptable’’ is used to describe excess lifetime cancer risks from exposure to radionuclides greater than a value in the range of about 10ⳮ1 to 10ⳮ3, the particular value depending on the exposure situation. Such risks normally must be reduced regardless of cost or other circumstances and, thus, are properly interpreted as intolerable (de manifestis). The term ‘‘acceptable’’ is used to describe risks below intolerable levels that also are ALARA. Risks just below ‘‘unacceptable’’ levels are regarded as barely tolerable and normally should be reduced substantially based on the ALARA principle. Risks that are ALARA may vary from one exposure situation to another;

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i.e., a risk that is ALARA is not a predetermined result that applies to all sources and practices. The term ‘‘negligible’’ is used to describe risks so low that further efforts at risk reduction using the ALARA principle generally are unwarranted; i.e., the risks are de minimis. However, achieving a negligible risk is not the goal of ALARA, and a risk that is ALARA (‘‘acceptable’’) can be substantially above negligible levels.

A negligible dose or risk from exposure to radionuclides has not been established in regulations under AEA. However, based on recommendations of NCRP and IAEA, excess lifetime cancer risks on the order of 10ⳮ4 or less generally could be considered negligible. The approach to risk management for hazardous chemicals developed under several environmental laws (e.g., Safe Drinking Water Act, RCRA, CERCLA) essentially is the opposite of the approach to regulating radionuclides under AEA described above. The approach for hazardous chemicals incorporates goals for acceptable risk and allowance for an increase (relaxation) in risks above the goals based primarily on considerations of technical feasibility and cost. In this approach, which also applies to radionuclides when they are regulated under laws addressing hazardous chemicals, risks or doses to individuals are divided into two categories of significance, which are commonly termed ‘‘acceptable’’ and ‘‘unacceptable.’’ ●

●

The term ‘‘acceptable’’ is used to describe excess lifetime cancer risks in the range of about 10ⳮ4 to 10ⳮ6, the particular value depending on the exposure situation, or intakes of noncarcinogens less than EPA’s reference doses (RfDs). The term ‘‘unacceptable’’ is used to describe lifetime cancer risks or intakes of noncarcinogens greater than ‘‘acceptable’’ levels.

RfDs are estimates of daily intakes of noncarcinogenic substances that are expected to be without an appreciable risk of deleterious health effects in sensitive population groups (e.g., children, the elderly). An RfD usually is derived from the highest dose without adverse effect in studies in humans or animals, referred to as the no-observed-adverse-effect level (NOAEL), using one or more safety and uncertainty factors that depend on the nature and quality of the data. Some RfDs are derived from the lowest dose at which a significant increase in adverse effects is observed, referred to as the lowest-observed-adverse-effect level (LOAEL), using an additional uncertainty factor that accounts for the uncertainty in extrapolating from a LOAEL to a NOAEL. RfDs are widely used in health protection of the public, but they do not represent threshold doses of noncarcinogenic hazardous chemicals.

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In the approach to risk management for hazardous chemicals, risks termed ‘‘acceptable’’ are properly interpreted as negligible, because action to reduce risks at these levels generally is not required. However, risks termed ‘‘unacceptable’’ are not necessarily intolerable because risks above ‘‘acceptable’’ levels often are permitted (e.g., in remediating contaminated sites under CERCLA). Rather, ‘‘unacceptable’’ refers to risks sufficiently high that risk reduction must be considered, but action to reduce risk is required only to the extent feasible. This approach does not explicitly include the concept of an intolerable risk that normally must be reduced regardless of cost or other circumstances. These considerations apply to noncarcinogens as well as carcinogens, owing to the large safety and uncertainty factors often applied in deriving RfDs. Based on these discussions, the commonly used terms ‘‘acceptable’’ and ‘‘unacceptable’’ clearly do not have the same meanings in the different approaches to risk management for radionuclides and hazardous chemicals. ‘‘Acceptable’’ risks or doses for hazardous chemicals generally correspond to negligible levels for radionuclides, whereas ‘‘acceptable’’ risks or doses for radionuclides can be well above negligible levels, provided they are ALARA. For hazardous chemicals, ‘‘unacceptable’’ essentially means ‘‘non-negligible’’ and this term does not distinguish between risks or doses so high that they are intolerable, where reductions normally would be required regardless of cost or other circumstances, and lower risks or doses above negligible levels where reductions are required only to the extent feasible. For radionuclides, ‘‘unacceptable’’ refers to risks or doses well above negligible levels that are intolerable under normal circumstances. These differences in meanings are important in understanding the approaches to risk management for radionuclides and hazardous chemicals, and they are summarized in Table 1.3. NCRP recognizes that both of the approaches to risk management described above are valid. NCRP believes, however, that the approach to risk management for radionuclides is more transparent in depicting how risk management decisions are made, because it explicitly includes a range of risks between negligible and intolerable levels where risks are managed based on the ALARA principle. In the approach to risk management for hazardous chemicals, the ALARA principle is only implicit in the proper interpretation of ‘‘unacceptable’’ given in Table 1.3, and the term ‘‘unacceptable’’ as commonly used in this approach is easily misinterpreted as referring to intolerable risks. The clear separation between negligible and intolerable risks in the approach to risk management for radionuclides is particularly relevant to waste classification because it allows the use of distinctly different levels of risk in classifying waste. Specifically,

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TABLE 1.3—Differences in interpretations of ‘‘acceptable’’ and ‘‘unacceptable’’ risks in approaches to risk management for radionuclides and hazardous chemicals.a

Description of Risk

Interpretation in Risk Management for Radionuclides b

Interpretation in Risk Management for Hazardous Chemicals c

‘‘Acceptable’’

Risks are below intolerable (de manifestis) levels and are ALARAd

Risks are negligible (de minimis); further reduction of risks usually need not be considered e

‘‘Unacceptable’’

Risks are intolerable; risks normally must be reduced regardless of cost or other circumstances f

Risks are above negligible levels; reduction of risks must be considered but is required only to the extent feasibleg

a Interpretations of commonly used terms also apply to dose when control of exposures is based on dose rather than risk; dose generally is used for noncarcinogenic hazardous chemicals and often is used for radionuclides. b Interpretations apply to control of exposures to radionuclides under AEA, but not to control of exposures to radionuclides under other environmental laws. c Interpretations also apply to control of exposures to radionuclides when they are regulated under laws addressing hazardous chemicals. d Excess lifetime cancer risks considered intolerable have values in the range of about 10ⳮ1 to 10ⳮ3 or greater, depending on the exposure situation, and are well above risks considered negligible (e.g., excess lifetime risks on the order of 10ⳮ4 or less). Risks that are ALARA depend on the particular exposure situation, and achieving a negligible risk is not the goal of ALARA. e Excess lifetime cancer risks considered negligible have values in the range of about 10ⳮ4 to 10ⳮ6 or below, depending on the exposure situation; intakes of noncarcinogenic hazardous chemicals less than RfDs are considered negligible. f Risks also are considered unacceptable if they are below intolerable levels but are not ALARA. g Approach to risk management for hazardous chemicals does not explicitly include concept of an intolerable risk that normally must be reduced regardless of cost or other circumstances.

with reference to the definitions of waste classes in Section 1.4.1, a negligible risk can be used to classify exempt waste and a substantially higher acceptable (barely tolerable) risk can be used to classify low-hazard waste. Recommendations on establishing negligible and

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acceptable risks for purposes of waste classification are discussed further in the following section.

1.5.4 Recommended Framework for Risk-Based Waste Classification NCRP’s recommendations on a framework for a risk-based classification system that is applicable to any waste containing radionuclides or hazardous chemicals are described in Table 1.4 and depicted in Figure 1.2. This framework follows from the assumption that waste classes should be defined in relation to types of disposal systems that are expected to be generally acceptable in protecting the public (Section 1.4.1), the definition of the risk index for any hazardous substance in terms of the risk that arises from disposal of that substance relative to a specified allowable risk for a particular waste class and associated disposal system (Section 1.5.1), the recommendation on the type of generic exposure scenario that should be used for purposes of waste classification (Section 1.5.2), and recognition of the distinction between a negligible and an acceptable risk (Section 1.5.3). The different waste classes are discussed in the following paragraphs. 1.5.4.1 Exempt Waste. Waste classified as exempt would be regulated as if it were nonhazardous, and would be generally acceptable for disposition as nonhazardous material (e.g., disposal in a municipal/ industrial landfill). As noted in Section 1.4.1, disposal is the only disposition of exempt materials considered in this Report. Limits on concentrations of hazardous substances in exempt waste would be derived based on an assumption that the risk or dose to a hypothetical inadvertent intruder at a disposal site should not exceed negligible levels. The use of a negligible risk or dose to determine exempt waste is based on an assumption that a disposal facility for nonhazardous waste could be released for unrestricted use by the public soon after the facility is closed. Negligible risks or doses used to classify exempt waste could be established based on a variety of considerations, consistent with the different approaches to risk management for radionuclides and hazardous chemicals described in Section 1.5.3. For noncarcinogenic hazardous chemicals, NCRP recommends that a negligible dose should be set at a small fraction (e.g., 10 percent) of a nominal threshold for deterministic responses in humans; the recommended approach to estimating this threshold is described in Section 1.5.5.3. For radionuclides, NCRP has recommended that an annual effective

Any nonexempt waste that is generally acceptable for disposal in dedicated nearsurface facility for hazardous wastes d

Any nonexempt waste that generally requires disposal system more isolating than dedicated near-surface facility for hazardous wastes f

Low-hazard waste

High-hazard waste

Concentrations of any hazardous substances that exceed limits for low-hazard waste

Based on a risk index less than unity for all hazardous substances and assumption that risk or dose to hypothetical inadvertent intruder at disposal site should not exceed acceptable (barely tolerable) levels e

Based on a risk index less than unity for all hazardous substances and assumption that risk or dose to hypothetical inadvertent intruder at disposal site should not exceed negligible levels

Determination of Boundary of Waste Class b

a Classification system applies to any waste that contains radionuclides or hazardous chemicals. Waste classification system does not provide a substitute for site-specific risk assessments in developing waste acceptance criteria at particular disposal facilities or in developing criteria for remediation of particular contaminated sites. b Boundaries of waste classes normally would be expressed as limits on concentrations of hazardous substances; there would be no such limits in high-hazard waste. The risk index for waste classification is described in Sections 1.5.1 and 1.5.5. c Disposal of nonhazardous waste in municipal/industrial landfills is currently permitted under Subtitle D of RCRA; however, classification of waste as exempt is not intended to preclude any beneficial uses or other means of disposal allowed by laws and regulations addressing disposition of nonhazardous materials. d Facilities for low-level radioactive waste currently permitted under AEA in accordance with NRC requirements in 10 CFR Part 61 (or compatible Agreement State requirements) or requirements in applicable DOE Orders, facilities for uranium or thorium mill tailings currently permitted under AEA in accordance with EPA requirements in 40 CFR Part 192 or facilities for hazardous chemical waste currently permitted under Subtitle C of RCRA. e Acceptable risks or doses generally would be substantially higher than negligible levels used to define exempt waste. f Examples of suitable facilities include geologic repositories for high-level and transuranic radioactive waste currently permitted under the Nuclear Waste Policy Act or the National Security and Military Applications of Nuclear Energy Act, but other types of greater confinement disposal facilities also could be considered; similar facilities for hazardous chemical waste have not been established under RCRA.

Any waste containing hazardous substances that is generally acceptable for disposition as nonhazardous material (e.g., disposal in municipal/industrial landfill) c

General Definition

/

Exempt waste

Class

TABLE 1.4—Framework for the recommended risk-based waste classification system.a

38 1. TECHNICAL SUMMARY

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Fig. 1.2. Depiction of waste classes defined in relation to acceptable disposal systems in recommended risk-based waste classification system.

dose of 10 ␮Sv, which corresponds to an estimated lifetime fatal cancer risk of about 4 ⳯ 10ⳮ5 for an assumed exposure time of 70 y, is a negligible individual dose for any source or practice; this dose also was used by IAEA to define an exempt class of radioactive waste (see Table 1.2). Similarly, EPA has proposed that a negligible lifetime risk of about 10ⳮ5 could be used to exempt waste that contains chemical carcinogens from requirements for disposal as hazardous waste under RCRA. As an alternative, RfDs established by EPA could be used to define negligible doses of noncarcinogenic hazardous chemicals. RfDs usually are derived from NOAELs or LOAELs by applying a safety and uncertainty factor between 100 and 10,000 depending on the nature and quality of the data, with values of at least 100 being most common. Thus, RfDs are intended to be well below thresholds for deterministic responses in humans. However, NCRP believes that RfDs should not be used without presenting NOAELs or LOAELs used to derive the values. In addition, when RfDs for important waste constituents are derived using large safety and uncertainty

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factors, thus indicating that the quality of the data is poor, NCRP believes that further studies should be undertaken to reduce uncertainties in the nominal threshold in humans, to avoid introducing undue levels of conservatism in classifying waste. To promote consistency in waste classification, NCRP believes that it would be desirable to define negligible doses of all noncarcinogens at approximately the same fraction of the nominal thresholds in humans. Negligible risks or doses for radionuclides and chemical carcinogens also could be established based on considerations of unavoidable risks from natural background. Since the average lifetime risks from exposure to natural background radiation and naturally occurring chemical carcinogens each are about 10ⳮ2, a negligible risk could be set at a small fraction (e.g., one percent) of the average background risk. Such a risk should be less than the variability in the background risk at any location due to differences in living habits. The negligible individual dose for radiation discussed above is consistent with this alternative, because an annual effective dose of 10 ␮Sv is about one percent of the dose due to natural background radiation, excluding radon. Exemption of waste that contains naturally occurring hazardous substances warrants further consideration because the negligible risks or doses for carcinogens described above may correspond to exemption levels that are less than background levels in soil or rock. This could be the case, for example, for radium, thorium, and arsenic. As a consequence, exemption of virtually any waste derived from earthen materials could be precluded, even when the concentrations of naturally occurring hazardous substances are not enhanced by human activities. In order to provide a practical system for exempting such wastes, NCRP believes that exemption levels for naturally occurring hazardous substances in waste should be based on considerations of background levels in surface soil and their associated health risks to the public, in addition to the negligible risks used to establish exemption levels for man-made substances. These additional considerations could be incorporated in the modifying factor (F) in Equation 1.1, which can be waste- and substance-specific. Disposal facilities for nonhazardous waste (e.g., municipal/industrial landfills) normally are constructed without substantial engineered barriers, such as a rock cover or cement waste forms, that would deter inadvertent intrusion into waste, and the waste itself often is in a readily accessible physical form. Therefore, in determining exempt waste, scenarios for inadvertent intrusion involving permanent occupancy of disposal sites and normal human activities that could access waste would be appropriate. Examples include excavation in the construction of homes and permanent residence on

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a site at any time after an excavation. Furthermore, these scenarios should be assumed to occur at the time of facility closure, because institutional control is not expected to be maintained at disposal sites for nonhazardous waste for a substantial period of time thereafter. These assumptions would apply to any allowable means of disposal of exempt waste on or near the land surface. An assumption that exempt waste would be sent to a landfill for nonhazardous waste permitted under Subtitle D of RCRA is not required. 1.5.4.2 Low-Hazard Waste. Waste classified as low-hazard would be generally acceptable for disposal in a dedicated near-surface facility for hazardous wastes. Limits on concentrations of hazardous substances in low-hazard waste would be derived based on an assumption that the risk or dose to a hypothetical inadvertent intruder at a disposal site should not exceed acceptable (barely tolerable) levels. Acceptable risks or doses used to determine low-hazard waste should be substantially higher than the negligible risks or doses used to determine exempt waste (see Section 1.5.3). As a result, limits on concentrations of hazardous substances in low-hazard waste generally should be substantially higher than in exempt waste. The use of higher risks or doses in classifying low-hazard waste can be justified on the following grounds. Institutional control is planned to be maintained over dedicated near-surface disposal sites for hazardous wastes until the sites can be released for unrestricted use by the public. Furthermore, allowable risks or doses used to define conditions for unrestricted release should be similar to the values that would be used to determine exempt waste. Therefore, prior to unrestricted release of a site, scenarios for inadvertent intrusion into near-surface disposal facilities for low-hazard waste should be regarded as accidental occurrences. Risks to inadvertent intruders, taking into account the probability that assumed exposure scenarios would occur with a specified duration of exposures, should be comparable to or less than risks resulting from unrestricted access to nearsurface disposal sites for exempt waste. Acceptable (barely tolerable) risks or doses used to classify low-hazard waste could be established based on a variety of considerations, consistent with the different approaches to risk management for radionuclides and hazardous chemicals described in Section 1.5.3. For noncarcinogenic hazardous chemicals, NCRP recommends that an acceptable dose should be set at a nominal threshold for deterministic responses in humans obtained as described in Section 1.5.5.3, or slightly below the threshold (e.g., by a factor of two or three) if an additional margin of safety is warranted. For

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radionuclides, the limit on annual effective dose to individual members of the public of 1 mSv recommended by NCRP, which corresponds to an estimated lifetime fatal cancer risk of about 4 ⳯ 10ⳮ3 for an assumed exposure time of 70 y, provides a suitable precedent. The limits on concentrations of radionuclides in waste that is generally acceptable for near-surface disposal established by NRC in 10 CFR Part 61, based on scenarios for inadvertent intrusion following an assumed loss of institutional control at 100 y after disposal, also could be used. An acceptable (barely tolerable) risk from exposure to chemical carcinogens has not been considered by EPA, but its value should be about the same as for radionuclides. As an alternative, multiples of RfDs established by EPA could be used to define acceptable (barely tolerable) doses of noncarcinogenic hazardous chemicals, because RfDs normally are intended to be well below nominal thresholds for deterministic responses in humans. However, the cautions about using RfDs discussed in the previous section, especially when RfDs are based on data of poor quality, also apply in establishing acceptable doses. As in establishing negligible doses of noncarcinogens, NCRP prefers an approach in which acceptable doses are based directly on nominal thresholds in humans and application of small safety factors, as appropriate, to promote transparency and consistency in waste classification. Acceptable risks or doses for radionuclides and chemical carcinogens also could be established based on considerations of unavoidable risks from natural background; as noted previously, these lifetime risks are about 10ⳮ2. For example, an acceptable risk could be set at a value corresponding approximately to the geographical variability in the background risk, because people normally do not consider this variability in deciding where to live. The assumed disposal systems for exempt waste and low-hazard waste both involve near-surface disposal, and either type of waste often would be emplaced sufficiently close to the surface that inadvertent intrusion into the waste could occur as a result of normal human activities. However, there are differences in the two types of disposal systems that should be taken into account in developing appropriate scenarios for inadvertent intrusion. Disposal facilities for low-hazard waste frequently include engineered barriers to deter inadvertent intrusion, impenetrable waste forms, or deliberate emplacement of more hazardous wastes at locations where access to the waste during normal human activities would be less likely. Most importantly, as noted previously, current plans call for institutional control to be maintained over hazardous waste disposal sites for a considerable period of time after facility closure, which allows for substantial

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decay of many radionuclides and the possibility of chemical transformations of other hazardous substances to less hazardous forms prior to the time permanent occupancy of disposal sites by the public could occur. Inadvertent intrusion could occur during the institutional control period, but credible scenarios would differ from scenarios involving permanent occupancy of disposal sites in regard to the relevant exposure pathways and the duration of exposures. The role of institutional control over near-surface disposal sites is particularly important in cases of very large volumes of waste, such as uranium mill tailings and wastes from mining and milling of ores to extract nonradioactive materials, that contain concentrations of naturally occurring hazardous substances (e.g., radium, heavy metals) far above background levels in Earth’s crust. For such wastes, the risk to an inadvertent intruder often would be well above any level that could be considered acceptable if permanent occupancy of near-surface disposal sites could occur. In the case of uranium mill tailings, however, disposal in facilities located well below the ground surface had been deemed impractical due to the volumes of waste involved. Rather, the intention is to maintain perpetual institutional control over near-surface disposal sites to prevent scenarios for inadvertent intrusion involving permanent site occupancy. Similar considerations could apply to large volumes of other mining and milling wastes. In developing generic scenarios for inadvertent intrusion into near-surface disposal facilities used to determine limits on concentrations of hazardous substances in exempt and low-hazard waste, consideration must be given to the question of how far into the future the scenarios should be applied, as well as the earliest time at which the scenarios could occur. This issue arises because the potential risk posed by some radionuclides (e.g., uranium) increases with time, due to the long-term buildup of radiologically significant decay products, and some hazardous chemicals could be transformed over time into more hazardous forms. NCRP believes that scenarios for inadvertent intrusion used to classify waste should be applied over a time period consistent with the time period for applying standards for protection of members of the public beyond the boundaries of waste disposal sites. 1.5.4.3 High-Hazard Waste. Waste classified as high-hazard would contain such high concentrations of radionuclides or hazardous chemicals that it would not be generally acceptable for nearsurface disposal in a dedicated facility for hazardous waste, but would require disposal in a facility located well below the ground surface. At the present time, geologic repositories are intended for

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disposal of most radioactive waste that is not generally acceptable for near-surface disposal (see Table 1.1 and Section 1.3.1.1), but there are no planned alternatives to near-surface disposal for highly hazardous chemical wastes. An important characteristic of acceptable disposal facilities for high-hazard waste is that inadvertent intrusion into a facility, such as by drilling, must be unlikely. Therefore, assessments of risk or dose to hypothetical inadvertent intruders based on exposure scenarios that are assumed to occur do not provide a suitable basis for determining acceptable disposals in facilities located well below the ground surface. 1.5.5 Calculation of the Risk Index The risk index for any hazardous substance in Equation 1.1 or 1.2 (see Section 1.5.1) is calculated based on assumed exposure scenarios for hypothetical inadvertent intruders at near-surface waste disposal sites and a specified negligible risk or dose in the case of exempt waste or acceptable (barely tolerable) risk or dose in the case of lowhazard waste. Calculation of the risk index also requires consideration of the appropriate measure of risk (health-effect endpoint), especially for carcinogens, and the appropriate approaches to estimating the probability of a stochastic response per unit dose for carcinogens and the thresholds for deterministic responses for noncarcinogens. Given a calculated risk index for each hazardous substance in a particular waste, the waste then would be classified using Equation 1.3. 1.5.5.1 Measure of Risk for Carcinogens. The health-effect endpoint most often calculated in risk assessments for radionuclides is cancer fatalities, whereas cancer incidence normally is calculated for chemical carcinogens. In principle, the same measure of risk should be used for all carcinogens in calculating the risk index. However, since about half or more of most cancers are fatal, the difference between cancer fatalities and cancer incidence usually is only about a factor of two or less. Such small differences generally should be unimpotant in classifying waste. 1.5.5.2 Estimates of Probability Coefficients for Carcinogens. The nominal probabilities of a stochastic response (primarily cancers) per unit dose used in risk assessments, which are referred to in this Report as probability coefficients, normally differ for radionuclides and chemical carcinogens in regard to the degree of conservatism incorporated in the assumed values and the number of organs or tissues at risk that are taken into account.

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The nominal probability coefficient for radionuclides normally used in radiation protection is derived mainly from maximum likelihood estimates (MLEs) of observed responses in the Japanese atomicbomb survivors. A linear or linear-quadratic dose-response model, which is linear at low doses, is used universally to extrapolate the observed responses at high doses and dose rates to the low doses of concern in radiation protection. The probability coefficient at low doses also includes a small adjustment that takes into account an assumed decrease in the response per unit dose at low doses and dose rates compared with the observed responses at high doses and dose rates. In contrast, nominal probability coefficients for chemical carcinogens are derived from upper 95 percent confidence limits of observed responses at high doses, mainly in studies in animals. In some studies, the difference between the upper 95 percent confidence limit and MLE of the observed responses at high doses is an order of magnitude or more. Furthermore, several models have been used to extrapolate the observed responses to the low doses of concern in health protection of the public, with the result that estimated probability coefficients at low doses can differ by several orders of magnitude depending on the extrapolation model chosen. Thus, the nominal probability coefficients at low doses of chemical carcinogens could be considerably more conservative (more likely to overestimate risk) than the probability coefficient for radionuclides. As a result, potential risks posed by chemical carcinogens could be given a disproportionate weight in classifying waste. For the purpose of classifying waste that contains radionuclides, NCRP reaffirms use of the nominal probability coefficient for fatal cancers (i.e., the probability of a fatal cancer per unit effective dose) of 0.05 Svⳮ1 normally assumed in radiation protection of the public. For chemical carcinogens, NCRP believes that MLEs of probability coefficients obtained from the linearized, multi-stage model should be used in classifying waste, in order to provide reasonable consistency with the probability coefficient for radionuclides. The use of MLEs for chemical carcinogens usually will result in substantially lower probability coefficients than the use of upper 95 percent confidence limits. The use of MLEs of probability coefficients, rather than upper confidence limits (UCLs), to classify waste can be justified, in part, on the grounds that the assumed exposure scenarios for hypothetical inadvertent intruders at waste disposal sites are expected to be conservative compared with likely on-site exposures at future times. However, uncertainties in probability coefficients should still be considered in classifying waste. When risk is calculated using MLEs of

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probability coefficients, judgments about allowable risk, which can be substance-specific, should take uncertainties in probability coefficients into account, along with such other factors as judgments about the quality of the data on dose-response, desired margins of safety in protecting the public, and the cost-benefit of different choices. This approach would provide a clear separation between risk assessment and risk management aspects of waste classification. Risk assessment would focus on central estimates of risk for assumed exposure scenarios, and risk management decisions could incorporate any desired degrees of conservatism in protecting the public beyond those embodied in the assumed scenarios. In risk assessments for radionuclides, the nominal probability coefficient is applied to the effective dose, which takes into account doses and stochastic responses in all irradiated organs or tissues. In contrast, probability coefficients for chemical carcinogens often are based on observed responses in a single organ and the possibility of significant responses in multiple organs is not taken into account. This difference cannot be eliminated at the present time, in part because the probability coefficients for most chemicals are based on studies in animals and the organs in which cancers are seen in the study animals often do not correspond to the organs at greatest risk in humans. However, the extent of underestimation of risks from exposure to chemical carcinogens is unlikely to be large when cancers presumably are induced only at sites of deposition in the body. For a few chemical carcinogens, the probability coefficients are based on observed responses in multiple organs. 1.5.5.3 Thresholds for Deterministic Effects. In controlling exposures to substances that induce deterministic health effects, the goal is to prevent such effects by limiting doses to levels considered safe, i.e., to levels below those known to cause adverse effects. Thus, the goal is to achieve zero risk. For radionuclides and noncarcinogenic hazardous chemicals, dose limits for the public are established by applying safety and uncertainty factors to nominal threshold doses estimated from studies in humans or animals. Thus, doses considered safe are substantially less than no-effects levels observed in the studies. For radionuclides, deterministic dose limits are unimportant in routine health protection of the public, and should be unimportant in classifying waste, because the limit on annual effective dose of 1 mSv from exposure to all man-made sources combined, which is intended to limit stochastic effects, generally ensures that equivalent doses in any organ or tissue will be substantially less than the limits intended to prevent deterministic effects. Therefore, the approach to estimating thresholds for deterministic effects for the

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purpose of classifying waste should be important only for hazardous chemicals. For noncarcinogenic hazardous chemicals, NCRP believes that the threshold for deterministic effects in humans should be estimated using EPA’s benchmark dose method, which is increasingly being used to establish allowable doses of noncarcinogens. A benchmark dose is a dose that corresponds to a specified level of effects in a study population (e.g., an increase in the number of effects of 10 percent); it is estimated by statistical fitting of a dose-response model to the dose-response data. A lower confidence limit of the benchmark dose (e.g., the lower 95 percent confidence limit of the dose that corresponds to a 10 percent increase in number of effects) then is used as a point of departure in establishing allowable doses. Consistent with EPA’s benchmark dose method, NCRP believes that a suitable representation of the threshold for deterministic effects in virtually all humans is a dose that is a factor of 10 lower than the lower confidence limit of the benchmark dose obtained in a high-quality human study or a dose that is a factor of 100 lower than the lower confidence limit of the benchmark dose obtained in a high-quality animal study. The reduction by a factor of 10 when data in humans are available takes into account the need to protect sensitive population groups (e.g., children, the elderly). This reduction is consistent with the approach used in radiation protection of the public, where deterministic dose limits are set at a factor of 10 lower than nominal thresholds for deterministic radiation effects in adults. The further reduction by a factor of 10 when data are available only in animals takes into account that the animals may be less sensitive than humans. The recommended approach acknowledges the considerable uncertainty in estimating the highest dose at which no significant effects would be observed in humans. However, the approach is not unduly conservative and, thus, should not give disproportionate weight to noncarcinogenic hazardous chemicals, compared with radionuclides and chemical carcinogens, in classifying waste. In traditional toxicological methods of determining virtually safe doses of hazardous chemicals, nominal thresholds for deterministic responses in humans are estimated based on a NOAEL obtained in human or animal studies. In most high-quality studies, NOAEL is approximately the same as the lower confidence limit of the benchmark dose that corresponds to a 10 percent increase in the number of responses. Thus, as an alternative to the benchmark dose method, the nominal threshold in humans could be set at a factor of 10 or 100 lower than NOAEL obtained in a high-quality human or animal study. However, the benchmark dose method preferred by NCRP

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generally should provide a more reliable estimate of the highest dose at which no effects would be observed, mainly because the method makes use of the full range of data on dose-response, rather than a single data point (NOAEL). The benchmark dose method can also address difficulties that arise when a NOAEL is not obtained in a high-quality study or is not included in a data set. Given the nominal threshold for deterministic effects in virtually all humans estimated as described above, NCRP believes that the negligible and acceptable (barely tolerable) doses of noncarcinogenic hazardous chemicals used in classifying waste should be set at appropriate fractions of the nominal thresholds (see Sections 1.5.4.1 and 1.5.4.2). NCRP’s preferred approach is transparent in presenting the nominal threshold in humans, and it encourages the use of reasonably consistent safety factors for all noncarcinogens in establishing negligible and acceptable doses. 1.5.5.4 Risk Index for Mixtures of Hazardous Substances. For the purpose of developing a comprehensive and risk-based hazardous waste classification system, a simple method of calculating the risk posed by mixtures of radionuclides and hazardous chemicals is needed. The method should account for the linear, nonthreshold dose-response relationships for radionuclides and chemical carcinogens (stochastic effects) and the threshold dose-response relationships for noncarcinogenic hazardous chemicals (deterministic effects). NCRP believes that a conceptually simple composite risk index for mixtures of hazardous substances can be developed that provides an adequate representation of risk for the purpose of waste classification. The composite risk index is written in terms of separate risk indexes for substances that induce stochastic (s) and deterministic (d) effects as: RI j ⳱ RI sj Ⳮ RI jd,

(1.4)

where j is an index indicating whether the denominator in the risk index (RI) (see Equation 1.1) represents a negligible or acceptable risk (i.e., whether a material is being evaluated for classification as exempt or low-hazard waste). The recommended approaches to evaluating Equation 1.4 are described in the following sections. 1.5.5.4.1 Risk index for mixtures of substances that cause stochastic effects (carcinogens). The risk index for mixtures of substances that cause stochastic effects (radionuclides and chemical carcinogens) takes into account the risk in all organs or tissues, and it assumes that the risk in any organ is independent of the risk in all other organs. The risk index for mixtures of substances causing stochastic effects can be represented as:

/

1.5 DEVELOPMENT OF THE RECOMMENDED SYSTEM

RI sj ⳱

兺兺兺 i

r

T

Fi

(risk from disposal) si, j, r,T , (allowable risk) si, j, r,T

49 (1.5)

where the index T denotes the different organs (tissues) at risk, r denotes the different stochastic health effects of concern (cancers and severe hereditary effects), and i again denotes each hazardous substance. The index j described following Equation 1.4 is included in the numerator, as well as the denominator, to indicate that the exposure scenarios for disposal of exempt waste can differ from those for disposal of low-hazard waste (see Section 1.5.4.2). In accordance with Equation 1.2, the risk index (RI) for mixtures of substances causing stochastic effects can be evaluated in terms of dose rather than risk. In practice, Equation 1.5 can be greatly simplified. For radionuclides, doses in all organs and tissues and the different health effects of concern are incorporated in the effective dose, and calculation of the risk index for mixtures is reduced to a single sum over all radionuclides of the ratio of a calculated effective dose from exposure to each radionuclide to the allowable effective dose applicable to the particular waste class (disposal technology) of concern. Furthermore, the denominator in the risk index normally would be the same for all radionuclides, and any differences in judgments about an allowable effective dose for different wastes in the same class could be included in the modifying factor which can be radionuclide-specific. For hazardous chemicals, substance-specific probability coefficients incorporate information on risks in single or, in a few cases, multiple organs and the stochastic effects of concern. Therefore, calculation of the risk index for mixtures again is reduced to a single sum over all substances of the ratio of a calculated to an allowable risk or dose. 1.5.5.4.2 Risk index for mixtures of substances that cause deterministic effects (noncarcinogens). The risk index for mixtures of substances causing deterministic effects (hazardous chemicals only) takes into account the threshold dose-response relationships for deterministic effects in any organ. The risk index for mixtures also assumes, first, that doses in any organ due to exposures to multiple substances are additive even though the deterministic effects induced in that organ may not be the same for each substance and, second, that the threshold doses in any organ are independent of doses in any other organ. Based on these assumptions, the risk index (RI) for mixtures of substances causing deterministic effects, which generally should be expressed in terms of dose rather than risk (see Section 1.5.1), can be represented as:

冤

RI jd ⳱ INTEGER MAXT

兺兺F

i

i

r

冥,

(dose from disposal) i,d j, r,T (allowable dose) i,d j, r,T

(1.6)

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where MAX is a function yielding the maximum value of a set of numbers and INTEGER is a function yielding the truncated integer value of a number. All indexes in this equation have been described previously, except r denotes the different deterministic health effects of concern. The procedure for evaluating Equation 1.6 is the following. For each substance, the critical organ or organs in which deterministic effects are assumed to occur are identified, and the ratio of a calculated dose to those organs in the assumed exposure scenario to the allowable dose to those organs for the waste class of concern is obtained. If a particular substance is assumed to induce deterministic effects in more than one organ, this ratio is calculated for all organs at risk from exposure to that substance. Then, for each organ, the ratios of calculated to allowable doses are summed over all substances that induce deterministic effects, without regard for any differences in the health effects induced in that organ by the different substances, and the maximum of the summed ratios in any organ is selected. Use of the MAX function is based on the assumption that induction of deterministic effects in any organ is independent of doses to other organs. Finally, the highest risk index in any organ is truncated using the INTEGER function. This operation takes into account the assumption that the risk of a deterministic response is zero if the dose to each organ is less than the allowable dose in the denominator of the risk index. The modifying factor (F) is allowed to depend on the particular hazardous substance of concern, but its value often would be the same for all substances that cause deterministic effects. 1.5.5.4.3 Use of the composite risk index in classifying waste. Given the risk indexes for mixtures of substances causing stochastic or deterministic effects calculated using Equations 1.5 and 1.6, respectively, the composite risk index for all hazardous substances is calculated using Equation 1.4. This procedure assumes that induction of stochastic effects is independent of exposures to substances causing deterministic effects, and vice versa. In accordance with Equation 1.3 (see Section 1.5.1) and as indicated in Figure 1.2 (see Section 1.5.4), classification of waste would proceed in the following way. First, if the composite risk index is less than unity when the denominator represents a negligible risk and the numerator is evaluated using an exposure scenario appropriate to disposal of nonhazardous waste, the waste would be classified as exempt, but the waste would be nonexempt if the composite risk index is unity or greater. Then, for nonexempt waste, if the

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composite risk index is less than unity when the denominator represents an acceptable (barely tolerable) risk and the numerator is evaluated using an exposure scenario appropriate to disposal in a dedicated near-surface facility for hazardous waste, the waste would be classified as low-hazard, but the waste would be classified as high-hazard if the composite risk index is unity or greater. As emphasized in Section 1.1, the recommended approach to classifying waste does not provide a basis for establishing waste acceptance criteria at specific disposal sites. NCRP expects, however, that waste classified as exempt or low-hazard in accordance with its recommendations should be acceptable for disposal in the associated type of disposal facility at well-chosen sites.

1.6 Implications of the Recommended Waste Classification System The recommended risk-based waste classification system has important implications in three areas: (1) the resulting classification of existing radioactive and hazardous chemical wastes, (2) subclassification of the basic waste classes, and (3) changes in existing laws and regulations that would be required to implement such a classification system. 1.6.1 Classification of Existing Hazardous Wastes As part of this study, NCRP investigated how the recommended waste classification system would affect the current classifications of radioactive and hazardous chemical wastes. The results of this investigation are summarized as follows: ●

●

●

Substantial quantities of waste that contains small amounts of radionuclides or hazardous chemicals could be exempted from regulatory control as hazardous waste. Most radioactive waste currently classified as low-level waste and most hazardous chemical waste would be classified as lowhazard waste, based on the expectation that these wastes would be generally acceptable for disposal in dedicated near-surface facilities for hazardous wastes. A possible exception is hazardous chemical waste that contains relatively high concentrations of heavy metals, which could be classified as high-hazard waste. Most uranium and thorium mill tailings that contain elevated levels of naturally occurring radionuclides could be classified as low-hazard waste, but only under conditions of perpetual

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institutional control. In the absence of institutional control over disposal sites, most mill tailings would be classified as highhazard waste. Management and disposal of the large volumes of uranium mill tailings would continue to require separate considerations regardless of how this type of waste is classified, because regulatory authorities have judged that disposal of these wastes below ground is impractical and unrestricted release of tailings piles could result in unacceptable health risks to the public. Similar considerations could apply to other wastes with large volumes that contain elevated levels of hazardous substances, especially heavy metals (e.g., wastes from mining or processing of ores to obtain nonradioactive materials). Most radioactive waste currently classified as spent fuel, highlevel waste, or transuranic waste, and most low-level waste currently subclassified as greater-than-Class-C would be classified as high-hazard waste, based on the expectation that these wastes usually would require disposal in a geologic repository or other disposal system providing a substantially greater degree of waste isolation than a near-surface facility. However, some of these wastes that contain relatively low concentrations of radionuclides could be classified as low-hazard waste.

Based on this investigation, the waste classification system recommended by NCRP appears to be largely consistent with the current classification systems for radioactive and hazardous chemical wastes and with plans for their disposal. Therefore, implementation of the new waste classification system should not be unduly disruptive or costly. In addition, the possibility that substantial quantities of waste currently classified as radioactive or chemically hazardous could be exempted from regulatory control, and thus managed as nonhazardous waste or considered for beneficial use, could result in significant cost savings without increasing risks to public health by more than a negligible amount.

1.6.2 Subclassification of Basic Waste Classes Various wastes that would be classified as low-hazard or highhazard in accordance with NCRP’s recommendations may have significantly different physical, chemical, radiological, or toxicological properties. To facilitate efficient management of wastes having different properties, subclassification of these waste classes may be desirable. For example, uranium mill tailings and other similar wastes with very large volumes could be distinguished from wastes with similar properties but much smaller volumes in subclassifying

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low-hazard waste, because large waste volumes necessitate a different approach to management and disposal. Similarly, it may be reasonable to subclassify smaller volumes of low-hazard waste that contains varying concentrations of hazardous substances, in a manner similar to the system for subclassifying low-level radioactive waste developed by NRC in 10 CFR Part 61. NCRP believes that subclassification of the basic waste classes would be appropriate as long as it is based on properties of waste that are related to health risks from disposal or considerations of the cost-benefit of different options for waste management and disposal. Other factors that have influenced waste classification in the past should not be used as a basis for waste subclassification. For example, the present distinction between radioactive waste that arises from operations of the nuclear fuel cycle and NARM waste should not be maintained in subclassifying waste, because this distinction is based solely on the source of the waste rather than significant differences in health risks from waste disposal or considerations of cost-benefit in waste management and disposal.

1.6.3 Legal and Regulatory Implications Development of the generally applicable and risk-based waste classification system recommended by NCRP would have a number of important implications with regard to current laws and regulations: ●

●

A general class of exempt waste, which could be regulated as nonhazardous material, would be established. Development of an exempt class of waste that contains low levels of hazardous substances has been controversial and currently is banned by law in the case of radioactive waste. Some radioactive and hazardous chemical wastes have been exempted on a case-by-case basis, but general principles for exempting radioactive or hazardous chemical wastes have not been established. In spite of these difficulties, however, a meaningful risk-based waste classification system must include a general class of exempt waste. The present difficulties with management and disposal of mixed radioactive and hazardous chemical wastes, which result from dual regulation of these materials under AEA and RCRA or other laws (e.g., TSCA) and the different approaches to waste management and disposal under the various laws, would be addressed by including all radioactive and hazardous chemical wastes in the same waste classification system. This approach to waste classification would require changes in existing laws and regulations that apply to mixed waste.

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Inclusion of NARM waste in the same classification system with radioactive waste that arises from operations of the nuclear fuel cycle would require a change in the scope of AEA, because management and disposal of commercial NARM waste cannot be regulated under AEA. Under current laws and regulations, many radioactive and hazardous chemical wastes are classified based on their source, rather than their radiological or toxicological properties. Development of a risk-based waste classification system would require elimination of source-based waste classifications. The recommended waste classification system would affect current disposal practices for hazardous chemical waste in two ways. First, the new system calls for risk assessments over long time frames in deciding whether waste is generally acceptable for near-surface disposal. Second, it allows the possibility that waste containing the highest concentrations of hazardous chemicals might be classified as high-hazard waste and, thus, would generally require a disposal system considerably more isolating than the type of near-surface facility currently used for all hazardous chemical waste.

1.7 Further Development of the Recommended Waste Classification System The waste classification system presented in this Report would apply to all radioactive and hazardous chemical wastes from any source, and it would be based on considerations of health risks to the public that arise from waste disposal. The recommended classification system differs from the existing waste classification systems in three respects: radioactive and hazardous chemical wastes would be included in the same classification system; all waste would be classified based on its properties, rather than its source; and the classification system would include a general class of exempt waste. Given these differences, NCRP believes that replacement of the existing waste classification systems by the classification system recommended in this Report should be undertaken carefully over time, and in recognition that the existing systems for waste classification and waste management, despite their shortcomings, have with few exceptions been more than adequate in protecting human health. In establishing a new hazardous waste classification system that would be an improvement over the existing systems, there is a

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need to ensure that current approaches to management and disposal of radioactive and hazardous chemical wastes would not be unduly disrupted. While some of the benefits of the recommended waste classification system could be obtained with incremental changes to the existing system (e.g., the addition of exempt classes of radioactive and hazardous chemical wastes), many other benefits, such as classification of waste based on risk and transparency of the system, will require full implementation. Many details would need to be considered in developing a new waste classification system based on the framework presented in this Report. Assumptions about generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites to be used in classifying waste and the time frames for applying the scenarios would be required. Decisions would need to be made about negligible and acceptable (barely tolerable) doses or risks that would be used in classifying waste as exempt or low-hazard, respectively. Inconsistencies in current approaches to cancer risk assessment for radionuclides and hazardous chemicals would need to be considered and resolved in developing a comprehensive waste classification system. Foremost among these is the difference between cancer risk estimates for radionuclides, which are based on MLEs of observed risks and a standard model for extrapolating observed risks at high doses to the low doses of concern in health protection of the public, and risk estimates for chemical carcinogens, which are based on upper 95 percent confidence limits of observed risks and have been derived using different risk-extrapolation models that can result in risk estimates at low doses that differ by several orders of magnitude. Another issue requiring consideration is the difference in the measure of risk normally used in cancer risk assessments, i.e., fatal cancers for radionuclides but cancer incidence for chemical carcinogens. For noncarcinogenic hazardous chemicals, an important issue requiring consideration is the most suitable approach to estimating nominal thresholds for deterministic health effects in humans. Consideration also needs to be given to the appropriate magnitude of safety and uncertainty factors that should be applied to nominal thresholds in determining negligible and acceptable doses of noncarcinogens. Deterministic effects from exposure to radionuclides should not be important in classifying waste. In addition to the effort required to develop a comprehensive and risk-based hazardous waste classification system based on NCRP’s recommendations, several legal and regulatory impediments would need to be addressed. However, the resulting classification system would be more transparent and understandable than the separate classification systems for radioactive and hazardous chemical wastes

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in use at the present time. In addition, the new system would be largely consistent with the existing classification systems in regard to the intended disposal technologies for the different waste classes. Such a classification system could lead to greater acceptance by the public of waste management and disposal activities.

2. Introduction

The purpose of this Report is to set forth the technical principles and framework for a comprehensive and risk-based hazardous waste classification system. In this context, waste is any material that has insufficient value to justify further beneficial uses, and thus must be managed at a cost. Hazardous waste is waste that can be harmful to biological organisms, due to the presence of radioactive substances or chemicals that are deemed hazardous, to the extent that it must be regulated. Hazardous waste excludes material that is simply useless (e.g., typical household trash). This work is comprehensive because it considers all hazardous wastes irrespective of their source.1 NCRP undertook a study of waste classification because of the importance and visibility of hazardous waste management in the United States coupled with the observation that the existing classification systems for hazardous wastes are increasingly complex and inefficient. This determination led to the independently conceived alternative approach to hazardous waste classification described in this Report.

2.1 Foundations and Directions This Section presents basic definitions and concepts necessary to undertake a detailed discussion of waste classification (more detailed technical background is provided in Section 3). It also describes the scope of this Report to indicate why further discussion of a number of issues that are important to waste classification is not required because they are outside the scope of this study.

1 Biohazardous wastes are not considered because they are conventionally rendered nonhazardous before disposal according to guidelines of EPA (1986a).

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2.1.1 Definition of Waste Classification The generic life cycle of materials containing radionuclides or hazardous chemicals is shown in Figure 2.1. The current approach to waste management is to prevent the generation of hazardous waste by substituting nonhazardous input materials to the extent practicable. Generation of hazardous waste also may be reduced by recycling

Fig. 2.1. Life cycle of hazardous materials.

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of hazardous materials, either within the generating facility (e.g., treatment of contaminated waste water and reuse of the purified water within the facility) or externally (e.g., recycling of lead contained in automobile batteries). To the extent that these practices are not implemented for whatever reason, hazardous wastes result. Waste management includes any activities associated with the disposition of waste products after they have been generated. Operations in waste management typically include: ●

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A wide variety of treatment technologies to reduce the volume, change the physical or chemical form (e.g., incineration, solidification of a liquid waste, neutralization of acidic or basic waste), and suitably package the waste for subsequent management steps. Storage (defined as holding a waste with the intent to retrieve it for further management operations) awaiting the accumulation of an economic quantity of material for subsequent steps (e.g., a full truck load of waste), allowing for decay of radioactive materials, or awaiting the development of appropriate treatment or disposal facilities. Transportation from generation to treatment or storage facilities and eventually to disposal facilities. Disposal of the waste by emplacing it in isolating surroundings, with no intent to retrieve it, for the purpose of preventing the hazardous substances from reaching the biosphere in unacceptable amounts (this does not mean that the waste is irretrievable if a particular disposal method does not prove satisfactory).

An important purpose of waste management is to dispose of hazardous waste safely and cost-effectively. If waste disposal is not costeffective within the constraint of protecting human health and the environment, then resources would be required that could better be spent on other beneficial activities. For the purposes of this Report, waste classification is defined as a grouping of wastes having similar attributes related to disposal. For example, one might seek to group highly toxic and long-lived wastes in one class destined for disposal in a geologic repository and lower-toxicity or shorter-lived wastes in another class destined for disposal in a regulated near-surface facility. A waste classification system could be expressed in terms of waste characteristics that define the boundaries between classes and the rules for using these defining characteristics. For example, a waste that contains more than 100 units of a hazardous substance per cubic meter of waste might be in waste Class X, a waste with 10 to 100 units per cubic meter in waste Class Y, and waste with less than 10 units per cubic

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meter in waste Class Z. Waste also could be classified for purposes other than disposal (e.g., transportation or storage), but such classifications are not addressed in this Report.

2.1.2 Purpose of Waste Classification The purpose of waste classification is to provide guidance at conceptual and operational levels on appropriate approaches to waste management and disposal for many kinds of waste exhibiting widely varying potential hazards. However, it may not be immediately obvious why a waste classification system is needed. Referring to Figure 2.1, it would appear possible to simply accumulate waste until a disposal facility is available and then send the waste to the facility. While there are many reasons for classifying waste (IAEA, 1994), there are two major reasons why accumulating waste and sending it to a single disposal facility is sufficiently impractical so as to require that wastes be classified. First, without waste classification there would be one large composite waste stream that is the aggregate of many input streams. These input streams could range from essentially nonhazardous (e.g., household and industrial trash) to highly hazardous (e.g., high-level radioactive waste). The composite stream would have a very large volume because of the large amounts of nonhazardous trash, and it would be managed on the basis of its higher-hazard substances, which could require the use of technologies for treatment (e.g., highintegrity packaging, vitrification) and disposal (e.g., engineered and monitored near-surface disposal, geologic repository) that are much more expensive than what is needed for nonhazardous trash. The need to manage waste based on the characteristics of constituents that pose the highest hazard would require the siting and construction of an impractically large number of treatment and disposal facilities to handle the large volume of aggregated waste. This approach would entail costs far beyond those required to protect human health and the environment. A second driving force for waste classification, which cannot be adequately reflected in Figure 2.1, is the sequential nature of waste management activities that take place over an extended period of time: ●

A wide range of hazardous waste is being generated and some management actions must be performed almost immediately (e.g., containment of hazardous materials, assuring proper storage). However, disposal is often many years in the future, as evidenced by the fact that a geologic repository for the most

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hazardous radioactive wastes will not begin operations until many decades after these wastes were first generated. As a consequence of timing differences for various waste management operations, decisions must be made now concerning actions that anticipate the disposal technology that will eventually be used, and these decisions must be clearly communicated to ensure consistency and continuity. Particularly important in this regard are decisions concerning the co-mingling of various waste streams,2 dilution of hazardous wastes,3 exemption of waste materials from requirements for management as hazardous waste,4 the relevant regulations and regulatory agency, and the nature of any immediate treatment in anticipation of disposal. If it is assumed that there will be multiple waste disposal technologies available to waste generators (e.g., municipal/industrial landfill, near-surface engineered facility, geologic repository), it is necessary to decide which wastes are generally acceptable for each type of disposal facility so that planners can determine the required capacity. Furthermore, planning for treatment facilities requires knowledge of the expected amounts of various wastes in different classes because the treatment required for wastes is usually determined by disposal requirements. Early knowledge of the cost of management operations for different waste classes allows engineers to optimize the design of waste generating facilities and treatment operations (e.g., production of a large volume of lower-hazard waste versus a small volume of higher-hazard waste). For example, knowledge of the costs of transporting and disposing of low-level and high-level radioactive wastes at DOE sites allows decisions to be made concerning the extent to which costly separations technology should be used to reduce the amount of the more expensive highlevel waste. Early knowledge of the types of facilities needed in the management sequence for each class of waste is necessary to plan for and establish appropriate regulations. Waste management facilities

2 It is usually less costly to manage a unit of lower-hazard waste than one with a higher hazard. Thus, a generator would not want to create a large volume of highhazard waste by adding a smaller volume of high-hazard waste to a large volume of low-hazard waste. 3 In the United States, it is deemed unacceptable to dilute a waste that poses a high-hazard for the purpose of reducing the hazard, including doing so by combining wastes of different hazard, unless such combination eliminates an inherently hazardous characteristic (e.g., ignitability, corrosivity). 4 It is desirable to identify and segregate exempt wastes at the earliest possible time to avoid unnecessary expenditure of resources to further manage them and to avoid subsequent contamination with hazardous materials.

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are expensive, and regulations need to be established in advance to provide public confidence that will allow resources to be committed to facility construction and operation, to provide the basis for facility design (e.g., effluent controls), and to allow prompt commencement of facility operations. In summary, a hazardous waste classification system is needed because (1) disposal of the composite unclassified waste would be prohibitively expensive and (2) the differences in timing between waste generation (now) and the development of treatment and disposal facilities (the future) require that wastes be segregated in anticipation of cost-effective means of waste management and disposal. Waste classification also allows consistent communication of the information needed to develop adequate treatment and disposal capacity and to develop appropriate regulations.

2.1.3 Bases for Waste Classification Numerous formal waste classification systems, or, equivalently, boundaries between classes of waste and rules for using them, have been developed over the years (see Section 4 for an extensive discussion). The bases for the boundaries also are numerous, with the following being the most common: ● ● ● ● ● ●

physico-chemical properties (for example, strong acid or base, pyrophoric) facility or process (i.e., source) generating the waste composition of the waste, especially its hazardous constituents environmental persistence or rate of degradation and decay environmental mobility or availability toxicity of hazardous substances in the waste (i.e., probability and severity of adverse health effects resulting from ingestion or inhalation of a unit amount of material)

Other factors that are sometimes taken into account in classifying hazardous waste include the quantity of the waste, containment (packaging) of the waste, potential for exposure to hazardous substances, control of potential releases, environmental and human health risks, economic factors, and sociopolitical aspects (IAEA, 1994). Most of these factors are, at best, indirectly related to risk (e.g., a material that does not degrade rapidly does not necessarily pose a larger risk) and, at worst, are unrelated to risk (e.g., waste from a particular source being treated as if it were hazardous irrespective of its constituents and their concentrations).

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As implied by Figure 2.1, the objective of a waste management system is to identify generally acceptable methods for disposition of hazardous wastes such that risks to human health and the environment will be sufficiently low. Given the need to categorize wastes (see previous section), it makes sense to define the categories so that the wastes in each category would pose roughly equivalent risks following disposition. The consequence of this is that wastes within a category could be managed in the same way to ensure adequate protection of human health and the environment while not committing resources to excessively protective measures. That is, the ideal waste classification system should be based on considerations of risk management. Unfortunately, achieving this ideal solution while trying to establish a practical waste classification system faces some major obstacles: ●

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Risk assessment (i.e., calculation of risk) is a complex, multistep process, and the results usually have a significant degree of uncertainty because of limitations in data and in the models of environmental and biological systems. In addition, for purposes of generally classifying waste, risk assessment must be generic; i.e., it is not intended to apply to disposition of a specific waste in a specific manner at a specific site. Establishing the boundaries in a risk-based waste classification system requires that one or more values of acceptable risk be specified. The values of acceptable risk are then used to establish the values of parameters that define the boundaries of the different waste classes. The process of establishing the value(s) of acceptable risk is part of risk management. Risk management is an essential aspect of establishing a waste classification system, but it has an important nontechnical component that reflects societal values. Existing waste classification systems now codified in law, regulation, and commerce evolved from times when risks that arise from waste disposal could only be evaluated qualitatively, due to inadequate knowledge of the long-term performance of waste disposal systems and the dose-response relationships for radionuclides and hazardous chemicals. Establishing a new, riskbased waste classification system will require changes in current laws and regulations. Despite the benefits of such a unified, transparent system, change is likely to be resisted because of legal and regulatory inertia.

Despite the challenges described above, NCRP believes that risk is the appropriate primary basis for a waste classification system, and risk will be used as the basis for the work described in this

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Report; thus, the phrase ‘‘risk-based waste classification system.’’ The desirability of basing a waste classification system on risk has been recognized for many years (DOE, 1980). While ‘‘risk’’ will be defined more precisely in subsequent sections, the following general definition is useful at this point (Garrick and Kaplan, 1995): Risk is composed of: ● ● ●

What can go wrong (an undesirable event)? How likely is it (probability)? What are the consequences (e.g., probability of induction of cancer)?

This means that development of a risk-based waste classification system must consider the events that could result in exposing biological organisms (e.g., humans) to hazardous substances placed in a disposal facility, the probability that each event will occur, and the consequences of the event if it does occur.

2.1.4 Shortcomings of Current Waste Classification Systems Given that waste classification systems presently exist, it is reasonable to ask whether an effort to develop the foundations of a new system would be beneficial. The short answer (expanded in Sections 4 and 5) is ‘‘yes’’ for the following reasons: ●

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For many wastes, there is no practical classification system for establishing a boundary (e.g., amount of hazardous substances) below which the waste is considered to be nonhazardous. This means that large volumes of waste are managed at considerable cost because the waste cannot be conclusively shown to contain no hazardous substances or, even more difficult, to contain an amount of hazardous substances (e.g., uranium) no greater than was initially present in a material before its use by humans. Significant portions of existing waste classification systems are not based on the primary objective of ensuring that the risk that arises from waste disposal is acceptable. The best example of this is wastes that are classified based solely on the nature of the generating process or facility (e.g., highlevel radioactive waste, chemical wastes from certain industries), irrespective of the content and concentration of hazardous substances. This results in resources being used unnecessarily on lower-risk situations when they could be better applied to higher-risk situations (hazardous waste disposal or otherwise). For example, billions of dollars have been spent in managing

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DOE’s high-level wastes as if they were among the most hazardous of all radioactive wastes. However, the concentrations of hazardous substances in some of these wastes are similar to those in low-level radioactive waste that is normally intended for disposal in near-surface facilities. In contrast, some chemical wastes that are highly hazardous, compared with other wastes, and nondegradable are being sent to near-surface disposal facilities. Both of these situations occur largely because of the sourcebased aspects of existing waste classification systems. To the extent that risk is used as a basis for waste classification, it is not used consistently. Different values for acceptable risk are assumed for different hazardous waste disposal situations. In addition, a variety of surrogate measures (e.g., ingestion toxicity, total radioactivity) having varying relationships to risk have been used to classify wastes. The requirements for managing hazardous chemical waste are sufficiently different from those for radioactive waste that treatment and disposal of waste that contains both types of substances is greatly impeded. Large volumes of waste that contains hazardous chemicals and radionuclides (referred to as ‘‘mixed waste’’) are presently being stored because the inconsistency in regulations has resulted in inadequate treatment and disposal capacity. The existing waste classification systems are becoming increasingly complex as additional waste streams are incorporated into a patchwork system that is not based on a consistent set of principles. Some wastes are classified based on their source (i.e., the nature of the process or facility that produces them), some based on their composition, and some based on their physico-chemical characteristics. Some wastes are defined by exclusion (i.e., by what they are not), not on the basis of their properties or associated risks. Low-level radioactive waste is defined as waste that is not high-level waste, spent fuel, transuranic waste, or uranium or thorium mill tailings. Because the excluded wastes are defined by their source, rather than their properties, the definition of low-level waste is not based on properties of the waste and wastes in this class can vary from essentially innocuous to highly hazardous over long time frames. Waste classification systems are not transparent or defensible. There exist numerous classification systems for different wastes having a variety of bases and implementation rules that are not tied to any consistent set of principles. As a consequence, the overall classification of hazardous waste is not transparent to

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anyone but the most knowledgeable of experts. Its defensibility is highly questionable because of the potential for inconsistencies among waste classification systems that provide fuel for legal challenges. 2.1.5 Focus on Classification of Waste This Report is concerned with classification of waste. However, NCRP’s assumption that hazardous waste should be classified based on considerations of health risks posed by its constituents also could be used in classifying hazardous materials for such other purposes as transportation or their beneficial use in commerce. The waste classification system developed in this Report includes a general class of exempt waste. Waste in this class would contain sufficiently small amounts of hazardous substances that it could be managed in all respects as if it were nonhazardous (e.g., as household trash). NCRP intends that exempt materials could be used or disposed of in any manner allowed by laws and regulations addressing disposition of nonhazardous materials. However, exempt waste would not necessarily be exempt for purposes of beneficial use without further analysis of the risks associated with anticipated uses. Materials could be exempted for purposes of disposal or beneficial use based on similar considerations of acceptable risk. However, based on differences in exposure scenarios for the two dispositions, limits on the amounts of hazardous substances that could be present in exempt materials intended for beneficial use could be substantially lower than the limits for disposal as exempt waste. Thus, disposal may be the only allowable disposition for some exempt materials based on considerations of risk. In addition, some exempt materials may consist of trash, rubble, and residues from industrial processes that would have no beneficial uses and must be managed as waste. Based on these considerations and the purpose of this study, the recommended approach to defining an exempt class of waste that contains low levels of hazardous substances focuses on disposal as the intended disposition of exempt material. Consideration of other dispositions of exempt material (e.g., recycling, reuse in commerce) is beyond the scope of this study. However, the principles used to exempt waste for purposes of disposal based on risk could be used to exempt such materials for any other purpose. 2.1.6 Classification of Waste for Purposes of Disposal The classification system for hazardous wastes developed in this Report is intended to be applied to waste prior to disposal. It is not

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intended to be applied to screening or ranking of contaminated sites, including sites at which hazardous wastes have previously been disposed. Site screening or ranking involves site-specific considerations that cannot be taken into account in a generally applicable waste classification system. However, remediation of contaminated sites may involve exhuming hazardous materials that require disposal elsewhere, and such wastes would be included in the proposed classification system.

2.2 Limits and Relationships The discussions in Section 2.1 outline the logic leading to the need for, and scope of, a risk-based waste classification system. Despite this presentation, there is the potential for confusion and misunderstanding concerning the limits of developing the foundations of a risk-based waste classification system and its relationship to other aspects of waste management. The following sections address these limits and relationships.

2.2.1 Regulatory Implications This Report culminates in the presentation of the principles and framework for a comprehensive and risk-based hazardous waste classification system. NCRP does not propose a particular implementation of the proposed classification system (e.g., a particular quantification in terms of limits on concentrations of hazardous substances in each waste class); this is most appropriately left to governmental policy organizations. The relationship of the proposed risk-based waste classification system to existing regulations is discussed in Section 7.2.

2.2.2 Risk Management Establishment of a risk-based waste classification system requires that one or more levels of acceptable risk be specified. A determination of acceptable risks depends on societal values, and is a task appropriately left to governmental policy makers and the public. As a result, this Report will not attempt to select or justify specific values for acceptable risk. However, in Sections 6 and 7, values of

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acceptable risk taken from the literature, including NCRP recommendations, are used to illustrate application of the proposed riskbased waste classification system. 2.2.3 Waste Classification in a Continuum of Waste Compositions Concentrations of hazardous substances in waste occur as a continuum, ranging from extremely dilute to extremely concentrated, rather than in discrete, well-separated amounts. Given that risks from waste disposal generally are proportional to the concentrations of hazardous constituents, it is not obvious how one can justify the establishment of boundaries separating waste classes. However, this approach can be justified by recognizing that waste disposal technologies intended to ensure acceptable risks to the public are discrete in regard to their expected capabilities for isolating waste from the human exposure environment. There are just a few types of disposal technologies that are generally available, including (1) municipal/ industrial landfills, (2) regulated near-surface disposal facilities for hazardous wastes located where human intrusion can readily occur in the absence of institutional control and water that may become contaminated with hazardous substances is relatively accessible, and (3) highly isolating disposal facilities located where human intrusion is much less likely and water that may become contaminated would be less accessible (e.g., geologic repositories). Thus, boundaries of waste classes can be established by determining waste properties that would result in an acceptable risk if waste disposal were to occur using one of the few available technologies. If a waste is not generally acceptable for disposal using a given technology, then a more isolating technology normally would be required. The fact that a waste just exceeding a boundary would be sent to a more confining facility than a waste just within the boundary can be reconciled by the fact that the design and analysis of disposal facilities is intended to be conservative (i.e., to provide increased margins of safety below regulatory requirements), and the capability of the less confining facility to maintain risks at or below an acceptable level should actually extend beyond the boundary. In addition, to the extent that the waste classification system is flexible, regulators can accommodate special cases in which the concentrations of contaminants in wastes are near the boundaries. 2.2.4 Subclassifications of Basic Waste Classes There often are valid reasons for developing subclassifications of basic waste classes (i.e., classifications subordinate to those based

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on risk that arises from waste disposal). Waste subclassifications are typically required for engineering purposes during pre-disposal operations and may be desirable to account for differing waste characteristics (e.g., waste forms) in a disposal context. For example, in waste operations, there is a clear need to distinguish between liquid and solid wastes and between heat-generating wastes and those with insignificant heat generation. Subclassifications based on different compositions of hazardous substances, resulting in different requirements for waste treatment to allow use of a particular disposal technology, also are justifiable. An example is the subclassification of low-level radioactive waste destined for near-surface disposal established by NRC (1982a; 1982b), which depends on the concentrations of different radionuclides in the waste and includes differing requirements on packaging and disposal of the different subclasses in the same type of facility. The principles of waste classification presented in this Report do not address a framework for subclassification of hazardous wastes. However, the relationship of existing subclassifications to the proposed framework is discussed.

2.2.5 Site-Specific Risk A risk-based waste classification system must focus on the inherent characteristics of waste, representative facilities, and generic events, because the system necessarily presumes that specific disposal sites and related waste treatment and disposal technologies have not yet been identified and characterized. NCRP emphasizes that the principles, framework, and implementation details of a riskbased waste classification system do not provide a substitute for sitespecific risk assessments. The two most important cases where site-specific risk must be estimated are (1) an assessment of risk for the spectrum of actual wastes at a specific disposal site for the purpose of establishing site-specific waste acceptance criteria, and (2) an assessment of risk posed by a prior waste disposal at a site for the purpose of determining whether the risk is unacceptable and, thus, whether remedial action is required at the site.

2.2.6 Ecological and Other Potential Impacts In developing a risk-based approach to waste classification, NCRP has focused exclusively on the potential for significant adverse health effects in humans. However, there are other potential adverse impacts

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that can result from disposal of radioactive and hazardous chemical wastes. These include adverse impacts on flora and fauna, damage to cultural or natural resources, and sensory impacts. With regard to radionuclides, standards for radiation protection of the public also are believed to be protective of nonhuman biota (IAEA, 1976; 1979; 1992; NCRP, 1991). However, some chemicals may pose greater risks to biota than to humans. An example is selenium, which is toxic to cattle at levels that are not generally a problem for humans. In addition, hazardous substances may accumulate in the environment in ways that could result in much higher doses to biota than to humans, especially in aquatic systems. In most cases, evaluations of ecological impacts are site-specific and, as a consequence, are not considered when establishing a generally applicable waste classification system. These impacts normally are addressed in disposal site selection, design, and operation, and they may be used in establishing waste acceptance criteria for the site. To the extent that ecological impacts can be evaluated generically, NCRP believes that the principles and framework for riskbased waste classification presented in this Report are sufficiently flexible to take them into account.

2.3 Conceptual Framework of This Report This Report is directed at a multidisciplinary audience with different levels of technical understanding. NCRP recognizes that readers having expertise in areas of radiation risk assessment and radioactive waste management may not be as knowledgeable about risk assessment and waste management for hazardous chemicals, and vice versa. Therefore, one of the aims of this Report is to present discussions on technical issues relevant to risk assessment and waste classification in sufficient detail to allow readers having different technical backgrounds to understand these issues without having to refer to other sources of information. The following summarizes the conceptual framework of the Report to provide initial points of reference for the various discussions. Section 3 provides technical background related to risk-based waste classification, including: ● ●

general background on the standard risk assessment process (NAS/NRC, 1983) more detailed information on the aspect of risk assessment of central import to this Report; namely, the assessment of adverse health effects in humans resulting from exposure to hazardous substances

2.3 CONCEPTUAL FRAMEWORK OF THIS REPORT ●

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approaches to cancer risk management for radionuclides and hazardous chemicals

Section 4 presents detailed information on existing classification systems for radioactive and hazardous chemical wastes, the relationships between waste classification and requirements for waste disposal, and the impacts of waste classification systems on management and disposal of mixed wastes. This Section also summarizes previous NCRP recommendations relevant to waste classification. Section 5 discusses the desirable attributes of a waste classification system and evaluates present classification systems with respect to these attributes. These discussions essentially summarize the rationale for the development of a comprehensive and risk-based hazardous waste classification system. Section 6 then establishes and discusses the principles and framework for a comprehensive and risk-based hazardous waste classification system in a number of steps: ●

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The conceptual foundation of the system is first established by addressing such issues as its focus, dose-response relationships, measures of response, and the applicable risk management paradigm; The framework for a risk-based waste classification system is then proposed; A risk index for waste classification to be used in conjunction with the framework is then developed, the combination of these constituting the recommended risk-based waste classification system; and Issues related to subclassification of basic waste classes, incorporation of conservative assumptions in applying the system, and future development needs regarding waste classification are discussed.

Section 7 then addresses the implications of the recommended risk-based waste classification system. By assuming key parameters (e.g., values of acceptable risk, characteristics of exposure scenarios) and applying the system to a variety of example waste streams, the question of how existing wastes would be classified in the new system is investigated. This Section also summarizes the legal and regulatory ramifications of the proposed hazardous waste classification system. Section 8 summarizes NCRP’s conclusions and recommendations on waste classification. This is followed by an extensive glossary and a list of references.

3. Technical Background on Risk Assessment and Risk Management Assessment of risks to human health posed by hazardous wastes and decisions about acceptable risks posed by hazardous wastes (i.e., risk management) are essential to the development of a risk-based waste classification system. The purpose of this Section is to provide technical information concerning risk assessment for radionuclides and hazardous chemicals and approaches to risk management for the two types of substances. It begins by defining risk in general terms and in terms relevant to disposal of hazardous waste, and by describing the process by which risks that arise from waste disposal would be assessed. The discussion then focuses on aspects of risk assessment concerned with estimating the probability that a significant adverse health effect, called a response, will result from a hypothetical exposure to a hazardous substance. This issue is discussed separately for radionuclides and hazardous chemicals because significantly different approaches have been used to estimate response probabilities for the two types of substances. This Section concludes with a discussion of the different approaches to risk management used in controlling exposures of the public to radionuclides and hazardous chemicals. The discussions on risk assessment, particularly the approaches to assessing the probability of a response from a given exposure (dose-response assessment), are presented in considerable detail to allow readers who are knowledgeable about risk assessments for ionizing radiation and the data that support them to become familiar with risk assessments for hazardous chemicals, and vice versa. The significant differences in the approaches to dose-response assessment for radionuclides and hazardous chemicals constitute a major issue requiring resolution in establishing a comprehensive and riskbased waste classification system. Therefore, even for readers knowledgeable about issues of risk assessment, the comparisons of the different approaches presented in Section 3.2.3 are important to an understanding of the waste classification system developed in this Report. 72

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Similar considerations apply to the discussions of approaches to risk management in Section 3.3. Readers who are knowledgeable about principles of radiation protection may not be familiar with the different approach to health protection used for hazardous chemicals, and vice versa, and an understanding and resolution of the different approaches to risk management is important in developing a comprehensive and risk-based waste classification system.

3.1 Assessment of Risk

3.1.1 Definition of Risk The term ‘‘risk’’ as used in this Report refers to the probability of harm, combined with the potential severity of that harm. In the context of impacts on human health resulting from disposal of hazardous waste, ‘‘risk’’ is the probability of a response in an individual or the frequency of a response in a population taking into account (1) the probability of occurrence of processes and events that could result in release of hazardous substances to the environment and the magnitude of such releases, (2) the probability that individuals or populations would be exposed to the hazardous substances released to the environment and the magnitude of such exposures, and (3) the probability that an exposure would produce a response. For example, ‘‘risk’’ refers to the probability that a member of the public living near a waste disposal site will develop a certain type of cancer as a result of emplacement of hazardous substances at the site. When expressed as a probability, risk is a number between zero and one, without units. In this Report, all values are risks to an individual over a normal lifetime. Risk can be calculated for individual radioactive and chemical substances in waste and for specific pathways by which release and exposure might occur. These component risks can be combined to yield an overall risk that arises from disposal of waste. Different measures of response can be used in estimating risk. For example, risk could refer to the probability of occurrence (incidence) of a particular response or the probability that death will result. The probabilities of these two endpoints will rarely be the same, because some adverse effects will be cured by medical treatment or the receptor will die by some other means before death is caused by exposure to a hazardous substance. In environmental health, risk is typically expressed in such terms as ‘‘the estimated incremental lifetime cancer risk to an individual

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that might live near a disposal site containing carcinogen A is one in 100,000 (0.00001 or 10ⳮ5).’’ This means in effect that, absent uncertainties in the estimated risk, one of every 100,000 people having the specified relationship (e.g., geographic proximity, living habits) to the disposal site would be expected to develop a specific cancer due to exposure to carcinogen A during their lifetime that they would not have developed if they were not located near the site.

3.1.2 Types of Responses from Exposure to Hazardous Substances Two types of responses from exposure to hazardous substances, called stochastic or deterministic,5 are of concern in risk assessment. The two types of responses are distinguished by the characteristic features of the dose-response relationship, i.e., the relationship between the dose of a hazardous substance and the probability (or frequency) of a response. Stochastic responses are those for which the probability, but not their severity, is a function of dose, without threshold. Because of the long latency period between exposure and the expression of a stochastic response, the existence of a causal relationship between dose and response can only be inferred on statistical grounds based, for example, on knowledge of the background incidence of the response of concern in unexposed populations. Severe hereditary (genetic) and many carcinogenic (e.g., genotoxic) responses are considered to be stochastic. Deterministic responses are those for which the severity varies with dose and for which a threshold usually exists. In some toxicology texts, this type of response is called a graded response, to reflect both the increase in incidence of the response and the increase in its severity that usually are observed as the dose increases above the threshold. If the dose does not exceed a certain threshold, the probability of occurrence of a particular response is presumed to be zero. Deterministic responses often occur soon after exposure, and a causal relationship between dose and response in such cases is easily established if the dose is sufficiently high. Deterministic responses resulting from exposure to chemical toxicants include, for example, increased protein in the urine, birth defects and sterility,

5 In common usage, the term ‘‘carcinogenic’’ often is used instead of ‘‘stochastic’’ because the vast preponderance of substances having a stochastic relationship of adverse biological effect to dose are carcinogens. Similarly, ‘‘noncarcinogenic’’ is commonly used instead of ‘‘deterministic.’’

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effects on the nervous system, and liver damage. Exposure to radiation can result in such deterministic responses as cataract in the lens of the eye, skin erythema, cell depletion in bone marrow resulting in hematological deficiencies, cell damage in the gonads leading to impairment of fertility, and damage to blood vessels or connective tissue in many organs. For some hazardous chemicals that exhibit carcinogenic responses, the dose-response relationship appears to be deterministic in character, in that an increase in the probability of cancer is not observed until certain doses are reached for a specific period of time. These substances are referred to as nongenotoxic because they do not affect deoxyribonucleic acid (DNA).

3.1.3 Definition of Risk Assessment If one conducts a literature search on the term ‘‘risk assessment,’’ a lengthy list of publications on a range of topics will be produced (NAS/NRC, 1983; 1994; Paustenbach, 1995), because this term has been used to describe estimates of the likelihood of a number of unwanted events. These include, for example, industrial explosions, workplace injuries, failures of machine parts, natural catastrophes, injury or death as a result of voluntary activities or lifestyle, diseases, and death from natural causes. For the purposes of this Report, a risk assessment is a written document wherein all the pertinent scientific information regarding the risk that arises from disposal of hazardous waste is assembled, critiqued, and interpreted. The goal of the assessment generally could be to calculate the likelihood of responses in humans, aquatic or terrestrial biota, or ecological systems that arise from disposal of hazardous wastes. In this Report, however, the focus is on assessment of health risks in humans (see Section 2.2.6). The magnitude of the risk depends on both the potency of hazardous substances and the amount of exposure, which is a function of the duration of exposure and the concentrations of hazardous substances.

3.1.4 Risk Assessment Process Estimates of risks to human health resulting from disposal of hazardous wastes will nearly always be calculated values based on models. Even if health effects were to occur in the future, they are likely to be unobservable in the background of similar effects from all causes. Therefore, mathematical predictions of risks are required. In general, risk assessment is the process by which toxicology data

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collected from animal studies and human epidemiology are combined with information about the amount of exposure to quantitatively predict the likelihood that a particular response will be seen in a specific human population (Paustenbach, 1995). The four steps involved in calculating risk are conventionally referred to as hazard identification, dose-response assessment, exposure assessment, and risk characterization (NAS/NRC, 1983; 1994; Paustenbach, 1995). These steps in the risk assessment process, as they would conceptually be applied to hazardous waste classification, are shown in Figure 3.1. 3.1.4.1 Hazard Identification. The process of determining whether exposure to a particular substance at any dose can cause a response in a biological organism and, if so, the type(s) of response is called hazard identification. Hazard identification typically involves doses of a substance that are much higher than would actually be experienced in routine exposures of the public, including exposures resulting from waste disposal. Once the hazardous nature of a substance is determined, the results are documented and the hazard identification process need not be repeated for other applications. 3.1.4.1.1 Radiation hazard identification. The hazard identification process is trivial in the case of radiation, because all types of ionizing radiation are assumed to be hazardous and, thus, all radionuclides are assumed to be hazardous substances (see Section 3.2.2). While some responses may not occur at low doses (e.g., damage to the lens of the eye), other responses are assumed to occur with some probability at any dose (e.g., cancer induction). 3.1.4.1.2 Chemical hazard identification. In contrast to radiation, most chemicals are thought not to be hazardous to human health at a sufficiently low dose. In the United States, the process of determining whether a chemical is hazardous relies upon principles established by EPA. These principles are used extensively, but not universally, in other countries. This Section describes the general principles used by EPA to identify hazardous chemicals. Hazard identification is related to the process of dose-response assessment for hazardous chemicals discussed in Section 3.2.1. Characterization and classification of chemical toxicity is complex because of the many possible responses a chemical might induce and the variability of the dose required to yield a response. Toxic responses can include acute effects on the function of various organs or long-term effects such as cancer. Occurrence of a response may be deterministic or stochastic. EPA treats chemicals showing deterministic responses as if there is a threshold below which there is no

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Fig. 3.1. Risk assessment process as applied to waste classification.

observable response. The basis of this assumption is EPA’s understanding of homeostatic, compensating, detoxifying, and adaptive mechanisms. The response occurs only after these mechanisms fail. For example, a toxicant must damage many nephrons in the kidney before clinical signs of kidney failure appear. In contrast, EPA usually treats carcinogenic and mutagenic responses as having a stochastic relationship to dose, but there are some exceptions to this.

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Evidence of possible toxicity in humans comes primarily from two sources: long-term animal tests and epidemiologic investigations. Results from these studies are supplemented with available information from short-term tests, pharmacokinetic studies, comparative metabolism studies, structure-activity relationships, and other relevant toxicologic studies. The question of how likely it is that a substance is a human toxicant is answered in the framework of a weightof-evidence judgment. Such a judgment involves consideration of the quality and adequacy of the data and the kinds and consistency of responses induced by a suspected hazardous substance. There are three major steps in characterizing the weight of evidence for toxicity in humans: (1) characterization of the evidence from human studies and animal studies individually; (2) combination of the characterizations from these two types of studies into an indication of the overall weight of evidence; and (3) evaluation of all supporting information to determine if the overall weight of evidence should be modified. Although there is the potential for a complex, multi-dimensional discussion in this area, the identification of chemicals that cause deterministic responses is discussed first, followed by a discussion of identifying chemicals that cause stochastic responses. An additional complexity is that a material may be hazardous due to its physical and chemical form. Thus, an additional section discusses the identification of hazardous chemical wastes, as opposed to their hazardous chemical constituents per se. Identification of Chemicals That Cause Deterministic Responses. Hazardous chemicals having a threshold in the dose-response relationship are identified using the following process: ● ● ● ● ● ●

consider toxic responses select the critical response select principal study (human or laboratory animal) judge ability of study to predict human toxicity judge appropriateness of route of administration, nature of exposure, and the approach used in each study consider overall weight of evidence from principal and supporting studies

There are significant judgmental aspects involved in this process. The following discussion of hazard identification is taken from EPA (1987a). In hazard identification, EPA considers the adverse toxic responses from all studies. A chemical may cause a variety of adverse effects depending on the magnitude of the dose and the duration of exposure. These may range from clearly defined effects, such as death, to more

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subtle biochemical, physiological, or pathological changes. The effects seen may also differ depending on whether the exposure is acute, subchronic, or chronic. EPA gives primary attention to the significant adverse effect that is seen at the lowest dose or that is of greatest biological significance to humans. EPA defines the critical response (often called the critical effect or the most sensitive toxic endpoint) as the significant adverse effect, or its known precursor, that occurs at the lowest dose. When selecting the critical response, EPA depends on professional judgment in determining whether an observed effect constitutes a response. EPA considers the statistical and biological significance of the effect when making this judgment. However, EPA gives precedence to biological significance and does not consider a statistically significant change lacking biological significance to be a response. For example, EPA has concluded that male rat nephropathy is not an appropriate effect to consider in a human health risk assessment because humans lack the precursor that produces the kidney damage. Other effects that EPA judges to be biologically insignificant include a decrease in body weight compared with controls of less than 10 percent, a change in liver weight of less than 20 percent without significant histopathological changes, and minor changes in clinical chemistry values that will not affect the physiological well-being of the animal. However, EPA relies on its review of all the data before deciding whether an effect is biologically significant and adverse, thus constituting a response to be considered further. Principal studies are those that are the most significant for determining whether a chemical is potentially toxic in humans. These studies are of two types: studies of human populations and studies using laboratory animals. EPA also uses the principal studies in the dose-response assessment (see Section 3.2.1). Human data often are useful in showing the presence of a response. When human studies provide information on the dose associated with toxicity, EPA gives priority to appropriately documented studies in the dose-response assessment. In epidemiologic studies, the investigator attempts to control and measure, within limits, recognized confounding factors. Case reports and acute doses showing severe adverse effects provide support for the choice of the critical response. However, these sources are often of limited utility in showing a quantitative relationship between dose and response. Epidemiologic and clinical studies may contain dose-response information that EPA can use in estimating response probabilities, but EPA must determine that the method of quantifying exposure is appropriate.

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In the absence of adequate human data, EPA selects data from laboratory animals, generally mammals. The animals used most often are the rat, mouse, rabbit, guinea pig, hamster, dog, and monkey. In a typical laboratory animal study, the investigator carefully controls the doses of the toxicant and reduces exposure to other toxicants. Laboratory animal studies also reduce problems associated with heterogeneity of exposed populations. When using these data, EPA must extrapolate from laboratory animals to humans and must account for human heterogeneity. When reviewing animal studies, EPA makes judgments on the ability of the study to predict the potential for toxicity in humans. EPA tries to select data from the species that is most relevant to humans using the most defensible biological rationale. EPA often will use comparative pharmacokinetic data for this decision. For example, dogs and rodents differ in their ability to excrete organic acids. Since humans resemble rodents more closely than dogs in this ability, studies in dogs to test the toxicity of organic acids may not predict the response in humans. Therefore, in these cases, studies in dogs are inappropriate as the basis for determining potential human toxicity. In the absence of a clearly most relevant species, EPA uses the most sensitive mammalian species (i.e., the species showing toxicity at the lowest dose). EPA makes this judgment because there is no assurance that humans are not at least as sensitive as the most sensitive species tested. In addition to the principal studies, supporting studies are used in evaluating chemical toxicity. These studies provide supportive, rather than definitive, information and can include data from a variety of sources. For example, studies of different durations or in different species may confirm the choice of the critical response. Metabolic and other pharmacokinetic studies can provide insights into the mechanism of action of a chemical. By comparing the metabolism of the chemical in the laboratory animal and in humans, EPA might be able to estimate equitoxic doses. In vitro studies can provide insights into the chemical’s potential for biological activity. The known toxicity of a structurally related compound and the use of structure-activity relationships can also provide clues to the chemical’s possible toxicity. EPA usually approaches hazard identification for a chemical with respect to a particular route of exposure (e.g., oral or inhalation). The most appropriate studies for assessing the toxicity of a chemical by a particular route of exposure are those in which the investigator administers the chemical by that route.

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In many cases, the data for a chemical do not include detailed testing for all routes of exposure. The toxicity of the chemical may depend on the route of exposure because of differences in mechanism of action, biochemistry, or absorption. For example, hexavalent chromium (CrⳭ6) can cause lung cancer when inhaled but is not assumed to be carcinogenic when ingested since it is converted to trivalent chromium (CrⳭ3), an essential dietary component, in the stomach. However, EPA’s judgment is that toxicity from one route of exposure suggests the potential for toxicity from another route, unless convincing evidence exists to the contrary. EPA considers potential differences in absorption or metabolism resulting from different routes of exposure. Whenever EPA has relevant data (e.g., comparative metabolism studies), EPA describes the quantitative effects of these differences on the chemical’s toxicity. The amount, frequency, and duration of exposure may vary considerably in different laboratory or epidemiologic studies. For studies in laboratory animals, investigators use a variety of conditions of exposure, typically an acute dose over 1 to 14 d, a subchronic dose over 90 d, or a chronic dose over 1 to 2 y. Dosing schedules are either single, intermittent, or continuous. EPA uses information from all of these types of studies in hazard identification. For example, overt neurotoxicity shown in high-dose, acute studies reinforces the finding of subtle neurological changes in low-dose, chronic studies. EPA gives special attention to studies involving low doses that are continuous and chronic, because such studies reflect the conditions of exposure for which EPA is trying to protect the public. Continuous exposure at low doses can elicit responses absent in studies involving high, short-term doses. Common mechanisms for this behavior include an accumulation of toxicants during chronic exposure or exceeding the repair capacity of a particular organ. If the chronic dose is below that resulting in toxicity, then EPA assumes that no toxicity will occur from any equivalent dose of shorter duration. An ideal study approach attempts to clearly delineate a hypothesis and follow a carefully prescribed protocol. In addition, the investigator provides a clear reporting of the data and describes the analysis to support the conclusions. Listed below are some of the factors that EPA considers in its review of a study: ● ● ● ● ● ●

identity of the substance(s) under study test species and its similarities to humans sex and age of test animals use of proper controls number of animals and doses tested spacing and choice of dose levels

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possible alteration in metabolism at high doses types of observations and methods of analysis nature of pathological changes

As the final step in hazard identification, the risk assessor considers the weight of evidence available from the principal and supporting studies. The results from different studies are examined to determine whether a consistent, plausible picture of toxicity emerges. Some of the factors that add weight to the evidence that the chemical poses a hazard to humans include: ● ● ● ● ● ●

similar effects shown in studies by different investigators similar effects shown across sex, strain, and species similar effects shown across different routes of exposure clear evidence of a dose-response relationship a plausible relationship between data on metabolism, a postulated mechanism of action, and the effect of concern similar toxicity shown by structurally related compounds

Identification of Chemicals That Cause Stochastic Responses. The following discussion on identification of chemicals that cause stochastic responses is based on guidelines issued in 1987 (EPA, 1987a); these guidelines have been used in most risk assessments. New guidelines were proposed in 1996 (EPA, 1996a), but they have not been issued in final form. Differences between the two guidelines are discussed at the end of this Section. Hazard identification for chemicals that cause stochastic responses is concerned with the process of determining whether exposure to a substance has the potential to increase the incidence of stochastic responses. Hazard identification should include a review of the following information to the extent that it is available. 1. Physico-chemical properties, and routes and patterns of exposure. Parameters relevant to identifying stochastic responses include physical state, physical and chemical properties, and exposure pathways in the environment. 2. Structure-activity relationships. Relevant structure-activity relationships can support or argue against the potential toxicity of a substance. These relationships are used to predict the toxicity or the chemical and physical properties of a substance based on its similarity in chemical structure with other substances with known toxicity or other properties (Enslein, 1988). 3. Metabolic and pharmacokinetic properties. This part of the hazard assessment should summarize relevant metabolic information. Such information as whether the substance is direct-acting

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or requires conversion to a reactive carcinogenic (e.g., electrophilic) substance, metabolic pathways for such conversions, macromolecular interactions, and fate (e.g., transport, storage, and excretion), as well as species differences, should be discussed and critically evaluated. Pharmacokinetic properties determine the biologically effective dose and may be relevant to hazard identification and other aspects of stochastic risk assessment. 4. Toxicologic effects. Other toxicologic effects that are relevant to the evaluation of the stochastic response of interest should be summarized. Interactions with other hazardous substances and with lifestyle factors (e.g., smoking) should be discussed. Prechronic and chronic toxicity evaluations, as well as other test results, may yield information on target organ effects, pathophysiological reactions, and pre-neoplastic lesions that bear on the evaluation of the toxicity of substances causing stochastic responses. Dose-response and time-to-response analysis of these reactions may also be helpful. 5. Short-term tests. Tests for point mutations, numerical and structural chromosome aberrations, DNA damage/repair, and in vitro transformation provide supportive evidence of stochastic responses and may give information on potential mechanisms of action. A range of tests for each of the above responses helps to characterize the response spectrum of a substance. Short-term in vivo and in vitro tests that can give an indication of initiation and promotion activity may also provide supportive evidence for a particular stochastic response. However, lack of positive results for genetic toxicity does not necessarily provide a basis for discounting positive results in long-term animal studies. 6. Long-term animal studies. Transplacental and multigenerational studies of stochastic responses, in addition to more conventional long-term animal studies, can yield useful information about the toxicity of hazardous substances. Criteria for the technical adequacy of animal studies have been published in references provided by EPA (1987a) and should be used to judge the acceptability of individual studies. It is recognized that chemicals that induce benign tumors can induce malignant tumors, and that benign tumors can progress to malignant tumors. Therefore, as a conservative measure, the incidence of benign and malignant tumors often is combined. For example, EPA generally will consider the combination of benign and malignant tumors to be scientifically defensible

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unless the benign tumors are not considered to have the potential to progress to the associated malignancies of the same histogenic origin. If an increased incidence of benign tumors is observed in the absence of malignant tumors, EPA considers such information to be limited evidence of carcinogenicity in most cases. The weight of evidence that a substance is likely to be carcinogenic in humans increases when there is (1) an increase in the number of tissue sites affected by the substance, (2) an increase in the number of animal species or sexes showing a stochastic response, (3) an increase in the number of experiments and doses showing a stochastic response, (4) a clear-cut dose-response relationship as well as a high level of statistical significance of the increased responses in treated subjects compared with controls, (5) a dose-related shortening of the time-to-incidence or time-todeath for the response, and (6) a dose-related increase in the proportion of malignant responses. Long-term animal studies using doses at or near the maximum tolerated dose (MTD) are used to ensure adequate power for the detection of toxicity (EPA, 1987a). Negative long-term animal studies at doses above MTD may not be acceptable if animal survival is so impaired that the sensitivity of the study is significantly reduced below that of a conventional chronic animal study at MTD. Positive studies at doses above MTD should be carefully reviewed to ensure that the responses are not due to factors that do not operate at doses below MTD. Evidence indicating that high doses alter responses by indirect mechanisms that may be unrelated to responses at lower doses should be dealt with on an individual basis. Stochastic responses under conditions of the experiment should be reviewed carefully with respect to the relevance of the evidence to humans (e.g., the occurrence of bladder tumors in the presence of bladder stones and implantation site sarcomas). Interpretation of animal studies is aided by the review of target organ toxicity and other effects (e.g., changes in the immune and endocrine systems) that may be noted in pre-chronic or other toxicologic studies. Time- and dose-related incidence of pre-neoplastic lesions may also be helpful in interpreting animal studies. To evaluate toxicity of substances causing stochastic responses, the primary comparison is responses in exposed animals relative to responses in contemporary matched controls. Historical

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control data often are valuable and could be used along with concurrent control data in the evaluation of responses (EPA, 1987a). For the evaluation of rare responses, even small response rates may be significant compared with historical controls. Data from all long-term animal studies are considered in the evaluation of toxicity. A positive response in one species/strain/ sex is not generally negated by negative results in another species/strain/sex. However, replicate negative studies that are essentially identical to a positive study may indicate that the positive results are spurious. Evidence of toxicity should be based on an observation of statistically significant responses in specific organs or tissues. Appropriate statistical analysis should be performed on data from long-term studies to help determine whether the responses are related to exposure to the study substance or possibly due to chance. This analysis should include, at a minimum, a statistical test for trend, including appropriate corrections for differences in survival. The weight to be given to the level of statistical significance (the p-value) and to other available information is a matter of scientific judgment. A statistically significant excess of responses of all types in the aggregate, in the absence of a statistically significant increase in any individual response, should be regarded as minimal evidence of toxicity unless there are persuasive reasons to the contrary. 7. Human studies. Epidemiologic studies provide unique information about the responses of humans who have been exposed to substances suspected of being hazardous. Descriptive epidemiologic studies are useful in generating hypotheses and providing supporting data but can rarely be used to make a causal inferences. Analytical studies of the case-control or cohort variety, on the other hand, are especially useful in assessing risks to humans. Criteria for the adequacy of epidemiologic studies are well recognized (Monson, 1990). They include, for example, proper selection and characterization of exposed and comparison groups, adequacy of the duration and quality of follow-up, proper identification and characterization of confounding factors, attention to potential methodologic biases, appropriate consideration of latency effects, valid ascertainment of the causes of morbidity and death, and the ability to detect specific responses. The statistical power to detect a particular response

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should be included in the assessment when it can be calculated. The weight of the epidemiologic evidence for toxicity depends, among other things, on the type of analysis and on the magnitude and consistency of the response. The weight of evidence increases rapidly with the number of adequate studies that show comparable results on populations exposed to the same substance under different conditions. Epidemiologic studies are inherently capable of demonstrating an association between exposure to a given agent and a disease, thereby allowing estimation of the dose-response relationship, only when the increase in occurrence of the disease is substantially above the background incidence. Negative results from such studies, while reassuring, cannot prove the absence of toxicity. However, negative results from a well-designed and well-conducted epidemiologic study that contains usable data on doses can serve to define upper limits on the possibility of a response. Such results are useful if animal evidence indicates that the substance is potentially toxic in humans. Taking into account the available information, the overall weight of evidence for carcinogenicity of a chemical is classified by EPA into five groups: Group Group Group Group Group

A: Human carcinogen B: Probable human carcinogen C: Possible human carcinogen D: Not classifiable as to human carcinogenicity E: Evidence of noncarcinogenicity in humans

Substances judged to be in Group A or B generally are regarded as suitable for quantitative risk assessment. Substances judged to be in Group C normally are regarded as suitable for quantitative risk assessment, but judgments about this may be made on a case-bycase basis. Substances judged to be in Group D or E are not subjected to quantitative risk assessment. In the hazard identification process for chemicals that cause stochastic effects described above (EPA, 1987a), the weight-of-evidence classification is determined primarily by observations of tumors in animals or humans. Other information about the properties of a chemical, structure-activity relationships for other chemicals that cause stochastic effects, and the influence of a chemical on the carcinogenic process often is limited and plays only a modulating role in the weight-of-evidence classification based on tumor findings. The approach to hazard identification in the proposed revision of the cancer risk assessment guidelines (EPA, 1996a) differs from the

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1987 guidelines in two respects. First, the five weight-of-evidence classes (Groups A through E) are replaced by three categories, which include standard descriptors of conclusions about the carcinogenicity of a substance in humans and a brief narrative description of the informational basis for that conclusion. The standard descriptors of the three categories are ‘‘known/likely,’’ ‘‘cannot be determined,’’ and ‘‘not likely.’’ The narrative explains the kinds of evidence and how they fit together in drawing conclusions. As an example, the narrative describes the carcinogenic potential by different routes of exposure, and it may include a description that a substance is likely to be carcinogenic by one route (e.g., inhalation) and not likely to be carcinogenic by another route (e.g., ingestion). Second, instead of basing the classification mainly on tumor findings in animals or humans, with other information playing only a modulating role in the classification, the conclusion about the weight of evidence for carcinogenicity is reached in a single step, wherein all the information is considered together. This change recognizes the growing sophistication of research methods, particularly in their ability to study modes of action of carcinogenic substances at cellular and subcellular levels, as well as toxicokinetic and metabolic processes. If such information is largely unavailable, cancer risk assessments under the proposed new guidelines will not differ significantly from assessments under the earlier guidelines. Identification of Hazardous Chemical Wastes. The foregoing discussions in this Section have considered the process of identifying substances that are hazardous to human health and the nature of any toxic effects. Two additional concerns arise in identifying hazardous chemical wastes. First, a waste (or any other material) may be hazardous due to its physical and chemical properties, rather than the presence of hazardous substances. For example, a material that is readily explosive or reactive (e.g., hydrogen gas, liquid sodium metal) clearly constitutes a hazard even though the constituent substances themselves may not be hazardous to human health. EPA has identified wastes as hazardous if they are ignitable, corrosive, or reactive. Second, not all chemical wastes that contain hazardous substances are deemed to be hazardous. EPA considers wastes that contain certain hazardous substances (heavy metals and organic compounds, including carcinogens and noncarcinogens) not to be hazardous if the leachability of the substances from the waste form is limited. This characterization of waste as nonhazardous is based on EPA’s judgment that potential risks to humans resulting from disposal of the waste would not exceed acceptable levels.

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The identification of hazardous chemical waste based on its physical and chemical characteristics is discussed in Section 4.2. 3.1.4.2 Dose-Response Assessment. Determining the relationship between the dose of a hazardous substance and the probability of a specific response is called dose-response assessment.6 This aspect of risk assessment is needed to extrapolate from responses observed in experiments or incidents involving high doses to the much lower potential doses relevant to waste disposal and other routine exposure situations. Dose-response assessment is a major issue in establishing the foundations of a risk-based waste classification system, and it is discussed in detail in Section 3.2. 3.1.4.3 Exposure Assessment. In exposure assessment, the population potentially exposed to hazardous substances and the pathways and routes through which exposure could occur are specified, and the magnitude, duration, and timing of the doses people might receive are quantified. The approach to exposure assessment for hazardous waste disposal can range from very sophisticated and complex (e.g., Wilson et al., 1994) to a multiplication of simple factors (e.g., Dornsife, 1995; EG&G, 1982; EPA, 1989; Smith et al., 1980). Exposure assessment for waste disposal is itself a multi-step process, and is discussed below. 1. Describe the conditions of waste disposal. Wastes in specified physical and chemical forms and having certain compositions or ranges of compositions of hazardous substances are assumed to be emplaced in certain ways in a disposal site having specified characteristics. The disposal site can be a real location or generic with hypothetical characteristics typical of real sites. The exposure assessment usually assumes that disposal operations have been completed and the site is closed, although the 6 In health risk assessments for ionizing radiation, the term ‘‘dose’’ generally refers to the energy imparted to organs or tissues from exposure to radionuclides or other sources of radiation modified by a quantity that represents the biological effectiveness of different radiation types, and the dose-response relationship in a particular organ or tissue gives the probability of an adverse health effect as a function of dose (e.g., X excess cancers per sievert). For hazardous chemicals, however, ‘‘dose’’ usually refers to mass intake, rather than an impact on an organ or tissue, and the dose-response relationship usually is an exposure-response relationship (e.g., Z excess cancers per milligram of a toxic chemical ingested per kilogram of body weight per day). This difference is a result of the existence of a unifying measure of radiological impact on humans (i.e., the sievert) and the absence of such a measure for hazardous chemicals, which can impact biological organisms in many ways. In spite of this difference, the phrase ‘‘dose-response’’ will be used in discussing both radionuclides and hazardous chemicals.

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assessment is most often conducted before operations commence as a part of the licensing or permitting process. 2. Describe possible mechanisms by which hazardous substances could be released from a disposal facility. A credible series of processes and events that could result in release of hazardous substances from the disposal site to a portion of the environment that is accessible to humans and the probability that these processes and events would occur, often called a release scenario, is developed. Release scenarios for waste disposal facilities generally should include considerations of inadvertent human intrusion resulting from normal activities, such as excavation or drilling, as well as releases to air and groundwater due to natural processes and events. Processes and events normally considered in developing release scenarios are shown in Figure 3.2. The thought process involves conceiving of ways in which the barriers to release of hazardous substances, such as waste containers or control of groundwater, might be compromised or circumvented. Examples of potentially important release scenarios include: (1) infiltration of water, which degrades waste containers, dissolves hazardous substances, and transports them to a location where exposure of humans can occur; and (2) inadvertent drilling into the waste site, resulting in hazardous material being brought to the surface where an intruder can be exposed. 3. Characterize possible mechanisms of exposure to hazardous substances. The pathways by which hazardous substances released from a disposal facility can be transported through the biosphere and the resulting routes of human exposure are specified, often along with their respective probabilities. To estimate exposures of humans at assumed receptor locations, dilution of contaminants by transport in air or water as well as concentration by various means, such as precipitation and uptake by intermediate biological organisms consumed by humans, must be considered. An example of the potentially complex web of exposure pathways is shown in Figure 3.3. 4. Develop models for scenarios and acquire data. The release and exposure scenarios described above are evaluated through modeling. The models embody the mathematical interrelationships of the possible steps in each scenario. Simplifications and approximations usually are introduced to reflect limitations in knowledge and data or the results of previous risk assessments that show certain scenarios and pathways to be negligible. The result often is a series of models describing (1) degradation of

Fig 3.2.

GEOLO GY

BACKFILL

T EMEN

IZATION INHIBIT BIL OR RPACK MO OVE NTAINER CO ABILIZER ST EF ST O

LA C

EARTH'S CORE

HAZARDOUS MATERIALS

GEOSPHERE

WASTE AND DISPOSAL FACILITY DRIVING FORCES • THERMAL • CHEMICAL • MECHANICAL • RADIOLOGICAL

NATURAL RESOURCES

HUMAN DRIVING FORCES • INTRUSION • TRANSPORT AGENTS • BIOSPHERE

BIOSPHERE

Considerations involved in establishing scenarios for release of hazardous substances from waste disposal sites.

WASTE DISPOSAL FACILITY

WASTE PACKAGE

P EM

LAND SURFACE

RM

TERRIGENIC DRIVING FORCES • THERMAL • GRAVITATIONAL • MECHANICAL

GROUNDWATER

SURROUNDING GEOLOGIC MEDIUM

COSMIC DRIVING FORCES • SOLAR • FOREIGN BODY KINETIC

SURFACE WATER

ATMOSPHERE

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SEAL OR COVER

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Fig. 3.3. Pathways for movement of hazardous substances through the environment to humans.

containers and release of hazardous substances or access to waste by inadvertent human intruders, (2) transport to the biosphere, and (3) biospheric transport and exposure of humans. Many models also include data to estimate doses and associated responses. Exogenous data, such as container corrosion rates, solubility and mobility of hazardous substances, and consumption rates of foodstuffs by humans, are required to use the models. As a result of risk being defined as the probability of a response, consideration of the probability of the processes

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and events in the release and exposure scenarios is required. In some risk assessments, probabilities of scenarios may be considered only qualitatively. For example, scenarios judged to be credible are assumed to occur with a probability of one, whereas scenarios judged not to be credible are assigned a probability of zero. This approach often is taken when the purpose of the assessment is to evaluate compliance with requirements on facility performance, rather than to estimate expected risks. In more complex analyses, probabilities of scenarios may be specified quantitatively and the input data or probabilities of processes and events may be specified as distributions that are statistically sampled to yield a distribution of risks. 5. Exercise the models to calculate exposure and risk. The models are typically implemented using computer programs, which are exercised using the acquired data. Simpler programs will calculate an exposure and associated response resulting from a waste disposal situation and multiply it by the probability of the initiating process or event, yielding an estimate of the risk for that situation. More complex programs calculate a distribution of risks based on a range of probabilities for the initiating processes and events and the input data. A summary depiction of the probabilistic exposure and risk assessment process as applied to waste disposal is shown in Figure 3.4 (Garrick and Kaplan, 1995). It should be recognized that probabilistic assessments are appropriate for use at specific sites and when the purpose is to estimate expected risks. For the purpose of classifying waste, however, risk assessment must be nonsite-specific (generic) and is necessarily much simpler. 3.1.4.4 Risk Characterization. As emphasized in the preceding discussions of hazard identification (Section 3.1.4.1), dose-response assessment (Section 3.1.4.2), and exposure assessment (Section 3.1.4.3), calculating risk involves numerous assumptions and simplifications, including significant extrapolation of data on dose-response. Risk characterization provides the capstone of a risk assessment by integrating and interpreting the information developed in these steps, identifying limitations and uncertainties in the models and data used to estimate human health risks, and then communicating the results appropriately (NAS/NRC, 1994). Integration of the results of the first three steps in a risk assessment typically results in a quantitative estimate of risk. Estimated

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1. UNDISTURBED CASE a) CLIMATE CHANGES b) WATER FLOW RATES AND DURATION 2. EPISODIC EVENTS a) EARTHQUAKES b) VOLCANIC ERUPTIONS c) EXTREME FLOODING/ EROSION d) METEORITES e) HUMAN INTRUSTIONS f) OTHER

INITIATING EVENTS

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WATER FLUX

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HAZARDOUS SUBSTANCE RELEASE STATES

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HAZARDOUS SUBSTANCE RELEASE STATES

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HAZARDOUS SUBSTANCE RELEASE STATES

PERFORMANCE MEASURES (HAZARDOUS SUBSTANCE INVENTORY AND HEALTH EFFECTS)

BIOSPHERE TRANSPORT (HAZARDOUS SUBSTANCE TRANSPORT THROUGH THE BIOSPHERE)

GEOSPHERE TRANSPORT (HAZARDOUS SUBSTANCE TRANSPORT THROUGH THE GEOSPHERE)

FREQUENCY OF EXCEEDANCE

t1 TIME

TIME

t2

t3

t1

t3

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NUMBER OF HEALTH EFFECTS H

RISK CURVES PROBABILITY

t2

RISK AT DISCRETE TIMES

AMOUNT

PROBABILITY

TOTAL AMOUNT RELEASED

DOSE RATE

FINAL RESULTS DOSE TO HUMANS

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Fig. 3.4. Depiction of probabilistic exposure and risk assessment process for hazardous waste disposal (Garrick and Kaplan, 1995).

INITIATING EVENT

ENGINEERED BARRIER BREACH (DEGRADATION AND FAILURE OF ENGINEERED BARRIERS)

INFILTRATION (WATERFLOW TO CONTAINER)

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risks can be expressed in a variety of ways, such as incidence or fatalities, deterministic or probabilistic, individual or population, occupational or public, relative or absolute. Comparison of estimated risks with other relevant risks to provide perspective also is a part of risk characterization. An important part of risk characterization is an interpretation of the results of a risk assessment, particularly in regard to evaluating the impact of uncertainties in the different steps of the assessment on the significance of the estimated risk. The appropriate treatment of uncertainty depends on the purpose of the assessment. If the purpose is to estimate actual or expected risks, the uncertainty analysis should consider the variability of the estimated risk about a measure of central tendency, such as a standard deviation of the mean. However, if the purpose of the assessment is to evaluate compliance with regulatory requirements, such as a specified level of acceptable risk, the uncertainty analysis should identify those parameters or assumptions which, when varied over their assumed ranges, could change the decision about compliance. Some aspects of uncertainty can be quantified, such as the distribution functions that result from probabilistic risk calculations (see Figure 3.4). However, for many important aspects of risk assessment, uncertainties can only be characterized qualitatively (see discussion of ‘‘weight of evidence’’ in Section 3.1.4.1.2). No distinction is made here between the type of uncertainty that results from an observed variability in a natural or biological system and the type of uncertainty that results from lack of knowledge about the behavior of a system (NAS/NRC, 1994). Uncertainty due to lack of knowledge about system behavior is likely to be particularly important in risk assessments of waste disposal systems. This type of uncertainty is difficult to quantify objectively. 3.1.4.5 Risk Management. Not part of the risk assessment process per se is the process by which the results of risk assessments are integrated with other information to make decisions about the need for, method of, and extent of risk reduction or limitation. This process is referred to as risk management. In a waste classification context, risk management would involve evaluating estimates of risks that arise from waste disposal and their attendant uncertainties with respect to values of acceptable risk, resulting in decisions about classification and acceptable methods of disposal (see Figure 3.1). Values of acceptable risk may be expressed using surrogates, such as release limits or maximum acceptable concentrations of hazardous

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substances in waste; such surrogates are calculated from the values of acceptable risk based on assumed exposure scenarios. The risk management process may involve consideration of socioeconomic and institutional factors, as well as the results of risk assessment. Risk management is a very important aspect of establishing a waste classification system and is discussed further in Section 3.3.

3.1.5 Use of Risk Assessment in Risk-Based Waste Classification Based on the discussions in Section 3.1.4, the process of estimating the risk that arises from disposal of a hazardous waste at a specific site is relatively straightforward from a conceptual standpoint. In practice, however, implementation of the risk assessment process can be difficult owing to several factors, including incomplete understanding and data concerning release, transport, exposure (dose), and response to hazardous substances; uncertainties in the data; and simplifications necessary for the models to be usable. However, since this Report is concerned with establishing the conceptual foundations of a waste classification system based on nonsite-specific (generic) risk assessments, the complexities of implementation can be greatly reduced. The two most important issues in implementing a generic risk assessment for purposes of waste classification involve assumptions about release and exposure scenarios and assumptions about dose-response relationships. These issues are introduced in the following two sections. 3.1.5.1 Risk Assessment of a Generic Site. The primary purpose of waste classification is to allow a grouping of wastes destined for the same disposal technology, so that waste can be managed before disposal with this objective in mind. The characteristics of specific disposal sites and site-specific waste acceptance criteria usually are not known at the time waste is being classified. The issue resulting from this situation is how to perform the risk assessment for disposal necessary to classify waste when waste may be sent to different disposal sites having unknown but potentially very different characteristics (e.g., arid versus humid). There are three potentially significant pathways through which exposure to hazardous substances emplaced in a waste disposal facility can occur: ●

Dissolution and transport of hazardous substances by groundwater. Release of hazardous substances into groundwater is relevant to virtually all hazardous substances, although it may not

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be significant for many substances that are short-lived, insoluble, or greatly retarded during migration. Water is assumed to infiltrate the disposal facility from above (downward percolation of precipitation) or the side (lateral flow of water into the wastebearing area). This water degrades waste containers, solubilizes hazardous substances, and flows away from the disposal facility to a point where the contaminated water enters the biosphere and is assumed to be accessible to plants, animals, and humans. This release pathway is very sensitive to the site selected, because the amounts of precipitation and groundwater vary widely and the behavior of the water and hazardous substances contained therein are strongly dependent on the physical and chemical characteristics of the soil and rock at the site. Release of hazardous substances directly to the atmosphere. Hazardous substances can be transported to the atmosphere as suspended particles or gases. However, the atmospheric release pathway usually is significant only for a limited number of hazardous substances that are volatile at temperatures common in the environment, including volatile organic chemicals and a few gaseous radionuclides, such as volatile 3H and 14C compounds and radon. Volatile substances typically are contained in a waste disposal facility by the use of caps composed of plastic, clay, or other impermeable materials. A site-specific risk assessment normally assumes that these caps fail or are compromised, resulting in the release of volatile substances. Because containment is provided primarily by engineered barriers, releases to the atmosphere are not very sensitive to the specific disposal site being considered. Inadvertent human intrusion into hazardous waste. Inadvertent human intrusion is relevant to disposal of virtually all hazardous substances, especially in near-surface facilities. Typical scenarios assume that an unknowing individual (1) digs or drills into the waste and brings some of it to the surface where it is then available for dispersal and uptake, or (2) lives on the disposal site after waste has been exhumed or the cover removed, and consumes contaminated plant and animal products. Scenarios for inadvertent intrusion usually are assumed to occur after some period of active institutional control over the disposal site, which is typically 100 to 300 y. Intrusion scenarios are not very sensitive to site-specific parameters because the nature of intrusion (by digging or drilling) effectively bypasses the site-specific protection features, such as small amounts of groundwater,

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impermeable soils, or the presence of engineered barriers to release and transport by natural processes.7 Thus, for purposes of waste classification, it appears possible to assess the risk from the intrusion and atmospheric release pathways at a generic hazardous waste disposal site. However, a generic assessment of risk from the water release pathway normally would incorporate assumptions that would be extremely conservative for many sites (e.g., the amount of water infiltration and travel times of hazardous substances to a nearby well). Fortunately, risk-based waste classification becomes possible because the risk from hazardous waste disposal, especially in nearsurface facilities, usually is dominated by the risk from inadvertent intrusion onto a disposal site after an assumed loss of institutional control, given an assumption that postulated scenarios for intrusion would occur. For disposal of radioactive waste in near-surface facilities, generic assessments (NRC, 1982b) have shown that, for most radionuclides, disposal limits based on the need to protect inadvertent intruders are more restrictive than limits based on requirements for protection of off-site individuals. That is, except in unusual cases of highly soluble, long-lived radionuclides, the calculated dose per unit amount of a radionuclide in disposed waste is substantially higher for hypothetical inadvertent intruders than for off-site individuals who are assumed to be exposed to contaminants released to water or air. The key feature of these analyses is that transport of contaminants from a disposal facility into the environment by the water pathway generally results in considerable reductions in concentrations compared with the concentrations in the disposal facility itself and, thus, potential exposures of off-site individuals compared with inadvertent intruders. Similar analyses have not been performed for hazardous chemicals, but the same general result should be obtained for substances that are not highly soluble. Thus, as a practical matter, it is possible to establish a risk-based hazardous waste classification system by focusing on intrusion scenarios that are essentially generic. In reaching this conclusion, it is 7 Inadvertent human intrusion continues to be controversial because it does not effectively discriminate among sites. Many view intrusion as something to be addressed by including certain features in the disposal system to discourage intrusion and warn the unknowing, but that intrusion scenarios should not be used as a basis for establishing site suitability. However, intrusion scenarios continue to be used as a basis for assessing the acceptability of waste disposal sites in most countries, including the United States, and it is used in this context in this Report. For further information, the reader is referred to a report by the Nuclear Energy Agency (NEA, 1995).

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recognized that there will be cases where disposal of relatively soluble contaminants should be limited based on an analysis of transport in water to off-site receptor locations, rather than an analysis of intrusion scenarios. However, such cases would be accounted for in establishing site-specific waste acceptance criteria, and the few exceptions should not negate the usefulness of a waste classification system based on generic analyses of scenarios for inadvertent intrusion. This is especially the case if analyses of intrusion scenarios are based on the conservative assumption that all contaminants are highly immobile and, thus, are retained in the disposal facility until the time intrusion is assumed to occur. Given the conclusion that the concept of a hypothetical inadvertent intruder at a waste disposal site is useful for purposes of classifying waste, a multitude of scenarios might be considered. Although the development of scenarios for inadvertent intrusion for the purpose of waste classification is properly the role of regulatory authorities, NCRP believes that the types of scenarios commonly assumed in risk assessments of near-surface disposal sites for low-level radioactive and hazardous chemical wastes (EPA, 1989; NRC, 1982b) or in screening of contaminated sites (NCRP, 1996; 1999a; NRC, 1996a) are appropriate. These types of intrusion scenarios involve such activities as building homes at a site, excavation or drilling into waste and exhuming its contents, and cultivation of the site by resident homesteaders after waste has been exhumed or uncovered. Although other intrusion scenarios might be envisioned, these probably capture the plausible range of scenarios relevant to classification of hazardous waste. Furthermore, they are likely to result in conservative estimates of risk compared with intrusion scenarios that might actually occur at any site, because the assumed scenarios usually involve pessimistic assumptions about the quantities and concentrations of contaminated materials to which an intruder would be exposed, the number of exposure pathways, and exposure times. The dominance of the risk to inadvertent intruders at near-surface waste disposal sites allows the use of this type of scenario to develop a risk-based waste classification system. However, NCRP recognizes that exposures of the public and protection of the environment also are of concern in determining acceptable disposal practices at specific sites. The potential for off-site releases of hazardous substances is the primary reason that classification of waste based on risks to hypothetical inadvertent intruders does not obviate the need for sitespecific risk assessments to determine waste acceptance criteria in the form of limits on disposal of particular hazardous substances.

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3.1.5.2 Dose-Response Relationships. The primary objective of this study is to set forth the foundations of a risk-based waste classification system that applies to hazardous chemicals and radionuclides. Most aspects of the risk assessment process that provide the basis for establishing this system are conceptually the same for chemicals and radionuclides, although the specific data (e.g., solubilities) may differ. One important exception is the assumed relationship of the probability of a response to a unit dose of a substance that causes stochastic effects, which is called the dose-response relationship.8 There are important conceptual differences in the way this relationship has been defined and used for hazardous chemicals and radionuclides, and these differences could pose a major impediment to development of a risk-based waste classification system that applies to both types of substances on a consistent basis. These differences are elucidated in the following section.

3.2 Assessment of Responses from Exposure to Hazardous Substances For any hazardous substance, estimates of the relationship of dose to response in humans are based on either animal or human data. For example, only about 20 of the approximately 300 chemical carcinogens regulated by EPA have dose-response relationships based on human data from epidemiologic studies; the remainder are based on animal bioassays. In contrast, the dose-response relationships for radiation are based primarily on the results of human epidemiologic studies. The doses of hazardous substances at which responses can be observed in humans or animals are higher (sometimes by large factors) than doses relevant to waste disposal and other routine exposure situations. Therefore, most dose-response relationships at the low doses of interest in protection of human health are calculated rather than measured; they are based not only on scientific data but also on various assumptions and extrapolation models which, while scientifically plausible, cannot yet be subjected to empirical study 8 In general, the relationship between dose and response can be represented by a variety of functional forms. At low doses of substances that cause stochastic effects, the dose-response relationship usually is assumed to be linear and, thus, can be expressed as a single probability coefficient. This coefficient is frequently referred to as a ‘‘risk’’ (or potency factor or unit risk factor or slope factor) in the literature. However, it is really the response (consequence) resulting from a dose of a hazardous substance, and it should not be confused with ‘‘risk’’ as defined and used in this Report.

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and validation. Indeed, the vast majority of responses alleged to be associated with low doses of hazardous substances are not generally testable with any practical epidemiologic study. Despite this difficulty, regulatory authorities make decisions based on the possibility that hypothetical responses predicted to occur at low doses may be real. Such actions are based partly on legal requirements and partly on prudence. In short, the rationale for making decisions based on such evidence is that the body of scientific information, assumptions, and extrapolation models used to develop dose-response relationships is considered sufficiently revealing on the question of risk to humans to prompt control measures. The following sections discuss dose-response relationships for hazardous chemicals and radionuclides. 3.2.1 Assessment of Responses from Exposure to Hazardous Chemicals Once the hazard identification process is completed for a chemical (see Section 3.1.4.1), a judgment is made concerning the appropriateness of conducting a dose-response assessment. If such an assessment is judged appropriate, based on the observation of sufficient responses in humans or animals, the process shown in Figure 3.1 is undertaken. First, the basis for the assessment must be established, as described in Section 3.2.1.1. The dose-response assessment then is undertaken using one of several approaches described in Sections 3.2.1.2 and 3.2.1.3, depending on the available data, nature of the responses (deterministic or stochastic), and applicable regulatory guidance. Characterization of the dose-response assessment is discussed in Section 3.2.1.4. Deficiencies and uncertainties are an inevitable part of this process, and these are discussed in Section 3.2.1.5. 3.2.1.1 Basis for a Dose-Response Assessment. At a minimum, the results of the hazard identification process are available as a basis for dose-response assessment. Additional data obtained from known occupational exposures or studies conducted specifically for the purpose of dose-response assessment may also be available. If available, adequate human epidemiologic data are preferred over data from animal studies. If there are adequate data on doses received in a well-designed and well-conducted negative epidemiologic study, it may be possible to obtain an upper-bound estimate of the response probability from that study. Estimates of upper bounds obtained from negative studies on animals, if available, also should be presented as supporting evidence. In the absence of appropriate human studies, data from an animal species expected to respond most like humans should be used, if this

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type of information is available. When several studies on a given substance are available, which may involve different species, strains, and sexes given several doses by different routes of exposure, the following approach to selecting the data sets is used: (1) the response data are separated according to organ affected and response type; (2) all biologically and statistically acceptable data sets are presented; (3) the range of the dose-response estimates is presented with due regard to biological relevance, particularly in the case of animal studies, and appropriateness of the route of exposure; and (4) the biologically acceptable data set from long-term animal studies showing the greatest sensitivity is generally given the greatest emphasis, again with due regard to biological and statistical considerations, because human sensitivity could be as high as the most sensitive responding animal species, in the absence of evidence to the contrary. When the route of exposure in the species from which the doseresponse information is obtained differs from the route occurring in environmental exposures, the considerations used in making the route-to-route extrapolation must be carefully described. All assumptions should be presented along with a discussion of the uncertainties in the extrapolation. Whatever procedure is adopted in a given case, it must be consistent with the existing metabolic and pharmacokinetic information on the chemical, such as the absorption efficiency in the gut and lung, doses to target organs, metabolic toxification or detoxification processes, and changes in placental transport throughout gestation for transplacental toxicity. When two or more significantly elevated response sites or types are observed in the same study, extrapolations may be conducted on selected sites or types. These selections will be made on biological grounds. To obtain an estimate of the total response probability in animals with two or more response sites or types showing significantly elevated occurrence, response probabilities often are pooled and used for extrapolation. Pooling of data results in an estimate of total risk that is applied to humans without regard for the particular organs or tissues at risk in humans. Such pooling increases the statistical power of a study, and it takes into account that observed cancers in study animals often do not correspond to cancers expected to occur in humans. Quantitative extrapolations of the dose-response relationship generally will not be made on the basis of totals that include response sites without statistically significant elevations. Chemical agents are not expected to increase the incidence of cancer in all, or even many, organs or tissues. Rather, it is thought that certain agents can cause an increase in the incidence of cancer in a single organ or, in some cases, two or three related tissues. This

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is the rationale for conducting quantitative risk assessments based on those organs or tissues in which there is a statistically related increase in cancer incidence between study populations and control groups, and generally only in those cases where there is an increase in tumor response with increasing dose. When more than one organ has an increased cancer incidence, several permutations of the data often are evaluated and the one that yields the strictest and lowest virtually safe dose is used for purposes of regulatory risk assessment and health protection. For example, if malignant tumors in the kidney and bladder are observed, and both carcinomas and adenomas are present in both organs, the arrangement of the data (e.g., the sum of both benign and malignant tumors in the kidney) that produces the lowest ‘‘safe’’ dose is the one used. There is no standard set of assumptions about which organs should be considered, the tumor types that can be added, the degree of dose-relatedness that must accompany the tumor incidence, or which animal model is preferred. Benign tumors generally should be combined with malignant tumors for the purpose of estimating the dose-response relationship, unless the benign tumors are not considered to have the potential to progress to the associated malignancies of the same histogenic origin. The contribution of the benign tumors to the total response should be indicated. Since responses at the low dose levels of concern in routine exposures of the public cannot be measured directly in animal or human epidemiologic studies, a number of approaches have been developed to extrapolate from high to low doses. Different extrapolation approaches may fit the observed data reasonably well but lead to large differences in the projected responses at low doses (see Section 3.2.1.5.2). 3.2.1.2 Dose-Response Assessment for Chemicals That Cause Deterministic Effects. For hazardous chemicals that cause deterministic effects and exhibit a threshold in the dose-response relationship, the purpose of the dose-response assessment is to identify the dose of a substance below which it is not likely that there will be an adverse response in humans. Establishing dose-response relationships for chemicals that cause deterministic effects has proved to be complex because (1) multiple responses are possible, (2) the dose-response assessment is usually based on data from animal studies, (3) thousands of such chemicals exist, and (4) the availability and quality of data are highly variable. As a consequence, the scientific community has needed to devise and adhere to a number of methods to quantify the most important (low or safe dose) part of the dose-response relationship.

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There are two possible approaches to estimating the human ‘‘safe’’ dose for chemicals that cause deterministic effects: the use of safety and uncertainty factors and mathematical modeling. The former constitutes the traditional approach to dose-response assessment for chemicals that induce deterministic effects. Biologically-based mathematical modeling approaches that more realistically predict the responses to such chemicals, while newer and not used as widely, hold promise to provide better extrapolations of the dose-response relationship below the lowest dose tested. 3.2.1.2.1 Dose-response concepts. Dose-response assessment for hazardous chemicals that can cause deterministic effects begins with the toxicology data developed during the hazard identification step described in Section 3.1.4.1.2. In many cases, hazard identification and dose-response assessment occur simultaneously. For each chemical, the critical response (a specific response in a specific organ) is identified in the hazard identification process. Using the available data for the critical response, one of the following is established: ●

●

A no-observed-adverse-effect level (NOAEL), which is the highest administered dose at which no biologically significant increase in the frequency or severity of the critical response between the exposed population and its control in the most sensitive test species is identified. The study may show some effect at this dose but the effect is not deemed a response because EPA judges that it is not adverse or is not a precursor to a specific adverse effect severe enough to be considered a response. If several doses in a study are NOAELs or if several studies indicate different NOAELs, EPA focuses on the highest dose without adverse effect; this leads to the common usage of the term NOAEL to mean the highest dose without significant adverse effect. A lowest-observed-adverse-effect level (LOAEL), which is the lowest administered dose of a chemical at which there is a biologically significant increase in frequency or severity of the critical response between the exposed population and its control.

LOAEL is, of course, higher than NOAEL. NOAEL (or LOAEL if NOAEL is not available) is used as a point of departure to calculate a reference dose (RfD), which is the highest dose of the chemical at which no statistically significant adverse effects are expected in the most sensitive humans. RfD of a particular substance is calculated from NOAEL (or LOAEL) by applying one or more safety and uncertainty factors. RfD usually is 100 to 1,000 times lower than NOAEL or 1,000 to 10,000 times lower than

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LOAEL, although a substantially smaller safety and uncertainty factor may be used when RfD is derived from a NOAEL obtained in a high-quality study in humans. The schematic relationship of NOAEL, LOAEL, and RfD is illustrated in Figure 3.5. Although RfDs are widely used in health protection of the public, it is important to understand that they do not represent thresholds for deterministic responses in humans. 3.2.1.2.2 Safety factor approach for chemicals that cause deterministic effects. Traditional toxicologic procedures for chemicals that can induce deterministic effects, which are assumed to have a threshold dose, define RfD for humans or animals as some fraction of NOAEL. This fraction is determined by establishing safety factors to account for weaknesses and uncertainties in the data and in the extrapolation from animals to humans. In the safety factor approach, doses below RfD are assumed not to result in a response because they are below the threshold of toxicity (Dourson and Stara, 1983; Renwick and Lazarus, 1998; Weil, 1972).

Fig. 3.5. Illustration of relationships of NOAEL, LOAEL, and RfD.

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EPA bases its procedures for estimating RfD on several assumptions, the most basic of which is that a threshold exists in the doseresponse relationship for the critical response. If the dose is above the threshold (not the same as RfD) and is of sufficient duration, EPA considers that the chemical will cause the response in some segment of the exposed population. The U.S. Food and Drug Administration uses a similar approach to identify safe levels of exposure to food additives and certain residues. Studies on many substances have shown that before toxicity occurs, the chemical must deplete a physiological reserve or overcome the various repair capacities in the human body (Klaassen et al., 1996). A second major assumption is that an RfD adequately protects particularly susceptible humans (e.g., infants and children, the elderly or infirm). To account for these population groups, EPA includes an uncertainty factor that represents the variation in the thresholds among individuals. For some chemicals, however, a few persons show evidence of hypersensitivity or chemical idiosyncrasy (NAS/NRC, 1994), and there is concern over whether using RfD to limit exposure will protect these subgroups. A third important assumption relates to selecting the critical response. EPA assumes that if the dose is below that required to cause the most sensitive response, then other deterministic responses will not occur. However, if other responses have shallower slopes in the dose-response curves near their thresholds, estimating RfD on the basis of the critical response may not be sufficiently protective to preclude a noncritical response from occurring. For this reason, EPA may use information on the slopes of dose-response curves to determine the critical response and the number of safety factors to be applied, although EPA rarely does so. Lastly, EPA usually assumes that using animals of different ages does not affect NOAEL and LOAEL. This assumption ignores possible differences in toxicity among animals of different ages. 3.2.1.2.3 Selection of the database. The types of studies that make up a complete database for estimating an RfD of high confidence for chemicals causing deterministic effects from data in laboratory animals include: ● ● ●

Two adequate chronic toxicity studies in different mammalian species by oral exposure; One adequate multi-generation reproductive toxicity study in a mammalian species by oral exposure; and Two adequate developmental toxicity studies in different mammalian species by oral exposure.

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These studies may show a need for special studies to assess, for example, neurotoxic or immunotoxic responses. In such cases, the database may not be complete without the special studies. EPA does not consider data on developmental toxicity, standing alone, as an adequate basis for estimating RfD. Investigators use acute or short-term exposures for these studies. Therefore, they are of limited use in estimating the threshold for deterministic responses. However, if developmental toxicity is the critical response for a chemical with a complete database, EPA will derive RfD from that study. EPA uses the following hierarchy for choosing the study, species, and NOAEL used to calculate an RfD: ● ●

●

The most appropriate NOAEL for the critical response from a well-conducted study in humans is identified. The most appropriate NOAEL for the critical response from a well-conducted study in a laboratory animal species that most resembles humans is chosen. The most sensitive species and study is identified. EPA bases this decision on a comparison of the available NOAELs and LOAELs. However, the data from one study may be unusual or inconsistent when compared with the results of other studies. If there is a convincing scientific argument that the response will not occur in humans, EPA may discount the unusual data in these cases.

A subchronic (90 d) bioassay in a mammalian species by oral exposure is the recommended minimum data for estimating an RfD. The study must meet EPA’s minimum standards of quality. Ideally, the study should identify a NOAEL and a LOAEL. In the absence of these minimum data, EPA assigns the chemical to a ‘‘not verifiable’’ group, and EPA then seeks or waits for additional data before estimating RfD for that chemical. 3.2.1.2.4 Determination of the reference dose. EPA determines RfD from NOAEL or LOAEL obtained from an animal study or, occasionally, a human study using the following equation: RfD ⳱ NOAEL or LOAEL/(UF ⳯ MF),

(3.1)

where UF is an uncertainty factor that accounts for uncertainties in extrapolating from the experimental data and MF is a modifying factor (see Table 3.1). The product of UF and MF is the safety factor, which also is referred to as the safety and uncertainty factor since it accounts for uncertainties in the data.

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TABLE 3.1—Guidelines for the use of uncertainty and modifying factors in deriving an RfD (EPA, 1989). Sub-Factor

Issue Addressed

Value and Comments

Uncertainty Factor (UF): H ⳯ A ⳯ S ⳯ L ⳯ D H

Interhuman

Generally use a factor of 10 when extrapolating from experimental results in studies using prolonged exposure of average healthy humans. EPA intends that this factor account for variation in sensitivity in humans.

A

Experimental: animals to humans

Generally use a factor of 10 when extrapolating from results of chronic studies on experimental animals. EPA intends that this factor account for uncertainty involved in extrapolating from laboratory animals to humans.

S

Subchronic to chronic

Generally use a factor of 10 when extrapolating from results of subchronic studies on experimental animals or humans. EPA intends that this factor account for uncertainty involved in extrapolating from subchronic NOAELs to chronic NOAELs.

L

NOAEL to LOAEL

Generally use a factor of 10 when deriving an RfD from a LOAEL, instead of a NOAEL. EPA intends that this factor account for uncertainty involved in extrapolating from a LOAEL to a NOAEL.

D

Incomplete database

Generally use a factor of 10 when data are incomplete. EPA intends that this factor account for inability of a study to adequately address all adverse effects. This factor reflects extent of information on the chemical to judge its toxicity in chronic, reproductive, and developmental settings.

Modifying Factor (MF) Other uncertainties and limitations

Use professional judgment to determine a factor which is ⬎0 and ⱕ10. The size of this factor depends on uncertainties in the study and database not explicitly treated in uncertainty factor. Default value is one.

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RfD is useful as a reference point for identifying whether a particular dose to humans poses a significant health hazard. Doses below RfD are not likely to cause any deterministic responses and are of little regulatory concern. As the frequency of exposure that exceeds RfD increases, the chance increases that the dose may cause a response. However, EPA cannot conclude that doses below RfD will not result in a response in a few individuals, or that doses above RfD will result in a response in any individual. 3.2.1.2.5 Selection of uncertainty and modifying factors. The choice of an uncertainty factor for a chemical that can cause deterministic effects is based on case-by-case judgment. This factor should account for the shortcomings and uncertainties in the scientific data. As noted in Table 3.1, EPA often uses up to a factor of 10 for each of five areas of uncertainty in establishing an RfD from a NOAEL or LOAEL. In practice, however, the magnitude of any uncertainty factor is dependent on professional judgment. If EPA has resolved uncertainties in all areas, which usually is not the case, an uncertainty factor of one is used to estimate RfD. When uncertainties exist in one, two, or three areas, EPA can use an uncertainty factor of 10, 100, or 1,000, respectively. When uncertainties exist in four areas, EPA often uses an uncertainty factor of 3,000. When uncertainties exist in five areas, EPA might use an uncertainty factor of 10,000. The justification for reducing the uncertainty factor in the latter two situations is EPA’s knowledge of interrelationships among the various areas of uncertainty. In these cases, the multiplication of four or five factors of 10 is likely to yield an unnecessarily low RfD. EPA occasionally uses an uncertainty factor of less than 10 if the available data reduce the need to account fully for a particular area of uncertainty. For example, if the lowest dose tested shows only a minor adverse effect, EPA might assign an intermediate value to the uncertainty factor. The usual intermediate value is three, which is the geometric mean of one (the lowest theoretical factor) and 10, rounded to one significant digit. This rounding procedure reflects the expected precision of the process. EPA uses the geometric mean because it judges that the biological processes involved are likely to show a log-normal probability distribution. EPA might use a value of three for the database factor if studies on developmental or reproductive toxicity are missing. EPA might also use a value of three for the subchronic-to-chronic factor if the database includes occupational studies of long duration (e.g., 7 to 15 y). EPA uses a modifying factor as an occasional adjustment in estimating an RfD. EPA intends that this factor account for areas of uncertainty not explicitly addressed by the usual factors. Its value

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may be less than one but not more than 10; the default value is one. EPA assigns the value using the same rules as for the uncertainty factors (i.e., 0.3, 1, 3, or 10). EPA uses a modifying factor when the usual uncertainty factors do not fully account for uncertainties in the data. For example, the fewer the number of laboratory animals used in a group, the more likely it is that the study will show a NOAEL. In such a case, EPA might raise the usual uncertainty factor of 100 to 300 to account for this deficiency. While this increase is scientifically reasonable, the public might perceive the change as arbitrary. EPA intends to avoid this perception by using a modifying factor. Thus, EPA might use an uncertainty factor of 100 and a modifying factor of three to arrive at an RfD. 3.2.1.2.6 Assigning confidence levels. EPA assigns confidence levels (low, medium or high) to the principal study, the database, and RfD. When assessing the level of confidence in the principal study, EPA considers several factors. One is the adequacy of study design including the route of exposure, sample size, duration of the study, analytical techniques, and biological responses measured. Other factors considered include the demonstration of a dose-response relationship and potential differences in response among different species. If EPA determines that a subchronic study meets high standards of quality, the study will receive a high confidence rating. EPA gives a low confidence rating to the database for a chemical lacking supporting studies in other species and lacking studies on reproductive or developmental toxicity. The confidence in RfD is a composite of the confidence in the principal study and in the database. In assigning confidence to RfD, EPA gives precedence to confidence in the database. If the principal study has a medium confidence rating and the database a low confidence rating, EPA assigns low confidence to RfD. The confidence level is a part of dose-response characterization discussed in Section 3.2.1.4. 3.2.1.2.7 Mathematical modeling and the benchmark dose method. Mathematical models may be applied to data on dose-response to reduce the uncertainty in identifying a reliable (statistically valid) NOAEL for chemicals inducing deterministic effects or, alternatively, an ED10 (the benchmark dose), which is the dose at which 10 percent of the study animals are expected to show a response (Krewski et al., 1989; Moolgavkar et al., 1999). Examples of relevant curve-fitting models include the probit and Weibull (Moolgavkar et al., 1999; Park and Snee, 1983; Paustenbach, 1995). These models take into account the uncertainty in dose-response data obtained

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from animal studies and use ‘‘best-fit’’ procedures to construct a dose-response curve. Sometimes these models are combined with physiologically-based pharmacokinetic (PB-PK) models (see Section 3.2.1.3.4) to predict the response across the doses tested (Reitz et al., 1996). Since about 1995, EPA and other agencies have begun to use the so-called benchmark dose method to estimate NOAEL and ED10 (Barnes et al., 1995; Crump, 1984; 1995; EPA, 1995a). As illustrated in Figure 3.6, a statistical fit of a dose-response model to the doseresponse data is used to identify an ED10, which is the central estimate of the dose that results in a response in 10 percent of the study animals. The lower 95 percent confidence limit of the benchmark dose (LED10) then is used as the point of departure for establishing allowable exposures to chemicals that cause deterministic effects, in a manner similar to the approach of determining RfDs from NOAELs by using safety and uncertainty factors. For example, a

Fig. 3.6. Illustration of use of benchmark dose method to estimate nominal thresholds for deterministic effects in humans. The benchmark dose (ED10) and LED10 are central estimate and lower confidence limit of dose corresponding to 10 percent increase in response, respectively, obtained from statistical fit of dose-response model to dose-response data. The nominal threshold in humans could be set at a factor of 10 or 100 below LED10, depending on whether the data are obtained in humans or animals (see text for description of projected linear dose below point of departure).

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nominal threshold for deterministic responses in humans could be estimated as a dose that is a factor of 10 or 100 less than the lower confidence limit of the benchmark dose, depending on whether the data are obtained in humans or animals, and allowable doses could be established by applying additional safety and uncertainty factors to the nominal threshold. The rationale supporting use of ED10 as the benchmark dose is that a 10 percent response is at or just below the limit of sensitivity in most animal studies. Use of the lower confidence limit of the benchmark dose, rather than the best (maximum likelihood) estimate (ED10), as the point of departure accounts for experimental uncertainty; the difference between the lower confidence limit and the best estimate does not provide information on the variability of responses in humans. In risk assessments for substances that induce deterministic effects, a dose at which significant effects are not observed is not necessarily a dose that results in no effects in any animals, due to the limited sample size. NOAEL obtained using most study protocols is about the same as an LED10. The benchmark dose method was developed to overcome difficulties with determining NOAEL based on dose-response data. The potential advantages of the method include the following (Crump, 1984; 1995): ● The benchmark dose method makes use of all the dose-response data by fitting a dose-response model to the data, whereas the determination of a NOAEL generally involves a comparison of responses at discrete and well separated doses with responses in control subjects. ● The benchmark dose reflects sample size more appropriately than a NOAEL because small studies tend to result in smaller benchmark doses, whereas the opposite is the case for NOAELs. ● A NOAEL is constrained to be one of the administered doses, but this is not the case with the benchmark dose method. ● A benchmark dose can be defined from a data set that does not include a NOAEL. ● The determination of a NOAEL generally involves dose data that are categorized into distinct groups, but this categorization is arbitrary in some studies. Grouping of data into distinct dose categories is not required in the benchmark dose method. The benchmark dose method can also be applied to chemicals that cause stochastic effects (Section 3.2.1.3.3). This is indicated by the projected linear response at doses below LED10 in Figure 3.6. 3.2.1.3 Dose-Response Assessment for Chemicals That Cause Stochastic Effects. For hazardous chemicals that do not have a threshold in the dose-response relationship, which is currently believed to

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be true for genotoxic carcinogens, the purpose of a dose-response assessment is to extrapolate the data obtained at high doses to the low-dose region of concern in routine exposure situations. This extrapolation is usually accomplished with mathematical models that are thought to be conservative (i.e., likely to overestimate the true responses). Although dose-response assessments for deterministic and stochastic effects are discussed separately in this Report, it should be appreciated that many of the concepts discussed in Section 3.2.1.2 for substances that cause deterministic effects apply to substances that cause stochastic effects as well. The processes of hazard identification, including identification of the critical response, and development of data on dose-response based on studies in humans or animals are common to both types of substances. Based on the dose-response data, a NOAEL or a LOAEL can be established based on the limited ability of any study to detect statistically significant increases in responses in exposed populations compared with controls, even though the dose-response relationship is assumed not to have a threshold. Because of the assumed form of the dose-response relationship, however, NOAEL or LOAEL is not normally used as a point of departure to establish ‘‘safe’’ levels of exposure to substances causing stochastic effects. This is in contrast to the common practice for substances causing deterministic effects of establishing ‘‘safe’’ levels of exposure, such as RfDs, based on NOAEL or LOAEL (or the benchmark dose) and the use of safety and uncertainty factors. 3.2.1.3.1 Introduction to mathematical modeling for chemicals that cause stochastic effects. Given the assumption of a nonthreshold dose-response relationship for chemicals causing stochastic effects (genotoxic), mathematical modeling is essential in estimating the response at doses below levels where dose-response data are available, or in estimating the dose that would be ‘‘safe,’’ i.e., a dose corresponding to an ‘‘acceptable’’ level of health risk (probability of a stochastic response) established by regulators. The following sections discuss dose-response modeling for chemicals that cause stochastic effects. Except for biologically-based models of cancer induction, the various models discussed also can be applied in doseresponse assessment for substances that cause deterministic effects, because the models do not depend on the particular form of the true dose-response relationship. Because of the statistical and biological problems inherent in the identification of a true no-effect level in any study of dose-response, most mathematical models for chemicals that cause stochastic effects have eliminated the concept of a threshold dose below which no

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response would be expected. Cornfield (1977) and Schaffer (1981), for example, have discussed kinetic models for cancer induction that lead to the existence of thresholds under steady-state conditions, but these have not been used in regulatory decision making in the United States. Other investigators have argued that numerous biological protective mechanisms surely exist for all mechanisms by which cancers can be initiated (Bus and Gibson, 1995), and that at some low dose a practical threshold for even the most potent carcinogen must exist (Gold et al., 1998; Stokinger, 1977). The basis for assuming the absence of a threshold is that one cannot dismiss the possibility of a response being induced, for example, if a reactive metabolite were produced and it then interacted with DNA (Brown, 1978); if this hypothesis is accepted, there is some probability of a response no matter how small the dose. 3.2.1.3.2 Statistical models. A number of statistical doseresponse extrapolation models have been discussed in the literature (Krewski et al., 1989; Moolgavkar et al., 1999). Most of these models are based on the notion that each individual has his or her own tolerance (absorbed dose that produces no response in an individual), while any dose that exceeds the tolerance will result in a positive response. These tolerances are presumed to vary among individuals in the population, and the assumed absence of a threshold in the dose-response relationship is represented by allowing the minimum tolerance to be zero. Specification of a functional form of the distribution of tolerances in a population determines the shape of the doseresponse relationship and, thus, defines a particular statistical model. Several mathematical models have been developed to estimate low-dose responses from data observed at high doses (e.g., Weibull, multi-stage, one-hit). The accuracy of the response estimated by extrapolation at the dose of interest is a function of how accurately the mathematical model describes the true, but unmeasurable, relationship between dose and response at low doses. For the most frequently used low-dose models, the ‘‘multi-stage’’ and ‘‘one-hit,’’ there is an inherent mathematical uncertainty in the extrapolation from high to low doses that arises from the limited number of data points and the limited number of animals tested at each dose (Crump et al., 1976). The statistical term ‘‘confidence limits’’ is used to describe the degree of confidence that the estimated response from a particular dose is not likely to differ by more than a specified amount from the response that would be predicted by the model if much more data were available. EPA and other agencies generally use the 95 percent upper confidence limit (UCL) of the dose-response data to estimate stochastic responses at low doses.

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By using UCL and assuming that the model accurately reflects the dose-response relationship at low doses, there is only a five percent chance that the true response is higher than the response predicted by the model. UCL takes into account measurement uncertainty in the study used to estimate the dose-response relationship, such as the statistical uncertainty in the number of tumors at each administered dose, but it does not take into account other uncertainties, such as the relevance of animal data to humans. It is important to emphasize that UCL gives an indication of how well the model fits the data at the high doses where data are available, but it does not indicate how well the model reflects the true response at low doses. The reason for this is that the bounding procedure used is highly conservative. Use of UCL has become a routine practice in dose-response assessments for chemicals that cause stochastic effects even though a best estimate (MLE) also is available (Crump, 1996; Crump et al., 1976). Occasionally, EPA will use MLE of the dose-response relationship obtained from the model if human epidemiologic data, rather than animal data, are used to estimate risks at low doses. MLEs have been used nearly universally in estimating stochastic responses due to radiation exposure. Although rarely presented in a dose-response assessment, in nearly all cases the lower bound on the incremental probability of a response will be zero or less (see Figure 3.7). That is, the statistical model that accounts for the uncertainty in the results of an animal study also accommodates the possibility that no response may occur at low doses and that, in fact, there may be fewer responses (e.g., cancers) than observed in the control population at some low doses. The possibility of reduced responses at low doses also is shown by the lower confidence limit of data on radiation-induced cancers in some organs of humans including, for example, the pancreas, prostate, and kidney (Thompson et al., 1994). The question of whether MLE or UCL of the dose-response relationship should be used to estimate health risks to the public for the purpose of regulatory decision making has been debated by many investigators (Bailar et al., 1988; Crump et al., 1976; Finkel, 1994; Sielken, 1985; Sielken et al., 1994). Although good reasons for adopting one approach or the other have been advanced, chemical risk assessments generally use UCLs and radiation risk assessments generally use MLEs. The difference between the two estimates usually is not trivial for chemicals that cause stochastic effects, and it can be of great regulatory significance. For example, for a number of chemicals that have been examined, UCL will rarely predict responses less than five times greater than those predicted by MLE,

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Fig. 3.7. Typical relationship of best estimate and statistical bounds of incremental probability of an adverse response from exposure to a hazardous chemical (Sielken, 1987).

and the predicted responses based on UCL can often be as much as 200 times greater than estimates based on MLE (Paustenbach, 1995). Finkel (1989; 1994) and others have maintained that this amount of possible conservatism or prudence is justified in light of the possible severity of the hazards, our lack of knowledge about mechanisms of action of most carcinogens, and the largely unknown effects of exposure to mixtures of hazardous substances. 3.2.1.3.3 Benchmark dose method. In recent years, confidence in the ability of statistical curve-fitting models to accurately predict cancer incidence in humans based exclusively on data obtained from animal studies has lessened (Crump, 1996). The reasons are manifold and include that (1) the pharmacokinetic behavior of chemicals in humans is often different than in animals, (2) the study animals

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do not live as long as humans, (3) MTDs in animals have little relationship to the doses that humans normally would experience, (4) biological repair mechanisms in humans are different than in animals, and (5) the study animals are in-bred with a predisposition to developing tumors. As a result of this lack of confidence, recent cancer guidelines (EPA, 1996a) include a recommendation that it would be appropriate to use the benchmark dose method discussed in Section 3.2.1.2.7, rather than traditional low-dose extrapolation models, when attempting to predict cancer risks at low doses. The benchmark dose method acknowledges that traditional models are only an approximation of the plausible response at low doses. In this method, LED10 (the lower 95 percent confidence limit of a dose associated with an increase in response of 10 percent) is considered to be within the range of observation, without any significant extrapolation (see Figure 3.6). The dose-response relationship for chemicals that cause stochastic effects often is assumed to be essentially linear at doses below LED10. The benchmark dose method is particularly useful when the mode of action of a chemical that causes stochastic effects is thought to be nonlinear. In these circumstances, the response is assumed to decrease more rapidly than linearly with decreasing dose. Alternatively, the mode of action may theoretically have a threshold; for example, the carcinogenicity of a substance may be a secondary effect of its toxicity or of an induced physiological change that is itself a threshold phenomenon. In the benchmark dose method, observed responses are not extrapolated to give an estimate of the probability of a response at low doses. Instead, a margin of exposure (MOE) analysis is used to evaluate the degree of concern about different levels of exposure (CRARM, 1997; EPA, 1996a). MOE usually is defined as LED10 divided by the level of exposure. For example, if an individual routinely ingests 10 ␮g (kg d)ⳮ1 and LED10 is 1,000 ␮g (kg d)ⳮ1, MOE is 100. Larger margins of exposure indicate exposures of lesser concern, and vice versa. The benchmark dose method and MOE analyses are essentially the same for substances that cause stochastic or deterministic effects. For both types of substances, the point of departure in the doseresponse curves for purposes of protecting human health is a dose at which some response is expected, either LED10 or some other human equivalent dose or concentration as the data support. For stochastic responses (e.g., cancers), the point of departure when animal data are used is a human equivalent dose or concentration

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obtained based on interspecies dose adjustment or toxicokinetic analysis. 3.2.1.3.4 Pharmacokinetic models. An important advance in risk assessment for hazardous chemicals has been the application of pharmacokinetic models to interpret dose-response data in rodents and humans (EPA, 1996a; Leung and Paustenbach, 1995; NAS/NRC, 1989; Ramsey and Andersen, 1984). Pharmacokinetic models can be divided into two categories: compartmental or physiological. A compartmental model attempts to fit data on the concentration of a parent chemical or its metabolite in blood over time to a nonlinear exponential model that is a function of the administered dose of the parent. The model can be rationalized to correspond to different ‘‘compartments’’ within the body (Gibaldi and Perrier, 1982). PB-PK models, sometimes referred to as biologically-based disposition models, allow for accurate extrapolation of rodent data to estimate human dose-response relationships (Paustenbach, 1995). PB-PK models, unlike compartmental models, have the capability of simulating a chemical’s behavior in biological systems. The purpose of a PB-PK model is to predict the human dose-response relationship based on animal data by quantitatively estimating the delivered dose of the biologically relevant chemical species in a target tissue (Andersen et al., 1987; Clewell et al., 1994; Leung and Paustenbach, 1995; Ramsey and Andersen, 1984). Models based on the PB-PK approach differ from compartmental models in that they incorporate actual physiology and the biochemical behavior of the hazardous chemical in the test animal. Instead of compartments defined by the experimental data, actual organ and tissue groups are modeled. After a conceptual model of the chemical’s behavior is developed, mass-balance differential equations are written to describe the behavior in each biologically relevant compartment. These models can be adapted to accommodate such biological processes as nonlinear tissue binding, Michaelis-Menten elimination, parallel organ-specific elimination, enzyme induction and inhibition, biliary recycling, diffusional resistance across cell membranes, and the number and affinity of receptors (Andersen and Krishnan, 1999; Andersen et al., 1987). Biokinetic models similar to PB-PK models for hazardous chemicals are widely used in estimating radiation doses to different organs or tissues following intakes of radionuclides into the body (see Section 3.2.2), although the models for radionuclides do not include biochemical processes that may be important for hazardous chemicals other than heavy metals. Physico-chemical processes and blood flows are modeled in the PB-PK approach (Ramsey and Anderson, 1984). Use of physicochemical and biochemical rate constants of the subject hazardous

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chemical allows the creation of a model that has greater predictive power than traditional compartmental models. Because actual metabolic parameters are used, dose extrapolations can be carried out over ranges in which saturation of metabolism occurs. Most importantly, because known physiological parameters are used, a different substance can be modeled by replacing the appropriate rate constants. Different routes of administration can be accounted for by adding equations describing the nature of the new route. PB-PK models have been developed for at least 60 industrial chemicals, including kepone, methylene chloride, styrene, perchloroethylene, polychlorinated dibenzofurans, carbon tetrachloride, dioxane, methyl chloroform, methylene chloride, ethylene dichloride and dioxin (Leung and Paustenbach, 1995). For illustrative purposes, the conceptual model for carbon tetrachloride is shown in Figure 3.8 (Paustenbach et al., 1988). The power of the PB-PK approach was demonstrated by Andersen et al. (1987) in an analysis of stochastic responses to low doses of methylene chloride and by Reitz et al. (1996) with vinyl chloride. Based on consideration of biological factors that occur only at high doses, these studies showed that in order to predict the low-dose response in humans, the model must correct for a number of metabolic, physiological, and pharmacokinetic factors. One of the key factors that PB-PK approaches can take into account is saturable metabolic pathways, which is the reason that many chemicals are carcinogenic in animal studies but would pose much less of a hazard to humans at low doses. After applying the appropriate corrections and developing a model that could accurately predict the chemical’s behavior in rodents and humans, these investigators determined that the highest plausible dose associated with a frequency of response of 1 in 1,000,000 was overestimated by more than a factor of 100 when a non-pharmacokinetic approach was used. In addition to describing complex phenomena needed to quantify and understand the dose-response relationship, PB-PK models can help provide physiological explanations for differences in response between animals and humans. For example, Leung et al. (1990) developed a PB-PK model for dioxin wherein they showed how differences in the number and affinity of receptors in the liver could explain observed differences in the dose-response relationship in two species of mice. Although the investigators did not discuss how this insight might affect the low-dose extrapolation, they noted that a PB-PK model for dioxin should be useful in assessing dose-response relationships in humans. A validated model that accounts for differences in dioxin concentrations in the liver among species might explain why rodents appear to be more sensitive to dioxin than humans, since

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Fig. 3.8. Schematic of a combination PB-PK and compartmental model for inhalation of radiocarbon tetrachloride (14CCl 4) (Paustenbach et al., 1988).

rodents have liver concentrations 10 to 100 times higher than humans exposed to similar doses (Leung et al., 1990). 3.2.1.3.5 Biologically-based models of cancer. Although the development and use of PB-PK models represents a significant improvement in achieving an understanding of the likely human response to hazardous chemical exposure, owing to their improved extrapolation capabilities, perhaps an even more promising contribution to stochastic dose-response assessment for hazardous chemicals is the development of biologically-based models of cancer. The difference between PB-PK models and more complex biologically-based models is that the latter attempt to address factors that are not easily measured but influence the tumorigenic progress. These factors

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include, for example, cell proliferation, cell death, immunization rates, and the progress of tumors to malignancy. The first of these models was developed by Moolgavkar and Knudson (1981); see also Moolgavkar et al. (1988; 1999). Despite the simplicity of the model compared with the diversity of the disease processes categorized under the heading of cancer, the model offers significant potential. It is a form of the two-stage model of cancer induction initially described by Armitage and Doll (1954). In this model, cancer is explained as the end result of two mutagenic events that correspond to mutations at a critical gene locus. In humans, these mutations are duplicated within the genetic material of the cell. As discussed by Andersen et al. (1987), the first event depicted in Figure 3.9, Step I, produces an intermediate cell type that may have different growth characteristics than the normal cell but is not aggressively malignant. A second irreversible event (Figure 3.9, Step II) is necessary to complete the cell transformation process, alter the second locus of the critical gene, and produce the cancer cell that is aggressively malignant and grows into a tumor by clonal expansion (Figure 3.9, Step III). This model can be used to explain how genotoxicants alter mutation rates, how cytotoxicants alter cell death and birth rates of the normal and intermediate cells, and how promoters convey growth advantages on the intermediate cell populations. Unfortunately, because most of the factors in these models cannot be measured at the present time, it is unlikely that the models will be very helpful in the foreseeable future. 3.2.1.3.6 Use of stochastic modeling results. EPA and other regulatory agencies review each dose-response assessment for substances causing stochastic effects with respect to the evidence on causative mechanisms and other biological or statistical evidence that indicates the suitability of a particular mathematical extrapolation. In the absence of adequate information to the contrary, the linearized multi-stage model is employed (Crump et al., 1976; EPA, 1996a). This model assumes that at each stage in the development of a cancer (see Figure 3.9), the response induced by exposure to an agent is additive to the response induced by all other external stimuli at that stage. The result is a linear dose-response model at low doses. Where appropriate, the results of using other extrapolation models may be useful for comparison with the linearized multi-stage procedure. When longitudinal (time-series) data on tumor development are available, time-to-tumor models may be used. The linearized multi-stage procedure leads to a plausible upper limit to the dose-response relationship at doses below the range of observation that is consistent with some proposed mechanisms

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Fig. 3.9. Biologically-based model of the cancer induction process used to estimate the dose-response relationship of chemicals causing stochastic effects (Andersen et al., 1987).

of carcinogenesis (Allen et al., 1988). However, this model does not necessarily give a realistic prediction of responses at low doses. The true relationship is unknown, and the response may be as low as zero. The range of responses defined by the upper and lower confidence limits given by the chosen model, which may include zero, should be explicitly stated in a dose-response assessment. A procedure for obtaining a ‘‘best estimate’’ of the response at a particular

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dose within the range of uncertainty defined by the upper and lower confidence limits has not yet been established, although the model can provide a value of MLE. Best estimates of true responses will be most feasible when human data are available and the data encompass expected exposures. When pharmacokinetic or metabolic data are available, or when other substantial evidence on the mechanistic aspects of the causative process exists, a low-dose extrapolation model other than the linearized multi-stage procedure might be considered more appropriate on biological grounds. When a different model is used, the dose-response assessment should discuss the nature and weight of evidence that led to the choice. Since considerable uncertainty will remain concerning the response at low doses, a UCL of the doseresponse relationship obtained using the linearized multi-stage procedure also should be presented in most cases when another model is used. Estimates of responses at low doses derived from data on laboratory animals and extrapolated to humans are complicated by a variety of factors that differ among species and potentially affect the response to hazardous chemicals. These factors include differences between humans and experimental test animals with respect to life span, body size, genetic variability, population homogeneity, existence of concurrent disease, such pharmacokinetic effects as metabolism and excretion patterns, and the dosing regimen. These factors are discussed further in Section 3.2.1.5. The usual approach to making interspecies comparisons has been to use standardized scaling factors. Commonly employed standardized dosage scales include mg kgⳮ1 body weight per day, ppm (parts per million) in the diet or water, mg mⳮ2 body surface area per day, and mg kgⳮ1 body weight per lifetime (Travis et al., 1990). In the absence of comparative toxicological, physiological, metabolic, and pharmacokinetic data for a given suspect substance that causes a stochastic effect, EPA (1996a) takes the position that an extrapolation on the basis of body weight to the three-fourths power is most appropriate. 3.2.1.4 Characterization of Dose-Response Estimates. Characterization of the estimated dose-response relationship involves a presentation of (1) the dose-response relationship per se and (2) a framework to help judge the significance of the relationship. Dose-response characterization includes an evaluation of exposure mechanisms during data acquisition, as well as how the dose-response relationship was established. If the dose-response relationship is linear at

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low doses, it may be presented as a unit-dose estimate (e.g., probability of a response per unit dose), which can be combined with a predicted exposure to hazardous chemicals for the purpose of assessing risk. Numerical estimates of the dose-response relationship at low doses can be presented in one or more of the following ways. ●

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Response to a unit dose. Under an assumption of low-dose linearity, the response per unit dose is independent of dose, and the response to a unit dose is often given. Typical units of dose include ppm or ppb (parts per billion) in food or water, mg (kg d)ⳮ1 by ingestion, or ppm or ␮g mⳮ3 in air. Dose corresponding to a given level of response. Presenting the dose corresponding to a given response can be useful, particularly when using nonlinear extrapolation models in which the response per unit dose depends on the dose. Individual and population responses. The dose-response relationship may be characterized either in terms of the lifetime probability that an individual exposed to a given dose will develop a cancer as a result of that dose, the excess number of responses per year in an exposed population, or both.

Whichever method of presentation is chosen, it is critical that the numerical estimates not be separated from the various assumptions upon which they are based and their uncertainties. The doseresponse characterization should contain a discussion and interpretation of the numerical estimates so that the risk manager gains significant insight into the extent to which the quantitative estimate reflects the true magnitude of potential human responses. The risk manager needs to understand that the true human health risk cannot be known with the degree of accuracy reflected in the numerical estimates. 3.2.1.5 Uncertainties and Deficiencies in Dose-Response Assessment. Any approach to determining the dose-response relationship for hazardous chemicals involves many attendant uncertainties that limit its accuracy. In addition, many dose-response assessments suffer from deficiencies in the way they are conducted, which further decreases accuracy. These two aspects of dose-response assessment, which in some ways have led to adoption of such conservative approaches as large safety factors and UCLs in applying the results to health protection of the public, are discussed in the following two sections.

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3.2.1.5.1 Uncertainties in dose-response assessment. Several aspects of dose-response assessment result in significant uncertainties in the accuracy of the resulting relationship. ●

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Dose-response data based largely on animal studies. It is noteworthy that rodent studies now used to predict the dose-response relationship in humans were never intended for that purpose (Barr, 1988). These studies were designed for purposes of hazard identification (see Section 3.1.4.1.2) and were not intended to be the basis for estimating human responses at low doses (Paustenbach, 1995). For example, there usually are significant differences between animals and humans with respect to the rate at which chemicals are metabolized, distributed, and excreted, and these are not taken into account when animal tests are designed. Also, animal tissues will frequently respond differently to toxicants than human tissue. Extrapolation to low doses. The doses used in animal tests are so high that they often produce responses that would not occur at doses to which humans might be exposed. Thus, a model or theory must be used to estimate responses in humans at doses that often are a factor of 100 to 1,000 below the lowest animal dose tested or the doses to which humans have been occupationally exposed (Krewski et al., 1989; Munro and Krewski, 1981). Extrapolation to low doses is probably the most uncertain aspect of assessing the dose-response relationship for chemicals, especially substances that produce stochastic effects. The response of humans exposed to many such substances at low doses may well be negligible because of the presence of protective biological mechanisms (Ames, 1987; Ames and Gold, 1995; Bus and Gibson, 1995), although this is often difficult or impossible to demonstrate unequivocally. Low-dose extrapolation models are the backbone of doseresponse assessments. Because they can play such a dominant role in the regulatory process, it is important to understand some of their characteristics. As shown in Figure 3.10, different extrapolation models usually fit the data in the observable dose region in animal tests about equally well (Krewski et al., 1989), but they often give quite different results in the unobserved lowdose region of interest in assessments of risk to human health. The results obtained by extrapolation of the most commonly used low-dose models usually vary in a predictable manner, because the models use different mathematical equations to describe the chemical’s likely behavior in the low-dose region.

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Fig. 3.10. Illustration of close correlation between observable doseresponse data and results of different statistical models but very different model extrapolations into the unobservable response range (Paustenbach, 1995).

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The one-hit and linearized multi-stage models usually will predict the highest response rates and the probit model the lowest (Paustenbach, 1989a; 1989b). Absence of mechanistic understanding. Substantial understanding of the mechanisms by which most chemicals cause responses is not yet available. To date, virtually none of the models used to identify safe or acceptable doses of contaminants in air, water, or soil have been based on assessments that attempt to account for biological phenomena quantitatively (Andersen et al., 1987; Paustenbach, 1995). As a consequence, conservative approaches are usually employed, and this introduces an unknown uncertainty in the estimation of the dose-response relationship.

3.2.1.5.2 Deficiencies in dose-response assessment. In addition to the sources of uncertainty in dose-response assessment described above, there are several important deficiencies in the way that the

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dose-response relationship for hazardous chemicals has been evaluated in most assessments (Paustenbach, 1989a). ●

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Presenting only the upper bound of the dose-response relationship. Bounding techniques are used in dose-response assessments in an attempt to account for the statistical uncertainty in the results of animal tests. However, the degree of potential conservatism incorporated in the bounding procedure and the fact that a negligible response can be nearly as likely as the estimated upper bound usually are not presented. As a result, risk managers often are not fully aware of the range of equally plausible response estimates (Sielken, 1985). For example, the cancer probability associated with exposure to chloroform in chlorinated drinking water has been reported to be as high as 1 in 10,000, based on an upper-bound estimate obtained from the multi-stage model (Reitz et al., 1988). However, using the same model, MLE is about 1 in 1,000,000 and the lower-bound estimate is virtually zero (about 1 in 10,000,000). Therefore, the range of plausible responses obtained from the model is between 1 in 10,000 and zero. When biological factors are considered, such as the pharmacokinetics and weak genotoxicity of chloroform, the stochastic response associated with low levels of chloroform in drinking water is most likely to be quite small or negligible (Corley et al., 1990). Based on such considerations, the establishment of drinking water standards for chloroform has been particularly controversial (EPA, 2000a). Reliance on results of only one mathematical model. Several different modeling approaches may need to be considered when estimating responses at low doses (see Section 3.2.1.3.6). Each model can yield results that are plausible, depending on the mechanism of action and pharmacokinetics of the chemical, as well as the characteristics of the dose-response relationship (Krewski et al., 1984; 1989; Sielken, 1985). As a result of an improved understanding of the genesis of responses and the shortcomings of statistical models, regulatory agencies have recently become more willing to consider models that can account for chemical-specific phenomena quantitatively (Andersen et al., 1987; Corley et al., 1990; Paustenbach, 1989b). However, support for flexibility in dose-response assessment has been criticized on the grounds that too little is known about response initiation to regulate in other than a very conservative manner (Perera, 1984; Perera and Boffetta, 1988; Silbergeld, 1988; 1993). Although some investigators have claimed that models lacking low-dose linearity are not appropriate for substances that cause

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stochastic effects, the scientific support for this assertion is not compelling, especially for nongenotoxic chemicals (Reitz et al., 1988). Except perhaps for chemicals that are known to be initiators or mutagens, no single statistical model can be expected to predict the low-dose response with greater certainty than another. One possible approach to resolving this problem is to present MLE of the dose-response relationship obtained from the two or three models that are considered most plausible, as well as the upper and lower confidence limits. Based on this information and considering statistical and biological factors, an appropriate dose-response relationship could be selected based on the ‘‘preponderance of evidence.’’ EPA attempted this approach for dioxin, but it failed to gain broad support (Paustenbach et al., 1992). Not giving sufficient weight to results of epidemiologic studies. There is a widely held belief that epidemiologic studies are almost never as statistically robust as animal studies and, therefore, are not very useful (Silbergeld, 1988). This assertion is too strong because epidemiologic studies can, at the very least, establish the degree of confidence that should be placed in the results of low-dose extrapolation models (Layard and Silvers, 1989). A difficulty with epidemiologic studies at low doses is the inability to adequately control for potentially confounding factors to the extent necessary to exclude spurious observations, either positive or negative. Epidemiologic studies are not capable of detecting increased responses unless the excess relative risk is on the order of 30 to 40 percent or higher. At the present time, many regulatory agencies and some scientists believe that not enough is known about responses at low doses of hazardous substances to consider using epidemiologic data, which typically give less conservative results than extrapolation models (Perera and Boffetta, 1988). For example, questions have been raised about the lack of understanding of the cancer process, the possibility that several mechanisms might occur simultaneously, and whether the risk from any incremental exposure to a carcinogen is additive to the much larger risk of cancer from all other causes. For some chemicals, investigators have noted that even the multi-stage model could underpredict actual responses at moderate doses (Bailar et al., 1988). Although these are legitimate concerns, the available data (Ikeda, 1988; NAS/NRC, 1988a) seem to indicate that the inherent conservatism in methods of dose-response assessment usually yields predictions of responses in the low-dose region that

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are greater than those indicated by epidemiologic data. The primary reasons for this are thought to be the use of the most sensitive animal species to estimate responses, the use of a surface-area scaling factor, and the inability of models to account for the many protective biological mechanisms that operate at low doses. Failure to quantitatively scale data from rodents in predicting responses in humans. When evaluating most toxicological effects, statisticians and biologists have generally assumed that at a given dose of a chemical, the response in humans will be nearly identical to the response observed in rodents. This assumption is usually accurate for deterministic responses. For substances that cause stochastic effects, however, several factors need to be considered when trying to predict how humans will respond compared with rodents. For example, the biological halflife of the chemical in rodents can be expected to be different from that in humans for virtually all chemicals. Often, for a given chemical, this difference will vary in a predictable manner based simply on the ratio of body weight to surface area and/or life span (D’Souza and Boxenbaum, 1988). This scaling may also be valid for those chemicals causing stochastic effects that require activation. Consequently, for regulatory purposes, surface-area corrections have been used in an attempt to adjust for pharmacokinetic differences between rodents and humans. However, there is ample work suggesting that body weight alone is probably a more valid scaling factor if no compelling information to the contrary is available (Allen et al., 1988), and EPA now uses body weight to the three-fourths power as a default scaling factor (EPA, 1996a). As an alternative to relying on simple scaling factors, PB-PK models described in Section 3.2.1.3.4 can be used to more accurately predict responses in humans based on rodent data (Corley et al., 1990; Reitz et al., 1988; 1996). The benefits of this approach have been so impressive that a special symposium was held by the National Academy of Sciences to encourage its use (Krewski et al., 1987). Failure to adjust dose-response estimates by considering biological information. In many dose-response assessments, potentially important biological information is not taken into account in selecting an extrapolation model. Examples of information often not included when a model is selected are the types of tumors, time to onset, and whether the chemical is genotoxic. Some

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chemicals for which this was a critical issue in low-dose extrapolation include ethylene oxide, formaldehyde, dioxin, the goitrogens, trimethylpentane, and nitrilotriacetic acid. Use of models that do not respond to dose-response relationship. As discussed by Sielken (1985), it does not seem appropriate to base important regulatory decisions on the results of models that are minimally responsive to the very costly information collected in standard lifetime rodent studies. Two terms are frequently used to describe the responsiveness of an extrapolation model to the data: ‘‘fragile’’ and ‘‘insensitive.’’ A fragile model varies too strongly with the data, whereas an insensitive model varies little irrespective of the response in rodents. For example, if the predicted probability of a response at low doses varies dramatically depending on whether a single animal in the study develops a tumor, then the model is too responsive. On the other hand, if the probability of a response does not change much irrespective of the observed tumor incidence, then the model is nonresponsive or insensitive. Scientists should not be constrained by the insensitivities of the statistical bounding methodology or the responsiveness of MLE to the data. Instead, decisions should be influenced by biological factors and scientific judgment. Clearly, toxicologists and risk managers need to be aware of the potential for a mathematical model to inadvertently over- or underestimate the significance of the data because, at times, such a tendency may have a dramatic effect on the regulatory decision. Excessive regulatory constraints. Estimation of the doseresponse relationship is also affected by regulatory regimes. Whenever adherence to strict regulatory guidance is required, the potential exists for the dose-response assessment to be so constrained that it cannot account for information excluded by the regulations that would dramatically alter the results. Some of the more commonly encountered problems in dose-response assessment caused by rigid regulatory policy have been discussed elsewhere (Paustenbach, 1989a; 1995).

3.2.2 Assessment of Responses from Radiation Exposure Estimation of the probability of a response from exposure to radionuclides (or any other source of ionizing radiation) is greatly facilitated by the knowledge that radiation dose is the common measure of insult to any organ or tissue for any exposure situation (e.g., see NCRP, 1993a; 1993b). All radiation dose or risk assessments are

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based on estimates of absorbed dose, given in grays (Gy), which is a physical quantity defined essentially as the energy imparted to matter by ionizing radiation divided by the mass irradiated. For purposes of radiation protection and assessing risk in general terms, the biologically significant dose is assumed to be the equivalent dose, given in sieverts (Sv), which is defined as the average absorbed dose in an organ or tissue (i.e., the total energy absorbed in an organ or tissue divided by its mass) modified by a radiation weighting factor that accounts for differences in biological effectiveness of different types of radiation. For example, a given absorbed dose of alpha particles is assumed to result in a greater biological response than the same absorbed dose of photons or electrons. The radiation weighting factor depends essentially on the density of ionization in matter, which often is given in terms of the linear energy transfer (LET) (ICRP, 1991). The utility of absorbed dose and equivalent dose in radiation risk assessments is the following. Once the response per unit absorbed dose of low-LET radiation (e.g., photons) in a particular organ or tissue is known, this relationship can be used to estimate the response per unit equivalent dose of any type of radiation. Given knowledge of the response per unit absorbed dose in the different organs or tissues of the body, the response from any exposure then can be estimated based on estimates of equivalent dose in all irradiated organs or tissues. The equivalent dose in different organs or tissues from any exposure can be estimated, as described below, using knowledge of the energies and intensities of the ionizing radiations of different types emitted in the decay of any radionuclide. Thus, in contrast to the situation for hazardous chemicals, separate studies to determine the response from exposure to each radionuclide of concern are not needed. Radiation dose may be delivered to organs or tissues when radionuclides located outside the body emit penetrating radiations, such as photons or higher-energy electrons, or when radionuclides are taken into the body by inhalation, ingestion, or (rarely) absorption through the skin. These two means of irradiation often are referred to as external and internal exposure. Doses to specific organs or tissues from external exposure to radionuclides in the environment are estimated using complex models of radiation transport (e.g., see Eckerman and Ryman, 1993). Doses from internal exposure are estimated using two types of models: (1) models for absorption, deposition, and retention of radionuclides taken into the body, which depend on the chemical and physical form of the radionuclide; and (2) dosimetry models involving complex calculations of radiation transport that give estimates of the dose delivered to each target

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organ or tissue per disintegration of a radionuclide in each of its sites of deposition in the body (e.g., lung, bone surfaces, thyroid) or during transit (e.g., in the gastrointestinal tract or in blood) [see ICRP (1996) and references therein]. The biokinetic models currently used to describe absorption, deposition, and retention of any radionuclide in the body are similar to the compartmental or PB-PK models developed for some hazardous chemicals (see Section 3.2.1.3.4), and they are based on studies in humans and animals. Given the models for estimating external or internal radiation doses in specific organs or tissues, the following sections consider the responses resulting from a given dose by any route of exposure. As is the case with hazardous chemicals, both stochastic and deterministic radiation effects can occur. 3.2.2.1 Deterministic Responses from Radiation Exposure. Based mainly on data in humans, a threshold dose-response relationship generally is assumed for radiation-induced deterministic responses. For purposes of radiation protection, deterministic responses generally are assumed not to occur if the annual equivalent dose is less than 150 mSv to the lens of the eye or 500 mSv to any other organs or tissues, including the skin and extremities (ICRP, 1977; 1991; NCRP, 1987a; 1993a). Dose limits for the public intended to ensure prevention of deterministic responses are set at one-tenth of the assumed thresholds, in order to provide an adequate margin of safety for nearly all individuals. However, deterministic responses are not expected to be of concern in routine exposures of the public, because the limit on annual effective dose of 1 mSv from exposure to all controlled sources combined, which is intended to limit the increase in stochastic responses, should ensure that deterministic responses would not occur, even when the dose from natural background radiation is included (ICRP, 1991; NCRP, 1993a). 3.2.2.2 Databases and Methods of Dose-Response Assessment for Stochastic Effects. At radiation doses below levels where cell killing occurs, a linear-quadratic relationship between dose and stochastic responses, including cancers and severe hereditary effects, generally is assumed, based on analyses of data in humans (NAS/NRC, 1988b; 1990). The importance of the quadratic term differs for low-LET radiations (photons and electrons) and high-LET radiations (e.g., alpha particles and neutrons). For purposes of radiation protection and assessing risk in general terms, a linear dose-response relationship, without threshold, generally is assumed in estimating stochastic responses at low doses. This Section discusses databases and methods used in estimating the relationship of radiation-induced

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stochastic responses to dose. More detailed discussions are given elsewhere (NCRP, 1993b; 2001). A substantial body of data on the frequency of radiation-induced cancers in humans has been obtained from studies in human populations that received doses considerably above levels of natural background; for recent reviews of these data see IARC (2000; 2001) and UNSCEAR (2000). These populations, and the associated types of cancers for which information on the dose-response relationships has been obtained, include the Japanese atomic-bomb survivors (cancers at many sites), uranium and other underground miners (lung cancer), medical patients irradiated with x rays (leukemia and breast, thyroid, and bone cancer) or injected with radium or thorium (leukemia, bone and liver cancer), and the radium dial painters (bone cancer). For low-LET radiations (e.g., photons) and for most organs and tissues, the Japanese atomic-bomb survivors are the most important source of data used to obtain estimates of cancer frequency per unit dose. These data were obtained under conditions of acute external exposure at high doses and dose rates. The observed doseresponse relationships in this population are assumed to apply to internal exposure, taking into account the different radiation weighting factors for high-LET radiations (e.g., alpha particles) as appropriate, and to exposures at lower doses and dose rates, taking into account information on the dependence of the frequency of responses on dose and dose rate. The Japanese atomic-bomb survivors also are a potentially important source of data on the dose-response relationship for severe hereditary responses. However, no evidence for inherited genetic effects has been observed in spite of nearly 50 y of study. In the absence of data in humans, estimates of the frequency of radiationinduced hereditary responses have been based primarily on data from studies in mice. In all studies of the relationship of radiation-induced stochastic responses to dose, the derivation of MLEs (mean values) of the doseresponse relationships has been emphasized (NAS/NRC, 1988b; 1990). Furthermore, for purposes of radiation protection, MLEs of the dose-response relationships, rather than UCLs, have been emphasized in extrapolating the observed dose-response data to lower doses beyond the range of observation (NCRP, 1975; 1999b). The use in radiation protection of MLEs of the relationships of stochastic responses to dose, rather than UCLs, is justified on the grounds that the probability of a response in most individuals is not likely to be significantly underestimated. Even if the probability were underestimated, the current framework for radiation protection

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of the public generally ensures that responses from exposure to manmade sources would be well below any responses from unavoidable exposures to natural background radiation. Features providing such assurance include a limit on annual effective dose of 1 mSv from exposure to all controlled sources combined and a requirement to maintain doses below the limit ‘‘as low as reasonably achievable’’ (ALARA). The average annual effective dose from natural background is about 3 mSv (NCRP, 1987b), and the geographical variability in the background dose is a substantial fraction of the average. In addition, epidemiologic studies have not consistently found evidence for increased cancer incidence below a dose on the order of 100 to 200 mSv, which is about the same as the average lifetime dose from natural background. Relationships between dose and stochastic responses obtained from data on human populations are subject to uncertainty resulting, e.g., from (1) uncertainties in doses received, (2) uncertainties in extrapolating data obtained at high doses and dose rates to the low doses and dose rates of concern in routine exposure situations, (3) incomplete expression of responses in study populations, particularly the Japanese atomic-bomb survivors who were young in 1945, resulting in uncertainty in projecting future responses in those populations, (4) differences in the dose-response relationships for different radiation types and different organs or tissues, and (5) effects of competing causes of the same response (e.g., smoking) and age at exposure. However, for external exposure of the whole body to lowLET radiations, the uncertainty in the dose-response relationship for induction of all cancers in humans at low doses appears to be less than an order of magnitude (EPA, 1994a; 1999a; 1999b; NAS/ NRC, 1990; NCRP, 1997). For external exposure at low doses and dose rates, NCRP has estimated that the 90 percent confidence interval of the probability coefficient for fatal cancers for lifetime exposure of the United States population is 0.012 to 0.088 Svⳮ1 (NCRP, 1997). The mean of the uncertainty distribution is 0.040 Sv ⳮ1 and the median (50th percentile) is 0.034 Svⳮ1. The conclusion about the uncertainty in the dose-response relationship for radiation stated above takes into account the uncertainty in extrapolating the data at high doses and dose rates in the Japanese atomic-bomb survivors to the lower doses and dose rates of concern in routine exposures of the public. The issue of extrapolation to low doses and dose rates is a matter of considerable controversy and is an important source of uncertainty (NCRP, 1997; 2001). For purposes of radiation protection, the frequency of responses at low doses and dose rates generally has been assumed to be a factor of two less than MLE of the frequency of responses in the Japanese atomic-bomb

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survivors (ICRP, 1991; NCRP, 1993a). This correction is called the dose and dose-rate effectiveness factor (DDREF). An evaluation of the uncertainty in DDREF is presented in another NCRP report (NCRP, 1997). In spite of uncertainties in the dose-response relationship for radiation discussed above, it is generally believed that radiation risks in humans can be assessed with considerably greater confidence than risks from exposure to most hazardous chemicals that cause stochastic effects. The state of knowledge of radiation risks in humans compared with risks from exposure to chemicals that cause stochastic effects is discussed further in Section 4.4.2. 3.2.2.3 Measures of Radiation-Induced Responses. This Section discusses the measures of response from radiation exposure generally used in radiation protection and assessments of radiation risk in general terms. 3.2.2.3.1 Measures of deterministic responses. Deterministic responses from radiation exposure are expected to occur only at doses much higher than doses that the general public might experience in routine exposure situations. The measure of deterministic responses used in radiation protection generally has been incidence of an adverse effect, although prompt fatalities also are of concern at very high doses (i.e., at doses well above thresholds for nonfatal deterministic responses). Prompt fatalities are considered, for example, in evaluating the potential consequences of severe radiation accidents. In radiation protection, incidence is the appropriate measure of deterministic response because the goal is to prevent such responses in any organ or tissue in almost all individuals. No attempts have been made to assign different weights to different deterministic responses, depending on their severity. Rather, all responses considered to be significant to human health are given equal weight in establishing deterministic dose limits in specific organs or tissues. As mentioned previously, deterministic responses from radiation exposure generally are not of concern in routine exposures of the public, because they should be precluded by the dose limit that is intended to ensure an acceptable increase in stochastic responses. 3.2.2.3.2 Measures of stochastic responses. The primary measure of stochastic responses used in radiation protection and radiation risk assessment by ICRP and NCRP has been fatalities (i.e., fatal cancers and severe hereditary effects). Fatalities have been emphasized essentially because this was the only health-effect endpoint for which data generally were available, both for study populations

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receiving high doses and for the background frequency of cancers and severe hereditary effects from all causes. Until recently, fatalities (especially latent cancer fatalities) was the only measure of stochastic response used in radiation protection and assessments of radiation risks in general terms (ICRP, 1977; NCRP, 1987a). No consideration was given to radiation-induced nonfatal stochastic responses or to the relative severity of different types of fatal responses (e.g., the expected length of life lost per fatality). In its current recommendations on radiation protection, ICRP (1991) developed a quantity called total detriment to describe stochastic responses (see also NCRP, 1993a). As summarized below, total detriment includes not only the probability of a fatal cancer or severe hereditary effect but also a contribution from nonfatal cancers and an additional adjustment that accounts for differences in expected length of life lost per fatal response in different organs or tissues. The term ‘‘detriment,’’ rather than ‘‘response’’ or ‘‘risk,’’ is used to describe this quantity because (1) the contribution from nonfatal cancers is not simply the probability of a nonfatal cancer, (2) severe hereditary responses are not experienced by exposed individuals but by their progeny, and (3) the adjustment for expected length of life lost per fatal response does not represent a probability of a fatality or incidence. ICRP (1991) has acknowledged that the modifications of the probability of a fatal response are necessarily judgmental and somewhat arbitrary, particularly the weight to be given to nonfatal cancers relative to fatal responses in assessing total detriment. Nonetheless, the following approach to assessing total detriment from radiation exposure for purposes of radiation protection was developed. ICRP (1991) has recommended that the detriment due to radiation-induced stochastic responses in any organ should include, in addition to the probability of a fatal response, the probability of a nonfatal response weighted by the lethality fraction (k). This adjustment is used only for cancers because all severe hereditary effects are assumed to be fatal. If the probability coefficient for fatal cancer (i.e., probability of a fatal cancer per unit equivalent dose) in a particular organ is denoted by F, then the probability coefficient for cancer incidence in that organ is F/k and the probability coefficient for nonfatal cancers is F/k ⳮ F ⳱ (1 ⳮ k)F/k. Then, using the lethality fraction (k) to weight the probability coefficient for nonfatal cancers, the contribution to the total detriment from nonfatal cancers is k(1 ⳮ k)F/k ⳱ (1 ⳮ k)F, and the total weighted detriment from fatal and nonfatal cancers is F Ⳮ (1 ⳮ k)F ⳱ F(2 ⳮ k). Finally, the total detriment in any organ, including the gonads, is obtained by multiplying the total weighted detriment for fatal and nonfatal

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responses by the factor ᐉ/l, where ᐉ is the expected years of life lost per fatal cancer in that organ or per fatal hereditary effect and l is the average expected years of life lost from all fatal responses. Therefore, in any organ, the total detriment due to stochastic responses recommended by ICRP (1991) is given by F(ᐉ/l)(2 ⳮ k). In estimating the total detriment due to stochastic responses in any organ as described above, the probability coefficient for fatal cancers (F) or severe hereditary responses is based on data in humans and animals described in Section 3.2.2.2, and the lethality fraction (k) and relative length of life lost per fatal response (ᐉ/l) are based on data on responses from all causes in various national populations. The values of F, k, and ᐉ/l for different organs, as well as the probability coefficient for severe hereditary responses, assumed by ICRP (1991) and the resulting estimates of total detriment, F(ᐉ/l)(2 ⳮ k), are summarized in Table 3.2. The two entries for ‘‘Total’’ in the last row represent the probability coefficient for

TABLE 3.2—Contributions from different organs to total detriment from radiation exposure of a general population.a

Organ

Fatal Cancers (F)b

Bladder Bone marrow Bone surface Breast Colon Liver Lung Esophagus Ovary Skin Stomach Thyroid Remainder Gonads Total

30 50 5 20 85 15 85 30 10 2 110 8 50 — 500

a

Severe Hereditary Effectsb

Lethality Fraction (k)c

Relative Length of Life Lost (ᐉ/l)

Total Detriment F(ᐉ/l)(2 ⳮ k)b

100 —

0.50 0.99 0.70 0.50 0.55 0.95 0.95 0.95 0.70 0.002 0.90 0.10 0.71 — —

0.65 2.06 1.00 1.21 0.83 1.00 0.90 0.77 1.12 1.00 0.83 1.00 0.91 1.33 —

29 104 6.5 36 102 16 80 24 15 4 100 15 59 133 730d

Adapted from Tables B-19 and B-20 of ICRP (1991). Values per 10,000 person-Sv. c Assumed fraction of all cancers in adults that are fatal. d The sum has been rounded. b

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fatal cancers and the total detriment resulting from uniform irradiation of the whole body, i.e., when all organs and tissues receive the same dose. From the expression for the total detriment given above and the data in Table 3.2, the following observations can be made. For cancers that are nearly always fatal (e.g., leukemia from irradiation of bone marrow), the total detriment is determined essentially by the probability of a fatal cancer, absent consideration of the relative length of life lost (ᐉ/l), and the contribution from weighted nonfatal cancers is insignificant. For cancers that are rarely fatal (e.g., skin or thyroid cancers), the total detriment exceeds the probability of a fatal cancer by no more than a factor of two, based on the assumption that nonfatal cancers should be weighted by the lethality fraction (k). In general, if the lethality fraction is less than about 0.1, the total detriment essentially is twice the probability of a fatal cancer, independent of the lethality fraction. For routine exposures of the public, ICRP recommends a total detriment per unit equivalent dose from uniform whole-body irradiation of 7.3 ⳯ 10ⳮ2 Svⳮ1, as shown in Table 3.2. Of this, the recommended probability coefficient for fatal cancers is 5.0 ⳯ 10ⳮ2 Svⳮ1, or about two-thirds of the total detriment, and the contributions from severe hereditary responses and weighted nonfatal cancers are 1.3 ⳯ 10ⳮ2 Svⳮ1 and 1.0 ⳯ 10ⳮ2 Svⳮ1, respectively. These probability coefficients are summarized in Table 3.3, and their use in radiation protection is discussed in the following section. As noted previously, the probability coefficient for weighted nonfatal cancers is not the same as the probability coefficient for incidence of nonfatal cancers. The probability coefficient for fatal cancers also gives the probability of a fatal cancer per unit effective dose. The effective dose was developed to describe nonuniform irradiations of the body and is discussed below.

TABLE 3.3—Nominal probability coefficients for stochastic responses due to radiation exposure of the general public.a

Fatal Cancer (10ⳮ2 Svⳮ1)

Weighted Nonfatal Cancerb (10ⳮ2 Svⳮ1)

Severe Hereditary Effects (10ⳮ2 Svⳮ1)

Total Detriment (10ⳮ2 Svⳮ1)

5.0

1.0

1.3

7.3

a

Adapted from Table 3 of ICRP (1991). Probability coefficient does not represent probability of a nonfatal cancer (see Section 3.2.2.3.2). b

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3.2.2.3.3 Effective dose. In radiation protection, all organs and tissues at risk from a given exposure are taken into account. This is accomplished by calculating a quantity called the effective dose (E), which is defined as a weighted sum (average) of equivalent doses in all organs and tissues (ICRP, 1991; NCRP, 1993a): E⳱

兺 T w T HT, 兺 T w T ⬅ 1,

(3.2)

where HT is the equivalent dose in organ or tissue (T) and wT is the weighting factor for tissue (T) described below. The equivalent dose in organ or tissue (T) is calculated as: HT ⳱

兺 R w R D T,R,

(3.3)

where wR is the radiation weighting factor for radiation of type (R) described below and DT,R is the average absorbed dose in organ or tissue (T) (i.e., the total energy absorbed divided by the total mass) from radiation (R). For external exposure, the effective dose represents the doses received in the different organs or tissues during the time of an exposure. However, since intakes of radionuclides continue to deliver a dose to target organs or tissues until the radionuclides are removed from the body by radioactive decay or biological elimination, even with no further intakes, the effective dose for internal exposure represents committed doses, i.e., the time-integral of the dose rates following an acute intake, in the different organs or tissues. For intakes by an adult, the effective dose used in radiation protection normally is the integrated dose over 50 y (i.e., the dose received to age 70 following an acute intake at age 20); for intakes by younger age groups, the effective dose normally is the integrated dose from the age at intake to age 70 (ICRP, 1996). The wR for a specified type and energy of radiation is chosen to represent the relative biological effectiveness of that radiation in inducing stochastic responses at low doses. For example, the radiation weighting factor is one for photons and electrons of any energy and 20 for alpha particles of any energy (ICRP, 1991; NCRP, 1993a). A complete listing of recommended radiation weighting factors is given in Table 1 of ICRP (1991) and Table 4.3 of NCRP (1993a). The wT used in calculating the effective dose is proportional to the total detriment given in Table 3.2 and, thus, takes into account fatal cancers and severe hereditary effects, weighted nonfatal cancers, and the relative severity of all fatal responses. When the whole body is irradiated uniformly, the value of wT for a particular organ is the fraction of the total detriment resulting from irradiation of that organ. Thus, the effective dose is intended to be proportional to total

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detriment for nonuniform irradiations of the whole body, such as often result from inhalation or ingestion of radionuclides, as well as for uniform whole-body irradiations; i.e., exposures with equal effective doses are assumed to correspond to equal total detriments regardless of the distribution of doses among different organs and tissues. The tissue weighting factors used in calculating the effective dose are given in Table 3.4. Both the effective dose and equivalent doses in specific organs and tissues, from which effective dose is calculated, are intended for use only in radiation protection (i.e., in setting limits on radiation dose and evaluating compliance with dose limits) or in assessments of risk in general terms, such as for prospective or hypothetical exposure situations. This caution is warranted because of the approximate nature of wT s used in estimating the effective dose (they are obtained as rounded values of the total detriment coefficients for specific organs or tissues given in Table 3.2), as well as the approximate nature of wR s used in estimating equivalent doses in each organ or TABLE 3.4—Tissue weighting factors used in calculating effective dose.a

a

Organ

wT

Gonads Red bone marrow Colon Lung Stomach Bladder Breast Liver Esophagus Thyroid Skin Bone surface Remainderb

0.20 0.12 0.12 0.12 0.12 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.05

Adapted from Table 2 of ICRP (1991). The ‘‘Remainder’’ category includes the adrenals, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, thymus, and uterus. The wT for remainder normally is applied to the average of the equivalent doses to these organs and tissues. In those exceptional cases in which a single one of the remainder organs or tissues receives an equivalent dose in excess of the highest dose in any of the 12 organs for which a weighting factor is specified, a weighting factor of 0.025 is applied to that organ or tissue and a weighting factor of 0.025 is applied to the average dose in the rest of the remainder. b

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tissue from calculated absorbed doses. Equivalent doses in specific organs or tissues and the effective dose generally are not intended for use in estimating probabilities of stochastic responses from actual exposures. In such cases, it is preferable to use estimates of absorbed dose, data on the relative biological effectiveness in each organ irradiated and for each radiation type of concern to the particular exposure situation, the sex of the exposed individuals, and age at exposure. Assessments of risk from waste disposal for the purpose of developing a generally applicable, risk-based waste classification system provide an example of an appropriate use of equivalent doses and the effective dose. It also is important to note that the total detriment developed by ICRP (1991) is intended to be used mainly in obtaining wT s in the effective dose (the values of wT in Table 3.4 are roughly proportional to the corresponding total detriments in Table 3.2). However, total detriment is not normally used in estimating responses from a given effective dose. ICRP and NCRP have continued to emphasize fatal cancers as the health effect of primary concern and have used the probability coefficient for fatal cancers of 5 ⳯ 10ⳮ2 Svⳮ1 given in Table 3.3 for this purpose. Total detriment is not used in estimating responses because, as noted previously, the detriment due to nonfatal cancers in Table 3.3 is not the probability of a nonfatal cancer and the detriment due to severe hereditary effects is not experienced by exposed individuals.

3.2.3 Comparison of Dose-Response Assessments for Radionuclides and Chemicals The discussions in Sections 3.2.1 and 3.2.2 have indicated that there are important differences in the approaches to dose-response assessment for radionuclides and hazardous chemicals. An understanding of these differences is important in developing a risk-based waste classification system that applies to both types of substances. A fundamental difference between radionuclides and hazardous chemicals in regard to dose-response assessment is the following. Estimates of responses from exposure to radionuclides can be based on estimates of absorbed dose and equivalent dose in all organs and tissues, and the dose-response relationships for different organs or tissues obtained from human or animal studies can be applied to any radionuclide and any exposure situation. Separate studies of responses from exposure to each radionuclide of concern are not needed. For hazardous chemicals, however, quantities analogous to absorbed dose and equivalent dose have not been developed; i.e.,

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the physical and biological quantities that relate response to dose delivered to target tissues have not yet been elucidated. Therefore, the dose-response relationship for most hazardous chemicals can only be determined by direct observation but cannot readily be inferred from the dose-response relationship for any other substance. Structure-activity relationships and pharmacokinetic models may be useful in inferring dose-response relationships for some hazardous chemicals based on data for another substance, but these methods have not been applied widely. The following sections present a comparison of approaches to estimating deterministic and stochastic dose-response relationships for radionuclides and hazardous chemicals. 3.2.3.1 Deterministic Responses. Prevention of deterministic responses is a basic principle of health protection for both radionuclides and hazardous chemicals; the goal is to achieve zero probability of such responses. Incidence is the primary measure of deterministic response for any hazardous substance, although prompt fatalities also are of concern at sufficiently high doses. In risk assessments and in establishing deterministic dose limits, no adjustments are made to take into account, for example, the relative severity of different responses with regard to consequent reductions in the quality of life. For purposes of health protection, the dose-response relationships for deterministic effects from exposure to radionuclides and hazardous chemicals are assumed to have a threshold. For either type of substance, the assumed thresholds are based on data for the most sensitive organ or tissue. However, there are potentially important differences in the way these thresholds are estimated and then applied in health protection of the public. First, the threshold for hazardous chemicals that cause deterministic effects is assumed for purposes of health protection to represent a lower confidence limit, taking into account uncertainties in the dose-response relationship (see Section 3.2.1.2.7). Depending, for example, on the slope of the dose-response relationship near the threshold, the chosen steps in the dosing regimen, and the magnitude of uncertainties in the data, the lower confidence limit of the assumed threshold can be substantially below MLE. In radiation protection, the estimated thresholds for deterministic effects are based on MLEs of dose-response relationships (ICRP, 1991). Second, in radiation protection of the public, deterministic dose limits are based mainly on data in humans and normally are set at a factor of 10 below the assumed thresholds. This safety factor is intended to ensure that deterministic responses would be precluded

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in almost all individuals, including those that might be unusually sensitive to radiation. A more conservative approach to health protection of the public normally is taken for hazardous chemicals that cause deterministic effects, in part because most such substances have been studied only in animals. Limits on acceptable dose often are defined by RfDs that usually are derived from lower confidence limits of the assumed thresholds, as represented by NOAELs or lower confidence limits of benchmark doses, by applying several safety and uncertainty factors (see Sections 3.2.1.2.5 and 3.2.1.2.7). A safety and uncertainty factor of at least 100 normally is applied in obtaining an RfD, and this factor may be as much as 5,000 for some substances. Based on these differences, the use of RfDs for hazardous chemicals that induce deterministic effects to define acceptable exposures of the public often may be considerably more conservative (provide a substantially larger margin of safety) than the dose limits for radiation induced deterministic effects. The likely degree of conservatism embodied in RfDs has important implications for establishing limits on allowable exposures to substances causing deterministic effects for the purpose of developing a risk-based waste classification system. Dose limits for deterministic effects for radiation should not be important in classifying waste (see Section 3.2.2.1). 3.2.3.2 Stochastic Responses. A basic principle of health protection for both radionuclides and hazardous chemicals is that the probability of a stochastic response, primarily cancers, should be limited to acceptable levels. For any substance that causes stochastic responses, a linear dose-response relationship, without threshold, generally is assumed for purposes of health protection. However, the probability coefficients for radionuclides and chemicals that induce stochastic responses that are generally assumed for purposes of health protection differ in two potentially important ways. First, the dose-response relationships for radiation used for purposes of health protection and the probability coefficients derived from those relationships are intended to be MLEs. In contrast, the dose-response relationships and probability coefficients for chemicals that induce stochastic responses are intended to be upper-bound estimates (UCLs), although MLEs also are available. In animal data from which the probability coefficients for most chemicals that cause stochastic responses are obtained, UCL can be greater than MLE by a factor that ranges from 5 to 100 or more. Second, the primary measure of stochastic response used in radiation protection and in most radiation risk assessments has been fatalities. In contrast, the measure of response for chemicals causing

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stochastic responses generally has been incidence. This difference results from the fact that dose-response relationships for radiation were based on data on radiation-induced fatalities in human populations, whereas the dose-response relationships for most hazardous chemicals that cause stochastic responses have been based on data on tumor incidence in animal studies. The total detriment, which takes into account nonfatal cancers as well as fatalities (ICRP, 1991), also is used in radiation protection. However, total detriment is used mainly to obtain the tissue weighting factors in the effective dose and is not normally used in radiation risk assessments (see Sections 3.2.2.3.2 and 3.2.2.3.3). Data on radiation-induced cancer incidence in the Japanese atomic-bomb survivors and comparisons of cancer incidence and mortality in this population are becoming available (Mabuchi et al., 1994; Preston et al., 1994; Ron et al., 1994; Thompson et al., 1994; UNSCEAR, 2000). These data could be used to derive the relationship between dose and cancer incidence at low doses, in which case the same measure of response could be used for radionuclides and stochastic chemicals. However, there are difficulties with using the data on cancer incidence for radiation, including uncertainties in (1) determining background rates of cancer incidence from all causes in various organs or tissues and (2) applying the results from the Japanese study population to other national populations in which the background rates of cancer incidence in some organs may be significantly different (e.g., the gastrointestinal tract). These concerns also apply, of course, to the data on cancer mortality in the atomic-bomb survivors. Radiation-induced cancer incidence also could be estimated using calculations of the probability of cancer incidence per unit activity intake of specific radionuclides by particular ingestion and inhalation pathways or the probability per unit activity concentration of specific radionuclides in the environment by particular pathways of external exposure (Eckerman et al., 1999); probabilities of fatal cancers for the different exposure pathways also have been calculated. These probability coefficients differ from those developed by ICRP (see Section 3.2.2.3.2) in that they are calculated with respect to activity of specific radionuclides rather than dose, and they thus bypass the need to estimate the effective dose. For external exposure, the methods used by Eckerman et al. (1999) and ICRP (1991) to estimate responses essentially are equivalent. However, there are significant differences in the methods used to estimate responses from intakes of radionuclides, and the results obtained by Eckerman et al. (1999) differ substantially in a few cases (e.g., intakes of 232Th)

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from those based on effective doses and probability coefficients developed by ICRP. The method used by Eckerman et al. is methodologically more rigorous (NAS/NRC, 1999a), mainly because the response estimates are based on calculated dose rates as a function of time after intake and age at intake. The approach of estimating responses from intakes of radionuclides based on committed effective doses for different ages at intake (ICRP, 1991) does not properly account for the distribution of doses over time after intake in cases where a long-lived radionuclide decays to radiologically significant progeny whose activity increases over a period of a few years or more. An additional difference in current approaches to dose-response assessment for radionuclides and stochastic chemicals is the following. Radiation is a much more general carcinogen, affecting cancer in many different organs and tissues, than most chemicals. Use of the effective dose in radiation protection takes into account all organs and tissues at risk for any exposure situation, regardless of whether the whole body is irradiated uniformly or nonuniformly. For most hazardous chemicals, however, only one organ or tissue at risk is taken into account, and responses in other organs or tissues are ignored. In only a few cases are risk estimates for hazardous chemicals based on observed responses in multiple organs. The development of PB-PK models for hazardous chemicals offers the possibility of estimating doses (concentrations) of chemicals in different organs or tissues, but such models have not yet been widely accepted by regulators. This difference in approaches to dose-response assessment for radionuclides and stochastic chemicals cannot be eliminated at the present time, in part because the probability coefficients for most chemicals are based on studies in animals and the organs in which cancers are seen in study animals often do not correspond to the organs at greatest risk in humans. However, this difference is unlikely to be important when chemicals presumably induce cancers only at sites of deposition in the body and most hazardous chemicals are not distributed widely in the body following an intake. Given the different approaches to dose-response assessment and the different measures of response normally used for radionuclides and chemicals that cause stochastic effects, estimates of responses from exposure to the two types of substances clearly are not equivalent, and the correspondence of the estimated frequency of responses to the frequency that might actually be experienced differs substantially. Specifically, if the results of experiments indicating chemicalinduced stochastic responses in animals are assumed to be indicative of stochastic responses in humans, estimates of responses for chemicals could be considerably more conservative (pessimistic) than estimates for radionuclides. This difference is primarily the result of

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using best estimates (MLEs) of response probabilities for radionuclides but upper bounds (UCLs) for chemicals that cause stochastic responses, because cancer fatalities and cancer incidence do not differ substantially in most organs and tissues (see Table 3.2). The difference between MLEs and UCLs of dose-response relationships is an important concern in developing a comprehensive and riskbased hazardous waste classification system.

3.3 Approaches to Risk Management for Radionuclides and Hazardous Chemicals That Cause Stochastic Effects Risk management is the process by which values of acceptable risk are established and the results of risk assessments are compared with these values, resulting in decisions concerning the acceptability of particular practices involving hazardous substances, including waste disposal. Risk management often involves consideration of economic, legal, and socio-political factors, and is typically performed by regulatory authorities. Consideration of suitable approaches to risk management clearly is important in establishing a risk-based waste classification system. The acceptable risks for substances that induce stochastic responses discussed in this Section are values in excess of unavoidable risks from exposure to the undisturbed background of naturally occurring agents that cause stochastic responses, such as many sources of natural background radiation and carcinogenic compounds produced by plants that are consumed by humans. This distinction is based on the assumption of a linear, nonthreshold doseresponse relationship for substances that cause stochastic responses and the inability to control many sources of exposure. Risk management can address exposures to naturally occurring substances that induce stochastic responses, but only when exposures are enhanced by human activities or can be reduced by reasonable means. In contrast, risk management for substances that cause deterministic effects must consider unavoidable exposures to the background of naturally occurring substances that cause such effects. Based on the assumption of a threshold dose-response relationship, the risk from man-made sources is not independent of the risk from undisturbed natural sources, and the total dose from all sources must be considered in evaluating deterministic risks. In the case of ionizing radiation, thresholds for deterministic responses are well above average doses from natural background radiation (see Section 3.2.2.1)

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and background can be neglected. This may not be the case, however, for some naturally occurring chemicals that induce deterministic effects (e.g., lead). In such cases, exposures to man-made sources comparable to background exposures could result in a significant increase in deterministic risks. As noted in Section 3.2.3.1, the approaches to management of deterministic risks are essentially the same for radionuclides and hazardous chemicals, although the degree of conservatism may differ for the two types of substances. Management of deterministic risks is not discussed further in this Section. This Section discusses approaches to risk management that are used in protecting the public from exposure to radionuclides and chemicals that cause stochastic responses in the environment. Different approaches to management of stochastic risks are used for radionuclides and chemicals. An understanding of the two approaches, including their differences and ways in which these differences can be reconciled, is important in developing a comprehensive and riskbased hazardous waste classification system. The different approaches to management of stochastic risks for radionuclides and hazardous chemicals are referred to in this Report as the radiation and chemical paradigms (EPA, 1992a). The following discussion of the two paradigms for management of stochastic risks is adapted from previous papers (Kocher, 1999; Kocher and Hoffman, 1991).

3.3.1 Radiation Paradigm for Risk Management of Stochastic Responses The radiation paradigm for management of stochastic risks is applied to control of radiation exposures under authority of AEA (1954). Thus, this approach to risk management applies only to regulation of radionuclides that arise from operations of the nuclear fuel cycle, but it does not apply to control of radiation exposures under authority of any other laws. For example, the radiation paradigm does not apply to regulation of radionuclides in public drinking water supplies under authority of the Safe Drinking Water Act (EPA, 1975; SDWA, 1974). Radionuclides in drinking water are regulated in accordance with the chemical paradigm discussed in the following section. The radiation paradigm for management of stochastic risks is based on the fundamental principles of radiation protection developed over many decades by ICRP and NCRP. As stated by NCRP (1993a), these principles include the following:

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the need to justify any activity involving radiation exposure on the basis that the expected benefits to society exceed the overall societal cost (principle of justification) the need to ensure that the total societal detriment from such justifiable activities or practices is maintained as low as reasonably achievable (ALARA), economic and social factors being taken into account (principle of optimization) the need to apply dose limits to individuals to ensure that the procedures of justification and ALARA do not result in exposures of individuals or groups of individuals that exceed levels of acceptable risk (principle of dose limitation)

As depicted in Figure 3.11, the principles of optimization (ALARA) and dose limitation embodied in the radiation paradigm may be thought of as defining a ‘‘top-down’’ approach to management of stochastic risks. Given that radiation exposures have been justified, the radiation paradigm has two basic elements:

Fig. 3.11. The radiation paradigm for management of stochastic risks (Kocher, 1999).

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1. a limit on radiation dose to individuals from exposure to all controlled sources combined, corresponding to a maximum allowable risk for any routine exposure situation; and 2. a requirement to reduce exposures to controlled sources as far below the limit as reasonably achievable (ALARA). The proper interpretation of the dose limit is that higher doses (and their associated risks) from controlled sources are regarded as unacceptable, meaning intolerable. Thus, the dose limit normally must be met in routine exposures to controlled sources regardless of cost or other circumstances. Application of the ALARA principle to further control of exposures takes into account, for example, the cost of reducing radiation doses in relation to the benefits in health risks averted and other societal concerns. Thus, in the radiation paradigm, doses are acceptable if they correspond to risks less than the maximum allowable risk and they are ALARA; compliance with the dose limit does not, by itself, determine acceptable risks. It is important to emphasize that doses that are ALARA may vary from one exposure situation to another; i.e., what is ALARA is not a pre-determined result that applies to all exposure situations. As indicated in Figure 3.11, doses that are less than the dose limit but are not ALARA are regarded as barely tolerable and are not considered to be acceptable in most cases. The radiation paradigm is embodied in current recommendations of ICRP (1991) and NCRP (1993a) and in radiation protection standards for the public established by NRC (1991) and DOE (1990). These recommendations and standards include a limit on annual effective dose of 1 mSv to individual members of the public from all sources of routine exposure combined, excluding natural background, indoor radon, and deliberate medical practices. Assuming a nominal probability coefficient for fatal cancers of 5.0 ⳯ 10ⳮ2 Svⳮ1 (see Table 3.3) and continuous exposure over an average lifetime of 70 y, the estimated lifetime fatal cancer risk corresponding to the dose limit is about 4 ⳯ 10ⳮ3. ICRP (1991) and NCRP (1993a) also recommend, however, that the lifetime risk from routine exposure to man-made sources normally should not exceed about 1 ⳯ 10ⳮ3. Risks from man-made sources having values in the range of (1 to 4) ⳯ 10ⳮ3 are regarded as barely tolerable, and risks below this range are regarded as reasonably achievable in most cases. The development of many standards that specify dose constraints for specific practices or sources at levels well below the annual dose limit of 1 mSv for all controlled sources combined (Kocher, 1988; Mills et al., 1988) is an important means of ensuring that the lifetime cancer risk from exposure to controlled sources normally will not

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exceed about 10ⳮ3. This type of standard is referred to as a source constraint (ICRP, 1991). Examples include EPA standards for operations of uranium fuel-cycle facilities in 40 CFR Part 190 (EPA, 1977) and NRC’s performance objectives for release of radionuclides from near-surface waste disposal facilities in 10 CFR Part 61 (NRC, 1982a). In the radiation paradigm, source constraints essentially represent generic applications of the ALARA principle; i.e., they are based primarily on judgments by regulatory authorities that the specified doses are reasonably achievable at any site. In practice, the lifetime fatal cancer risk due to any single controlled source often is much less than 10ⳮ3 (NCRP, 1987b), due to vigorous site-specific applications of the ALARA principle beyond the requirements of source constraints. The radiation paradigm also is applied to other situations including cleanup of sites contaminated with uranium or thorium mill tailings, mitigation of indoor radon, remediation of elevated levels of naturally occurring radionuclides other than radon, and responses to radiation accidents. In these applications, the maximum acceptable risk has a value in the range of about 10ⳮ1 to 10ⳮ3 (Kocher, 1999). In addition to the dose limit that defines a maximum acceptable risk and the requirement to reduce doses below the limit using the ALARA principle, there has long been the concept in radiation protection of a dose so low that the associated risk would be considered negligible (de minimis), as indicated in Figure 3.11. At such low doses, efforts at further reductions in dose using the ALARA principle generally would be unwarranted. A widely discussed de minimis dose for individual members of the public from any man-made source is an annual effective dose of 0.01 mSv (IAEA, 1988; NCRP, 1993a). This dose is one percent of the dose limit for individual members of the public, and it corresponds to a lifetime fatal cancer risk of about 4 ⳯ 10ⳮ5. The concept of a negligible dose is discussed further in Sections 4.1.2.5, 4.1.3.2, and 4.4.1.2; such a dose is not yet incorporated in radiation protection standards for the public in the United States (DOE, 1990; NRC, 1991). If the concept of a negligible risk is included, the radiation paradigm for management of stochastic risks (‘‘top-down’’) depicted in Figure 3.11 defines three regions of risk: 1. a risk so high that it is unacceptable (intolerable, de manifestis) and normally must be reduced regardless of cost or other circumstances; i.e., an excess lifetime cancer risk above a value in the range of about 10ⳮ1 to 10ⳮ3, the particular value depending on the exposure situation;

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2. a risk so low that it generally is negligible regardless of the cost-benefit for dose reduction; i.e., an excess lifetime risk less than about 10ⳮ4 for any exposure situation; and 3. risks between intolerable and negligible levels that are acceptable (i.e., tolerable) if they are ALARA but are unacceptable otherwise. It is important to emphasize that achieving a negligible risk is not the goal of ALARA, because any non-negligible risk between intolerable and negligible levels is acceptable if it is ALARA.

3.3.2 Chemical Paradigm for Risk Management of Stochastic Responses The chemical paradigm for management of stochastic risks is applied to control of exposures to stochastic chemicals under authority of several environmental laws. The chemical paradigm also applies to control of radiation exposures when these exposures are regulated under authority of any laws other than AEA. The chemical paradigm for management of stochastic risks essentially is the opposite of the radiation paradigm (‘‘top-down’’) described in Section 3.3.1, and thus may be thought of as ‘‘bottom-up.’’ The chemical paradigm depicted in Figure 3.12 has two basic elements: (1) a goal for acceptable risk, and (2) allowance for an increase (relaxation) in risks above the goal, based primarily on considerations of technical feasibility and cost. The extent to which the goal for acceptable risk may be relaxed generally depends on the particular situation. The use of risk goals and allowance for an increase (relaxation) in risks is fundamentally different from the approach in the radiation paradigm of establishing a limit on dose (and therefore risk) and requiring reductions in dose below the limit based on the ALARA principle. Thus, the goal for acceptable risk in the chemical paradigm clearly does not have the same meaning as the limit on acceptable risk in the radiation paradigm. Indeed, a noteworthy feature of the chemical paradigm is that it does not explicitly incorporate the concept of an intolerable risk that normally must be reduced regardless of cost or other circumstances. The chemical paradigm also differs from the radiation paradigm in that there are no standards that apply to all controlled sources of exposure and all hazardous substances combined, as in radiation protection standards. Regulations for hazardous chemicals generally apply only to specific release pathways (e.g., the atmosphere) or

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Fig. 3.12. The chemical paradigm for management of stochastic risks (Kocher, 1999).

exposure pathways (e.g., drinking water), or to all release and exposure pathways at specific sites (e.g., standards for cleanup of contaminated sites). In some cases (e.g., standards for atmospheric releases and drinking water), each hazardous chemical of concern is regulated separately, and there is no standard that specifies an acceptable risk from exposure all regulated substances combined. The chemical paradigm for management of stochastic risks is exemplified by standards for contaminants in public drinking water supplies established by EPA under authority of the Safe Drinking Water Act (EPA, 1975; SDWA, 1974). EPA must first establish maximum contaminant level goals (MCLGs), which are non-enforceable health goals for drinking water that must correspond to levels where no known or anticipated health effects would occur. Thus, based on the assumption of a linear, nonthreshold dose-response relationship, MCLG for all known substances that induce stochastic responses, including radionuclides, must be zero. This goal cannot be achieved at any cost. Then, EPA must establish maximum contaminant levels

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(MCLs), which are the legally enforceable standards for contaminants in drinking water. MCLs must be set as close to MCLGs as possible, taking into account technical feasibility and cost. Thus, MCLGs are the goals for health risk, and MCLs are the allowable relaxations above the goals. Under authority of the Safe Drinking Water Act, EPA has established MCLs for substances that cause stochastic responses, including radionuclides, in public drinking water supplies. The MCLs usually (but not always) correspond to lifetime cancer risks having values in the range of about 10ⳮ4 to 10ⳮ6. This range of acceptable risks from contaminants in drinking water also has been embodied in other regulations established by EPA, including standards for airborne emissions of radionuclides and chemicals (EPA, 1992a) developed under authority of the Clean Air Act (CAA, 1963), goals for cleanup of radionuclides and chemicals at hazardous waste sites (EPA, 1990a) developed under authority of CERCLA (1980), and requirements for corrective actions at disposal sites for hazardous chemical waste in 40 CFR Part 264 (EPA, 1980a) developed under authority of RCRA (1976). It is important to understand that the limits on lifetime cancer risk in the range of about 10ⳮ4 to 10ⳮ6 as embodied, for example, in drinking water standards are conceptually different from the dose limit (and associated limit on risk) in radiation protection standards described in Section 3.3.1. In the radiation paradigm, the dose limit is regarded as necessary for protection of public health and, thus, normally must be met in routine exposure situations regardless of cost or other circumstances. In contrast, the standards (MCLs) for hazardous substances in drinking water, although they also are legally enforceable limits, are based primarily on considerations of contaminant levels that are reasonably achievable, taking into account technical feasibility and cost, rather than levels that must be met to protect public health regardless of cost or other circumstances. Thus, drinking water standards and other standards established under the chemical paradigm that embody limits on lifetime cancer risk in the range of about 10ⳮ4 to 10ⳮ6 are analogous to the source constraints that are widely used as one means of applying the ALARA principle in the radiation paradigm. As another example, risk goals having a value in the range of 10ⳮ4 to 10ⳮ6 for cleanup of contaminated sites under CERCLA (EPA, 1990a) do not define a limit that must be met without regard for cost or other circumstances, because CERCLA and its implementing regulations (EPA, 1990a) specify many conditions for waiving compliance with the goals. Rather, CERCLA risk goals define risks above which action to reduce risk must be considered, but reduction of

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risks above the goals is required only to the extent feasible. Furthermore, EPA (1991a) has indicated that lifetime cancer risks below about 10ⳮ4 are properly interpreted as negligible, because action to reduce risks at these levels generally is not required. Indeed, in cleanup decisions involving radioactively contaminated sites under CERCLA, the risk levels achieved in many cases have values in the range of about 10ⳮ2 to 10ⳮ4 (EPA, 1994b) and, thus, are substantially above the goals. This result is indicative of how CERCLA risk goals should be interpreted. The chemical paradigm for management of risks (‘‘bottom-up’’) depicted in Figure 3.12 often is interpreted as defining two regions of stochastic risk: (1) an ‘‘acceptable’’ risk, i.e., any excess lifetime cancer risk less than a value in the range of about 10ⳮ4 to 10ⳮ6, the particular value depending on the exposure situation; and (2) an ‘‘unacceptable’’ risk, i.e., any risk greater than an ‘‘acceptable’’ risk. However, as indicated above, this interpretation is misleading because it does not properly convey how risk-management decisions usually are made in the chemical paradigm. Although risks less than a value in the range of about 10ⳮ4 to 10ⳮ6 clearly are acceptable, risks at these levels are more properly interpreted as negligible because there usually is no requirement for further reduction of risks based, for example, on considerations of technical feasibility and cost, even when such reductions would be cost-effective. Furthermore, ‘‘unacceptable’’ risks (any risks greater than a value in the range of about 10ⳮ4 to 10ⳮ6) clearly are not intolerable and are not required to be mitigated regardless of cost or other circumstances, because risks above these levels often have been permitted based primarily on considerations of cost-benefit (e.g., in cleanups of CERCLA sites). These conclusions about the proper interpretations of the significance of different levels of risk in the chemical paradigm are supported, for example, by a review of EPA regulatory decisions prior to 1985 (Travis et al., 1987). As depicted in Figure 3.13, this review showed the following. First, EPA always declined to regulate when the risk was less than a value in the range of about 10ⳮ4 to 10ⳮ6, depending on the size of the exposed population. Thus, EPA clearly regarded risks at these levels as negligible. Second, when the risk was greater than a value in the range of about 10ⳮ4 to 10ⳮ6 but less than a value in the range of about 10ⳮ2 to 10ⳮ3 (the middle region in Figure 3.13), EPA required risk reduction in some cases but declined to do so in others, depending primarily on the cost-benefit for risk reduction. Thus, risks greater than a value in the range of

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about 10ⳮ4 to 10ⳮ6 clearly are not unacceptable without regard for other circumstances. This interpretation is not changed by more recent laws and regulations implementing the chemical paradigm for management of stochastic risks. The upper region in Figure 3.13 is discussed in the following section. The chemical paradigm for risk management also is used in regulating exposures to hazardous chemicals that cause deterministic effects and exhibit a threshold in the dose-response relationship. For these substances, RfDs, which are often used to define acceptable exposures, represent negligible doses, because RfDs usually are well below assumed thresholds for deterministic responses in humans and action to reduce doses below RfDs generally is not required. This interpretation is supported by cases where doses above an RfD are allowed when achieving RfD is not feasible. A particular example

Fig. 3.13. Summary of EPA regulatory decisions prior to 1985 on whether to regulate carcinogenic hazardous materials (Travis et al., 1987); lower-right region comprising high population risks is excluded based on assumed United States population.

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involves regulation of thallium in drinking water. In this case, the standard (MCL) for limiting concentrations in drinking water established by EPA in 40 CFR Part 141 (EPA, 1975) is substantially above the goal (MCLG) that is intended to ensure a negligible dose (a dose below RfD) because achieving the goal is not feasible using existing technology for water treatment. Thus, RfDs clearly do not define intolerable doses.

3.3.3 Comparison of the Radiation and Chemical Paradigms The radiation paradigm for management of stochastic risk (‘‘topdown’’) described in Section 3.3.1 and depicted in Figure 3.11 clearly is quite different, at least conceptually, from the chemical paradigm (‘‘bottom-up’’) described in Section 3.3.2 and depicted in Figure 3.12. The radiation paradigm essentially involves a limit on ‘‘acceptable’’ (meaning barely tolerable) risk and reductions in risk below the limit using the ALARA principle, whereas the chemical paradigm essentially involves a goal for ‘‘acceptable’’ (meaning negligible) risk and allowance for relaxation of risks above the goal based primarily on considerations of technical feasibility and cost. The acceptable risks embodied in the radiation and chemical paradigms—i.e., lifetime cancer risks less than values in the range of about 10ⳮ1 to 10ⳮ3 in the radiation paradigm but less than about 10ⳮ4 to 10ⳮ6 in the chemical paradigm—appear to be inconsistent. This seeming inconsistency, if not properly understood, leads to the misleading and improper conclusion that risk management based on the chemical paradigm is more stringent (achieves lower levels of risk). However, this inconsistency is more perceived than real, and it results essentially from the different meanings of ‘‘acceptable’’ and ‘‘unacceptable’’ in the two paradigms. The different meanings of these terms are summarized in Table 3.5 and discussed below. In the radiation paradigm, ‘‘unacceptable’’ clearly means ‘‘intolerable’’ because this term describes risks so high that they normally must be reduced regardless of cost or other circumstances, and ‘‘acceptable’’ is used to describe risks below intolerable levels that also are ALARA, i.e., risks that are ‘‘optimized.’’ The radiation paradigm also includes the concept of a risk so low that further reductions in risk using the ALARA principle would be unwarranted, but such low risks are termed ‘‘negligible,’’ rather than ‘‘acceptable,’’ to distinguish them from higher risks that are acceptable if they are ALARA. In contrast, ‘‘acceptable’’ in the chemical paradigm usually means ‘‘negligible’’ because further reductions in risk usually need not be considered even if they would be cost-effective, and ‘‘unacceptable’’

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TABLE 3.5—Differences in interpretations of ‘‘acceptable’’ and ‘‘unacceptable’’ risks in radiation and chemical paradigms for management of stochastic risks.a Description of Risk

Interpretation in Radiation Paradigmb

Interpretation in Chemical Paradigmc

‘‘Acceptable’’

Risks are below intolerable (de manifestis) levels and are ALARAd

Risks are negligible (de minimis); further reduction of risks usually need not be considerede

‘‘Unacceptable’’

Risks are intolerable; risks normally must be reduced below intolerable levels regardless of costf

Risks are above negligible levels; reduction of risks must be considered but is required only to the extent feasibleg

a

Differences in interpretations of ‘‘acceptable’’ and ‘‘unacceptable’’ in the two paradigms also apply to dose when regulations are expressed in terms of dose rather than risk; dose is commonly used in regulating radionuclides under either paradigm. b Interpretations apply to control of exposures to radionuclides under AEA, but not to control of exposures to radionuclides under other environmental laws. c Interpretations also apply to control of exposures to radionuclides when they are regulated under laws addressing hazardous chemicals. d Lifetime cancer risks considered intolerable have values in the range of about 10ⳮ1 to 10ⳮ3 or greater, with the particular value depending on the exposure situation, and are well above risks considered negligible (e.g., lifetime risks having a value less than about 10ⳮ4). Risks that are ALARA depend on the particular exposure situation, and achieving a negligible risk is not the goal of ALARA. e Lifetime cancer risks considered negligible have values in the range of about 10ⳮ4 to 10ⳮ6 or below, with the particular value depending on the exposure situation. f Risks also are considered unacceptable if they are below intolerable levels but are not ALARA. g Approach to risk management for hazardous chemicals does not explicitly include concept of intolerable risk that normally must be reduced regardless of cost or other circumstances.

usually refers to any risks sufficiently high that they are not unconditionally acceptable. That is, while reduction of risks above ‘‘acceptable’’ levels must be considered, risk reduction usually is not required unless it would be practicable based, for example, on considerations

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of technical feasibility and cost. It clearly is not the case that any risks above ‘‘acceptable’’ levels are intolerable. Thus, perhaps the most important substantive difference in the two approaches to management of stochastic risks is that the radiation paradigm clearly and explicitly incorporates the concept of an intolerable risk that normally must be reduced in any exposure situation regardless of cost or other circumstances, whereas the chemical paradigm does not. Only the radiation paradigm explicitly recognizes that there is a wide range of risks between intolerable and negligible levels where risk management decisions are made based on considerations of cost-benefit and other societal concerns (the ALARA principle). The concept of an intolerable risk that is well above negligible levels has been acknowledged, at least implicitly, by EPA in regulating hazardous chemicals. As shown in the analysis of case-by-case regulatory decisions prior to 1985 in Figure 3.13, EPA always acted to reduce risks having a value above a range of about 10ⳮ2 to 10ⳮ3, thus defining a de facto maximum tolerable risk. As noted previously, for risks in the middle region of Figure 3.13, reduction of risks was required in some cases, but not in others, based primarily on considerations of cost-benefit (Travis et al., 1987). As indicated in Figure 3.13 and previous discussions, there is an important similarity in the radiation and chemical paradigms that overrides any differences, both perceived and real. In spite of the difference in the basic approach to risk management (limits plus ALARA in the radiation paradigm, in contrast to goals plus allowance for relaxation in the chemical paradigm) and in spite of the different meanings attached to the terms ‘‘acceptable’’ and ‘‘unacceptable’’ risk in the two paradigms, application of the ALARA principle essentially is the basis for almost all risk-management decisions, without regard for the particular paradigm being applied (Kocher, 1999; NAS/NRC, 1999a). Application of the ALARA principle is an explicit requirement in the radiation paradigm, and it has been so successful that the dose limit defining a maximum acceptable (barely tolerable) risk from exposure to all controlled sources combined essentially plays no role in regulating routine public exposures to man-made sources. In the chemical paradigm, application of the ALARA principle is the primary basis for virtually all risk-management decisions, particularly when the goal for acceptable (i.e., negligible) risk cannot be achieved at any cost but also when standards have been established that define allowable exposures for particular situations based on considerations of technical feasibility and cost (e.g., drinking water standards). This is the case even though the term ALARA does not appear explicitly in laws or regulations that embody the chemical paradigm.

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3.3.4 Reconciliation of the Radiation and Chemical Paradigms Based on discussions similar to those in Section 3.3.3 on the similarities and differences in the radiation and chemical paradigms for management of stochastic risks, Kocher and Hoffman (1991) developed a proposed regulatory framework that would apply to all substances in the environment that cause stochastic effects, including those that are naturally occurring, and to any routine or accidental exposure situation. This framework is intended to be consistent with the two risk-management paradigms while also addressing the seeming inconsistencies and ambiguities in their present applications. In the framework for regulating public exposures to all substances that cause stochastic effects proposed by Kocher and Hoffman (1991), three regions of risk depicted in Figure 3.14 are defined:

Fig. 3.14. Unified framework for regulating all radionuclides and chemicals in the environment that cause stochastic effects proposed by Kocher and Hoffman (1991).

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1. a negligible (de minimis) excess lifetime risk having a value in the range of about 10ⳮ4 to 10ⳮ6 and below, where risks from any substance and source of exposure would be so low that efforts at further risk reduction would be unwarranted; 2. an intolerable (de manifestis) excess lifetime risk having a value in the range of about 10ⳮ1 to 10ⳮ3 and above, where reduction of risks normally would be required regardless of cost or other circumstances; and 3. excess lifetime risks between de minimis and de manifestis levels, where efforts to reduce risk would be based on application of the ALARA principle, with the proviso that achieving a de minimis risk is not the goal of ALARA. The use of ranges for the de manifestis and de minimis risks, rather than single values, would allow consideration of the size of an exposed population. For example, higher levels could be used when only a few individuals are at risk, but lower levels could be used for large populations. The use of ranges also would allow considerable flexibility in accommodating the kinds of subjective societal judgments involved in applying the ALARA principle to particular exposure situations. Kocher (1999) has shown that this regulatory framework is consistent with all current regulatory policies for limiting routine or accidental exposures of the public to radionuclides and chemicals that cause stochastic effects. Perhaps the most important consideration in developing a consistent approach to regulating all substances in the environment that induce stochastic effects would be to recognize the primary importance of the ALARA principle in risk-management decisions, without regard for the particular risk management paradigm being applied. Another important consideration would be to achieve consensus on a clear and unambiguous meaning of the term ‘‘unacceptable’’ in regard to risk of stochastic effects. A consistent regulatory approach would be greatly aided if ‘‘unacceptable’’ were used only to describe an intolerable risk, rather than any risk above negligible levels. Such a consistent interpretation would address the widespread confusion concerning the difference between a dose (risk) limit in the radiation paradigm and a risk goal in the chemical paradigm. Similarly, a consistent approach to regulation would be aided by an understanding that ‘‘acceptable’’ means not only that a risk is negligible but also that a risk is below intolerable levels and is ALARA. Such interpretations of these terms would be completely consistent with most risk management decisions that have been made using the radiation and chemical paradigms.

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3.3.5 Application of Risk Management Paradigms to Waste Classification A proper reconciliation of the radiation and chemical paradigms for risk management is important to the development of a comprehensive and risk-based hazardous waste classification system. In particular, the proposed waste classification system developed in Sections 6.2 and 6.3 of this Report is based fundamentally on the concept that an acceptable risk generally can be substantially greater than a negligible risk. This distinction is used to define different classes of waste that pose an increasing hazard.

3.4 Summary Section 3 has discussed issues of risk assessment and risk management for radionuclides and hazardous chemicals. These discussions provide important background information for the development of a comprehensive and risk-based hazardous waste classification system. A basic premise of this Report is that waste that contains hazardous substances should be classified based on considerations of risks resulting from disposal. In the context of waste classification, ‘‘risk’’ is the probability that an adverse health effect (response) would result from disposal of hazardous waste, taking into account (1) the probabilities of all processes and events that could result in exposure of humans, (2) the magnitude of such exposures, and (3) the probability that an exposure would result in a response. Probabilities of processes or events that could result in exposures may be considered only qualitatively (e.g., as credible or non-credible occurrences), but the probability of a response from a given exposure generally must be considered quantitatively. Waste would be classified based on risk by comparing an estimated risk resulting from disposal of a unit amount or concentration of a hazardous substance using a particular disposal technology with a specified allowable risk, thus yielding an allowable amount or concentration of the substance in the waste class associated with that disposal option. In classifying waste based on considerations of risks resulting from disposal, hypothetical and generic disposal sites or abstractions of real sites must be considered, because waste may be classified before disposal sites are chosen and multiple disposal sites may be used for similar wastes. An important characteristic of waste disposal systems is that risks resulting from release of hazardous substances and transport beyond the site boundary are highly site-specific,

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whereas risks resulting from inadvertent human intrusion into a disposal facility are much less dependent on the disposal site. Furthermore, assessments of near-surface waste disposal facilities have shown that risks to inadvertent intruders generally are higher than risks to individuals beyond the site boundary, given an assumption that exposures of intruders would occur according to postulated scenarios. Thus, assessments of risk associated with scenarios for inadvertent intrusion at waste disposal sites can be used to develop a risk-based waste classification system. Risks to members of the public beyond the site boundary also are an important consideration in determining acceptable disposal practices, and these risks are taken into account in developing site-specific waste acceptance criteria. A second basic premise of this Report is that a waste classification system should apply to waste that contains radionuclides, hazardous chemicals, or mixtures of the two, and that the approaches to risk assessment and risk management used in classifying waste should be reasonably consistent for the two types of substances. The process of risk assessment per se is quite similar for radionuclides and hazardous chemicals, and there are important similarities in the ways that deterministic and stochastic responses are treated in risk assessment and risk management. Deterministic responses generally are treated by identifying a threshold in the dose-response relationship and applying safety and uncertainty factors to limit exposures to levels well below the threshold. Furthermore, incidence is the measure of deterministic response used for all hazardous substances. Similarly, stochastic responses generally are treated by assuming that the probability of a response is linearly proportional to dose, without threshold, and this relationship is used to establish limits on exposure that are intended to limit the probability of occurrence of stochastic responses. However, as summarized below, there are important differences in the ways that the dose-response relationships for radionuclides and hazardous chemicals are used in risk assessment and risk management. 1. In setting limits on exposure intended to prevent the occurrence of deterministic responses, the safety and uncertainty factors that are applied to the assumed thresholds for hazardous chemicals that cause deterministic effects usually are considerably larger (by at least a factor of 10) than the safety factor normally applied to the thresholds for deterministic responses from exposure to radiation. Furthermore, the assumed threshold usually is more conservative for hazardous chemicals than for radiation (i.e., a lower confidence limit of the threshold often is used for

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chemicals, but a best estimate generally is used for radiation). These differences result, in part, from the much greater reliance on studies in animals for hazardous chemicals compared with radiation and the uncertainty in applying animal data to humans. 2. In establishing dose-response relationships for stochastic effects, primarily cancers, for use in health protection, there are three differences in the approaches used for radionuclides and hazardous chemicals: ● The dose-response relationship for radionuclides is intended to be a best estimate, whereas the dose-response relationship for chemicals that cause stochastic effects is intended to be an upper-bound estimate (UCL). ● The primary measure of stochastic responses for radionuclides used in radiation protection has been fatalities, whereas incidence is the universal measure of stochastic responses for hazardous chemicals. ● Assessments of stochastic responses for radionuclides take into account all organs and tissues at risk from a given exposure, whereas assessments for hazardous chemicals that cause stochastic effects usually are based on observed responses in a single organ or tissue in study animals. Deterministic responses from exposure to hazardous chemicals generally are of concern in health protection of the public because many of the exposure limits derived from the assumed thresholds and the applied safety and uncertainty factors fall within the range of potential routine exposures. However, the possibility that the large safety and uncertainty factors normally used in setting exposure limits are quite conservative (pessimistic) could be taken into account in developing a risk-based waste classification system. Deterministic responses from exposure to radionuclides should not be of concern in health protection of the public or in classifying waste, because the dose limits intended to prevent deterministic responses are substantially higher than the dose limit intended to limit the occurrence of stochastic responses. Stochastic responses from exposure to radionuclides and hazardous chemicals generally are of concern in health protection of the public and in classifying waste. Of the three differences in approaches to dose-response assessment identified above, the most important is the use of a best estimate (MLE) of the dose-response relationship for radionuclides but upper-bound estimates (UCLs) for hazardous chemicals that cause stochastic effects. UCL in the doseresponse relationship for chemicals that cause stochastic effects normally exceeds MLE by a factor of 5 to 100 or more. If this difference

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is not reconciled, unequal weight would be given to radionuclides and chemicals in classifying waste, and the weight given to chemicals could be far out of proportion to the potential stochastic risks. The difference between using fatalities or incidence as the measure of stochastic response is unlikely to be important in classifying waste because, on average, about 60 to 70 percent of all stochastic responses are fatal. The difference in approaches to accounting for the organs and tissues at risk would be important only if several organs were at risk from exposure to chemicals and the probability of a response were about the same in all organs at risks. This situation is expected to occur only rarely, if ever. An essential consideration in developing a risk-based waste classification system is the levels of acceptable risk that should be assumed in classifying waste. Therefore, an important concern in developing a comprehensive waste classification system is the different approaches to management of stochastic risks that have been used for radionuclides and hazardous chemicals. The radiation paradigm for management of stochastic risks is based on the principles of radiation protection developed by ICRP and NCRP. In this paradigm, stochastic risks are managed by (1) establishing a limit on dose (and therefore risk) from routine exposure to all controlled sources combined, which has the interpretation that doses (risks) above the limit normally are intolerable and must be reduced regardless of cost or other circumstances, and (2) requiring that doses be reduced below the limit ALARA, taking into account cost-benefit and other societal concerns. The approach of establishing a limit and requiring reductions below the limit is referred to as ‘‘top-down.’’ The radiation paradigm also includes the concept that there are risks so low that they generally need not be reduced (i.e., the risks are negligible), but this concept has not been incorporated in laws and regulations in the United States that implement the radiation paradigm. The chemical paradigm for management of stochastic risks essentially is the opposite of the radiation paradigm, and is referred to as ‘‘bottom-up.’’ In this approach, a goal for acceptable risk is established, but the goal may be increased (relaxed) based, for example, on considerations of cost-benefit and technical feasibility. The degree of allowable relaxation in the goal for acceptable risk depends on the exposure situation. Thus, the goal for acceptable risk in the chemical paradigm clearly does not have the same meaning as the limit on acceptable risk in the radiation paradigm. The chemical paradigm does not explicitly include the concept of an intolerable risk that normally must be reduced regardless of cost or other circumstances, as defined by the dose limit in the radiation paradigm. The

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two paradigms for management of stochastic risks also differ in that regulations established under the chemical paradigm apply only to particular substances, release or exposure pathways, or sources, whereas the dose limit in the radiation paradigm applies to all controlled sources combined. The two paradigms for management of stochastic risks can be reconciled based on two considerations. The first is a recognition that the terms ‘‘unacceptable’’ and ‘‘acceptable’’ in regard to risk have different meanings in the two paradigms. The term ‘‘unacceptable’’ means ‘‘intolerable’’ in the radiation paradigm but ‘‘non-negligible’’ in the chemical paradigm, whereas ‘‘acceptable’’ essentially means ALARA in the radiation paradigm but ‘‘negligible’’ in the chemical paradigm. These differences are a particularly important consideration in applying the two risk management paradigms to waste classification. The second important consideration in reconciling the two risk management paradigms is the realization that the ALARA principle is the single most important factor in risk management decisions for radionuclides and hazardous chemicals, without regard for the particular risk management paradigm being applied.

4. Existing Classification Systems for Hazardous Wastes

Wastes have been classified for decades for a variety of purposes. This Section discusses the historical development of classification systems for radioactive and hazardous chemical wastes and the resulting classification systems in use at the present time. The relationship between waste classification and requirements for disposal of different classes of hazardous waste is emphasized. The framework for this discussion is the top-level system for waste classification in the United States shown in Figure 4.1. Within this framework, it is first determined whether a waste is nonhazardous (e.g., municipal waste); these wastes are not addressed in this Report. If a waste is deemed hazardous, it is so classified due to the presence of radionuclides or hazardous chemicals. Mixed radioactive and hazardous chemical waste is not a separate class of waste. However, mixed waste has been an important concern as a result of differences in requirements for management and disposal of radioactive and hazardous chemical wastes. Section 4.1 addresses classification and disposal of radioactive waste, and is followed by discussions of classification and disposal of hazardous chemical waste in Section 4.2 and approaches to management of mixed radioactive and hazardous chemical waste in Section 4.3. Finally, Section 4.4 summarizes previous NCRP recommendations relevant to waste classification. The discussions of classification of radioactive and hazardous chemical wastes and management of mixed waste in Sections 4.1 to 4.3 are presented in considerable detail to facilitate understanding of these issues by readers who may not be knowledgeable in these areas. The existing hazardous waste classification systems and the historical developments underlying them are complex. NCRP believes that an appreciation of these complexities is important in gaining an understanding of the need for a new hazardous waste classification system and the benefits it would provide. 165

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Fig. 4.1. Top-level waste classification system in the United States.

4.1 Classification and Disposal of Radioactive Waste This Section discusses the historical development and current approaches to classification and disposal of radioactive waste. Classification and requirements for disposal of different radioactive wastes in the United States are emphasized, particularly the relationship between waste classification and requirements for disposal; much of this discussion is adapted from a previous paper (Kocher, 1990). Proposals for alternative radioactive waste classification systems are reviewed. Classification systems developed by the International Atomic Energy Agency (IAEA) and the relationship between waste classification and disposal requirements in IAEA recommendations are discussed in some detail. Waste classification systems developed in other countries are briefly mentioned.

4.1.1 Background Classification of radioactive waste has been facilitated by two considerations. The first is that radiation dose provides a common measure of potential health impacts from exposure to any radionuclide and for any exposure situation (see Section 3.2.2). All classification systems for radioactive waste take into account, at least to some

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degree, potential doses to individuals who might be exposed to waste materials. A second consideration that has been important, at least implicitly, in developing classification systems for radioactive waste is natural background radiation. The presence of a ubiquitous and unavoidable background of radiation and its description in terms of radiation dose provide a measure of the significance of potential exposures of radiation workers and members of the public to any radioactive waste. Levels of radiation in waste materials compared with levels of natural background radiation have played an important role in radioactive waste classification.

4.1.2 Radioactive Waste Classification in the United States 4.1.2.1 Introduction. This Section reviews the classification system for radioactive waste that has been developed in the United States. The historical development of radioactive waste classes is emphasized to provide an understanding of the present classification system and an appreciation of why a new system would be beneficial. Descriptions of waste classes developed prior to the current definitions in laws and regulations are discussed first, followed by statutory and regulatory definitions developed over the last three decades. Requirements for permanent disposal of different classes of radioactive waste also are described. The relationship between requirements for waste disposal and its classification is important in developing an understanding of the present system for classifying radioactive waste in the United States. To provide a focus for these discussions, the current definitions of different classes of radioactive waste in the United States and the intended disposal systems (technologies) for the different waste classes are summarized in Table 4.1. The use of nuclear reactors to produce fissionable materials for defense purposes, beginning in the 1940s, and to generate electric power in the commercial sector, beginning in the 1950s, has been the most important source of radioactive waste requiring management and disposal. In operations of the nuclear fuel cycle,9 radioactive waste produced in the commercial sector often is distinguished from waste that arises from atomic energy defense activities, but this distinction usually is not important in classifying waste. However, 9 For the purposes of this Report, the term ‘‘nuclear fuel cycle’’ encompasses the production, utilization, and disposition of the fuel used in fission reactors for electricity generation, research and development, or production of nuclear materials for any purpose, and any byproduct materials that arise from or are associated with these activities.

Geologic repositorye

Primary waste from chemical reprocessing of spent nuclear fueld Waste that contains more than 4 kBq gⳮ1 of alpha-emitting transuranium radionuclides with half-lives greater than 20 y, excluding high-level waste Any waste not classified as spent fuel, highlevel waste, transuranic waste, or uranium or thorium mill tailings Residues from chemical processing of ores for their source material (i.e., uranium or thorium) content

High-level waste

Transuranic waste

Low-level waste

Mill tailings

No coordinated federal policy for disposalh No radiological restrictions

Any waste that does not arise from operations of nuclear fuel cycle Determined on case-by-case basisi

NARM waste

Exempt waste

Near-surface disposal in situ or at processing site;g small volumes may be managed as low-level waste

Near-surface disposal system or, for highactivity, longer-lived waste, geologic repositoryf

Geologic repositoryc

Irradiated nuclear fuel that has not been chemically reprocessed

Geologic repositoryc

Intended Disposal Systema

Spent fuelb

Fuel-cycle waste

Definition

/

Class

TABLE 4.1—Summary of current definitions of radioactive waste classes and intended disposal systems for different waste classes in the United States.

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Under current law and regulations, particular disposal systems generally are not required for any waste class, but only certain systems are authorized in law. Disposal systems other than those listed, such as greater confinement disposal at depths intermediate between a near-surface facility and a geologic repository, may be used for some wastes. b Spent fuel is not waste until it is so declared. In some laws and regulations, spent fuel is not distinguished from high-level waste for purposes of waste classification. c The Yucca Mountain site in Nevada is the only candidate disposal facility authorized in law. d Certain incidental wastes that arise in fuel reprocessing or further processing of reprocessing wastes have been excluded from high-level waste on a case-by-case basis. e Waste Isolation Pilot Plant (WIPP) in New Mexico is only disposal facility authorized in law; commercial transuranic waste also may be acceptable for near-surface disposal on a case-by-case basis. f For commercial low-level waste, limits on concentrations of radionuclides that are generally acceptable for near-surface disposal are the Class-C limits specified in NRC’s 10 CFR Part 61 (NRC 1982a). g Large volumes of mill tailings are not intended for disposal in dedicated facilities for low-level waste. h At DOE sites, large volumes of waste that contains relatively low concentrations of naturally occurring radionuclides normally are managed as mill tailings, and small volumes of waste that contains relatively high concentrations of NARM are usually managed as low-level waste. i Many specific wastes that contain small amounts of radionuclides have been exempted from regulatory control, but a general class of exempt waste has not been established in law or regulations. NRC currently is prohibited by law from establishing such a waste class.

a

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radioactive wastes also arise from activities not associated with the nuclear fuel cycle, and the distinction between fuel-cycle and NARM waste has been important in waste classification. The present system for classification of radioactive waste in the United States may be depicted in the form of a hierarchy shown in Figure 4.2. As indicated at the top of the figure, the first distinction is between waste associated with the nuclear fuel cycle and any other radioactive waste. The latter category includes any waste that contains naturally occurring radioactive material (NORM) other than that associated with the nuclear fuel cycle (e.g., radium waste produced in treating drinking water) or radioactive material produced in an accelerator. These two types of radioactive material not associated with the nuclear fuel cycle are called naturally occurring and accelerator-produced radioactive material (NARM). The distinction between fuel-cycle and NARM waste originates in law, as described below, but is largely artificial with regard to requirements for safe management and disposal of waste because this distinction is based on the source of the waste rather than its radiological properties. Radioactive wastes that arise from operations of the nuclear fuel cycle are divided into five classes, called spent nuclear fuel, highlevel waste, transuranic waste, low-level waste, and uranium or thorium mill tailings. At the present time, NARM wastes are not formally divided into different classes (see Section 4.1.2.4). The division of all radioactive waste into fuel-cycle and NARM waste and the division of fuel-cycle waste into five classes constitutes the basic classification system for radioactive waste in the United States.

Fig. 4.2. Current United States radioactive waste classification system.

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The lowest level of the hierarchy in Figure 4.2 represents further classifications of transuranic waste and low-level waste that arises from operations of the nuclear fuel cycle and a further classification of NORM waste that represents various state regulations specifying concentrations of naturally occurring radionuclides, especially radium, below which the materials are not regulated as radioactive waste. This level of the hierarchy is not part of the basic radioactive waste classification system in the United States that identifies broad categories of waste. The further classifications of transuranic waste, low-level waste, and NORM waste are used primarily for planning purposes in developing specific systems for waste management and disposal, rather than for the purpose of identifying, in general terms, the type of disposal technology that might be acceptable for a broad class of waste. The distinction between radioactive waste associated with the nuclear fuel-cycle and NARM waste shown in Figure 4.2 arises from provisions of AEA (1954), which governs the production and use of so-called source, special nuclear, and byproduct materials for defense and peaceful purposes. Source material is defined as (1) uranium or thorium, or (2) ores that contain more than 0.05 percent by weight of either of these elements, except source material does not include special nuclear material. Thus, source material essentially is the raw material from which nuclear fuel is made. Excluding source material, special nuclear material is defined as (1) plutonium, 233U, or uranium enriched in 233 U or 235 U, or (2) materials artificially enriched by any of these isotopes. Thus, special nuclear material is the fissionable material used in nuclear reactors or nuclear weapons. Byproduct material is defined as (1) any radioactive material, except special nuclear material, resulting from production or use of special nuclear material, and (2) uranium or thorium mill tailings. Because these are the only radioactive materials defined in AEA, the Act governs classification and disposal of radioactive wastes only if they arise from operations of the nuclear fuel cycle, but the Act does not govern classification and disposal of NARM waste. The important distinction between fuel-cycle materials and NARM originated in the security and safeguards aspects of the early nuclear weapons program. AEA governs the processing and use of source, special nuclear, and byproduct materials in the commercial sector under licenses issued by NRC or Agreement States (states that enter into licensing agreements with NRC). Since all licensing activities of NRC are performed under authority of AEA, NRC has no licensing authority over NARM waste generated in the commercial sector (Agreement and non-Agreement States may regulate NARM waste under state

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statutes). AEA, as amended by the Energy Reorganization Act of 1974 (ERA, 1974) and DOE Organization Act of 1977 (DEOA, 1977), also governs atomic energy defense and research and development activities of DOE. Under AEA, DOE also is responsible for management and disposal of any NARM waste that arises from its authorized activities. EPA could regulate commercial or DOE NARM waste under TSCA (1976) or RCRA (1976). EPA also regulates NARM waste at Superfund sites subject to remediation under CERCLA (1980). However, for all practical purposes, federal regulation of NARM waste at the present time depends on whether the waste originates in commercial activities, where it is regulated only by the states, or in DOE activities, in which case it is self-regulated by DOE. 4.1.2.2 Early Descriptions of Radioactive Waste Categories. The following sections discuss the earliest categories of radioactive waste that were developed prior to the current legal and regulatory definitions of waste classes. These categories applied only to waste that arises from operations of the nuclear fuel cycle. 4.1.2.2.1 Liquid wastes. Historically, the most important radioactive wastes have been liquid wastes that arise from chemical reprocessing of spent nuclear fuel for defense production purposes, i.e., for the purpose of extracting plutonium for use in nuclear weapons. These wastes contain varying concentrations of many radionuclides, primarily fission products and long-lived, alpha-emitting transuranium isotopes. Three categories of liquid radioactive waste from fuel reprocessing, containing decreasing concentrations of radionuclides, were first described and used at U.S. Atomic Energy Commission (AEC) production sites in the late 1950s (Lennemann, 1972). High-level waste contained the highest concentrations of radionuclides and required confinement and storage in underground tanks. Liquid high-level waste contained high concentrations of fission products, including 90 Sr and 137Cs, and long-lived radionuclides, principally alpha-emitting transuranium isotopes such as 239Pu and 241Am. Liquid highlevel waste was further categorized as self-boiling or non-boiling. Self-boiling waste was waste with high levels of decay heat that required engineered cooling systems during storage, whereas nonboiling waste was waste with lower levels of decay heat that required only natural cooling during storage. Medium- or intermediate-level waste contained lower concentrations of radionuclides than highlevel waste and could be released to underground structures or seepage basins. Finally, low-level waste contained the lowest concentrations of radionuclides in liquid reprocessing waste and could be released to holding ponds and lagoons or directly to surface waters.

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It is important to emphasize that the earliest classification system for radioactive waste described above was not based on considerations of potential impacts on public health or the environment following permanent disposal of waste. Rather, these descriptions of categories of liquid wastes from reprocessing of spent nuclear fuel were based primarily on operational requirements for safe handling and storage of waste, taking into account the widely varying levels of radioactivity in different waste streams (Lennemann, 1972). That is, the primary impetus for waste classification was the need to protect workers from radiation exposure during waste operations. Each AEC site that reprocessed spent nuclear fuel developed its own limits on radionuclide concentrations in the three categories of liquid waste described above, based on site-specific operating practices and environmental conditions (Beard and Godfrey, 1967; Marter, 1967). At the Hanford site, for example, the following limits were used to classify liquid wastes from fuel reprocessing at 100 to 200 d after discharge from a reactor (Beard and Godfrey, 1967): high-level waste contained total activity concentrations greater than 3.7 TBq mⳮ3, intermediate-level waste contained total activity concentrations between 1.9 MBq mⳮ3, and 3.7 TBq mⳮ3 and low-level waste contained total activity concentrations less than 1.9 MBq mⳮ3. Since this classification of liquid wastes was based on the total activity concentration of all radionuclides, classification of these wastes was based primarily on the concentrations of relatively short-lived fission products. These radionuclides are quite important in protection of workers, but they are relatively unimportant in regard to long-term impacts on public health and the environment following permanent disposal of waste. 4.1.2.2.2 Solid wastes. The development of different categories of solid radioactive waste began in 1960 when AEC initiated interim shallow-land burial services for solid wastes generated in the private sector (e.g., at nuclear power plants) until disposal facilities for commercial waste could be developed. The following three categories of solid waste that was acceptable for shallow-land burial, containing decreasing concentrations of radionuclides, were defined (Lennemann, 1967): high-level waste contained total activity concentrations greater than 1,300 TBq mⳮ3, intermediate-level waste contained total activity concentrations between 13 and 1,300 TBq mⳮ3, and low-level waste contained total activity concentrations less than 13 TBq mⳮ3. As in the earliest classifications of liquid reprocessing wastes discussed in the previous section, these descriptions were based primarily on operational requirements for protection of workers during waste handling at generating sites, rather than requirements for protection

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of public health and the environment at disposal sites. By comparison with the descriptions of liquid waste classes at the Hanford site given in the previous section, the earliest descriptions of different classes of solid waste were not based on those for the corresponding classes of liquid waste. The differing descriptions of classes of liquid and solid waste resulted primarily from differences in radionuclide compositions (i.e., the source of the waste) and methods of handling liquid and solid wastes. In the late 1960s, a fourth category of solid waste, first called alphabearing waste but later transuranic waste, came into use at AEC sites. Transuranic waste arose primarily from processing of materials containing plutonium or 233U that were obtained from chemical reprocessing of spent uranium or thorium fuel.10 This category of solid waste contained relatively high concentrations of long-lived, alpha-emitting transuranium radionuclides or 233U but generally lower concentrations of beta/gamma-emitting fission products than higher-activity liquid wastes from fuel reprocessing. As shown in Figure 4.2, transuranic waste was further classified as ‘‘contact-handled’’ if it required little or no shielding or ‘‘remotely-handled’’ if it required shielding or remote handling to protect workers from high levels of external photon or neutron radiation. The subclasses of transuranic waste thus were based only on requirements for safe handling and storage but were not based on requirements for permanent disposal. In 1970, AEC established a policy that solid waste with concentrations of certain alpha-emitting radionuclides, including long-lived transuranium radionuclides and 233U, greater than 0.4 kBq gⳮ1 was not acceptable for shallow-land burial but required storage or burial in a retrievable manner (Hollingsworth, 1970). Transuranic waste thus referred to solid waste with concentrations of alpha-emitting radionuclides greater than 0.4 kBq gⳮ1. The concentration limit for shallow-land burial of solid waste that contains certain alpha-emitting radionuclides was based on the higher concentrations of radium, also an alpha-emitting radionuclide, that occur naturally in Earth’s crust. That is, shallow-land burial of waste with concentrations of alpha-emitting radionuclides less than 0.4 kBq gⳮ1 was regarded as acceptable because the resulting radiation doses to the public should not be significantly greater than the unavoidable dose due to naturally occurring radium and its decay products in surface soil. This 10 According to strict interpretation of ‘‘transuranic,’’ these wastes could be so classified only if they contained sufficient amounts of elements having an atomic number greater than 92. Despite this, wastes that contained sufficient amounts of 233U and other alpha-emitting non-transuranium radionuclides often were classified and managed as transuranic waste because their specific activities (activities per unit mass) are more similar to those of the transuranium elements than to natural uranium or thorium.

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definition of transuranic waste represented the first quantification of a waste class based primarily on considerations of protection of public health following permanent disposal of solid waste. As discussed in the previous section, low-level waste originally included any waste with concentrations of radionuclides less than those in high-level or intermediate-level waste, and the descriptions of low-level waste were based primarily on operational requirements for protection of workers at waste generating sites, rather than requirements for protection of public health and the environment following permanent disposal. However, as described in more detail in Section 4.1.2.3.3, the definition of low-level waste later was broadened to include any radioactive waste that arises from operations of the nuclear fuel cycle other than spent nuclear fuel, high-level waste, transuranic waste, or uranium or thorium mill tailings. This definition was no longer related to requirements for safe handling and storage of waste or permanent disposal, because low-level waste was no longer restricted to containing relatively low concentrations of any radionuclides and could include waste destined for different disposal systems (e.g., a near-surface disposal facility or a geologic repository). Indeed, low-level waste could contain very high concentrations of relatively short-lived beta/gamma-emitting radionuclides as well as high concentrations of long-lived fission or activation products, for example. Thus, low-level waste could require extensive shielding to protect workers during waste operations or storage and disposal far below the ground surface to protect public health. 4.1.2.2.3 Summary of bases for early descriptions of radioactive wastes. The earliest descriptions of different classes of radioactive waste were based primarily on operational requirements for safe handling and storage of liquid wastes that arise from a particular source, namely, chemical reprocessing of spent nuclear fuel. These descriptions were extended to reflect operational requirements for safe handling and storage of solid waste as well. Thus, protection of public health and the environment following permanent disposal of waste was not a primary consideration in developing the earliest descriptions of different waste classes. Later on, a description of transuranic waste was developed and quantified based on considerations of protecting public health following shallow-land burial of solid waste that contains alpha-emitting radionuclides. 4.1.2.3 Classification and Disposal of Wastes from the Nuclear Fuel Cycle. This Section discusses the different classes of radioactive waste that arise from operations of the nuclear fuel cycle that have been defined in laws and regulations over the last three decades and

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requirements for permanent disposal of the different classes of fuelcycle waste. The discussions particularly emphasize the relationship between waste classification and requirements for disposal. 4.1.2.3.1 High-level waste and spent fuel. The earliest descriptions of different classes of liquid and solid radioactive wastes were based primarily on requirements for safe operations, rather than requirements for permanent disposal (Section 4.1.2.2). These descriptions have greatly influenced the present classification system for radioactive waste that arises from operations of the nuclear fuel cycle in the United States, even though permanent disposal of solid waste is now the desired endpoint of radioactive waste management. The definition of high-level waste is the most important because the definitions of all other classes of fuel-cycle waste, except uranium or thorium mill tailings, depend on the definition of high-level waste. Statutory and Regulatory Definitions. The first regulatory definition of high-level waste was developed by AEC in 1970 and is contained in 10 CFR Part 50, Appendix F (AEC, 1970). Specifically: High-level waste is the aqueous wastes resulting from operation of a first-cycle solvent extraction system, or equivalent, and concentrated wastes from subsequent extraction cycles, or equivalent, in a facility for fuel reprocessing. High-level waste thus includes the concentrated wastes that arise from reprocessing of commercial or defense nuclear fuel that contain virtually all the fission products and transuranium radionuclides (except plutonium) in spent fuel. However, the definition does not mention the constituents of the waste, and it is only qualitative because ‘‘concentrated’’ is not quantified and the minimum fuel burnup that would yield high-level waste is not specified. Although the definition given above referred only to liquid (aqueous) waste, it is clear from further discussions in 10 CFR Part 50, Appendix F (AEC, 1970), that AEC intended that high-level waste also would include concentrated solid waste derived from liquid high-level waste that was suitable for permanent disposal. The definition of high-level waste developed by AEC was based on the traditional source-based description of high-level waste discussed in Section 4.1.2.2.1; i.e., high-level waste is the primary waste from fuel reprocessing. Thus, the definition implies that high-level waste (1) produces high levels of decay heat and external radiation, due primarily to the high concentrations of shorter-lived fission products, and (2) requires long-term isolation from the biosphere in order to protect public health (AEC, 1969a), due primarily to the high

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concentrations of long-lived, alpha-emitting transuranium radionuclides. High-level waste was defined in terms of its source, rather than its radiological properties, mainly because reprocessing of spent nuclear fuel was the only significant source of waste with these properties at that time. Because the definition of high-level waste developed by AEC was only qualitative, there was some ambiguity regarding materials from fuel reprocessing that would be included in high-level waste. AEC and, later, NRC have indicated that, in their view, high-level waste does not include the following: (1) metal cladding used to contain fuel material and other irradiated and contaminated fuel structural hardware; (2) incidental wastes that arise in operations of reprocessing plants, such as ion-exchange beds or sludges; or (3) incidental wastes generated in further treatment of high-level waste, such as decontaminated salts containing substantially lower concentrations of 90Sr, 137Cs, and plutonium than first-cycle solvent extraction wastes (AEC, 1969b; NRC, 1987). Wastes that have been excluded from high-level waste generally have lower concentrations of fission products and transuranium radionuclides than wastes that arise directly from fuel reprocessing. However, general principles, such as limits on concentrations of radionuclides or levels of external radiation, have not been established for excluding incidental wastes that arise from fuel reprocessing from high-level waste, and decisions regarding classification of such wastes have been made only on a case-bycase basis. Early statutory definitions of high-level waste are contained in the Marine Protection, Research and Sanctuaries Act of 1972 (MPRSA, 1972) and the West Valley Demonstration Project Act of 1980 (WVDPA, 1980). These definitions are consistent with the definition developed by AEC. NRC’s current regulatory definition of high-level waste is contained in 10 CFR Part 60 (NRC, 1983). Specifically: High-level waste is: (1) irradiated reactor fuel; (2) liquid wastes resulting from operation of a first-cycle solvent extraction system, or equivalent, and concentrated wastes from subsequent extraction cycles, or equivalent, in a facility for fuel reprocessing; and (3) solids into which such liquid wastes have been converted. NRC thus has retained the qualitative, source-based definition of high-level waste first developed by AEC, and spent fuel is considered to be a form of high-level waste. The Nuclear Waste Policy Act of 1982 [NWPA (1982)] as amended in 1987, contains the current statutory definitions of spent nuclear

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fuel and high-level waste. Spent fuel is defined separately from highlevel waste as follows: Spent nuclear fuel is fuel that has been withdrawn from a nuclear reactor following irradiation, the constituent elements of which have not been separated by reprocessing. However, spent fuel is not a waste until it is so declared. As in the definitions of high-level waste discussed previously, the constituents of spent fuel and the minimum fuel burnup or concentrations of radionuclides produced by irradiation are not specified. High-level waste then is defined in two parts as: Clause (A): highly radioactive material from fuel reprocessing, including liquid waste produced directly in reprocessing and any solid material derived from such liquid waste that contains fission products in sufficient concentrations; and Clause (B): other highly radioactive material that NRC, consistent with existing law, determines by rule requires permanent isolation. In the context of NWPA, ‘‘requires permanent isolation’’ means disposal in a geologic repository, or in an alternative system that would provide equivalent capabilities for isolation of the waste from the biosphere, with no intention of retrieving the waste after facility closure. The definition of high-level waste in Clause (A) of NWPA given above follows the traditional, source-based description although, for the first time, the presence of fission products is mentioned explicitly. However, the definition remains qualitative because ‘‘highly radioactive’’ material and ‘‘sufficient concentrations’’ of fission products are not quantified, nor are the minimum concentrations of alpha-emitting transuranium radionuclides. The definition of high-level waste in Clause (B) of NWPA given above represents a potentially significant departure from previous definitions in that it allows the development of a generally applicable definition of high-level waste that is not based on the source of the waste. However, as in Clause (A), ‘‘highly radioactive’’ and ‘‘requires permanent isolation’’ in Clause (B) are not quantified. In 1987, NRC announced its intent to develop a quantitative and generally applicable definition of high-level waste in response to the definition in Clause (B) of NWPA (NRC, 1987). NRC indicated that the definition would specify minimum concentrations of radionuclides constituting high-level waste and would be based primarily

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on analyses of risks from waste management and disposal. In particular, ‘‘highly radioactive’’ in Clause (B) would be defined in terms of minimum concentrations of shorter-lived radionuclides that produce high levels of decay heat and external radiation, and ‘‘requires permanent isolation’’ would be defined in terms of minimum concentrations of long-lived radionuclides that require disposal systems providing a high degree of isolation from the biosphere (a geologic repository or equivalent). Thus, while the definition in Clause (B) would apply to waste with radiological properties similar to those of high-level waste from fuel reprocessing, the definition would incorporate these properties explicitly, and the definition essentially would be risk-based rather than source-based. In considering a new definition of high-level waste in accordance with Clause (B) of NWPA, an important issue for NRC was whether this definition should encompass and quantify the traditional, source-based definition in Clause (A). Such a definition would quantify ‘‘sufficient concentrations’’ of fission products and the minimum concentrations of alpha-emitting transuranium radionuclides in high-level waste from fuel reprocessing. NRC indicated its preference that the definition in Clause (B) should not apply to the primary wastes from fuel reprocessing and that the definition in Clause (A) should continue to apply to all wastes previously considered to be high-level waste in accordance with source-based definitions (NRC, 1987). In 1988, NRC announced its intention to abandon efforts to develop a quantitative and generally applicable definition of high-level waste in response to Clause (B) of NWPA (NRC, 1988). This decision was based on the following argument. First, the definition in Clause (B) should not be applied to the primary wastes from fuel reprocessing defined in Clause (A). Given this, there is little need for a new definition of high-level waste, given the current institutional framework for management of high-level waste and other radioactive wastes associated with the nuclear fuel cycle and the small volumes of waste from sources other than fuel reprocessing that likely would be defined as high-level waste under Clause (B). Furthermore, considerable effort would be required to quantify ‘‘requires permanent isolation’’ in the context of NWPA on the basis of analyses of risks from waste disposal and to develop licensing criteria for disposal of higher-activity wastes that do not ‘‘require permanent isolation’’ (disposal in a geologic repository or equivalent) but would not be acceptable for disposal in a near-surface facility. This effort would be difficult to justify, given the small amounts of waste involved. In 1989, NRC confirmed its decision to retain the qualitative, source-based definition of high-level waste and not to define any

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radioactive wastes from sources other than fuel reprocessing as highlevel waste (NRC, 1989). Thus, all definitions of high-level waste developed in accordance with NWPA apply only to waste from chemical reprocessing of spent nuclear fuel. Waste with similar radiological properties that arises from any other source is not included in highlevel waste. EPA’s current definition of high-level waste was first developed in 1985 (EPA, 1985) and is contained in 40 CFR Part 191 (EPA, 1993a). This definition defers to NWPA, and spent fuel is defined separately from high-level waste as in the Act. Thus, EPA has adopted the traditional, source-based definition of high-level waste. DOE also has used the traditional, source-based definition of highlevel waste. In contrast to other definitions, the definition adopted in 1988 was explicit that high-level waste contains transuranium radionuclides (DOE, 1988a). Later, however, DOE essentially adopted the definition in NWPA given above (DOE, 1999a). In summary, in accordance with current laws and regulations, high-level radioactive waste essentially can be defined as follows: High-level waste is the primary waste (either liquid or solid) that arises from chemical reprocessing of spent nuclear fuel. This definition is based on the source of the waste, but certain incidental wastes that arise from fuel reprocessing that contain lower concentrations of fission products and alpha-emitting transuranium radionuclides than the primary reprocessing wastes have been excluded on a case-by-case basis. Spent nuclear fuel is a form of high-level waste in some definitions [e.g., NRC’s 10 CFR Part 60 (NRC, 1983)] but not in others [e.g., the Nuclear Waste Policy Act (NWPA, 1982)]. This inconsistency is not important, because spent fuel and the primary waste from fuel reprocessing have similar radiological properties and require similar precautions for safe handling, storage, and disposal. Spent fuel is not a waste until it is so declared. NWPA authorizes NRC to define radioactive materials other than the primary waste from fuel reprocessing as high-level waste, but NRC has chosen not to do so. As a consequence, waste from sources other than fuel reprocessing with equivalent levels of decay heat or external radiation, due to high concentrations of shorter-lived radionuclides, and requiring an equivalent degree of long-term isolation from the biosphere for protection of public health (such as disposal in a geologic repository), due to high concentrations of longlived radionuclides, are not included in high-level waste. Thus, the definition of high-level waste clearly is not risk-based.

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Requirements for Disposal. The first requirements for disposal of commercial high-level waste were developed by AEC in 10 CFR Part 50, Appendix F (AEC, 1970). AEC specified that liquid high-level waste shall be converted to dry solids and transferred to a federal repository, to be designated later, for permanent disposal. NWPA (1982), as amended, governs disposal of spent fuel and high-level waste. This statute established the current DOE program for disposal of commercial spent fuel and high-level waste in geologic repositories, subject to licensing by NRC. Its requirements apply to any repository not used exclusively for (1) disposal of defense spent fuel or high-level waste or (2) DOE research and development activities. This statute also specifies that liquid high-level waste from fuel reprocessing must be converted to a solid prior to permanent disposal. The 1987 amendments to NWPA designated the site at Yucca Mountain, Nevada, as the sole candidate to be studied for its potential to host the first geologic repository. NWPA authorizes but does not require disposal of commercial spent fuel and high-level waste in a geologic repository. Indeed, the Act directed DOE to investigate alternative technologies for disposal of these wastes (e.g., subseabed disposal), but DOE is not authorized to construct or operate alternative disposal facilities. The Act also called for an evaluation of the merits of disposing of defense highlevel waste in the same repository to be used for commercial spent fuel and high-level waste. Following a study by DOE (1985a), codisposal of defense high-level waste with commercial spent fuel and high-level waste in a single repository was recommended, and this is the current policy. In 1985, EPA established the first environmental standards for disposal of spent fuel and high-level waste in 40 CFR Part 191 (EPA, 1985); these standards were revised in 1993 (EPA, 1993a). The EPA standard was intended to apply to disposal of spent fuel and highlevel waste at any site and using any technology. NRC’s first licensing criteria that govern DOE activities at geologic repositories were developed in 10 CFR Part 60 (NRC, 1983). This regulation applied to disposal of spent fuel and high-level waste only if a geologic repository were used, and it also applied to any other radioactive waste that might be sent to a repository. NRC’s licensing criteria for geologic repositories were intended to be compatible with EPA’s first environmental standards in 40 CFR Part 191 (EPA, 1985). In 1992, Congress directed EPA to issue a new environmental standard for disposal of spent fuel and high-level waste that would apply only to the candidate geologic repository at the Yucca Mountain site in Nevada (NEPA, 1992). Thus, the existing EPA standards in

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40 CFR Part 191 (EPA, 1993a), as well as NRC’s licensing criteria in 10 CFR Part 60 (NRC, 1983), no longer apply to this site. EPA’s environmental standards for the Yucca Mountain site were established in 40 CFR Part 197 (EPA, 2001a) and NRC has issued its final licensing criteria for the site in 10 CFR Part 63 (NRC, 2001). Under authority of AEA, DOE has established policies for management and disposal of defense high-level waste and any other materials which, because of their highly radioactive nature, require similar handling (DOE, 1988a; 1999a). In addition to specifying that disposal of these wastes in a geologic repository shall comply with requirements of NWPA, general environmental standards developed by EPA, and NRC’s licensing criteria, DOE policies address (1) storage of high-level waste in doubly-contained and singly-contained tank systems, principally at the Hanford, Washington, and Savannah River, South Carolina, sites, and (2) options for disposal of defense high-level waste that is not readily retrievable. Current policies and requirements for disposal of spent fuel and high-level waste thus can be summarized as follows. A geologic repository at the Yucca Mountain site in Nevada is the only candidate facility for disposal of commercial spent fuel and high-level waste. A geologic repository is expected to provide a high degree of isolation of the waste from the biosphere, and the need for such a disposal system is based primarily on the high concentrations of long-lived, alpha-emitting radionuclides in spent fuel and high-level waste. The Yucca Mountain site will be developed and operated by DOE. The facility must comply with EPA’s environmental standards in 40 CFR Part 197 (EPA, 2001a), and it will be licensed by NRC in accordance with 10 CFR Part 63 (NRC, 2001). Defense high-level waste will be co-disposed in the same repository with commercial spent fuel and high-level waste, and the two types of waste will be subject to the same environmental standards and licensing criteria. 4.1.2.3.2 Transuranic waste. As described in Section 4.1.2.2.2, transuranic waste originally was defined by AEC as solid waste that contains long-lived, alpha-emitting transuranium radionuclides or 233 U in concentrations greater than 0.4 kBq gⳮ1. Transuranic waste so defined was not generally acceptable for shallow-land burial. Statutory and Regulatory Definitions. The earliest statutory definitions of transuranic waste were contained in AEA (1954), the National Security and Military Applications of Nuclear Energy Authorization Act (NSMA, 1980), and the Low-Level Radioactive Waste Policy Act (LLRWPA, 1980). All of these laws defined transuranic waste in terms of concentrations of long-lived, alpha-emitting

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transuranium radionuclides greater than 0.4 kBq gⳮ1 as in AEC’s original definition. In 1982, federal agencies concurred with a recommendation to increase the lower limit on concentrations of long-lived, alphaemitting transuranium radionuclides in transuranic waste from 0.4 to 4 kBq gⳮ1 (Steindler, 1982). This change in the definition of transuranic waste was made in response to difficulties in routinely measuring levels of alpha activity near 0.4 kBq gⳮ1 in bulk solid waste and analyses which indicated that risks to public health from shallowland burial of transuranium radionuclides in concentrations up to 4 kBq gⳮ1 should be acceptable. In 1985, EPA developed a regulatory definition of transuranic waste in 40 CFR Part 191 (EPA, 1985) that incorporated the increase in the lower limit on concentrations of long-lived, alpha-emitting transuranium radionuclides. This definition was retained when 40 CFR Part 191 was repromulgated in 1993 (EPA, 1993a). EPA’s definition is the same as the current statutory definition described below. The current statutory definition of transuranic waste is contained in the WIPP Land Withdrawal Act of 1992 (WIPPLWA, 1992). Specifically: Transuranic waste is waste that contains more than 4 kBq gⳮ1 of alpha-emitting transuranium isotopes, with half-lives greater than 20 y, except for: 1. high-level radioactive waste; 2. waste that the Secretary of DOE has determined, with the concurrence of the Administrator of EPA, does not need the degree of isolation required by the disposal regulations in 40 CFR Part 191 (EPA, 1985); or 3. waste that NRC has approved for disposal on a case-by-case basis in accordance with 10 CFR Part 61 (NRC, 1982a). In addition to specifying the lower limit on concentrations of alphaemitting transuranium radionuclides, this definition specifies their minimum half-life. In contrast to the earliest definition developed by AEC, this definition does not include waste that contains high concentrations of long-lived, alpha-emitting non-transuranium radionuclides (e.g., 233U). The current statutory definition is explicit that transuranic waste excludes high-level waste, which also contains high concentrations of long-lived, alpha-emitting transuranium radionuclides. The other two exceptions in the definition allow that some wastes, other than high-level waste, that contain concentrations of long-lived, alphaemitting transuranium radionuclides greater than 4 kBq gⳮ1 may

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be excluded from transuranic waste. The second exception refers to waste, particularly DOE’s defense waste that is not regulated by NRC, with concentrations of such radionuclides sufficiently low that disposal in a geologic repository would not be required to protect public health, and a less isolating disposal system could be used. The third exception could apply to commercial waste with concentrations of such radionuclides sufficiently low that shallow-land disposal would provide adequate protection of public health (NRC, 1982b). Thus, by definition, transuranic waste contains sufficient concentrations of longer-lived, alpha-emitting transuranium radionuclides. These radionuclides usually are the most important in transuranic waste. However, transuranic waste also may contain high concentrations of fission products (e.g., 137Cs) and long-lived, alpha-emitting non-transuranium radionuclides (e.g., 232Th, 233U), and these constituents can determine the radiological properties of transuranic waste and the risk it poses (DOE, 1997a). Most transuranic waste has been generated in DOE’s atomic energy defense activities (DOE, 1997a); this waste is not subject to licensing by NRC. Prior to the current statutory definition in the WIPP Land Withdrawal Act, DOE developed its own definition of transuranic waste (DOE, 1988b). This definition included waste contaminated with alpha-emitting transuranium radionuclides with half-lives greater than 20 y and concentrations greater than 4 kBq gⳮ1 and it also specified that other alpha-contaminated wastes could be managed as transuranic waste. Based on this definition, DOE sites managed waste that contained high concentrations of long-lived, alpha-emitting non-transuranium radionuclides (e.g., 233 U) or high concentrations of alpha-emitting transuranium radionuclides with half-lives less than 20 y (e.g., 244Cm and 252Cf) as transuranic waste (DOE, 1997a). However, in accordance with the current statutory and regulatory definition described above, these wastes cannot be classified as transuranic waste unless they also contain more than 4 kBq gⳮ1 of alpha-emitting transuranium radionuclides with half-lives greater than 20 y; this definition has been adopted by DOE (1999b). Such wastes that are not transuranic waste under the current definition would be classified as low-level waste (see Section 4.1.2.3.3), regardless of the concentrations of alpha-emitting radionuclides. Defense transuranic waste is further categorized as ‘‘contacthandled’’ if it requires little or no shielding or ‘‘remotely-handled’’ if it requires shielding or remote handling due to high levels of photon or neutron radiation (DOE, 1996a). An external dose-equivalent rate at the surface of a waste package of 2 mSv hⳮ1 is used to distinguish the two subclasses of transuranic waste. This subclassification is

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based on requirements for protection of workers during waste operations, rather than requirements for protection of public health and the environment following permanent disposal. NRC has not developed a definition of transuranic waste, primarily because only small amounts of transuranic waste subject to licensing by NRC are generated in the commercial sector. NRC regards commercial transuranic waste as a form of higher-activity low-level waste. However, NRC’s licensing criteria for near-surface disposal of radioactive waste in 10 CFR Part 61 (NRC, 1982a) acknowledge the current statutory and regulatory definition of transuranic waste, because commercial waste that contains more than 4 kBq gⳮ1 of alpha-emitting transuranium radionuclides, with half-lives greater than 5 y, is not generally acceptable for near-surface disposal. The minimum half-life of 5 y for alpha-emitting transuranium radionuclides specified by NRC differs from the value of 20 y specified in the WIPP Land Withdrawal Act and EPA’s 40 CFR Part 191 (EPA, 1985); the minimum half-life of 20 y specified by EPA would apply in classifying any commercial waste as transuranic waste. In summary, in accordance with current laws and regulations, transuranic waste essentially can be defined as follows: Transuranic waste is waste, except for high-level waste, that contains alpha-emitting transuranium radionuclides, with half-lives greater than 20 y, in concentrations greater than 4 kBq gⳮ1. Although this definition specifies a lower limit on the concentration of particular radionuclides, it also depends on the qualitative, sourcebased definition of high-level waste and, thus, is not strictly quantitative. Alpha-emitting transuranium radionuclides with half-lives greater than 20 y are expected to be the principal constituents of most transuranic waste, but the definition does not specify any limits on the concentrations of other radionuclides that may occur in transuranic waste, including fission products, alpha-emitting nontransuranium radionuclides, and alpha-emitting transuranium radionuclides with half-lives less than 20 y. Requirements for Disposal. The National Security and Military Applications of Nuclear Energy Authorization Act (NSMA, 1980) established the current DOE program for disposal of defense transuranic waste at the WIPP facility in New Mexico. The Act specifically authorized test emplacements of waste for purposes of research and development. WIPPLWA (1992) then authorized permanent disposal of defense transuranic waste at this facility. The Act specifies that the WIPP facility may not be used for disposal of high-level waste, commercial transuranic waste, or any DOE non-defense transuranic

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waste, which is transuranic waste generated in DOE activities not related to national security such as production of nuclear weapons. This facility also may not be used for disposal of commercial or DOE low-level waste. Defense transuranic waste sent to the WIPP facility is emplaced in a bedded-salt formation located far below ground. Thus, the WIPP facility is similar to a geologic repository for spent fuel and highlevel waste in its expected waste-isolation capabilities. Disposal of defense transuranic waste at the WIPP facility is not licensed by NRC. However, the National Security and Military Applications of Nuclear Energy Authorization Act (NSMA, 1980) created the Environmental Evaluation Group as an agency of the state of New Mexico to provide independent technical oversight of DOE activities at the WIPP facility. Disposal of defense transuranic waste at WIPP must comply with EPA’s general environmental standards in 40 CFR Part 191 (EPA, 1993a). EPA also developed criteria in 40 CFR Part 194 (EPA, 1996b) that DOE must use in certifying that the WIPP facility complies with the disposal standards. In 1998, EPA ruled that disposal of DOE’s defense transuranic waste at WIPP complies with the standards in 40 CFR Part 191 (EPA, 1998a), and permanent disposal of waste at the site has begun. DOE (1988b; 1999b) has established policies for management and disposal of its transuranic waste. In addition to activities associated with permanent disposal at the WIPP facility, these policies address waste storage at DOE sites and alternatives for long-term management of transuranic waste that was buried at DOE sites prior to 1970. An unresolved issue at the present time is the development of facilities for permanent disposal of DOE waste that has been managed as transuranic waste but is not currently classified as transuranic waste; these wastes contain relatively high concentrations of such radionuclides as 233U, 244Cm, or 252Cf but concentrations of long-lived, alpha-emitting transuranium radionuclides less than 4 kBq gⳮ1. Wastes that are not classified as transuranic waste, as defined in WIPPLWA (1992), are not acceptable for disposal at the WIPP facility (DOE, 1996a). Any non-defense transuranic waste generated by DOE also cannot be sent to WIPP. DOE has used socalled greater confinement disposal systems, which are intermediate in depth and waste-isolation capabilities between near-surface facilities and geologic repositories, for small volumes of selected transuranic waste (DOE, 1997b). However, greater confinement disposal has not been developed to the extent needed to accept all transuranic waste that cannot be sent to the WIPP facility. There are no laws that explicitly address disposal of commercial transuranic waste, again because little such waste has been generated. At the present time, there are two alternatives for disposal of

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commercial transuranic waste. The first is near-surface disposal, on a case-by-case basis, in accordance with NRC’s licensing requirements in 10 CFR Part 61 (NRC, 1982a). This alternative could be appropriate for small volumes of waste with concentrations of longerlived, alpha-emitting transuranium radionuclides only slightly greater than 4 kBq gⳮ1. The second alternative is disposal in the candidate geologic repository for spent fuel and high-level waste at the Yucca Mountain site in accordance with EPA’s environmental standards in 40 CFR Part 197 (EPA, 2001a) and NRC’s licensing criteria in 10 CFR Part 63 (NRC, 2001). 4.1.2.3.3 Low-level waste. Low-level radioactive waste is produced in many commercial and non-commercial activities, and these wastes vary widely in radionuclide compositions and concentrations. Statutory and Regulatory Definitions. Current statutory definitions of low-level waste are contained in NWPA (1982) and the Low-Level Radioactive Waste Policy Amendments Act (LLRWPAA, 1986). In the Nuclear Waste Policy Act, low-level waste is defined as radioactive waste that: Clause (A): is not high-level waste, spent fuel, transuranic waste, or byproduct material as defined in Section 11(e)(2) of AEA; and Clause (B): NRC, consistent with existing law, classifies as lowlevel waste. In Clause (A), the byproduct material defined in Section 11(e)(2) of AEA (1954) essentially is uranium or thorium mill tailings. LLRWPAA contains a similar definition, except transuranic waste is not excluded. Thus, the two laws differ in regard to whether transuranic waste is distinct from low-level waste. The statutory definitions of low-level waste apply only to radioactive waste that arises from operations of the nuclear fuel cycle; i.e., to waste that contains source, special nuclear, or byproduct material as defined in AEA (see Section 4.1.2.1). This restriction, although not explicit in the definitions, is indicated by the applicability of NWPA and LLRWPAA to fuel-cycle waste only and by the reference to NRC, which can only regulate fuel-cycle waste. Thus, low-level waste does not include NARM waste. DOE has defined low-level waste as in Clause (A) above (DOE, 1988c; 1999c). In the earlier definition (DOE, 1988c), test specimens of fissionable material irradiated for purposes of research and development could be classified as low-level waste, provided the concentration of long-lived, alpha-emitting transuranium radionuclides was

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less than 4 kBq gⳮ1. However, the revised definition (DOE, 1999c) does not include this provision. Thus, DOE’s current definition is in accordance with existing legal definitions. DOE’s current definition also specifies that low-level waste excludes NORM other than uranium or thorium mill tailings. As noted above, this provision is not explicit in current legal definitions. Based on the current legal definitions, DOE waste that contains alpha-emitting radionuclides that has been managed as transuranic waste but cannot be classified as transuranic waste under current law (see Section 4.1.2.3.2) is a form of low-level waste. An important example is waste that contains high concentrations of 233U. EPA has not yet developed a regulatory definition of low-level waste. Such a definition presumably would be developed in the course of establishing general environmental standards for land disposal of low-level waste. NRC has developed licensing criteria for near-surface disposal of waste that contains source, special nuclear, or byproduct materials in 10 CFR Part 61 (NRC, 1982a). These regulations are intended to apply primarily to disposal of commercial low-level waste. They do not include a definition of low-level waste but essentially defer to the current statutory definition in the Low-Level Radioactive Waste Policy Amendments Act of 1985. Thus, low-level waste can include wastes with high concentrations of radionuclides that are not generally acceptable for near-surface disposal in accordance with the licensing criteria in 10 CFR Part 61 (NRC, 1982a). In summary, in accordance with current laws and regulations, low-level waste is defined only by exclusion and essentially as follows: Low-level waste is any radioactive waste that arises from operations of the nuclear fuel cycle except for spent fuel, high-level waste, transuranic waste, and uranium or thorium mill tailings. This definition clearly depends on the source-based definition of highlevel waste. Some definitions of low-level waste differ from the one summarized above. In particular, transuranic waste is not excluded in the definition in the Low-Level Radioactive Waste Policy Amendments Act of 1985, and transuranic waste thus is a form of low-level waste. However, this inconsistency has little practical significance, because the Amendments Act governs disposal of commercial low-level waste only, unless DOE waste is sent to a commercial facility, and there is very little commercial transuranic waste requiring disposal. NRC has statutory authority to define radioactive materials as low-level waste, consistent with existing law, but has not done so. Given that NRC can only regulate radioactive materials defined in

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AEA, and given the current statutory definition of low-level waste by exclusion and the applicability of this definition only to wastes regulated under AEA, it is not evident how NRC could develop a new definition of low-level waste that would be different from the current exclusionary definition unless NRC first developed a new definition of high-level waste in accordance with Clause (B) of NWPA (see Section 4.1.2.3.1). Because low-level waste is defined by exclusion of other types of waste, this waste class does not necessarily contain relatively low concentrations of radionuclides, in contrast to the earliest descriptions discussed in Section 4.1.2.2. In addition to very high concentrations of short-lived radionuclides, such as 60Co and short-lived fission products, low-level waste can contain high concentrations of longlived, non-transuranium radionuclides (e.g., 99Tc, 232Th) such that the risks posed by disposal of the waste are comparable to the risks posed by disposal of some high-level and transuranic wastes. The definition does not describe the constituents or properties of lowlevel waste and, thus, is not related in any way to requirements for safe handling and storage or disposal. Requirements for Disposal. The Low-Level Radioactive Waste Policy Act of 1980 (LLRWPA, 1980), as amended by the Policy Amendments Act (LLRWPAA, 1986), governs disposal of commercial low-level waste. A particular disposal technology is not specified, but shallowland burial was presumed in accordance with contemporary practices. The original Act (LLRWPA, 1980) directed NRC to identify alternatives to shallow-land burial for commercial low-level waste and to establish technical guidance and requirements for licensing of alternative disposal methods. NRC published technical studies of alternative disposal technologies (Bennett, 1985; Bennett and Warriner, 1985; Bennett et al., 1984; Miller and Bennett, 1985; Warriner and Bennett, 1985), but specific licensing criteria for these alternatives have not been established. Near-surface disposal of commercial low-level radioactive waste is licensed in accordance with criteria established by NRC in 10 CFR Part 61 (NRC, 1982a) or compatible licensing requirements established by Agreement States. These regulations do not apply to disposal of (1) specified wastes by individual licensees in accordance with provisions in 10 CFR Part 20 (NRC, 1991), (2) high-level waste in a geologic repository in accordance with licensing criteria in 10 CFR Part 60 (NRC, 1983) or 10 CFR Part 63 (NRC, 2001), (3) uranium or thorium mill tailings, or (4) waste not regulated under authority of AEA (NARM waste). In addition, these regulations do

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not require that materials must be classified as low-level waste to be acceptable for near-surface disposal. NRC’s licensing criteria in 10 CFR Part 61 (NRC, 1982a) and compatible Agreement State requirements include a waste classification system that is intended mainly to provide protection of inadvertent intruders at near-surface disposal sites. NRC’s waste classification system specifies limits on concentrations of radionuclides that are generally acceptable for near-surface disposal under specified conditions. These limits are based on: (1) assumed exposure scenarios for intrusion into disposal facilities at 100 to 500 y after disposal; (2) assumed limits on radiation dose to intruders; (3) requirements on institutional controls, the waste form, and disposal methods; and (4) consideration of reported radionuclide concentrations in commercial low-level waste. The following waste classes, with increasing limits on concentrations of radionuclides and increasingly stringent requirements on the waste form and disposal methods, are defined: (1) Class-A, -B and -C wastes that contain radionuclides with half-lives less than about 30 y; and (2) Class-A and -C wastes that contain longer-lived radionuclides. Waste with concentrations of radionuclides greater than the Class-C limits is not generally acceptable for near-surface disposal. However, nearsurface disposal of greater-than-Class-C waste may be approved by NRC or an Agreement State on a case-by-case basis. The further classification of low-level waste by NRC is indicated at the bottom of Figure 4.2. It is important to note that NRC’s waste classification system in 10 CFR Part 61 (NRC, 1982a) does not constitute a definition of low-level waste. Rather, it is a subclassification of waste developed primarily for purposes of facilitating management and disposal of commercial low-level waste in nearsurface facilities. Following establishment of NRC’s waste classification system for near-surface disposal described above, the Low-Level Radioactive Waste Policy Amendments Act (LLRWPAA, 1986) assigned responsibility for disposal of commercial greater-than-Class-C low-level waste to DOE, subject to licensing by NRC. A subsequent DOE study did not resolve the issue of acceptable alternatives to near-surface disposal for the small volumes of these wastes (DOE, 1987a). In accordance with an amendment to NRC’s licensing criteria in 10 CFR Part 61 (NRC, 1989), disposal of commercial greater-than-Class-C waste in a geologic repository now is required, unless disposal elsewhere is approved by NRC on a case-by-case basis. Current DOE policy also permits near-surface disposal of most of its low-level waste (DOE, 1988c; 1999c). Disposal of DOE’s low-level waste is not licensed by NRC, unless the waste is sent to a licensed

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facility intended primarily for disposal of commercial waste. DOE has not adopted NRC’s waste classification system for near-surface disposal in 10 CFR Part 61 (NRC, 1982a) discussed earlier, except low-level waste that would be classified as greater-than-Class-C in accordance with 10 CFR Part 61 normally is handled as special cases. DOE has used greater confinement disposal (see Section 4.1.2.3.2) for small volumes of selected high-activity low-level waste, including waste that is not acceptable for near-surface disposal at the generating site (DOE, 1997b). In summary, current laws and regulations do not specify that particular disposal technologies must be used for low-level waste. Most low-level waste is intended for disposal in near-surface facilities, except the small volumes of commercial greater-than-Class-C waste, as defined by NRC, are intended for disposal in a geologic repository. DOE’s low-level waste that would be classified as greaterthan-Class-C and any other waste that is not acceptable for nearsurface disposal at the generating site also require a disposal technology considerably more confining than a near-surface facility, either a geologic repository or a greater confinement disposal system. 4.1.2.3.4 Uranium or thorium mill tailings. Mill tailings are the residues resulting from extraction or concentration of uranium or thorium from any ore processed primarily for its source material content. Mill tailings do not include residues from mining operations (e.g., uranium mine overburden) or other chemical extraction industries, such as wastes from radium processing and phosphogypsum waste piles. Mill tailings are a form of byproduct material as defined in Section 11(e)(2) of AEA (1954). The principal concern with mill tailings is the relatively high concentrations of radium and emanation rates of radon. Under current law, mill tailings are not a form of low-level waste (see Section 4.1.2.3.3), even though the concentrations of uranium, thorium, and radium generally are much less than the concentrations of long-lived, alpha-emitting radionuclides in high-level waste and transuranic waste. Mill tailings are not included in low-level waste primarily because the very large volumes of these materials (DOE, 1997a) necessitate different approaches to management and disposal from those used for low-level waste. Management and disposal of most uranium or thorium mill tailings are governed by the Uranium Mill Tailings Radiation Control Act of 1978 (UMTRCA, 1978). This Act is concerned with the control and stabilization of mill tailings for protection of public health and the environment. It addresses (1) remedial actions at inactive uranium or thorium processing sites or on properties in the vicinity of

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such sites, which are performed by DOE with the concurrence of NRC, and (2) disposal of uranium or thorium mill tailings at active processing sites. Regulations for management and disposal of uranium or thorium mill tailings have been established by EPA in 40 CFR Part 192 (EPA, 1983; 1995b). In contrast to requirements for disposal of low-level waste, the Uranium Mill Tailings Radiation Control Act and its implementing regulations emphasize control and stabilization of mill tailings in place. If removal of residual radioactive material from the vicinity of mill properties is required to protect public health and the environment, the Act calls for permanent disposal and stabilization of these materials at or near processing sites. Thus, most mill tailings are not intended for disposal in facilities for commercial or DOE lowlevel waste. Small volumes of DOE waste that contains uranium or thorium mill tailings have been managed as low-level waste (DOE, 1988d). 4.1.2.3.5 Characteristics of the system for classification and disposal of fuel-cycle waste. The current classification system for radioactive waste that arises from operations of the nuclear fuel cycle in the United States and the current requirements for disposal of waste in the different classes have the important characteristics discussed below. Definitions of Different Classes of Fuel-Cycle Waste. The definitions of the different classes of radioactive waste that arises from operations of the nuclear fuel cycle in the United States may be summarized as in Table 4.1 (see Section 4.1.2.1). These definitions apply only to waste regulated under AEA, i.e., to waste that contains source, special nuclear, or byproduct material. The classification system for fuel-cycle waste in the United States has the following important characteristics: ● ●

● ●

Most of the definitions are not explicit in regard to the primary constituents of the waste or its radiological properties. The definitions of the different waste classes are not quantitative, i.e., expressed strictly in terms of limits on concentrations of radionuclides or other waste properties. The definitions are not generally applicable to any fuel-cycle waste, regardless of its source (i.e., how the waste is generated). The definitions are not based primarily on considerations of risk, particularly risks resulting from waste disposal.

These characteristics result from three factors: (1) the qualitative definition of high-level waste as waste from a particular source (fuel

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reprocessing); (2) the origin of the definition of high-level waste in operational requirements for safe management of liquid waste, rather than requirements for permanent disposal of solid waste; and (3) the dependence of the definitions of transuranic waste and lowlevel waste on the definition of high-level waste. The classification system for fuel-cycle waste does not distinguish unambiguously between waste in different classes. For example, high-level waste, transuranic waste, and low-level waste can have similar radiological properties and require similar methods of safe management and disposal. Such similarities are a consequence of definitions that depend on the source of the waste (high-level waste) or the presence of particular radionuclides (transuranic waste) and a definition by exclusion only (low-level waste). Low-level waste can contain high concentrations of long-lived radionuclides (e.g., 14C, 94 Nb, 99Tc, and 233U) and can pose long-term risks similar to those of high-level waste and transuranic waste that contains high concentrations of long-lived, alpha-emitting transuranium radionuclides. As another example, low-level waste that contains mostly naturally occurring radionuclides (e.g., uranium) can resemble mill tailings. The definition of low-level waste only by exclusion is particularly problematic. The term ‘‘low-level’’ gives the impression that waste in this class contains low concentrations of radionuclides or low radiation levels compared, for example, with high-level waste. However, this is not necessarily the case because low-level waste can contain the highest concentrations of radionuclides of any waste, including high concentrations of radionuclides with half-lives of about 30 y or greater. Since waste in this class can range from innocuous to highly hazardous, the definition of low-level waste is not related in any way to its radiological properties or to requirements for safe management and disposal. The lack of a definition of what low-level waste is also has undesirable social and political ramifications, in that it presents a barrier to public understanding and public discourse on waste issues and, thus, may foster mistrust of waste management and disposal activities (Wiltshire and Dow, 1995). Requirements for Disposal and Their Relationship to Waste Classification. Under current laws and regulations, spent fuel, high-level waste, transuranic waste, and low-level waste generally do not require particular disposal systems. However, only certain types of disposal systems are authorized for some types of waste (see Table 4.1). In particular: (1) spent fuel, high-level waste, transuranic waste, and greater-than-Class-C low-level waste normally are intended for disposal in a geologic repository, such as the proposed Yucca Mountain facility and the Waste Isolation Pilot Plant; and

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(2) low-level waste with concentrations of radionuclides less than the Class-C limits normally is intended for disposal in near-surface facilities, such as the currently operating commercial and DOE facilities. Thus, the current statutory and regulatory framework emphasizes two options for disposal of radioactive waste. In addition, DOE has utilized greater confinement disposal systems, which are intermediate in depth and waste-isolation capabilities between near-surface facilities and a geologic repository, for some of its transuranic waste and low-level waste. However, this type of facility has not been used for large volumes of DOE waste and has not been developed for commercial waste. In managing uranium or thorium mill tailings, current laws and regulations emphasize control and stabilization in place, rather than shipment to dedicated disposal facilities. Although mill tailings resemble some low-activity low-level wastes in their radiological properties, the very large volumes of mill tailings necessitate a different approach to management and disposal of most of these wastes compared with the approaches used for low-level waste. However, small volumes of mill tailings may be disposed of in facilities intended primarily for low-level waste. These considerations lead to an important conclusion regarding the relationship between classification of fuel-cycle wastes and requirements for their disposal—namely, that the selection of acceptable systems for disposal of fuel-cycle wastes does not depend on the definitions of waste classes. Rather, the types of disposal systems that are expected to provide adequate protection of public health (e.g., a near-surface facility or a geologic repository) are selected based on the radiological properties of waste, essentially without regard for how the waste is classified. Thus, general requirements for disposal are not affected by the qualitative, source-based, and ambiguous definitions in the classification system for fuel-cycle waste. 4.1.2.4 Naturally Occurring and Accelerator-Produced Radioactive Material. NARM includes any radioactive material other than source, special nuclear, or byproduct material as defined in AEA (1954). Thus, NARM refers to any radioactive material not associated with the nuclear fuel cycle. NARM waste generally is divided into waste that contains NORM and waste produced in an accelerator (see Figure 4.2). These two categories are not formally defined in federal law or regulations. Rather, they are based mainly on the different properties of the two types of waste and the fact that they would rarely, if ever, be generated at the same site. NORM waste, especially waste that arises in

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various mining and energy-related activities, often resembles uranium or thorium mill tailings in having large volumes but relatively low concentrations of radionuclides, although some wastes can have relatively small volumes but considerably higher concentrations of radionuclides (e.g., waste from treatment of drinking water, radium needles used in cancer therapy, pipe scale from extraction of oil). Accelerator-produced waste, such as accelerator targets or wastes that arise from production of certain medical isotopes, generally occurs only in small volumes, and it usually resembles forms of low-level waste in which most of the activity is due to short-lived radionuclides and the concentrations of longer-lived radionuclides are relatively low. However, because of the way that radioactive materials are defined in AEA, diffuse NORM wastes are not a form of mill tailings, and the more concentrated NORM and acceleratorproduced wastes are not forms of low-level waste. At the present time, most commercial NARM waste is not subject to federal regulation and, thus, is regulated only by the states. An exception is phosphogypsum materials, which are regulated by EPA for their radium content and radon emissions (EPA, 1992b) under the Clean Air Act (CAA, 1963). States generally regulate commercial accelerator-produced waste as low-level waste (Jacobi, 2000).11 A variety of approaches have been taken in regulating commercial NORM waste, particularly waste produced in mining, energy exploitation, and other industrial activities (NAS/NRC, 1999a). Some states do not currently regulate these forms of NORM waste as radioactive waste. States that do regulate NORM waste generally specify concentrations of radium below which materials are exempt from regulation as radioactive waste, but the concentrations of radium that distinguish regulated and unregulated NORM waste vary from state to state. The distinction between regulated and unregulated (including exempt) waste is indicated by the two subclasses of NORM waste shown in Figure 4.2. DOE is responsible for management and disposal of all NARM waste generated in any of its authorized activities, based on the provision of AEA (1954) that requires DOE to protect public health and safety in any such activity. DOE’s NORM waste usually is managed as uranium or thorium mill tailings, except small volumes may be managed as low-level waste, and DOE’s accelerator-produced waste generally is managed as low-level waste (DOE, 1988d; 1999c).

11

Jacobi, W. (2000). Personal communication (Colorado Department of Public Health and Environment, Denver, Colorado) to Kocher, D.C. (Oak Ridge National Laboratory, Oak Ridge, Tennessee.)

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EPA may develop general environmental standards for management and disposal of NARM waste under authority of TSCA (1976); e.g., see Cameron (1996). Such standards would subject commercial NARM waste to federal regulation, and they would also apply to DOE’s NARM waste. In principle, EPA also could regulate NARM waste under RCRA (1976), because the exclusion of radioactive waste from regulation under RCRA applies only to waste that contains source, special nuclear, or byproduct material as defined in AEA (1954). However, NARM waste can be regulated under RCRA only if it is included in the definition of hazardous waste in 40 CFR Part 261 (EPA, 1980b). The current definition of hazardous waste (see Section 4.2.1) specifically excludes many important wastes from mining and energy exploitation activities that contain naturally occurring radionuclides substantially above average background levels. 4.1.2.5 Exempt Radioactive Waste. The classes of waste discussed in Sections 4.1.2.3 and 4.1.2.4 generally are presumed to require disposal in facilities dedicated to radioactive waste in order to protect public health and the environment. It has long been recognized, however, that there are materials containing such low amounts of radioactivity that they could be managed in all respects as if they were nonradioactive and still protect public health and the environment. Such considerations have led to the concept of an exempt class of radioactive waste. The primary advantage of establishing exemption levels for radioactive waste would be the considerably lower costs of waste disposal (e.g., in a municipal/industrial landfill) compared with the cost of disposal in a dedicated facility for radioactive waste. This Section describes the concepts used in exempting waste that contains radioactive material and discusses efforts in the United States to establish exemption levels for radioactive waste. 4.1.2.5.1 Concepts and definitions. Two concepts are potentially useful in establishing exemption levels for radioactive waste. The first is the concept of a generally applicable negligible (de minimis) dose or risk, and the second is the concept of amounts of radionuclides that are exempt or below regulatory concern (BRC) for particular practices or sources. A negligible dose would be generally applicable to all man-made sources of radiation and would define a dose below which further control of sources by regulatory authorities is deemed to be unwarranted. If all doses were below a negligible level, no further reductions in dose using the ALARA principle would be attempted (see

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Section 3.3.1). A negligible dose is based on consideration of a negligible risk from radiation exposure, without regard for whether such a dose is reasonably achievable for any particular exposure situation. In contrast to a generally applicable negligible dose based on consideration of a negligible risk, radioactive materials that are exempt or BRC represent doses judged by regulatory authorities to be ALARA for specific practices at any site (e.g., waste disposal). Because doses that are ALARA may depend on the particular exposure situation, levels of radioactivity that are exempt or BRC may vary from one practice to another. Exemption levels for specific practices generally could be higher than levels corresponding to a generally applicable negligible dose based, for example, on considerations of cost-benefit in choosing among options for management and disposal of waste. 4.1.2.5.2 Exemption levels for radioactive waste. This Section discusses exemption levels for radionuclides in waste materials that have been established or were proposed by NRC. Exemption levels for radioactive waste have not been established by DOE or EPA. Established Exemption Levels. NRC’s radiation protection standards in 10 CFR Part 20 (NRC, 1991) include limits on concentrations or annual releases of radionuclides for unrestricted discharge into sanitary sewer systems, except any excreta from individuals undergoing medical treatment with radioactive material are exempt from the limits. These regulations also include an exemption for land disposal of liquid scintillation materials and animal carcasses that contain 2 kBq gⳮ1 (0.05 ␮Ci gⳮ1) or less of 3H or 14C, although the exempted scintillation materials must be managed in accordance with RCRA requirements due to the presence of toluene. Current NRC regulations for source material in 10 CFR Part 40 (AEC, 1961) and byproduct material in 10 CFR Part 30 (AEC, 1965a) specify conditions for exemption of many products or materials that contain small amounts of radioactive material (see also Schneider et al, 2001). These exemptions apply to commercial or specialized industrial uses of radioactive materials, as well as their disposal, and they include many common consumer products (e.g., timepieces, smoke detectors, thorium gas mantles). These exemptions were established based on judgments by AEC and NRC that the benefits of exempt uses far outweighed the risks to public health. NRC regulations described above represent a case-by-case approach to establishing exemption levels for radioactive material. Although the various exemption levels are expected to correspond to low doses from use and disposal of materials compared, for example, with dose limits in radiation protection standards for the public

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or doses due to natural background radiation (AEC, 1965b; NCRP, 1987c), many of the exemption levels are not clearly related to dose to the public. Furthermore, the doses associated with use and disposal of the different exempt materials vary widely (Schneider et al., 2001) and, in many cases, are well above doses that might be regarded as de minimis. In addition to the exemptions established in regulations, NRC issued guidance on concentration limits for disposal of residual thorium or uranium from past operations with no restrictions on burial method (NRC, 1981). There would be no restrictions on burial method if the concentrations were less than (1) 0.4 Bq gⳮ1 for natural thorium or uranium with its decay products present and in activity equilibrium, (2) 1.3 Bq g ⳮ1 for depleted uranium, and (3) 1 Bq g ⳮ1 for enriched uranium. These concentration limits were intended to provide criteria for remediation of contaminated sites to permit unrestricted use by the public, but they could be applied to waste disposal as well. The exemption levels for residual thorium or uranium in NRC guidance described above are more than an order of magnitude greater than average levels of naturally occurring thorium or uranium in surface soil (NCRP, 1984a). Since the average annual dose from exposure to naturally occurring thorium, uranium, and their decay products, including radon, is about 2 mSv (NCRP, 1987b) and is greater than the annual dose limit for continuous exposure of members of the public to man-made sources of 1 mSv established by NRC (1991), the exemption levels for natural thorium or uranium in particular clearly do not correspond to doses (and risks) that would be widely regarded as de minimis. Proposed Generic Policy on Below Regulatory Concern. To provide a common risk basis for exempting specific practices or sources regulated by NRC, and to replace the present system of case-by-case exemptions described above, NRC (1990) issued a proposed policy on doses from certain sources or practices by its licensees that would be BRC. This policy was intended to apply, for example, to consumer products, recycle/reuse, and waste disposal. NRC proposed that sources or practices would be BRC if (1) the annual dose to individuals would be 100 ␮Sv or less for practices affecting a limited number of individuals or 10 ␮Sv or less for practices affecting a large number of individuals and (2) the annual collective dose would be 10 person-Sv or less, with annual doses to individuals less than 1 ␮Sv not needing consideration in estimating collective dose. The exemption level of 10 ␮Sv for practices affecting a large number of individuals is one percent of the dose limit for members of the public (NRC, 1991) and

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is about 0.3 percent of the average dose from natural background radiation (NCRP, 1987b). Following widespread public objection and in accordance with a provision of the National Energy Policy Act (NEPA, 1992) revoking NRC’s authority under AEA to exempt broad classes of radioactive material from its licensing requirements, NRC withdrew its generic policy on BRC (NRC, 1993). The objections to the BRC policy were not based on technical or scientific arguments, but were related to the process used to develop the policy and the perception that the policy represented an abrogation of NRC’s responsibilities to protect public health and safety. NRC will continue to address requests for exemption from licensing requirements for radioactive material on a case-by-case basis using the criteria and guidance issued previously (AEC, 1965b). 4.1.2.5.3 NCRP recommendation on a negligible individual dose. NCRP’s current recommendations on radiation protection (NCRP, 1993a) include a recommendation that annual effective doses to individual members of the public of 10 ␮Sv or less from any practice or source are negligible. The recommendation on a negligible individual dose was based on considerations of the magnitude of the dose and its associated risk, the difficulty in detecting and measuring doses and associated responses at very low doses, and the estimated risk associated with the mean and variance of doses from natural background radiation. For continuous exposure over a 70 y lifetime, the recommended negligible individual dose corresponds to a fatal cancer risk of about 4 ⳯ 10ⳮ5 (see Table 3.3). The NCRP recommendation on a negligible individual dose could be used to establish exemption levels for radioactive waste. However, a negligible individual dose of 10 ␮Sv yⳮ1 would be useful mainly in establishing exemption levels for man-made radionuclides, because exposure to naturally occurring radionuclides (e.g., radium, thorium, and uranium) in their undisturbed state results in much higher doses of about 2 mSv yⳮ1 (NCRP, 1987b) when the contributions from their radiologically significant shorter-lived decay products are taken into account. Exemption levels for naturally occurring radionuclides could be established based on considerations other than a negligible individual dose as indicated, for example, by NRC’s guidance on unrestricted disposal of residual thorium or uranium from past operations (NRC, 1981) discussed in the previous section. 4.1.2.5.4 Summary of exemptions for radioactive waste in the United States. At the present time, exemption levels for radionuclides in waste materials, or materials intended for beneficial use,

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have been established by NRC only on a case-by-case basis. Furthermore, there is no clear relationship between the existing exemption levels and doses (risks) to the public from unrestricted use or disposal of exempt materials. NCRP has developed a recommendation on a negligible individual dose that could be used to establish exemption levels for radioactive waste for such purposes as disposal and recycle/reuse, but this recommendation has not been adopted by regulatory authorities. Indeed, NRC is prohibited by law from implementing a proposed generic policy on exemption of radioactive materials that was consistent with the NCRP recommendation. 4.1.2.6 Proposals for Alternative Radioactive Waste Classification Systems. This Section discusses a number of proposals in the United States for developing alternative classification systems for radioactive waste. These proposals illustrate difficulties with the existing classification system, and they indicate approaches that could be used to overcome these difficulties. 4.1.2.6.1 NRC discussion on definition of high-level waste. In 1985, NRC described an approach to developing a quantitative definition of high-level waste in response to the definition in Clause (B) of NWPA (1982) discussed in Section 4.1.2.3.1 (Fehringer, 1985). In accordance with the Clause (B) definition, high-level waste would be any waste that arises from operations of the nuclear fuel cycle, other than the primary waste from chemical reprocessing of spent nuclear fuel, that is ‘‘highly radioactive’’ and ‘‘requires permanent isolation.’’ NRC’s primary concern in this study was to identify concentrations of radionuclides that require permanent isolation, i.e., disposal in a geologic repository or equivalent. From an evaluation of radionuclide concentrations in commercial and defense high-level waste from fuel reprocessing, NRC suggested that it might be appropriate to consider other waste with concentrations of radionuclides greater than 30 times the Class-C limits for near-surface disposal, as specified in 10 CFR Part 61 (NRC, 1982a) (see Section 4.1.2.3.3), to be highlevel waste. However, NRC did not undertake further analyses to investigate the feasibility of this approach (NRC, 1988; 1989). 4.1.2.6.2 Generally applicable waste classification system proposed by Kocher and Croff. In response to the definition of high-level waste in Clause (B) of NWPA (1982) discussed in Section 4.1.2.3.1, Kocher and Croff (1987; 1988) developed a proposal for a quantitative, generally applicable, and risk-based radioactive waste classification system that addresses the definitions of high-level waste in

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Clauses (A) and (B) of the Act, as well as the definitions of other waste classes. In this proposal, all radioactive waste would be placed into one of three classes, which are defined conceptually as follows: ● ● ●

High-level waste is any waste that is highly radioactive and requires permanent isolation. Transuranic waste and equivalent is any waste that requires permanent isolation but is not highly radioactive. Low-level waste is any waste that does not require permanent isolation, without regard for whether it is highly radioactive.

In these definitions, ‘‘highly radioactive’’ refers to high levels of decay heat and external radiation, due primarily to shorter-lived radionuclides, and ‘‘requires permanent isolation’’ refers to high concentrations of long-lived radionuclides; i.e., these terms have the same interpretations as in the definitions of high-level waste in NWPA. Kocher and Croff then suggested the following implementation of the conceptual definitions of the three waste classes given above, based on analyses of risks from waste management and disposal: ●

●

‘‘Highly radioactive’’ means a thermal power density (decay heat) in the waste greater than 50 W mⳮ3 or an external dose-equivalent rate at a distance of 1 m from unshielded waste greater than 1 Sv hⳮ1. ‘‘Requires permanent isolation’’ means concentrations of radionuclides greater than those that would be generally acceptable for near-surface disposal.

The concentration limits for near-surface disposal either are the Class-C limits specified in NRC’s 10 CFR Part 61 (NRC, 1982a) and discussed in Section 4.1.2.3.3 or, for other radionuclides, are the Class-C limits calculated using NRC’s risk analysis methodology for near-surface disposal (Oztunali and Roles, 1986; Oztunali et al., 1986). The generally applicable and risk-based radioactive waste classification system proposed by Kocher and Croff would have the following consequences. High-level waste would include most waste presently classified as high-level waste because of its source (fuel reprocessing) as well as waste from any other source with similar properties, such as greater-than-Class-C low-level waste with high levels of decay heat or external radiation. Transuranic waste and equivalent would include most waste presently classified as transuranic waste as well as waste from any source with similar properties, such as greater-than-Class-C low-level waste with low levels of decay heat or external radiation. Low-level waste would include commercial Class-A, -B, or -C waste, most DOE waste presently classified

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as low-level waste, and any other waste that is generally acceptable for near-surface disposal, including most mill tailings and most NARM waste. Thus, the proposed waste classification system not only quantifies the definitions of waste classes based on risk, but it also associates waste classes with particular disposal technologies, either near-surface disposal systems for low-level waste or a considerably more isolating system for high-level waste or transuranic waste and equivalent (e.g., a geologic repository). 4.1.2.6.3 Generally applicable waste classification system proposed by Smith and Cohen. Smith and Cohen (1989) developed a proposal for a comprehensive and risk-based radioactive waste classification system largely in response to the definition in Clause (B) of NWPA (1982) that high-level waste is ‘‘highly radioactive’’ and ‘‘requires permanent isolation’’ (see Section 4.1.2.3.1). As in the proposal by Kocher and Croff discussed in the previous section, this proposal associates waste classes with particular disposal technologies. Four waste classes containing increasing levels of radioactivity and/or increasing duration (persistence) of the hazard from waste disposal were defined. These waste classes are described as follows: ●

●

●

●

BRC waste is waste with such low concentrations of radionuclides that the waste can be managed according to its nonradiological characteristics. Low-level waste is waste with concentrations of radionuclides less than the Class-C limits specified in NRC’s 10 CFR Part 61 (NRC, 1982a) and, thus, is generally acceptable for near-surface disposal. Intermediate-level waste is waste with concentrations of radionuclides greater than NRC’s Class-C limits but which does not pose a sufficient long-term hazard to justify disposal in a geologic repository. High-level waste is waste with such high concentrations of longlived radionuclides that disposal in a geologic repository or equivalent is required.

Smith and Cohen did not perform a detailed risk analysis to quantify the boundaries of the different waste classes. However, as an example, if concentration is used as the measure of radioactivity in waste, the following 239Pu-equivalent concentrations (concentrations for which the hazard would be equivalent to that of 239Pu) were suggested for use in quantifying the different waste classes: (1) BRC waste would be any waste which, after 10 y of decay, has an equivalent concentration less than 40 MBq mⳮ3; (2) low-level waste would be any non-BRC waste which, after 100 y of decay, has an equivalent

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concentration less than 4 GBq mⳮ3; (3) intermediate-level waste would be any waste more radioactive than low-level waste which, after 1,000 y of decay, has an equivalent concentration less than 0.4 TBq mⳮ3; and, (4) high-level waste would be any waste which, after 1,000 y of decay, has an equivalent concentration greater than 0.4 TBq mⳮ3. 4.1.2.6.4 Waste classification system proposed by LeMone and Jacobi. LeMone and Jacobi (1993) developed a proposed classification system for radioactive waste based primarily on a classification system developed previously by IAEA (1981) (see Section 4.1.3.1). The proposed system includes four classes of radioactive waste, which are described as follows: ●

●

●

●

BRC waste is waste with such low concentrations of radionuclides that the waste would be unregulated with respect to its radioactivity. BRC waste generally would correspond to very low concentrations of low-level, short-lived waste and low-level, long-lived waste as defined by IAEA (1981). Low-level waste is waste with only low concentrations of intermediate-level, short-lived waste or intermediate-level, long-lived waste as defined by IAEA (1981). Low-level waste would be suitable for disposal in a municipal/industrial landfill that met current EPA standards and would include relatively low-activity Class-A waste, as defined in NRC’s 10 CFR Part 61 (NRC, 1982a). Intermediate-level waste is waste with high concentrations of intermediate-level, short-lived waste or intermediate-level, long-lived waste (IAEA, 1981). Such waste would be suitable for disposal in a near-surface facility incorporating engineered barriers and would include higher-activity Class-B and Class-C waste, as defined in NRC’s 10 CFR Part 61 (NRC, 1982a). High-level waste is waste with high concentrations of long-lived radionuclides (IAEA, 1981). High-level waste would require a disposal system considerably more confining than a near-surface facility (e.g., a geologic repository).

This proposal differs from the others discussed previously in that the first three waste classes all would include waste that is generally acceptable for disposal in a near-surface facility. However, these three classes differ in the extent to which engineered barriers would be relied upon to inhibit migration of radionuclides and exposures of inadvertent intruders. LeMone and Jacobi also suggested that the proposed waste classification system could be quantified by means of the following limits

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on annual dose to maximally exposed individuals: BRC waste could be defined by a limit from unregulated disposal of 1 ␮Sv, low-level waste by a limit from disposal in a municipal landfill approved by EPA of 1 to 10 ␮Sv, and intermediate-level waste by a limit from disposal in a licensed facility using engineered barriers of 10 to 250 ␮Sv. High-level waste would include any waste that could not meet the dose limit for intermediate-level waste. LeMone and Jacobi did not implement their proposal to derive radionuclide-specific concentration limits for the different waste classes.

4.1.3 IAEA Recommendations on Radioactive Waste Classification and Exemption Principles IAEA has been developing recommendations on classification of radioactive waste and principles for exempting radioactive waste from regulatory requirements for radioactive material for more than 30 y. This Section briefly reviews these developments. Not discussed in this Section are radioactive waste classification systems developed in other countries, particularly in Europe. Waste classification systems in European countries are discussed, for example, in a report of the Commission of the European Communities (CEC, 1990), and waste classification systems in a number of countries have been reviewed by Numark et al. (1995). The waste classification systems developed in other countries often have been based, at least in part, on the source-based classification system in the United States or the various IAEA recommendations discussed in this Section; they generally do not include any new concepts of waste classification. 4.1.3.1 Recommendations on Waste Classification. The earliest classification systems proposed by IAEA (1970; 1981) placed radioactive waste into one of three classes, which were defined as follows: ●

●

High-level waste is (1) the highly radioactive liquid, containing mainly fission products as well as some actinides, which is separated during chemical reprocessing of irradiated fuel; i.e., the aqueous waste from the first solvent extraction cycle and those waste streams combined with it; (2) any other waste with radioactivity levels intense enough to generate significant amounts of heat by the radioactive decay process; and (3) spent nuclear fuel, provided it is declared a waste. Intermediate-level waste is waste which, because of its radionuclide content, requires shielding but needs little provision for heat dissipation during handling and transport.

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Low-level waste is waste which, because of its low radionuclide content, does not require shielding during normal handling and transportation.

Within the low-level and intermediate-level waste classes, a further distinction was made between short- and long-lived waste, as well as alpha-bearing waste (IAEA, 1981). Short-lived waste referred to waste that would decay to low activity levels during the time period of perhaps a few centuries when administrative control over the waste can be expected to last, and long-lived waste referred to waste that would not decay to low levels during an administrative control period. Alpha-bearing waste referred to waste that contains one or more alpha-emitting radionuclides in amounts above acceptable limits established by national authorities. Although the waste classification system described above was useful for general purposes, it had several limitations. First, the classification system was not clearly linked to safety aspects of radioactive waste management, particularly disposal. Second, it was not consistent with definitions of radioactive wastes developed in some countries, particularly when waste was classified according to the facilities in which the waste is generated or by the processes that generate the waste. Third, it lacked quantitative boundaries between classes. Fourth, it lacked recognition of a class of waste that contains so little radioactive material that it may be exempted from control as radioactive waste. Finally, it lacked recognition of wastes, such as those from mining and milling of uranium ore, that contain low levels of naturally occurring radionuclides but occur in very large volumes. To address the limitations of the waste classification system described above, new recommendations on waste classification were developed (IAEA, 1994). A particular aim of the new system was to associate waste classes with intended disposal technologies (options), at least to some degree. The recommended classification system includes the following three major classes of waste: exempt waste, low- and intermediate-level waste, and high-level waste. These waste classes and the associated disposal options are summarized in Table 4.2 and described as follows. Exempt waste would be defined as waste that contains such low concentrations of radionuclides that it could be exempted from regulatory control as radioactive material because the radiological hazards associated with disposal of the waste would be negligible. The basis for defining exempt radioactive waste recommended by IAEA is a limit on annual dose to individuals from waste disposal of 10 ␮Sv (see Section 4.1.3.2).

Disposal Options

Thermal power density greater than about 2 kW mⳮ3 and concentrations of long-lived, alpha-emitting radionuclides that exceed restrictions for short-lived waste Contains uranium, thorium, or radium, e.g., from mining and milling of ores or decommissioning of nuclear facilitiesd

High-level waste

Waste that contains longlived, naturally occurring radionuclidesc

b

Distinction between short- and long-lived radionuclides is half-life of about 30 y. Range of disposal options may be acceptable, due to variety of radionuclides and wide range of concentrations that may be present. c Waste class is not part of basic waste classification system, but large volumes of waste that contains long-lived, naturally occurring radionuclides are given additional consideration. d Waste from decommissioning also may contain man-made radionuclides. e Disposal option would depend on results of safety assessments for particular wastes.

a

Geologic repository

Concentrations of long-lived, alpha-emitting radionuclides that exceed restrictions for short-lived waste

Long-lived waste

No radiological restrictions or systems similar to those for short-lived wastee

Geologic repository

Concentrations of long-lived, alpha-emitting radionuclides restricted to 4 kBq gⳮ1 in individual waste packages and average of 0.4 kBq gⳮ1 over all waste packages

Near-surface disposal system or geologic repositoryb

No radiological restrictions

Short-lived wastea

Concentrations of radionuclides above exempt levels and thermal power density (decay heat) less than about 2 kW mⳮ3

Low- and intermediatelevel waste

Typical Characteristics

Concentrations of radionuclides at or below levels corresponding to annual dose to members of the public from waste disposal of 10 ␮Sv

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Exempt waste

Class

TABLE 4.2—Summary of characteristics of radioactive wastes and disposal options in the waste classification system recommended by IAEA (1994).

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In recommending a single waste class called low- and intermediatelevel waste, IAEA recognized that the previous distinction between low-level and intermediate-level waste is of secondary importance in developing a waste classification system that is closely linked to safety aspects of waste disposal. In the past, low-level waste often was defined as radioactive waste that does not require shielding during normal handling and transportation, whereas radioactive waste that required shielding but needed little or no provision for heat dissipation was classified as intermediate-level waste. An external dose rate at the surface of the waste of 2 mSv hⳮ1 often was used to distinguish between the two classes. For purposes of waste disposal, however, classification should be related to amounts of individual radionuclides taking into account the exposure routes (e.g., ingestion) of greatest importance in post-closure scenarios for releases from a disposal facility. In IAEA’s new recommendations, low- and intermediate-level waste thus contains concentrations of radionuclides above those for exempt waste but still sufficiently low that heat dissipation is not a concern in ensuring safe disposal. IAEA recommends that the thermal power density for this class of waste be restricted to about 2 kW mⳮ3. This class would cover a wide range of radionuclide concentrations, and a variety of disposal methods may be appropriate depending on the radiological properties of the waste. IAEA continues to recommend that low- and intermediate-level waste be further classified as short-lived or long-lived. Short-lived waste could contain high concentrations of shorter-lived radionuclides, with half-lives less than about 30 y, subject to the restriction on thermal power density of about 2 kW mⳮ3. However, concentrations of long-lived, alpha-emitting radionuclides in short-lived waste should be limited to 4 kBq gⳮ1 in individual waste containers and to an average of 0.4 kBq gⳮ1 in all containers in a disposal facility. Short-lived low- and intermediate-level waste thus would contain mainly short-lived radionuclides that decay appreciably during the period of institutional control over the disposal facility. Short-lived waste often should be acceptable for disposal in a near-surface facility; but disposal in a geologic repository could be considered based on the results of safety analyses or if co-disposal of short-lived and long-lived wastes is anticipated. Long-lived waste would contain concentrations of long-lived radionuclides greater than the restrictions on short-lived waste; such waste normally would require disposal in a geologic repository. Finally, high-level waste would include any waste with (1) a thermal power density greater than about 2 kW mⳮ3, due mainly to high concentrations of short-lived radionuclides, and (2) concentrations

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of long-lived radionuclides greater than the restrictions on shortlived low- and intermediate-level waste described above. High-level waste would require disposal in a geologic repository. The first part of this description takes into account that provisions for heat dissipation would be needed in the design of waste disposal facilities for high-level waste, and the second part takes into account the significant long-term radiological hazard. IAEA also has given some consideration to waste that contains long-lived, naturally occurring radionuclides (uranium, thorium, or radium) that may be generated by mining and milling of ores or decommissioning of nuclear facilities. Although these wastes contain long-lived radionuclides, and decommissioning waste may contain man-made radionuclides as well, they are not expected to require disposal in a geologic repository. In some cases, the radionuclide concentrations may be sufficiently low that the waste can be exempted; in other cases, disposal options similar to those for shortlived low- and intermediate-level waste may be considered, depending on the results of safety assessments. However, waste that contains long-lived, naturally occurring radionuclides is not considered to be part of the basic waste classification system consisting of exempt waste, low- and intermediate-level waste, and high-level waste. In its recommendations, IAEA emphasizes that waste classification, even if it focuses on waste disposal, does not provide an adequate substitute for site-specific safety assessments of particular disposal systems to ensure the acceptability of waste disposal. IAEA also recognizes the role of national authorities in implementing waste classification systems and ensuring the safety of waste disposal, and that different countries may choose to classify waste in different ways depending on their particular situations. However, IAEA believes that, if for no other reason than to facilitate communication, it would be desirable to achieve some level of uniformity of waste classification systems in different countries. IAEA recommends that it is particularly important to obtain an international consensus on the boundary for determining unconditionally exempt material that may be transferred from one country to another, especially for purposes of recycle/reuse. 4.1.3.2 Recommendations on Exemption Principles. IAEA has developed recommendations on general principles for exemption of radioactive material from regulatory control (IAEA, 1988). Any practice or source could be exempted from regulatory control if (1) the annual dose to individuals would be less than 10 ␮Sv and (2) the annual collective dose from an unregulated practice would be less

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than 1 person-Sv. The negligible dose to individuals is the same as the value recommended by NCRP (1993a) and the value for practices affecting a large number of individuals contained in NRC’s proposed generic policy on BRC (NRC, 1990), and the negligible dose to exposed populations is an order of magnitude less than the value contained in NRC’s generic policy (see Sections 4.1.2.5.2 and 4.1.2.5.3). Based on the negligible annual dose to individuals of 10 ␮Sv and assumed scenarios for unrestricted disposal of waste, IAEA has developed recommendations on exemption levels for radionuclides in solid waste (IAEA, 1995); the recommended exempt concentrations have values in the range of about 0.1 to 104 Bq gⳮ1 depending on the radionuclide. IAEA also has issued recommendations on total activities and activity concentrations of radionuclides that could be exempted from any requirements for notification, registration, or licensing of sources or practices, based on the same exemption principles and assumed scenarios for exposure of the public (IAEA, 1996). The recommended exemption levels for naturally occurring radionuclides are limited to their incorporation in consumer products, use as a radioactive source, or use for their elemental properties.

4.1.4 Comparison of the United States and IAEA Radioactive Waste Classification Systems The present classification system for radioactive waste in the United States summarized in Table 4.1 (see Section 4.1.2.1) and IAEA’s recommended classification system summarized in Table 4.2 have certain similarities, but they also have important differences. These are briefly summarized below. The radioactive waste classification systems developed in the United States and by IAEA are similar in some of their practical implications. In the United States system, different classes of waste have the following characteristics: most (but not all) low-level waste contains relatively low concentrations of radionuclides; transuranic waste contains relatively high concentrations of long-lived, alphaemitting transuranium radionuclides and usually (but not always) contains relatively low concentrations beta/gamma-emitting radionuclides; and, high-level waste contains relatively high concentrations of beta/gamma-emitting fission products and long-lived, alphaemitting radionuclides. Thus, most waste classified in the United States as low-level or transuranic waste would be similar to waste classified by IAEA as low- and intermediate-level waste, and waste classified as high-level waste in the United States would be similar to waste classified as high-level waste by IAEA. Furthermore, the

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intended disposal systems for most of these wastes are the same in the two classification systems. However, the two radioactive waste classification systems differ in several important respects. First, in the United States, NARM waste is not included in the classification system for waste that arises from operations of the nuclear fuel cycle. In IAEA’s classification system, fuel-cycle and NARM wastes are included in the same classification system. Second, high-level waste as defined in the United States includes only the primary waste from reprocessing of spent nuclear fuel. In IAEA’s classification system, any waste from sources other than fuel reprocessing with similar radiological properties would be included in high-level waste. Third, low-level waste as defined in the United States can contain very high concentrations of shorter-lived beta/gamma-emitting radionuclides, resulting in high levels of thermal power density (decay heat) and external dose rate. In IAEA’s classification system, concentrations of shorter-lived radionuclides in low-level waste would be limited by imposing a limit on thermal power density. Lowlevel waste as defined in the United States also can contain high concentrations of long-lived, beta/gamma-emitting radionuclides (e.g., 14C, 94Nb, and 99Tc) such that the risk from waste disposal would be comparable to or greater than the risk corresponding to the concentration limits for long-lived, alpha-emitting radionuclides in low- and intermediate-level waste as defined by IAEA. Waste that contains high concentrations of long-lived, beta/gamma-emitting radionuclides is not accounted for explicitly in IAEA’s classification system. Similarly, low-level waste as defined in the United States can contain high concentrations of long-lived, alpha-emitting nontransuranium radionuclides (e.g., 233U), but the concentrations of these radionuclides are limited in low- and intermediate-level waste as defined by IAEA. Fourth, the definitions of waste classes in the United States are not related to requirements for disposal. In IAEA’s waste classification system, there is some linkage between the definitions of waste classes and the types of disposal technologies that would be required, particularly for high-level waste. However, not all waste classes in IAEA’s system are linked to required disposal technologies, because lowand intermediate-level waste could be acceptable for near-surface disposal or could require disposal in a geologic repository depending, for example, on the concentrations of long-lived radionuclides. Finally, recommendations on principles that could be used to exempt radioactive waste from regulatory control as radioactive material have been developed both in the United States (NCRP,

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1993a) and by IAEA (1988), and IAEA has implemented its recommendations by developing concentration limits of radionuclides in exempt waste (IAEA, 1995). In the United States, however, NRC is forbidden by law from implementing generally applicable exemption principles, and radioactive waste presently can be exempted only on a case-by-case basis.

4.2 Classification and Disposal of Hazardous Chemical Waste This Section discusses the current system for classification and disposal of hazardous chemical waste in the United States. Most hazardous chemical waste is managed under RCRA (1976) and its implementing regulations established by EPA, and the system for classification and disposal of hazardous chemical waste established under RCRA is discussed in this Section. However, some hazardous chemical wastes are regulated under other environmental laws. Examples include wastes that contain dioxins, PCBs, or asbestos, which are regulated by EPA under TSCA (1976), and sewage sludge, which is regulated by EPA under the Clean Water Act (CWA, 1972).

4.2.1 Classification System for Hazardous Chemical Waste Under the Resource Conservation and Recovery Act This Section discusses the definitions of hazardous chemical waste developed under RCRA (1976) and its implementing regulations. RCRA was not preceded by substantial federal involvement in the management of hazardous chemical waste (EPA, 1978). Thus, formal systems for defining and classifying such waste had not been developed previously. RCRA and its implementing regulations were established based, in part, on a Congressional finding that disposal of hazardous chemical waste in or on the land without careful planning and management had endangered human health and the environment. The classification system for hazardous chemical waste was developed independently of the radioactive waste classification system discussed in Section 4.1.2. In contrast to the classification system for radioactive waste, the classification system for hazardous chemical waste was not developed in recognition of the unavoidable risks from exposure to naturally occurring hazardous chemicals in the environment (see Section 6.3.1.2.1).

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4.2.1.1 Description of EPA’s Hazardous Waste Classification System. RCRA (1976) and its implementing regulations (EPA, 1980b) have defined ‘‘solid waste’’ and ‘‘hazardous waste.’’ As defined in Section 1004(27) of RCRA, a solid waste is any garbage, refuse, or sludge from a waste treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, including solid, liquid, semisolid, or contained gaseous material resulting from industrial, commercial, mining, and agricultural operations and from community activities. Solid waste does not include solid or dissolved material in domestic sewage, or solid or dissolved material in irrigation return flows or industrial discharges that are point sources subject to permits under the Clean Water Act (CWA, 1972). Also specifically excluded from solid waste are source, special nuclear, and byproduct materials as defined in AEA (1954). A more detailed definition of ‘‘solid waste’’ and description of materials excluded from solid waste are given in the implementing regulations (EPA, 1980b). The exclusion of radioactive materials defined in AEA from regulation under RCRA leads to the concept of ‘‘mixed waste’’ that contains radioactive and hazardous chemical waste. Management and disposal of mixed waste is discussed in Section 4.3. Given the definition of ‘‘solid waste’’ described above, Section 1004(5) of RCRA (1976) then defines ‘‘hazardous waste’’ as follows: Hazardous waste is a solid waste, or combination of solid wastes, which because of its quantity, concentration, or physical, chemical or infectious character may: Clause (A): cause, or significantly contribute to, an increase in mortality or an increase in serious irreversible, or incapacitating reversible, illness; or Clause (B): pose a substantial present or potential hazard to human health or the environment when improperly treated, stored, transported, or disposed of, or otherwise managed. This definition is further amplified by Section 3001(a) of RCRA (1976), which specifies that EPA shall develop and promulgate criteria for identifying the characteristics of hazardous waste and for listing hazardous waste, taking into account its toxicity, persistence, and degradability in nature, potential for accumulation in tissue, and other related factors such as flammability, corrosiveness, and other hazardous characteristics. Based on the legal definition of ‘‘hazardous waste’’ given above, EPA developed a definition of hazardous chemical waste in 40 CFR Part 261 (EPA, 1980b). Hazardous chemical waste is defined as a

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solid waste that meets at least one of several criteria: it exhibits the characteristic of ignitability, corrosivity, reactivity, or toxicity; or it is a specifically listed waste. Biohazardous waste is not included because such waste normally is rendered nonhazardous before disposal according to EPA guidelines (EPA, 1986a). The various types of hazardous chemical waste defined by EPA under RCRA are described below. An ignitable waste is a solid waste that meets one of the following criteria: (1) it is a liquid, other than an aqueous solution containing less than 24 percent alcohol, that has a flash point less than 60 °C; (2) it is not a liquid and is capable, under standard temperature and pressure, of causing fire through friction, absorption of moisture or spontaneous chemical changes and, when ignited, burns so vigorously and persistently that it creates a hazard; (3) it is an ignitable compressed gas; or (4) it is a specified oxidizing agent. A corrosive waste is a solid waste that meets one of the following criteria: (1) it is an aqueous waste with a pH of less than or equal to two or greater than or equal to 12.5; or (2) it corrodes SAE 1020 steel at a rate greater than 6.35 mm yⳮ1 at 55 °C. A reactive waste is a solid waste that meets one of the following criteria: (1) it is normally unstable and readily undergoes violent change without detonating; (2) it reacts violently with water; (3) it forms potentially explosive mixtures with water; (4) it generates toxic gases, vapors or fumes in a quantity sufficient to pose a danger to human health or the environment when mixed with water; (5) it is a cyanide or sulfide that releases toxic gases, vapors or fumes when in contact with materials at a pH between 2 and 12.5; (6) it is capable of detonation or explosive reaction when subjected to a strong initiating source or heated under confinement; (7) it is readily capable of detonation or explosive decomposition at standard temperature and pressure; or (8) it is a forbidden, Class-A, or Class-B explosive as defined by the U.S. Department of Transportation. A toxic waste is a solid waste which, when tested using the toxicity characteristic leaching procedure (EPA, 1980b, Appendix II), yields an extract containing any of 40 contaminants (heavy metals and organic compounds) at or above specified concentrations (EPA, 1980b, Table 1). The contaminants considered by EPA in defining the toxicity characteristic were mainly (but not exclusively) those included in drinking water standards (EPA, 1975) developed under the Safe Drinking Water Act (SDWA, 1974). Thus, toxic waste, as defined under RCRA, is waste which, when placed in a landfill, could result in contamination of groundwater at a nearby well above drinking water standards, based on modeling of transport of leached contaminants to the well. However, the toxicity characteristic does

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not take into account the presence of many hazardous chemicals in waste materials that could be leached into groundwater in significant amounts. EPA originally intended that the toxicity characteristic leaching procedure would be used to identify hazardous chemical waste that contains substances other than those included in drinking water standards, but EPA was unable to do so because concentrations of concern in drinking water had not been established for many other substances (EPA, 1980b). In addition to chemical waste that may be classified as hazardous based on one or more of the characteristics described above, a chemical waste may be classified as hazardous if it is specifically listed (EPA, 1980b). Chemical wastes are listed based on their source or the presence of specific hazardous substances. Listed hazardous wastes include wastes from nonspecific sources (the so-called ‘‘F’’ list), wastes from specific sources (‘‘K’’ list), acutely toxic hazardous waste from any source (‘‘P’’ list), and toxic (other than acute) waste from any source (‘‘U’’ list). When a hazardous chemical waste is mixed with a nonhazardous waste, the entire waste is considered hazardous if the initial hazardous waste is a listed waste or if the final waste exhibits a hazardous characteristic (the so-called ‘‘mixture rule’’) (EPA, 1980b; 1992c; 2001b). Mixing of a hazardous chemical waste with a nonhazardous waste can result in a waste that is nonhazardous only if the initial hazardous waste is not a listed waste and mixing eliminates any hazardous characteristics in the initial hazardous waste. Any waste derived from processing of a listed waste also is a listed waste, without regard for the amounts of listed substances, until it is delisted, which allows the waste to be managed as a nonhazardous solid waste (the so-called ‘‘derived-from rule’’) (EPA, 1992c; 1993b; 2001b). Based on the descriptions of hazardous chemical waste given above, a waste is either hazardous or it is not, and there is no further classification of hazardous waste with respect to degree of hazard. Some states have defined a category of extremely hazardous waste (see Section 4.2.1.3), and extremely hazardous substances are specified by EPA (1987b) under the Emergency Response and Community Right-to-Know Act, which is a free-standing title of CERCLA (1980). However, these designations have not affected how hazardous waste is classified, managed, and disposed of under RCRA. 4.2.1.2 Discussion of EPA’s Hazardous Waste Classification System. Waste that is hazardous because it is ignitable, corrosive, or reactive, as defined above, must be treated to remove these characteristics prior to disposal. Examples of appropriate treatment methods

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include neutralization of acidic or basic corrosive waste, chemical reaction of a reactive waste to render it nonreactive, or incineration of an ignitable waste. A waste that is hazardous only because it is ignitable, corrosive, or reactive is no longer considered hazardous after treatment to remove these characteristics. However, ignitable, corrosive, or reactive waste may still be considered hazardous after treatment to remove these characteristics if, prior to and following treatment, the waste exhibits the toxicity characteristic or it contains a listed substance. Waste that is hazardous because it exhibits the toxicity characteristic also must be treated to remove this characteristic prior to disposal. Techniques to remove the toxicity characteristic include, for example, destruction of organic compounds by incineration or incorporation of the waste in an immobilizing waste form (e.g., grout). However, in contrast to ignitable, corrosive, or reactive waste, a properly treated toxic waste may still be considered hazardous in some cases, even if it is not characteristically hazardous after treatment and does not contain any listed substances. For example, a waste that is toxic because it contains high levels of heavy metals could be treated to reduce the leachability of the metals to acceptable levels by incorporation in an appropriate waste form, but the treated waste may still be considered hazardous when the toxic substances of concern are not destroyed by treatment and the possibility exists that their leachability from the waste form could increase substantially after disposal. As noted previously, a listed hazardous waste cannot be rendered nonhazardous by treatment or by dilution or mixing with nonhazardous material. EPA has issued proposals related to establishing exemption levels for listed hazardous waste that contains small amounts of listed substances (EPA, 1992d; 1995c; 1999c), but exemption provisions for listed hazardous waste have not yet been established. At the present time, specific hazardous chemical wastes can be exempted from RCRA requirements in one of two ways. First, a waste generator may petition EPA to ‘‘delist’’ a listed hazardous waste, and the waste would be exempted from RCRA requirements if the petition were approved (EPA, 1993b). Thus, listed wastes are delisted on a case-by-case basis. Second, EPA has exempted certain materials from requirements for regulation as hazardous waste under RCRA and other laws, including the Clean Water Act (CWA, 1972) and Clean Air Act (CAA, 1963), to permit their beneficial use. Examples of such materials include ash and sludge from coalburning power plants used in construction materials and in cement and concrete products (EPA, 2000b), sewage sludge used as fertilizer (EPA, 1999d), and phosphogypsum used in outdoor agriculture (EPA,

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1992b). Coal ash and sewage sludge contain elevated levels of heavy metals and naturally occurring radionuclides, and phosphogypsum contains elevated levels of radium that result in elevated levels of radon in air. Of the various ways of designating a solid waste as hazardous described above, only the toxicity characteristic is based on a quantitative assessment of potential risks resulting from waste disposal. The specifications of ignitable, corrosive, and reactive waste are based on qualitative considerations of risk, in that the presence of materials with these characteristics in a disposal facility clearly constitutes a hazard that could compromise the ability of the facility to protect public health. The specifications of listed hazardous wastes are based on risk in the sense that the listed substances have been identified as potentially hazardous to human health. However, requirements for treatment and disposal of listed waste discussed in Section 4.2.2 do not distinguish between different wastes based on considerations of risk from disposal. Although EPA’s hazardous waste classification system developed under RCRA applies to a great many chemical wastes, the system is not comprehensive because it does not apply to all potentially hazardous wastes. Some wastes are not regulated under RCRA because they are regulated under other environmental laws, including TSCA (1976) and the Clean Water Act (CWA, 1972). In addition, many potentially important wastes containing hazardous chemicals that are not regulated under other environmental laws are specifically excluded from the definition of hazardous waste in 40 CFR Part 261 (EPA, 1980b) and, thus, may not be regulated under RCRA. Examples include: certain wastes from combustion of coal or other fossil fuels; drilling fluids and other wastes associated with the exploration, development, or production of crude oil, natural gas, or geothermal energy; and a variety of wastes from the extraction, beneficiation, and processing of ores and minerals. These wastes can contain concentrations of heavy metals well above average background levels in soil and rock, as well as hazardous substances introduced in processing of materials. The exclusions from hazardous waste specified in 40 CFR Part 261 (EPA, 1980b) generally are based on the source of the waste rather than the risk associated with management or disposal. 4.2.1.3 State Programs. Many states have been authorized to administer their own hazardous waste management programs. Each of these states uses the definition of hazardous waste described above (Lathrop, 1992a; 1992b). Details of selected state programs are given in a report of the Office of Technology Assessment (OTA, 1981).

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The states of Washington and California have considered a classification of hazardous chemical waste based on risk and have developed a category of extremely hazardous waste (California, 1999; Mehlhaff et al., 1979; NAS/NRC, 1999b). However, the requirements for treatment and disposal of extremely hazardous waste differ little from those applied to other hazardous waste. Thus, the designation of a class of extremely hazardous waste based on relative hazard has had little effect on waste management and disposal.

4.2.2 Treatment and Disposition of Hazardous Chemical Waste There generally are three available dispositions for hazardous chemical waste: incineration, deep-well injection, and disposal in a near-surface facility. Incineration involves high-temperature burning to destroy the hazardous constituents of the waste. This leaves only a small residual ash that is typically nonhazardous, unless it contains high levels of heavy metals; in some cases, incinerator ash may be rendered nonhazardous by use of a waste form (e.g., grout) or by delisting of the waste, depending on the nature of the hazard. Incineration is most commonly used to destroy liquid hazardous waste, although certain kinds of solid hazardous waste also may be incinerated. Deep-well injection involves drilling a borehole into a permeable geologic formation below all potential sources of groundwater of usable quality and quantity. Waste then is pumped into the formation and the borehole is sealed. This method is most commonly used for large quantities of contaminated wastewater but not for solidified hazardous waste. Incineration and deep-well injection are not considered further in this Report, because they are intended for the destruction or disposal of a narrow range of hazardous chemical wastes. All waste classified as hazardous under RCRA, including waste that has been treated to remove the toxicity characteristic but is still considered hazardous, is managed under Subtitle C of RCRA and implementing regulations established by EPA in 40 CFR Parts 260–268 (EPA, 1986b). Emplacement in a near-surface burial site (hazardous waste disposal facility) is the usual disposition of solidified hazardous chemical waste. Burial sites must meet stringent location requirements with respect to seismic considerations, floodplains, salt domes and beds, underground mines and caves, and potential sources of drinking water. Each disposal facility must be constructed with an appropriate liner system, leachate collection and removal system, and leak detection system. The requirements on the design, construction, and operation of a disposal facility reflect

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the importance of preventing contamination of groundwater. A burial site is to be actively maintained for 30 y after closure, at which time the operator can cease active maintenance and rely on passive safety features if EPA so approves. Institutional control generally must be maintained over disposal sites for as long as the waste remains hazardous. However, the basis for judging that waste at a disposal site is no longer hazardous has not been established, and no RCRA burial sites have gone through this process. Certain restrictions are imposed on hazardous wastes that may be placed in a disposal facility or in the same disposal cell at a site. The applicable regulations in 40 CFR Part 268 (EPA, 1986b) contain an extensive catalog of hazardous chemical wastes for which disposal is prohibited, called land disposal restrictions (LDRs). LDRs require treatment of these wastes before disposal. The required treatment may be incineration, stabilization, extraction of metals, or other appropriate technologies, depending on the nature of the waste. Ignitable, corrosive, or reactive waste generally requires treatment to remove the hazardous characteristic prior to disposal. Incompatible wastes, or incompatible wastes and materials, cannot be placed in the same cell at a disposal site. Certain listed wastes (F020, F021, F022, F023, F026, and F027), which may contain 2,3,7,8-tetrachlorodibenzodioxin, may not be buried unless EPA has approved a disposal plan (EPA, 1986b). From a practical viewpoint, an operator of a hazardous chemical waste disposal site may segregate the waste received in any way seen fit to best utilize the facility, as long as the conditions of the operating permit are not violated. The requirements on treatment and disposal of hazardous chemical waste established by EPA under RCRA, especially LDRs specified in 40 CFR Part 268 (EPA, 1986b) and the intention to limit contamination of groundwater, are based on a desire to limit risks to public health and the environment. However, these requirements are not based on long-term projections of health risks to the public beyond the site boundary, nor is any consideration given to potential risks to individuals who might inadvertently intrude onto the disposal site after institutional control ceases. Rather, in addition to the detailed technical requirements on waste treatment and disposal that apply at any site, the approach to protection of public health and the environment under RCRA relies on monitoring of releases of hazardous substances, especially releases into groundwater, corrective actions if releases exceed specified standards, and an intention to maintain institutional control over disposal sites for as long as the waste remains hazardous. Based on the foregoing discussions, management of hazardous chemical waste is essentially not risk-based for the following reasons:

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The same management system is applied to all hazardous chemical waste, regardless of the degree of hazard or potential risk. Essentially all solidified hazardous chemical waste is intended for disposal in near-surface facilities, with prescribed actions to prevent unacceptable releases of hazardous material (e.g., leachate collection and treatment). However, these facilities have been developed and operated essentially without consideration of the potential long-term risks posed by the waste in the absence of active monitoring and maintenance, including potential risks to future inadvertent intruders, or the requirements on site closure and release from institutional control that would ensure long-term protection of public health and the environment. Exclusion or exemption of waste that contains hazardous substances from the management system for hazardous chemical waste has been based primarily on the source of the waste rather than the risk that it poses.

Section 3019 of RCRA (1976) calls for risk-based analyses to provide justification for closing hazardous waste disposal sites, and EPA’s implementing regulations in 40 CFR Part 268 (EPA, 1986b) incorporate risk-based groundwater protection standards. However, these types of risk analyses and groundwater protection standards generally have been applied at hazardous waste disposal sites only on a real-time or near-term basis. They have not been applied prospectively over long time periods in the future. RCRA (1976) also addresses nonhazardous waste, and disposal of nonhazardous waste in sanitary (municipal/industrial) landfills is governed under Subtitle D. This type of waste includes household trash, various industrial wastes, and characteristically hazardous waste that has been treated and is no longer considered hazardous. In current EPA regulations implementing Subtitle D in 40 CFR Part 258 (EPA, 1991b), requirements on siting, design, operation, and closure of landfills are similar to the requirements that apply to hazardous waste disposal facilities regulated under Subtitle C of RCRA. Therefore, management and disposal of hazardous and nonhazardous wastes differ mainly in regard to requirements on waste generators and the storage and treatment of waste, as well as requirements on institutional control after closure of a disposal facility. 4.3 Regulation of Mixed Radioactive and Hazardous Chemical Waste Mixed waste is waste that contains both radionuclides and hazardous chemicals; it is not a separate class of waste. This Section discusses mixed waste issues as they relate to waste classification and

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waste management and disposal. Particular emphasis is given to the historical development of current approaches to regulating mixed waste. Rather than waste classification per se, the issues discussed in this Section are largely concerned with difficulties that have been encountered in the management of mixed waste.

4.3.1 Introduction In 1992, Congress enacted the Federal Facility Compliance Act (FFCA, 1992), which defined mixed waste as waste that contains both hazardous chemicals regulated under RCRA (1976) and source, special nuclear, or byproduct material regulated under AEA (1954). Although the primary purpose of the Federal Facility Compliance Act was to waive sovereign immunity under RCRA for federal facilities, the Act contains special provisions for mixed waste that address a range of legal and institutional issues associated with mixed waste generated by the federal government. Also, states may have definitions of hazardous chemical waste, and thus mixed waste, that vary from the federal definitions. For example, several states currently include PCBs in their RCRA statutes concerning hazardous chemical wastes, whereas PCBs are hazardous under federal regulations only as a result of provisions of TSCA (1976). Federal facilities that generate mixed waste in these states need to accommodate such differences. Most mixed waste in the United States is the responsibility of DOE, which has about 525,000 m3 currently in storage and is expected to generate approximately 30,000 m3 in the next 35 y (DOE, 1997a). DOE’s mixed waste is about 71 percent high-level waste, 14 percent low-level waste, and 15 percent transuranic waste, and it consists of aqueous liquids, inorganic sludges and particulates, assorted debris, and a variety of other forms (Bloom and Berry, 1994). The volumes of mixed high-level waste and transuranic waste have remained fairly constant over time, but there has been a sharp decline in the amount of mixed low-level waste. Non-DOE (commercial) mixed waste is generated at a rate of about 4,000 m3 yⳮ1; since commercial mixed waste is more amenable to treatment using existing capabilities, only about 2,100 m3 is currently in storage. Because mixed waste contains radionuclides and hazardous chemicals, it is subject to dual regulation under AEA and RCRA. Mixed waste generated in commercial activities is regulated under EPA or state RCRA requirements and NRC or comparable Agreement State requirements under AEA. Mixed waste generated by DOE is regulated by DOE under AEA and by EPA or a state under RCRA. This dual regulatory framework has created difficulties for mixed waste

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managers in attempting to understand and comply with requirements under the two laws. In addition, uncertainties in the volume and characteristics of mixed low-level waste have been a significant barrier to development of treatment and disposal facilities for mixed waste.

4.3.2 Establishing Dual Regulation of Mixed Waste Prior to the mid-1980s, most mixed waste was managed and disposed of as radioactive waste. Although the issue of mixed waste surfaced during development of NRC’s licensing requirements for land disposal of low-level waste in 10 CFR Part 61 (NRC, 1982a), until 1985 mixed waste was regulated principally for its radiological characteristics. Many mixed waste managers believed that the exclusion of source, special nuclear, and byproduct material from the definition of solid waste at Section 1004(27) of RCRA (1976) excluded mixed waste from the requirements of RCRA. In addition, many believed that there were inconsistencies in the requirements of AEA and RCRA and that Section 1006(a) of RCRA12 would result in mixed waste not being subject to RCRA. Finally, it was commonly believed that, in many cases, mixed waste could be managed according to the predominant hazard associated with the waste, which usually was determined to be the radioactive material. During the mid-1980s, several significant actions changed the way in which mixed waste was managed. In 1984, the U.S. District Court for the Eastern District of Tennessee established that hazardous chemical waste generated at DOE’s Y-12 Plant in Oak Ridge, Tennessee, was subject to the requirements of RCRA (LEAF, 1984). The plaintiffs in this case, the Legal Environmental Assistance Foundation (LEAF), charged that DOE was in violation of RCRA and the Clean Water Act (CWA, 1972). In response, DOE argued that application of RCRA to the Y-12 Plant was inconsistent with AEA because (1) AEA precluded state regulation of DOE activities but RCRA subjected federal facilities to state regulation, (2) AEA gave the authority to set waste disposal standards to DOE while RCRA gave this authority to EPA and to state and local authorities, and (3) AEA restricted the dissemination of 12

Section 1006(a) of RCRA (1976) states that ‘‘Nothing in this Act shall be construed to apply to (or authorize any State, interstate, or local authority to regulate) any activity or substance which is subject to . . . the Atomic Energy Act of 1954 (42 U.S.C. 2011 and following) except to the extent that such application (or regulation) is not inconsistent with the requirements of such Acts.’’

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data pertaining to nuclear weapons while RCRA would require the public disclosure of this information. The Court found in favor of LEAF by concluding that application of RCRA to the Y-12 Plant was not inconsistent with AEA. The Court also concluded that the most reasonable reconciliation of the two statutes was that facilities regulated under AEA are subject to RCRA, except RCRA does not apply to those materials that are expressly regulated under AEA (i.e., source, special nuclear, and byproduct materials). A second action occurred as an outcome of legislative debates on the Low-Level Radioactive Waste Policy Amendments Act (LLRWPAA, 1986). During these debates, various proposals were made to assign jurisdiction for regulation of mixed low-level waste to a single agency, in order to provide relief to mixed waste generators and ease the burdens associated with dual regulation of mixed waste. There also was a concern that delays in issuing permits for mixed waste treatment and disposal facilities would undermine state efforts to comply with schedules for developing new disposal capacity for low-level radioactive waste. Because of the complexity of the issue and the desire to avoid creating a special class of hazardous chemical waste that would not be regulated under RCRA, Congress encouraged NRC and EPA to resolve mixed waste issues administratively by using the flexibility in the regulatory programs under both statutes. Thus, consideration of sole agency jurisdiction over mixed waste was suspended. This ensured that commercial mixed waste would remain subject to dual regulation by NRC and EPA. In 1985, DOE published a notice of proposed rulemaking (the Byproduct Material Rule) that would have established an interpretive rule clarifying RCRA’s applicability to DOE’s radioactive waste (DOE, 1985b). This rule would have established a distinction between ‘‘direct process’’ waste, which is waste directly yielded in or necessary to the process of producing and utilizing special nuclear material, and other radioactive waste. Direct process waste, even when it contained hazardous chemical waste, would have been regulated solely as byproduct material. Any non-direct process waste that contained hazardous chemical waste would have been managed as mixed waste. Based on additional operational experience and comments on the proposed rule, DOE chose to abandon an attempt to distinguish direct process waste from other radioactive waste and adopted a narrower interpretation of the definition of byproduct material, which limited the authority of AEA to the radionuclides alone (DOE, 1987b). In 1986, EPA published a notice that a state must have the authority to regulate mixed waste in order for that state to obtain and

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maintain authorization to administer and enforce a hazardous chemical waste program pursuant to Subtitle C of RCRA (EPA, 1986c). As part of this notice, EPA discussed the applicability of RCRA to mixed waste, concluding that waste containing hazardous chemicals and radionuclides is subject to RCRA regulations. Prior to this notice, EPA had not established that mixed waste also is subject to the requirements of RCRA, and states with RCRA authorization were not required to obtain the authority to regulate the hazardous chemical portion of mixed waste under their RCRA programs. EPA also concluded that states that had previously obtained RCRA authorization did not have authorization to regulate mixed waste until they obtained specific authorization to do so. This notice essentially recognized that mixed waste contains both a radioactive component subject to AEA and a hazardous chemical component subject to RCRA, and it specified that states would need to recognize this dual regulatory framework in order to maintain their RCRA programs. DOE acknowledged the dual regulatory framework for mixed waste in 1987 with a notice clarifying the definition of byproduct material (DOE, 1987b). In this notice, DOE issued a final interpretive rule establishing that the exclusion of byproduct material at Section 1004(27) of RCRA applied only to the radionuclides in mixed waste and that the nonradioactive portion of the waste was subject to RCRA. In addition, in 1987, DOE recognized that RCRA LDRs (see Section 4.2.2) and other RCRA requirements applied to transuranic waste intended for disposal at the Waste Isolation Pilot Plant (see Section 4.1.2.3.2). Thus, by the late 1980s, it was firmly established that mixed waste was subject to dual regulation under AEA and RCRA, and that generators of mixed waste would be required to comply with regulations for the control of radioactive material and hazardous chemical waste under both statutes. Because compliance with two sets of regulations would be difficult, many generators believed that relief from dual regulation might be obtained by identifying inconsistencies in requirements of AEA and RCRA. Throughout the late 1980s, however, NRC and EPA repeatedly concluded that no inconsistencies existed in the regulatory programs established under the two statutes that could not be resolved through the existing flexibility within each agency’s programs. The agencies also advanced the position that if such inconsistencies existed, relief from RCRA requirements would be limited to those specific requirements that were inconsistent with those of AEA. In the face of such regulatory uncertainties, private companies have been largely unwilling to take the financial risk to design, build, and operate disposal facilities that would accept all mixed low-level waste. A near-surface disposal facility in Clive,

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Utah, which has a radioactive waste disposal license and a RCRA permit from the state, accepts large volumes of mixed waste containing low concentrations of radionuclides that resembles uranium or thorium mill tailings.

4.3.3 Facilitating Compliance with Dual Regulation of Mixed Low-Level Waste In 1981, during the development of licensing requirements for near-surface disposal of radioactive waste, principally low-level waste, in 10 CFR Part 61, NRC stated that the standards for radioactive waste were adequate for hazardous chemical waste as well, and that NRC would work with EPA to ensure that mixed low-level waste was disposed of in a manner that met both agencies’ requirements (NRC, 1982a). In response to NRC’s request for comments on the proposed low-level waste disposal requirements, several commenters suggested that NRC’s waste classification system (the Class-A, -B, and -C limits and associated disposal requirements; see Section 4.1.2.3.3) should incorporate a ‘‘total hazard’’ approach that would consider both the radiological hazard and the chemical hazard of the waste. NRC stated publicly that if it were technically feasible to classify waste by total hazard, then it would make ‘‘eminently good sense’’ to do so. NRC also stated that although it was not aware of a scheme for such classification, it appeared that DOE intended to support research into the development of a classification system for hazardous chemical waste that might be compatible with 10 CFR Part 61 (NRC, 1982a). In the final disposal standards, NRC established that waste posing nonradiological hazards be treated, to the maximum extent practicable, to eliminate or minimize such hazards, although NRC does not regulate these constituents. After Congress declined to address mixed low-level waste in the Low-Level Radioactive Waste Policy Amendments Act (LLRWPAA, 1986), NRC and EPA identified several issues that required resolution through the development of joint guidance documents, including the issues of what the two agencies considered to be mixed waste and what environmental and design criteria should be considered in developing mixed waste disposal facilities. The agencies’ objective in developing these guidance documents was to facilitate compliance with the dual regulatory framework and to remove institutional barriers to the development of commercial low-level radioactive waste disposal facilities in accordance with the schedules in LLRWPAA. In addition, mixed low-level waste generators identified the need for guidance on the testing and storage of mixed waste.

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To address these issues, EPA and NRC developed and issued guidance documents on: ● ● ● ● ● ●

the definition and identification of mixed low-level waste (EPA/ NRC, 1989) siting of mixed low-level waste disposal facilities (EPA/NRC, 1987a) the design of mixed low-level waste disposal facilities (EPA/ NRC, 1987b) testing of mixed low-level waste (NRC, 1992) the elements of a mixed waste minimization program (NRC, 1994a) storage of mixed waste (NRC, 1995)

By developing these joint guidance documents, NRC and EPA attempted to provide assistance to mixed low-level waste managers faced with the complicated task of understanding and complying with the requirements of the two agencies. While these guidance documents were developed for commercial mixed waste, the concepts also apply to management of mixed low-level waste generated at DOE facilities. In 1990, EPA published the first treatment standards for mixed waste, including LDRs, as part of the ‘‘Third Thirds’’ rule (EPA, 1990b). 13 The remaining parts of this rule were issued later in 40 CFR Part 268 (EPA, 2001b). Under LDRs, untreated hazardous chemical waste may not be disposed of in a land disposal facility, except under limited conditions such as demonstrating that there will be no migration of the waste from the disposal unit for as long as the waste remains hazardous. LDRs prescribe treatment standards for hazardous chemical waste as either concentrations of specific hazardous substances in the treated waste (or waste extract) or as a specified technology [best demonstrated available technology (BDAT)]. Examples of these mixed wastes and their BDAT treatment standards include vitrification of high-level radioactive waste generated during the reprocessing of spent nuclear fuel, macroencapsulation of lead solids, amalgamation of radioactively contaminated elemental mercury, and incineration of hydraulic oil contaminated with mercury. In addition to promulgating the treatment standards for most mixed waste in 1990, EPA established a 2 y National Capacity Variance for mixed waste for which there was no available treatment or 13

LDRs specified in RCRA required EPA to develop treatment standards for hazardous chemical waste and established deadlines for EPA to develop treatment standards for those wastes for which treatment standards did not exist. Congress divided LDR hazardous waste into several categories: solvents and dioxins; California listed wastes; first, second, and third listed wastes; and characteristically hazardous wastes.

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disposal capacity. Under this variance, which expired in May 1992, mixed waste generators were not required to treat their mixed wastes for which treatment capacity was not available, provided the waste was disposed of in a facility that met EPA’s minimum technology standards. However, in the absence of disposal facilities having RCRA permits, such disposal did not occur during the capacity variance. In 1991, because of a lack of adequate treatment capacity for most mixed waste, EPA adopted a policy that assigned a lower enforcement priority to violations of storage prohibitions under Section 3004(j) of RCRA (1976), provided certain conditions were met (EPA, 1991c). RCRA requirements prohibit storage of LDR-restricted hazardous waste unless such storage is solely for the purpose of accumulating sufficient quantities to allow proper recovery, treatment, or disposal. EPA policy on enforcement was limited to facilities that generated less than 28 m3 yⳮ1 of LDR-restricted mixed waste, was limited in duration, and initially expired on December 31, 1993. EPA has extended its enforcement policy at 2 y intervals (EPA, 1994c; 1996c; 1998b), because the limited availability of treatment and disposal capacity had not changed substantially. In order for facilities to be afforded the lower enforcement priority, they must demonstrate to EPA that they are managing their mixed waste in an environmentally sound manner. EPA indicated that it would consider a variety of indicators of environmentally responsible management in determining the civil enforcement priority at individual facilities. These indicators include, but are not limited to, whether the facility has: ●

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conducted an inventory of its mixed waste storage areas to assess and assure compliance with all other applicable RCRA storage facility standards; identified and kept records of its mixed waste, including sources, waste codes, generation rates, and volumes in storage; developed a mixed waste minimization plan, or can demonstrate that waste minimization is not technically feasible for its waste; and documented periodically that it has made good faith efforts to ascertain the availability of treatment capacity for its mixed waste.

EPA policy concerning relaxed enforcement of prohibitions on storage of mixed waste does not extend to Executive Branch federal facilities, including DOE facilities. In 1992, Congress amended Section 6001 of RCRA (1976) through the Federal Facility Compliance Act (FFCA, 1992) to clarify that federal facilities are subject to administrative orders and civil and administrative penalties and

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fines related to management and disposal of hazardous chemical waste. The Federal Facility Compliance Act allows EPA to take administrative enforcement action pursuant to the enforcement authority in RCRA. The Act requires that DOE submit to EPA and the governor of each state in which DOE stores or generates mixed waste a report containing a national inventory of mixed waste and a report containing a national inventory of mixed waste treatment capacities and technologies. DOE also must develop a plan for establishing treatment capacities and technologies to treat mixed waste at each of its facilities. This plan must contain a schedule of the actions necessary to accomplish the treatment and disposal of mixed waste or, for mixed waste for which treatment capacity does not exist, a schedule of actions necessary to initiate research and development of this technology. In 1992, NRC and EPA issued the National Profile on commercially generated mixed low-level waste (Klein et al., 1992) in response to a request from the Host State Technical Coordinating Committee (Alvarado, 1990). In its request, the committee stated that the information was needed by states, compact officials, private developers, and federal agencies to plan and develop treatment and disposal facilities for commercial mixed low-level waste. The National Profile was based on a survey of over 1,300 licensed nuclear facilities. The National Profile indicated that commercial nuclear facilities licensed by NRC or an Agreement State generated about 4,000 m3 of mixed low-level waste in 1990. Industrial facilities produced the largest amount (1,400 m3) and nuclear utilities the least (400 m3). Liquid scintillation fluids comprised the largest portion of commercial mixed low-level waste in 1990, about 71 percent. In addition, 2,100 m3 of commercial mixed low-level waste was in storage as of the end of 1990. The National Profile indicated that adequate treatment capacity existed for much of the commercial mixed lowlevel waste, although additional treatment capacity was required to treat all the waste generated or in storage as of the end of 1990. Wastes that contain chlorinated fluorocarbons, lead, or mercury were identified as needing additional treatment capacity. Since 1990, additional treatment capacity using vitrification, steam reforming, and molten metal technology has been developed to address some of this capacity shortfall (Kirner et al., 1995). Significant quantities of mixed low-level waste have been generated and are in storage at more than 40 DOE sites, including national laboratories and naval shipyards. These wastes contain materials listed as hazardous or having hazardous characteristics under RCRA and wastes that are considered hazardous under TSCA (1976). At the end of 1994, DOE sites had in storage 91,000 metric tons of

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RCRA-contaminated mixed low-level waste and 15,000 metric tons of TSCA-contaminated mixed low-level waste. The volumes of these wastes are about 138,000 m3 and 24,000 m3, respectively. The largest component of these wastes is inorganic solids and contaminated soils. DOE’s Integrated Data Base Report (DOE, 1997a) contains more information on the locations, characteristics, and projected generation rates of DOE’s mixed low-level waste. Future cleanups at DOE sites may greatly increase the volumes of contaminated soils that need to be treated as mixed waste. Treatment capabilities for both DOE and commercial mixed low-level wastes are described in a report by DOE’s National Low-Level Waste Management Program (DOE, 1996b). As indicated above, the volumes of commercial mixed low-level waste are much less than the volumes at DOE sites. Therefore, in the interest of cost-effective waste management, consideration was given to the possibility of DOE accepting the commercial waste for treatment and disposal (Owens et al., 1993). However, given the difficulties that DOE has experienced in managing its own mixed low-level waste in accordance with legal and regulatory requirements under RCRA and the Federal Facility Compliance Act and the additional complications that would arise in accepting responsibility for commercial mixed waste, this possibility has not been pursued to any significant extent. In 1993, the U.S. Court of Appeals for the District of Columbia Circuit denied a petition filed by the Edison Electric Institute and other plaintiffs concerning EPA’s prohibition at Section 3004(j) of RCRA (1976) on indefinite storage of mixed waste for which treatment and disposal capacity did not exist (EEI, 1993). EPA’s interpretation of the statute was that it was unlawful to store waste for indefinite periods of time pending the development of adequate treatment or disposal capacity. The plaintiffs contended that this interpretation was inconsistent with the statute and unreasonable as it applied to generators of mixed waste. The Court denied the petition and ruled that EPA’s interpretation was permissible and, in fact, mandated by the statute. In its opinion, the Court stated the following: ‘‘Thus, we conclude that . . . Section 3004(j) clearly proscribes the indefinite storage of wastes pending the development of treatment and disposal capacity. We wish to emphasize that we are not unsympathetic to the hardships that this decision implies for mixed waste generators. They find themselves in the unenviable position of having no choice but to violate the law. Nevertheless, the possibility that such hardships will occur is inherent in statutes such as RCRA that are expressly designed to force technology by threatening extreme sanctions. Moreover, the fact that technology may not keep

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up with time-tables established by Congress does not mean that Courts are at liberty to ignore them, however burdensome the resulting enforcement. Accordingly, if the petitioners are to obtain relief it must come from Congress.’’ In 2001, EPA issued a new regulation that provides increased flexibility to facilities that manage mixed low-level waste by reducing the burden of dual regulation under AEA and RCRA (EPA, 2001c). This regulation specifies conditions under which storage and treatment of commercial mixed low-level waste at the generating site is exempt from RCRA requirements when the generator is licensed by NRC or an Agreement State, and conditions under which commercial mixed low-level waste is exempt from RCRA requirements on waste manifests, transportation, and disposal, except LDR treatment standards under RCRA (see Section 4.2.2) remain in effect. The provisions of this regulation also apply to chemically hazardous waste that contains NARM waste, provided the radioactive material is regulated by a state (see Section 4.3.5). The relaxation of requirements for storage, treatment, transportation, and disposal of mixed waste does not apply to any DOE mixed waste. The specified exemptions from RCRA requirements for management and disposal of mixed low-level waste were prompted by several concerns (EPA, 1999e; 2001c): ●

● ●

●

● ●

the burdensome, duplicative, and costly requirements of dual regulation that do not provide greater protection of human health and the environment than achieved under a single regulatory regime; increased radiation exposure of workers at storage and treatment facilities; the lack of availability of disposal facilities that can accept certain kinds of commercial mixed low-level waste and the very low possibility of siting a new disposal facility that could accept all commercial mixed low-level waste; the continued limited capacity for treatment of commercial mixed low-level waste and the unwillingness of treatment facilities to accept waste for which there is no viable disposal option; the continued limited capacity for treatment and disposal of mixed low-level waste at DOE sites; and the continual need for EPA to extend its policy on lower enforcement priority of the prohibition on storage of mixed waste at Section 3004(j) of RCRA (1976), due to the lack of adequate treatment and disposal capacity.

The exemptions reflect EPA’s assessment that NRC and Agreement State regulations for storage and disposal of low-level radioactive

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waste under AEA would not compromise human health and environmental protection from chemical risks when the specified conditions on management and disposal of mixed low-level waste are met (EPA, 1999e; 2001c).

4.3.4 Dual Regulation of Other Fuel-Cycle Wastes The previous section mainly considered the considerable impacts of dual regulation of mixed waste on management and disposal of mixed low-level waste. High-level waste, transuranic waste, and uranium or thorium mill tailings also may be subject to dual regulation under AEA (1954) and RCRA (1976). This Section briefly considers the impacts of dual regulation on these wastes. 4.3.4.1 High-Level Waste. DOE currently manages its high-level radioactive waste produced in chemical reprocessing of spent fuel as mixed waste (DOE, 1999a). Liquid waste from fuel reprocessing and sludges resulting from settling or further processing of the liquid waste are classified as hazardous under RCRA because they exhibit some of the characteristics of hazardous waste, including corrosivity or toxicity, and they contain high concentrations of toxic heavy metals that cannot be removed by waste treatment. Dual regulation of high-level waste presents a number of challenges. First, long-term storage of reprocessing wastes in underground tanks at various DOE sites would appear to be in violation of the RCRA prohibition on indefinite storage of mixed waste discussed in the previous section. DOE has begun the process of converting waste liquids and sludges in the storage tanks to a vitrified waste form (borosilicate glass) suitable for permanent disposal, but it will be many years before treatment of DOE’s high-level waste currently in storage is completed and the waste is ready for disposal. Second, solidified forms of high-level waste intended for permanent disposal are subject to LDRs for hazardous waste (EPA, 1986b; 1990b) discussed in the previous section (see also Section 4.2.2). LDRs specify that vitrified high-level waste is an acceptable waste form under RCRA, but there are as yet no such provisions for other forms of high-level waste that might be intended for disposal. Finally, if high-level waste is considered to be hazardous waste under RCRA, requirements on construction, operation, and closure of a disposal facility, including the provision of a liner system, leachate collection and removal system, and leak detection system (see Section 4.2.2), would need to be addressed. Such requirements are impractical at a geologic repository for disposal of high-level waste

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and could have an adverse impact on the long-term performance of a facility. However, as noted in Section 4.3.3, LDRs allow waiver of many requirements on the waste form and disposal facility if EPA finds that there will be no significant migration of waste from the facility for as long as the waste remains hazardous. Obtaining such a ‘‘no-migration’’ variance is one option for essentially exempting a geologic repository from requirements for disposal of hazardous chemical waste under RCRA. Exemption from RCRA requirements also could be based on a finding that all forms of solidified high-level waste and spent fuel are acceptable waste forms under RCRA, or a finding that compliance with EPA standards for the radioactive component of the waste (see Section 4.1.2.3.1) fulfills the RCRA requirement concerning no significant migration of waste (see following section). 4.3.4.2 Transuranic Waste. Much of DOE’s transuranic radioactive waste is classified as hazardous waste under RCRA and is managed as mixed waste (DOE, 1999b). Many transuranic wastes are hazardous due to the presence of toxic heavy metals or organic chemicals introduced into the waste during processing of plutonium. In contrast to high-level waste, much of the existing transuranic waste is loose trash and is not prepared for disposal using standard waste forms (DOE, 1997a). The variety of transuranic wastes presents a challenge in meeting RCRA requirements on waste characterization, in that sampling and analysis of hazardous waste as called for in EPA regulations (EPA, 1986b) can lead to increased radiation exposures of workers and, thus, a potential conflict with requirements to maintain exposures ALARA established under AEA (DOE, 1993a; NRC, 1991). Given the variety of transuranic wastes requiring disposal, there are no specifications in RCRA regulations for exempting mixed transuranic waste from LDRs (EPA, 1986b). As in the case of high-level waste disposal in a geologic repository, it is impractical to apply some RCRA requirements on construction, operation, and closure of a hazardous waste disposal facility to WIPP, which will be used for disposal of DOE’s defense transuranic waste (see Section 4.1.2.3.2). These concerns were addressed in a 1996 amendment to WIPPLWA (1992). This amendment exempted the WIPP facility from LDRs for hazardous chemical waste, thus allowing disposal of mixed transuranic waste to proceed if the state of New Mexico approves plans for managing the hazardous component prior to termination of institutional control. This exemption was based on a finding by EPA (1998a) that the WIPP facility complies with standards for disposal of the radioactive component of transuranic waste in 40 CFR Part 191,

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which apply for 10,000 y (EPA, 1993a). It was the sense of Congress that the LDRs for the hazardous component of the waste are redundant with the standards for radionuclides; i.e., that compliance with the standards for radionuclides satisfies the conditions for a ‘‘no-migration’’ variance from LDRs for the hazardous component (EPA, 1996d). 4.3.4.3 Uranium or Thorium Mill Tailings. In contrast to highlevel waste, transuranic waste, and low-level waste, regulations for management and disposal of uranium or thorium mill tailings were developed in recognition that these radioactive wastes also are chemically hazardous, due mainly to the presence of toxic heavy metals. EPA regulations established under AEA (EPA, 1983) specify that operations and closure at mill tailings sites must conform to RCRA requirements for hazardous chemical waste. The regulations do not require liner systems but they include requirements on (1) caps to control infiltration of water, as well as releases of radon, (2) monitoring of groundwater, and (3) mitigation of releases of radionuclides and hazardous chemicals to groundwater if standards for groundwater protection, which are consistent with drinking water standards in 40 CFR Part 141 (EPA, 1975), are exceeded. 4.3.5 Dual Regulation of Naturally Occurring and AcceleratorProduced Radioactive Material Waste Issues of dual regulation also arise in management and disposal of waste that contains NARM and waste classified as hazardous under RCRA. This type of waste is subject to dual regulation essentially because the definition of hazardous waste developed by EPA under RCRA (EPA, 1980b) does not include NARM waste (Section 4.2.1.2). Waste that contains NARM can be regulated under RCRA only if it is specifically included in the definition of hazardous waste, even though the exemption of radioactive materials defined in AEA from regulation under RCRA does not apply to NARM. Although NARM is not a radioactive material defined in AEA, DOE is responsible for management and disposal of NARM waste generated by any of its authorized activities, based on the provision of AEA that all DOE activities must be protective of public health and safety (AEA, 1954). Current DOE policy specifies that NARM waste is to be managed as mixed waste under AEA and RCRA or TSCA (1976) if the waste is hazardous under either of the latter two laws (DOE, 1999c). Thus, all issues that arise in management and disposal of DOE’s mixed low-level waste (see Section 4.3.3) also apply to DOE’s mixed NARM waste.

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In the commercial sector, issues of dual regulation of mixed NARM waste arise whenever a state regulates NARM as radioactive material and the waste also is hazardous under RCRA or TSCA. States generally regulate accelerator-produced waste as low-level waste, but not all states regulate waste that contains elevated levels of NORM as radioactive material and the states that do regulate NORM waste have taken a variety of approaches (see Section 4.1.2.4). EPA (2001c) has exempted mixed NARM waste in the commercial sector from certain RCRA requirements if the radioactive component of the waste is regulated by a state and specified conditions are met (see Section 4.3.3). In particular, the radioactive component of mixed NARM waste would need to be acceptable for disposal in a facility for low-level radioactive waste licensed by NRC or an Agreement State under 10 CFR Part 61 (NRC, 1982a).

4.3.6 Summary of Mixed Waste Issues In managing and disposing of waste that contains radionuclides and hazardous chemicals, it is now well established that AEA applies only to the radionuclides in the waste (i.e., to source, special nuclear, and byproduct materials), and that all other hazardous constituents of the waste are subject to regulation under other laws, principally RCRA but also TSCA for waste that contains, for example, dioxins, PCBs, or asbestos. The concept of mixed waste also extends to waste that contains NARM and hazardous chemicals when the radioactive component of the waste is regulated by DOE or a state. Requirements for management and disposal of hazardous chemical waste under RCRA or TSCA were developed largely independently of requirements for radioactive waste developed under AEA. Dual regulation has led to costly and inefficient approaches to management and disposal of mixed wastes, including substantial delays in their treatment and disposal, without commensurate improvements in protection of public health and the environment. Dual regulation of mixed waste is an important concern for radioactive waste that arises from operations of the nuclear fuel cycle, because until the 1980’s policies and regulations for management and disposal of high-level waste, transuranic waste, and low-level waste were developed under AEA largely without regard for the possible presence of hazardous chemicals and without taking into account requirements for management and disposal of hazardous chemical waste that were being developed under RCRA or TSCA. Only uranium and thorium mill tailings were regulated taking into

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account the presence of hazardous chemicals and requirements for managing hazardous chemical waste. Dual regulation of mixed waste has no effect on classification of the radioactive component of the waste, and classification of waste as chemically hazardous is not affected by the presence of radionuclides. Rather, dual regulation mainly affects requirements on management and disposal of waste that had previously been managed as if it were radioactive but not chemically hazardous. In essence, the approach to protecting public health and the environment under AEA has been based on numerical standards that specify acceptable overall performance of a radioactive waste management or disposal system. The standards for acceptable system performance are in the form of limits on radiation dose to members of the public or other related criteria. Radioactive waste generators and radioactive waste management and disposal facilities are afforded considerable flexibility in meeting these standards, and there are few prescriptive technical requirements that apply to all facilities. The approach to management and disposal of hazardous chemical waste under RCRA is quite different. As does AEA, RCRA requires that public health be protected in management and disposal of hazardous chemical waste. RCRA and its implementing regulations particularly emphasize protection of groundwater in accordance with drinking water standards in meeting this requirement. However, the approach to protection of public health under RCRA is based largely on detailed and highly prescriptive technical standards for waste generators and waste treatment, storage, and disposal facilities. Each type of facility must meet the same technical standards, largely without regard for the nature of the hazardous wastes or local environmental conditions. Thus, in contrast to facilities regulated under AEA, facilities regulated under RCRA have little flexibility in meeting the overall objective of protecting public health. In spite of differences in the regulatory approaches under AEA and RCRA, dual regulation of mixed waste does not present any insurmountable technical obstacles to waste management and disposal. Rather, the need for generators of radioactive waste and facilities for management, storage, and disposal of radioactive waste, which had previously been regulated only under AEA, to comply with RCRA requirements when the waste also is chemically hazardous has been the main impediment to successful management and disposal of mixed waste. Regulation of hazardous waste based on detailed and prescriptive technical requirements that apply at all stages from generation to disposal and to any facility, as well as the sometimes complex and difficult procedural requirements for

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obtaining operating permits under RCRA or waivers of RCRA requirements (e.g., delisting of hazardous waste or a ‘‘no-migration’’ variance for disposal of hazardous waste), represent a regulatory approach for which the radioactive waste management community was initially unprepared. Similar considerations apply to mixed waste regulated under AEA and TSCA. Legal and regulatory actions in recent years have succeeded in easing the burdens of dual regulation of mixed waste, especially mixed high-level waste, transuranic waste, and commercial low-level waste. However, relief has not been extended to many important mixed wastes, such as DOE’s mixed low-level waste.

4.4 NCRP Recommendations Relevant to Waste Classification This Section briefly reviews previous recommendations of NCRP that are potentially relevant to the development of a risk-based waste classification system. The topics discussed include NCRP’s recommendations on radiation protection of the public and the comparative hazards of ionizing radiation and chemicals.

4.4.1 Recommendations on Radiation Protection of the Public An important function of NCRP is to develop basic recommendations on radiation protection; NCRP’s current recommendations are contained in Report No. 116 (NCRP, 1993a). With regard to radiation protection of the public, two recommendations are potentially relevant to the development of a risk-based waste classification system. These recommendations involve limits on radiation dose and a negligible individual dose. 4.4.1.1 Radiation Dose Limits. For routine exposure of individual members of the public to all man-made sources of radiation combined (i.e., excluding exposures due to natural background, indoor radon, and deliberate medical practices), NCRP currently recommends that the annual effective dose should not exceed 1 mSv for continuous or frequent exposure or 5 mSv for infrequent exposure. The quantity ‘‘effective dose’’ is a weighted sum of equivalent doses to specified organs and tissues (ICRP, 1991), which is intended to be proportional to the probability of a stochastic response for any uniform or nonuniform irradiations of the body (see Section 3.2.2.3.3).

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The recommended dose limits for the public define limits on the probability of stochastic responses that are regarded as necessary for protection of public health. Doses above the limits are regarded as intolerable and normally must be reduced regardless of cost or other circumstances, except in the case of accidents or emergencies (see Section 3.3.1). For continuous exposure over a 70 y lifetime, and assuming a nominal probability coefficient for fatal cancers (i.e., the probability of a fatal cancer per unit effective dose) of 5 ⳯ 10ⳮ2 Svⳮ1 (ICRP, 1991; NCRP, 1993a), the dose limit for continuous exposure corresponds to an estimated lifetime fatal cancer risk of about 4 ⳯ 10ⳮ3. However, meeting the dose limits is not sufficient to ensure that routine exposures of the public to man-made sources would be acceptable. In addition to requiring that doses to individuals should not exceed specified limits, an important principle of radiation protection is that all doses should be maintained ALARA, economic and other societal concerns being taken into account (ICRP, 1991; NCRP, 1993a). In the past, application of the ALARA principle emphasized considerations of cost-benefit in optimizing collective doses to affected populations [e.g., see 10 CFR Part 50, Appendix I (NRC, 1977), and DOE (1991)]. However, in controlling exposures of the public, the ALARA principle has increasingly been implemented in part by means of standards for specific practices or sources of exposure, called source constraints (ICRP, 1991), that limit the dose from each practice or source to a fraction of the dose limits for all man-made sources combined. Source constraints often represent judgments by regulatory authorities about doses that are reasonably achievable for specific practices or sources at any site, and they provide a practical basis for ensuring that the dose limits for all man-made sources combined will be met (Kocher, 1988). NCRP (1993a) also has emphasized the importance of source constraints in radiation protection of the public. NCRP has reaffirmed a previous recommendation (NCRP, 1984b; 1987a) that whenever the potential exists for routine exposure of an individual member of the public to exceed 25 percent of the limit on annual effective dose as a result of irradiation attributable to a single site, the site operator should ensure that the annual effective dose to the maximally exposed individual from all man-made sources combined does not exceed 1 mSv on a continuous basis. Alternatively, if such an assessment is not conducted, no single source or set of sources under one control should result in an individual receiving an annual effective dose of more than 0.25 mSv. The recommended limit on annual effective dose of 0.25 mSv per source corresponds to an estimated lifetime fatal cancer risk of about

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1 ⳯ 10ⳮ3. Annual effective doses in the range of 0.25 to 1 mSv from all man-made sources combined are acceptable if they are ALARA. However, doses toward the upper end of this range are regarded as only barely tolerable (ICRP, 1991), and doses below this range are expected to be justifiable and achievable in most cases, based on site-specific application of the ALARA principle. Therefore, lifetime risks from routine exposure to all man-made sources combined usually should not exceed about 1 ⳯ 10ⳮ3. 4.4.1.2 Negligible Individual Dose. The second NCRP recommendation that is potentially relevant to developing a risk-based waste classification system is concerned with a negligible individual dose (NCRP, 1993a). A negligible dose is based on the concept of a negligible (de minimis) risk, and it defines a level below which further reductions in dose using the ALARA principle generally would not be warranted. NCRP has recommended that annual effective doses to individuals from any practice or source of 10 ␮Sv or less are negligible (see Section 4.1.2.5.3). This dose is one percent of the dose limit for continuous exposure to all man-made sources combined discussed in the previous section, and it also is about one percent of the dose from natural background radiation, excluding radon (NCRP, 1987b). The recommended negligible individual dose corresponds to an estimated lifetime fatal cancer risk of about 4 ⳯ 10ⳮ5. 4.4.1.3 Application of NCRP Recommendations to Waste Classification. NCRP’s recommendations on dose limits and a negligible dose for individual members of the public, and their associated cancer risks, could be used in developing a risk-based waste classification system. Specifically, the dose limits applicable to all man-made sources of exposure combined could be used in establishing concentration limits of radionuclides or hazardous chemicals in dedicated hazardous waste disposal facilities based on assumed scenarios for exposure of the public. Similarly, the negligible individual dose could be used in establishing concentration limits of radionuclides in disposal facilities for nonhazardous waste. These applications are discussed in Sections 6.2 and 6.3 where NCRP’s recommendations on risk-based waste classification are presented.

4.4.2 Comparative Carcinogenicity of Ionizing Radiation and Chemicals NCRP has published an evaluation of the extent to which principles and methods that have been developed for use in assessing

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cancer risks from exposure to ionizing radiation are applicable to chemical carcinogens (NCRP, 1989). Some of the conclusions of this study are summarized below. 1. The carcinogenic effects of certain chemicals in man and laboratory animals are similar to those of ionizing radiation. 2. Cancers induced by ionizing radiation and chemicals are individually indistinguishable from those of the same type induced by other causes. Thus, in either case, induction of cancers can only be inferred from statistical analyses of a dose-dependent increase in their frequency in exposed populations. 3. From studies of human populations exposed to certain chemicals, available data are sufficient to characterize the doseincidence relationships for some types of cancer at high dose levels. However, as in the case of ionizing radiation, the data are not sufficient to define the dose-incidence relationships precisely for any form of cancer over a wide range of doses and dose rates. Therefore, the probability of cancer induction that may be associated with low doses of chemicals that would be of primary concern in protection of public health can be estimated only by interpolation and extrapolation of data at higher doses and dose rates, based on assumptions about the dose-incidence relationships and mechanisms of toxicity. For the few chemicals for which incidence data are available over a range of doses, the dose-incidence relationship is not inconsistent with linearity, but this result does not constitute proof of linearity. 4. Few chemicals identified as carcinogens in laboratory animals are known to cause cancer in humans, and the dose to affected tissues for these chemicals usually is not known well enough to define the dose-response relationship except in a general way. In this respect, the carcinogenic effects of most chemicals in humans are far less well known than are those of ionizing radiation. 5. Dosimetry (i.e., the dose delivered to target tissues per unit intake of material) generally is more uncertain for chemicals than for radionuclides, because the dose of a chemical at its biological site of action may depend on a number of metabolic and pharmacokinetic factors that are not relevant for radionuclides. 6. Because chemicals differ widely in molecular structure, biological activity, and mode of action and because the relationships among these properties are poorly understood, the toxic effects of one chemical usually cannot be predicted with confidence from those of another.

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7. An understanding of chemical toxicity is complicated by the large differences in potency among chemicals and in the stages of the carcinogenic process at which they act. For example, some chemicals predominantly affect late stages of carcinogenesis, whereas others affect earlier stages. 8. The interactive responses of two or more carcinogens may be independent, synergistic, or antagonistic, but few such interactions are well characterized. Furthermore, the combined effects of exposure to complex mixtures of chemicals, such as are typically encountered in human life, are virtually unexplored. 9. In spite of the differences among carcinogens, the principles of dose-response assessment that have proven to be useful for ionizing radiation appear to be applicable, within limits, to chemicals, particularly those chemicals that resemble radiation in genotoxicity, cytotoxicity, and in the stages of carcinogenesis that are affected. 10. For a given exposure situation, the choice of a dose-incidence model for risk assessment is a matter of scientific judgment which must be based on consideration of all pertinent epidemiological and experimental data, including the results of shortterm tests where applicable. 11. Dose-response assessments for chemical carcinogens generally are more uncertain than dose-response assessments for ionizing radiation. In developing a risk-based waste classification system, the primary emphasis would be on risk management, rather than estimation of risk for actual exposure situations. However, differences in the state of knowledge of the carcinogenicity of ionizing radiation and chemicals could be taken into account in establishing limits on allowable doses (hypothetical risks) for radionuclides and chemicals to be used in classifying waste.

4.5 Summary Classification systems for radioactive waste and requirements for disposal of different classes of radioactive waste have been developed largely independently of classification systems and disposal requirements for hazardous chemical waste. This Section has discussed the classification systems for radioactive and hazardous chemical wastes and the relationships between waste classification and requirements for disposal. Impacts of the different systems for waste classification and disposal on management and disposal of waste that contains

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mixtures of radionuclides and hazardous chemicals also has been discussed. The current definitions of the different classes of radioactive waste and the intended disposal system (technology) for waste in each class in the United States are summarized in Table 4.1 (see Section 4.1.2.1). The system for classification and disposal of radioactive waste in the United States has several important characteristics. First, the classification system is not comprehensive, because NARM waste is not included in the classification system for waste that arises from operations of the nuclear fuel cycle. This distinction is based solely on the source of the waste, rather than its radiological properties or requirements for safe management and disposal. Second, the classification system for fuel-cycle waste is qualitative (i.e., the different waste classes are not defined solely in terms of limits on concentrations of radionuclides or other properties of the waste), it is sourcebased (i.e., the different classes of waste are distinguished mainly on the basis of how the waste is generated, rather than its properties), and it is not based on considerations of risk, especially risks resulting from waste disposal. Third, waste classes are not defined in relation to the type of disposal system that is expected to be acceptable (e.g., a near-surface facility or geologic repository). As a result of the second and third characteristics, the different classes of fuel-cycle waste are not defined unambiguously, and waste in different classes can have similar radiological properties and require similar approaches to waste management and disposal. A number of alternatives to the qualitative and source-based classification system for radioactive waste in the United States have been proposed. The alternative waste classification systems have three important features in common. First, they are comprehensive, in that NARM waste and nuclear fuel-cycle waste are included in the same classification system. Second, they are based on the concept that waste classes should be defined primarily on the basis of risk, particularly the risk resulting from waste disposal. Finally, to some degree, they associate waste classes with particular disposal systems that are expected to be generally acceptable. None of these features is embodied in the radioactive waste classification system in the United States. In addition, some proposed classification systems include an exempt class of radioactive waste that contains negligibly small amounts of radionuclides. Waste in this class would be regulated in all respects as if it were nonhazardous. A general class of exempt waste is not included in the radioactive waste classification system in the United States. Classification and disposal of hazardous chemical waste is based mainly on EPA regulations and guidance developed under RCRA,

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although individual states have supplemented EPA policies in some cases. Some hazardous chemical wastes, such as sewage sludge or waste that contains dioxins, PCBs, or asbestos, are classified and managed under other laws, including TSCA or the Clean Water Act. Hazardous chemical waste is defined in RCRA regulations as a solid waste that exhibits the characteristic of ignitability, corrosivity, reactivity, or toxicity, or is a specifically listed waste. The definition of hazardous waste specifically excludes radioactive material (source, special nuclear, or byproduct material) defined in AEA. Under current EPA regulations, a chemical waste is either hazardous or it is not, and there is no further classification of hazardous chemical waste with respect to the degree of hazard. Some states have defined classes of hazardous chemical waste (e.g., extremely hazardous waste) but, in practice, the requirements on management and disposal of all hazardous wastes have resulted in essentially the same approaches being used regardless of hazard. When a hazardous chemical waste is mixed with a nonhazardous solid waste, the entire waste is classified as hazardous unless the former is a characteristically hazardous waste that does not contain any listed waste and mixing with the nonhazardous waste removes the hazardous characteristic. Emplacement in a near-surface disposal facility is the common disposition of solidified hazardous chemical waste, regardless of the hazard posed by the waste. Disposal sites must meet location requirements, and they must be provided with appropriate liner, leachate collection and removal, and leak detection systems. The system for classification and disposal of hazardous chemical waste under RCRA is neither comprehensive nor is it based strictly on considerations of risks posed by waste. As noted above, all hazardous chemical wastes are managed alike, regardless of hazard, and this policy extends to waste that contains only minuscule amounts of listed hazardous substances. In addition, many potentially important wastes that contain hazardous chemicals are excluded from the definition of hazardous waste and, thus, are not presently regulated under RCRA. The distinction between regulated and unregulated wastes is based primarily on the source of the waste, rather than its hazard. The term ‘‘mixed waste’’ refers mainly to waste that contains radionuclides regulated under AEA and hazardous chemical waste regulated under RCRA. Mixed waste is subject to dual regulation as a result of the exclusion of radioactive materials defined in AEA from regulation under RCRA. Dual regulation of mixed waste also extends to waste that contains NARM and hazardous chemicals, since NARM waste is not defined as a hazardous waste under RCRA, and to

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radioactive waste that contains hazardous chemicals that are regulated under other laws (e.g., TSCA). By the early 1990s, dual regulation of mixed waste was firmly established as a result of court decisions and federal law. Dual regulation of mixed waste does not affect classification of its radioactive and hazardous chemical constituents. However, successful management and disposal of mixed waste has been a formidable challenge. Difficulties in managing mixed waste have not resulted from technical obstacles, such as fundamental differences in (1) the nature of radioactive and hazardous chemical wastes and how they should be managed or (2) requirements on waste management and disposal related to protection of public health and the environment. Rather, the main impediment to successful management and disposal of mixed waste has been the difficulties in obtaining operating permits for waste treatment, storage, and disposal facilities under RCRA and in obtaining waivers from RCRA requirements. These difficulties are due mainly to the significant differences between the detailed and prescriptive technical requirements in RCRA regulations and the less prescriptive, outcome-based requirements normally imposed on facilities that manage or dispose of radioactive waste under AEA. The result has been costly and inefficient approaches to management and disposal of mixed wastes that have not led to improvements in protection of public health and the environment.

5. Desirable Attributes of a Waste Classification System Previous sections have presented technical and historical information on radiation and chemical risk assessment and on classification of radioactive and hazardous chemical wastes. This information provides important perspectives for establishing the foundations of a new hazardous waste classification system. Before establishing these foundations, it is useful to specify the attributes that an ideal waste classification system should possess. The following sections identify the desirable attributes of a waste classification system including that the system should be risk-based, it should allow for exemption of waste, and it should be comprehensive, consistent, intrinsic, comprehensible, quantitative, compatible with existing systems, and flexible. These attributes should be recognized as goals that are not all likely to be fully realized in a practical waste classification system. For many years, the hazardous waste classification systems that existed at a particular time performed adequately, although there clearly were inconsistencies and occasional difficulties. In recent years, however, increased interest in protecting and cleaning up the environment has resulted in a proliferation of waste classification systems (generic and situation-specific) and application of these in ways that have increased the undesirable legal, sociopolitical, and economic ramifications of existing waste classification systems. As a consequence, difficulties that were previously minor have assumed major proportions. The following sections also summarize some of these difficulties for the purpose of illustrating the need for a consolidated approach to waste classification and identifying some of the major issues that must be addressed by a new system.

5.1 Risk-Based Society desires that waste disposition activities be conducted in a manner that provides long-term protection of human health. Many measures of ‘‘long-term protection’’ have been developed over the 243

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years to aid in this endeavor. However, the most fundamental measure of the extent to which protection is provided is the incremental health risk to the public associated with disposition of waste. Thus, a waste classification system should be based on risk (Adam and Rogers, 1978; Waters et al., 1993), in order to facilitate disposition of hazardous wastes by means that are expected to limit risks to the public to levels deemed acceptable. There are two possible alternatives to using risk directly as the basis for waste classification: non-risk-based systems and surrogate systems. Non-risk-based systems could use any conceivable attribute of hazardous waste as a basis for classification, including its source (see Sections 4.1 and 4.2 for examples) or the date it was produced. These bases are at best somewhat related to risk and at worst are totally unrelated. Because of this variable relationship, the use of non-risk-based approaches to waste classification could result in an unacceptable risk if the waste is managed in a way that does not provide adequate long-term protection, or an inappropriate allocation of resources if relatively innocuous wastes are managed in the same way as much more hazardous wastes. Surrogate systems attempt to compensate for the shortcomings of non-risk-based systems by using a subset of the attributes of waste that determine risk. One of the more common measures is the toxicity of hazardous substances (the probability of a response per unit dose). Less frequently, such other parameters as radioactivity, persistence, or mobility are employed. While these represent an improvement over non-risk-based approaches to waste classification, they are still inadequate. Examples of shortcomings include that highly toxic materials may not persist long enough to pose a significant risk or that persistent materials may be so immobile that human exposures are virtually nil. Based on these considerations, classification of waste based on risk is the preferred approach. However, it is not without shortcomings. By far the most important is that estimation of risks to human health is often a complicated, multi-step process involving many assumptions and attendant uncertainties. It is this shortcoming that spawned the use of surrogates. However, in applications related to waste classification, risk assessment is significantly simplified because it is by definition not site-specific. While this results in some uncertainties related to classification of wastes, these can be accommodated by introducing prudent degrees of conservatism in generic risk assessments. Additionally, the magnitude of uncertainties and their potential impact are taken into account in site-specific risk assessments performed in the process of licensing particular waste disposal facilities.

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Existing waste classification systems have a highly variable relationship to the risk posed by waste. In some cases, waste classification is based on the source of the waste rather than risk. Examples include high-level radioactive waste (see Section 4.1) and some of the hazardous chemical wastes listed in, or excluded from, regulations implementing RCRA (see Section 4.2). Some high-level radioactive wastes have decayed and been chemically processed to the point that they could be managed as low-level waste under NRC regulations, and other high-level wastes are similar to defense transuranic waste (Kocher and Croff, 1987). All listed chemical wastes must be managed as hazardous waste without regard for the amounts of listed substances present, and many potentially important wastes are excluded from the definition of hazardous waste based on their source rather than their properties. A risk-based waste classification system would result in these wastes being more appropriately grouped, which should result in more cost-effective management commensurate with the risks posed by the wastes. Some existing waste classification systems are risk-based to a substantial degree. For example, the present system for subclassifying commercial low-level radioactive waste (NRC, 1982a) identifies waste that is generally acceptable for near-surface disposal based on a set of dose limits for a hypothetical inadvertent intruder that provide a surrogate for risk. However, the relationship of the dose limits to the radionuclide concentrations that quantify the classification system is not always evident or consistent, although there may be valid reasons for this (Kocher and Croff, 1987). In addition, if unanticipated low-level wastes are generated by future processes, including them in NRC’s classification system on a consistent basis may be difficult. For example, NRC did not anticipate that commercial low-level waste might include large volumes of waste that contains elevated levels of long-lived, naturally occurring radionuclides (NRC, 1994b). Most importantly, because the general class of lowlevel waste is defined only by exclusion (see Section 4.1.2.3.3) and the definitions of the excluded waste classes are not risk-based, the definition of low-level waste is not risk-based. For hazardous chemical waste, there is no federal classification system other than a specification that the waste is hazardous or that it can be managed as if it were nonhazardous because it has been shown not to be characteristically hazardous or has been delisted or specifically excluded.14 Hazardous chemical waste that is not 14

A few states have developed a category of high-hazard chemical waste called ‘‘extremely hazardous’’ or similar term (California, 1999; Mehlhaff et al., 1979; NAS/ NRC, 1999b; OTA, 1981) (see Section 4.2.1.3). However, extremely hazardous wastes appear to be managed in much the same way as other hazardous wastes, primarily because of the increasingly stringent regulations being applied to the latter.

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destroyed by incineration normally is disposed of in dedicated nearsurface facilities. However, there presumably are hazardous chemical wastes that would pose an unacceptable risk from near-surface disposal in the absence of perpetual institutional control (Okrent and Xing, 1993). Such waste could require a disposal technology more isolating than a near-surface facility. This subset of hazardous chemical waste is not yet identified or managed according to its long-term risk, primarily because risk assessments for disposal of hazardous chemical waste tend to have a relatively short time horizon (Doty and Travis, 1989). Limits on allowable risk established by regulatory authorities have been used to derive maximum acceptable concentrations or inventories of waste constituents in particular disposal situations. However, risk has been expressed in a variety of ways. For example, is the risk to an individual (NRC, 1982a) or population (EPA, 1993a)? Maximum or average? Annual or lifetime? Critical organ or weighted responses in multiple organs? To the public, workers, or inadvertent intruders? Adults or children? Humans, animals, or plants? Several of these approaches to expressing risk have been used in practice, which means that existing waste classification systems ostensibly based on risk may differ substantially. A properly constituted riskbased waste classification system will address such issues.

5.2 Exemption Ideally, the incremental costs of waste treatment and disposal should not exceed the resulting benefits in health risks averted (Waters et al., 1993), given that the resources that can be allocated to reducing risks are finite. Use of acceptable methods for disposal of nonhazardous waste is one option that should be considered to appropriately match the costs and benefits of waste disposal. For example, municipal solid waste is sent to an approved municipal/ industrial landfill, and other wastes that pose no greater risk to the public should be acceptable for disposal in a similar facility, even when these wastes contain small amounts of hazardous chemicals or radionuclides. Similarly, consideration of reuse of slightly contaminated materials should be allowed if the benefits clearly outweigh the costs, including the incremental health risks. Viewed more broadly, resources spent on waste treatment and disposal are not available for use in achieving other desirable outcomes, such as health care or other types of environmental interventions, so the tradeoff ultimately is reduction of risks in one area at the expense of neglecting risks in other areas.

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Allowing for exemption of waste materials that contain sufficiently small amounts of hazardous substances is a potentially important means of balancing the resources required to manage waste and the benefits in health risks averted. As a consequence of the discussion in Section 5.1, it is desirable that the definition of waste that can be exempted and, thus, managed as if it were nonhazardous should be risk-based. Furthermore, waste should be exempted based on the consideration that the associated risks should not exceed levels generally regarded as negligible. Existing waste classification systems in the United States do not include general principles for exempting waste that contains small amounts of hazardous substances from requirements for management and disposal as hazardous waste. Regulations governing hazardous chemical waste include provisions allowing delisting of listed hazardous waste on a case-by-case basis (EPA, 1993b), but the regulations do not provide a clear indication of conditions under which a hazardous waste might be delisted and the process can be difficult. A few chemically hazardous wastes have been exempted to allow their beneficial use. Exemptions for radioactive waste or beneficial uses of radioactive materials also have been established only on a case-by-case basis (see Section 4.1.2.5.2), and the existing exemptions correspond to potential doses to the public that vary widely (Kennedy et al., 1992; Schneider et al., 2001). The lack of general exemption principles for radioactive and hazardous chemical wastes has important consequences for the costs of waste treatment and disposal. The ubiquitous nature of radioactivity in combination with the sensitivity of modern analytical techniques often makes it impossible to determine whether the radioactivity in a material is naturally occurring or resulted from some operation. This leads to expensive undertakings such as the common DOE practice of assuming that any material that has been in a radiation area is contaminated and must be managed as radioactive material. The amounts of such material are expected to increase greatly in the future as aging nuclear facilities (DOE and commercial) are decontaminated and dismantled. A risk-based waste classification system would address such issues by specifying a risk below which a material would be exempt from regulation as hazardous waste, thus alleviating a significant expense in the hazardous waste management system while still protecting public health. 5.3 Comprehensive A risk-based waste classification system should apply to all wastes that contain hazardous substances. That is, it should apply irrespective of the nature of the hazardous substances in the waste (chemical,

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radioactive, or both), the process generating the waste, or the organization generating the waste. This attribute is a necessary ramification of the waste classification system being based on risk: if the classification system is not comprehensive, it will differentiate among wastes based on attributes other than risk. The existing waste classification systems for radioactive and hazardous chemical wastes clearly are not comprehensive. At a fundamental level, entirely separate and quite different classification systems have been developed for the two types of hazardous waste. In addition, each classification system is not comprehensive in the context of the general type of waste to which each system applies. In the existing radioactive waste classification system, waste that arises from operations of the nuclear fuel cycle is classified separately from NARM waste. The existing classification system for hazardous chemical waste excludes many potentially important wastes that contain hazardous chemicals.

5.4 Consistent Another desirable attribute of a waste classification system that is a corollary of the system being risk-based is that it treat wastes that pose similar health risks consistently. A chemically hazardous waste estimated to pose a certain risk should be in the same waste class as a radioactive waste that poses an equivalent risk, and similarly for mixed waste. Consistency also implies that wastes posing similar risks could be disposed of using essentially the same technology (municipal/industrial landfill, licensed near-surface facility for hazardous waste, or geologic repository). Differences in the approaches to classification of radioactive and hazardous chemical wastes have resulted in inconsistent waste management practices. Radioactive waste classification is more complex, with categories for mainly short-lived waste, which is commonly called low-level waste, and long-lived waste, such as defense transuranic waste (among others). Shorter-lived waste that contains sufficiently small amounts of long-lived radionuclides is typically suitable for disposal in near-surface facilities. Long-lived waste is destined for disposal in deeper facilities (e.g., geologic repositories) that are expected to provide substantially greater waste isolation than nearsurface facilities. Chemical wastes are deemed hazardous or not under the federal classification system, and hazardous chemical waste usually is sent to a near-surface facility. Thus, disposal of a long-lived chemical waste, such as a heavy metal or degradation-resistant pesticide, uses the same technology as disposal of a short-lived (e.g.,

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readily degradable) waste, rather than a technology similar to that used for its conceptual radioactive counterpart (transuranic waste). The inconsistency inherent in emplacing long-lived chemical waste in near-surface facilities could lead to unacceptably high risks (Okrent and Xing, 1993) in the absence of perpetual institutional control. Waste classification can impede waste management when materials are classified under multiple systems that contain incompatible provisions concerning waste management. As discussed in Section 4.3, management of solidified mixed waste is clearly impeded at the present time, as evidenced by the following: ●

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Substantial amounts of mixed low-level waste continue to be stored and inventories are increasing. Little of this waste is being sent to facilities intended for permanent disposal because few exist and those that are operating have restrictive waste acceptance criteria. The capacity for treating mixed low-level waste to meet applicable (primarily RCRA) regulations is inadequate to the point that EPA has had to extend its policy concerning relaxed enforcement of requirements that limit the time that mixed waste can be stored without treatment (EPA, 1994c). However, the decision to continue this policy is not binding on the many states that are authorized to regulate mixed waste. This situation mainly impacts DOE’s mixed low-level waste, which comprises the bulk of such waste requiring treatment and disposal. Under terms of the Federal Facility Compliance Act (FFCA, 1992), DOE was subject to RCRA requirements beginning in October 1995, including individual states’ variations thereof. DOE was expected to have mixed low-level waste treatment plans for each site approved by the host state, but adequate treatment capacity for some mixed waste will not be available until far beyond the time when its storage becomes non-compliant. As a result, solidified mixed low-level waste is and will continue to be stored in a noncompliant manner for times longer than those allowed under RCRA. This practice continues to be tolerated by EPA for responsible generators because of the lack of practical alternatives, but only until such time as appropriate treatment or disposal capacity becomes available. DOE intends to dispose of its mixed defense transuranic waste at WIPP. This facility is located hundreds of meters underground in a bedded salt formation, which clearly is much more isolating and protective than the near-surface facilities in which most RCRA hazardous waste is currently emplaced. Nonetheless, the

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need to obtain a waiver from certain RCRA requirements on waste treatment and the construction, operation, and closure of the WIPP facility constituted a time-consuming and costly impediment to obtaining the necessary permits for disposal of mixed waste. Although the issue of disposing of mixed transuranic waste at the WIPP facility was resolved successfully (EPA, 1996d; 1998a), the effort expended did little, if anything, to improve the capability of the facility to protect public health and the environment. The extent to which differences in waste classification and approaches to waste management may impede the disposal of high-level radioactive waste and spent nuclear fuel is not yet clear because of uncertainties in the final waste forms intended for disposal and the fact that siting and licensing of a repository is still in the investigative phase.

The above examples of waste management actions that have been impeded represent the manifest symptoms of the wastes being classified and regulated under separate and inconsistent frameworks. The root cause of these impediments is the separate waste classifications under AEA (low-level waste, transuranic waste, high-level waste) and RCRA (chemically hazardous waste) and the different requirements for waste management and disposal under the two laws. EPA has recognized these difficulties and has begun to address them (EPA, 2001c), but relief from requirements imposed by dual regulation is not yet established for many mixed wastes.

5.5 Intrinsic A risk-based waste classification system should do what its name implies—namely, classify waste so that some value of risk deemed acceptable in a given context would not be exceeded. The purpose of the system is not to classify waste containers, disposal sites containing waste, or processes that generate waste. The logical ramification of this is that the classification system must be based on intrinsic properties of waste. Additionally, it is desirable that these properties be readily measurable, especially in bulk solid waste. This necessity is reinforced by the arguments in Section 2.1.2, which point out that waste often must be classified before the method of packaging or the intended disposal site is known. Basing waste classification on intrinsic properties of waste provides significant advantages because in a mature system the generator would be able to manage waste in the interim with confidence

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that one or more disposal options suitable for waste in a particular class would be available. Similarly, waste disposal sites would not have to be concerned about the source of waste when it is properly classified. Significant parts of the existing waste classification systems are based on intrinsic properties of waste. The system for subclassifying low-level waste in 10 CFR Part 61 (NRC, 1982a) and the determination of whether a chemical waste is characteristically hazardous (see Section 4.2.1.1) are examples of waste classification based on intrinsic properties. However, some important waste classifications are based on attributes not intrinsic to the waste. A prime example is waste that is classified based on its source (process or organization), such as highlevel radioactive waste and many listed hazardous chemical wastes. The undesirable effects of this are summarized previously and are not repeated here.

5.6 Comprehensible A classification system can be risk-based and comprehensive and still not be very useful if it is complex and difficult to use. At a minimum, the waste classification system must be comprehensible to those involved in managing and regulating the disposition of waste. Otherwise, waste may not be properly classified. It also is desirable that the waste classification system be transparent to nonexperts outside the waste management system, ranging from potentially impacted individuals to the general population (Wiltshire and Dow, 1995). Transparency would greatly enhance acceptability of the classification system and the associated waste management system. Comprehensibility would be greatly facilitated if the waste classification system applies to all waste (i.e., there is only one system) and is simple in its concepts and applications. Unfortunately, however, it is difficult for anyone to fully comprehend the existing classification systems for radioactive and hazardous chemical wastes. These systems are not based on clearly stated principles from which a logical and transparent classification system might follow, and the two systems approach classification and disposal of hazardous waste in different ways. The systems intermix legal and technical considerations in ways that sometimes defy logic. A few examples of the incongruities in the waste classification systems that result in a lack of transparency and difficulties in comprehension are described below.

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In regulations implementing RCRA, a ‘‘solid waste’’ is defined to include solid, liquid, semisolid, and contained gaseous materials. While there is a reasonable rationale for such a definition, it runs counter to common understanding of the meaning of ‘‘solid,’’ and it immediately signals that the system for classifying and managing hazardous chemical waste will be quite complex and difficult for operators, as well as regulators and the public, to understand. In many respects, the system for classifying and managing hazardous chemical waste under RCRA makes no distinction between highly hazardous waste and virtually innocuous waste that contains very low levels of hazardous substances. Furthermore, many wastes that contain hazardous chemicals, as well as radionuclides not regulated under AEA, are excluded from the definition of hazardous waste based on the source of the waste, even though the excluded wastes can be just as hazardous as other wastes that are deemed hazardous under RCRA. Some high-level radioactive waste can be less hazardous than high-activity (Class-C or greater-than-Class-C) low-level waste in regard to the levels of radioactivity due to shorter-lived radionuclides and the long-term risks that arise from disposal due to long-lived radionuclides. Some low-level radioactive wastes can be more radioactive than any other type of waste (e.g., high-activity 60Co sources, 90Sr or 137 Cs capsules). Surplus nuclear materials that consist almost entirely of 233U by mass and also contain high activity concentrations of 232 U and its short-lived, photon-emitting decay products, if declared to be waste, would be classified as low-level waste, rather than transuranic or high-level waste (Bereolos et al., 1998a; 1998b). Wastes that contain high concentrations of long-lived radionuclides are destined for highly isolating disposal facilities, such as a geologic repository, whereas similar chemical wastes are destined for near-surface disposal.

Such incongruities only serve to confuse the public (Wiltshire and Dow, 1995), and this confusion leads to mistrust that is manifest in unwarranted obstruction of the facilities and activities required to manage hazardous wastes. A straightforward, consistent system based on a few simple principles would serve to make waste classification and approaches to waste management more transparent and understandable. A further impediment to comprehension of waste classification and waste management systems is the practice of assigning different

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meanings to important nomenclature. An important example for purposes of this Report is the differing meanings attached to ‘‘acceptable’’ and ‘‘unacceptable’’ levels of risk in the chemical and radiation paradigms for risk management (see Section 3.3).

5.7 Quantitative It is highly desirable that a waste classification system be expressed in quantitative terms. More specifically, the intrinsic waste characteristics that define the boundaries between waste classes should be stated numerically. Qualitative definitions of waste classes, such as the definition of high-level radioactive waste discussed in Section 4.1.2.3.1, simply defer the issue of waste classification to a subsequent definition of the qualitative terms or to caseby-case determinations that typically occur after waste is generated. Quantitative definitions of waste classes make the classification system relatively unambiguous and also enhance the comprehensibility of the system. Exceptions to the rule can be handled by allowing regulatory authorities to include wastes that exceed a boundary or exclude wastes within a boundary on a case-by-case basis. Some existing waste classification systems are quantitative. For example, the concentrations of radionuclides defining the different subclasses of low-level radioactive waste that is generally acceptable for near-surface disposal are clearly stated in the regulations (NRC, 1982a), as are the quantitative conditions defining ignitable, corrosive, reactive, and toxic hazardous chemical wastes (see Section 4.2.1.1). However, there are significant instances of nonquantitative definitions. Important examples include the qualitative definitions of the different classes of radioactive waste that arises from operations of the nuclear fuel cycle and the definitions of listed hazardous chemical wastes. If a waste classification system is not quantitative, the inevitable result is uncertainty or inappropriate classification, the typical manifestations of which are paralysis (e.g., storage of increasing amounts of waste) and legal challenges to proposed or ongoing activities. Waste classification systems that are based on the source of the waste, including some listed chemical wastes and essentially all radioactive wastes, yield the undesirable effects discussed above. Even definitions of waste classes that contain specific qualitative language are inadequate because the interpretation of important terms is ambiguous. An example is the legal definition of high-level radioactive waste as material that is ‘‘highly radioactive’’ and ‘‘requires permanent isolation.’’ The absence of quantification makes

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these terms largely meaningless (see Section 4.1.2.3.1), resulting in perpetuation of the traditional source-based definition of this waste class.

5.8 Compatible The waste classification system should be developed in recognition of the types of information that are available and likely to be obtainable, and it should be specified to maximize compatibility with available information consistent with maintaining the fundamental integrity of the system. Establishment of a risk-based waste classification system must begin with the existing classification systems and associated databases (e.g., toxicity of hazardous substances). These would be expanded and refined as needed. However, if the foundations of a risk-based waste classification system or its implementation involve radically new concepts or call for data that cannot feasibly be obtained, the effort will be for naught. A realistic waste classification system must use the existing base of concepts and data to achieve the desired result. Existing waste classification systems are generally compatible with available data. Compatibility is the result of the databases having been acquired to meet the needs of the waste classification systems.

5.9 Flexible The waste classification system must be flexible so that it can accommodate special circumstances without need of a continuing series of separate classifications or ad hoc solutions. Common instances where flexibility is required include (1) taking the presence or absence of engineered waste forms into account, (2) providing for classification of small amounts of highly hazardous materials, (3) dealing with situations where the cost of disposing of a legacy waste to meet acceptable risk values is prohibitive, and (4) classifying new hazardous substances or types of waste. It is highly desirable, however, that flexibility be applied in a reasonably consistent manner to different special circumstances. This means that the rationale for exceptions should be developed using the precepts of the risk-based waste classification system, and that decisions should be documented in a form that is readily available so as to constitute a body of precedent.

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Essentially all existing waste classifications systems include some provisions to incorporate flexibility. However, such provisions often are established by ad hoc decisions of regulatory authorities. Litigation intended to either engender or challenge flexibility is often involved. The results typically are exceptions to the rule and are codified in separate sections of the regulations covering waste classification, instead of being related to and justified in the context of the framework for classification.

6. Principles and Framework for a Comprehensive and RiskBased Hazardous Waste Classification System This Section develops NCRP’s recommendations on the principles and framework for a comprehensive and risk-based hazardous waste classification system. Implementation of the system also is discussed. These recommendations focus on classification of waste that contains hazardous substances for purposes of permanent disposal. The proposed waste classification system was developed to address deficiencies in the existing waste classification systems discussed in Sections 2, 4 and 5. The basic framework for the waste classification system developed in this Report is depicted in Figure 6.1. Starting with the objectives that the classification system should apply to any waste that contains radionuclides or hazardous chemicals and that all such waste should be classified based on risks to the public posed by its hazardous constituents, the fundamental principle of the proposed system is that hazardous waste should be classified in relation to disposal systems (technologies) that are expected to be generally acceptable in protecting public health. This principle leads to the definitions of three classes of waste, and to quantification of the boundaries of the different waste classes based on considerations of risks that arise from different methods of disposal. The boundaries normally would be specified in terms of limits on concentrations of hazardous substances. At the present time, nearly all hazardous and nonhazardous wastes are intended for disposal in a near-surface facility or a geologic repository, and these are the two types of disposal systems assumed in classifying waste. The three waste classes and their relationship to acceptable disposal systems are described in more detail in Section 6.2. Given the assumed types of disposal systems (near-surface facilities or geologic repositories), waste would be classified as exempt, low-hazard, or high-hazard based on the magnitude of its risk index, 256

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Fig. 6.1. Basic framework for proposed hazardous waste classification system.

a quantity which is introduced in Section 6.2 and further developed in Sections 6.3 and 6.4. For any hazardous substance, the risk index essentially is the ratio of a calculated risk based on a postulated exposure scenario for an assumed type of disposal system to a specified limit on allowable risk for that disposal system. If the risk index is less than a specified value (e.g., unity), the risk posed by the waste is within acceptable bounds for the assumed type of disposal system

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and the waste would be classified accordingly. If the risk index is not less than the specified value, the waste would be assigned to a class containing more hazardous waste, except in the case of highhazard waste that contains the most hazardous materials. NCRP reiterates that the risk-based waste classification system developed in this Report does not, and cannot, obviate the need to establish waste acceptance criteria at each hazardous waste disposal site based on the characteristics of the site, the particular disposal technology, and characteristics of the wastes that are intended for disposal at the site. NCRP expects that most waste that would be assigned to a particular class will be acceptable for disposal using the associated type of disposal technology indicated in Figure 6.1. However, the disposal capabilities of particular sites and engineered systems can vary substantially and can depend on the waste characteristics. The primary function of any waste classification system is to facilitate development of cost-effective approaches to waste management and disposal and effective communication on waste matters (see Section 2.1.2).

6.1 Issues of Risk Assessment and Risk Management Previous sections of this Report have discussed concepts, precedents, and technical information that are important to development of NCRP’s recommendations on a comprehensive and risk-based hazardous waste classification system. This Section discusses selected aspects of this background information that are critical to establishing the principles and framework for the recommended hazardous waste classification system. The topics discussed involve technical aspects of risk assessment and issues of risk management.

6.1.1 Measures of Response from Exposure to Hazardous Substances Development of a comprehensive and risk-based hazardous waste classification system requires assumptions about the measure or measures of response (adverse health effects) from exposure to radionuclides and hazardous chemicals that should be used in classifying waste. Possible measures of response discussed in Section 3.2.3 include fatalities, incidence, or some combination of the two, such as total detriment (ICRP, 1991). The following sections discuss the measures of response from exposure to hazardous subtances that

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have been used in health protection and NCRP’s recommendations on the measure of response that should be used in classifying waste. 6.1.1.1 Measures of Response for Substances Causing Deterministic Responses. For purposes of health protection in routine exposure situations, incidence has been the primary measure of deterministic response for both radionuclides and hazardous chemicals. Fatalities also are of concern for substances that cause deterministic responses, but only at doses substantially above the thresholds for nonfatal responses. Given that the objective of standards for health protection is to prevent the occurrence of deterministic responses, incidence is not modified by any subjective factors that take into account, for example, the relative severity of different nonfatal responses with respect to a diminished quality of life. Judgments about the importance of deterministic responses are applied only in deciding which responses are sufficiently adverse to warrant consideration in setting protection standards. For the purpose of developing a risk-based hazardous waste classification system, prevention of deterministic responses should be of concern only for hazardous chemicals, but not for radionuclides. Deterministic responses from exposure to radionuclides can be ignored because radiation dose limits for the public intended to limit the occurrence of stochastic responses are sufficiently low that the doses in any organ or tissue would be well below the thresholds for deterministic responses (see Section 3.2.2.1). 6.1.1.2 Measures of Response for Substances Causing Stochastic Responses. Stochastic responses from exposure to both radionuclides and hazardous chemicals must be taken into account in developing a comprehensive and risk-based waste classification system. Such responses are assumed to occur with some probability at any dose and the responses of concern (primarily cancers) often are fatal. Therefore, consideration must be given to the question of whether fatalities, incidence, or some combination of the two is the most appropriate measure of response for substances causing stochastic responses. The following sections discuss the advantages and disadvantages of the three options. 6.1.1.2.1 Incidence. In the first option, the common measure of stochastic response from exposure to radionuclides and hazardous chemicals would be incidence, without any modifications to account for such factors as differences in lethality fractions for responses in different organs or tissues or expected years of life lost per fatality. Such modifications are intended to represent differences in the severity of different stochastic responses.

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This option offers a number of advantages. First, incidence is the measure of response that generally has been used for chemicals that induce stochastic responses, mainly because dose-response data for most chemicals obtained from studies in animals are reported only in terms of incidence. Second, incidence is the only measure of response that is generally applicable to any hazardous subtances because many deterministic responses of concern are nonfatal. Therefore, with this option, the same measure of response would be used for all hazardous substances in classifying waste, regardless of the associated type of response. Third, incidence is a simple measure of response that is easily understood by the public. It may provide the best representation of societal concerns about hazardous substances, although cancer perhaps would not be so fearful if it were not often fatal. Fourth, incidence does not depend greatly on the availability and intensity of medical care. In general, if incidence were used as the common measure of response for any hazardous substance, waste classification essentially could be based on the simple notion of avoidance of harm. However, this option presents some difficulties for radionuclides, because studies of radiation effects in human populations have focused on cancer fatalities as the measure of response and probability coefficients for radiation-induced cancer incidence have not yet been developed by ICRP or NCRP for use in radiation protection. Probabilities of cancer incidence in the Japanese atomic-bomb survivors have been obtained in recent studies (see Section 3.2.3.2), but probability coefficients for cancer incidence appropriate for use in radiation protection would need to take into account available data on cancer incidence rates from all causes in human populations of concern, which may not be as reliable as data on cancer fatalities. Thus, in effect, if incidence were used as the measure of stochastic response for radionuclides, the most technically defensible database on radiation effects in human populations available at the present time (the data on fatalities in the Japanese atomic-bomb survivors) would be given less weight in classifying waste. Another possible disadvantage of using incidence is that stochastic responses that are rarely fatal (e.g., skin and thyroid cancers) would be given the same weight as responses that are almost always fatal, even though the latter presumably are of greater concern. Although any effort to weight the severity of nonfatal responses relative to fatal responses necessarily involves subjective judgment, this point is addressed by ICRP (1991) in its recommendation that nonfatal responses should be weighted by the lethality fraction in assessing total detriment from radiation exposure.

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6.1.1.2.2 Fatalities. In the second option, the common measure of stochastic response from exposure to radionuclides and hazardous chemicals would be fatalities, without any modifications to account for such factors as differences in lethality fractions for responses in different organs or tissues or expected years of life lost per fatality. This option is particularly advantageous for radionuclides, because fatalities is the measure of response provided by the most scientifically defensible database on stochastic radiation effects in humans. Fatalities is the measure of response normally emphasized in radiation risk assessments. A disadvantage of using fatalities is that it does not take into account the effects of nonfatal cancers on the quality of life. Although cancer is fearful mainly because it is often fatal, treatments even in cases that are rarely fatal (e.g., thyroid and skin cancer) generally are not welcomed, and some successful cures can significantly affect an individual’s quality of life. The use of fatalities also does not take into account that cures for cancer depend greatly on the availability and intensity of medical care, which in turn depends on socioeconomic conditions. Another disadvantage of this option is that it might be difficult to implement for chemicals that cause stochastic responses. At the present time, probability coefficients for nearly all substances that cause stochastic responses are based on data on incidence only, and the data are obtained mostly from studies in animals. This difficulty could be addressed if the observed responses in animals were the same as known responses in humans. In such cases, the lethality fraction (k) for cancers in different organs or tissues (see Table 3.2) could be used to convert cancer incidence to fatalities. In most cases, however, the organs or tissues in which cancers are induced in study animals are not the same as the cancer sites in humans, or the estimates of cancer incidence in animals are based on pooled responses at all sites. In other cases, the observed responses in animals are not known to occur in humans. In either of these situations, judgment would be needed in applying a lethality fraction to the data on cancer incidence obtained from studies in animals. An assumption of a lethality fraction of 0.6 to 0.7, which is the average for all types of cancers in many organs, should be reasonable in these cases, because the actual value could not be underestimated by more than 30 to 40 percent. Difficulties in converting data on cancer incidence to fatalities would not occur when the data on cancer incidence are obtained from studies in humans. However, human data are available for only a few chemicals that induce stochastic responses. 6.1.1.2.3 ICRP’s total detriment. In the third option, the common measure of stochastic response from exposure to radionuclides and

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hazardous chemicals would be total detriment, as developed by the ICRP (1991) for use in radiation protection (see Section 3.2.2.3.2). Total detriment is calculated from probability coefficients for fatal responses by applying modifying factors that take into account nonfatal responses, which are weighted by the lethality fraction for responses in each organ or tissue, and the expected years of life lost from fatal responses in each organ or tissue relative to the expected years of life lost from all fatal responses. The basic assumption used in calculating total detriment is that fatalities are the primary stochastic responses of concern, but that nonfatal responses also warrant consideration in health protection. This option does not appear to be advantageous for either radionuclides or chemicals that cause stochastic responses. In radiation protection, total detriment is used mainly to develop the tissue weighting factors in the effective dose (see Section 3.2.2.3.3), but ICRP and NCRP have continued to emphasize fatal responses as the primary health effect of concern in radiation protection and radiation risk assessments. Since total detriment is based on an assumption that fatalities are the primary health effect of concern, the same difficulties described in the previous section would occur if this measure of response were used for chemicals that induce stochastic responses. Other disadvantages of using total detriment include that detriment is not a health-effect endpoint experienced by an exposed individual and the approach to weighting nonfatal responses in relation to fatalities is somewhat arbitrary. Furthermore, total detriment is not as simple and straightforward to understand as either incidence or fatalities. 6.1.1.3 Recommendations on Selection of a Measure of Response. For purposes of waste classification, NCRP believes that it would be desirable, in principle, to use the same measure of response for all hazardous substances, essentially because this approach would help give equal weight to all such substances in classifying waste. Incidence is the common measure of response for all substances that cause a deterministic effect, including radionuclides, used in routine health protection, and there is no evident reason to change this. As indicated by the discussions in the previous sections, arguments can be advanced in favor of using either incidence or fatalities as the common measure of stochastic response. Use of ICRP’s total detriment appears to be disadvantageous, compared with either incidence or fatalities, and is not considered further. In classifying waste based on risk, incidence appears to be the most logical common measure of response for all hazardous substances, primarily because incidence is the only measure that is generally

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applicable to all subtances that cause stochastic or deterministic responses. Furthermore, this approach would involve the fewest subjective judgments about the relative importance of fatal and nonfatal responses, although it clearly incorporates a judgment for substances that cause stochastic responses that the incidence of any type of response would be of equal concern, regardless of the likelihood that the response would be fatal. However, given the current state of knowledge and methods of dose-response assessment for substances that cause stochastic responses, there appear to be important technical and institutional impediments to the use of either incidence or fatalities exclusively. Data on radiation-induced cancer incidence and chemical-induced cancer fatalities for use at the low doses and dose rates relevant to health protection are not readily available, and current regulatory guidance calls for calculation of cancer incidence for hazardous chemicals. Since use of a common measure of response for all substances that cause stochastic responses may not be practical in the near term, both measures (fatalities for radionuclides and incidence for hazardous chemicals) could be used in the interest of expediency. The primary advantage of this approach is that the measures of stochastic response for radionuclides and hazardous chemicals would be based on the best available information from studies in humans and animals, and it would involve the fewest subjective modifying factors. This approach also would be the easiest to implement. The approach of using fatalities for radionuclides but incidence for hazardous chemicals clearly would not result in a consistent measure of stochastic response for all substances of concern. However, cancer incidence and fatalities do not differ by more than a factor of two to three in most organs or tissues except the thyroid and skin (see Table 3.2). Thus, the difference between incidence and fatalities would not be large for most substances that cause stochastic responses, particularly compared with uncertainties in the data on which the estimated probabilities of stochastic responses are based, uncertainties in extrapolating data from animals to humans, and uncertainties in extrapolating from data at high doses and dose rates to the low doses and dose rates of concern in routine exposures of the public. Furthermore, the difference between incidence and fatalities would be insignificant in most cases compared with differences discussed in the following section that result from the different approaches to establishing probability coefficients for stochastic responses from exposure to radionuclides and hazardous chemicals. 6.1.2 Dose-Response Relationships Development of a comprehensive and risk-based hazardous waste classification system requires assumptions about thresholds in the

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dose-response relationships for hazardous chemicals that cause deterministic effects and probability coefficients for induction of stochastic responses from exposure to radionuclides and hazardous chemicals. Establishment of dose-response relationships for radionuclides and hazardous chemicals is discussed in Section 3.2. The following sections discuss NCRP’s recommendations on suitable approaches to addressing these issues for purposes of waste classification. 6.1.2.1 Deterministic Responses. At the low levels of exposure of concern to waste classification, deterministic responses generally are important only for hazardous chemicals, but not for radionuclides. For chemicals that cause deterministic effects, NCRP believes that threshold doses in humans should be estimated using the benchmark dose method; the benchmark dose is a dose that corresponds to a 10 percent increase in the number of responses and is obtained by statistical fitting of a dose-response model to the dose-response data in the region where the number of responses is increased (see Section 3.2.1.2.7). Specifically, NCRP believes that a suitable representation of the threshold in the dose-response relationship in virtually all humans is a dose that is a factor of 10 lower than the lower confidence limit of the benchmark dose obtained in a high-quality human study or a dose that is a factor of 100 lower than the lower confidence limit of the benchmark dose obtained in a high-quality animal study. The reduction by a factor of 10 when the benchmark dose is obtained in a human study takes into account the need to protect sensitive population groups (e.g., infants and children, the elderly and infirm). This reduction is consistent with the approach used in radiation protection of the public, where deterministic dose limits are set at a factor of 10 lower than nominal thresholds for deterministic radiation effects in adults. The further reduction by a factor of 10 when the benchmark dose is obtained in an animal study takes into account that the animals may be less sensitive than humans. The recommended approach to estimating threshold doses of chemicals that induce deterministic effects in humans acknowledges the considerable uncertainty in estimating the highest dose at which no significant effects would be observed. However, the approach should not be unduly conservative and, thus, should not give disproportionate weight to chemicals that induce deterministic effects, compared with radionuclides and chemicals that cause stochastic effects, in classifying waste. Characteristics of high-quality studies in animals are discussed in Section 3.1.4.1.2. As an alternative to using the benchmark dose method, the more traditional approach of estimating threshold doses of substances that cause deterministic effects based on NOAELs could be used. In

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most high-quality studies, the two methods are largely equivalent because NOAEL is approximately the same as the lower confidence limit of the benchmark dose that corresponds to a 10 percent increase in response. Thus, the nominal threshold in humans could be set at a factor of 10 or 100 lower than NOAEL obtained in a high-quality human or animal study, respectively. However, NCRP prefers the benchmark dose method, mainly because the method makes use of the full range of data on dose-response, rather than a single data point (NOAEL). The benchmark dose method also can address difficulties that arise when NOAEL is not obtained in a high-quality study or is not included in a data set. Estimated thresholds for deterministic responses in virtually all humans based on lower confidence limits of benchmark doses or NOAELs, as described above, would be used as points of departure in establishing allowable doses of chemicals that induce deterministic responses for purposes of waste classification. NCRP’s recommendations on the magnitude of safety and uncertainty factors that should be applied to benchmark doses or NOAELs in classifying waste are described in Section 6.3.1.1. 6.1.2.2 Stochastic Responses. Consideration of the dose-response relationships and the nominal probability coefficients for induction of stochastic responses at low doses is important for both radionuclides and hazardous chemicals. For radionuclides, NCRP reaffirms use of a best estimate (MLE) of the response probability obtained from a linear or linear-quadratic model as derived from data in humans, principally the Japanese atomic-bomb survivors. This model essentially is linear at the low doses of concern to waste classification. Specifically, for purposes of health protection of the public, NCRP reaffirms use of a probability coefficient for fatal cancers (probability per unit effective dose) of 0.05 Svⳮ1 (ICRP, 1991; NCRP, 1993a). Although this probability coefficient is less rigorous for intakes of some long-lived radionuclides that are tenaciously retained in the body than for other exposure situations, such as external exposure or intakes of short-lived radionuclides (Eckerman et al., 1999), it is adequate for the purpose of generally classifying waste, especially when the lack of data on cancer risks in humans for most chemicals is considered. For chemicals that cause stochastic responses, NCRP believes that a linearized multi-stage model should be used to estimate risks at low doses based on data at high doses in humans or animals. Furthermore, NCRP believes that best estimates (MLEs) of response probabilities obtained from that model should be used for purposes of riskbased waste classification, rather than UCLs that are used nearly

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universally in health risk assessments for chemicals that cause stochastic responses. The recommended approach should provide estimates of probability coefficients for chemicals that are reasonably consistent with the value for radionuclides given above. Thus, comparable weight would be given to the two types of substances in classifying waste. NCRP believes that the use of MLEs of probability coefficients for radionuclides and chemicals that cause stochastic effects can be justified on the grounds that the types of exposure scenarios that would be assumed for purposes of classifying waste (see Section 6.1.3 below) are likely to provide considerable overestimates of actual exposures at waste disposal sites. However, uncertainties in probability coefficients should not be ignored in classifying waste. When risk is calculated using MLEs of probability coefficients, judgments about allowable risk that are required in classifying waste based on risk should take uncertainties in probability coefficients into account, along with such other factors as judgments about the quality of the data on dose-response, desired margins of safety in protecting the public, and the cost-benefit of different choices. This approach would provide a clear separation between risk assessment and risk management aspects of waste classification. Risk assessment would focus on central estimates of risk for assumed exposure scenarios, and risk management decisions based on judgments about allowable risk, which can be substance-specific, could incorporate any desired degrees of conservatism in protecting the public beyond those embodied in the assumed scenarios.

6.1.3 Exposure Scenarios for Waste Classification Assumptions about exposure scenarios are required in developing a risk-based waste classification system. These scenarios would be used to calculate potential risks posed by hazardous wastes for purposes of waste classification. An exposure scenario essentially is a set of assumptions about events and processes that could result in exposure of humans. In principle, for any type of disposal system that could be assumed for purposes of classifying waste, such as a near-surface disposal facility for hazardous wastes, a multitude of exposure scenarios might be considered. However, NCRP believes that only a single type of exposure scenario should be considered in classifying waste. Specifically, NCRP believes that the concept of a hypothetical inadvertent intruder at waste disposal sites provides a suitable basis for

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defining exposure scenarios relevant to risk-based waste classification. Scenarios for inadvertent intrusion are appropriate, in part, because they often do not depend greatly on the characteristics of a particular disposal site and facility. Given the concept of a hypothetical inadvertent intruder at waste disposal sites, a multitude of scenarios for exposure of such individuals could be considered. NCRP believes that the types of scenarios commonly used in risk assessments at near-surface disposal sites for low-level radioactive waste (NRC, 1982b) or cleanup of sites contaminated with radionuclides or hazardous chemicals (EPA, 1989; Kennedy and Strenge, 1992) would be appropriate. These scenarios generally assume permanent residence at a disposal site at any time after loss of institutional control and, thus, should provide conservative overestimates of risks to inadvertent intruders that are reasonably likely to occur at any site, including risks associated with scenarios involving short-term exposure that might occur during the period of institutional control. Although other scenarios could be envisioned that might result in higher estimates of exposure, such scenarios would not be appropriate for the purpose of classifying waste if they were not reasonably likely to occur for the types of disposal systems of concern. NCRP does not believe that riskbased waste classification should be based on implausible, worstcase assumptions. NCRP also recognizes that potential exposures of members of the public beyond the boundaries of disposal sites generally are of concern in determining acceptable disposal practices at any site. However, at most disposal sites, off-site releases of many hazardous substances and resulting exposures of the public are determined primarily by the movement of water and, thus, are expected to be highly site-specific. The dependence of exposure scenarios involving off-site release of contaminants on site-specific characteristics makes these types of scenarios inappropriate for use in classifying waste based on generic assumptions about disposal systems. The general concern about limiting off-site releases of hazardous substances is the primary reason why classification of waste based on risks to hypothetical inadvertent intruders at waste disposal sites does not provide a substitute for site-specific risk assessments when determining acceptable disposal practices. Nonetheless, experience with risk assessments at near-surface disposal sites for low-level radioactive waste has indicated that, for most radionuclides, disposal limits that provide adequate protection of future inadvertent intruders should provide adequate protection of the public and the environment at off-site locations as well. Exceptions are expected to occur only in unusual cases of long-lived and highly mobile radionuclides.

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Similar conclusions should apply to hazardous chemical waste, especially waste that contains heavy metals. Thus, basing waste classification on scenarios for inadvertent intrusion should not result in large quantities of waste that would not be acceptable for disposal using the intended disposal technology for the particular waste class at well chosen sites. The types of scenarios for inadvertent intrusion that could be used in classifying waste are discussed further in Section 6.3.2.

6.1.4 Approaches to Risk Management Classification of hazardous waste based on risk requires assumptions about allowable risks from exposure to hazardous substances. Therefore, an understanding of current approaches to risk management for radionuclides and hazardous chemicals, especially their differences and how they can be reconciled, is important in classifying waste. Two different approaches to risk management, referred to as the radiation and chemical paradigms, are embodied in current laws and regulations addressing hazardous substances in the environment (see Section 3.3). The radiation paradigm involves establishing limits on acceptable risk and requiring reductions in risk below the limits to levels that are ALARA, economic and social factors being taken into account. In contrast, the chemical paradigm involves establishing goals for acceptable risk and allowing relaxations in risks above the goals based primarily on considerations of technical feasibility and cost. As a result of this difference, the two paradigms attach different meanings to the terms ‘‘acceptable’’ and ‘‘unacceptable’’ commonly used to describe risk (see Table 3.5). NCRP believes that a single paradigm for risk management should be applied in developing a risk-based classification system for waste that contains radionuclides or hazardous chemicals, and NCRP recommends use of the radiation paradigm for this purpose. In making this recommendation, NCRP recognizes that there is the appearance of allowing higher stochastic risks than might be permitted if the chemical paradigm were used in classifying waste. However, NCRP emphasizes that this is not necessarily the case because (1) stochastic risks regarded as ‘‘unacceptable’’ in the chemical paradigm (i.e., excess lifetime cancer risks above negligible levels in the range of about 10ⳮ4 to 10ⳮ6) often are permitted, based essentially on the same application of the ALARA principle as in the radiation paradigm, and (2) application of the ALARA principle in the radiation paradigm usually reduces stochastic risks to levels well below those that are

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regarded as intolerable (i.e., well below lifetime risks in the range of about 10ⳮ1 to 10ⳮ3). Indeed, in the radiation and chemical paradigms, the ALARA principle is the primary basis for most decisions about stochastic risk management, without regard for any limits in the radiation paradigm or goals in the chemical paradigm (Kocher, 1999; NAS/NRC, 1999a). The ALARA principle also has been used in decisions about risk management for chemicals that cause deterministic effects. RfDs often are used to define acceptable exposures to such substances. However, given the large safety and uncertainty factors often used in deriving RfDs from a NOAEL or LOAEL (see Section 3.2.1.2.4), RfDs generally correspond to doses considered negligible, and doses above an RfD may be permitted in particular situations if RfD is not achievable at a reasonable cost (see Section 3.3.2). In recommending use of the radiation paradigm in classifying waste, NCRP does not mean to imply that the chemical paradigm is inappropriate for use in risk management. Indeed, NCRP recognizes that the chemical paradigm is a valid approach to risk management if the risk goals are properly interpreted as defining negligible risks. However, NCRP believes that the radiation paradigm offers three important advantages compared with the chemical paradigm: (1) a clear concept of risks that are intolerable and normally must be reduced regardless of cost or other circumstances, (2) explicit recognition of the importance of the ALARA principle in reducing risks below levels regarded as barely tolerable, and (3) a clear distinction between risks that are negligible and higher risks that are acceptable provided they are ALARA. For purposes of waste classification, the radiation paradigm, which defines risks that are unacceptable (intolerable), acceptable if ALARA, and negligible, permits the use of different limits on allowable risk to define different waste classes. Specifically, as indicated in Figure 6.1, a negligible risk can be used to distinguish between exempt and low-hazard waste, and a substantially higher acceptable (barely tolerable) risk can be used to distinguish between low-hazard and high-hazard waste.

6.1.5 Legal and Regulatory Constraints NCRP recognizes that if the waste classification system described in this Report is to gain acceptance, it must be broadly compatible with current approaches to management and disposal of hazardous wastes. However, NCRP believes that development of a new waste classification system to address deficiencies in the existing systems should not be constrained by provisions of current laws or regulations

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that might discourage such a system. Examples of such constraints include: the distinction between radioactive waste that arises from operations of the nuclear fuel-cycle and other (NARM) waste (see Section 4.1.2.1); the provision of the National Energy Policy Act that prohibits NRC from establishing dose criteria that could be used to develop exemption levels for radionuclides in any waste (see Section 4.1.2.5.2); and the ‘‘mixture’’ and ‘‘derived-from’’ rules for listed hazardous chemical wastes under RCRA (Section 4.2.1.1). NCRP believes that its recommendations should focus on the technical basis for risk-based hazardous waste classification without regard for any legal or regulatory constraints that are largely unrelated to risks posed by waste.

6.2 Framework for Risk-Based Waste Classification This Section presents NCRP’s recommendations on a framework for a comprehensive and risk-based hazardous waste classification system. These recommendations focus primarily on the concepts and principles embodied in the new system. Approaches to implementing the waste classification system by specifying quantitative boundaries of different waste classes in the form of limits on concentrations of hazardous substances are discussed in this Section and in Section 6.4.5, and numerical examples are developed in Section 7.1. However, NCRP believes that the task of specifying such boundaries is properly the role of regulatory authorities, and specific recommendations on limits on concentrations of hazardous substances in different waste classes are not presented.

6.2.1 Framework of the Proposed Waste Classification System The framework for the comprehensive and risk-based waste classification system developed in this Report is depicted in Figure 6.1 at the beginning of Section 6. Classification of waste is based on specific objectives and the fundamental principle of defining waste classes in relation to acceptable disposal systems, and these lead to the definitions of three basic waste classes. The remainder of this Section through Section 6.5 discusses the framework for the hazardous waste classification system depicted in Figure 6.1 and its expected consequences in more detail. The merits of this general framework in the context of radioactive waste classification and its basis in the three distinct types of disposal systems have been recognized for many years (Adam and Rogers,

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1978; King and Cohen, 1977). The possibility of defining subclasses of the three basic waste classes is discussed in Section 6.6.

6.2.2 Framework for Waste Classification The basic element of the recommended framework for a comprehensive and risk-based waste classification system is the assumption that any waste that contains sufficiently small amounts of radionuclides or hazardous chemicals should be classified as exempt, or essentially nonhazardous. Waste that contains greater amounts of hazardous substances then would be classified as nonexempt, and further classification of nonexempt wastes, based also on the amounts of hazardous substances present, would be appropriate. Limits on amounts of hazardous substances in each waste class would be calculated based on values of the so-called risk index for each hazardous substance in the waste and the composite risk index for mixtures of hazardous substances. For the purpose of describing the recommended framework for a risk-based hazardous waste classification system, the risk index is generally defined as: RI ⳱ F

(risk from disposal) , (allowable risk)

(6.1)

where F is a modifying factor described below (F ⬎ 0). RI essentially is the ratio of a calculated risk that arises from disposal of a given waste using a particular type of disposal system (technology) to a specified limit on allowable risk for that type of disposal system. The calculated risk in the numerator would be based on an assumed exposure scenario that is appropriate to the assumed type of disposal system. If the modifying factor is omitted, the risk index is in the form of a hazard quotient, which often is used to describe exposures to chemicals that induce deterministic effects (EPA, 1989). The modifying factor in the risk index represents any considerations of importance to waste classification other than those that are directly incorporated in the calculated risk from disposal and the specified allowable risk. The modifying factor can take into account, for example, the probability of occurrence of assumed exposure scenarios used in classifying waste, uncertainties in the assessment of risk from disposal and in the data required to evaluate the risk index, levels of naturally occurring hazardous substances in surface soil and their associated health risks to the public, and the costs and benefits of different means of waste disposal. The modifying factor is discussed further in Section 6.3.3.

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The basic definitions of exempt, low-hazard, and high-hazard waste shown in Figure 6.1 are considered in the following sections. Recommendations on approaches to calculating the risk from waste disposal in the numerator of the risk index and recommendations on specifying allowable risks in the denominator of the risk index for the purpose of classifying waste are discussed in Section 6.3. 6.2.2.1 Exempt Waste. The class of exempt waste embodies the concept that there are amounts of hazardous substances in waste which are so low that the associated risks to the public for any method of disposal generally would not be of concern. Thus, if waste that contains radionuclides were classified as exempt, the waste could be disposed of as if it were nonradioactive, and similarly for waste that contains hazardous chemicals or mixtures of the two. Further, mixed wastes that contain exempt amounts of radionuclides could be managed based on their hazardous chemical content, and vice versa. NCRP believes that different classes of waste should be defined in relation to general types of disposal systems that presently exist or are likely to be developed in the future. In accordance with current waste disposal practices, the exempt class of waste (essentially nonhazardous) thus is defined as any waste containing sufficiently small amounts of hazardous substances that the waste would be generally acceptable for disposal in a municipal/industrial landfill (or equivalent) for nonhazardous materials. This type of disposal facility is regulated under Subtitle D of RCRA (1976). Because disposal of exempt waste would be unregulated in regard to its hazardous constituents, NCRP recommends that limits on concentrations of radionuclides and hazardous chemicals in exempt waste should be defined on the basis of a negligible risk to hypothetical inadvertent intruders at near-surface waste disposal sites. That is, the allowable risk in the denominator of Equation 6.1 should correspond to a negligible risk. Waste would be classified as exempt if the risk index calculated in this way were less than unity, but otherwise would be nonexempt.15 NCRP again notes that, in accordance with the radiation paradigm for risk management, a negligible risk is distinct from, and considerably less than, an acceptable (barely tolerable) risk.

15

Expressing the decision rule for waste classification in terms of a risk index less than unity conforms to the approach used by NRC in its classification system for near-surface disposal of radioactive waste in 10 CFR Part 61 (NRC, 1982a).

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6.2.2.2 Nonexempt Waste. NCRP recommends that nonexempt waste—i.e., waste that contains amounts of radionuclides, hazardous chemicals, or both greater than the allowable amounts in exempt waste—be placed in one of two classes, called low-hazard waste and high-hazard waste. 6.2.2.2.1 Low-hazard waste. NCRP recommends that low-hazard waste be defined as any nonexempt waste that is generally acceptable for disposal in a dedicated near-surface facility for hazardous wastes. Examples of such facilities include licensed or permitted for disposal of low-level radioactive waste under AEA (1954) or disposal of hazardous chemical waste under Subtitle C of RCRA (1976). Because nonexempt waste would be carefully regulated with respect to its hazardous constituents, NCRP recommends that limits on concentrations of radionuclides and hazardous chemicals in lowhazard waste should be defined on the basis of an acceptable (barely tolerable) risk to hypothetical inadvertent intruders at near-surface waste disposal sites. That is, the allowable risk in the denominator of Equation 6.1 should correspond to an acceptable risk. NCRP reiterates that, in accordance with the radiation paradigm for risk management, an acceptable risk is distinct from, and considerably greater than, a negligible risk that would be used as a basis for classifying exempt waste. As a result, limits on concentrations of hazardous substances in low-hazard waste generally would be substantially higher than in exempt waste. Waste would be classified as low-hazard if the risk index calculated in this way were less than unity. Otherwise, the waste would be classified as high-hazard. The use of an acceptable (barely tolerable) risk to classify nonexempt waste can be justified, in part, on the following grounds. Disposal facilities for exempt and low-hazard waste both are located near the ground surface, and many scenarios for inadvertent intrusion into municipal/industrial landfills for nonhazardous waste also would be credible occurrences at disposal sites for low-hazard waste. However, these types of scenarios should be less likely to occur at hazardous waste sites, compared with sites for disposal of nonhazardous waste, given the intention to maintain institutional control and records of past disposal activities for a considerable period of time after closure of hazardous waste sites and the possibility that societal memory of disposal activities will be retained long after institutional control is relinquished. Thus, the risk to future inadvertent intruders at dedicated hazardous waste disposal sites, taking into account the probability that exposures according to postulated scenarios would actually occur, should be comparable to the risk at disposal sites for nonhazardous waste.

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6.2.2.2.2 High-hazard waste. NCRP recommends that highhazard waste be defined to include any nonexempt waste with concentrations of hazardous substances greater than those that are generally acceptable for disposal in a dedicated near-surface facility for hazardous wastes. High-hazard waste would require a disposal system considerably more isolating than a near-surface facility for hazardous waste. An essential feature of acceptable disposal systems for high-hazard waste is that the waste would need to be isolated so that inadvertent intrusion into the waste as a result of normal human activities, such as drilling or excavation, would be highly unlikely. Therefore, an analysis of the potential consequences to an inadvertent intruder, given an assumption that a highly unlikely intrusion scenario would actually occur, does not provide a reasonable basis for determining disposal systems that should be generally acceptable for highly hazardous wastes (NAS/NRC, 1995a). Disposal systems for high-hazard waste also need to be capable of limiting environmental releases of highly concentrated wastes to acceptable levels for long periods of time using natural and engineered barriers. In this regard, the consequences of an unlikely intrusion event, such as drilling, for increasing potential releases to the biosphere by the groundwater pathway are a legitimate concern in licensing disposal facilities for high-hazard waste (EPA, 2001a; NAS/NRC, 1995a). However, these considerations are site-specific, and they do not provide an appropriate basis for classifying waste in general terms. At the present time, a geologic repository is the intended disposal system for most radioactive waste that is not acceptable for nearsurface disposal. Alternatives to near-surface disposal facilities have not been considered for hazardous chemical waste that contains unusually high concentrations of persistent substances (e.g., heavy metals).

6.3 Development of the Risk Index for Individual Hazardous Substances For the purpose of developing a comprehensive and risk-based hazardous waste classification system, a simple method of calculating the risk posed by mixtures of radionuclides and hazardous chemicals in waste is needed. The method should properly account for the presence of subtances in waste that cause deterministic or stochastic responses. As a first step in accomplishing this, it is necessary to specify a method for calculating the risk posed by individual substances with associated deterministic or stochastic responses.

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NCRP assumes that the risk from disposal of any hazardous substance in waste can be described by means of a dimensionless risk index. The risk index for the ith hazardous substance is defined as the calculated risk from disposal of that substance, based on an assumed exposure scenario, relative to a specified allowable risk for the assumed type of disposal system. Based on this definition, the risk index is written as: RI i ⳱ Fi

(risk from disposal) i , (allowable risk) i

(6.2)

where F is a modifying factor introduced with Equation 6.1 and discussed in more detail in Section 6.3.3. The modifying factor can depend on the particular hazardous substance. The risk index in Equation 6.2 is expressed in terms of risk (i.e., the probability that an adverse response will occur during an individual’s lifetime). This definition is consistent with the fundamental objective of developing a risk-based hazardous waste classification system. However, the use of health risk per se in calculating the risk index presents some difficulties because risk is not proportional to dose for substances that cause deterministic effects. For this type of substance, the risk is presumed to be zero at any dose below a nominal threshold. Since the allowable dose should always be less than the threshold in order to prevent the occurrence of adverse responses, expressing the risk index in terms of risk would result in an indeterminate value and, more importantly, a lack of distinction between doses near the nominal thresholds and lower doses of much less concern. For any hazardous substance, including carcinogens for which risk is assumed to be proportional to dose without threshold, it is generally useful to express the risk index as the ratio of a calculated dose [e.g., sieverts, mg (kg d)ⳮ1] to an allowable dose that corresponds to an allowable risk: RI i ⳱ Fi

(dose from disposal) i , (allowable dose) i

(6.3)

NCRP believes that use of a risk index expressed in terms of dose is acceptable and desirable as long as (1) the units of the numerator and denominator are consistent at a conceptual level, (2) the assumptions embodied in the proportionality constants between dose and response for substances that cause stochastic responses are clearly stated, and (3) the allowable doses are adjusted when the proportionality constants between dose and response for substances that cause stochastic responses or the thresholds for substances that cause deterministic responses change significantly.

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6.3.1 Establishing Allowable Risks or Doses of Individual Substances Establishing allowable risks or doses in the denominator of the risk index involves two major steps. First, societal judgments about an allowable risk for the general types of disposal systems assumed in classifying waste are required. Second, when the risk index is expressed in terms of dose, an allowable risk must be related to dose. For chemicals that cause deterministic effects, these steps involve establishing an appropriate threshold in the dose-response relationship and applying judgments about safety and uncertainty factors that should be used in setting allowable doses at levels below the threshold. For radionuclides and chemicals that cause stochastic effects, the second step involves an assumption about the probability of a response per unit dose. These probability coefficients, when applied to an assumed allowable risk, determine allowable doses. NCRP’s recommendations on specifying an allowable risk or dose in the denominator of the risk index for the purpose of classifying waste are discussed in the following two sections. A general discussion of dose-response assessment for hazardous chemicals and radionuclides is presented in Section 3.2. 6.3.1.1 Establishing Allowable Doses of Substances That Cause Deterministic Responses. The risk index for substances that cause deterministic responses normally should be expressed in terms of dose, rather than risk, given the assumption of a threshold doseresponse relationship. The allowable dose of substances that cause deterministic responses in the denominator in Equation 6.3 should be related to thresholds for induction of deterministic responses in different organs or tissues. NCRP recommends continued use of the current approach to controlling exposures to substances that cause deterministic responses so that the probability of a response is essentially zero. Thus, doses corresponding to an allowable risk should be set so that average individuals in the most sensitive population subgroups (e.g., infants and children) would not be expected to experience a significant deterministic response. As noted previously, the issue of establishing allowable doses of substances that induce deterministic effects for the purpose of classifying waste should be of concern only for hazardous chemicals. NCRP’s recommendations on establishing negligible and acceptable doses of chemicals that cause deterministic responses are described in the following two sections. 6.3.1.1.1 Dose corresponding to a negligible risk. For the purpose of classifying waste as exempt, NCRP believes that a negligible dose

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of a chemical that causes a deterministic effect should be set at a small fraction (e.g., 10 percent) of the nominal threshold in virtually all humans estimated using the benchmark dose method as described in Section 6.1.2.1. Furthermore, under the presumption that the data on dose-response from studies in humans or animals are of sufficiently high quality that the nominal threshold can be estimated with considerable confidence, the negligible dose should be set at the same fraction of the nominal threshold for all chemicals that cause deterministic effects. The recommended approach to establishing negligible doses of chemicals that induce deterministic effects should ensure that such effects would be precluded in almost all individuals with a substantial margin of safety. RfDs established by EPA also could be used to define negligible doses of noncarcinogenic hazardous chemicals because RfDs are intended to be well below thresholds for deterministic responses in humans. However, NCRP believes that RfDs should not be used without presenting NOAELs or LOAELs used to derive the values. In addition, when RfDs for important waste constituents are derived using large safety and uncertainty factors, thus indicating that the quality of the data is poor, NCRP believes that further studies should be undertaken to reduce uncertainties in the nominal threshold in humans, to avoid introducing undue levels of conservatism in classifying waste. To promote consistency in waste classification, NCRP believes that it would be desirable to define negligible doses of all substances that cause deterministic responses at approximately the same fraction of the nominal thresholds in humans. 6.3.1.1.2 Dose corresponding to an acceptable risk. For the purpose of classifying waste as low-hazard or high-hazard, NCRP believes that an acceptable (barely tolerable) dose of a chemical that causes deterministic effects should be set at the nominal threshold in virtually all humans estimated as described in Section 6.1.2.1, or perhaps slightly below the nominal threshold (e.g., within a factor of two to three) if an additional margin of safety is warranted. When the data on dose-response are of sufficiently high quality that the nominal threshold can be estimated with considerable confidence, the recommended approach to establishing acceptable (barely tolerable) doses of chemicals that induce deterministic effects should ensure that average individuals in the most sensitive population groups would be adequately protected. The use of multiples of RfDs established by EPA to define acceptable (barely tolerable) doses of chemicals that cause deterministic responses also could be considered because RfDs normally are intended to be well below nominal thresholds for deterministic

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responses in humans. However, the cautions about using RfDs discussed in the previous section also apply in establishing acceptable doses, especially when the quality of the data on dose-response is poor. As in establishing negligible doses of substances that cause deterministic effects, NCRP prefers an approach in which acceptable doses are based directly on nominal thresholds in humans and application of small safety factors, as appropriate, to promote transparency and consistency in waste classification. 6.3.1.2 Establishing Allowable Risks or Doses of Substances That Cause Stochastic Responses. Given the assumption of a linear doseresponse relationship for substances that cause stochastic responses without threshold, either risk or dose may be used to calculate the risk index. The following two sections discuss suitable approaches to establishing negligible and acceptable risks or doses of substances that cause stochastic responses. 6.3.1.2.1 Establishing a negligible risk or dose. There are a number of precedents for establishing a negligible risk or dose of substances that cause stochastic responses for the purpose of classifying exempt waste. For radionuclides, NCRP has recommended that an annual effective dose to individuals of 0.01 mSv generally can be considered negligible (see Section 4.4.1.2). Assuming a probability coefficient of 0.05 Svⳮ1, the recommended negligible dose corresponds to an estimated lifetime fatal cancer risk of about 4 ⳯ 10ⳮ5. An annual dose of 0.01 mSv also has been used by IAEA to define an exempt class of radioactive waste and, more generally, to define radioactive material that can be exempted from regulatory control (see Section 4.1.3). Similarly, a negligible lifetime risk of about 1 ⳯ 10ⳮ5 was proposed, but not implemented, by EPA (1992d) for the purpose of exempting waste that contains chemicals that cause stochastic responses from requirements of RCRA. A negligible risk or dose of substances that cause stochastic responses consistent with the precedents described above can be supported by considering the unavoidable risks due to natural background radiation and naturally occurring chemicals. The average annual dose due to natural background radiation, including indoor radon, is about 3 mSv (NCRP, 1987b). This dose corresponds to an estimated lifetime fatal cancer risk of about 10ⳮ2. The risk from exposure to naturally occurring chemicals (e.g., carcinogenic heavy metals such as arsenic, natural pesticides in food, and organic compounds in soil) is not as well characterized but, based on available information, the estimated lifetime cancer risk also is about 10ⳮ2 (Ames and Gold, 1995; Travis and Hester, 1990). Given the magnitude of the unavoidable cancer risks from exposure to the natural

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background of radiation and chemicals, a negligible risk from exposure to man-made sources reasonably could be set at a small fraction (e.g., one percent) of the average background risk. Such a risk (e.g., an excess lifetime cancer risk of about 10ⳮ4) should be less than the variability in the background risk at any location due to differences in living habits. The negligible individual dose of 0.01 mSv discussed above corresponds to about one percent of the annual dose due to natural background radiation, excluding radon. Exemption of waste that contains naturally occurring substances that cause stochastic responses warrants further consideration because the negligible cancer risks or doses described above may correspond to exemption levels that are less than background levels in soil or rock. For example, average background levels of radium and thorium in soil (NCRP, 1984a; 1987b) correspond to annual effective doses well above 0.01 mSv (NCRP, 1999a) and estimated lifetime cancer risks well above 10ⳮ4. Similarly, average background levels of arsenic in soil (Buonicore, 1995) correspond to estimated lifetime cancer risks well above 10ⳮ5 (EPA, 1996e). As a consequence, application of these exemption criteria to all wastes could preclude exemption of virtually any waste derived from earthen materials, even when the concentrations of naturally occurring hazardous substances are not enhanced by human activities. In order to provide a practical system for exempting such wastes, NCRP believes that exemption levels for naturally occurring substances that cause stochastic responses should be based on considerations of background levels in surface soil and their associated health risks to the public, in addition to the negligible risks that would be used to establish exemption levels for man-made substances. Similar considerations could apply to naturally occurring substances that cause deterministic responses for which normal intakes are a substantial fraction of the nominal threshold in humans. 6.3.1.2.2 Establishing an acceptable risk or dose. There also are a number of precedents for establishing an acceptable (barely tolerable) risk or dose of substances that cause stochastic responses for the purpose of classifying waste as low-hazard or high-hazard. For radionuclides, the annual dose limit for the public of 1 mSv currently recommended by ICRP (1991) and NCRP (1993a) and contained in current radiation protection standards (DOE, 1990; NRC, 1991) could be applied to hypothetical inadvertent intruders at licensed near-surface disposal facilities for low-hazard waste. This dose corresponds to an estimated lifetime fatal cancer risk of about 4 ⳯ 10ⳮ3. Alternatively, the limits on concentrations of radionuclides in radioactive waste that is generally acceptable for near-surface disposal,

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as established by NRC in 10 CFR Part 61 (NRC, 1982a), could be applied directly to waste classification, because these concentration limits were based on scenarios for inadvertent intrusion following an assumed loss of institutional control at 100 y after disposal (see Section 4.1.2.3.3). An acceptable (barely tolerable) risk from exposure to chemicals that cause stochastic responses has not formally been considered by EPA. However, based on an analysis of case-by-case regulatory decisions prior to 1985 discussed in Sections 3.3.2 and 3.3.3, EPA generally acted to reduce risks in the range of about 10ⳮ2 to 10ⳮ3, the particular value depending in part on the size of the exposed population. Thus, there are precedents for setting an acceptable risk from exposure to chemicals that cause stochastic responses at about the same level as the value for radionuclides. An acceptable risk or dose of substances that cause stochastic responses consistent with the precedents described above can be supported by available information on cancer risks from exposure to natural background radiation and naturally occurring chemicals. As noted in the previous section, the estimated lifetime cancer risks due to the background of radiation and chemicals each are about 10ⳮ2. An acceptable risk could be set at a value that corresponds approximately to the geographical variability in the background risk, because people normally do not consider this variability in deciding where to live. For example, excluding indoor radon, the standard deviation of the geographical distribution of the dose to an average individual due to natural background radiation is a few tens of percent of the mean dose (NCRP, 1987b).

6.3.2 Developing Exposure Scenarios for Purposes of Waste Classification In general, calculation of the risk or dose from waste disposal in the numerator of the risk index in Equation 6.2 or 6.3 involves the risk assessment process discussed in Section 3.1.5.1. As summarized in Section 6.1.3, NCRP recommends that generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites should be used in calculating risk or dose for purposes of waste classification. Implementation of models describing exposure scenarios for inadvertent intruders at waste disposal sites and their associated exposure pathways generally results in estimates of risk or dose per unit concentration of hazardous substances in waste. These results then are combined with the assumptions about allowable risk discussed in the previous section to obtain limits on concentrations of hazardous substances in exempt or low-hazard waste.

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6.3.2.1 Exposure Scenarios for Classifying Exempt Waste. Based on the definition of exempt waste as any waste that would be generally acceptable for disposal in a municipal/industrial landfill for nonhazardous waste, scenarios for inadvertent intrusion appropriate to this type of facility should be used in determining whether a waste would be classified as exempt. A municipal/industrial landfill for nonhazardous waste normally is constructed without engineered barriers that would deter inadvertent intrusion into the waste during normal human activities on the ground surface. Furthermore, the waste itself often is in a readily penetrable physical form. Therefore, scenarios for inadvertent intrusion involving permanent occupancy of disposal sites and normal human activities that could access waste, such as excavation of waste in the construction of homes and residence on top of exposed waste, would be appropriate. These types of scenarios have been used in evaluating inadvertent intrusion at near-surface disposal facilities for radioactive waste (NRC, 1982b; Oztunali and Roles, 1986; Oztunali et al., 1986) and hazardous chemical waste (Okrent and Xing, 1993), and they are used in risk assessments of contaminated sites subject to remediation under CERCLA (EPA, 1989) or AEA (Kennedy and Strenge, 1992). Since institutional control is not expected to be maintained for a substantial period of time after closure of a landfill for nonhazardous waste, intrusion scenarios involving permanent occupancy of a site could be assumed to occur essentially at the time of facility closure. The assumptions about exposure scenarios described above would apply to any allowable means of disposal of exempt waste on or near the ground surface. An assumption that exempt waste would be sent to a disposal facility for nonhazardous waste permitted under Subtitle D of RCRA (1976) is not required. 6.3.2.2 Exposure Scenarios for Classifying Low-Hazard Waste. The assumed technologies for disposal of low-hazard and exempt waste are similar in that both involve near-surface facilities. Thus, scenarios for inadvertent intrusion used in classifying low-hazard waste could be the same in many respects as those used in classifying exempt waste. However, there are important differences that should be taken into account in developing intrusion scenarios at nearsurface facilities for low-hazard waste. This type of disposal facility frequently includes engineered barriers, impenetrable waste forms, or deliberate placement of more hazardous wastes at relatively inaccessible locations (e.g., at greater depths or beneath other wastes). All of these features are intended to deter inadvertent intrusion for some time after disposal or to make it less likely that waste would

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be accessed by normal human activities after occupancy of the site could occur. Furthermore, given the intention to maintain institutional control over hazardous waste disposal sites for a considerable period of time after facility closure, substantial decay of many radionuclides, as well as significant chemical transformations of other hazardous substances to less hazardous forms, could occur prior to the time that permanent occupancy of disposal sites by the public would be possible. All of these factors should be taken into account in developing generic scenarios for inadvertent intrusion, and their associated exposure pathways, for the purpose of classifying lowhazard waste. An additional consideration in classifying low-hazard waste based on scenarios for inadvertent intrusion at a near-surface facility is that some forms of intrusion would be credible even during the institutional control period. However, appropriate intrusion scenarios during the institutional control period would differ from scenarios involving permanent occupancy of disposal sites after loss of institutional control in regard to the credible exposure pathways and the duration of exposures. For example, exposure pathways involving consumption of contaminated foodstuffs obtained from the disposal site, which are credible when permanent occupancy of a site could occur, would not be credible during the institutional control period, and exposures reasonably could occur for only a small fraction of the time during a year. The role of institutional control over near-surface hazardous waste disposal sites is particularly important in cases of very large volumes of waste, such as uranium mill tailings and wastes from mining and milling of ores to extract nonradioactive materials, that contain concentrations of naturally occurring hazardous substances, such as radium and heavy metals, far above background levels in Earth’s crust (see Section 7.1.5). For such wastes, the risk to an inadvertent intruder often would be well above any level that could be considered acceptable if permanent occupancy of near-surface disposal sites could occur. In the case of the large volumes of uranium mill tailings, however, disposal in facilities located well below the ground surface is not considered practical at the present time (EPA, 1982). Therefore, the intention is to maintain perpetual institutional control over near-surface disposal sites for these wastes to prevent scenarios for inadvertent intrusion involving permanent site occupancy. Similar considerations could apply to large volumes of other mining and milling wastes. 6.3.2.3 Classification as High-Hazard Waste. Waste that would not be generally acceptable for near-surface disposal in dedicated

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facilities for hazardous wastes, based on assessments of risk or dose for the types of scenarios for inadvertent intrusion described in the previous section, would be classified as high-hazard waste. Waste in this class generally would be intended for disposal in a facility that provides substantially greater isolation than a near-surface facility for low-hazard waste (e.g., a geologic repository). As discussed in Section 6.2.2.2.2, assessments of risk or dose to hypothetical inadvertent intruders do not provide a reasonable basis for determining acceptable disposals of waste in facilities located far below the ground surface, and there would be no limits on concentrations of hazardous substances in high-hazard waste because more confining waste disposal concepts are not evident.

6.3.3 Application of the Modifying Factor in Risk Index The modifying factor in the risk index in Equation 6.2 or 6.3 represents any factors deemed important in classifying waste other than those explicitly accounted for in the calculated risk or dose from waste disposal in the numerator or the allowable risk or dose for the waste class of concern in the denominator. The modifying factor generally can be substance- or waste-specific. The inclusion of a modifying factor in the risk index is intended to represent the essential role of judgment in classifying waste, even though the objective is to develop a classification system based on sound science. For example, generic scenarios for exposure of hypothetical inadvertent intruders at waste disposal sites could take into account not only the calculated exposure of an individual, given that a postulated scenario occurs, but the probability that the assumed scenario might occur as well. This probability could be included in the modifying factor and could depend, for example, on assumptions about the ability of an engineered disposal facility or the intended placement of waste in the facility to deter inadvertent intrusion into the waste. The modifying factor also could take into account, for example, the quality of the data underlying the assumed doseresponse relationships, background levels of naturally occurring hazardous substances of concern, considerations of cost-benefit in waste disposal, and societal concerns about particular wastes. In principle, the modifying factor could assume any value. A value less than unity would represent factors judged to mitigate risk, a value of unity would mean that there are no significant factors that were not already taken into account in calculating risk, and a value greater than unity would represent factors judged to enhance risk.

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The use of the modifying factor in the risk index is illustrated by assumptions used by NRC in developing the concentration limits of radionuclides in Class-C low-level radioactive waste in 10 CFR Part 61 (NRC, 1982a; 1982b) (see Section 4.1.2.3.3). The concentration limits for the relatively small volumes of Class-C waste incorporate several assumptions that were not used in developing the concentration limits for the much larger volumes of Class-A waste. Specifically, the Class-C limits incorporate assumptions that the use of engineered barriers and waste forms or the selective emplacement of Class-C waste at greater depths than other waste would delay access to the waste for a period of 500 y after facility closure, and that such measures would also reduce the probability of exposure to Class-C waste by a factor of 10. In addition, the Class-C limit for 137Cs was further increased based on knowledge of the volumes of waste that contains high concentrations of this radionuclide and considerations of the balance of costs and benefits of different options for managing and disposing of that waste. The modifying factor also can represent more qualitative considerations. An example is provided by current federal policies regarding disposal of uranium and thorium mill tailings. As noted in Section 6.3.2.2, these materials contain such high concentrations of radium that disposal in a near-surface facility would result in intolerable risks to an inadvertent intruder if unrestricted access to tailings piles were allowed. Nonetheless, the decision was made to dispose of most uranium and thorium mill tailings on or near the ground surface (EPA, 1982; UMTRCA, 1978), based primarily on the consideration that disposal of the very large volumes of these wastes far below ground appeared to be impractical and might not provide adequate protection of the environment, especially groundwater. Thus, an intention to maintain perpetual institutional control over tailings piles to prevent intrusion scenarios involving long-term exposures is a crucial consideration in this decision. In this case, the modifying factor essentially represents an assumption that scenarios for inadvertent intrusion should be restricted to those involving short-term access to a tailings pile, rather than permanent residence on a disposal site. NCRP notes that the modifying factor in the risk index should be applied independently of the requirement to achieve a negligible risk or dose for exempt waste or an acceptable (barely tolerable) risk or dose for nonexempt waste, in order to provide regulatory flexibility in classifying particular wastes. NCRP believes that such flexibility is highly desirable to promote cost-effective management and disposal of waste, provided it is applied in a transparent manner.

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6.4 Development of the Composite Risk Index for Multiple Substances The risk index defined in Equation 6.1 (see Section 6.2.1) is intended to provide a measure of the potential risk that arises from disposal of any waste that contains hazardous substances. In Section 6.3, the general definition of the risk index is elaborated and recommendations on suitable approaches to calculating the risk index for individual hazardous substances are presented. For purposes of developing a comprehensive and risk-based waste classification system, a simple method of calculating the risk from disposal of mixtures of hazardous substances is needed. The method must take into account that the allowable concentrations of particular hazardous substances in waste of a given class generally will be lower when multiple substances are present than when only a single substance is present. Such a method is presented and discussed in this Section. NCRP believes that a conceptually simple approach to calculating the risk from disposal of mixtures of hazardous substances can be developed which, although it may not be scientifically rigorous in estimating health risks from exposure to multiple hazardous substances, is adequate for the purpose of classifying waste. Because the dose-response relationships for substances that cause stochastic or deterministic responses are fundamentally different (see Section 3.2), care must be taken in developing a risk index for multiple hazardous substances. In recognition of this difference, separate risk indexes are developed for mixtures of either type of substance in Section 6.4.1. These risk indexes are combined in Section 6.4.2 to yield a composite risk index for waste that contains mixtures of any hazardous substances. Development of the risk index is completed in Section 6.4.3 by reiterating the need to specify risk indexes based on both negligible and acceptable risks for the purpose of classifying waste as exempt or low-hazard, respectively. Examples of how the composite risk index for mixtures of hazardous substances is calculated using hypothetical data are presented in Section 6.4.4. The process by which boundaries between waste classes might be established is discussed in Section 6.4.5. Finally, potential shortcomings and advantages of the risk index are described in Section 6.4.6.

6.4.1 Risk Indexes for Mixtures of Hazardous Substances Hazardous wastes can contain mixtures of substances that cause stochastic or deterministic responses, or a single substance can cause both types of responses (e.g., arsenic, uranium). The two types of

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hazardous substances differ in important ways (see Section 3.2). Specifically, a threshold dose-response relationship is assumed for substances that cause deterministic responses, whereas a linear, nonthreshold dose-response relationship is assumed for substances that cause stochastic responses. As a consequence, the objective of risk management for substances that cause deterministic responses is to keep the dose sufficiently below the threshold in any organ or tissue that the occurrence of adverse responses is prevented in the most sensitive population groups, whereas the objective of risk management for substances that cause stochastic effects is to limit the frequency of occurrence of adverse responses, taking into account responses in all organs or tissues. The implication of the difference described above is that the mathematical form of the risk index for the two types of hazardous substances must be different. Thus, while NCRP believes that it is appropriate to develop a single risk index that accounts for mixtures of substances that cause stochastic or deterministic responses, separate risk indexes for these two types of substances are formulated first. 6.4.1.1 Risk Index for Multiple Substances That Cause Stochastic Responses. The risk index for mixtures of substances that cause stochastic responses (radionuclides and chemicals) is based on an assumption of a linear, nonthreshold dose-response relationship. This risk index takes into account the stochastic risk in all organs or tissues, and it assumes that the risk in any organ is independent of risks in any other organs. Based on these conditions, and expressing the risk index for a single hazardous substance in terms of dose (see Equation 6.3), the risk index for mixtures of substances that cause stochastic responses, denoted by RI s, can be expressed as: RI sj ⳱

兺兺兺F

i

i

r

T

(dose from disposal) si, j, r,T , (allowable dose) si, j, r,T

(6.4)

where j is an index indicating whether the denominator in the risk index represents a negligible or an acceptable (barely tolerable) dose (i.e., whether the waste is being evaluated for classification as exempt or low-hazard), the index i again denotes the particular hazardous substance, T denotes each organ or tissue at risk, and r denotes the different stochastic responses of concern (e.g., cancers and severe hereditary effects). The index j is included in the numerator, as well as the denominator, to indicate that exposure scenarios used to calculate risk can be different for disposal of exempt waste in municipal/ industrial landfills (or equivalent means of disposal) compared with disposal of low-hazard waste in dedicated near-surface facilities (see

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Section 6.3.2). Equation 6.4 is expressed in terms of dose, rather than risk, mainly because this form is consistent with the preferred form of the risk index for mixtures of substances that cause deterministic responses presented in the following section. Both ways of expressing the risk index are equivalent for substances that cause stochastic responses when a linear, nonthreshold dose-response relationship is assumed. The order in which the various summations in Equation 6.4 are executed is arbitrary, because the probability of a stochastic response in a particular organ or tissue from exposure to any such substance is assumed to be independent of any other responses caused by that substance and independent of exposures to any other substances that cause stochastic or deterministic responses. The method of calculating the risk index for mixtures of substances that cause stochastic responses also ignores possible synergistic or antagonistic effects. Information on any such responses, which is rarely available, could be taken into account for particular wastes either in establishing the allowable dose of each substance or in evaluating the modifying factor which can be substance-specific. In practice, Equation 6.4 would be greatly simplified. For radionuclides, doses in all organs or tissues (T) and the different responses of concern (r) are incorporated in the effective dose (see Section 3.2.2.3.3). Thus, calculation of the risk index for mixtures of radionuclides is reduced to a single summation over all radionuclides of the ratio of a calculated effective dose from exposure to each radionuclide to the allowable effective dose for the particular waste class of concern. Furthermore, the denominator normally would be the same for all radionuclides in a given waste, and any differences in judgments about an allowable effective dose for different wastes in the same class could be included in the modifying factor. Similarly for chemicals that cause stochastic responses, information on responses in single or multiple organs or tissues at risk is incorporated in substance-specific probability coefficients, and the summations over all organs and tissues (T) and responses (r) thus are reduced to a single ratio of a calculated dose to an allowable dose. The simplified form of the risk index for mixtures of chemicals that cause stochastic responses also could be expressed in terms of risk. Therefore, for waste that contains mixtures of substances that cause stochastic responses, the risk index in Equation 6.4 generally can be reduced to a single summation over all such substances (i) of the ratio of a calculated dose to an allowable dose. The risk index for such mixtures of substances thus is in the form of a simple sumof-fractions rule. An example calculation is described in Section 6.4.4.

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The modifying factor (F ) in Equation 6.4 generally can be substance-specific. However, in a given waste, its value often would be the same for all substances that cause stochastic responses. 6.4.1.2 Risk Index for Multiple Substances That Cause Deterministic Responses. The risk index for mixtures of substances that cause deterministic responses should be expressed in terms of dose, rather than risk, because risk is not proportional to dose and the goal of risk management is to limit doses to less than the threshold in the dose-response relationship (see discussion of Equation 6.2 in Section 6.3). As noted previously, deterministic responses from exposure to radionuclides should not be of concern in classifying waste, in which case only the risk index for chemicals that induce deterministic responses needs to be considered. Formulation of the risk index for mixtures of substances that cause deterministic effects is considerably more complex than in the case of substances that cause stochastic effects discussed in the previous section. The added complexity arises from the threshold doseresponse relationship for these substances and the need to keep track of the dose in each organ or tissue at risk in evaluating whether the dose in each organ is less than the allowable dose in that organ. For substances that cause deterministic responses, the index T can refer not only to a specific organ or tissue (e.g., the liver or skin) but also to a body system that may be affected by a particular chemical, such as the immune or central nervous system. Taking into account the threshold dose-response relationship, the risk index for mixtures of substances that cause deterministic responses is based on the following assumptions. First, the doses in any organ due to multiple substances are assumed to be additive, even though the deterministic responses induced in that organ may not be the same for each substance. Second, the threshold doses for deterministic responses in any organ caused by any substance are assumed to be independent of doses in all other organs due to any substance that cause deterministic (or stochastic) responses and independent of doses in the same organ due to all other substances with deterministic (or stochastic responses). Based on these assumptions, the risk index for mixtures of substances that cause deterministic responses, denoted by RId, can be represented as:

冤

RI dj ⳱ INTEGER MAXT

兺兺F

i

i

r

冥 , (6.5)

(dose from disposal) di, j, r,T (allowable dose) di, j, r,T

where MAX is a function yielding the maximum value of a set of numbers and INTEGER is a function yielding the truncated integer

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value of a number. As in Equation 6.4, the index i denotes the particular hazardous substance, j is an index indicating whether the waste is being evaluated for classification as exempt or low-hazard, T denotes each organ or tissue at risk, and r denotes the different responses of concern. In essence, the procedure for evaluating Equation 6.5 involves calculation of a separate risk index for each organ at risk and a comparison of the results. The various steps in the procedure are described as follows: ●

●

●

●

For each substance, the organ or organs (including tissues or body systems) in which deterministic responses can be induced are identified. If a substance can induce responses in more than one organ, all such organs are included in calculating the risk index. For each substance, the ratio of a calculated dose in each organ at risk to an allowable dose in that organ is obtained, based on an assumed exposure scenario for the waste class (disposal system) of concern (i.e., exempt or low-hazard). If a substance can induce responses in more than one organ, the allowable dose can depend on the particular organ, because the threshold generally will not be the same in all organs. The result of this step is a set of substance-specific and organ-specific ratios of calculated doses to allowable doses. For each organ at risk, the substance-specific ratios of calculated doses in that organ to the corresponding allowable doses are summed over all substances, without regard for any differences in the deterministic responses induced by the different substances. This calculation is based on an assumption that doses in any organ due to multiple substances that cause deterministic responses are additive, even though the responses induced in that organ may not be the same for each substance. The result of this step is a set of organ-specific ratios of calculated doses to allowable doses in which the particular substances in the waste that cause deterministic responses and their associated responses are no longer distinguished. By examination of the organ-specific ratios of calculated doses to allowable doses obtained in the previous step, the maximum value of these ratios is selected. Application of the MAX function to these organ-specific ratios is based on an assumption that induction of deterministic responses in any organ is independent of doses in any other organs or, equivalently, that the threshold in the dose-response relationship for any substance that causes deterministic responses is not affected by exposure to multiple

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substances that cause deterministic (or stochastic) responses. The result of this step is a single number representing the risk index for the organ at greatest risk (the critical organ), taking into account all substances in the waste that cause deterministic responses and all organs at risk from exposure to those substances. The highest deterministic risk index in any organ selected in the previous step is truncated using the INTEGER function. Truncation of the highest risk index in any organ, rather than rounding to the nearest integer value, is based on an assumption that the probability of a deterministic response is zero if the doses in all organs are less than the corresponding allowable doses, but is unity otherwise.

The order in which the summations over the responses (r) and substances (i) of concern are executed in the second and third steps above is arbitrary. However, these steps must be executed before the MAX and INTEGER functions are applied to the result. If the risk index for substances causing deterministic responses were based on calculations of health risk per se, rather than dose, the INTEGER function in Equation 6.5 would not be necessary, because the risk would be zero whenever a dose is below the threshold. Again, however, evaluation of the risk index for substances that cause deterministic responses based on dose is recommended when the doseresponse relationship is assumed to have a threshold. The use of dose is supported by the observation that the dose-response relationship above the threshold generally is nonlinear. The method of calculating the risk index for mixtures of substances that cause deterministic effects described above assumes that all deterministic responses that are taken into account in determining allowable doses in any organ are equally undesirable. The method also ignores possible synergistic or antagonistic effects of exposure to multiple substances. While it would be desirable to take such effects into account, there are few data that could be used to support particular assumptions. The possibility of synergistic or antagonistic effects could be taken into account, if so desired, in the safety and uncertainty factors applied to the assumed threshold doses in establishing the allowable dose of each substance for the waste class of concern or in the substance-specific modifying factor used to determine the risk index. It must be emphasized that the sum-of-fractions rule for substances that cause stochastic responses (see Equation 6.4) generally does not apply in calculating the risk index for mixtures of substances that cause deterministic responses. That is, based on an assumption of a

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threshold dose-response relationship for substances that cause deterministic responses, it generally is inappropriate to simply sum the ratios of calculated doses in each organ to the corresponding allowable doses without regard for the particular organs at risk. For example, consider a case of exposure to two substances that cause deterministic responses, each of which affects only a single organ, and suppose that the ratio of the calculated dose in the critical organ to the allowable dose is 0.6 for each substance. If the critical organ is the same for both substances, the risk index for the mixture of the two before applying the INTEGER function would be 0.6 Ⳮ 0.6 ⳱ 1.2, based on the assumption that doses in any organ are additive without regard for any differences in the responses induced in that organ by the different substances. However, if the critical organ is not the same for the two substances, the dose in each organ would be less than the corresponding allowable dose and the risk index for the mixture of the two would be zero, based on the assumption that the induction of deterministic responses in any organ is independent of doses in other organs. Thus, the risk of a deterministic response due to multiple substances generally depends on the particular organs at risk for each substance causing deterministic effects, and the sum-of-fractions rule does not apply when multiple organs are at risk. Additional examples of calculating the risk index for mixtures of substances causing deterministic responses, which illustrate the need to keep track of the different organs and tissues at risk, are given in Section 6.4.4. As in the case of the risk index for mixtures of substances that cause stochastic responses discussed in the previous section, the modifying factor (F) in Equation 6.5 generally can be substancespecific, but its value often would be the same for all substances in a given waste that cause deterministic responses.

6.4.2 Composite Risk Index for All Hazardous Substances Hazardous waste generally can contain mixtures of substances that cause stochastic or deterministic responses. The composite risk index for any mixture of hazardous substances in a given waste can be represented as the sum of risk indexes for multiple substances that cause stochastic or deterministic responses given in Equations 6.4 and 6.5: RI j ⳱ RI sj Ⳮ RI jd .

(6.6)

Again, the index j denotes whether the waste is being investigated for classification as exempt or low-hazard. It is included because

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both the numerator (the calculated risk) and the denominator (allowable risk) in Equations 6.4 and 6.5 depend the assumed disposal technology and the waste class of concern. Given the form of the deterministic risk index in Equation 6.5, which results in zero or integer values, the composite risk index for all hazardous substances in Equation 6.6 also can be expressed as the maximum of the separate risk indexes for multiple substances causing stochastic or deterministic responses: RI j ⳱ MAX [RI sj , RI jd ].

(6.7)

If the risk index for all substances that cause deterministic responses in the waste (RId) in Equation 6.5 is zero (i.e., the doses of all substances that cause deterministic responses are less than the allowable values), classification is determined solely by the risk index for all substances that cause stochastic responses (RI s) in Equation 6.4; the latter must be nonzero based on the assumption of a linear, nonthreshold dose-response relationship. On the other hand, if the risk index for all substances that cause deterministic responses is unity or greater, the calculated risk exceeds the allowable risk for the waste class of concern without the need to consider the risk posed by substances that cause stochastic effects. The only advantage of the form of the composite risk index in Equation 6.6 is that it indicates more explicitly that the total risk posed by a given waste is the sum of the risks posed by the two types of hazardous constituents, however approximate that representation may be.

6.4.3 Implications of the Framework for Calculating the Risk Index The risk-based waste classification system described in Section 6.2.2 contains two boundaries: one between exempt waste and low-hazard waste, based on a negligible risk, and one between low-hazard and high-hazard waste, based on an acceptable (barely tolerable) risk. Consequently, there are two separate sets of Equations 6.4, 6.5 and 6.6, which are distinguished by different meanings of the index j. One set of equations is used to evaluate the general acceptability of disposal in a municipal/industrial landfill for nonhazardous waste, and the other is used to evaluate the general acceptability of disposal of nonexempt waste in a dedicated near-surface facility for hazardous waste. As discussed in Section 6.2.2, the exposure scenario used in classifying exempt waste generally can differ from the exposure scenario used in classifying low-hazard waste, even when disposal in a near-surface facility is assumed in both cases.

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6.4.4 Example Calculations of the Risk Index This Section provides example calculations of the composite risk index for a simple, hypothetical waste that contains a mixture of substances that cause stochastic or deterministic effects. Application of the risk index in classifying real wastes is considered in Section 7.1. For the purpose of illustrating how the composite risk index in Equation 6.6 would be used to classify a hypothetical waste, it is helpful to simplify Equations 6.4 and 6.5. This is done by assuming that the summation over all responses (index r) has been calculated, that only one waste classification boundary represented by the index j is being considered (i.e., the boundary between exempt and low-hazard waste, based on a negligible risk, or the boundary between low-hazard and high-hazard waste, based on an acceptable risk), and that the modifying factor (F) is unity. Further, the calculated dose in the numerator of the risk index is denoted by D and the allowable dose in the denominator is denoted by L. Then, the composite risk index for all hazardous substances in the waste, expressed in the form of Equation 6.6, can be written as:

冤

RI j ⳱ INTEGER MAXT

冥

d D i,T

兺L i

d i,T

Ⳮ

D si,T

兺兺L i

T

s i,T

.

(6.8)

Calculation of the composite risk index for the purpose of waste classification based on the simplified Equation 6.8 is illustrated using the hypothetical data given in Table 6.1. Consistent with the form of the risk index in Equations 6.3 and 6.8, risk indexes for individual hazardous substances in Table 6.1 are expressed as the ratio of a

TABLE 6.1—Hypothetical values of risk indexes for individual substances and organs in first example calculation of a composite risk index. (D d /Ld) a

(D s /Ls ) b

Substance (i)

Organ A

Organ B

Organ A

Organ B

1 2 3

0.4 0.8 —

— — 1.6

0.2 0.1 —

0.3 0.2 —

a

Ratio of calculated dose (D) to allowable dose (L) of a substance that causes deterministic responses (d). b Ratio of calculated dose (D) to allowable dose (L) of a substance that causes stochastic responses (s).

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calculated dose based on an assumed exposure scenario to an allowable dose for the waste class of concern (i.e., the risk index is not expressed in terms of risk per se). We then suppose that the waste would be placed in so-called Class 1 if the composite risk index were less than unity, but would be placed in Class 2 otherwise. Based on the information given above, the composite risk index for the waste can be calculated and the resulting waste classification obtained. Substituting the values in Table 6.1 into Equation 6.8 results in the following: RI ⳱ INTEGER {MAX [(0.4Ⳮ0.8), 1.6]} Ⳮ [(0.2Ⳮ0.1) Ⳮ (0.3Ⳮ0.2)] RI ⳱ INTEGER (1.6) Ⳮ 0.8 RI ⳱ 1 Ⳮ 0.8 ⳱ 1.8

(6.9)

Substances that cause deterministic responses (the first term in Equation 6.8) contribute a value of one to the composite risk index of 1.8, and substances that cause stochastic responses account for the remaining 0.8. Thus, the presence of the substances that cause deterministic responses alone would be sufficient to place this waste in Class 2. This result also would be indicated if the alternative form of the composite risk index in Equation 6.7 were used. As another example, suppose that each risk index for the individual hazardous substances given in Table 6.1 were a factor of two lower. Then, by the procedure described above, the composite risk index for the waste would be determined as follows: RI ⳱ INTEGER {MAX [(0.2Ⳮ0.4), 0.8]} Ⳮ [(0.1Ⳮ0.05) Ⳮ (0.15Ⳮ0.1)] RI ⳱ INTEGER (0.8) Ⳮ 0.4 RI ⳱ 0 Ⳮ 0.4 ⳱ 0.4

(6.10)

The maximum deterministic risk index for any organ (0.8 for Organ B) is less than unity, which means that the doses due to all substances that cause deterministic responses are below the allowable doses in each organ at risk. Thus, the composite risk index is 0.4, due solely to substances that cause stochastic responses, and substances that cause deterministic responses do not contribute. This result also would be indicated if the alternative form of the composite risk index in Equation 6.7 were used. In this example, the waste would be placed in Class 1. The second example illustrates the importance of identifying and keeping track of the specific organs at risk from exposure to substances that cause deterministic responses. If the deterministic risk indexes for each substance were simply summed without regard for the organs at risk, the risk index for all substances that cause

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deterministic responses in this example would be 1.2, thus indicating that the dose due to all such substances would exceed the allowable dose. However, this is not the proper interpretation when the dose in each organ is less than the corresponding allowable dose and deterministic responses in any organ are assumed to be independent of doses in other organs. In this case, the proper interpretation is that the risk of a deterministic response in any organ is essentially zero.

6.4.5 Establishing a Waste Classification System Based on the Framework and Risk Index The foregoing development of the foundations and framework for a comprehensive and risk-based hazardous waste classification system began with a discussion of fundamental principles of waste classification and the basic definitions of waste classes, and eventually arrived at detailed conceptual equations to be used in classifying waste. However, establishment of the proposed waste classification system probably would not involve waste generators undertaking the calculations implied by the formulas constituting the risk index. The more likely approach to implementation would be undertaken by regulatory authorities, and this approach is outlined in Section 6.4.5.1. Following this discussion, the questions of when a waste should be classified and the time frame following disposal over which a risk assessment should be performed for the purpose of classifying waste are addressed in Sections 6.4.5.2 and 6.4.5.3, respectively. Finally, implementation of the proposed waste classification system over time, to replace the existing classification systems for radioactive and hazardous chemical wastes, is discussed in Section 6.4.5.4. 6.4.5.1 Process of Implementing the Waste Classification System. Taken together, the framework for waste classification discussed in Section 6.2 and the risk index developed in Section 6.3 and this Section constitute the foundations of a comprehensive and risk-based hazardous waste classification system. Such a waste classification system could be established by regulatory authorities using the following general process: ●

For substances that cause stochastic effects (radionuclides and hazardous chemicals), specify negligible and acceptable (barely tolerable) risks to be used in classifying waste. Then, establish the corresponding negligible and acceptable dose of each substance of concern based on an assumed probability coefficient (risk per unit dose).

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For each substance of concern that causes deterministic effects (hazardous chemicals only), establish nominal thresholds for induction of deterministic responses in humans, taking into account all organs and tissues at risk. Then, establish organspecific negligible and acceptable doses of each substance by applying appropriate safety and uncertainty factors to the assumed thresholds. Establish generic exposure scenarios for inadvertent human intrusion into a municipal/industrial landfill for disposal of exempt waste and intrusion into a dedicated near-surface facility for disposal of low-hazard waste. For each generic exposure scenario to be used in classifying waste, and taking into account all relevant exposure pathways in each scenario, calculate the dose per unit concentration of each hazardous substance in the waste. These doses generally would be the highest values calculated over an assumed time frame for the risk assessment (see Section 6.4.5.3), taking into account the time-dependence of the concentrations of hazardous substances in the waste. For example, the quantity calculated for radionuclides would be the annual effective dose (sievert) per unit activity concentration (Bq mⳮ3), and the quantity calculated for hazardous chemicals would be the dose (intake, mg kgⳮ1 dⳮ1) per unit concentration (kg mⳮ3). Divide the negligible and acceptable doses of each hazardous substance by the corresponding doses per unit concentration, resulting in limits on the concentrations of each hazardous substance in exempt and low-hazard waste, respectively. Waste that contains concentrations of hazardous substances greater than the limits in low-hazard waste would be classified as highhazard. Specify rules for applying the concentration limits, such as the sum-of-fractions rule for waste that contains mixtures of substances that cause stochastic effects (NRC, 1982a) and the rules for combining risk indexes for mixtures of substances that cause deterministic effects taking into account the substance-specific organs or tissues at risk. Promulgate tables of concentration limits of hazardous substances that cause stochastic or deterministic effects in exempt and low-hazard waste and the rules for using the tables. For example, concentration limits of substances that cause deterministic effects should include an identification of the organ or organs at risk from exposure to each substance, so that the risk index for multiple substances that cause deterministic effects can be evaluated properly.

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For substances that cause stochastic effects, the risk index can be expressed in terms of risk, rather than dose. In this case, the risk per unit dose would be incorporated in the calculated risk in the numerator, based on the assumed exposure scenario, rather than in the denominator. However, the effective dose provides a convenient surrogate for risk for radionuclides, because all organs at risk and all stochastic responses of concern are taken into account, and the use of dose for all substances that cause stochastic effects is consistent with the form of the risk index for substances that cause deterministic effects, which generally should be expressed in terms of dose based on the assumption of a threshold dose-response relationship. In effect, using the risk index to establish a risk-based waste classification system involves performing the risk assessment process described in Section 3.1 in reverse order by beginning with an assumed allowable risk and ending with the calculation of concentrations of hazardous substances in waste that are generically equivalent to that risk. This process is essentially the same as that used by NRC to define subclasses of low-level radioactive waste that are generally acceptable for near-surface disposal in licensed facilities (NRC, 1982a; 1982b) (see Section 4.1.2.3.3). Use of the risk index in classifying waste requires that adequate data be available to allow estimation of dose-response relationships for substances that induce stochastic or deterministic responses. The availability of suitable data is a potential problem only for hazardous chemicals. If suitable data are not available for particular hazardous substances, there is no satisfactory approach that could be used to include these substances in classifying waste. However, this would be an important deficiency only if substances with inadequate data on dose-response posed an important hazard in the waste. NCRP does not expect that the most important hazardous substances in waste in regard to potential risks would be lacking information on the dose-response relationship. 6.4.5.2 Time When Waste Should be Classified. Time is an important issue in waste classification in two respects. The first issue discussed in this Section is the question of when waste should be classified. The second issue, which is discussed in the following section, is the question of the time frame following disposal of waste over which risk assessments should be carried out for the purpose of classifying waste. Even after acknowledging that waste must often be classified well before a specific method of disposal is known (see Section 2.1.2), the time at which such classification occurs is an important issue in a risk-based waste classification system because the hazard of many

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wastes changes with time. The hazard of many wastes declines as a result of radioactive decay or chemical degradation, but the hazard from some wastes increases because the decay or degradation products are more hazardous than their parents. In general, material deemed to be waste should be classified when it is so declared or is ready for disposal. Many materials are declared to be waste at the time of generation, and classification of waste at that time is appropriate. However, some materials, such as spent nuclear fuel and surplus special nuclear materials, have economic value and often are not declared to be waste until long after they are generated. In cases where the predominant hazard is due to hazardous substances with relatively high decay or degradation rates (e.g., half-lives of a few years or less), a strategy of storage to allow decay or degradation to occur would be appropriate if it would result in the waste being in a lower class. 6.4.5.3 Time Frame for Risk Assessment in Classifying Waste. In classifying waste based on assessments of potential risks from disposal, the time frame over which the assumed exposure scenarios should be evaluated is an important consideration. There are two issues involving this time frame. The first is the earliest time after disposal at which exposures could occur according to assumed scenarios. The second is the time following the assumed onset of exposures over which risk assessments should be carried out for the purpose of determining the highest potential dose or risk at any time. It is this dose or risk that normally would be used in classifying waste. The earliest time at which exposures could occur according to assumed scenarios is important because the concentrations of many hazardous substances, such as shorter-lived radionuclides and biodegradable organic chemicals, decrease significantly over time. Therefore, the concentration limits of these types of substances in exempt and low-hazard waste will depend on assumptions about the earliest times at which exposure to waste in the different classes could occur. For example, based on expectations embodied in current laws and regulations, the period of institutional control over waste disposal sites following closure may range from essentially zero at landfills for nonhazardous waste to about 30 to 100 y at dedicated nearsurface facilities for hazardous wastes. This difference by itself would result in substantially higher limits on concentrations of shorterlived radionuclides in low-hazard waste compared with exempt waste. The issue of the time following the assumed onset of exposures over which risk assessments should be carried out arises because the risk posed by some hazardous substances increases substantially

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over time. For example, the risk posed by depleted uranium, which consists primarily of 238U, increases for about 106 y, due to very long half-life of the uranium and buildup of the activity of its decay products 234U, 230Th, and 226Ra over that time. Long-term buildup of radiologically significant decay products also is potentially important for other long-lived radionuclides (e.g., 237Np, 233U). Similarly, it is possible that some persistent hazardous chemicals (e.g., heavy metals) could be transformed into more hazardous chemical compounds over time. Therefore, consideration needs to be given to the maximum time after disposal over which risks should be assessed for the purpose of waste classification. The question of appropriate time frames for risk assessments to be used in classifying waste is discussed, in part, in Section 6.3.2, where the types of exposure scenarios that could be used in classifying exempt and low-hazard waste are described. These discussions and other considerations by NCRP may be summarized as follows: ●

●

●

In assessing risks based on scenarios for exposure of hypothetical inadvertent intruders at municipal/industrial landfills for nonhazardous waste (i.e., in determining whether a waste would be classified as exempt or nonexempt), scenarios involving permanent occupancy of a disposal site should be assumed to occur beginning at the time of facility closure, based on the expectation that institutional control will not be maintained over this type of facility for a significant period of time after closure. In assessing risks based on scenarios for exposure of hypothetical inadvertent intruders at dedicated near-surface disposal facilities for hazardous wastes (i.e., in determining whether a waste would be classified as low-hazard or high-hazard), scenarios involving permanent occupancy of disposal sites should be assumed to occur beginning at the time after facility closure when institutional control over a disposal site is assumed to cease. In addition, credible scenarios involving temporary (shortterm) access to a disposal site during the institutional control period and exposure pathways appropriate to such short-term events should be considered. The more restrictive of the two types of scenarios should be used to establish limits on concentrations of hazardous substances in low-hazard waste. The maximum time after disposal over which generic exposure scenarios should be evaluated for the purpose of classifying waste should not be any longer than the maximum time over which potential exposures of members of the public are evaluated for the purpose of determining the acceptability of specific wastes for disposal at specific sites. This time could range from

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hundreds to thousands of years, especially when classifying nonexempt waste. In general, the question of the time frame for risk assessments to be used in classifying waste is a matter of judgment to be addressed by regulatory authorities. 6.4.5.4 Implementation of the Waste Classification System Over Time. NCRP recognizes that developing and promulgating a comprehensive and risk-based hazardous waste classification system to replace existing classification systems for radioactive and hazardous chemical wastes probably cannot happen quickly or in one step, owing to the continuing generation of hazardous wastes and the need to manage them safely without interruption. NCRP recommends that a risk-based waste classification system be established by using the foundations and framework provided in this Report as a road map to the ultimate objective, and that existing waste classification systems be addressed one at a time under these unifying principles. Such a process could take many years. One likely outcome of this approach is that during the transition period there will be a need for risk-based waste classifications to acknowledge and be integrated with existing classification systems, which are not risk-based, to sustain interim operations. Any provisions that are deemed necessary to facilitate the transition should be explicitly identified so that they can be readily eliminated when no longer needed.

6.4.6 Shortcomings and Advantages of the Risk Index Risk indexes for mixtures of substances that induce stochastic or deterministic responses given in Equations 6.4 and 6.5, respectively, have at least one possible shortcoming. Specifically, both assume that the responses in a given target organ or tissue due to all such hazardous substances can be added, even when the nature of the responses and mechanisms of action in that organ or tissue might be different. This implies that all responses in a particular organ are equally important, and that the hazardous substances and responses are not synergistic or antagonistic. Thus, the approach is not scientifically rigorous to the extent that these assumptions are invalid. However, this shortcoming is not normally addressed in risk assessments for radionuclides or hazardous chemicals and is generally ignored in setting health protection standards for workers and the public, due to a lack of information on how different substances interact in causing responses when exposure to multiple

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substances occurs. Therefore, in this regard, the recommended approach to waste classification is consistent with current approaches to health protection. In addition, the reliance on generic scenarios for inadvertent intrusion in classifying waste cannot, by definition, represent site-specific risks. However, this is not a serious shortcoming because such scenarios have been used in establishing subclasses of low-level radioactive waste for disposal in near-surface facilities (NRC, 1982a). Furthermore, as emphasized in this Report, establishment of a riskbased waste classification system using particular exposure scenarios does not obviate the need to perform site-specific risk assessments for the purpose of establishing waste acceptance criteria at each disposal site. NCRP also believes that the recommended approach to waste classification has two important benefits. First, the approach is conceptually simple and transparent. Specifically, it is based on conceptually simple definitions of waste classes in relation to disposal technologies that are expected to be acceptable in protecting the public, it is implemented using a conceptually simple risk index to describe exposure to any hazardous substance, and risk indexes for substances that induce deterministic or stochastic responses are clearly related to the assumed form of the dose-response relationship in each case. Second, the risk index for either type of substance includes a substantial margin of safety in protecting the public if prudently conservative (pessimistic) assumptions are used in selecting and evaluating exposure scenarios. For example, an assumption that exposures of individuals would occur continuously over a lifetime in accordance with scenarios for inadvertent intrusion involving permanent occupancy of waste disposal sites should overestimate exposures of nearly all individuals who might actually access a disposal site at some time in the future. Finally, NCRP emphasizes that calculated risk indexes for substances that induce deterministic or stochastic responses are not intended to be used as predictors of the probability of a response for any actual or hypothetical exposure situation. The risk index is nothing more than a simple, dimensionless representation of the risk posed by hazardous substances in waste to be used for purposes of waste classification.

6.5 Expected Classification of Existing Wastes There is sufficient information available to allow NCRP to anticipate the classification of a number of existing wastes that would

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result from implementation of the risk-based waste classification system proposed in this Report. This evaluation assumes that precedents concerning suitable values of negligible and acceptable risks or doses to be used in classifying waste discussed in Section 6.3.1.2 would prevail. The discussion in this Section should be taken as a very preliminary indication of how risk-based waste classification might impact current waste classifications and management. The evaluation of different wastes in the following sections is based, in part, on analyses of specific wastes presented in Section 7.1.

6.5.1 Wastes Expected to be Classified as Exempt In contrast to the cases of low-hazard and high-hazard waste discussed in the following two sections, NCRP did not investigate in any detail the kinds and quantities of radioactive or hazardous chemical wastes that might be classified as exempt, based on the definition that disposal of waste in this class in a municipal/industrial landfill for nonhazardous waste would pose no more than a negligible risk to a hypothetical inadvertent intruder. However, based on the substantial number of case-by-case exemptions for radioactive materials in NRC regulations (see Section 4.1.2.5), assessments of risk from disposal of waste that contains low levels of radionuclides (EPRI, 1989; Schneider et al., 2001), and studies in support of proposed regulations to establish exemption levels for listed hazardous chemical wastes (EPA, 1992d; 1995c; 1999c), NCRP expects that substantial quantities of waste currently managed as radioactive or chemically hazardous waste could be classified as exempt for purposes of disposal.

6.5.2 Wastes Expected to be Classified as Low-Hazard By defining low-hazard waste as any radioactive or hazardous chemical waste that is generally acceptable for disposal in a dedicated near-surface facility for hazardous waste, NCRP expects that this class would include most radioactive waste presently classified in the United States as low-level waste. Because low-hazard waste would include radioactive waste from any source, this class also should include most NARM waste. Most uranium or thorium mill tailings presumably could be classified as low-hazard waste, but only under conditions of perpetual institutional control over near-surface disposal sites. In the absence

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of institutional control, however, most mill tailings would be classified as high-hazard waste. Regardless of how mill tailings are classified, NCRP recognizes that a distinction between mill tailings and other low-hazard or high-hazard wastes would be reasonable, because the much larger volumes of mill tailings have necessitated different approaches to waste management and disposal from those that have been used for other hazardous wastes. A distinction between mill tailings and other hazardous wastes would not be part of the basic waste classification system developed in this Report, but it could be taken into account in developing subclassifications of basic waste classes (see Section 6.6). Similar considerations could apply to other wastes with large volumes that are produced in mining and milling of ores to obtain nonradioactive materials and contain elevated levels of NORM or heavy metals. NCRP also expects that many hazardous chemical wastes presently generated in the United States would be classified as lowhazard waste. Indeed, for hazardous chemical waste, low-hazard waste essentially would correspond to the one waste class presently defined in the United States—namely, solid hazardous waste (see Section 4.2.1). Although most hazardous chemical waste generated in the United States is considered acceptable for disposal in a regulated nearsurface facility, NCRP emphasizes that the concept of permanent disposal—i.e., placement in a facility based on the results of longterm performance assessments with no intent to retrieve the waste or maintain perpetual institutional control even though the waste may remain hazardous for a very long time—generally has not been applied to hazardous chemical waste. In addition, the concept of a hypothetical inadvertent intruder at waste disposal sites, which would be the basis for defining low-hazard chemical waste, has not been used to determine the acceptability of hazardous chemical waste for disposal in a regulated near-surface facility. Potential risks to inadvertent intruders apparently could be of concern for some wastes that contain heavy metals (Okrent and Xing, 1993). Thus, when disposal facilities for hazardous chemical waste are considered for closure after the active management period (assumed to be 30 y), the available options would appear to be removal of waste that poses unacceptable risks to inadvertent intruders or continuation of active site management essentially in perpetuity. Perpetual institutional control over near-surface disposal sites also is envisioned for uranium mill tailings, on account of the unacceptably high risks that could result if tailings piles were released from control and the view that disposal of the very large volumes of these wastes in underground facilities is not feasible (EPA, 1982;

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1983). However, there is an important difference between mill tailings and hazardous chemical wastes. In the case of mill tailings, the need for permanent institutional control was recognized during the development of regulations, not only to protect against inadvertent intrusion but also to limit releases of radon to the air and releases of radionuclides and heavy metals to groundwater. In the case of hazardous chemical waste, however, the potential long-term implications of near-surface disposal, including potential impacts on inadvertent intruders, were not considered in any detail in developing regulations governing the design, operation, and closure of disposal facilities. 6.5.3 Wastes Expected to be Classified as High-Hazard By defining high-hazard waste as any radioactive or hazardous chemical waste that generally would require a disposal system considerably more isolating than a near-surface facility, NCRP expects that this waste class would include most radioactive waste presently classified in the United States as high-level waste (including spent nuclear fuel when it is declared to be waste), transuranic waste, and any other radioactive waste with similar properties, such as greater-than-Class-C low-level waste as defined by NRC (1982a). In accordance with current practices in the United States, most highhazard radioactive waste would be intended for disposal in a geologic repository. However, greater confinement disposal systems, which have depths intermediate between a near-surface facility and a geologic repository, also could be appropriate for some high-hazard waste. Again, for purposes of waste classification, a critical feature of an acceptable disposal system for high-hazard waste is that inadvertent intrusion into the waste as a result of expected human activities should be highly unlikely. A number of dispositions could be acceptable for high-hazard chemical waste, including destruction (e.g., incineration), treatment to reduce the hazard to levels that would be acceptable for near-surface disposal, or disposal using a technology considerably more isolating than a near-surface facility. At the present time, there are no planned alternatives to near-surface facilities for disposal of high-hazard chemical wastes in the United States.16 However, there do not appear 16 Facilities located at a considerable depth below the ground surface, such as mined cavities, are used in some countries (e.g., Germany) for disposal of hazardous chemical wastes, as well as low-level radioactive waste. However, the selection of a deep disposal system often is based on general land-use policies that prohibit disposal of hazardous wastes on or near the land surface, as well as a desire to protect public health and the environment, and no distinction is made between wastes that pose a lesser or greater hazard in selecting such disposal systems and in developing site-specific waste acceptance criteria.

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to be any substantive technical reasons why high-hazard radioactive and chemical wastes could not be placed in a similar disposal facility, such as a geologic repository as presently intended for most highhazard radioactive waste. The types and amounts of chemical waste that might be classified as high-hazard cannot be estimated until a quantitative definition of low-hazard chemical waste is developed and implemented. NCRP notes that there are at least two precedents for defining a class of high-hazard chemical waste. The first is a classification system developed for the state of Washington (Mehlhaff et al., 1979), which designates the most toxic chemical wastes as ‘‘extremely hazardous waste.’’ The second is regulations of the state of California, which include a definition of ‘‘extremely hazardous waste’’ (Pilorin, 1994),17 and a proposed revision of these regulations (California, 1999; NAS/ NRC, 1999b). However, in neither case is the distinction between extremely hazardous waste and less hazardous waste based on assessments of risks from waste disposal, nor is a separate and more isolating system for disposal of extremely hazardous chemical waste identified. Indeed, the state of Washington currently sends extremely hazardous waste to the same disposal site as all other less toxic chemical wastes.

6.6 Subclassification of Basic Waste Classes The proposed framework for risk-based classification of all radioactive and hazardous chemical wastes developed in Section 6.2.2 represents waste classification in its broadest, most general terms. Thus, this classification system can be viewed as the highest level of a possible hierarchy of hazardous waste classifications (e.g., see Figure 4.2). Further subclassification of these broadly defined waste classes may be desirable for such purposes as protection of workers during waste operations, protection of public health and the environment following waste disposal, and development of efficient methods of waste management taking into account the characteristics of actual wastes. Subclassifications of broadly defined waste classes are commonplace in the existing classification systems for radioactive and hazardous chemical wastes in the United States (see Sections 4.1 and 4.2 17

Pilorin, R. (1994). Personal communication from Pilorin, R. (California Environmental Protection Agency, Sacramento, California) to Croff, A.G. (Oak Ridge National Laboratory, Oak Ridge, Tennessee).

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and Figure 4.2). For example: low-level radioactive waste is further classified as Class-A, -B, -C, or greater-than-Class-C, depending on the concentrations of particular radionuclides (NRC, 1982a); transuranic waste is further classified as contact-handled or remotelyhandled, depending on the levels of external photon and neutron radiation (DOE, 1996a); listed hazardous chemical waste is divided into four subclasses depending on the source of the waste, the presence of particular substances, or the nature of toxic effects caused by a substance; and, highly hazardous chemical waste sometimes is distinguished from other chemical waste, depending primarily on the intrinsic toxicity of its hazardous constituents (Mehlhaff et al., 1979). In addition, the radioactive waste classification system recommended by IAEA (see Section 4.1.3.1) includes a subclassification of the class of low- and intermediate-level waste, which is based on the concentrations of long-lived, alpha-emitting radionuclides. Recommendations on subclassifications of the basic classes of exempt, low-hazard, and high-hazard waste defined in Section 6.2.2 are not developed in this Report. However, NCRP acknowledges that subclassifications of basic waste classes would be reasonable, particularly in the case of low-hazard and high-hazard wastes. NCRP believes that any such subclassifications should be consistent with the physical, chemical, radiological, and toxicological properties of waste, and with requirements for safe management and disposal. NCRP believes that extrinsic and non-risk-related factors, such as the source of a waste, should not be used in subclassifying risk-based waste classifications. As indicated by the current subclassifications of existing waste classes summarized above, a variety of waste properties could be used to develop meaningful subclassifications of broadly defined waste classes. These properties include, for example, waste volumes, levels of decay heat and external radiation, and the long-term persistence of the hazard posed by waste constituents. Subclassifications of waste classes also could be based on the presence of particular hazardous substances. However, if the broadly defined waste classes are based on risk, as in the classification system proposed in this Report, the intrinsic toxicity of hazardous substances normally would not provide a basis for subclassification, because this property already is accounted for in determining the basic classification of any waste. Examples of possible approaches to subclassifying the basic waste classes are discussed in the following paragraphs. Volumes of particular wastes are relevant to subclassification of basic waste classes, especially when the very large volumes of some wastes in a particular class necessitate different approaches to management and disposal than the much smaller volumes of other wastes

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in the same class. An example of such a subclassification is the present distinction between low-level radioactive waste and most uranium and thorium mill tailings; this distinction also could apply to large volumes of other wastes from mining and processing of ores (see Section 6.5.2). A waste class could be subclassified based on the levels of decay heat and external radiation, in order to distinguish between wastes that require special protection systems for workers during waste handling and storage, such as active or passive cooling systems and extensive shielding, and wastes that do not require such protection systems, even when both types of waste would require essentially the same type of disposal system. Decay heat also could be considered in subclassifying a waste class for purposes of disposal, because emplacement of waste in a disposal facility should take into account temperature increases in the host environment and their effects on the waste isolation capabilities of a site. An example of a subclassification based on considerations of external exposure of workers is the present distinction in the United States between contact-handled and remotely-handled transuranic waste. An example of a subclassification based on considerations of the effects of decay heat on waste disposal is the distinction between high-level waste and long-lived, intermediate-level waste in the radioactive waste classification system recommended by IAEA (see Section 4.1.3.1). Long-term persistence of the hazard posed by waste constituents could be used to subclassify basic waste classes. For example, different radioactive wastes that presumably would be classified as highhazard in accordance with NCRP’s recommendations, such as spent fuel and high-level waste, transuranic waste, and greater-thanClass-C low-level waste, often have substantially different concentrations of long-lived radionuclides. Many transuranic wastes contain much lower concentrations of long-lived alpha-emitting radionuclides than spent fuel or high-level waste, and some greaterthan-Class-C low-level waste consists mostly of radionuclides with half-lives of about 30 y or less. Even though all of these wastes would require disposal well below the ground surface, disposal systems less confining and less costly than a geologic repository could be acceptable for waste containing relatively low concentrations of longlived radionuclides. Similar considerations could apply in distinguishing waste that contains degradable hazardous chemicals from waste that contains nondegradable (persistent) substances, such as heavy metals. A waste class could be subclassified based primarily on its particular hazardous constituents, even though essentially the same disposal system might be used for all waste in that class. For example,

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since the presence of organic materials can enhance the mobility of radionuclides and heavy metals in the environment, it could be reasonable to distinguish between wastes that contain mainly organic hazardous chemicals and wastes that contain mainly inorganic hazardous materials, based on considerations of risks from waste disposal. Given the present legal and regulatory distinction between radioactive and hazardous chemical wastes, it also might be reasonable to consider subclassifications of low-hazard and highhazard waste that distinguish between radionuclides and hazardous chemicals, in order to facilitate management of waste in these broad classes under some aspects of current laws and regulations that do not conflict with the precepts of risk-based waste classification. However, such subclassifications should be based on considerations of the risk posed by all hazardous substances in a particular waste. Finally, a waste class could be subclassified based on multiple factors. For example, NRC’s classification system for near-surface disposal of low-level radioactive waste in 10 CFR Part 61 (NRC, 1982a), which includes concentration limits for Class-A, -B, and -C waste and separate requirements for disposal of waste in each class within the same facility, takes into account, among several factors, the long-term persistence of the hazard posed by its constituents and the expected volumes of waste in each class. As indicated in Section 6.5 and discussed further in Section 7.1, NCRP believes that its proposed waste classification system would not have serious adverse impacts on existing classification systems for radioactive and hazardous chemical wastes. Indeed, there generally is a clear and logical correspondence between existing classes of radioactive and hazardous chemical wastes and the waste classification system proposed in this Report, even though the definitions of existing and proposed waste classes have quite different bases. Therefore, the various subclassifications of existing classes of radioactive and hazardous chemical wastes should be compatible with the proposed waste classes. For example, given the proposed definition of low-hazard waste as waste that is generally acceptable for disposal in a dedicated near-surface facility for hazardous wastes, NRC’s classification system for near-surface disposal of low-level radioactive waste (NRC, 1982a) and current disposal practices for such waste should be unaffected. 6.7 Future Development Needs for Risk-Based Waste Classification Previous discussions have indicated that a number of technical, social, legal, and regulatory issues would need to be addressed and

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resolved in establishing the waste classification system proposed in this Report. These issues are discussed further in this Section.

6.7.1 Standardization of Nomenclature In developing the waste classification system presented in this Report, NCRP found that differences in the meanings of common terms used by practitioners in the fields of radioactive and hazardous chemical materials, including materials management and risk assessment, constituted a significant initial impediment to progress. Even among experts in the different fields, a substantial amount of time was required to develop an understanding of the differences in terms and to establish a mutually comprehensible nomenclature. NCRP believes that making meaningful and efficient progress toward establishing a comprehensive and risk-based hazardous waste classification system would be helped by an initial effort to standardize the relevant nomenclature. The most important difference in nomenclature is the different meanings attached to ‘‘acceptable’’ and ‘‘unacceptable’’ risks in the radiation and chemical paradigms for risk management (see Sections 3.3.3 and 3.3.4). Reconciling the two risk management paradigms depends critically on developing a common understanding of the meanings of these two terms (Kocher, 1999). This understanding is important if a consistent approach to risk management is to be applied to all hazardous substances in developing a risk-based waste classification system. In particular, a common understanding that an ‘‘acceptable’’ risk is not necessarily negligible and that risks above negligible levels are not necessarily ‘‘unacceptable’’ is an essential aspect of the waste classification system developed in this Report. This issue is discussed further in Section 6.7.5.

6.7.2 Approaches to Estimating Dose-Response Relationships for Radionuclides and Hazardous Chemicals An important technical issue that requires resolution in developing a comprehensive and risk-based waste classification system concerns the approaches that should be used to estimate health risks from a given exposure to radionuclides and hazardous chemicals. NCRP believes that reasonably consistent approaches should be used for all hazardous substances. Otherwise, some hazardous substances

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could be assigned disproportionate risks and the classification system would not provide a meaningful correspondence with protection of the public, which is the fundamental objective of a risk-based system. 6.7.2.1 Approaches to Estimating Dose-Response Relationships for Substances That Cause Stochastic Responses. The approach to estimating the probability of a response from a given dose (probability coefficient) is of concern in classifying waste that contains substances that induce stochastic effects. This Report has discussed several differences in current approaches to estimating probability coefficients for radionuclides and chemicals that induce stochastic effects that complicate the development of a reasonably consistent approach to risk assessment. These differences include: (1) the use of fatalities as the primary measure of response for radionuclides but incidence as the measure of response for chemicals that induce stochastic responses, (2) the use of best estimates (MLEs) of probability coefficients for radionuclides but upper-bound estimates (UCLs) for chemicals that induce stochastic responses, (3) general acceptance of a single risk-extrapolation model for radionuclides but at least occasional use of a variety of extrapolation models for chemicals that induce stochastic responses that can give very different estimates of risks at the low doses of concern to waste classification, and (4) an accounting of stochastic responses in essentially all organs or tissues for radionuclides but estimation of responses for most chemicals that cause stochastic responses based on observed responses in a single organ in laboratory animals. The use of MLEs of probability coefficients for radionuclides but UCLs for chemicals that induce stochastic responses is the most important issue that would need to be resolved to achieve a consistent approach to estimating risks for the purpose of waste classification. For some chemicals, the difference between MLE and UCL can be a factor of 100 or more. The difference between using fatalities or incidence as the measure of response is unlikely to be important. Use of the linearized, multistage model to extrapolate the doseresponse relationship for chemicals that induce stochastic effects, as recommended by NCRP, should be reasonably consistent with estimates of the dose-response relationship for radionuclides, and this model has been used widely in estimating probability coefficients in chemical risk assessments. The difference in the number of organs or tissues that are taken into account, although it cannot be reconciled at the present time, should be unimportant.

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NCRP’s recommendation that MLEs of probability coefficients should be used in classifying waste based on risk, rather than UCLs (see Section 6.1.2.2) is based on an assumption that point estimates of probability coefficients, as well as point estimates of parameters used to calculate exposure and dose in an assumed scenario, would be used to calculate risk. This recommendation can be justified, in part, on the grounds that the assumed exposure scenarios for hypothetical inadvertent intruders at waste disposal sites should be pessimistic compared with actual exposure scenarios at future times. The use of conservative exposure scenarios helps compensate for the possibility that MLEs of probability coefficients may underestimate actual responses from a given exposure. However, the use of UCLs of probability coefficients in conjunction with point estimates of dose based on conservative exposure scenarios could result in estimates of risk that are unreasonably biased for purposes of cost-effective risk management. As noted in Section 6.1.2.2, NCRP believes that risk assessments used in classifying waste should focus on central estimates of risk for assumed exposure scenarios. Any desired degree of conservatism in protecting the public, beyond those embodied in the assumed scenarios, should be incorporated in risk management decisions made by regulatory authorities, which are represented by the allowable risk and modifying factor in the risk index. Ideally, risk assessments used in classifying waste, or for any other purpose, should take into account the full range of uncertainty in probability coefficients (e.g., the 90 percent confidence interval), not just point estimates of MLEs or UCLs. Furthermore, confidence intervals should be incorporated in all other aspects of a risk assessment, including the definitions of exposure scenarios and the estimates of exposure and dose for those scenarios. Some of these estimates would be highly subjective, such as the confidence interval to be assigned to the probability of occurrence of defined exposure scenarios. Nonetheless, only in this way can the full weight of information about potential risks and their uncertainties be brought to bear in making transparent and cost-effective risk management decisions. 6.7.2.2 Approaches to Estimating Dose-Response Relationships for Substances That Cause Deterministic Responses. Most of the factors that must be considered in developing reasonably consistent approaches to estimating risk for radionuclides and chemicals that induce stochastic responses discussed in the previous section do not apply to substances that induce deterministic responses. For purposes of health protection, incidence generally is the appropriate measure of response for substances that cause deterministic responses. Furthermore, an accounting of deterministic responses

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in multiple organs or tissues, which occur for some substances (e.g., heavy metals), can be based on observed dose-response relationships in each organ or tissue at risk. Finally, the development of riskextrapolation models to predict responses at low doses is not an important concern for substances that induce deterministic effects, due to the threshold nature of the dose-response relationship. In classifying waste, deterministic responses generally should be of concern only for hazardous chemicals (see Section 3.2.2.1). Therefore, the only important issue for risk assessment is the most appropriate approach to estimating thresholds for induction of responses in humans. The primary concern here is that consistent approaches should be used for all substances that induce deterministic effects. NCRP’s recommendation that nominal thresholds in humans should be estimated using the benchmark dose method and a safety factor of 10 or 100, depending on whether the data were obtained in a study in humans or animals (see Section 6.1.2.1), is intended to provide consistency in estimating thresholds for all substances that cause deterministic effects. For most chemicals that induce deterministic effects, the nominal threshold in humans or animals has been estimated based on NOAELs or LOAELs. However, the benchmark dose method should provide more reliable estimates of thresholds (see Section 3.2.1.2.7). Therefore, whenever the nominal threshold in humans for an important chemical in waste that induces deterministic effects has been estimated based on NOAELs or LOAELs, NCRP believes that the data should be re-evaluated using the benchmark dose method to promote greater consistency in classifying waste. As in the case of chemicals that induce stochastic effects discussed in the previous section, NCRP believes that uncertainties in the data beyond those incorporated in the benchmark dose method should be taken into account, if need be, in setting allowable exposures, rather than in an estimate of the nominal threshold.

6.7.3 Allowable Risks from Exposure to Substances That Cause Stochastic or Deterministic Effects The risk-based waste classification system developed in this Report is based fundamentally on the concepts of negligible (de minimis) and acceptable (barely tolerable) risks from exposure to radionuclides and hazardous chemicals, with the crucial distinction that acceptable risks generally can be considerably higher than negligible risks. Therefore, in implementing the waste classification system, decisions would need to be made by regulatory authorities about

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appropriate values of negligible and acceptable risks or doses of substances that cause stochastic or deterministic responses. For substances that induce stochastic effects, decisions about negligible and acceptable risks reflect societal judgments as well as technical information on dose-response relationships at high doses and appropriate methods of extrapolating observed effects to the low doses of concern to waste classification. Precedents for establishing negligible and acceptable stochastic risks for purposes of risk-based waste classification are discussed in Section 6.3.1.2. NCRP particularly emphasizes that any such risks should be reasonably consistent with those used in other risk management activities for substances that induce stochastic effects, such as control of routine releases from operating facilities. In establishing negligible and acceptable doses of hazardous chemicals that induce deterministic effects, decisions are needed about the magnitude of safety and uncertainty factors that should be applied to estimates of nominal threshold doses in humans. NCRP’s recommendations are discussed in Section 6.3.1.1. NCRP particularly emphasizes that RfDs should be used with caution, especially when they incorporate large safety and uncertainty factors, even though RfDs are widely used in health protection of the public. When a substance that causes deterministic effects is important in classifying waste but the quality of the data is poor, NCRP believes that it would be preferable to undertake additional studies in an effort to reduce uncertainties in the data rather than to simply incorporate large safety and uncertainty factors in a risk assessment. NCRP believes that the goal should be to use reasonably consistent safety and uncertainty factors in defining negligible and acceptable doses of all substances that induce deterministic effects, in order to give about the same weight to all such substances in classifying waste. 6.7.4 Selection of Exposure Scenarios In implementing the risk-based waste classification system developed in this Report, the selection of exposure scenarios appropriate to waste disposal is an important technical issue that must be addressed. NCRP believes that scenarios for inadvertent intrusion into near-surface disposal facilities are appropriate in classifying waste for purposes of disposal and, further, that scenarios involving permanent occupancy of disposal sites after loss of institutional control would be appropriate (see Section 6.1.3); such scenarios are commonly used in regulating near-surface disposal of low-level radioactive waste and in risk assessments at hazardous waste sites subject to remediation under CERCLA.

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However, alternatives to exposure scenarios involving permanent occupancy of disposal sites also should be considered in the case of dedicated near-surface facilities that are intended for disposal of low-hazard waste. Some form of institutional control, such as fences, warning signs, and occasional surveillance, presumably will be established over dedicated hazardous waste disposal sites with the intention to maintain such control for a considerable period of time after facility closure (e.g., 30 to 100 y). Nonetheless, it is reasonable to assume that some type of accidental inadvertent intrusion would occur during that time, because institutional control should not be assumed to be completely effective in deterring unwanted intrusion onto a disposal site. Thus, for the purpose of classifying waste as lowhazard or high-hazard, it would be appropriate to develop exposure scenarios involving activities of short duration at disposal sites during the institutional control period (see Section 7.1 for examples). The more restrictive of scenarios involving short-term exposure during an institutional control period and chronic exposure after the institutional control period then could be used to classify nonexempt waste. NCRP also emphasizes that credible scenarios, rather than implausible, worst-case assumptions, should be used in classifying waste, because the probability that exposures of inadvertent intruders will occur according to postulated scenarios is less than unity.

6.7.5 Legal and Regulatory Development Needs Development and implementation of the comprehensive and riskbased hazardous waste classification system presented in this Report would be facilitated by changes in the current legal and regulatory framework for managing radioactive and hazardous chemical wastes in the United States. A number of examples have been discussed previously in this Report and are summarized below. The present distinction between radioactive waste that arises from operations of the nuclear fuel cycle and NARM waste provides an unnecessary impediment to development of a classification system that applies to all radioactive wastes. This distinction is not based on considerations of protection of public health but is based only on the source of the waste. NCRP notes that EPA’s proposed guidance on radiation protection of the public (EPA, 1994d) encourages elimination of this legal distinction, because the guidance specifies that dose limits for all sources of radiation exposure combined and authorized limits for individual sources or practices should be applied to essentially all controllable sources, excluding indoor radon, not just to sources associated with the nuclear fuel cycle.

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The provision of the National Energy Policy Act (NEPA, 1992) that prohibits NRC from establishing dose criteria that could be used to exempt radioactive wastes from licensing requirements for disposal clearly is an impediment to development of generally applicable exemption levels for radioactive waste. An exempt class of radioactive and hazardous chemical waste is the cornerstone of the risk-based waste classification system developed in this Report, and any legal and regulatory impediments to establishment of generally applicable exemption levels would need to be removed. Development of a comprehensive hazardous waste classification system based on the distinct concepts of negligible and acceptable risks would be facilitated by an appropriate reconciliation of the radiation and chemical paradigms for control of exposures to radionuclides and chemicals that induce stochastic effects (Section 3.3). Such a reconciliation can be based on the recognition that differences in the approaches to management of stochastic risks under the two paradigms are more a matter of perception and differences in nomenclaure than reality (Kocher, 1999; NAS/NRC, 1999a). Risks achieved using the radiation paradigm often are well below the limit on acceptable (barely tolerable) risk, and risks achieved using the chemical paradigm often are well above the risk goals. In reality, risk management decisions using either paradigm are based primarily on the principle that exposures should be ALARA, largely without regard for the regulatory limits or goals in either case. The radiation and chemical paradigms for management of stochastic risks can be reconciled in the following way. First, the concept of an intolerable (de manifestis) risk—i.e., a risk so high that it normally must be reduced regardless of cost or other circumstances— should be incorporated in the chemical paradigm. Current laws and regulations that apply the chemical paradigm do not distinguish between risks so high that action to reduce risk normally should be required and lower risks that only warrant consideration of whether risk reduction is feasible (e.g., cost-effective) and should be undertaken on that basis. Such a distinction would emphasize that goals for acceptable risk in the chemical paradigm essentially define negligible risks, rather than limits on acceptable (barely tolerable) risk. Second, the ALARA principle could be incorporated more explicitly in the chemical paradigm to emphasize, as noted above, that application of the ALARA principle is the basis for almost all risk management decisions. Finally, the concept of a generally applicable exemption level, such as a negligible individual dose, should be incorporated explicitly in the radiation paradigm (NCRP, 1993a). At the present time, radiation practices or sources are exempted only on a case-by-case basis.

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In managing and disposing of mixed radioactive and hazardous chemical wastes under AEA and RCRA (see Section 4.3), there are no irreconcilable technical inconsistencies between the two laws. However, there are differences in the approaches to regulating waste disposal under the two laws that could impede efforts to develop a comprehensive waste classification system if they are not reconciled. First, the goal of zero release to the environment under RCRA, which is based on the chemical paradigm for risk management, clearly is different from the concept of limits on allowable releases under AEA, which is based on the radiation paradigm. Second, the RCRA requirement for monitoring and maintenance of disposal facilities for 30 y after facility closure if hazardous waste remains at the site, which essentially calls for perpetual care when the waste does not degrade chemically, differs from the concept of permanent disposal under AEA, in which it is assumed that disposal sites will be abandoned after an institutional control period with no intent to retrieve the waste even though substantial amounts of hazardous substances may remain at the site. Third, acceptable disposals of radioactive waste at any site are based in part on long-term projections of the performance of disposal facilities in limiting releases of hazardous substances and potential exposures of the public, but no such projections have been used in determining acceptable disposals of hazardous chemical waste at specific sites. Finally, the issuance of permits for hazardous waste disposal sites under RCRA does not yet take into account the concept of a hypothetical inadvertent intruder after institutional control is ended. The concept of inadvertent intrusion provides an important basis for developing acceptance criteria for disposal of radioactive waste in near-surface facilities. An additional constraint under RCRA that would need to be addressed in implementing the waste classification system presented in this Report involves solid waste that is identified as hazardous by listing (see Section 4.2.1). At the present time, any solid waste that is hazardous by listing cannot be rendered nonhazardous by treatment. Rather, in accordance with the ‘‘mixture’’ and ‘‘derivedfrom’’ rules in 40 CFR Part 261 (EPA, 1980b; 1992c; 2001b), any listed waste is considered to be hazardous regardless of the concentrations of listed hazardous substances, unless the waste is specifically ‘‘delisted.’’ The waste classification system developed in this Report, which includes an exempt class of waste as an essential element, could be implemented only if these rules were revised to allow establishment of exemption levels for listed hazardous chemical wastes.

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6.8 Summary of the Proposed Risk-Based Waste Classification System The hazardous waste classification system recommended by NCRP is depicted in Figure 6.1 at the beginning of Section 6. This proposal was developed with two fundamental objectives in mind. First, all wastes that contain radionuclides, hazardous chemicals, or mixtures of the two should be included in the same classification system. A comprehensive hazardous waste classification system should be developed to replace the separate, and quite different, classification systems for radioactive and hazardous chemical wastes, as well as the separate classification systems for radioactive waste that arises from operations of the nuclear fuel cycle and NARM waste. Second, all hazardous wastes should be classified based on considerations of risks to the public that arise from disposition of the material. In this Report, permanent disposal in a permitted facility for hazardous or nonhazardous waste is the assumed disposition of waste containing hazardous substances that has no further use to its present custodian. An important consequence of these two objectives is that the same rules should apply in classifying any waste that contains hazardous substances. Based on these objectives, the fundamental principle embodied in the proposed classification system is that waste should be classified in relation to disposal systems (technologies) that are expected to be generally acceptable in protecting public health. The types of disposal systems assumed in classifying waste should represent current or planned practices for radioactive or hazardous chemical wastes. Based on the principle that hazardous waste should be classified in relation to disposal systems (technologies) that are expected to be generally acceptable in protecting the public, three basic classes of hazardous waste are defined: (1) exempt waste is any waste that would be generally acceptable for disposal in a municipal/industrial landfill for nonhazardous waste; (2) low-hazard waste is any nonexempt waste that would be generally acceptable for disposal in a dedicated near-surface facility for hazardous waste; and (3) highhazard waste is any waste that requires a disposal system considerably more isolating than a near-surface hazardous waste facility (e.g., a geologic repository). In a general way, these qualitative definitions clearly relate waste classification to risks that arise from disposal of waste. Based on the waste isolation capabilities of the three types of disposal systems specified in the definitions, exempt waste would contain the lowest concentrations of hazardous substances and highhazard waste the highest.

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Given the qualitative definitions of the three waste classes, the boundaries of the waste classes would be quantified based on explicit descriptions of how the definitions are related to risk. The boundaries would be expressed in terms of limits on amounts (concentrations) of individual hazardous substances, with specified rules for how to classify waste that contains mixtures of hazardous substances, such as the sum-of-fractions rule for mixtures of substances that induce stochastic effects. Specifically, waste would be classified as exempt if the risk that arises from disposal in a municipal/industrial landfill for nonhazardous waste does not exceed negligible (de minimis) levels. Use of a negligible risk to quantify limits on concentrations of hazardous substances in exempt waste is appropriate because the waste would be managed in all respects as if it were nonhazardous. Nonexempt waste would be classified as low-hazard if the risk that arises from disposal in a dedicated near-surface facility for hazardous wastes does not exceed acceptable (barely tolerable) levels. An essential condition of the definitions of exempt and low-hazard waste is that an acceptable (barely tolerable) risk must be substantially greater than a negligible risk. Waste would be classified as highhazard if it would pose an unacceptable (de manifestis) risk when placed in a dedicated near-surface facility for hazardous wastes. The boundaries between different waste classes would be quantified in terms of limits on concentrations of hazardous substances using a quantity called the risk index, which is defined in Equation 6.1. The risk index essentially is the ratio of a calculated risk that arises from waste disposal to an allowable risk (a negligible or acceptable risk) appropriate to the waste class (disposal system) of concern. The risk index is developed taking into account the two types of hazardous substances of concern: substances that cause stochastic responses and have a linear, nonthreshold dose-response relationship, and substances that cause deterministic responses and have a threshold dose-response relationship. The risk index for any substance can be expressed directly in terms of risk, but it is more convenient to use dose instead, especially in the case of substances that cause determinstic responses for which risk is a nonlinear function of dose and the risk at any dose below a nominal threshold is presumed to be zero. The risk index for mixtures of substances that cause stochastic or deterministic responses are given in Equations 6.4 and 6.5, respectively, and the simple rule for combining the two to obtain a composite risk index for all hazardous substances in waste is given in Equation 6.6 or 6.7 and illustrated in Equation 6.8. The risk (dose) that arises from waste disposal in the numerator of the risk index is calculated based on assumed scenarios for exposure of hypothetical

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inadvertent intruders at landfills for nonhazardous waste or at dedicated near-surface facilities for hazardous wastes. Use of the risk index in classifying waste is illustrated in Figure 6.2. Classification of waste essentially is a two-step process. The first step involves a determination of whether a waste can be classified as exempt, based on an assumed negligible risk and an exposure scenario for inadvertent intruders appropriate to disposal of waste in a municipal/industrial landfill for nonhazardous waste. If the waste is not exempt, the second step involves a determination of whether a waste can be classified as low-hazard, based on an assumed acceptable (barely tolerable) risk and an exposure scenario for inadvertent intruders appropriate to disposal in a dedicated nearsurface facility for hazardous wastes. An important issue in developing a risk-based hazardous waste classification system is the degree of conservatism in protecting public health that should be embodied in the foundations and framework of the system and its implementation. The specific issues are, first, the extent to which calculations of risk in the numerator of the risk index should deliberately overestimate expected risks that arise from disposal of hazardous waste and, second, the extent to which the

Fig. 6.2. Decision diagram for classification of hazardous waste using the risk index (RI).

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assumed allowable (negligible or acceptable) risks in the denominator of the risk index should incorporate margins of safety. In many respects, the foundations and framework of the proposed risk-based hazardous waste classification system and the recommended approaches to implementation are intended to be neutral in regard to the degree of conservatism in protecting public health. With respect to calculations of risk or dose in the numerator of the risk index, important examples include (1) the recommendation that best estimates (MLEs) of probability coefficients for stochastic responses should be used for all substances that cause stochastic responses in classifying waste, rather than upper bounds (UCLs) as normally used in risk assessments for chemicals that induce stochastic effects, and (2) the recommended approach to estimating threshold doses of substances that induce deterministic effects in humans based on lower confidence limits of benchmark doses obtained from studies in humans or animals. Similarly, NCRP believes that the allowable (negligible or acceptable) risks or doses in the denominator of the risk index should be consistent with values used in health protection of the public in other routine exposure situations. NCRP does not believe that the allowable risks or doses assumed for purposes of waste classification should include margins of safety that are not applied in other situations. NCRP also recognizes that it would be reasonable to incorporate significant degrees of conservatism in implementing the proposed waste classification system. An important example involves the selection of exposure scenarios to be used in calculating risk in the numerator of the risk index. Assuming that disposal in a near-surface facility is the intended disposition of exempt and low-hazard waste, NCRP believes that it would be reasonable to assume for purposes of waste classification that inadvertent intrusion into waste at a disposal site would occur immediately after an assumed loss of institutional control and that an intruder would permanently occupy the site over a normal (70 y) lifetime while engaging in activities typical of a self-sufficient homesteader. These assumptions should be pessimistic compared with exposure conditions that are likely to occur at waste disposal sites in the future, thus providing a margin of safety in classifying waste. For this reason, NCRP does not believe that implausible, worst-case assumptions should be used in developing and implementing models of relevant exposure pathways in the selected exposure scenarios. In general, degrees of conservatism could be incorporated in a risk-based waste classification system to account for such factors as uncertainties in assumptions, models, and data, as well as the need to protect sensitive population groups (e.g., infants and children). If

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regulatory authorities choose to incorporate intentionally conservative assumptions in implementing the waste classification system, NCRP recommends that these assumptions be explicitly identified and justified based, for example, on a quantitative uncertainty analysis. An open approach would foster consistency and transparency in developing the system and in applying it to the wide variety of hazardous wastes. Furthermore, NCRP believes that conservative assumptions beyond those incorporated in assumed exposure scenarios should be applied to the risk management aspect of waste classification (establishment of allowable risks), rather than in calculating risk based on assumed exposure scenarios. Ideally, risk-based waste classification, and any other activities in health protection or risk assessment, should be based on a full accounting of uncertainties in all the supporting information and assumptions.

7. Implications of the Recommended RiskBased Waste Classification System NCRP’s recommendations on the principles and framework for a comprehensive and risk-based hazardous waste classification system are given in Section 6. In this system, waste would be classified based in large part on the value of the risk index, which essentially is the ratio of the calculated risk that arises from an assumed disposition to a specified allowable risk for that disposition. Simple examples of how the composite risk index for waste that contains mixtures of hazardous substances would be calculated using hypothetical data are provided in Section 6.4.4. However, to explore the implications of the recommended principles and framework for classifying hazardous wastes, it is necessary to apply the risk index using typical compositions of existing wastes. Example waste classifications are presented in Section 7.1. The recommendations in Section 6 also have implications for the existing legal and regulatory framework for classifying hazardous wastes. These are discussed in Section 7.2.

7.1 Example Applications of the Risk-Based Waste Classification System This Section provides example applications of the recommended risk-based waste classification system to a variety of hazardous wastes to illustrate its implementation and potential ramifications. Disposal is the only disposition of waste considered in these examples. In Section 7.1.1, a general set of assumptions for assessing the appropriate classification of hazardous wastes is developed, including a variety of assumed exposure scenarios for inadvertent intruders at waste disposal sites and assumed negligible and acceptable risks or doses from exposure to radionuclides and hazardous chemicals. Subsequent sections apply the methodology to several example wastes. 322

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It is not NCRP’s intent to recommend specific boundaries between waste classes. Rather, the examples illustrate that the recommended framework has the potential to be practical and to result in an implementable waste classification system when a variety of plausible assumptions are used. Many assumptions are made in developing the examples. NCRP endorsement or disapproval should not be construed from the use or absence of specific assumptions about exposure scenarios and allowable doses or risks. It is the responsibility of the appropriate regulatory authorities to develop and guide implementation of any waste classification system.

7.1.1 General Approach to the Example Applications The approach to waste classification presented in this Report involves assessments of risk to hypothetical inadvertent intruders at generic waste disposal sites. When implementing NCRPs recommendations on classification of hazardous wastes, it should be recognized that maintaining the operational integrity of a disposal facility and meeting requirements for protection of the public and the environment are an integral part of the design and operation of specific facilities. These considerations are of limited importance to waste classification because they require foreknowledge of site-specific characteristics. Irrespective of how a waste is classified, appropriate designs and controls based on site-specific considerations are assumed to be in place to ensure that applicable standards for protection of the public and the environment will be met. Waste classification is necessarily based on non-site-specific factors, including assumptions about an appropriate level of protection for a hypothetical inadvertent intruder. There are two primary considerations in selecting the appropriate level of protection for intruders: the difficulty of an intruder accessing disposed waste and the extent of exposures and health effects resulting from such access. Classification of a given waste is based on an evaluation of the risk index specified in Equations 6.4, 6.5 and 6.6 for assumed types of disposal systems. If the risk index is less than unity, the waste is acceptable for inclusion in the associated waste class; otherwise, the waste generally requires a more protective disposal system and would be placed in a class for more hazardous wastes. The appropriate classification depends on the level of protection required which, in turn, depends on the characteristics of the waste relative to the capabilities of assumed disposal technologies. This concept is a fundamental part of the risk index. General assumptions about

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disposal technologies, exposure scenarios, and allowable doses or risks used in the example waste classifications based on the risk index are summarized in Table 7.1 and discussed below. Other assumptions about exposure scenarios not listed in this table are included in some of the example waste classifications to illustrate the importance of plausible alternatives. 7.1.1.1 Exempt Waste. When wastes that contain small amounts of radionuclides are considered for disposal in a landfill for nonhazardous waste, the allowable dose may be the same as that specified TABLE 7.1—Summary of assumptions used in example hazardous waste classifications.

Waste Classification Exempt

Low-hazard

High-hazard

Disposal Technology

Scenario and Period of Exposurea

Allowable Dose or Riskb 0.02 mSv yⳮ1

Disposal in a near-surface facility suitable for nonhazardous wastes

Residential (with gardening) or commercial

10ⳮ5 lifetime risk

25 or 30 y

RfD

Disposal in a near-surface facility suitable for hazardous wastes

Intruder drills into waste resulting in 1,000 h of exposure once during a lifetime

20 mSv per event

Disposal in a geologic facility suitable for highly hazardous wastes

Intrusion considered unlikely

10ⳮ3 lifetime risk per event 10 ⳯ RfD Not applicable to waste classification

Comments Long-term occupancy of disposal site is assumed to be possible Duration of intrusion is limited by occasional surveillance and recognition of waste by intruder Most protective disposal practice; any waste not acceptable in other classes is included in this class

a Other assumptions about exposure scenarios are included in discussions of example waste classifications. b These assumptions are not necessarily recommendations of NCRP but represent possible alternatives that are reasonable. Decisions regarding allowable risks or doses to be used in defining the boundaries between waste classes are the responsibility of regulatory authorities. Other examples of allowable risks or doses are discussed in considering classification of specific wastes.

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in standards for protection of the public that apply to a single source of radiation exposure (e.g., a small fraction of the annual dose limit of 1 mSv, corresponding to a lifetime risk of about 10ⳮ5 for an assumed exposure time of 30 y). In developing exposure scenarios at a disposal facility for nonhazardous waste, fewer access restrictions should be assumed than in the case of disposal in a regulated facility for radioactive waste, and the ultimate use of the site could be residential, commercial or resident-farmer. Selection of the end use should be based on plausibly conservative but realistic assumptions about future land uses. Similar considerations should apply to waste that contains small amounts of hazardous chemicals that might be sent to a disposal facility for nonhazardous waste. Allowable doses could correspond to a negligible lifetime risk of about 10ⳮ5 in the case of substances that induce stochastic effects or an intake at an RfD (Section 3.2.1.2) in the case of substances that induce deterministic effects. The considerations of exposure scenarios should be the same as in the case of radioactive wastes. 7.1.1.2 Low-Hazard Waste. In a scenario involving disposal of hazardous waste in a licensed facility, the allowable risk or dose can be higher than in cases of disposal in a facility for nonhazardous waste, based on the reduced duration of exposure that probably would result from occasional surveillance of the site or recognition of waste by an intruder. In addition, it is not likely that the public will gain long-term access to hazardous waste disposal sites for the foreseeable future. Therefore, in the examples that follow, a higher (less restrictive) allowable risk or dose is assumed to be appropriate when evaluating wastes for classification as low-hazard (e.g., a radiation dose of 20 mSv for a one-time exposure of 1,000 h, corresponding to a risk of about 10ⳮ3). In the case of near-surface disposal at hazardous waste sites, it is assumed in the following examples that an intruder gains access to the disposal site, drills into the waste, thus bringing some waste to the surface, and is subsequently exposed to the waste constituents for one-half of a working year (1,000 h). This assumption is believed to be conservative because (1) the postulated occasional surveillance at licensed facilities makes it unlikely that an activity of the magnitude required to intrude into disposed waste could actually continue for half a year and (2) the discovery of waste drums or other unusual barriers or features of the disposal site would alert the intruder and measures likely would be taken to minimize the exposure time. It is assumed that the drilling activity results in mixing of the waste with clean soil or cover material, thereby diluting the waste.

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7.1.1.3 High-Hazard Waste. No scenarios are developed for inadvertent intrusion into a disposal facility for high-hazard waste, essentially because intrusion into suitable facilities for these wastes should be highly unlikely (see Section 6.2.2.2.2) and evaluations of highly unlikely scenarios do not provide a reasonable basis for determining the general acceptability of waste disposal systems (NAS/NRC, 1995a). Any waste that is not generally acceptable for near-surface disposal in a licensed facility for hazardous waste would be relegated to the high-hazard waste class. 7.1.1.4 Development of Examples. The example classifications of existing wastes presented in Sections 7.1.2 through 7.1.8 use the risk-based approach generally developed in Section 6 and the assumptions shown in Table 7.1 and summarized earlier. Alternative assumptions about exposure scenarios are used in some cases. Several risk/dose estimation procedures are used to determine the appropriate classification for the waste compositions used in the examples to illustrate possible approaches to implementation. The goal is to demonstrate the feasibility of the concept and various procedures for employing it. The procedures range from simple screening methods requiring a minimum of effort (e.g., use of conservative assumptions for the contaminants that are the principle contributors to risk) to more detailed risk or dose assessments for all hazardous materials in the waste. In presenting the examples, qualitative considerations that are important in applying the recommended risk-based waste classification system also are discussed. While the classification system is based on risk assessment, its establishment involves major risk management aspects. Risk assessment as manifested in the risk index is a tool in making good risk management decisions; however, if used alone without clear recognition of the implications of the risk estimates, including their limitations and uncertainties, risk assessment can be just as likely to result in a poor decision as a good one.

7.1.2 Consideration of Exempt Wastes NCRP did not undertake a detailed investigation into the kinds and quantities of radioactive or hazardous chemical wastes containing low levels of hazardous substances that might be classified as exempt, based on the consideration that allowable dispositions should pose no more than a negligible risk or dose. Rather, published studies are cited to indicate that substantial quantities of waste currently managed as radioactive or chemically hazardous waste

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are potential candidates for exemption, especially for purposes of disposal. 7.1.2.1 Radioactive Wastes. In 1989, the Electric Power Research Institute (EPRI) issued a study of wastes that arise from operations of nuclear power plants that potentially would be below regulatory concern (BRC) and, thus, would be acceptable for disposal in a landfill for nonhazardous waste (EPRI, 1989). NRC’s proposed criteria for defining BRC waste are discussed in Section 4.1.2.5.2; they are consistent with the precedents for defining a negligible dose discussed in Section 6.3.1.2.1 and the assumption about an allowable risk given in Table 7.1. Based on assumptions used in EPRI study that (1) the total activity concentration of all photon-emitting radionuclides in the waste would not exceed 40 Bq gⳮ1, (2) each reactor station would produce 180 metric tons (about 100 m3) of potentially exemptible waste annually, and (3) the waste from each station would be mixed with 54,000 metric tons of nonradioactive waste prior to disposal, the estimated annual dose to a resident homesteader at a disposal site after closure of the site was about 3 ␮Sv. This estimate is about a factor of 30 less than NRC’s BRC criterion of 100 ␮Sv for practices affecting only a few individuals. The wastes considered in this study included most of the so-called dry-active waste generated at nuclear power plants. This type of waste includes rags, paper, plastic floor covering or bags, protective clothing, contaminated tools, wood, and discarded plant equipment and hardware. Other wastes considered in the analysis included contaminated soil, secondary ion-exchange resins, grit blast material, and sludge from water treatment. NRC has issued an assessment of potential doses to the public associated with the distribution, use, and disposal of exempt products or materials containing low levels of source or byproduct material (Schneider et al., 2001) (see Section 4.1.2.5.2). In a case involving disposal of large volumes of zircon sand produced in processing of zirconium-bearing minerals, the estimated annual dose to a future on-site resident at a disposal site was 100 ␮Sv, due to the elevated levels of thorium and uranium. In all other cases, however, the estimated annual dose was substantially less than 10 ␮Sv. Since the volumes of exempt material were large in many cases, this analysis indicates that substantial volumes of waste that contains low levels of radionuclides are potentially exemptible. A noteworthy result of NRC’s analysis of its present exemptions is that doses to individual members of the public during use of exempt products or materials generally are higher than doses that arise from disposal (Schneider et al., 2001). This is due in part to differences in the assumed exposure scenarios for use and disposal; the dilution

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of radionuclide concentrations by mixing with uncontaminated materials prior to closure of a disposal facility also is an important factor. This result indicates that materials that would be exempt for purposes of disposal would not necessarily be exempt for any other purpose. 7.1.2.2 Hazardous Chemical Wastes. NCRP has not considered studies of particular wastes containing low levels of hazardous chemicals that are potential candidates for exemption. However, studies in support of proposed regulations to establish exemption levels for listed hazardous wastes (EPA, 1992d; 1995c; 1999c) indicate that substantial quantities of waste currently managed as chemically hazardous waste could be classified as exempt for purposes of disposal.

7.1.3 DOE Low-Level Radioactive Waste DOE currently operates a number of disposal facilities at its sites that receive low-level radioactive wastes from DOE’s weapons complex and energy research facilities. One of the largest disposal facilities is on the Hanford site in the state of Washington. The waste inventory for 1990 (DOE, 1993b) was used in this example to determine if this waste is acceptable for near-surface disposal under the example assumptions given in Table 7.1. The concentrations of radionuclides in this waste clearly exceed those that would be permitted for disposal in a landfill for nonhazardous waste, and the waste would not be exempt. Therefore, potential doses were evaluated assuming temporary intrusion by unknowing individuals working in the disposal area. The risk index normally is determined by computing the risk or by using dose as a surrogate for risk. In the example in Section 7.1.3.1, the calculated dose associated with intrusion into the waste is divided by the assumed maximum allowable dose to estimate the risk index. In the example in Section 7.1.3.2, limits on acceptable concentrations are developed as surrogates for the allowable risk. The concentrations in the waste are then divided by these allowable concentrations to determine the risk index. The same approach is used in the example in Section 7.1.3.3, except the allowable concentrations are lower because a less protective disposal option is evaluated. The consequences of alternative assumptions about intrusion scenarios on classification of the Hanford waste are considered in Section 7.1.3.4. 7.1.3.1 Classification by Calculation of Total Dose. Exposure pathways considered in this analysis involve external exposure, ingestion of waste materials, and inhalation. Doses were estimated

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based on the concentrations of radionuclides shown in Table 7.2 using the RESRAD18 code (Yu et al., 1993). It was assumed that the intrusion event occurs at 100 y after disposal of the waste. The assumed time of intrusion takes into account the intention that active institutional controls (e.g., fences and guards) will be maintained for at least 100 y after closure of the facility (DOE, 1988c; 1999c). For the purposes of this example, it was assumed that the waste was placed 4 m deep and covered with a cap and soil that was at least 3 m thick. As a consequence, the assumed scenario was an onsite drilling event. The dose analysis assumes a two-fold volume increase (50 percent dilution) of the drill tailings by uncontaminated material. The mixture of waste and uncontaminated cover material is spread on the surface of the site, and individuals working in the area are exposed to the tailings for 1,000 h. The thickness of the layer of contaminated drill tailings is assumed to be about 5 cm and the area to be about 3.3 m2. Using dose as a surrogate for risk, analysis of this scenario yields a dose of 0.002 mSv from all radionuclides. Since the assumed allowable dose is 20 mSv (see Table 7.1), the risk index would be 0.002/20 ⳱ 10ⳮ4, which is well below the value of unity, and the waste would be classified as low-hazard. 7.1.3.2 Classification Using Pre-Established Limiting Concentrations. Another approach to classifying the Hanford low-level waste TABLE 7.2—Hanford low-level waste radionuclide contents and the risk index for drilling scenario. Radionuclide Content in Waste

Nuclides

Activity (TBq)

Co-60 Cs-137 Ni-63 Plutonium Th-232 Uranium (depleted) Uranium (enriched) All others

3.6 ⳯ 103 8.9 3.5 ⳯ 103 2.2 ⳯ 10ⳮ2 2.0 ⳯ 10ⳮ4 0.28 0.037 —

Total risk index

18

Risk-Based Concentration Concentration (Bq gⳮ1) Limit (Bq gⳮ1)

1.7 ⳯ 103 4.1 1.6 ⳯ 103 0.01 9.6 ⳯ 10ⳮ5 0.13 0.17 —

5.9 4.8 2.1 1.1 3.7 3.7 7.5

⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ —

108 104 109 104 102 102 103

Risk Index

2.9 8.7 7.8 9.3 2.6 1.1 2.2 1.7

⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯

10ⳮ6 10ⳮ5 10ⳮ7 10ⳮ7 10ⳮ7 10ⳮ4 10ⳮ6 10ⳮ7

2.0 ⳯ 10ⳮ4

The RESRAD code was used to illustrate implementation of the proposed waste classification system. NCRP did not evaluate the code or its underlying assumptions and database, and its use should not be construed to constitute endorsement.

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is to establish radionuclide-specific concentration limits obtained from the RESRAD calculations described in Section 7.1.3.1 as a surrogate for the allowable risk. Table 7.2 lists the concentration limits resulting from this calculation. This approach is generally conservative because some of the concentration limits for radionuclides having very long half-lives exceed the maximum theoretical concentrations based on the specific activity of the radionuclides themselves. As shown in Table 7.2, the risk index calculated using this approach is 2 ⳯ 10ⳮ4 and, thus, the waste is again classified as low-hazard. The benefit of this approach is that it avoids the need to model the potential doses or risks directly if concentration limits for different waste classes have been established. Classification of waste by using pre-established limiting concentrations would be the typical approach (NRC, 1982a). 7.1.3.3 Classification Using Pre-Established Limiting Concentrations and Enhanced Access. The same process as in the previous calculation was used to screen this example waste under conditions where intruder access to the waste would be enhanced. For example, if the cover for the waste is less than 2 m, it would be appropriate to consider that an intruder would remove sufficient cover material to expose the waste. The maximum allowable concentrations of selected radionuclides for this scenario calculated using the RESRAD code are listed in Table 7.3. The analysis assumes that the intruder is exposed via external exposure, ingestion, and inhalation for a period of 1,000 h. Using the sum of the ratios of the radionuclide concentrations in the waste to the maximum allowable concentrations as a TABLE 7.3—Hanford low-level waste radionuclide contents and the risk index for enhanced-access intrusion scenario. Radionuclide Content in Waste

Nuclides

Am-241 Co-60 Cs-137 Gd-153 Ni-63 Plutonium Ra-226 Uranium (depleted) Total risk index

Activity (TBq)

0.041 3.6 ⳯ 8.9 2.1 ⳯ 3.5 ⳯ 2.1 ⳯ 3.5 ⳯ 0.29

3

10

10ⳮ5 103 10ⳮ2 10ⳮ3

Risk-Based Concentration Concentration (Bq gⳮ1) Limit (Bq gⳮ1)

0.018 1.7 ⳯ 4.1 1.0 ⳯ 1.6 ⳯ 1.0 ⳯ 1.6 ⳯ 0.13

3

10

10ⳮ5 103 10ⳮ2 10ⳮ3

1.7 ⳯ 2.4 ⳯ 2.3 ⳯ 0.78 1.0 ⳯ 1.0 ⳯ 8.1 ⳯ 2.7 ⳯

103 107 103 108 103 101 103

Risk Index

1.1 6.8 1.8 1.3 1.6 1.0 2.0 4.9

⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯

10ⳮ5 10ⳮ5 10ⳮ3 10ⳮ5 10ⳮ5 10ⳮ5 10ⳮ5 10ⳮ5

2.0 ⳯ 10ⳮ3

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surrogate for risk, the risk index was estimated to be 0.002. This result is still much less than unity but is significantly greater than in the example assuming deeper burial of the waste. This example assumes some dilution of the uncovered waste because (1) waste emplacement procedures generally include the use of clean fill between containers (about a factor of two dilution) and (2) excavation of the cover material will likely dilute the waste (about a factor of 2 to 10). However, because the screening analysis indicated a risk index nearly three orders of magnitude less than unity, a more detailed analysis of this scenario is not warranted. 7.1.3.4 Alternative Exposure Scenarios. In the scenarios for inadvertent intrusion at a radioactive waste disposal facility considered in Sections 7.1.3.1 through 7.1.3.3, intrusion is assumed to be a onetime event occurring at 100 y after disposal. This Section considers alternative scenarios and their impacts on classification of the Hanford waste. The assumption that an intrusion event would not occur until 100 y after disposal is based on an intention to maintain institutional control over the disposal site for at least this long (DOE, 1988c; 1999c). However, even during the period of institutional control, inadvertent and unnoticed access to a disposal site cannot be ruled out completely. The consequences of inadvertent intrusion during the institutional control period can be bounded by assuming that a drilling or limited excavation event would occur essentially at the time of facility closure (i.e., 100 y earlier) and that the amount of waste brought to the surface is the same as the amount assumed in the drilling scenario at 100 y analyzed in Sections 7.1.3.1 and 7.1.3.2. A large-scale excavation, such as assumed in the analysis in Section 7.1.3.3, would not to be credible during the institutional control period. The exposure time is assumed to be 1,000 h, as in the previous examples. The risk index for the assumed scenario at the time of facility closure can be obtained from the results in Table 7.2 by adjusting for 100 y of radioactive decay. For example, for 137Cs, which has a half-life of 30 y, the risk index in Table 7.2 would be increased by a factor of 10 because the concentration at the time of facility closure would be a factor of 10 higher than at 100 y after closure. Taking into account similar increases in the risk index for 60Co (5.3 y) and 63 Ni (100 y), the resulting risk index for exposure at the time of facility closure would be 1.5, due almost entirely to 60Co. However, since drilling or limited excavation at the time of facility closure and exposure for 1,000 h to exhumed waste at that time are unlikely to occur (i.e., the assumed scenario is conservative), the estimated dose for any reasonably likely scenario during the institutional control

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period should be considerably less. Thus, this result supports the conclusion that the waste would be classified as low-hazard. Another credible assumption is that permanent access to the site could occur at the end of the 100 y period of institutional control. This assumption has been used in establishing waste acceptance criteria at all DOE low-level waste disposal sites (DOE, 1988c; 1999c), including the Hanford site, based on an acceptable dose from chronic exposure of an inadvertent intruder of 1 mSv yⳮ1. Therefore, the waste acceptance criteria for the Hanford site already take into account an acceptable dose to an inadvertent intruder from permanent site occupancy, so the waste is acceptable for near-surface disposal as low-hazard waste according to this scenario without the need for further analysis.

7.1.4 Average Commercial Low-Level Radioactive Waste Another example involves classification of average commercial low-level radioactive waste. The total activities of the dominant radionuclides in the waste, as obtained from data for all Class-A, -B and -C low-level waste emplaced in near-surface disposal facilities in the United States in 1990, are given in Table 7.4 (DOE, 1993b). These estimates do not account for decay that would occur during the 100 y institutional control period. That is, inadvertent intrusion is assumed to occur at the time of facility closure. Radionuclide concentrations are based on the estimated volume of waste and an

TABLE 7.4—Average commercial low-level waste radionuclide concentrations and the risk index for drilling intrusion scenario. Radionuclide Content in Waste

Nuclides

Am-241 Cs-137 Th-232 U-238 Uranium (depleted) All others Total risk index a

Risk-Based Concentration Concentration Activity (TBq) (Bq gⳮ1)a Limit (Bq gⳮ1)

1.2 3.5 1.3 7.4 4.1

⳯ ⳯ ⳯ ⳯ ⳯ —

102 105 104 103 103

2.4 7.4 2.6 1.5 8.1

⳯ ⳯ ⳯ ⳯ —

103 103 102 101

1.7 4.3 3.7 1.2 1.2

⳯ ⳯ ⳯ ⳯ ⳯ —

102 104 102 103 103

Risk Index

1.5 1.5 7.1 1.2 6.7 4.1

⳯ ⳯ ⳯ ⳯ ⳯ ⳯

10ⳮ2 10ⳮ2 10ⳮ1 10ⳮ1 10ⳮ2 10ⳮ3

9.3 ⳯ 10ⳮ1

Based on volume of 32,400 m3 and assumed density of 1.5 g cmⳮ3.

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assumed density of 1.5 g cmⳮ3. The limiting radionuclide concentrations calculated using the RESRAD code, assuming a drilling intrusion scenario as described in Section 7.1.3.1, are given in Table 7.4. Summing risk indexes for individual radionuclides yields a composite risk index of 0.93 for this waste, indicating that it is a low-hazard waste that is generally acceptable for near-surface disposal. Because the risk index calculated using the concentration-based screening approach is very close to unity, additional review was conducted to ensure that the approach was acceptable and to illustrate how additional factors could be taken into account for wastes near boundaries. As noted in Section 7.1.3.4, an assumption that drilling intrusion would occur at the time of facility closure should be conservative. For example, as a result of the relatively short halflife of 137Cs, the concentration will be halved every 30 y, which would reduce the risk index for 137Cs from 0.015 to 0.002 after an assumed institutional control period of 100 y (NRC, 1982a; 1982b). Additionally, review of the amounts of 232Th assumed to be in the waste indicate that the inventory may be overestimated by as much as a factor of 10 (DOE, 1993b). Since NRC does not specify limits on concentrations of 232Th that are generally acceptable for near-surface disposal (NRC, 1982a) and the presence of this radionuclide does not have a significant effect on predicted doses to off-site individuals, there is little incentive for generators to report more realistic inventories. These two factors alone would reduce the estimated the risk index to less than 0.3. Also, as noted in Section 7.1.3.2, use of the concentration-limit approach may be somewhat conservative (e.g., a factor of two in the earlier example case). A scenario involving permanent occupancy of a disposal site following loss of institutional control at 100 y after disposal also could be considered in classifying commercial low-level waste (see Section 7.1.3.4). As in the case of DOE waste discussed above, the limits on concentrations of radionuclides in commercial Class-A, -B, and -C waste that may be sent to a near-surface facility, as established by NRC in 10 CFR Part 61, are based largely on analyses of this type of scenario (NRC, 1982a; 1982b). Thus, average commercial low-level waste would be classified as low-hazard waste according to this scenario without the need for further analysis.

7.1.5 Typical Uranium Mill Tailings Current environmental standards for uranium mill tailings (EPA, 1983) permit surface disposal of these wastes with appropriate engineered controls. The controls include use of a cap that typically

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consists of about 1 m or more of clay covered by a layer of rip-rap and soil. Mill tailings disposal sites will ultimately be owned by the federal government or the host states. Uranium mill tailings contain a variety of naturally occurring radionuclides and toxic heavy metals. The most significant of these are 226Ra and its decay products. Concentrations of 226Ra in uranium mill tailings produced in the United States are in the range of about 2 to 37 Bq gⳮ1 (EPA, 1982). These concentrations are at least two orders of magnitude higher than the average concentration in surface soil of about 0.02 Bq gⳮ1 (NCRP, 1984a). As is apparent in the previous examples, it is not always necessary to consider all hazardous substances in a waste, because a few constituents often dominate the risk. This example considers the risk due only to 226Ra. The average concentration of 226Ra in surface soil results in an average dose from external exposure of about 0.07 mSv yⳮ1 (NAS/ NRC, 1999a). Since the concentration of 226Ra in mill tailings is at least a factor of 100 higher than the average concentration in surface soil, as noted above, it is obvious without further analysis that unrestricted release of mill tailings disposal sites would result in non-negligible doses and risks to individuals who might reside on a site. Therefore, mill tailings could not possibly be classified as exempt waste. Furthermore, since the dose due only to external exposure during permanent residence on a tailings pile would exceed the current dose limit for the public of 1 mSv yⳮ1 (DOE, 1990; NRC, 1991) by about an order of magnitude or more, mill tailings could be classified as low-hazard waste only if perpetual institutional control were maintained over disposal sites to prevent long-term occupancy by the public. An assumption of institutional control is used in the following example. In this analysis, it was conservatively assumed that an individual could enter a disposal site and remove a portion of the protective surface cover (about 200 m2) and expose on-site workers to the tailings.19 It was further assumed that such an unlikely scenario would occur only once in an individual’s lifetime, and that exposure would occur over a period of 1,000 h. Exposure pathways considered in this analysis involve external exposure, inhalation of outdoor radon and its short-lived decay products, inhalation of particulates, and ingestion of waste. The activity concentrations of 226Ra and its decay products in the waste were conservatively assumed to be 37 Bq gⳮ1, a 19

Such an action is mitigated by two considerations. First, it is expected that even with closed disposal facilities, annual visual inspections, if not caretaker activities, will occur. Furthermore, the recommended disposal cell design (including rip-rap and clay cover) will discourage the assumed activity. However, no credit was given for these controls in this assessment.

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value which is at the upper end of the range of concentrations in domestic mill tailings (EPA, 1982). Based on the assumptions described above, the risk calculated using the RESRAD code (Yu et al., 1993) and EPA slope factors (HEAST, 1991; 1995) is a probability of cancer incidence of about 7 ⳯ 10ⳮ4. The assumed acceptable risk for low-hazard waste is 10ⳮ3 (see Table 7.1), resulting in a risk index of 0.7. Hence, domestic uranium mill tailings could be classified as low-hazard waste acceptable for licensed near-surface disposal under conditions of perpetual institutional control over disposal sites. An alternate analysis was performed using dose as a surrogate for risk. Using the same assumptions given above and the RESRAD code, the dose to an intruder was estimated to be 15 mSv. Assuming an allowable dose of 20 mSv (see Table 7.1), the risk index was estimated to be 0.8. Therefore, as expected, either approach yields about the same result. The example analysis used to classify uranium mill tailings as low-hazard waste depends critically on the assumption of perpetual institutional control over disposal sites. An alternative approach to classification of mill tailings could be considered in which permanent residence on disposal sites is assumed to be plausible at some time in the future, as in classifying low-level radioactive waste in previous examples. As noted above, the doses and risks based on such a scenario clearly would be intolerable and mill tailings would be classified as high-hazard waste. However, disposal of the very large volumes of mill tailings far below the ground surface, as intended for other high-hazard radioactive wastes, is considered to be impractical (EPA, 1982; 1983). Instead of taking the high risk to an inadvertent intruder at an uncontrolled near-surface disposal site into account by assuming perpetual control in classifying mill tailings, regulatory authorities could grant an exception to disposal requirements for this type of high-hazard waste. The net result would be the same in either approach—namely, the need to maintain institutional control over tailings piles essentially forever. This alternative also could be considered for other hazardous wastes that contain highly elevated levels of naturally occurring hazardous substances and occur in very large volumes, such as wastes from mining and processing of various mineral ores to extract nonradioactive materials.

7.1.6 Residues from Processing of High-Grade Uranium Ore During the early years of the nuclear energy and weapons development programs, ores containing unusually high concentrations of

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uranium from nondomestic sources were processed at various locations in the United States. Some of these ores contained as much as 65 percent uranium oxide. Processing of these ores and concentrates produced about 11,000 m3 of residues with 226Ra concentrations that range from tens of Bq gⳮ1 to more than 18,000 Bq gⳮ1 (NAS/NRC, 1995b). The exposure scenario described in the previous example of domestic uranium mill tailings was used to classify the high-radium residues. The risk and dose assessments indicated a probability of radiation-induced cancer incidence of about 0.6, potential doses in excess of 10 Sv, and a risk index between 50 and 100. Thus, these residues would be classified as high-hazard waste, even under conditions of perpetual institutional control over near-surface disposal sites, and they would require some form of greater confinement disposal well below the ground surface. This conclusion is consistent with recommendations for disposition of these residues (NAS/ NRC, 1995b).

7.1.7 Mixed Waste: Electric Arc Furnace Dust Electric arc furnace dust is a listed hazardous chemical waste. This material is deemed hazardous because it contains relatively high concentrations of heavy metals. The waste consists of the emission control dust or sludge collected from electric arc furnaces during the manufacture of iron and steel. The principle chemicals of concern and their concentrations are listed in Table 7.5 (EPA, 1988). The first part of the following analysis considers the toxic metals in this waste and evaluates the waste for near-surface disposal. In a second part of this example, the waste is presumed to be contaminated by 137Cs from sources inadvertently included in scrap metals that are recycled into the manufacturing process. 7.1.7.1 Introduction to Analysis. All previous examples involved waste in which radionuclides were assumed to be the only hazardous substances. However, the contaminants of concern in electric arc furnace dust include chemicals that induce stochastic and deterministic effects. Furthermore, the deterministic chemicals affect different organs, and some affect more than one organ. The composite risk index for mixtures of substances that cause stochastic or deterministic effects is shown in its most general form in Equations 6.4, 6.5 and 6.6 (see Sections 6.4.1 and 6.4.2), and is restated in a simpler and more convenient form in Equation 6.7 (see Section 6.4.4). In calculating the risk index for mixtures of substances

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TABLE 7.5—Concentrations of hazardous chemicals in untreated, high-zinc electric arc furnace dust waste. Range of Measured Concentrations

Chemical

Low (ppm)

High (ppm)

Arithmetic Mean (ppm)a

Antimony Arsenic Barium Beryllium Cadmium Chromium Lead Mercury Nickel Selenium Silver Vanadium Zinc

52 40 24 0.15 12 400 500 0.0002 10 5 2.5 24 4,400

290 400 400 1.5 5,000 12,000 140,000 41 6,900 20 59 140 320,000

170 220 210 0.82 2,500 6,300 70,000 21 3,400 13 31 81 160,000

a

Average of low and high values.

that induce deterministic effects, it is necessary to identify the organ or organs potentially affected by exposure to each such substance, taking into account that a given substance may induce deterministic responses in more than one organ, and the allowable dose for each combination of organ and substance. For this example, RfD for each chemical that induces a deterministic effect given in Table 7.6 (ATSDR, 1987; HEAST, 1995; IRIS, 1988) was assumed to be the allowable dose; RfD generally depends on the route of intake. For chemicals that induce stochastic effects, the slope factors (probability coefficients for cancer incidence) given in Table 7.7 were used to obtain the allowable doses based on an assumed allowable risk. As discussed in Sections 3.2.3, 3.3, and 6.1, RfDs and slope factors are intended to provide conservative estimates of risk. Therefore, they are most suitable for use in establishing a negligible dose, i.e., in determining whether a waste could be classified as exempt. To reduce the amount of conservatism to a degree appropriate to establishing an acceptable risk for the purpose of evaluating whether a particular waste would be classified as low-hazard or high-hazard, RfDs are multiplied by a factor of 10 and the slope factors are divided by a factor of 10.

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TABLE 7.6—Organ-specific RfDs for toxic metals in electric arc furnace dust.

Chemical

Antimony Arsenic Barium Beryllium Cadmium Chromiumb Leadc Mercury Nickel Selenium Silver Vanadium Zinc

Route of Intake and Critical Organ

Oral-cardiovascular Oral-dermal/ocular Oral-cardiovascular Inhalation-multiple Oral-multiple Inhalation-respiratory Oral-renal Oral-multiple Inhalation-respiratory Oral-multiple Oral-renal Inhalation-renal Oral-weight decrease Oral-hepatic Oral-dermal/ocular Oral-renal Oral-weight decrease

RfDa [mg (kg d)ⳮ1]

4.0 3.0 7.0 1.4 2.0 5.7 1.0 3.0 3.0 4.0 3.0 8.6 2.0 5.0 5.0 7.0 3.0

⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯ ⳯

10ⳮ4 10ⳮ4 10ⳮ2 10ⳮ4 10ⳮ3 10ⳮ6 10ⳮ3 10ⳮ3 10ⳮ5 10ⳮ4 10ⳮ4 10ⳮ5 10ⳮ2 10ⳮ3 10ⳮ3 10ⳮ3 10ⳮ1

a

Based on data given in EPA’s Integrated Risk Information System (www.epa.gov/iris), current as of March 1999, except as noted. b Assumed to be hexavalent form. c RfD is based on drinking water standard (ATSDR, 1987).

TABLE 7.7—Slope factors for metals that induce stochastic effects in electric arc furnace dust.

Chemical

Arsenic Beryllium Cadmium Chromium (VI) Nickel

Ingestion slope factora [mg (kg d)ⳮ1]

Inhalation slope factora [mg (kg d)ⳮ1]

1.5

15 8.4 6.3 42 0.84b

a Based on data given in EPA’s Integrated Risk Information System (www.epa.gov/iris), current as of March 1999, except as noted. b Value obtained from HEAST (1995).

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7.1.7.2 Evaluation as Exempt Waste. The subject waste was cursorily evaluated against soil screening criteria developed by EPA (1996e) to ascertain whether it might qualify as exempt waste. The concentrations of antimony, arsenic, beryllium, cadmium, chromium, and lead all were orders-of-magnitude above the screening criteria. The maximum concentrations of most of the other elements listed in Table 7.6 also exceed the screening values. The purpose of the screening criteria is to determine if further evaluation is needed. While these screening criteria are conservative and do not in themselves indicate that a material is unacceptable for disposal as nonhazardous waste, the magnitude of the differences is sufficient to indicate the need for classification as at least a low-hazard waste. 7.1.7.3 Approach to Example Analysis. Similar to the previous examples involving radioactive wastes, these residues were assumed to be placed in a typical near-surface disposal facility having a RCRA Subtitle C permit. In this example, it is assumed that an inadvertent intruder excavates an area of the disposal site of approximately 200 m2. This excavation is sufficient to reach the waste, and the exposure pathways considered involve inhalation of resuspended waste, ingestion of waste, and dermal absorption. The intrusion is identified and halted prior to any structures being constructed on the disposal site and before any farming activity can be developed. As in the similar scenarios used in the radioactive waste examples, exposure is assumed to continue for 1,000 h. The analysis of exposures to hazardous chemicals for this example was in accordance with EPA guidance on evaluating human health risks from exposure to chemicals in soil. The calculations were performed using the RESRAD-CHEM code (Cheng and Yu, 1993), which is similar to the RESRAD code for estimating doses and risks from exposure to radionuclides (Yu et al., 1993). Intake rates for individual pathways were calculated for each chemical (element) of interest in the waste, assuming a unit concentration of 1 mg kgⳮ1 (i.e., ppm). The estimated intake rates per ppm in waste by dust inhalation and soil ingestion were 6.2 ⳯ 10ⳮ9 and 3.3 ⳯ 10ⳮ8 mg (kg d)ⳮ1, respectively, for all chemicals. For dermal absorption, the intake rate depends on the particular chemical element and ranged from 3.3 ⳯ 10ⳮ9 and 1 ⳯ 10ⳮ9 mg (kg d)ⳮ1 for cadmium and lead, respectively, to about 1 ⳯ 10ⳮ10 mg (kg d)ⳮ1 or less for all other elements. Thus, the total intake rate by all routes of exposure is determined primarily by soil ingestion, and is about 4 ⳯ 10ⳮ8 mg (kg d)ⳮ1 per ppm in the waste for all chemicals. 7.1.7.4 Deterministic Risk Index for Hazardous Chemical Constituents. In accordance with Equation 6.5, the risk index for all substances in the waste that induce deterministic responses is

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calculated taking into account all potentially affected organs and all responses of concern. A risk index for each organ at risk is calculated, taking into account all substances affecting a given organ, the maximum risk index for any organ is selected, and the result is truncated to the nearest integer value to obtain the risk index for all substances in the waste that cause deterministic responses. This calculation must take into account that a given chemical can affect more than one organ. As indicated in Table 7.6, all hazardous chemicals in electric arc furnace dust are assumed to induce deterministic responses. The possible responses include renal toxicity, effects on the cardiovascular system, dermal or ocular effects, decrease in body weight, hepatic toxicity, and respiratory toxicity. Decrease in body weight is not a response in a particular organ but is assumed to be a health effect of concern. All deterministic responses are assumed to be induced by more than one chemical in the waste. Furthermore, some of the chemicals (barium, beryllium, chromium, and lead) are assumed to induce all responses. Results of the calculations of organ- and endpoint-specific risk indexes for the substances that cause deterministic responses in the waste are shown in Table 7.8. For each substance and organ or endpoint of concern, the calculated dose in the numerator of the risk index is the product of the arithmetic mean concentration in the waste in Table 7.5 and the intake rate per unit concentration in the waste of about 4 ⳯ 10ⳮ8 mg (kg d)ⳮ1 per ppm obtained as described in the previous section. The acceptable dose of each deterministic substance assumed for the purpose of classifying the waste as lowhazard or high-hazard is a factor of 10 higher than RfD given in Table 7.6 (see Section 7.1.7.1 and Table 7.1). When a hazardous substance affects more than one organ, the same RfD is used to calculate the risk index for all such organs or endpoints. This assumption is conservative when RfD is based on the lowest threshold for deterministic responses in any organ and the thresholds for responses in other organs are higher. The results in Table 7.8 indicate that the organ- and endpointspecific risk indexes are about 0.7 to 0.8 in all cases, due mainly to intakes of lead. The maximum risk index for any organ or endpoint is about 0.8. Truncating this result using the INTEGER function, as indicated in Equation 6.5, gives a risk index for all deterministic hazardous chemicals in the waste of zero. This result means that the calculated dose in all organs and for all endpoints due to exposure to all deterministic substances that cause deterministic responses in the waste is less than the assumed acceptable dose of 10 times RfDs. Therefore, based only on consideration of substances that

a

⳯ ⳯ ⳯ ⳯ ⳯ ⳯ 10ⳮ3 10ⳮ6 10ⳮ2 10ⳮ3 10ⳮ1 10ⳮ4

7.2 ⳯ 10ⳮ1

4.5 ⳯ 10ⳮ5

5.9 1.6 1.1 8.2 6.9 2.7

Renal

7.0 ⳯ 10ⳮ1

8.2 ⳯ 10ⳮ3 6.9 ⳯ 10ⳮ1

1.2 ⳯ 10ⳮ5 1.6 ⳯ 10ⳮ6

1.7 ⳯ 10ⳮ3

Cardiovascular

Calculations are described in Section 7.1.7.4.

Organ total

Antimony Arsenic Barium Beryllium Cadmium Chromium (VI) Lead Mercury Nickel Selenium Silver Vanadium Zinc

Toxic Element

7.1 ⳯ 10ⳮ1

2.4 ⳯ 10ⳮ5

8.2 ⳯ 10ⳮ3 6.9 ⳯ 10ⳮ1

2.9 ⳯ 10ⳮ3 5.9 ⳯ 10ⳮ3 1.6 ⳯ 10ⳮ6

Dermal and Ocular

7.1 ⳯ 10ⳮ1

2.1 ⳯ 10ⳮ3

6.7 ⳯ 10ⳮ4

8.2 ⳯ 10ⳮ3 6.9 ⳯ 10ⳮ1

5.9 ⳯ 10ⳮ3 1.6 ⳯ 10ⳮ6

Weight Decrease

Contribution to Organ or Endpoint Risk Index

7.0 ⳯ 10ⳮ1

1.0 ⳯ 10ⳮ5

8.2 ⳯ 10ⳮ3 6.9 ⳯ 10ⳮ1

5.9 ⳯ 10ⳮ3 1.6 ⳯ 10ⳮ6

Hepatic

TABLE 7.8—Deterministic risk index components for electric arc furnace waste.a

8.3 ⳯ 10ⳮ1

1.3 ⳯ 10ⳮ1 6.9 ⳯ 10ⳮ1

5.9 ⳯ 10ⳮ3 1.6 ⳯ 10ⳮ6

Respiratory

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cause deterministic responses and the assumptions about intakes of hazardous chemicals and acceptable doses, this waste could be classified as low-hazard. 7.1.7.5 Stochastic Risk Index for Hazardous Chemical Constituents. Calculation of the risk index for all hazardous chemicals in the waste that cause stochastic effects is performed in the same manner as in the previous examples for radioactive wastes. The calculated risk for each such substance, based on the assumed exposure scenario, is summed and then divided by the acceptable lifetime risk of 10ⳮ3 for classification as low-hazard waste (see Table 7.1). The risk for each chemical is calculated by multiplying the arithmetic mean of the concentration in the waste given in Table 7.5 by the intake rate from ingestion, inhalation, or dermal absorption per unit concentration discussed in Section 7.1.7.3 and 10 percent of the appropriate slope factor in Table 7.7 (see Section 7.1.7.1) adjusted for the exposure time. Since the slope factors assume chronic lifetime exposure, they must be reduced by a factor of 70 based on the assumption that the exposure scenario at the hazardous waste site occurs only once over an individual’s lifetime. In addition, a simplifying assumption is made that whenever more than one slope factor is given for a hazardous substance in Table 7.7, the higher value was applied to the total intake rate by all routes of exposure of about 4 ⳯ 10ⳮ8 mg (kg d)ⳮ1 per ppm. This assumption should be conservative. Based on the assumptions described above, the results of the calculation of stochastic risk for all hazardous chemicals in the waste are shown in Table 7.9. From the calculated lifetime risk of 1.7 ⳯ 10ⳮ5 and the assumed acceptable risk of 10ⳮ3, the risk index for all hazardous chemicals that cause stochastic effects is (1.5 ⳯ 10ⳮ5)/10ⳮ3, or about 0.02. Thus, based only on consideration of these substances, the waste would be classified as low-hazard. TABLE 7.9—Stochastic risk for toxic metals in electric arc furnace waste. Chemical

Arsenic Beryllium Cadmium Chromium (VI) Nickel Total stochastic risk a

Slope Factora [mg (kg d)ⳮ1]

1.5 0.84 0.63 4.2 0.084

Stochastic Risk

1.8 3.8 9.5 1.4 1.6

⳯ ⳯ ⳯ ⳯ ⳯

10ⳮ7 10ⳮ10 10ⳮ7 10ⳮ5 10ⳮ7

1.5 ⳯ 10ⳮ5

Values taken from Table 7.7 and multiplied by 0.1 (see Section 7.1.7.1).

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7.1.7.6 Stochastic Risk Index for Radionuclides. NRC (1996b) has proposed that certain wastes that contain incidental amounts of radionuclides be allowed for disposal in facilities for hazardous chemical wastes permitted under Subtitle C of RCRA. The wastes included in the NRC proposal are those related to emission control dust contaminated with 137Cs as a result of inadvertent melting of a 137Cs source in scrap metal processed in an electric arc furnace or at a foundry. NRC proposed that packaged wastes that contain up to 4.8 Bq gⳮ1 of 137Cs or bulk waste that contains up to 3.7 Bq gⳮ1 of 137 Cs may be sent to Subtitle C disposal facilities. NRC or an Agreement State would be required to monitor such disposition to ensure that no Subtitle C facility receives more than 37 GBq of 137Cs from all such sources. To classify this mixed radioactive and hazardous chemical waste based on risk, a composite risk index giving the sum of the risk indexes for chemicals that cause deterministic effects, chemicals that cause stochastic effects, and radionuclides must be evaluated. The first two elements of the composite risk index were evaluated in Sections 7.1.7.4 and 7.1.7.5, respectively. In this Section, the stochastic risk index for 137Cs in the waste is computed. A relatively simple calculation can be used to bound the risk from exposure to 137Cs in the electric arc furnace waste. The disposed waste is assumed to contain 4.8 Bq gⳮ1 of 137Cs, which is the maximum allowable concentration in packaged waste. This concentration is conservative because it assumes that exposure would occur at the time of disposal and it does not take into account the uncontaminated material that would be mixed with the waste after disposal. When a radionuclide emits high-energy photons, which is the case for 137Cs and its short-lived decay product 137mBa in activity equilibrium, external exposure is by far the most important and intakes by inhalation and soil ingestion are negligible; this assumption was verified by calculations using the RESRAD code. The external dose to an inadvertent intruder who is assumed to be exposed to uncovered waste for a period of 1,000 h at the time of facility closure can be estimated as follows. For a 137 Cs source assumed to be uniformly distributed in surface soil with its decay product 137mBa in activity equilibrium, and taking into account the decay branching fraction of 0.946 (Kocher, 1981), the external dose rate per unit concentration is 2.9 ⳯ 10ⳮ11 Sv sⳮ1 per Bq gⳮ1 (Eckerman and Ryman, 1993). Multiplying this external dose coefficient by the assumed concentration of 137Cs (4.8 Bq gⳮ1) and exposure time (1,000 h) gives a total dose for the assumed scenario of 5 ⳯ 10ⳮ4 Sv.

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The stochastic risk corresponding to the estimated dose is obtained using the nominal probability coefficient for fatal cancers of 0.05 Svⳮ1 (ICRP, 1991; NCRP, 1993a). Since this coefficient is intended to represent a best estimate, rather than a conservative upper bound, the value is not increased by a factor of 10, as in the adjustment of the slope factors for chemicals that induce stochastic effects (Section 7.1.7.5). Therefore, the calculated stochastic risk due to 137Cs in the waste is (5 ⳯ 10ⳮ4 Sv)(5 ⳯ 10ⳮ2 Svⳮ1) ⳱ 2.5 ⳯ 10ⳮ5. Given the assumption that an acceptable stochastic risk from disposal in a hazardous waste facility is about 10ⳮ3 (see Table 7.1), the stochastic risk index due to the presence of radionuclides in the electric arc furnace waste is (2.5 ⳯ 10ⳮ5)/10ⳮ3 ⳱ 0.025. Since this result is much less than unity, the waste clearly would be classified as low-hazard due only to the presence of 137Cs, and there is no need to perform a less conservative analysis. 7.1.7.7 Calculation of the Composite Risk Index. The composite risk index for the chemical and radioactive components of electric arc furnace waste is given by: RI ⳱ RI d (chemicals) Ⳮ RI s (chemicals) Ⳮ RI s (radionuclides).

(7.1)

Using the separate risk indexes for chemicals that cause deterministic effects, chemicals that cause stochastic effects, and radionuclides obtained in Sections 7.1.7.4 to 7.1.7.6, the composite risk index for all hazardous substances in the waste is given by: RI ⳱ 0 Ⳮ 0.02 Ⳮ 0.025 ⳱ 0.045.

(7.2)

This result is much less than unity. Therefore, based on the assumptions used in this analysis, the electric arc furnace waste would be classified as low-hazard. 7.1.7.8 Consideration of Alternative Assumptions. Two aspects of the example analysis for electric arc furnace waste warrant further consideration. The first is the assumption that an acceptable dose of each chemical that induces deterministic effects is 10 times its RfD. The second is the assumption that exposures would occur only once over a lifetime and for a period of 1,000 h. The analysis for chemicals that induce deterministic effects presented in Section 7.1.7.4 and summarized in Table 7.8 indicates that lead is the most important such constituent. Furthermore, the risk index for lead of about 0.7 is only marginally below the value of unity used to define the boundary between low-hazard and highhazard waste. Therefore, the assumption that an acceptable dose of

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lead would be 10 times its RfD is important in classifying this waste. This assumption may not be warranted given (1) the possibility that the assumed RfD in Table 7.6 is based on outdated information on the dose-response relationship, (2) the small safety and uncertainty factor typically used in deriving an RfD from a NOAEL or LOAEL when the data are obtained from studies in humans (see Sections 3.2.1.2.4 and 3.2.1.2.5), and (3) the heightened interest by regulators and the public in reducing levels of lead in children, as evidenced by EPA’s current assumption that there is essentially no threshold for induction of adverse effects in the young (IRIS, 1988). For example, if an acceptable dose of lead were assumed to be three times its RfD, the waste would be classified as high-hazard based on this analysis. As an alternative to the assumption of a one-time exposure for 1,000 h at the time of facility closure, permanent occupancy of a disposal site following loss of institutional control could be assumed (see Section 7.1.3.4). The assumption of chronic lifetime exposure would affect the analysis for hazardous chemicals that induce deterministic effects only if estimated intakes due to additional pathways, such as consumption of contaminated vegetables or other foodstuffs produced on the site, were significant. Based on the results for lead in Table 7.8, an intake rate from additional pathways of about 50 percent of the assumed intake rate by soil ingestion, inhalation, and dermal absorption would be sufficient to increase the deterministic risk index above unity. The importance of additional pathways was not investigated in this analysis, but they clearly would warrant consideration. The increase in exposure time during permanent occupancy does not otherwise affect the analysis for chemicals that induce deterministic effects, provided RfDs are appropriate for chronic exposure, because chronic RfDs incorporate an assumption that the levels of contaminants in body organs relative to the intake rate (dose) are at steady state. For substances that induce stochastic effects, an assumption of permanent site occupancy would increase the lifetime risk in proportion to the increase in exposure time. The risk indexes for chemicals that induce stochastic effects and radionuclides assuming 1,000 h of exposure obtained previously are 0.02 and 0.025, respectively. If permanent occupancy of a disposal site were assumed to occur after 100 y of institutional control, the risk index for radionuclides would be reduced by a factor of 10, due to the half-life of 137Cs. Then, if exposure were assumed to occur for 4,000 h yⳮ1 (about half of the time during a year) for a period of 30 y (see Table 7.1), the estimated stochastic risk index would be about 0.02 ⳯ (4,000/1,000) ⳯ 30 ⳱ 2.4, due mainly to the chemicals that induce stochastic effects. The

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risk index would be higher if other exposure pathways, such as consumption of contaminated vegetables, were included. This result indicates that the waste would be classified as high-hazard under the assumed conditions of unrestricted release of a disposal site. Based on the example analysis for electric arc furnace waste, the use of different assumptions about exposure scenarios or allowable doses of chemicals that induce deterministic effects could result in a difference in the resulting classification of the waste. This example thus illustrates the importance of judgment in classifying waste.

7.1.8 Hazardous Chemical Waste Okrent and Xing (1993) analyzed the cancer risk resulting from inadvertent intrusion into a RCRA facility for hazardous chemical waste. The facility was assumed to contain waste from production of veterinary pharmaceuticals and other wastes that resulted in concentrations of 1,000 mg kgⳮ1 of arsenic and 100 mg kgⳮ1 of beryllium, cadmium, chromium, and nickel. A scenario for inadvertent intrusion involving permanent site occupancy similar to the scenario used by NRC to develop the Class-A, -B, and -C limits for near-surface disposal of radioactive waste (NRC, 1982b) was used to estimate the human health consequences of the postulated intrusion. Okrent and Xing (1993) estimated the lifetime cancer risk to a future resident at a hazardous waste disposal site after loss of institutional control. The assumed exposure pathways involve consumption of contaminated fruits and vegetables, ingestion of contaminated soil, and dermal absorption. The slope factors for each chemical that induces stochastic effects were obtained from the IRIS (1988) database and, thus, represent upper bounds (UCLs). The exposure duration was assumed to be 70 y. Based on these assumptions, the estimated lifetime cancer risk was 0.3, due almost entirely to arsenic. If the risk were reduced by a factor of 10, based on the assumption that UCLs of slope factors for chemicals that induce stochastic effects should be reduced by this amount in evaluating waste for classification as low-hazard (see Section 7.1.7.1), the estimated risk would be reduced to 0.03. Either of these results is greater than the assumed limit on acceptable risk of 10ⳮ3 (see Table 7.1). Thus, based on this analysis, the waste would be classified as high-hazard in the absence of perpetual institutional control to preclude permanent occupancy of a disposal site. Many of the examples presented in Sections 7.1.3 through 7.1.7 incorporate an assumption that exposures of inadvertent intruders would occur only for a period of 1,000 h during a single year. This

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assumption is based on the possibility that an intruder would recognize the nature of the waste or that occasional monitoring and surveillance of the site would effectively preclude permanent occupancy. The scenario involving a one-time exposure over 1,000 h does not include the exposure pathway involving consumption of contaminated vegetables and fruits. The analysis by Okrent and Xing (1993) can be adjusted to represent this scenario in the following way. The estimated risk due only to exposure by dermal contact and soil ingestion over 70 y obtained by Okrent and Xing is 0.12. Reducing this estimate by a factor of 70 to account for the assumption of exposure during a single year and by a factor of 1,000/8,760 to account for the fraction of the year during which exposure occurs gives an estimated risk of 2 ⳯ 10ⳮ4. This estimate would be reduced by a factor of 10 if the slope factor were adjusted to represent a best estimate, rather than a UCL. Either of these estimates is less than the assumed limit on acceptable risk of 10ⳮ3. As in the example of electric arc furnace waste in the previous section, this result for a hazardous waste that contains heavy metals indicates the importance of an intention to maintain perpetual institutional control over hazardous waste disposal sites in allowing the waste to be classified as low-hazard.

7.1.9 Discussion of Example Analyses The example analyses for electric arc furnace dust in Section 7.1.7 and a hazardous chemical waste in Section 7.1.8 lead to an important conclusion about these particular wastes. The concentrations of heavy metals, especially lead, in the electric arc furnace waste clearly are sufficiently high that long-term exposure to the waste by an inadvertent intruder may need to be precluded in order to ensure that deterministic effects would not occur. In addition, for either waste, the stochastic risk that could result from unrestricted release of a disposal site might exceed acceptable levels, due to the concentrations of heavy metals that induce stochastic effects. Both of these factors indicate that these wastes may be classifiable as low-hazard only if perpetual control would be maintained over near-surface disposal sites to prevent long-term exposures of inadvertent intruders. Such a conclusion also was obtained in the example of uranium mill tailings discussed in Section 7.1.5. It is not NCRP’s intent to develop specific recommendations concerning classification of wastes based on the example analyses discussed in Sections 7.1.3 through 7.1.8. This is especially the case when wastes contain high concentrations of heavy metals (NCRP

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did not investigate the classification of wastes that contain high concentrations of organic hazardous chemicals). Rather, the intent of the example analyses is to indicate the importance of assumptions about exposure scenarios and allowable doses or risks in classifying waste. These assumptions are largely matters of judgment.

7.2 Legal and Regulatory Ramifications If implemented, the risk-based waste classification system presented in this Report would have impacts on the current waste classification systems for radioactive and hazardous chemical wastes. While it is expected that most wastes would be classified in accordance with current plans for their disposal, there would be some notable impacts on waste classification and waste management.

7.2.1 Establishment of an Exempt Waste Class The most profound change in waste classification that would result from implementation of the system proposed in this Report would be the establishment of an exempt class of waste. This class would be defined based on the principle that waste could be regulated as if it were nonhazardous if the hazardous constituents were present in amounts sufficiently low that the risk from disposal would be negligible (de minimis). At present, EPA has not established general provisions for exemption of listed hazardous chemical wastes regulated under RCRA, and efforts by NRC to establish general conditions for exemption of radioactive wastes were halted at the direction of Congress. The establishment of general exemption principles would provide a major incentive for generators to modify or create processes that result in smaller volumes of concentrated hazardous wastes and larger volumes of exempt wastes, with disposition of the latter potentially ranging from disposal in municipal/industrial landfills for nonhazardous waste to recycle or reuse as laws and regulations permit. This approach to waste management should be more cost-effective by allowing exempt materials to be managed at considerably less cost, commensurate with the risks they pose. It would also have other environmental benefits associated with reducing the need to acquire fresh resources, thus heading in the direction of sustainability.

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7.2.2 Elimination of Source-Based Waste Classifications In a risk-based hazardous waste classification system, waste would be classified based on its intrinsic characteristics. Radioactive wastes (most notably high-level waste) presently are classified based essentially on their source, and this would change in a risk-based classification system. In the case of high-level waste, this change should have few significant impacts, because much of what is now in this waste class would be classified as high-hazard waste and still require disposal in a geologic repository. Some wastes from fuel reprocessing and reprocessing wastes that are old or have had a significant fraction of the radionuclides removed might be classified as low-hazard waste. Similarly, some waste that is not presently classified as highlevel waste would probably be classified as high-hazard waste because of its relatively high concentrations of radionuclides; an example is waste now classified as greater-than-Class-C low-level waste. Most transuranic radioactive waste probably would be classified as high-hazard waste, but this would have no effect on management practices because most transuranic waste is destined for geologic disposal. Elimination of source-based waste classifications would also have some impact on classification and management of hazardous chemical wastes. For example, the identification of some listed hazardous wastes under RCRA based on the source of the waste (the ‘‘F ’’ and ‘‘K’’ lists) and the distinction between hazardous wastes regulated under RCRA and those regulated under TSCA would be eliminated. The most significant impact of eliminating source-based waste classifications is likely to be in the area of classifying and managing large-volume NORM wastes from mining and processing of mineral ores to extract nonradioactive materials. These wastes presently are classified and managed separately from uranium and thorium mill tailings having similar properties, and many NORM wastes essentially are unregulated. As in the case of mill tailings, many of these wastes could require special consideration because deep disposal or disposal in near-surface facilities currently used for radioactive or hazardous chemical wastes might be impractical. Thus, the distinction between large-volume low-hazard wastes and other low-hazard radioactive and chemical wastes would need to continue under a riskbased classification system, and this distinction could be specified by subclassification of low-hazard waste. 7.2.3 Recognition of Permanent Disposal of Hazardous Chemical Wastes Present methods of disposal of hazardous chemical wastes involve emplacement in a near-surface facility with the stipulation that

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(1) the facility be actively maintained and monitored for as long as the waste remains hazardous, (2) leachate be collected and treated, and (3) the future of the facility be considered at 30 y after closure. The long-term performance of a facility is not analyzed when it is permitted and waste acceptance criteria are established. As a consequence, there is no assurance that hazardous waste facilities will be acceptable for permanent disposal of the potentially longlived chemical wastes they contain. The approach to near-surface disposal of radioactive wastes is essentially the opposite of this. These facilities are planned to be permanent disposal sites, with waste acceptance criteria established at each site that would ensure adequate long-term protection of the public (including inadvertent intruders) in the absence of institutional control, which is assumed to cease at about 100 y. A risk-based waste classification system would be established by focusing on risks that arise from disposal of hazardous wastes. Thus, the amounts of hazardous chemical wastes that would be acceptable for near-surface disposal over the longer term would need to be evaluated. While NCRP believes that many hazardous chemical wastes would continue to be acceptable for near-surface disposal, it should be anticipated that this will not be the case for some wastes that contain high concentrations of heavy metals; e.g., see Okrent and Xing (1993). As a result, some hazardous chemical wastes could be classified as high-hazard (see next section), and such a classification also could also mean that perpetual institutional control will be required at some existing burial sites.

7.2.4 Establishing the Potential for High-Hazard Chemical Wastes At the present time, there is essentially only one class of hazardous chemical waste (i.e., a waste either is hazardous or it is not), without regard for the amounts of hazardous substances in the waste. Establishment of a risk-based waste classification system would allow for the possibility of two classes of hazardous chemical waste based on the amounts of hazardous substances, consistent with the present situation for radioactive waste, with the attendant implication that high-hazard chemical waste that contains the highest amounts of hazardous substances would require a disposal technology substantially more isolating than a near-surface system. High-hazard chemical waste could result from relatively high concentrations of hazardous organic chemicals (e.g., dioxins) or persistent toxic substances (e.g., heavy metals). Some wastes may be

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rendered less hazardous by treatment, such as incineration of some wastes that contain organic chemicals, but other wastes probably cannot be treated effectively to reduce the long-term hazard. NCRP did not investigate in any detail the amounts of hazardous chemical waste that might be classified as high-hazard. However, based on the study by Okrent and Xing (1993) discussed in Section 7.1.8 and previous efforts to develop a category of extremely hazardous chemical waste (see Section 4.2.1.3), NCRP expects that some amount of hazardous chemical waste would be classified as highhazard. This waste may require a disposition significantly different from the present practice of emplacement in near-surface facilities. Deeper disposal facilities for solid hazardous chemical wastes do not presently exist in the United States, and there are no plans for their development. In considering options for deeper disposal of highhazard chemical wastes, the costs and benefits would need to weighed against the monetary costs and health risks associated with maintaining perpetual institutional control at near-surface disposal facilities.

7.2.5 Elimination of the Mixed Waste Category Differences in current approaches to management of hazardous chemical and radioactive wastes come into full play in the case of mixed waste, with the result being major procedural and institutional impediments to effective management of these wastes. Either of two major features of a risk-based waste classification system would essentially eliminate these impediments. In some cases, establishment of an exemption principle could allow either the chemical or radioactive component of mixed waste to be exempt from regulation as hazardous waste. These wastes then would be considered hazardous by virtue of their radioactive or hazardous chemical constituents but not both (the waste would no longer be ‘‘mixed’’). As a result, the wastes could be managed in a relatively straightforward manner. Beyond this, the full impact of risk-based waste classification would be the elimination of regulatory differences in approaches to management of radioactive and hazardous chemical wastes. This should lead to a single, consistent set of regulations for management and disposal of hazardous waste, regardless of whether it is classified as such because of the presence of hazardous chemicals, radionuclides, or both. If this is achieved, the issue of mixed waste becomes moot. Achieving the minimum benefits of exemption would require few changes in the existing regulatory infrastructure other than those

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noted in Section 7.2.1. Achieving the full benefits of risk-based waste classification would require significant changes in existing regulations to eliminate important differences in approaches to risk assessment and risk management in general, approaches to treatment and characterization of waste, and approaches to ensuring adequate longterm protection of the public, including inadvertent intruders, from disposal in near-surface facilities.

7.2.6 Elimination of the Category of Waste Containing Naturally Occurring and Accelerator-Produced Radioactive Material NARM waste largely falls outside the existing federal infrastructure for management of radioactive wastes (see Section 4.1.2.4), except when this type of waste is the responsibility of a federal agency (e.g., DOE). Establishment of a risk-based waste classification system would necessarily include NARM waste within the same regulatory structure as other radioactive wastes. NCRP expects that much of this waste, especially diffuse NORM waste that contains relatively low concentrations of naturally occurring radionuclides, would be classified as exempt or low-hazard waste, resulting in very little change in current waste management practices. However, some discrete NARM wastes that contain relatively high concentrations of long-lived alpha-emitting radionuclides (e.g., radium) might be classified as high-hazard waste and could require a more isolating disposal technology than near-surface disposal. Perhaps the greatest impact of establishing a comprehensive waste classification system on management of NARM waste would be to encourage the development of a consistent set of regulatory requirements for all such waste, instead of the variety of federal and state regulations for these wastes that exist at the present time.

7.2.7 Impact on Subclassification of Waste Classes Existing hazardous waste classification systems frequently include subclassifications of basic waste classes to facilitate waste management (see Sections 2.2.4, 4.1.2 and 6.6). Examples include Class-A, -B, and -C commercial low-level waste and remotelyhandled and contact-handled transuranic waste. These waste subclassifications are not expected to be significantly affected by a riskbased classification system unless particular wastes would not be generally acceptable for the disposal using the intended technology. For example, there is no inherent incompatibility with the system

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for classifying Class-A, -B, and -C low-level waste unless the waste would be classified as exempt or high-hazard based on its intrinsic characteristics. Transuranic waste could be a subclass of highhazard waste based, for example, on the heat generation rate (see Section 4.1.3.1), and transuranic waste could be further subclassified as remotely-handled or contact-handled based on the need to protect workers. If a waste were reclassified at the highest level (e.g., from low-level to exempt), then existing subclassifications would be affected or, more likely, would become moot.

8. Conclusions and Recommendations

8.1 Conclusions Based on the background information presented in Sections 2 through 5 and the discussions on development of a new hazardous waste classification system in Sections 6 and 7, NCRP has reached the following conclusions: ●

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Despite the best efforts of pollution prevention and recycling programs, hazardous wastes are being, and will continue to be, generated. Classification of hazardous waste is necessary for cost-effective waste management. The most appropriate primary basis for classification of hazardous waste is the risk to human health posed by waste. Furthermore, the health risks of primary concern in classifying hazardous wastes are risks to the public that arise from waste disposal, since permanent disposal is the intended disposition of most waste materials having no further use to their present custodian. Although the existing classification systems for radioactive and hazardous chemical wastes have worked adequately in many respects, they have resulted in a number of undesirable outcomes, such as excessive costs in managing and disposing of some wastes, considerable difficulties in managing and disposing of mixed wastes, an increasing need to accommodate exceptional wastes that were not considered when the classification systems were developed, and unwarranted neglect of some potentially important wastes. Existing systems are deficient primarily because most are not based on risk, the collective system does not unambiguously classify wastes, and some potentially important wastes are not given due consideration. Classification of waste based on risk requires assessments of health risks posed by waste. While many aspects of risk assessment are the same for radionuclides and hazardous chemicals, there are important differences that complicate the establishment of a comprehensive and risk-based waste classification 354

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system. The most important of these differences are summarized as follows: – Dose-response relationships for substances that induce stochastic effects: best estimates (MLE) for radionuclides but upper bounds (UCLs) for chemicals that induce stochastic effects; – Safety and uncertainty factors used in establishing exposure limits that are intended to prevent deterministic responses: normally much larger for hazardous chemicals that induce deterministic effects than for radionuclides; – Primary measure of stochastic response: cancer fatalities for radionuclides but cancer incidence for hazardous chemicals that induce stochastic effects; and – Accounting of organs at risk: all organs and tissues at risk for radionuclides but estimates of risk for chemicals that induce stochastic effects based usually on observed responses in a single organ in laboratory animals. Of these, the difference between best estimates of dose-response relationships for radionuclides but UCLs for chemicals that induce stochastic effects is the most significant for risk-based waste classification. The eventual use of best estimates of doseresponse relationships and incidence of health effects as the primary measure of response is preferred by NCRP, although an acceptable interim waste classification system might be established using different approaches for radionuclides and chemicals that induce stochastic effects. Differences in the accounting of organs and tissues at risk are not expected to be important because, in contrast to radionuclides, it is unlikely that many chemicals that induce stochastic effects would induce health effects in several organs with a significant probability. Classification of waste based on risk requires assumptions about allowable risks from various waste dispositions, i.e., decisions about approaches to risk management. The paradigms for managing risk from exposure to radionuclides and hazardous chemicals are fundamentally different. After a practice is justified in terms of a positive net benefit to society, risks from exposure to radionuclides are managed by establishing a limit on acceptable (barely tolerable) dose (and, therefore, risk) and then requiring that doses (risks) be reduced below the limit ALARA. In contrast, risks from exposure to hazardous chemicals are managed by establishing goals for acceptable (negligible) risk and allowing an increase (relaxation) of risks above the goals based on the specific circumstances of a particular exposure situation (e.g.,

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cost-benefit). This difference has resulted in considerable confusion and misunderstanding about risks that are ‘‘acceptable’’ and those that are ‘‘unacceptable.’’ An ideal system for classifying hazardous wastes should be riskbased, applicable to all wastes that contain radionuclides or hazardous chemicals, internally consistent, based on intrinsic waste properties, comprehensible, quantitative, and compatible with existing or feasible data and methods. To the extent that these attributes are lacking in a waste classification system, undesirable consequences are likely to result. Given an assumption that waste materials have no further beneficial use, a risk-based hazardous waste classification system should focus on classification of waste for purposes of disposal. Therefore, waste should be classified in relation to one of the three types of disposal systems (technologies) in current use or under development that are expected to be generally acceptable in protecting public health: municipal/industrial landfills; dedicated near-surface facilities for hazardous wastes; and deeper, highly isolating facilities, such as a geologic repository. Materials containing very low concentrations of hazardous substances also could be considered for other dispositions. The framework for a risk-based waste classification system should include three classes of waste: exempt waste, which would be managed as if it were nonhazardous and would be generally acceptable for disposal in a municipal/industrial landfill; low-hazard waste, which is any nonexempt waste that would be generally acceptable for disposal in a dedicated near-surface facility for hazardous wastes; and high-hazard waste, which would generally require disposal in a facility considerably more isolating than a near-surface facility for hazardous wastes. Exempt wastes also could be considered for recycling and beneficial use in commerce, consistent with laws and regulations governing allowable dispositions of nonhazardous materials. The boundaries between waste classes should be quantified by use of a risk index, which essentially is the ratio of a calculated risk from disposal of a hazardous waste using a generic type of disposal system to an allowable risk appropriate to the assumed disposal system. In establishing the boundary between exempt and low-hazard waste, the allowable risk should be based on a negligible stochastic risk, and the deterministic risk should be zero with an ample margin of safety. In establishing the boundary between low-hazard and high-hazard waste, the allowable risk should be based on an acceptable (barely tolerable) stochastic risk substantially above a negligible risk and a deterministic

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risk that is expected to be zero but with less conservatism than in the boundary between exempt and low-hazard waste. The boundaries between waste classes normally would be expressed in terms of limits on concentrations of hazardous substances, and rules for applying the limits to waste that contains mixtures of hazardous substances would be specified. The concept of a hypothetical inadvertent intruder at a nearsurface waste disposal site, including permanent occupants of a site after an assumed loss of institutional control, provides a suitable basis for defining exposure scenarios that would be used to calculate risks that arise from waste disposal and the boundaries between waste classes. For other dispositions of waste, alternative scenarios would need to be developed and evaluated. As with existing waste classification systems, a risk-based waste classification system should be flexible and continue to include provisions for regulators to make exceptions on a case-by-case basis with appropriate due process. Development of a comprehensive and risk-based hazardous waste classification system, in which waste classes are defined in relation to types of disposal systems that are expected to be generally acceptable in protecting public health, would not obviate the need to establish waste acceptance criteria at each disposal site based on the characteristics of the site and engineered disposal facility and the properties of wastes intended for disposal therein. The primary purposes of a hazardous waste classification system are to facilitate cost-effective management and disposal of waste and effective communication on waste matters. Establishing subclassifications of the basic waste classes to facilitate waste management and disposal is likely to be desirable. Subclassifications of basic waste classes should be based on considerations of risk and could take into account, for example, differences in engineered systems required to manage wastes in the same class with different physical, chemical or radiological properties. Consideration of cost-benefit in managing and disposing of wastes that pose similar risks but have greatly different volumes also would be important in subclassifying basic waste classes. A risk-based waste classification system should include explicitly justified degrees of conservatism in protecting public health. Differences in the meanings of commonly used terms between the radiation and hazardous chemical communities presently

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constitute a significant impediment to establishing a comprehensive and risk-based hazardous waste classification system. The most important example, as noted above, is the different meanings attached to the terms ‘‘acceptable’’ and ‘‘unacceptable’’ in describing risks. Examples of how existing wastes might be classified under the risk-based system recommended in this Report, based on suitable precedents for defining allowable risks associated with different types of disposal systems, indicate that many hazardous wastes would continue to be managed essentially in the same manner as presently foreseen. However, NCRP expects that significant volumes of waste currently managed as radioactive or chemically hazardous waste could be classified as exempt, based on the low concentrations of hazardous substances they contain, thus resulting in substantially less expenditures of resources in managing these materials. NCRP also expects that some of the most hazardous chemical wastes could be classified as highhazard based on a conclusion that they may not be generally acceptable for near-surface disposal. In a risk-based hazardous waste classification system, classification and disposal of uranium mill tailings would continue to require special considerations, due to the high concentrations of radium and emanation rates of radon compared with average soil and rock and the very large waste volumes. NCRP believes that most uranium mill tailings could be classified as low-hazard waste, but only under conditions of perpetual institutional control over disposal sites to preclude permanent occupancy by an inadvertent intruder. If perpetual institutional control is not assumed, most mill tailings would be classified as high-hazard waste based on the high risk to a permanent resident on a tailings pile. Regardless of how mill tailings are classified, however, near-surface disposal probably will continue to be the preferred option, because disposal of the very large volumes of these materials far below ground has been deemed impractical. Similar considerations could apply to large volumes of other ore processing wastes that contain highly elevated levels of naturally occurring radionuclides or hazardous chemicals.

8.2 Recommendations As a result of the foregoing conclusions, NCRP recommends that the present classification systems for radioactive and hazardous

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chemical wastes be replaced over time by the comprehensive and risk-based hazardous waste classification system described in this Report. Such a replacement would need to be undertaken carefully and in recognition that existing systems for waste classification and waste management, despite their shortcomings, have been adequate in protecting human health. In establishing a new hazardous waste classification system that would be an improvement on the existing systems, there is a need to ensure that current approaches to management and disposal of radioactive and hazardous chemical wastes would not be unduly disrupted. The principal elements of an approach to establishing the hazardous waste classification system recommended in this Report should include the following: ● ●

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Use of the foundations and framework for risk-based waste classification as a blueprint for an improved classification system; A forthright approach to establishing such a system by planning the entire process at the outset, included needed changes in existing laws and regulations, as well as additional needs of data or scientific understanding; A striving to embody all the desired attributes of the new system, while recognizing that this may take many years and that a number of important benefits can be obtained by interim implementation of parts of the system. The most important areas in which interim implementations are likely to be beneficial include the establishment of exemption levels for radionuclides and hazardous chemicals in waste, to allow hazardous wastes to be managed as nonhazardous material or to allow mixed waste to be managed as radioactive or hazardous chemical waste only, and the elimination of source-based definitions of hazardous wastes, especially radioactive wastes. Ensuring that a new hazardous waste classification system, although it may be an improvement over existing classification systems in its principles and approaches to waste classification, does not become so rigid as to preclude the types of judgment and flexibility that are needed to accommodate the special situations and practicalities that will inevitably arise.

Glossary absorbed dose, chemicals: The amount of a substance crossing a specific absorption barrier (e.g., the exchange boundaries of the gastrointestinal tract, lungs, and skin) through uptake processes. absorbed dose, ionizing radiation (D): The quotient of d⑀¯ by dm, where d⑀¯ is the mean energy imparted by ionizing radiation to the matter in a volume element and dm is the mass of the matter in that volume element: D ⳱ d⑀¯ /dm. For purposes of radiation protection and assessing health risks in general terms, the quantity normally calculated is the average absorbed dose in an organ or tissue (T): DT ⳱ d⑀¯ T/mT, where d⑀¯ T is the total energy imparted in an organ or tissue of mass mT. The SI unit of absorbed dose is the joule per kilogram (J kgⳮ1), and its special name is the gray (Gy). In conventional units often used by federal and state agencies, absorbed dose is given in rads; 1 rad ⳱ 0.01 Gy. activity: The rate of transformation (or ‘‘disintegration’’ or ‘‘decay’’) of radioactive material. The SI unit of activity is the reciprocal second (sⳮ1), and its special name is the becquerel (Bq). In conventional units often used by federal and state agencies, activity is given in curies (Ci); 1 Ci ⳱ 3.7 ⳯ 1010 Bq. acutely toxic hazardous waste: A listed hazardous chemical waste regulated under the Resource Conservation and Recovery Act (RCRA) and designated by Hazard Code ‘‘H’’ in 40 CFR Part 261, Subpart D (EPA, 1980b) including all ‘‘P’’ listed wastes (waste codes beginning with ‘‘P’’) and F020, F021, F022, F023, F026, and F027 listed wastes. Acutely toxic hazardous waste is subject to more stringent requirements on accumulation and generation than other types of hazardous chemical waste regulated under RCRA. administered dose, chemicals: The amount of a substance given to a test subject (human or animal), especially by ingestion or inhalation, usually for the purpose of determining dose-response relationships. agent: An active force (e.g., ionizing radiation) or substance producing an effect. Agreement State: Any state with which the U.S. Nuclear Regulatory Commission has entered into an effective licensing agreement under Section 274(b) of the Atomic Energy Act to enable the state to regulate source, special nuclear, and byproduct materials. alpha radiation: Energetic nuclei of helium atoms, consisting of two protons and two neutrons, emitted spontaneously from nuclei in the decay of some radionuclides. Alpha radiation is weakly penetrating, and can be stopped by a sheet of paper or the outer dead layer of skin. Also called alpha particle and sometimes shortened to alpha (e.g., alpha-emitting radionuclide).

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annual dose, ionizing radiation: As normally used in radiation protection, the sum of the dose received in a year from external radiation and the committed dose due to intakes of radionuclides in a year. Definition applies most often to annual effective dose or annual effective dose equivalent, but also may be applied to annual dose equivalent and annual equivalent dose when prevention of deterministic responses in individual organs or tissues is of concern. antagonistic: Situations in which the total response from simultaneous exposure of an organism to two or more hazardous agents is less than the sum of the responses from separate exposures to each agent. applied dose, chemicals: The amount of a substance in contact with the primary absorption boundaries of an organism (e.g., gastrointestinal tract, lung, skin) and available for absorption. as low as reasonably achievable (ALARA): An approach to radiation protection in which radiation exposures (both individual and collective, to the workforce and general public) are maintained as low as social, technical, economic, practical, and public policy considerations permit. ALARA is not a dose limit but is a process, which has the objective of reducing doses as far below applicable limits as reasonably achievable. Atomic Energy Act (AEA): Law passed originally in 1946 and extensively revised in 1954 that governs the production and use of radioactive materials (i.e., source material, special nuclear material, and byproduct material) for defense and peaceful purposes and the regulation of such radioactive materials to protect public health and safety. Act provides the authority for licensing of commercial nuclear activities by the U.S. Nuclear Regulatory Commission and Agreement States, and regulation by the U.S. Department of Energy of its atomic energy defense, research, and development activities. background level: Levels (e.g., concentrations) of agents, especially hazardous agents in the environment, whose occurrence is not related to human activities at a site. Background sources may be naturally occurring or man-made (e.g., global fallout from atmospheric testing of nuclear weapons). background radiation: Ionizing radiation that occurs naturally in the environment, including: cosmic radiation; radiation emitted by naturally occurring radionuclides in air, water, soil, and rock; radiation emitted by naturally occurring radionuclides in the tissues of organisms (e.g., due to ingestion or inhalation); and radiation emitted by man-made materials containing incidental amounts of naturally occurring radionuclides (e.g., building materials). In the United States, the average annual effective dose due to natural background radiation is about 1 mSv, excluding the dose due to indoor radon, and the average annual effective dose due to indoor radon is about 2 mSv. becquerel (Bq): The special name for the SI unit of activity; 1 Bq ⳱ 1 sⳮ1. below regulatory concern (BRC): Definable amounts of hazardous substances in a material such that the material can be exempted from regulations governing particular practices or sources (e.g., management and disposal of hazardous wastes) based on considerations that the costs of

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regulating the materials generally are disproportionate to the low health risks to the public posed by the materials (application of ALARA principle). Amounts of hazardous substances that are BRC can depend on the particular practice or source, and they can be substantially above levels generally considered de minimis. benchmark dose, chemicals: A dose of a hazardous substance corresponding to a specified level of response in a study population, obtained by statistical fitting of a dose-response model to dose-response data. The benchmark dose often is taken to be the dose resulting in a response of 10 percent. For purposes of health protection, the lower 95 percent confidence limit of the benchmark dose often is taken as the point of departure in establishing a safe dose of a hazardous substance. best demonstrated available technology (BDAT): Technologies for treatment of hazardous materials that have been shown to yield the greatest environmental benefit among competing, practically available technologies. beta radiation: An energetic electron or positron emitted spontaneously from nuclei in the decay of some radionuclides. Beta radiation is not highly penetrating, and the highest-energy radiations can be stopped by a few centimeters of plastic or aluminum. Also called beta particle and sometimes shortened to beta (e.g., beta-emitting radionuclide). biokinetic model: A model describing the time course of the absorption, distribution, metabolism, and excretion of a substance (e.g., a drug or hazardous substance) introduced into the body of an organism. biologically effective dose, chemicals: The amount of a deposited or absorbed substance that reaches the cells or target tissues where an adverse effect occurs, or where the substance interacts with a membrane surface. biosphere: The life zone of Earth, including the lower part of the atmosphere, the hydrosphere, soil, and the lithosphere to a depth of about 2 km. buried waste: Waste that has been emplaced in a near-surface facility. byproduct material: (1) Any radioactive material (except special nuclear material) yielded in, or made radioactive by, exposure to the radiation incident to the process of producing or utilizing special nuclear material; and (2) the tailings or waste produced by the extraction or concentration of uranium or thorium from any ore processed primarily for its source material content. Ore bodies depleted by uranium solution extraction operations and which remain underground do not constitute byproduct material. cap: The soil applied over waste at the end of each working day at a landfill; a permanent layer of impervious material (e.g., clay, polyethylene or polyvinyl chloride liner) installed above the waste upon closure of a landfill. carcinogen: An agent that can cause cancer; frequently used as a synonym for stochastic agent. characteristically hazardous waste: A hazardous chemical waste regulated under the Resource Conservation and Recovery Act (RCRA) and designated by Hazard Code ‘‘D’’ in 40 CFR Part 261, Subpart C (EPA, 1980b). A waste is hazardous by characteristics if it is ignitable, corrosive, reactive, or toxic.

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commercial waste: Waste generated in any activity by a nongovernmental entity. committed dose, ionizing radiation: The dose received over a specified time period following an intake of a radionuclide by inhalation, ingestion, or dermal absorption. For adults, the committed dose usually is the dose received over 50 y; for children, the committed dose usually is the dose received from age at intake to age 70. Definition applies to committed dose equivalent, committed effective dose, committed equivalent dose, and committed effective dose equivalent. Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA): Law, also known as ‘‘Superfund,’’ passed in 1980 and amended by the Superfund Amendments and Reauthorization Act (SARA) of 1986 and later amendments, that governs federal response and compensation for unpermitted and uncontrolled releases, including threats of release, of hazardous substances to the environment. An ‘‘unpermitted’’ release is any release that is not properly regulated under other laws. An important focus of CERCLA/SARA is remediation of old, unpermitted waste disposal sites that are closed or inactive. Basic objectives of the Superfund program are to protect human health and the environment in a cost-effective manner, maintain this protection over time, and minimize the amounts of untreated waste in the environment. confidence interval: A measure of the extent to which an estimate of risk, dose, or other parameter is expected to lie within a specified interval. For example, a 90 percent confidence interval of a risk estimate means that, based on the available information, the probability is 0.9 that the true but unknown risk lies within the specified interval (see lower confidence limit and upper confidence limit). contact-handled transuranic waste: Containerized transuranic waste for which the external dose-equivalent rate at the surface of the container does not exceed 2 mSv hⳮ1. containment: The confinement of waste within a designated boundary (see isolation). contaminant: Includes, but is not limited to, any chemical element, substance, compound, or mixture, including disease-causing substances, which after release into the environment and upon external exposure, ingestion, inhalation, or assimilation into any organism, either directly from the environment or indirectly by ingestion through food chains, will or may reasonably be anticipated to cause death, disease, behavioral abnormalities, cancer, genetic mutation, physiological malfunctions including malfunctions in reproduction, or physical deformations in such organisms or their offspring. corrosive: A characteristic of solid hazardous waste regulated under the Resource Conservation and Recovery Act (RCRA) and defined in 40 CFR Part 261.22 (EPA, 1980b). A solid waste is corrosive if it (1) is aqueous and has a pH ⱕ2 or ⱖ12.5, or (2) is a liquid and corrodes SAE 1020 steel at a rate ⬎6.35 mm yⳮ1 at a test temperature of 55 °C. cosmic radiation: Ionizing radiation, including electromagnetic radiation and energetic particles, originating in space or produced by interactions of such radiation with constituents of Earth’s atmosphere.

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cover: (see cap). critical organ: The most sensitive organ to the toxic effects of a hazardous agent; the organ receiving the highest dose or experiencing the highest risk of an adverse effect from exposure to a hazardous agent. curie: (see activity). deep-well injection: The subsurface emplacement of fluids through a bored, drilled or driven well, or through a dug well whose depth is greater than the largest surface dimension. defense waste: Radioactive waste generated in any activity performed in whole or in part in support of the U.S. Department of Energy’s atomic energy defense activities. degradation rate: The rate at which a chemical is broken down in the environment by hydrolysis, photodegradation, or soil metabolism. delisting: The process of exempting a listed hazardous chemical waste, a mixture of a listed and solid waste, or a ‘‘derived-from’’ waste from requirements for regulation as hazardous waste under the Resource Conservation and Recovery Act (RCRA), as specified in 40 CFR Part 260.20 and 260.22 (EPA, 1986b). Delisting provisions do not apply to characteristically hazardous waste, which must be treated to remove any hazardous characteristics. delivered dose, chemicals: The amount of a substance available for interaction with a particular organ or cell. de manifestis: As applied to hazardous substances, a dose or risk that would generally be considered so high that action to reduce dose or risk normally should be undertaken without regard for cost or other circumstances. de minimis: As applied to hazardous substances, a dose or risk that would generally be considered negligible for any exposure situation, without regard for whether such a dose or risk is reasonably achievable for a particular source or practice. If doses or risks are below de minimis levels, efforts to control exposures generally would be unwarranted (see below regulatory concern). derived-from rule: Rule established under the Resource Conservation and Recovery Act (RCRA) in 40 CFR Part 261.3(c) (EPA, 1980b), which states that any solid waste derived from the treatment, storage, or disposal of a listed hazardous chemical waste is itself a hazardous waste, regardless of the concentrations of listed wastes, unless such waste is delisted. deterministic agent: An agent that can produce a deterministic response in organisms. deterministic response: An adverse effect on organisms for which the severity varies with the magnitude of the dose, and for which a threshold usually exists. Deterministic responses often occur relatively soon after an exposure. detriment: A measure of stochastic response from exposure to ionizing radiation which takes into account the probability of fatal cancers, the probability of severe hereditary effects, the probability of nonfatal cancers weighted by the lethality fraction, and relative years of life lost per fatal health effect (ICRP, 1991). disposal: Placement of waste in a facility designed to isolate waste from the exposure environment of humans without an intention to retrieve the waste, irrespective of whether such isolation permits recovery of the waste.

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disposal facility: The land, structures, and equipment used for the disposal of waste. disposal, geologic: Isolation of waste using a system of engineered and natural barriers at a depth of up to several hundred meters below ground in a geologically stable formation (see geologic repository). disposal, near-surface: Disposal of waste, with or without engineered barriers, on or below the ground surface, where the final protective covering is on the order of a few meters thick, or in mined openings within a few tens of meters of Earth’s surface (see land disposal facility). disposition: Reuse, recycling, sale, transfer, storage, treatment, consumption, or disposal. dosage, chemicals: Dose or dose rate normalized to body weight of an exposed organism; e.g., mg kgⳮ1 or mg (kg d)ⳮ1. dose: General term used to quantify extent of exposure to hazardous agents in assessing health risks to humans or other organisms; usually refers to administered dose or dosage (but sometimes to absorbed dose, applied dose, delivered dose, or potential dose) for hazardous chemicals, or to average absorbed dose, equivalent dose (or average dose equivalent), or effective dose (or effective dose equivalent) for ionizing radiation. dose assessment: (see exposure assessment). dose equivalent, ionizing radiation (H): The absorbed dose (D) at a point in tissue weighted by the quality factor (Q) for the type and energy of the radiation causing the dose: H ⳱ D ⳯ Q. For purposes of radiation protection and assessing health risks in general terms, and especially prior to introduction of the equivalent dose and as used by federal and state agencies, dose equivalent often refers to the average absorbed dose in an organ or tissue (T) weighted by the average quality factor (Q) for the particular type of radiation: HT ⳱ DT ⳯ Q. The SI unit of dose equivalent is the joule per kilogram (J kgⳮ1), and its special name is the sievert (Sv). In conventional units often used by federal and state agencies, dose equivalent is given in rem; 1 rem ⳱ 0.01 Sv. dose rate: Dose per unit time; often expressed as an average over some time period (e.g., a year or a lifetime) (see dosage). dose-response assessment: A determination of the relationship between the dose of a hazardous agent and the probability that a specific response will occur in an organism during its lifetime. Dose-response assessment is the second step of a risk assessment. effect: An observable change in an organ or tissue resulting from exposure to a hazardous agent (see response). effective dose equivalent, ionizing radiation (HE): The sum over specified organs and tissues (T) of the average dose equivalent in each tissue weighted by the tissue weighting factor (wT): HE ⳱ 兺 wTHT, where 兺 wT ⬅ 1 (ICRP, 1977) (now superseded by the effective dose, but often used by federal and state agencies). effective dose, ionizing radiation (E): The sum over specified organs and tissues (T) of the equivalent dose in each tissue weighted by the tissue weighting factor (wT): E ⳱ 兺 wTHT, where 兺 wT ⬅ 1 (ICRP, 1991) (supersedes the effective dose equivalent).

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effluents: Waste materials discharged into the environment. Emergency Response and Community Right-To-Know Act (ERCRA): Title III of the Superfund Amendments and Reauthorization Act (SARA) of 1986, but a free-standing title and not part of CERCLA (Superfund). Act addresses emergency planning for releases of hazardous substances, community right-to-know reporting of hazardous chemicals, and reportable quantities of emissions. engineered barrier: A man-made structure or device intended to improve the capability of a disposal facility to contain or isolate waste. equivalent dose, ionizing radiation (H): A quantity developed for purposes of radiation protection and assessing health risks in general terms, defined as the average absorbed dose in an organ or tissue (T) weighted by the radiation weighting factor (wR) for the type and energy of the radiation causing the dose: H ⳱ DT ⳯ wR (ICRP, 1991). The SI unit of equivalent dose is the joule per kilogram (J kgⳮ1), and its special name is the sievert (Sv). In conventional units often used by federal and state agencies, equivalent dose is given in rem; 1 rem ⳱ 0.01 Sv (see dose equivalent). exempt material: Material that is excluded from regulation as hazardous material. exposure: General term describing contact of an organism with a hazardous physical, chemical, radiological, or biological agent through dermal absorption, inhalation, ingestion, or external irradiation. Exposure is quantified as the amount of the hazardous agent at the exchange boundaries of an organism (e.g., gut, lungs, skin) and available to give a dose. exposure assessment: A specification of the population potentially exposed to hazardous agents and the pathways and routes by which exposure can occur, and quantification of the magnitude, duration, and timing of the exposures and resulting doses that organisms might receive; also may be referred to as dose assessment. Exposure assessment is the third step of a risk assessment. exposure pathway: The physical course a hazardous agent takes from its source to an exposed organism. exposure route: The means of intake of a substance by an organism (e.g., ingestion, inhalation, or dermal absorption). exposure scenario: A credible series of events that could result in exposure of organisms to hazardous agents (e.g., after emplacement of hazardous waste in a disposal facility and closure of the facility). external exposure, ionizing radiation: Exposure of organs or tissues of an organism due to radiation sources outside the body. extremely hazardous substance: A hazardous substance regulated under the Emergency Response and Community Right-To-Know Act (Title III of SARA) that when released at levels above its reportable quantity specified in 40 CFR Part 355 (EPA, 1987b) requires emergency notification of local and state emergency response authorities. Extremely hazardous substances could cause serious irreversible health effects from accidental releases and are most likely to induce serious acute reactions following short-term exposure; these substances have a median lethal concentration (LC50) in body tissues of ⱕ50 mg kgⳮ1 or an oral median lethal dose (LD50) of ⱕ25 mg kgⳮ1.

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367

More generally, any hazardous substance having unusually high toxicity compared with other hazardous substances. gamma radiation or gamma ray: The electromagnetic radiation emitted in de-excitation of an atomic nucleus, and frequently occurring in the decay of radionuclides. Sometimes shortened to gamma (e.g., gamma-emitting radionuclide). High-energy gamma radiation is highly penetrating and requires thick shielding, such as up to 1 m of concrete or a few tens of centimeter of steel (see photon and x ray). geologic repository: A system intended for disposal of hazardous wastes in excavated geologic media; includes a subterranean mined facility for the disposal of waste and the portion of the geologic setting that provides a barrier to the movement of hazardous substances in the waste (see disposal, geologic). gray (Gy): The special name for the SI unit of absorbed dose; 1 Gy ⳱ 1 J kgⳮ1. greater confinement disposal: Land disposal in a facility located at intermediate depths between those of a near-surface facility and a geologic repository. groundwater: Water below the land surface in a zone of saturation. half-life: The time period in which the activity of a radioactive material decreases by half. Measured half-lives of radionuclides vary from millionths of a second to billions of years. hazard: An act or phenomenon that has the potential to produce harm or other undesirable consequences to humans or what they value. Hazards may arise from physical phenomena (e.g., radioactivity, sound waves, magnetic fields, fire, floods, explosions), chemicals (e.g., ozone, mercury, dioxins, carbon dioxide, drugs, food additives), organisms (e.g., viruses, bacteria), commercial products (e.g., toys, tools, automobiles), or human behavior (e.g., drunk driving, skiing, firing guns). hazard identification: A qualitative process of determining whether exposure to an agent has the potential to increase the occurrence of adverse effects in organisms. Hazard identification is the first step of a risk assessment. Hazardous and Solid Waste Amendments: Amendments to the Resource Conservation and Recovery Act (RCRA) passed in 1984, which added the land disposal restrictions, minimum technology requirements, and expanded corrective action authorities to the law. hazardous waste: General term describing waste that is deemed to be a hazard to the health of humans or other organisms, due to the presence of radionuclides or hazardous chemicals, to the extent that it must be regulated. Hazardous waste does not include biological, medical, or infectious wastes. hazardous waste, chemical: Solid hazardous waste regulated under Subtitle C of the Resource Conservation and Recovery Act (RCRA) and defined in 40 CFR Part 261.3 (EPA, 1980b) (see characteristically hazardous waste and listed hazardous waste). high-level radioactive waste: (1) The highly radioactive material resulting from the reprocessing of spent nuclear fuel, including liquid waste produced directly in processing and any solid material derived from such liquid waste that contains fission products in sufficient concentrations; and (2) other highly radioactive material that the U.S. Nuclear Regulatory Commission,

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consistent with existing law, determines by rule requires permanent isolation. ignitable: A characteristic of solid hazardous waste regulated under the Resource Conservation and Recovery Act (RCRA) and defined in 40 CFR Part 261.21 (EPA, 1980b). A solid waste is ignitable if it (1) is a liquid, other than an aqueous solution containing less than 24 percent alcohol by volume, and has a flash point less than 60 °C, or (2) is not a liquid and is capable, under standard temperature and pressure, of causing fire through friction, absorption of moisture or spontaneous chemical changes and, when ignited, burns so vigorously and persistently that it creates a hazard. impact: (see effect). inadvertent intruder: A person who might occupy a waste disposal site after facility closure and engage in normal activities, such as agriculture, dwelling construction, permanent residence, or other pursuits, which might result in the person being unknowingly exposed to hazardous agents in the waste. institutional control: Control of a waste disposal site or other facility by an authority or institution designated under the laws of a country, state, or local authority. Institutional control may be active (e.g., monitoring of effluents, surveillance, guards, fences, or remedial activities) or passive (e.g., warning signs). internal exposure, ionizing radiation: Exposure of organs or tissues of an organism due to intakes of radionuclides (e.g., by ingestion, inhalation, or dermal absorption). ionizing radiation: Any radiation capable of displacing electrons from atoms or molecules, thereby producing ions. Examples include alpha radiation, beta radiation, gamma radiation or x rays, and cosmic rays. The minimum energy of ionizing radiation is a few electron volts (eV); 1 eV ⳱ 1.6 ⳯ 10ⳮ19 J. isolation: The emplacement of waste at locations apart from the human exposure environment, especially in a disposal facility (see containment). isotopes: Different forms of a chemical element distinguished by having different numbers of neutrons in the atomic nucleus. An element may have many stable or unstable (radioactive) isotopes. land disposal facility: The land, buildings, and equipment intended to be used for the disposal of wastes in a subsurface facility within the upper 30 m of Earth’s surface or in an above-grade facility. A geologic repository is not considered a land disposal facility. landfill: (see municipal/industrial landfill). linear energy transfer (LET): The quotient of dE by dᐉ, where dE is the energy lost by a charged particle in traversing a distance dᐉ in a material: LET ⳱ dE/dᐉ. The SI unit of LET is the joule per meter (J mⳮ1). For purposes of radiation protection, LET normally is specified in water and given in units of keV ␮mⳮ1. listed hazardous waste: A hazardous chemical waste regulated under the Resource Conservation and Recovery Act (RCRA) and designated as hazardous in 40 CFR Part 261.31–33 (EPA, 1980b), including: (1) wastes from nonspecific sources (‘‘F’’ wastes), (2) wastes from specific sources (‘‘K’’ wastes), and (3) discarded commercial chemicals from any source (‘‘P’’ and ‘‘U’’ wastes).

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lower bound: An estimate of exposure, dose, risk or other parameter that is expected to be lower than that experienced by any individual in a population. lower confidence limit: A measure of the extent to which an estimate of risk, dose, or other parameter is not expected to be less than a specified value. For example, the lower 95 percent confidence limit of a risk estimate means that, based on the available information, the probability is 0.05 that the true but unknown risk is less than the specified value (see confidence interval and upper confidence limit). low-level radioactive waste: Radioactive waste that (A) is not high-level radioactive waste, spent nuclear fuel, transuranic waste, or byproduct material as defined in Section 11(e)(2) of the Atomic Energy Act, and (B) the U.S. Nuclear Regulatory Commission, consistent with existing law, classifies as low-level radioactive waste. The byproduct material referred to in Clause (A) essentially is uranium or thorium mill tailings. Low-level radioactive waste does not include waste that contains naturally occurring and accelerator-produced radioactive material (NARM). lowest-observed-adverse-effect level (LOAEL): In dose-response experiments, the lowest dose of a hazardous agent at which there are statistically or biologically significant increases in the frequency or severity of adverse effects between the exposed population and an appropriate control group. maximally exposed individual: An individual assumed to be at greatest risk from a given hazard. maximum likelihood estimate (MLE): An estimate of the most probable level of a response resulting from a dose of a hazardous agent, or an estimate of the most probable dose, exposure, or other parameter; often synonymous with ‘‘best estimate.’’ migration: The transport of substances in the environment, usually by means of movement of air, surface water, or groundwater. mill tailings: The residues from chemical processing of uranium or thorium ores for their source material content. Mill tailings are a form of byproduct material, as defined in Section 11(e)(2) of the Atomic Energy Act. mixed waste: Waste that contains radionuclides (i.e., source, special nuclear, or byproduct material), as defined in the Atomic Energy Act, and hazardous chemical waste regulated under the Resource Conservation and Recovery Act (RCRA). Mixed waste also may include (1) waste that contains radionuclides defined in the Atomic Energy Act and hazardous chemical waste regulated under the Toxic Substances Control Act (TSCA) and (2) waste that contains naturally occurring and accelerator-produced radioactive material (NARM) and hazardous chemical waste regulated under RCRA or TSCA. mixture rule: Rule established under the Resource Conservation and Recovery Act (RCRA) in 40 CFR Part 261.3(a) (EPA, 1980b), which states that (1) any mixture of a solid waste and a characteristically hazardous waste is itself a hazardous waste if the mixture retains the hazardous characteristic and (2) any mixture of a solid waste and a listed hazardous waste is itself a hazardous waste, unless such waste is delisted.

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municipal/industrial landfill: A facility for disposal of solid waste that meets regulatory criteria established under Subtitle D of the Resource Conservation and Recovery Act (RCRA) or is otherwise acceptable for disposal of nonhazardous waste. Also may be referred to as a sanitary landfill in the literature. naturally occurring and accelerator-produced radioactive material (NARM): Any naturally occurring radioactive material (NORM) or any radioactive material produced in an accelerator. naturally occurring radioactive material (NORM): Any naturally occurring radioactive material that is not source material, special nuclear material, or byproduct material. near-surface disposal facility: (see land disposal facility). negligible: Same as de minimis in regard to doses or health risks. noncarcinogen: A hazardous agent that does not cause cancer; frequently used as a synonym for deterministic agent. no-observed-adverse-effect level (NOAEL): In dose-response experiments, a dose of a hazardous agent at which there are no statistically or biologically significant increases in the frequency or severity of adverse health effects between the exposed population and an appropriate control group. Some effects may be produced at this dose, but they are not considered to be adverse or precursors of specific adverse effects. In an experiment with more than one NOAEL, NOAEL normally is the highest dose without adverse effect. nuclear fuel cycle: Activities associated with the production, utilization, and disposition of fuel for nuclear reactors, including power reactors, research reactors, and isotope production reactors, and byproducts related to such activities. PCBs (polychlorinated biphenyls): A family of chemicals composed of biphenyl molecules that have been chlorinated to varying degrees. performance assessment: A type of risk assessment in which the potential long-term impacts of hazardous waste disposal on human health and the environment are evaluated for the purpose of determining whether disposal of specific wastes at specific sites should be acceptable. persistence: The length of time that a contaminant persists in the environment. pharmacokinetic model: (see biokinetic model). photon: A quantum of electromagnetic radiation, having no charge or mass, that exhibits both particle and wave behavior, especially a gamma ray or an x ray. post-closure: Times subsequent to cessation of waste emplacement activities at a disposal facility and actions (e.g., construction of impermeable caps, seals, surface markers) to prepare the disposal site for long-term waste isolation. potential dose, chemicals: The amount of a substance ingested, inhaled, or applied to the skin. probabilistic risk assessment: A type of risk assessment in which probabilistic methods are used to describe processes, events, and their consequences and to derive a distribution of risk based on repeated random sampling of distributions of input variables.

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probability coefficient: The nominal probability of an adverse stochastic response per unit dose of a hazardous agent assuming a linear, nonthreshold dose-response relationship; often called a ‘‘risk factor’’ or ‘‘risk coefficient’’ in the literature (see slope factor). pyrophoric: Term applied to materials that will ignite spontaneously in air at a temperature of 54 °C (130 °F) or lower. quality factor (Q): A dimensionless factor developed for purposes of radiation protection and assessing health risks in general terms which accounts for the relative biological effectiveness of different radiations in producing stochastic responses and is used to relate absorbed dose (D) at a point to dose equivalent (H): H ⳱ D ⳯ Q. The quality factor is a prescribed function of linear energy transfer (LET) in water (ICRP, 1991), and is defined with respect to the particular type and energy of radiation incident on tissue at the point of interest. Prior to introduction of the radiation weighting factor and as often used by federal and state agencies, an average quality factor (Q) for any energy of a particular radiation type (e.g., one for all photons and electrons, 20 for all alpha particles) is used to relate the average absorbed dose in an organ or tissue (DT) to the average dose equivalent in that organ or tissue (HT): HT ⳱ DT ⳯ Q. rad: (see absorbed dose). radiation weighting factor (wR): A dimensionless factor developed for purposes of radiation protection and assessing health risks in general terms which accounts for the relative biological effectiveness of different types (and, in some cases, energies) of radiations in producing stochastic responses and is used to relate the average absorbed dose in an organ or tissue (T) to equivalent dose: HT ⳱ DT ⳯ wR (ICRP, 1991). The radiation weighting factor is intended to supersede the average quality factor (Q), and is defined with respect to the type and energy of the radiation incident on the body or, in the case of sources within the body, emitted by the source. Values of wR include one for all photons and electrons and 20 for all alpha particles. The radiation weighting factor (wR) is independent of the tissue weighting factor (wT). radioactive waste: Solid, liquid, or gaseous materials of no value containing radionuclides in sufficient amounts that the waste must be regulated as a hazardous material. radioactivity: The property or characteristic of an unstable atomic nucleus to spontaneously transform with the emission of energy in the form of radiation. radionuclide: A naturally occurring or artificially produced radioactive element or isotope. radon: A colorless, odorless, naturally occurring, and radioactive gaseous element formed by radioactive decay of isotopes of radium. reactive: A characteristic of solid hazardous waste regulated under the Resource Conservation and Recovery Act (RCRA) and defined in 40 CFR Part 261.23 (EPA, 1980b). A solid waste is reactive if it has any of the following properties: (1) it is normally unstable and readily undergoes violent change without detonating; (2) it reacts violently with water; (3) it forms potentially explosive mixtures with water; (4) when mixed with water, it generates toxic gases, vapors or fumes in a quantity sufficient

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to present a danger to human health or the environment; (5) it is a cyanide or sulfide bearing waste which, when exposed to pH conditions between 2 and 2.5, can generate toxic gases, vapors or fumes in a quantity sufficient to present a danger to human health and the environment; (6) it is capable of detonation or explosive reaction if it is subjected to a strong initiating source or if heated under confinement; (7) it is readily capable of detonation or explosive decomposition or reaction at standard temperature and pressure; or (8) it is a forbidden, Class-A, or Class-B explosive as defined by the U.S. Department of Transportation. recycling: The use or reuse of a waste material as an effective substitute for a commercial product, as an ingredient, or as feedstock in an industrial or energy producing process; the reclamation of useful constituent fractions within a waste material; removal of contaminants from a waste to allow it to be reused. reference dose (RfD): An estimate, with uncertainty spanning perhaps an order of magnitude or greater, of a daily exposure level for the human population, including sensitive subpopulations, that is likely to be without an appreciable risk of deleterious effects. RfDs normally are estimated only for noncarcinogenic (deterministic) hazardous chemicals, and may be developed for chronic (7 y to lifetime), subchronic (two weeks to 7 y), or acute (single event) exposures. RfDs are obtained from LOAELs or NOAELs by applying various safety and uncertainty factors, and are not intended to represent thresholds for adverse health effects. relative biological effectiveness: For a specific radiation (A), the ratio of the absorbed dose of a reference radiation required to produce a specific level of response in a biological system to the absorbed dose of radiation (A) required to produce an equal response. The reference radiation normally is gamma rays or x rays with an average linear energy transfer (LET) of 3.5 keV ␮mⳮ1 or less. rem: (see dose equivalent or equivalent dose). remotely-handled transuranic waste: Containerized transuranic waste for which the external dose-equivalent rate at the surface of the container exceeds 2 mSv hⳮ1. repository: (see geologic repository). Resource Conservation and Recovery Act (RCRA): Law passed in 1976, and amended in 1980 and again in 1984 by the Hazardous and Solid Waste Amendments, that governs the generation, transport, treatment, storage, and disposal of solid hazardous waste and disposal of nonhazardous solid waste in municipal/industrial landfills. Solid hazardous wastes regulated under RCRA are defined in 40 CFR Part 261, Subpart A (EPA, 1980b), and specifically exclude source, special nuclear, and byproduct material as defined in the Atomic Energy Act. Objectives of RCRA include protection of human health and the environment, expeditious reduction or elimination of the generation of hazardous waste, and conservation of energy and natural resources (i.e., material recycling and recovery). response: A significant adverse effect on an organism resulting from exposure to a hazardous agent. The determination of whether an effect is

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373

significant or adverse involves subjective aspects. Often referred to as an ‘‘impact,’’ ‘‘biological endpoint,’’ or ‘‘effect.’’ retrieval: The intentional removal of waste from the location of its emplacement for disposal. risk: The probability of harm, combined with the potential severity of that harm. In the context of impacts on human health resulting from disposal of hazardous waste, ‘‘risk’’ is the probability of a response in an individual or the frequency of a response in a population taking into account: (1) the probability of occurrence of processes and events that could result in release of hazardous substances to the environment and the magnitude of such releases, (2) the probability that individuals or populations would be exposed to the hazardous substances released to the environment and the magnitude of such exposures, and (3) the probability that an exposure would produce a response. risk assessment: An analysis of the potential adverse impacts of an event (e.g., the release or threat of release of a hazardous substance) upon the well-being of an individual or population. Risk assessment is a process by which information or experience concerning causes and effects under a set of circumstances is integrated with the extent of those circumstances to quantify or otherwise describe risk. The process is multi-step and consists of hazard identification, dose-response assessment, exposure assessment, and risk characterization. risk characterization: An integration and interpretation of the information developed during hazard identification, dose-response assessment, and exposure assessment to yield an estimate of risk to human health or other organisms, including an identification of limitations and uncertainties in the models and data. Risk characterization is the last step of a risk assessment. risk management: The process by which results of risk assessments are integrated with other information (e.g., results of cost-benefit analysis, societal concerns) to make decisions about the need for, method of, and extent of risk reduction or control. sievert (Sv): The special name for the SI unit of dose equivalent and equivalent dose; 1 Sv ⳱ 1 J kgⳮ1. slope factor: For hazardous chemicals that induce stochastic effects, a plausible upper-bound estimate (e.g., upper 95 percent confidence limit) of the probability of cancer incidence per unit intake (e.g., milligram intake per kilogram body weight per day) of a hazardous substance over a lifetime. For radionuclides, the age-averaged best estimate of the probability of cancer incidence per unit activity intake of a radionuclide or, in the case of external exposure, per unit activity concentration of a radionuclide in the environment; values apply to any intake (chronic or acute) of any duration (see probability coefficient). solid waste: Material regulated under the Resource Conservation and Recovery Act (RCRA) and defined in 40 CFR Part 261.2 and 261.4 (EPA, 1980b); solid waste includes, but is not restricted to, material that has been discarded, abandoned, or is inherently waste-like, and such waste can be a solid, liquid, or gas.

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source material: (1) Uranium or thorium or any combination of uranium or thorium in any physical or chemical form, or (2) ores that contain, by weight, 0.05 percent or more of uranium, thorium, or any combination of uranium or thorium. Source material does not include special nuclear material. special nuclear material: (1) Plutonium, uranium enriched in the isotope 233 or 235, and any other material that the U.S. Nuclear Regulatory Commission determines to be special nuclear material, or (2) any material artificially enriched in any of the foregoing. Special nuclear material does not include source material. statistical significance: An inference that the probability is low that an observed difference in quantities being measured is due to variability in the data rather than an actual difference in the quantities themselves. The inference that an observed difference is statistically significant normally is based on a test to reject one hypothesis and accept another. stochastic agent: An agent that can produce a stochastic response in organisms. stochastic response: An adverse effect on organisms for which the probability of occurrence, but not the severity, is a function of dose without threshold (e.g., cancer). In humans, stochastic responses may not occur for many years after an exposure. storage: Retention of waste with the intent to retrieve it for subsequent use, processing, or disposal. Superfund: The common name for the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). Superfund Amendments and Reauthorization Act (SARA): Amendments to CERCLA (Superfund) passed in 1986, which also include freestanding provisions in Title III (The Emergency Response and Community Right-To-Know Act), Title IV (The Radon Gas and Indoor Air Quality Research Act), and Title V amending the Internal Revenue Code (The Superfund Revenue Act). synergistic: Situations in which the total response from simultaneous exposure of an organism to two or more hazardous agents is greater than the sum of the responses from separate exposures to each agent. tissue weighting factor (wT): A dimensionless factor which represents the ratio of the stochastic responses attributable to a specific organ or tissue (T) to the total stochastic responses attributable to all organs and tissues when the whole body receives a uniform exposure to ionizing radiation. When calculating effective dose equivalent, the tissue weighting factor represents the probability of fatal cancers or severe hereditary effects (ICRP, 1977). When calculating effective dose, the tissue weighting factor represents the total detriment (ICRP, 1991). toxic: (1) Capable of producing injury, illness, or damage to living organisms through ingestion, inhalation, or absorption through any body surface. (2) A characteristic of solid hazardous waste regulated under the Resource Conservation and Recovery Act (RCRA) and defined in 40 CFR Part 261.24 (EPA, 1980b). A solid waste is toxic if, when using the toxicity characteristic leaching procedure, the extract from a representative sample of

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375

the waste contains any of 40 contaminants (7 metals and 33 organic compounds) at a concentration equal to or greater than the specified values. When the waste contains less than 0.5 percent filterable solids, the waste itself, after filtering, is considered to be the extract for the purpose of determining whether it is toxic. toxicity characteristic leaching procedure: A testing procedure specified in 40 CFR Part 261, Appendix II (EPA, 1980b), used to determine whether a solid waste is toxic under the Resource Conservation and Recovery Act (RCRA). Toxic Substances Control Act (TSCA): Law passed in 1976 that governs the regulation of toxic substances in commerce, with the objective of preventing human health and environmental problems before they occur. The manufacturing, processing, or distribution in commerce of toxic substances may be limited or banned if EPA finds, based on results of toxicity testing and exposure assessments, that there is an unreasonable risk of injury to human health or the environment. Important hazardous chemicals regulated under TSCA include, for example, dioxins, PCBs, and asbestos. transportation: The movement of material by air, rail, highway, or water. transuranic waste: Radioactive waste containing more than 4 kBq gⳮ1 of alpha-emitting transuranium isotopes, with half-lives greater than 20 y, except for (1) high-level radioactive waste, (2) radioactive waste that the Secretary of the U.S. Department of Energy has determined, with concurrence of the Administrator of the U.S. Environmental Protection Agency, does not need the degree of isolation required by the disposal regulations in 40 CFR Part 191 (EPA, 1993a), or (3) radioactive waste that the U.S. Nuclear Regulatory Commission has approved for near-surface disposal on a case-by-case basis in accordance with 10 CFR Part 61 (NRC, 1982a; 1989). transuranium element: Chemical element with an atomic number greater than that of uranium (92) including, among others, neptunium, plutonium, americium, and curium (often referred to as transuranic element in the literature). treatment: Any method, technique, or process designed to change the physical or chemical character of a hazardous material to render it less hazardous, safer to transport, store or dispose of, or to reduce its volume. uncertainty: A lack of sureness or confidence about parameters, results of measurements, predictions of models, or other factors. Uncertainty often can be reduced through further study (see variability). upper bound: An estimate of exposure, dose, risk or other parameter that is expected to be higher than that experienced by any individual in a population. upper confidence limit (UCL): A measure of the extent to which an estimate of risk, dose, or other parameter is not expected to be greater than a specified value. For example, the upper 95 percent confidence limit of a risk estimate means that, based on the available information, the probability is 0.05 that the true but unknown risk is greater than the specified value (see confidence interval and lower confidence limit).

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variability: A heterogeneity, diversity, or range that characterizes a parameter (e.g., differences in body weight in a population) or response (e.g., differences in sensitivity to a hazardous agent in a population). Further study cannot reduce variability but may provide greater confidence in quantitative characterizations of variability (see uncertainty). waste classification: Any grouping wastes having similar attributes. waste management: All activities associated with the disposition of waste products after they have been generated, as well as actions to minimize the production of waste. waste minimization: Reduction, to the extent practicable, of the volume or toxicity of hazardous waste prior to its treatment, storage, or disposal. weight-of-evidence classification: A classification system for characterizing the extent to which available data indicate that an agent is a human carcinogen. x ray: The electromagnetic radiation emitted in de-excitation of bound atomic electrons, and frequently occurring in the decay of radionuclides, referred to as characteristic x rays, or the electromagnetic radiation produced in the deceleration of energetic charged particles (e.g., beta radiation) in passing through matter, referred to as continuous x rays or bremsstrahlung. Higher-energy x rays are penetrating and may require shielding of up to a few tens of centimeter of concrete or a few centimeters of steel (see gamma radiation and photon).

Acronyms AEA ALARA BDAT BRC CERCLA CFR DDREF DNA ED10 FR HEAST IRIS LDR LEAF LED10 LET LLRWPAA LOAEL MCL MCLG MF MLE MOE MTD NARM NOAEL NORM NWPA PB-PK PCBs RCRA RfD

Atomic Energy Act as low as reasonably achievable best demonstrated available technology below regulatory concern Comprehensive Environmental Response, Compensation, and Liability Act Code of Federal Regulations dose and dose-rate effectiveness factor deoxyribonucleic acid central estimate of dose causing increase in effects (responses) of 10 percent Federal Register Health Effects Assessment Summary Tables Integrated Risk Information System land disposal restriction Legal Environmental Assistance Foundation lower confidence limit of dose causing increase in effects (responses) of 10 percent linear energy transfer Low-Level Radioactive Waste Policy Amendments Act lowest-observed-adverse-effect level maximum contaminant level maximum contaminant level goal modifying factor maximum likelihood estimate margin of exposure maximum tolerated dose naturally occurring and accelerator-produced radioactive material no-observed-adverse-effect level naturally occurring radioactive material Nuclear Waste Policy Act physiologically-based pharmacokinetic polychlorinated biphenyls Resource Conservation and Recovery Act reference dose 377

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RI TSCA UCL UF WIPP WIPPLWA

ACRONYMS

risk index Toxic Substances Control Act upper confidence limit uncertainty factor Waste Isolation Pilot Plant Waste Isolation Pilot Plant Land Withdrawal Act

References

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THOMPSON, D.E., MABUCHI, K., RON, E., SODA, M., TOKUNAGA, M., OCHIKUBO, S., SUGIMOTO, S., IKEDA, T., TERASAKI, M., IZUMI, S. and PRESTON, D.L. (1994). ‘‘Cancer incidence in atomic bomb survivors. Part II: Solid tumors, 1958–1987,’’ Radiat. Res. 137, S17–S67. TRAVIS, C.C. and HESTER, S.T. (1990). ‘‘Background exposure to chemicals: What is the risk?’’ Risk Anal. 10, 463–466. TRAVIS, C.C., RICHTER, S.A., CROUCH, E.A.C., WILSON, R. and KLEMA, E.D. (1987). ‘‘Cancer risk management,’’ Environ. Sci. Technol. 21, 415–420. TRAVIS, C.C., WHITE, R.K. and WARD, R.C. (1990). ‘‘Interspecies extrapolation of pharmacokinetics,’’ J. Theor. Biol. 142, 285–304. TSCA (1976). Toxic Substances Control Act. Public Law 94-469 (October 11), 90 Stat. 2003, as amended (U.S. Government Printing Office, Washington). UMTRCA (1978). Uranium Mill Tailings Radiation Control Act. Public Law 95-604 (November 11), 92 Stat. 3021, as amended (U.S. Government Printing Office, Washington). UNSCEAR (2000). United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and Effects of Ionizing Radiation, UNSCEAR Report to the General Assembly, with Scientific Annexes, E.00.IX.6 (United Nations, New York). WARRINER, J.B. and BENNETT, R.D. (1985). Alternative Methods for Disposal of Low-Level Radioactive Wastes—Task 2a: Technical Requirements for Below-Ground Vault Disposal of Low-Level Radioactive Waste, U.S. Nuclear Regulatory Commission Report NUREG/CR-3774, Volume 2 (National Technical Information Service, Springfield, Virginia). WATERS, R.D., CRUTCHER, M.D. and PARKER, F.L. (1993). ‘‘Hazard ranking systems for chemical wastes and chemical waste sites,’’ pages 115 to 170 in Hazard Assessment of Chemicals, Volume 8, Saxena, J., Ed. (Academic Press, New York). WEIL, C.S. (1972). ‘‘Statistics versus safety factors and scientific judgment in the evaluation of safety for man,’’ Toxicol. Appl. Pharmacol. 21, 454–463. WILSON, M.L., BARNARD, R.W., GAUTHIER, J.H., BARR, G.E., DOCKERY, H.A., DUNN, E., EATON, R.R., GUERIN, D.C., LU, N., MARTINEZ, M.J., NILSON, R., RAUTMAN, C.A., ROBEY, T.H., ROSS, B., RYDER, E.E., SCHENKER, A.R., SHANNON, S.A., SKINNER, L.H., HALSEY, W.G., GANSEMER, J., LEWIS, L.C., LAMONT, A.D., TRIAY, I.R., MEIJER, A. and MORRIS, D.E. (1994). Total-System Performance Assessment for Yucca Mountain: SNL Second Iteration (TSPA-1993). Executive Summary, Sandia National Laboratories Report SAND 93-2675 (National Technical Information Service, Springfield, Virginia). WILTSHIRE, S. and DOW, K. (1995). ‘‘Social and political considerations,’’ pages 97 to 105 in Radioactive and Mixed Waste—Risk as a Basis for Waste Classification, NCRP Symposium Proceedings No. 2 (National Council on Radiation Protection and Measurements, Bethesda, Maryland). WIPPLWA (1992). Waste Isolation Pilot Plant Land Withdrawal Act. Public Law 102-579 (October 30), 106 Stat. 4777, as amended (U.S. Government Printing Office, Washington).

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WVDPA (1980). West Valley Demonstration Project Act. Public Law 96-368 (October 1), 94 Stat. 1347 (U.S. Government Printing Office, Washington). YU, C., ZIELEN, A.J., CHENG, J.J., YUAN, Y.C., JONES, L.B., LEPOIRE, D.J., WANG, Y.Y., LOUREIRO, C.O., GNANAPRAGASAM, E., FAILLACE, E., WALLO, A., III, WILLIAMS, W.A. and PETERSON, H. (1993). Manual for Implementing Residual Radioactive Material Guidelines Using RESRAD, Version 5.0, Argonne National Laboratory Report ANL/EAD/LD2 (National Technical Information Service, Springfield, Virginia).

The NCRP The National Council on Radiation Protection and Measurements is a nonprofit corporation chartered by Congress in 1964 to: 1. Collect, analyze, develop and disseminate in the public interest information and recommendations about (a) protection against radiation and (b) radiation measurements, quantities and units, particularly those concerned with radiation protection. 2. Provide a means by which organizations concerned with the scientific and related aspects of radiation protection and of radiation quantities, units and measurements may cooperate for effective utilization of their combined resources, and to stimulate the work of such organizations. 3. Develop basic concepts about radiation quantities, units and measurements, about the application of these concepts, and about radiation protection. 4. Cooperate with the International Commission on Radiological Protection, the International Commission on Radiation Units and Measurements, and other national and international organizations, governmental and private, concerned with radiation quantities, units and measurements and with radiation protection. The Council is the successor to the unincorporated association of scientists known as the National Committee on Radiation Protection and Measurements and was formed to carry on the work begun by the Committee in 1929. The participants in the Council’s work are the Council members and members of scientific and administrative committees. Council members are selected solely on the basis of their scientific expertise and serve as individuals, not as representatives of any particular organization. The scientific committees, composed of experts having detailed knowledge and competence in the particular area of the committee’s interest, draft proposed recommendations. These are then submitted to the full membership of the Council for careful review and approval before being published. The following comprise the current officers and membership of the Council:

Officers President Vice President Secretary and Treasurer Assistant Secretary

Thomas S. Tenforde Kenneth R. Kase William M. Beckner Michael F. McBride

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Members John F. Ahearne Larry E. Anderson Benjamin R. Archer Mary M. Austin-Seymour Harold L. Beck Eleanor A. Blakely John D. Boice, Jr. Thomas B. Borak Andre´ Bouville Leslie A. Braby Davi J. Brenner Antone L. Brooks Jerrold T. Bushberg John F. Cardella Shih-Yew Chen Chung-Kwang Chou Mary E. Clark James E. Cleaver J. Donald Cossairt Allen G. Croff Francis A. Cucinotta Paul M. DeLuca Carter Denniston Gail de Planque John F. Dicello Sarah S. Donaldson William P. Dornsife Stephen A. Feig H. Keith Florig Kenneth R. Foster John F. Frazier Thomas F. Gesell

Ethel S. Gilbert Joel E. Gray Andrew J. Grosovsky Raymond A. Guilmette William R. Hendee John W. Hirshfeld David G. Hoel F. Owen Hoffman Geoffrey R. Howe Kenneth R. Kase Ann R. Kennedy David C. Kocher Ritsuko Komaki Amy Kronenberg Charles E. Land Susan M. Langhorst Richard W. Leggett Howard L. Liber James C. Lin Jill Lipoti John B. Little Jay H. Lubin C. Douglas Maynard Claire M. Mays Barbara J. McNeil Fred A. Mettler, Jr. Charles W. Miller Jack Miller Kenneth L. Miller William F. Morgan John E. Moulder David S. Myers

Bruce A. Napier Carl J. Paperiello Ronald C. Petersen R. Julian Preston Jerome S. Puskin Marvin Rosenstein Lawrence N. Rothenberg Henry D. Royal Michael T. Ryan Jonathan M. Samet Stephen M. Seltzer Roy E. Shore Edward A. Sickles David H. Sliney Paul Slovic Daniel J. Strom Louise C. Strong Thomas S. Tenforde Lawrence W. Townsend Lois B. Travis Robert L. Ullrich Richard J. Vetter Daniel Wartenberg David A. Weber F. Ward Whicker Chris G. Whipple J. Frank Wilson Susan D. Wiltshire Marco Zaider Pasquale Zanzonico Marvin C. Ziskin

Honorary Members Lauriston S. Taylor, Honorary President Warren K. Sinclair, President Emeritus; Charles B. Meinhold, President Emeritus S. James Adelstein, Honorary Vice President W. Roger Ney, Executive Director Emeritus

Seymour Abrahamson Edward L. Alpen Lynn R. Anspaugh John A. Auxier William J. Bair Bruce B. Boecker Victor P. Bond Robert L. Brent Reynold F. Brown Melvin C. Carter Randall S. Caswell Frederick P. Cowan James F. Crow Gerald D. Dodd

Patricia W. Durbin Keith F. Eckerman Thomas S. Ely Richard F. Foster R.J. Michael Fry Robert O. Gorson Arthur W. Guy Eric J. Hall Naomi H. Harley Donald G. Jacobs Bernd Kahn Roger O. McClellan Dade W. Moeller A. Alan Moghissi

Robert J. Nelsen Wesley L. Nyborg John W. Poston, Sr. Andrew K. Poznanski Chester R. Richmond William L. Russell Eugene L. Saenger William J. Schull J. Newell Stannard John B. Storer John E. Till Arthur C. Upton George L. Voelz Edward W. Webster

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Lauriston S. Taylor Lecturers Herbert M. Parker (1977) The Squares of the Natural Numbers in Radiation Protection Sir Edward Pochin (1978) Why be Quantitative about Radiation Risk Estimates? Hymer L. Friedell (1979) Radiation Protection—Concepts and Trade Offs Harold O. Wyckoff (1980) From ‘‘Quantity of Radiation’’ and ‘‘Dose’’ to ‘‘Exposure’’ and ‘‘Absorbed Dose’’—An Historical Review James F. Crow (1981) How Well Can We Assess Genetic Risk? Not Very Eugene L. Saenger (1982) Ethics, Trade-offs and Medical Radiation Merril Eisenbud (1983) The Human Environment—Past, Present and Future Harald H. Rossi (1984) Limitation and Assessment in Radiation Protection John H. Harley (1985) Truth (and Beauty) in Radiation Measurement Herman P. Schwan (1986) Biological Effects of Non-ionizing Radiations: Cellular Properties and Interactions Seymour Jablon (1987) How to be Quantitative about Radiation Risk Estimates Bo Lindell (1988) How Safe is Safe Enough? Arthur C. Upton (1989) Radiobiology and Radiation Protection: The Past Century and Prospects for the Future J. Newell Stannard (1990) Radiation Protection and the Internal Emitter Saga Victor P. Bond (1991) When is a Dose Not a Dose? Edward W. Webster (1992) Dose and Risk in Diagnostic Radiology: How Big? How Little? Warren K. Sinclair (1993) Science, Radiation Protection and the NCRP R.J. Michael Fry (1994) Mice, Myths and Men Albrecht Kellerer (1995) Certainty and Uncertainty in Radiation Protection Seymour Abrahamson (1996) 70 Years of Radiation Genetics: Fruit Flies, Mice and Humans William J. Bair (1997) Radionuclides in the Body: Meeting the Challenge! Eric J. Hall (1998) From Chimney Sweeps to Astronauts: Cancer Risks in the Workplace Naomi H. Harley (1999) Back to Background S. James Adelstein (2000) Administered Radioactivity: Unde Venimus Quoque Imus Wesley L. Nyborg (2001) Assuring the Safety of Medical Diagnostic Ultrasound R. Julian Preston (2002) Developing Mechanistic Data for Incorporation into Cancer Risk Assessment: Old Problems and New Approaches

Currently, the following committees are actively engaged in formulating recommendations: SC 1

SC 9

Basic Criteria, Epidemiology, Radiobiology and Risk SC 1-4 Extrapolation of Risks from Non-Human Experimental Systems to Man SC 1-7 Information Needed to Make Radiation Protection Recommendations for Travel Beyond Low-Earth Orbit SC 1-8 Risk to Thyroid from Ionizing Radiation SC 1-10 Review of Cohen’s Radon Research Methods SC 1-11 Radiation Protection and Measurement for Neutron Surveillance Scanners SC 1-12 Exposure Limits for Security Surveillance Devices Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies Up to 10 MeV

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Operational Radiation Safety SC 46-8 Radiation Protection Design Guidelines for Particle Accelerator Facilities SC 46-13 Design of Facilities for Medical Radiation Therapy SC 46-16 Radiation Protection in Veterinary Medicine SC 46-17 Radiation Protection in Educational Institutions SC 57-15 Uranium Risk SC 57-17 Radionuclide Dosimetry Models for Wounds Environmental Issues SC 64-22 Design of Effective Effluent and Environmental Monitoring Programs SC 64-23 Cesium in the Environment Radiation Protection in Mammography Risk of Lung Cancer from Radon Radioactive and Mixed Waste SC 87-1 Waste Avoidance and Volume Reduction SC 87-3 Performance Assessment SC 87-5 Risk Management Analysis for Decommissioned Sites Nonionizing Electromagnetic Fields SC 89-3 Biological Effects of Extremely Low-Frequency Electric and Magnetic Fields SC 89-4 Biological Effects and Exposure Recommendations for Modulated Radiofrequency Fields SC 89-5 Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Fields SC 89-6 Wireless Telecommunications Safety Issues for Building Owners and Managers Radiation Protection in Medicine SC 91-1 Precautions in the Management of Patients Who Have Received Therapeutic Amounts of Radionuclides SC 91-2 Radiation Protection in Dentistry Public Policy and Risk Communication Radiation Measurement and Dosimetry

In recognition of its responsibility to facilitate and stimulate cooperation among organizations concerned with the scientific and related aspects of radiation protection and measurement, the Council has created a category of NCRP Collaborating Organizations. Organizations or groups of organizations that are national or international in scope and are concerned with scientific problems involving radiation quantities, units, measurements and effects, or radiation protection may be admitted to collaborating status by the Council. Collaborating Organizations provide a means by which the

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NCRP can gain input into its activities from a wider segment of society. At the same time, the relationships with the Collaborating Organizations facilitate wider dissemination of information about the Council’s activities, interests and concerns. Collaborating Organizations have the opportunity to comment on draft reports (at the time that these are submitted to the members of the Council). This is intended to capitalize on the fact that Collaborating Organizations are in an excellent position to both contribute to the identification of what needs to be treated in NCRP reports and to identify problems that might result from proposed recommendations. The present Collaborating Organizations with which the NCRP maintains liaison are as follows: Agency for Toxic Substances and Disease Registry American Academy of Dermatology American Academy of Environmental Engineers American Academy of Health Physics American Association of Physicists in Medicine American College of Medical Physics American College of Nuclear Physicians American College of Occupational and Environmental Medicine American College of Radiology American Dental Association American Industrial Hygiene Association American Institute of Ultrasound in Medicine American Insurance Services Group American Medical Association American Nuclear Society American Pharmaceutical Association American Podiatric Medical Association American Public Health Association American Radium Society American Roentgen Ray Society American Society for Therapeutic Radiology and Oncology American Society of Health-System Pharmacists American Society of Radiologic Technologists Association of University Radiologists Bioelectromagnetics Society Campus Radiation Safety Officers College of American Pathologists Conference of Radiation Control Program Directors, Inc. Council on Radionuclides and Radiopharmaceuticals Defense Threat Reduction Agency Electric Power Research Institute Federal Communications Commission Federal Emergency Management Agency Genetics Society of America Health Physics Society Institute of Electrical and Electronics Engineers, Inc.

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Institute of Nuclear Power Operations International Brotherhood of Electrical Workers National Aeronautics and Space Administration National Association of Environmental Professionals National Electrical Manufacturers Association National Institute for Occupational Safety and Health National Institute of Standards and Technology Nuclear Energy Institute Office of Science and Technology Policy Oil, Chemical and Atomic Workers Radiation Research Society Radiological Society of North America Society for Risk Analysis Society of Chairmen of Academic Radiology Departments Society of Nuclear Medicine Society of Skeletal Radiology U.S. Air Force U.S. Army U.S. Coast Guard U.S. Department of Energy U.S. Department of Housing and Urban Development U.S. Department of Labor U.S. Department of Transportation U.S. Environmental Protection Agency U.S. Navy U.S. Nuclear Regulatory Commission U.S. Public Health Service Utility Workers Union of America The NCRP has found its relationships with these organizations to be extremely valuable to continued progress in its program. Another aspect of the cooperative efforts of the NCRP relates to the Special Liaison relationships established with various governmental organizations that have an interest in radiation protection and measurements. This liaison relationship provides: (1) an opportunity for participating organizations to designate an individual to provide liaison between the organization and the NCRP; (2) that the individual designated will receive copies of draft NCRP reports (at the time that these are submitted to the members of the Council) with an invitation to comment, but not vote; and (3) that new NCRP efforts might be discussed with liaison individuals as appropriate, so that they might have an opportunity to make suggestions on new studies and related matters. The following organizations participate in the Special Liaison Program: Australian Radiation Laboratory Bundesamt fu¨r Strahlenschutz (Germany) Canadian Nuclear Safety Commission Central Laboratory for Radiological Protection (Poland)

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China Institute for Radiation Protection Commisariat a` l’Energie Atomique Commonwealth Scientific Instrumentation Research Organization (Australia) European Commission Health Council of the Netherlands International Commission on Non-Ionizing Radiation Protection Japan Radiation Council Korea Institute of Nuclear Safety National Radiological Protection Board (United Kingdom) Russian Scientific Commission on Radiation Protection South African Forum for Radiation Protection World Association of Nuclear Operations The NCRP values highly the participation of these organizations in the Special Liaison Program. The Council also benefits significantly from the relationships established pursuant to the Corporate Sponsor’s Program. The program facilitates the interchange of information and ideas and corporate sponsors provide valuable fiscal support for the Council’s program. This developing program currently includes the following Corporate Sponsors: 3M Corporate Health Physics Amersham Health Duke Energy Corporation ICN Biomedicals, Inc. Landauer, Inc. Nuclear Energy Institute Philips Medical Systems Southern California Edison The Council’s activities are made possible by the voluntary contribution of time and effort by its members and participants and the generous support of the following organizations: 3M Health Physics Services Agfa Corporation Alfred P. Sloan Foundation Alliance of American Insurers American Academy of Dermatology American Academy of Health Physics American Academy of Oral and Maxillofacial Radiology American Association of Physicists in Medicine American Cancer Society American College of Medical Physics American College of Nuclear Physicians American College of Occupational and Environmental Medicine American College of Radiology American College of Radiology Foundation

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American Dental Association American Healthcare Radiology Administrators American Industrial Hygiene Association American Insurance Services Group American Medical Association American Nuclear Society American Osteopathic College of Radiology American Podiatric Medical Association American Public Health Association American Radium Society American Roentgen Ray Society American Society of Radiologic Technologists American Society for Therapeutic Radiology and Oncology American Veterinary Medical Association American Veterinary Radiology Society Association of University Radiologists Battelle Memorial Institute Canberra Industries, Inc. Chem Nuclear Systems Center for Devices and Radiological Health College of American Pathologists Committee on Interagency Radiation Research and Policy Coordination Commonwealth Edison Commonwealth of Pennsylvania Consolidated Edison Consumers Power Company Council on Radionuclides and Radiopharmaceuticals Defense Nuclear Agency Eastman Kodak Company Edison Electric Institute Edward Mallinckrodt, Jr. Foundation EG&G Idaho, Inc. Electric Power Research Institute Electromagnetic Energy Association Federal Emergency Management Agency Florida Institute of Phosphate Research Florida Power Corporation Fuji Medical Systems, U.S.A., Inc. Genetics Society of America Health Effects Research Foundation (Japan) Health Physics Society Institute of Nuclear Power Operations James Picker Foundation Martin Marietta Corporation Motorola Foundation National Aeronautics and Space Administration National Association of Photographic Manufacturers

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National Cancer Institute National Electrical Manufacturers Association National Institute of Standards and Technology New York Power Authority Picker International Public Service Electric and Gas Company Radiation Research Society Radiological Society of North America Richard Lounsbery Foundation Sandia National Laboratory Siemens Medical Systems, Inc. Society of Nuclear Medicine Society of Pediatric Radiology U.S. Department of Energy U.S. Department of Labor U.S. Environmental Protection Agency U.S. Navy U.S. Nuclear Regulatory Commission Victoreen, Inc. Westinghouse Electric Corporation Initial funds for publication of NCRP reports were provided by a grant from the James Picker Foundation. The NCRP seeks to promulgate information and recommen-dations based on leading scientific judgment on matters of radiation protection and measurement and to foster cooperation among organizations concerned with these matters. These efforts are intended to serve the public interest and the Council welcomes comments and suggestions on its reports or activities from those interested in its work.

NCRP Publications

Information on NCRP publications may be obtained from the NCRP website (http://www.ncrp.com), e-mail ([email protected]), by telephone (800-229-2652, Ext. 25), or fax (301-907-8768). The address is: NCRP Publications 7910 Woodmont Avenue Suite 400 Bethesda, MD 20814-3095 Abstracts of NCRP reports published since 1980, abstracts of all NCRP commentaries, and the text of all NCRP statements are available at the NCRP website. Currently available publications are listed below.

NCRP Reports No. 8 22

25 27 30 32 35 36 37 38 40 41 42

Title Control and Removal of Radioactive Contamination in Laboratories (1951) Maximum Permissible Body Burdens and Maximum Permissible Concentrations of Radionuclides in Air and in Water for Occupational Exposure (1959) [Includes Addendum 1 issued in August 1963] Measurement of Absorbed Dose of Neutrons, and of Mixtures of Neutrons and Gamma Rays (1961) Stopping Powers for Use with Cavity Chambers (1961) Safe Handling of Radioactive Materials (1964) Radiation Protection in Educational Institutions (1966) Dental X-Ray Protection (1970) Radiation Protection in Veterinary Medicine (1970) Precautions in the Management of Patients Who Have Received Therapeutic Amounts of Radionuclides (1970) Protection Against Neutron Radiation (1971) Protection Against Radiation from Brachytherapy Sources (1972) Specification of Gamma-Ray Brachytherapy Sources (1974) Radiological Factors Affecting Decision-Making in a Nuclear Attack (1974)

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Krypton-85 in the Atmosphere—Accumulation, Biological Significance, and Control Technology (1975) Alpha-Emitting Particles in Lungs (1975) Tritium Measurement Techniques (1976) Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies Up to 10 MeV (1976) Environmental Radiation Measurements (1976) Cesium-137 from the Environment to Man: Metabolism and Dose (1977) Medical Radiation Exposure of Pregnant and Potentially Pregnant Women (1977) Protection of the Thyroid Gland in the Event of Releases of Radioiodine (1977) Instrumentation and Monitoring Methods for Radiation Protection (1978) A Handbook of Radioactivity Measurements Procedures, 2nd ed. (1985) Physical, Chemical, and Biological Properties of Radiocerium Relevant to Radiation Protection Guidelines (1978) Radiation Safety Training Criteria for Industrial Radiography (1978) Tritium in the Environment (1979) Tritium and Other Radionuclide Labeled Organic Compounds Incorporated in Genetic Material (1979) Influence of Dose and Its Distribution in Time on Dose-Response Relationships for Low-LET Radiations (1980) Management of Persons Accidentally Contaminated with Radionuclides (1980) Radiofrequency Electromagnetic Fields—Properties, Quantities and Units, Biophysical Interaction, and Measurements (1981) Radiation Protection in Pediatric Radiology (1981) Dosimetry of X-Ray and Gamma-Ray Beams for Radiation Therapy in the Energy Range 10 keV to 50 MeV (1981) Nuclear Medicine—Factors Influencing the Choice and Use of Radionuclides in Diagnosis and Therapy (1982) Radiation Protection and Measurement for Low-Voltage Neutron Generators (1983) Protection in Nuclear Medicine and Ultrasound Diagnostic Procedures in Children (1983) Biological Effects of Ultrasound: Mechanisms and Clinical Implications (1983) Iodine-129: Evaluation of Releases from Nuclear Power Generation (1983) Exposures from the Uranium Series with Emphasis on Radon and Its Daughters (1984) Evaluation of Occupational and Environmental Exposures to Radon and Radon Daughters in the United States (1984)

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Neutron Contamination from Medical Electron Accelerators (1984) Induction of Thyroid Cancer by Ionizing Radiation (1985) Carbon-14 in the Environment (1985) SI Units in Radiation Protection and Measurements (1985) The Experimental Basis for Absorbed-Dose Calculations in Medical Uses of Radionuclides (1985) General Concepts for the Dosimetry of Internally Deposited Radionuclides (1985) Mammography—A User’s Guide (1986) Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Fields (1986) Use of Bioassay Procedures for Assessment of Internal Radionuclide Deposition (1987) Radiation Alarms and Access Control Systems (1986) Genetic Effects from Internally Deposited Radionuclides (1987) Neptunium: Radiation Protection Guidelines (1988) Public Radiation Exposure from Nuclear Power Generation in the United States (1987) Ionizing Radiation Exposure of the Population of the United States (1987) Exposure of the Population in the United States and Canada from Natural Background Radiation (1987) Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources (1987) Comparative Carcinogenicity of Ionizing Radiation and Chemicals (1989) Measurement of Radon and Radon Daughters in Air (1988) Quality Assurance for Diagnostic Imaging (1988) Exposure of the U.S. Population from Diagnostic Medical Radiation (1989) Exposure of the U.S. Population from Occupational Radiation (1989) Medical X-Ray, Electron Beam and Gamma-Ray Protection for Energies Up to 50 MeV (Equipment Design, Performance and Use) (1989) Control of Radon in Houses (1989) The Relative Biological Effectiveness of Radiations of Different Quality (1990) Radiation Protection for Medical and Allied Health Personnel (1989) Limit for Exposure to ‘‘Hot Particles’’ on the Skin (1989) Implementation of the Principle of As Low As Reasonably Achievable (ALARA) for Medical and Dental Personnel (1990) Conceptual Basis for Calculations of Absorbed-Dose Distributions (1991) Effects of Ionizing Radiation on Aquatic Organisms (1991) Some Aspects of Strontium Radiobiology (1991)

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111 Developing Radiation Emergency Plans for Academic, Medical or Industrial Facilities (1991) 112 Calibration of Survey Instruments Used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination (1991) 113 Exposure Criteria for Medical Diagnostic Ultrasound: I. Criteria Based on Thermal Mechanisms (1992) 114 Maintaining Radiation Protection Records (1992) 115 Risk Estimates for Radiation Protection (1993) 116 Limitation of Exposure to Ionizing Radiation (1993) 117 Research Needs for Radiation Protection (1993) 118 Radiation Protection in the Mineral Extraction Industry (1993) 119 A Practical Guide to the Determination of Human Exposure to Radiofrequency Fields (1993) 120 Dose Control at Nuclear Power Plants (1994) 121 Principles and Application of Collective Dose in Radiation Protection (1995) 122 Use of Personal Monitors to Estimate Effective Dose Equivalent and Effective Dose to Workers for External Exposure to LowLET Radiation (1995) 123 Screening Models for Releases of Radionuclides to Atmosphere, Surface Water, and Ground (1996) 124 Sources and Magnitude of Occupational and Public Exposures from Nuclear Medicine Procedures (1996) 125 Deposition, Retention and Dosimetry of Inhaled Radioactive Substances (1997) 126 Uncertainties in Fatal Cancer Risk Estimates Used in Radiation Protection (1997) 127 Operational Radiation Safety Program (1998) 128 Radionuclide Exposure of the Embryo/Fetus (1998) 129 Recommended Screening Limits for Contaminated Surface Soil and Review of Factors Relevant to Site-Specific Studies (1999) 130 Biological Effects and Exposure Limits for ‘‘Hot Particles’’ (1999) 131 Scientific Basis for Evaluating the Risks to Populations from Space Applications of Plutonium (2001) 132 Radiation Protection Guidance for Activities in Low-Earth Orbit (2000) 133 Radiation Protection for Procedures Performed Outside the Radiology Department (2000) 134 Operational Radiation Safety Training (2000) 135 Liver Cancer Risk from Internally-Deposited Radionuclides (2001) 136 Evaluation of the Linear-Nonthreshold Dose-Response Model for Ionizing Radiation (2001) 137 Fluence-Based and Microdosimetric Event-Based Methods for Radiation Protection in Space (2001) 138 Management of Terrorist Events Involving Radioactive Material (2001) 139 Risk-Based Classification of Radioactive and Hazardous Chemical Wastes (2002) 140 Exposure Criteria for Medical Diagnostic Ultrasound: II. Criteria Based on All Known Mechanisms (2002)

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141 Managing Potentially Radioactive Scrap Metal (2002) 142 Operational Radiation Safety Program for Astronauts in LowEarth Orbit: A Basic Framework (2002) Binders for NCRP reports are available. Two sizes make it possible to collect into small binders the ‘‘old series’’ of reports (NCRP Reports Nos. 8-30) and into large binders the more recent publications (NCRP Reports Nos. 32-142). Each binder will accommodate from five to seven reports. The binders carry the identification ‘‘NCRP Reports’’ and come with label holders which permit the user to attach labels showing the reports contained in each binder. The following bound sets of NCRP reports are also available: Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume Volume

I. NCRP Reports Nos. 8, 22 II. NCRP Reports Nos. 23, 25, 27, 30 III. NCRP Reports Nos. 32, 35, 36, 37 IV. NCRP Reports Nos. 38, 40, 41 V. NCRP Reports Nos. 42, 44, 46 VI. NCRP Reports Nos. 47, 49, 50, 51 VII. NCRP Reports Nos. 52, 53, 54, 55, 57 VIII. NCRP Report No. 58 IX. NCRP Reports Nos. 59, 60, 61, 62, 63 X. NCRP Reports Nos. 64, 65, 66, 67 XI. NCRP Reports Nos. 68, 69, 70, 71, 72 XII. NCRP Reports Nos. 73, 74, 75, 76 XIII. NCRP Reports Nos. 77, 78, 79, 80 XIV. NCRP Reports Nos. 81, 82, 83, 84, 85 XV. NCRP Reports Nos. 86, 87, 88, 89 XVI. NCRP Reports Nos. 90, 91, 92, 93 XVII. NCRP Reports Nos. 94, 95, 96, 97 XVIII. NCRP Reports Nos. 98, 99, 100 XIX. NCRP Reports Nos. 101, 102, 103, 104 XX. NCRP Reports Nos. 105, 106, 107, 108 XXI. NCRP Reports Nos. 109, 110, 111 XXII. NCRP Reports Nos. 112, 113, 114 XXIII. NCRP Reports Nos. 115, 116, 117, 118 XXIV. NCRP Reports Nos. 119, 120, 121, 122 XXV. NCRP Report No. 123I and 123II XXVI. NCRP Reports Nos. 124, 125, 126, 127 XXVII. NCRP Reports Nos. 128, 129, 130 XXVIII. NCRP Reports Nos. 131, 132, 133 XXIX. NCRP Reports Nos. 134, 135, 136, 137

(Titles of the individual reports contained in each volume are given above.)

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NCRP Commentaries No.

Title

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Krypton-85 in the Atmosphere—With Specific Reference to the Public Health Significance of the Proposed Controlled Release at Three Mile Island (1980) Guidelines for the Release of Waste Water from Nuclear Facilities with Special Reference to the Public Health Significance of the Proposed Release of Treated Waste Waters at Three Mile Island (1987) Review of the Publication, Living Without Landfills (1989) Radon Exposure of the U.S. Population—Status of the Problem (1991) Misadministration of Radioactive Material in Medicine— Scientific Background (1991) Uncertainty in NCRP Screening Models Relating to Atmospheric Transport, Deposition and Uptake by Humans (1993) Considerations Regarding the Unintended Radiation Exposure of the Embryo, Fetus or Nursing Child (1994) Advising the Public about Radiation Emergencies: A Document for Public Comment (1994) Dose Limits for Individuals Who Receive Exposure from Radionuclide Therapy Patients (1995) Radiation Exposure and High-Altitude Flight (1995) An Introduction to Efficacy in Diagnostic Radiology and Nuclear Medicine (Justification of Medical Radiation Exposure) (1995) A Guide for Uncertainty Analysis in Dose and Risk Assessments Related to Environmental Contamination (1996) Evaluating the Reliability of Biokinetic and Dosimetric Models and Parameters Used to Assess Individual Doses for Risk Assessment Purposes (1998)

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Title Perceptions of Risk, Proceedings of the Fifteenth Annual Meeting held on March 14-15, 1979 (including Taylor Lecture No. 3) (1980) Critical Issues in Setting Radiation Dose Limits, Proceedings of the Seventeenth Annual Meeting held on April 8-9, 1981 (including Taylor Lecture No. 5) (1982) Radiation Protection and New Medical Diagnostic Approaches, Proceedings of the Eighteenth Annual Meeting held on April 6-7, 1982 (including Taylor Lecture No. 6) (1983)

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NCRP PUBLICATIONS

Environmental Radioactivity, Proceedings of the Nineteenth Annual Meeting held on April 6-7, 1983 (including Taylor Lecture No. 7) (1983) Some Issues Important in Developing Basic Radiation Protection Recommendations, Proceedings of the Twentieth Annual Meeting held on April 4-5, 1984 (including Taylor Lecture No. 8) (1985) Radioactive Waste, Proceedings of the Twenty-first Annual Meeting held on April 3-4, 1985 (including Taylor Lecture No. 9) (1986) Nonionizing Electromagnetic Radiations and Ultrasound, Proceedings of the Twenty-second Annual Meeting held on April 2-3, 1986 (including Taylor Lecture No. 10) (1988) New Dosimetry at Hiroshima and Nagasaki and Its Implications for Risk Estimates, Proceedings of the Twenty-third Annual Meeting held on April 8-9, 1987 (including Taylor Lecture No. 11) (1988) Radon, Proceedings of the Twenty-fourth Annual Meeting held on March 30-31, 1988 (including Taylor Lecture No. 12) (1989) Radiation Protection Today—The NCRP at Sixty Years, Proceedings of the Twenty-fifth Annual Meeting held on April 5-6, 1989 (including Taylor Lecture No. 13) (1990) Health and Ecological Implications of Radioactively Contaminated Environments, Proceedings of the Twenty-sixth Annual Meeting held on April 4-5, 1990 (including Taylor Lecture No. 14) (1991) Genes, Cancer and Radiation Protection, Proceedings of the Twenty-seventh Annual Meeting held on April 3-4, 1991 (including Taylor Lecture No. 15) (1992) Radiation Protection in Medicine, Proceedings of the Twentyeighth Annual Meeting held on April 1-2, 1992 (including Taylor Lecture No. 16) (1993) Radiation Science and Societal Decision Making, Proceedings of the Twenty-ninth Annual Meeting held on April 7-8, 1993 (including Taylor Lecture No. 17) (1994) Extremely-Low-Frequency Electromagnetic Fields: Issues in Biological Effects and Public Health, Proceedings of the Thirtieth Annual Meeting held on April 6-7, 1994 (not published). Environmental Dose Reconstruction and Risk Implications, Proceedings of the Thirty-first Annual Meeting held on April 12-13, 1995 (including Taylor Lecture No. 19) (1996) Implications of New Data on Radiation Cancer Risk, Proceedings of the Thirty-second Annual Meeting held on April 3-4, 1996 (including Taylor Lecture No. 20) (1997) The Effects of Pre- and Postconception Exposure to Radiation, Proceedings of the Thirty-third Annual Meeting held on April 2-3, 1997, Teratology 59, 181–317 (1999)

NCRP PUBLICATIONS

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21

22

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Cosmic Radiation Exposure of Airline Crews, Passengers and Astronauts, Proceedings of the Thirty-fourth Annual Meeting held on April 1-2, 1998, Health Phys. 79, 466–613 (2000) Radiation Protection in Medicine: Contemporary Issues, Proceedings of the Thirty-fifth Annual Meeting held on April 7-8, 1999 (including Taylor Lecture No. 23) (1999) Ionizing Radiation Science and Protection in the 21st Century, Proceedings of the Thirty-sixth Annual Meeting held on April 5-6, 2000, Health Phys. 80, 317–402 (2001) Fallout from Atmospheric Nuclear Tests—Impact on Science and Society, Proceedings of the Thirty-seventh Annual Meeting held on April 4-5, 2001, Health Phys. 82, 573–748 (2002)

Lauriston S. Taylor Lectures No. 1 2 3

4

5

6

7

8

9 10

Title The Squares of the Natural Numbers in Radiation Protection by Herbert M. Parker (1977) Why be Quantitative about Radiation Risk Estimates? by Sir Edward Pochin (1978) Radiation Protection—Concepts and Trade Offs by Hymer L. Friedell (1979) [Available also in Perceptions of Risk, see above] From ‘‘Quantity of Radiation’’ and ‘‘Dose’’ to ‘‘Exposure’’ and ‘‘Absorbed Dose’’—An Historical Review by Harold O. Wyckoff (1980) How Well Can We Assess Genetic Risk? Not Very by James F. Crow (1981) [Available also in Critical Issues in Setting Radiation Dose Limits, see above] Ethics, Trade-offs and Medical Radiation by Eugene L. Saenger (1982) [Available also in Radiation Protection and New Medical Diagnostic Approaches, see above] The Human Environment—Past, Present and Future by Merril Eisenbud (1983) [Available also in Environmental Radioactivity, see above] Limitation and Assessment in Radiation Protection by Harald H. Rossi (1984) [Available also in Some Issues Important in Developing Basic Radiation Protection Recommendations, see above] Truth (and Beauty) in Radiation Measurement by John H. Harley (1985) [Available also in Radioactive Waste, see above] Biological Effects of Non-ionizing Radiations: Cellular Properties and Interactions by Herman P. Schwan (1987) [Available also in Nonionizing Electromagnetic Radiations and Ultrasound, see above]

418

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12 13

14

15 16

17

18 19 20 21 22 23

24 25

NCRP PUBLICATIONS

How to be Quantitative about Radiation Risk Estimates by Seymour Jablon (1988) [Available also in New Dosimetry at Hiroshima and Nagasaki and its Implications for Risk Estimates, see above] How Safe is Safe Enough? by Bo Lindell (1988) [Available also in Radon, see above] Radiobiology and Radiation Protection: The Past Century and Prospects for the Future by Arthur C. Upton (1989) [Available also in Radiation Protection Today, see above] Radiation Protection and the Internal Emitter Saga by J. Newell Stannard (1990) [Available also in Health and Ecological Implications of Radioactively Contaminated Environments, see above] When is a Dose Not a Dose? by Victor P. Bond (1992) [Available also in Genes, Cancer and Radiation Protection, see above] Dose and Risk in Diagnostic Radiology: How Big? How Little? by Edward W. Webster (1992)[Available also in Radiation Protection in Medicine, see above] Science, Radiation Protection and the NCRP by Warren K. Sinclair (1993)[Available also in Radiation Science and Societal Decision Making, see above] Mice, Myths and Men by R.J. Michael Fry (1995) Certainty and Uncertainty in Radiation Research by Albrecht M. Kellerer. Health Phys. 69, 446–453 (1995). 70 Years of Radiation Genetics: Fruit Flies, Mice and Humans by Seymour Abrahamson. Health Phys. 71, 624–633 (1996). Radionuclides in the Body: Meeting the Challenge by William J. Bair. Health Phys. 73, 423–432 (1997). From Chimney Sweeps to Astronauts: Cancer Risks in the Work Place by Eric J. Hall. Health Phys. 75, 357–366 (1998). Back to Background: Natural Radiation and Radioactivity Exposed by Naomi H. Harley. Health Phys. 79, 121–128 (2000). Administered Radioactivity: Unde Venimus Quoque Imus by S. James Adelstein. Health Phys. 80, 317–324 (2001). Assuring the Safety of Medical Diagnostic Ultrasound by Wesley L. Nyborg. Health Phys. 82, 578–587 (2002)

Symposium Proceedings No. 1

2

Title The Control of Exposure of the Public to Ionizing Radiation in the Event of Accident or Attack, Proceedings of a Symposium held April 27-29, 1981 (1982) Radioactive and Mixed Waste—Risk as a Basis for Waste Classification, Proceedings of a Symposium held November 9, 1994 (1995)

NCRP PUBLICATIONS

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Acceptability of Risk from Radiation—Application to Human Space Flight, Proceedings of a Symposium held May 29, 1996 (1997) 21st Century Biodosimetry: Quantifying the Past and Predicting the Future, Proceedings of a Symposium held on February 22, 2001, Radiat. Prot. Dosim. 97, No. 1, 7–80 (2001)

NCRP Statements No. 1 2

3

4

5 6 7 8 9

Title ‘‘Blood Counts, Statement of the National Committee on Radiation Protection,’’ Radiology 63, 428 (1954) ‘‘Statements on Maximum Permissible Dose from Television Receivers and Maximum Permissible Dose to the Skin of the Whole Body,’’ Am. J. Roentgenol., Radium Ther. and Nucl. Med. 84, 152 (1960) and Radiology 75, 122 (1960) X-Ray Protection Standards for Home Television Receivers, Interim Statement of the National Council on Radiation Protection and Measurements (1968) Specification of Units of Natural Uranium and Natural Thorium, Statement of the National Council on Radiation Protection and Measurements (1973) NCRP Statement on Dose Limit for Neutrons (1980) Control of Air Emissions of Radionuclides (1984) The Probability That a Particular Malignancy May Have Been Caused by a Specified Irradiation (1992) The Application of ALARA for Occupational Exposures (1999) Extension of the Skin Dose Limit for Hot Particles to Other External Sources of Skin Irradiation (2001)

Other Documents The following documents of the NCRP were published outside of the NCRP report, commentary and statement series: Somatic Radiation Dose for the General Population, Report of the Ad Hoc Committee of the National Council on Radiation Protection and Measurements, 6 May 1959, Science, February 19, 1960, Vol. 131, No. 3399, pages 482-486 Dose Effect Modifying Factors In Radiation Protection, Report of Subcommittee M-4 (Relative Biological Effectiveness) of the National Council on Radiation Protection and Measurements, Report BNL 50073 (T-471) (1967) Brookhaven National Laboratory (National Technical Information Service Springfield, Virginia)

Index negligible 3, 28, 33–41, 48, 55, 147, 149–150, 153, 155–159, 163–164, 268–269, 272–273, 276–279, 295–296, 309, 312–313, 315, 318–320, 355–356, 364, 370 nomenclature issues 29, 33–36, 155–159, 163–164, 252–253, 268, 309, 355–358 noncarcinogenic responses see deterministic responses stochastic responses 33–34, 36, 39–42, 278–280, 312–313 trivial see negligible see also ALARA; Reference dose; Risk management paradigms; Unacceptable dose or risk Animal studies protocols for 81–85 use in hazard identification and dose-response assessment 47, 78–87, 99–102, 104–107, 122, 124, 126–129, 261, 263–265, 312 see also Dose-response assessment, basis Asbestos 22, 211, 233, 241 Atomic Energy Act (AEA) 8, 23–25, 28, 33–34, 36, 53–54, 146, 150, 156, 171–172, 182, 187–189, 191–192, 194–195, 212, 220–223, 229–235, 241–242, 250, 252, 273, 281, 316, 361

Absorbed dose, radiation 360 use in risk assessment 129–130, 138, 140 use in waste classification 166–167 Accelerator-produced waste see Radioactive materials, definitions, NARM; Waste classification system, existing radioactive, NARM waste ALARA (as low as reasonably achievable) 33–36, 133, 147–150, 152, 155–159, 163–164, 196–197, 231, 236–237, 268–269, 315, 355, 361 Allowable dose or risk 30–33, 160, 246, 266, 271, 275–276, 278, 318, 321, 324–325 acceptable 3, 28–29, 33–39, 41–42, 48, 55, 147–150, 153, 155–159, 164, 268–269, 272–273, 277–280, 295–296, 309, 312–313, 315, 318–320, 355–356, 358 background risks, use in establishing 40, 42, 145–146, 278–280, 283 barely tolerable see acceptable carcinogenic responses see stochastic responses de minimis see negligible deterministic responses 34, 36, 39–42, 276–278, 312–313 goals 34, 150–153, 163, 268–269 limits 33, 147–149, 155–157, 163, 268–269

Below regulatory concern (BRC) 196–199, 202–204, 209, 327, 361

420

INDEX

Benchmark dose definition and determination 47, 109–111, 362 use in cancer risk assessment 115–117 use in estimating threshold doses 47–48, 142, 264–265, 312, 320 use in exempt waste classification 37, 276–277 use in low-hazard waste classification 41, 277 see also Waste classification system, NCRP Beneficial uses of waste 2, 14–15, 21, 27, 38, 52, 66, 166, 198–200, 208, 215–216, 247, 356 Biohazardous waste 57, 213 Biokinetic models see Pharmacokinetic models Biologically-based models of cancer 103, 112, 119–121 Byproduct material see Radioactive materials, definitions Cancer lethality fractions 135–137, 259–262 models see Biologically-based models of cancer Carbon tetrachloride 118–119, 338 CERCLA (Comprehensive Environmental Response, Compensation, and Liability Act) 33–35, 152–153, 172, 214, 281, 313, 363 Characteristically hazardous waste see Waste classification system, existing chemical Chromium, toxicity 81 Classification of existing wastes see Waste classification system, NCRP, implications

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421

Clean Air Act (CAA) 152, 195, 215–216 Clean Water Act (CWA) 211–212, 215–216, 221, 241 Coal-burning power plant waste 21, 215–216 Committed dose, radionuclides 138, 144, 363 Consumer products 14, 197–198, 209 Corrosive waste 363 see Waste classification system, existing chemical, characteristically hazardous waste Critical effect 78–80, 103–106, 112 see also Hazard identification, chemical Critical organ, chemicals 50, 246, 289, 291, 338 see also Critical effect Critical response see Critical effect

Decommissioning waste 19, 206, 208 Deep-well injection 217 Delisting see Waste classification system, existing chemical, listed waste de manifestis dose or risk see Unacceptable dose or risk Derived-from rule see Waste classification system, existing chemical, listed waste Deterministic responses, definition see Dose-response assessment Dioxins 22, 118–119, 127, 129, 211, 218, 225, 233, 241, 350 Disposal technologies see Waste disposal technologies Department of Energy (U.S.) Organization Act 172

422

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INDEX

Dose meanings for chemicals and radiation 29, 88, 365 utility for radiation risk assessment 129–131, 138–140 Dose and dose-rate effectiveness factor 133–134 Dose-response assessment basis for 99, 102 carcinogens see stochastic responses characterization 122–123 comparison for chemicals and radionuclides, deterministic responses 29, 141–142, 161–162, 311–312 see also use in risk assessment and management comparison for chemicals and radionuclides, stochastic responses 29, 44–46, 114, 142–145, 162–163, 237–239, 310–311, 320 see also use in risk assessment and management confidence level in 109 database 105, 132 deficiencies, chemicals 123–129 definition 88, 365 deterministic responses, chemicals 102–111 deterministic responses, definition 74, 318, 364 deterministic responses, estimation of thresholds 110–111, 131, 264–265 deterministic responses, radionuclides 131 epidemiological studies 75–76, 78–79, 81, 85–86, 99–100, 102, 114, 127–128, 131–134, 239 extrapolation models 99–102, 122, 124–125

linearized-multistage model 45, 120–122, 125, 265, 310 lower confidence limit or bound see upper or lower confidence limits or bounds maximum likelihood estimates (MLE) 114–115, 122, 126–127, 132–133, 141–142, 145, 265–266, 310–311, 320, 369 modeling, use of 109–111, 120–122, 126–127 statistical models 113–115 stochastic responses, chemicals 111–122, 265–266 stochastic responses, definition 74, 318, 374 stochastic responses, radionuclides 131–134, 265–266 threshold see deterministic responses, estimation of thresholds uncertainties 123–125, 133–134, 141 uncertainties, comparison for radionuclides and chemicals 134, 239 upper or lower confidence limits or bounds 110, 113–115, 122, 126–127, 141–142, 145, 161–162, 265–266, 310–311, 320, 369, 375 use in risk assessment and management 75–77, 145–146, 161–163 Dose-response relationship see Dose-response assessment

Effective dose 49, 138–140, 143–144, 235, 287, 297, 365 Emergency Response and Community Right-to-Know Act 21, 214, 366 Energy Reorganization Act 172 Environmental standards air 152, 195

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423

drinking water 126, 151–152, 154–155, 157, 213–214, 232, 234, 338 remediation 151–153, 198 Epidemiological studies see Dose-response assessment Equivalent dose 130–131, 137–140, 235, 366 Exemptions, existing chemical waste 21–22, 215–216, 246–247 radioactive waste 11, 14, 168–169, 171, 195, 197–200, 247 source and byproduct materials 14, 197–200, 247, 302, 327–328, 362, 374 see also Waste classification system, existing chemical; Waste classification system, existing radioactive Exempt waste 366 see Below regulatory concern; Exemptions, existing; IAEA; Waste classification system, NCRP Exposure assessment 88–92 applied to waste classification 96–98 see also Inadvertent intrusion, general; Waste classification system, NCRP, exposure scenarios Extremely hazardous waste 21, 214, 217, 245, 305, 351, 366

weight-of-evidence judgments 78, 82, 84, 86–87 Hazard identification, radiation 76 Hazardous and Solid Waste Amendments 367 see RCRA Hazardous, definition 6, 57, 367 Hazardous material life cycle 58–59 Hazardous waste chemical waste identification 87–88 definition 20–21, 212–214, 367 see also Waste classification system, existing chemical; Waste classification system, existing radioactive Hazardous waste sites, screening or ranking 7, 66–67, 98 Health effects, types see Measure of response Heat generating wastes 17, 19, 69, 172, 176, 179–180, 201, 204, 206–208, 210, 306–307, 353 Heavy metals, waste containing 20–21, 24–25, 51–52, 307–308, 334, 336–342, 344–348, 350 High-hazard waste see Waste classification system, NCRP High-level waste 367 see IAEA; Waste classification system, existing radioactive

Federal Facility Compliance Act 220, 226–228, 249

IAEA (International Atomic Energy Agency), recommendations on radioactive waste classification comparison with U.S. system 17, 20, 209–211 early recommendations 203–205 exempt waste 17–18, 20, 39, 205–206, 278 exemption principles 34, 149, 208–211

Geologic repository 367 see Waste disposal technologies Greater confinement disposal 367 see Waste disposal technologies Hazard identification, chemical 76–87, 367 deterministic responses 78–82 stochastic responses 82–87

424

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INDEX

long-lived waste see low- and intermediatelevel waste low- and intermediate-level waste 17–19, 205–207, 306–307 high-level waste 17, 19, 205–208, 307 relationship to disposal technologies 17, 205, 207–208 short-lived waste see low- and intermediatelevel waste waste containing long-lived, naturally occurring radionuclides 19–20, 206, 208 ICRP (International Commission on Radiological Protection), recommendations on radiation protection 135–140 Ignitable waste 368 see Waste classification system, existing chemical, characteristically hazardous waste Immobilization of hazardous waste 20–21, 59, 191–192, 194, 215, 218 Inadvertent intrusion, general 32–33, 96–98 see also Waste classification system, NCRP, exposure scenarios Incidental waste 9–11, 168–169, 177, 180 Incineration of hazardous waste 20, 59, 215, 217–218, 225, 245–246, 350–351 Industrial waste management see Waste disposal technologies, municipal landfill Institutional control 368 chemical waste facilities 21–23, 25–26, 41–43, 218–219, 246, 249, 273, 281–282, 298, 303–304, 314, 316, 349–351 radioactive waste facilities 12, 23, 41–43, 190, 207, 231, 273,

279–280, 282, 284, 298, 303–304, 314, 316, 350 role in waste classification 12–13, 32, 41–43, 51–52, 96–97, 190, 207, 267, 273, 279–282, 298–299, 302–303, 313–314, 320, 329, 331–336, 345–347, 350, 357–358 Intolerable dose or risk see Unacceptable dose or risk Land disposal see Waste disposal technologies, near-surface disposal Land disposal restrictions, chemical waste 218, 223, 225–226, 229–232 Lethality fractions see Cancer Life cycle see Hazardous material life cycle Linearized-multistage doseresponse model see Dose-response assessment Liquid scintillation materials 14, 197, 227 Liquid wastes 69, 172–178, 180–181, 193, 204, 212–213, 217, 220, 227, 230, 252 Listed hazardous waste see Waste classification system, existing chemical LOAEL (lowest-observed-adverseeffects level) definition and determination 34, 103–105, 109–112, 369 use in estimating threshold doses 312 use in health protection 34, 106–108, 112, 269, 345 use in waste classification system, NCRP 39, 277 Low-hazard waste see Waste classification system, NCRP Low-Level Radioactive Waste Policy Act 182, 187, 189

INDEX

Low-Level Radioactive Waste Policy Amendments Act 187–190, 222 Low-level waste see Waste classification system, existing radioactive Margin of exposure analysis 116–117 Marine Protection, Research and Sanctuaries Act 177 Maximum tolerated dose 84, 116 Measure of response fatalities 29, 44, 55, 73, 94, 134–135, 141–143, 145, 162–163, 258–263, 310, 355 incidence 29, 44, 55, 73–74, 82–84, 94, 101–102, 115, 129, 133–135, 137, 141–143, 145, 161–163, 238–239, 258–263, 310–311, 335–338, 355, 364 total detriment 135–140, 143, 258, 260–262 Mill tailings see Waste classification system, existing radioactive Mining wastes 1, 14, 19, 43, 52, 191, 194–196, 205–206, 208, 212, 282, 303, 307, 335, 349 Mixed waste compliance difficulties 25, 53, 65, 220–221, 225–229, 233–235, 242, 249–250, 354 definition 24, 165–166, 212, 219–224, 233, 241, 369 dual regulation, consequences 24, 220–235, 249–250 exemption from dual regulation 24, 229–232, 250 National Capacity Variance 225–226 regulatory guidance 224–225 sources and amounts 220, 227–228 state regulation 222–223, 233 treatment standards 24, 225, 229 Mixture rule

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425

see Waste classification system, existing chemical, listed waste Modifying factor see Reference dose; Waste classification system, NCRP, Risk index Municipal waste management see Waste disposal technologies, municipal landfill NARM (naturally occurring and accelerator-produced radioactive material) see Radioactive materials, definitions; Waste classification system, existing radioactive National Energy Policy Act 28, 199, 270, 315 National Security and Military Applications of Nuclear Energy Authorization Act 38, 182, 185–186 Natural background risks chemicals 40, 42, 145–146, 278–280 radiation 40, 42, 131, 133, 145–146, 199, 237, 278–280 use in classifying waste 40, 42, 278–280 Naturally occurring hazardous substances chemicals 1, 25, 31, 40, 43, 52, 145–146, 216, 278–279, 282, 334 radionuclides 1, 19–20, 25, 31, 40, 43, 51, 145, 149, 171, 174, 196, 198–199, 205–206, 208–209, 216, 245, 279, 282, 334 Naturally-occurring radionuclides, waste containing see NARM, NORM NCRP recommendations application to waste classification 37, 39, 41–42, 237, 278–279

426

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INDEX

comparative carcinogenicity of radiation and chemicals 237–239 negligible individual dose 37, 39, 237, 278 radiation protection 33, 41–42, 235–237, 279 see also Waste classification system, NCRP Near-surface disposal 368, 370 see Waste disposal technologies NOAEL (no-observed-adverseeffects level) definition and determination 34, 103–105, 109–112, 370 use in estimating threshold doses 47–48, 104–106, 264–265, 312 use in health protection 34, 106–108, 112, 142, 269, 345 use in waste classification system, NCRP 39, 277 Noncritical effect 104–105 NORM (naturally occurring radioactive material wastes) see Radioactive materials, definitions; Waste classification system, existing radioactive Nuclear fuel cycle 8, 167, 370 Nuclear Waste Policy Act (NWPA) 38, 177–182, 187, 189, 200–202 PCBs (polychlorinated biphenyls) 22, 211, 220, 233, 241, 370 Pharmacokinetic models compartment models 117, 131 general 82–83, 126, 128, 141, 370 physiologically-based models 110, 117–119, 131, 144 Phosphogypsum 191, 195, 215–216 Probability coefficient, stochastic responses 44–46, 49, 99, 133, 135–137, 139–140, 142–144, 148, 236, 260–266, 276, 278,

287, 295, 310–311, 320, 337, 344, 371 see also Dose-response assessment, stochastic responses

Radiation protection standards see ICRP; NCRP recommendations, radiation protection; Risk management paradigms, stochastic responses, radionuclides Radiation weighting factor 130, 132, 138–140, 371 Radioactive materials, definitions byproduct material 171, 362 NARM 8, 170, 194, 370 NORM 8, 170, 194–195, 370 source material 171, 374 special nuclear material 171, 374 RCRA (Resource Conservation and Recovery Act) 20–25, 28, 33–34, 38–39, 41, 53, 152, 172, 196–197, 211–224, 226–235, 240–242, 245, 249–250, 252, 270, 272–273, 278, 281, 316, 339, 343, 346, 348–349 Reactive waste 371–372 see Waste classification system, existing chemical, characteristically hazardous waste Recycling and reuse see Beneficial uses of waste Reference dose (RfD) definition and determination 34–36, 103–110, 142, 372 modifying factor, U.S. Environmental Protection Agency 106–109 uncertainty factor, U.S. Environmental Protection Agency 106–109 use in exempt waste classification 39–40, 277, 313, 324–325, 337–338

INDEX

use in health protection 34, 105–108, 112, 142, 154–155, 269, 337, 345 use in low-hazard waste classification 42, 277–278, 313, 324, 337–338, 340, 342, 344–345 Risk assessment, definition 75, 373 Risk assessment process description 75–77 site-specific 3, 5–6, 32, 38, 63, 69, 95–98, 160–161, 208, 244, 267, 274, 301, 357 use in waste classification 63, 95–99, 296–297 see also Hazard identification; Dose-response assessment; Exposure assessment; Risk characterization Risk characterization 76, 92, 94, 373 Risk, general considerations basis for waste classification 1, 6, 26, 28, 63–65, 72, 160, 243–246, 256–258 definition 64, 73, 160, 373 measures of 29, 44, 55, 65, 73–74, 246 nomenclature issues see Allowable dose or risk, nomenclature issues see also Measure of response; Waste classification system, NCRP, development needs Risk index see Waste classification system, NCRP Risk management ALARA, importance of 157, 159, 164, 269 see also Risk management paradigms, stochastic responses, reconciliation of differences relationship to risk assessment 94–95, 145, 326

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use in waste classification 31, 46, 63, 67–68, 72, 77, 94–95, 145, 160, 163, 239, 266, 268–269, 295–296, 311, 321 see also Risk management paradigms Risk management paradigms, deterministic responses chemicals 34–35, 46, 141, 154–155, 276, 286, 288 comparison of chemicals and radionuclides 46–47, 141–142, 146, 276 radionuclides 46, 131, 141, 276, 286, 288 Risk management paradigms, stochastic responses chemicals 34–35, 150–154, 163–164, 268, 355–356 comparison of chemicals and radionuclides 29, 33, 35–36, 150–152, 155–157, 163–164, 253, 268–269, 355–356 organs accounted for 29, 44, 46, 48–49, 101–102, 136–140, 144, 162–163, 287, 297, 310, 355 see also Critical organ; Effective dose radionuclides 33–34, 146–150, 163, 268, 272–273, 355 reconciliation of differences 35, 157–159, 164, 309, 315 Safe Drinking Water Act 34, 146, 151–152, 213 Safety factor approach see Reference dose Scrap metals, contaminated 2, 343 Selenium, toxicity 70 Sewage sludge 21, 211–212, 215–216, 241 Shallow-land burial see Waste disposal technologies, near-surface disposal Single organ, chemicals see Critical organ, chemicals

428

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INDEX

Slope factor 99, 335, 337–338, 342, 344, 346–347, 373 see also Probability coefficient, stochastic responses Solid waste, definition 24, 212, 221, 252 Source constraints 148–149, 236–237 see also Environmental standards Source material see Radioactive materials, definitions Special nuclear material see Radioactive materials, definitions Stabilization of hazardous waste see Immobilization of hazardous waste Stochastic responses, definition see Dose-response assessment Structure-activity relationships 78, 80, 82, 86, 141 Subclassification of waste classes see Waste classification system, existing chemical; Waste classification system, existing radioactive; Waste classification system, general; Waste classification system, NCRP Subseabed disposal 181 Superfund see CERCLA Tissue weighting factor, radiation 138–140, 143, 374 Toxic Substances Control Act (TSCA) 22, 172, 196, 211, 220, 227–228, 232–233, 241, 375 Transuranic waste see Waste classification system, existing radioactive Unacceptable dose or risk 33–36, 148–150, 153, 155–159, 163–164, 268–269, 309, 318, 356, 358, 364

Uncertainty 375 treatment in risk assessment 63, 94, 100–101, 103–111, 113–115, 122–126, 133–134, 141–142, 161–162, 263–264, 266, 269, 271, 311–313, 320–321, 345 types 94 Uncertainty factor see Reference dose (RfD), uncertainty factor, U.S. Environmental Protection Agency Uranium Mill Tailings Radiation Control Act 191–192 Waste, definition 5, 57 Waste classification, general bases for waste classification 62–63, 65 definition 5, 59, 376 purpose 5, 60–62, 357 see also Waste classification system, general Waste classification system, alternative radioactive characteristics 240 Kocher and Croff 200–202 LeMone and Jacobi 203–204 Smith and Cohen 202–203 U.S. Nuclear Regulatory Commission 200 see also IAEA Waste classification system, existing chemical bases 1, 6, 54, 64–65, 216, 241, 245 characteristically hazardous waste 20–22, 213–218, 241, 362–363, 368, 371–372 deficiencies 1–2, 4, 7, 25–26, 64–66, 216, 241, 245–248, 251–253 disposal requirements, relationship to waste classification 4, 21, 23, 218, 241, 303, 305 exclusions 215–216, 241, 248

INDEX

exemption 21, 53, 215, 219, 247, 302, 328 hazardous waste, definition 212–214, 241, 367 implications 23, 65–66, 214, 216, 241, 245, 247, 249–253 industrial waste see municipal waste listed waste 21, 214–219, 234–235, 241, 245, 247, 270, 316, 364, 368–369 municipal waste 22, 38, 41, 219, 272, 281 state programs 21, 216–217, 241 subclassification 23, 214, 241, 306 waste treatment, effect on classification 20–21, 214–215 see also Extremely hazardous waste; Land disposal restrictions; Mixed waste; Waste classification system, general Waste classification system, existing mixed waste see Mixed waste Waste classification system, existing radioactive bases 1, 6, 9, 13, 16, 54, 64–65, 173, 175–177, 240, 245, 251, 253–254 comparison with IAEA recommendations 17, 20, 209–211 deficiencies 1–2, 4, 7, 15–17, 25–26, 64–66, 240, 245, 247–248, 251–254, 314–315 disposal requirements, relationship to waste classification 4, 12–13, 16–17, 175, 178–179, 193–194, 210, 240 distinction between nuclear fuel-cycle and other wastes 8, 28, 170–172, 240, 248, 270, 314 exclusions 195

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exemptions 11, 14, 168–169, 196–200, 247, 270, 302, 327–328, 366 see also Below regulatory concern; IAEA greater-than-Class-C waste see low-level waste, greaterthan-Class-C high-level waste, current definition 10–11, 168–169, 177–180, 367 see also IAEA high-level waste, disposal requirements 12, 181–182, 193–194 high-level waste, historical definitions 172–174, 176 high-level waste, sources and properties 9, 176–177, 209–210 implications 9, 12–13, 180, 189, 192–193, 209, 240, 247, 251–254 incidental waste 9, 11, 169, 177, 180, 343, 349 intermediate-level waste see IAEA international see IAEA low-level waste, current definition 9–10, 65, 168, 187–189, 369 see also IAEA low-level waste, disposal requirements 12, 189–191, 193–194 low-level waste, greater-thanClass-C 8, 12–13, 32, 52, 190–191, 193, 201, 252, 304, 306–307, 349 low-level waste, historical definitions 172–175, 182–183 low-level waste, sources and properties 187, 189, 209–210, 307 low-level waste, subclassification 8, 13, 30–32,

430

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53, 69, 170–171, 190, 245, 251, 284, 306, 308, 352–353 mill tailings, definition 10, 168, 191, 369 see also IAEA mill tailings, disposal requirements 12, 43, 191–192, 194, 282, 284, 303–304, 335 mill tailings, properties 191, 194, 284, 303–304, 306–307, 334, 358 NARM waste 8, 11, 13–14, 168, 170–172, 194–196, 240, 352 see also NORM waste NORM waste 8, 11, 13–14, 170–171, 194–195 see also NARM waste reprocessing wastes see high-level waste; Liquid wastes source-based definitions see bases spent nuclear fuel, definition 10–11, 177–178 see also high-level waste, current definition summary 7–8, 10–11, 168–171 transuranic waste, current definition 10, 168, 183–185, 375 transuranic waste, disposal requirements 12, 174–175, 185–187, 193–194 transuranic waste, historical definition 174–175 transuranic waste, sources and properties 184–185, 209 transuranic waste, subclassification 8, 13, 170–171, 174, 184–185, 306–307, 352–353 see also Mixed waste; Waste classification system, general Waste classification system, general comparison of systems for chemical and radioactive wastes 22–23

deficiencies 25–26, 64–66, 243–255, 354 description 165–166 desirable attributes 63, 243–255, 356 disposal technologies assumed 61, 68 generic disposal sites, focus on 63, 69, 77, 92, 160–161, 244 subclassifications 68–69 see also Waste classification, general; Waste classification system, existing chemical; Waste classification system, existing radioactive Waste classification system, NCRP advantages 1–2, 4, 55–56, 301, 359 allowable dose or risk, assumptions on 33, 272, 276, 278, 312, 320 see also Allowable dose or risk allowable dose or risk, acceptable for classifying lowhazard waste 3, 28, 41–44, 48, 277–280, 312–313 allowable dose or risk, negligible for classifying exempt waste 3, 28, 37, 39–40, 44, 48, 276–279, 312–313 applicability 7, 27, 66–67, 69–70, 256, 258, 357 association with disposal technologies 1, 4, 26, 66, 68, 256, 270, 272, 317, 356–357 assumptions 27–28, 258, 263–264, 266, 268, 317 basic principles and framework 1–3, 6, 26–27, 37–39, 54, 63–64, 256–258, 270–274, 317, 354, 356 benchmark dose, use in classifying waste 37, 41, 47–48, 276–277 see also Benchmark dose

INDEX

boundaries between waste classes, establishing 7, 28, 270, 280, 295–297, 318–320, 356–357 challenges in developing 28–29, 354–356 classification of existing wastes see implications conservative assumptions 40, 45–47, 85, 98, 244, 266–267, 277, 301, 311, 319–321, 357 development needs, doseresponse assessment 55, 309–312 development needs, establishing negligible and acceptable risks 55, 312–313 development needs, estimating health impacts 55, 312–313 development needs, exposure scenarios 55, 313–314 development needs, legal and regulatory 314–316 development needs, nomenclature 309, 357–358 see also Allowable dose or risk, nomenclature issues development needs, reconciliation of risk management paradigms 315 dose-response relationships, deterministic responses 28, 47–48, 264–265, 286, 288, 312, 320 dose-response relationships, maximum likelihood estimates versus upper confidence limits 45–46, 265–266, 311, 320 dose-response relationships, stochastic responses 28, 45–46, 265–266, 286–287, 310–311 ecological impacts, neglect of 69–70 example applications, assumptions 323–337, 339–340, 342–347

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example applications, chemical waste 346–347 example applications, commercial low-level radioactive waste 332–333 example applications, U.S. Department of Energy lowlevel waste 328–332 example applications, electric arc furnace dust (mixed waste) 336–347 example applications, exempt wastes 326–328 example applications, highgrade uranium ore residues 335–336 example applications, uranium mill tailings 333–335 exempt waste, definition 2, 26, 257, 272, 281, 317, 356 exempt waste, determination 3, 37, 257, 272, 318–319, 356–357 exempt waste, disposal technology 2, 26, 37, 40, 257, 272, 281, 317, 356 exposure scenarios, general 3, 32–33, 266–267, 274, 280, 320, 357 exposure scenarios, inadvertent intrusion 32–33, 40–44, 281–283 see also Inadvertent intrusion, general exposure scenarios, site-specific considerations 3, 267–268 exposure scenarios, time period for applying 43, 298–300 high-hazard waste, definition 2, 26, 257, 274, 317, 356 high-hazard waste, determination 3, 43, 257, 274, 318, 356–357 high-hazard waste, disposal technology 2, 26, 43–44, 257, 274, 317, 356 implementation 295–300

432

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implications, chemical wastes 3–4, 51–52, 54, 301–305, 308, 358 implications, exemption of waste 51, 53, 315, 348, 358 implications, laws and regulations 53–54 implications, mill tailings 43, 51–52, 282, 302–304, 349, 358 implications, mixed wastes 53, 316, 351–352, 359 implications, NARM and NORM wastes 54, 302–303, 314, 349, 352 implications, source-based classifications 54, 349 implications, subclassification 69, 352–353, 357 legal and regulatory impediments 28, 269–270 low-hazard waste, definition 2, 26, 257, 273, 317, 356 low-hazard waste, determination 3, 41, 273, 318, 356–357 low-hazard waste, disposal technology 2, 26, 41–42, 257, 273, 317, 356 measures of response in classifying waste 44, 262–263, 310–311 see also Measure of response nonexempt waste 2, 26–27, 38, 50–51, 271–274, 284, 292, 299–300, 314, 317, 356 see also high-hazard waste; low-hazard waste Risk index, composite 48–51, 285–292 Risk index, definition 30, 271, 275, 318, 356 Risk index, deterministic responses 31, 49–50, 275, 288–291, 318 Risk index, modifying factor 30–31, 40, 49–50, 271, 275,

283–284, 287–288, 290–291, 293, 311 Risk index, stochastic responses 31, 48–49, 275, 286–288, 318 Risk index, sum-of-fractions rule 287, 290–291, 296, 318 Risk index, use in classifying waste 31, 44, 50–51, 256–258, 291–295, 318–319, 356–357 risk management paradigm 35–37, 268–269 risk, use in classifying waste 1, 3, 6, 26, 28–31, 63–64, 256–258, 274–275 shortcomings 300–301 subclassification 52–53, 303, 305–308 summary 2–3, 38, 257, 317–321, 356–357 Waste disposal, approaches and regulations chemical waste 20–22, 214–215, 217–219, 241, 303, 349–350 see also Waste classification system, existing chemical, characteristically hazardous waste; Waste classification system, existing chemical, listed waste comparisons of chemical and radioactive wastes 23–24, 32–33, 248–250, 252, 349–350 radioactive waste 12, 43, 181–182, 185–187, 189–194, 249–250, 282, 284, 303–304 see also Waste classification system, existing radioactive Waste disposal technologies geologic repository 10, 12–13, 38–39, 68, 168–169, 178–182, 190–191, 206–208, 230–232, 256–257, 274, 283, 324, 367

INDEX

greater confinement disposal 11, 38, 194, 304, 367 industrial landfill see municipal landfill municipal landfill 37–41, 68, 196, 219, 257, 272–273, 281, 324–325, 370 near-surface disposal 10–13, 38–39, 68–69, 96–98, 161, 168–169, 189–191, 217–219, 223–224, 256–257, 266–267, 273–274, 279–283, 313–314, 324–325, 349–352, 368, 370 shallow-land burial see near-surface disposal

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Waste Isolation Pilot Plant 24, 169, 185–186, 193, 223, 231, 249–250 Waste Isolation Pilot Plant Land Withdrawal Act 183–186, 231–232 Waste management definition 59, 376 description 58–59 steps in 60–62 West Valley Demonstration Project Act 177 Yucca Mountain repository 169, 181–182, 187, 193