The Chernobyl Accident and its Implications for the United Kingdom: Watt Committee: report no 19 (Watt Committee Report No 19)

The Chernobyl Accident and its Implications for the United Kingdom Watt Committee Report Number 19

Members of The Watt Committee on Energy Working Group on the Chernobyl Accident and its Implications for the United Kingdom This report has been compiled by the Working Group on the Chernobyl Accident and its Implications for the United Kingdom. The members of the Working Group were: N.G.Worley (Chairman) Dr F.R.Allen F.J.L.Bindon R.Bulloch Prof. A.Charlesby Dr D.R.Cope Prof. P.M.S.Jones Dr J.D.Lewins

G.Lewis Dr. G.K.C.Pardoe P.D.Potter Dr F.B.Smith Prof. G.N.Walton J.G.Mordue (Secretary) G.F.Oliver (Information Officer)

The Chernobyl Accident and its Implications for the United Kingdom Edited by

NORMAN WORLEY C Eng, BSc (Eng) (Chem Eng) Lond, ACGI, M Inst of Energy Deputy Chairman of The Watt Committee on Energy Chairman of the Working Group on the Chernobyl Accident and its Implications for the United Kingdom appointed by The Watt Committee on Energy and

JEFFERY LEWINS MA, MSc, PhD, DSc (Lond), C Eng, PPINucE, F Am Nuc S Lecturer in Nuclear Engineering at the University of Cambridge

Report Number 19

Published on behalf of THE WATT COMMITTEE ON ENERGY by ELSEVIER APPLIED SCIENCE PUBLISHERS LONDON and NEW YORK

ELSEVIER SCIENCE PUBLISHERS LTD Crown House, Linton Road, Barking, Essex IG11 8JU, England This edition published in the Taylor & Francis e-Library, 2003. Sole Distributor in the USA and Canada ELSEVIER SCIENCE PUBLISHING CO., INC. 52 Vanderbilt Avenue, New York, NY 10017, USA WITH 14 TABLES AND 35 ILLUSTRATIONS © 1988 THE WATT COMMITTEE ON ENERGY Savoy Hill House, Savoy Hill, London WC2R 0BU British Library Cataloguing in Publication Data The Chernobyl accident and its implications for the United Kingdom.—(Watt Committee report; no. 19). 1. Ukraine. Chernobyl. Nuclear power stations. Accidents, 1986 I. Worley, Norman G. II. Lewins, Jeffery D. 1930– III. Watt Committee on Energy IV. Series 363.1’79 ISBN 0-203-21644-X Master e-book ISBN

ISBN 0-203-27264-1 (Adobe eReader Format) ISBN 1-85166-219-7 (Print Edition) Library of Congress Cataloging-in-Publication DatA The Chernobyl accident and its implications for the United Kingdom/ edited by Norman G.Worley and Jeffery D.Lewins. p. cm.—(Watt Committee report; no. 19) “This report has been compiled by the Working Group on the Chernobyl Accident and its Implications for the United Kingdom”— P. “Published on behalf of the Watt Committee on Energy by Elsevier Applied Science Publishers.” ISBN 1-85166-219-7 1. Nuclear power plants—Government policy—Great Britain. 2. Nuclear power plants—Accidents—Environmental aspects. 3. Chernobyl Nuclear Accident. Chernobyl. Ukraine, 1986. I. Worley, Norman G. II. Lewins, Jeffery D. III. Working Group on the Chernobyl Accident and its Implications for the United Kingdom. IV. Series. HD9698.G72C47 1988 363.1’79–dc19 88–7286 CIP The views expressed in this Report are those of the authors of the papers and contributors to the discussion individually and not necessarily those of their institutions or companies or of The Watt Committee on Energy. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Special regulations for readers in the USA This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside the USA, should be referred to the publisher. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher.

Foreword There can have been few occasions when an accident to a major work of engineering of any kind was of such immediate concern to governments and peoples throughout the world as was the nuclear power-plant accident at Chernobyl. The collapse of a bridge or a fire in a coal mine, for example, may win headlines in many countries and cause more immediate casualties, but neither civil accidents like these nor natural disasters like earthquakes and famines are likely to be major stories everywhere for so long. For the ordinary man and woman, nuclear power is incomprehensible and horrifying, even if it is possible to distinguish it from the nuclear weapons issue; even for professionally qualified readers, some half million of whom, in the United Kingdom, are represented by the professional institutions constituting The Watt Committee on Energy, understanding is often difficult and opinions range right through the spectrum from enthusiasm for nuclear power to outright opposition. The Watt Committee Executive, when it met soon after the accident occurred in April 1986, saw at once that the implications of what had happened, both immediately and over a period of perhaps years to come, would be far-reaching: perhaps more so because the possible benefits and dangers of nuclear power were already a matter of fierce public controversy, in the United Kingdom as in many other advanced countries, and it was appreciated that important national decisions were being made and could be affected. The importance of these decisions was not limited to the few thousands who would construct a nuclear power station, work in it and live near it; they would have an impact on the technological base, and therefore on the economic prosperity, of the whole country, and on the consumers of energy—specifically of electric power which, in the United Kingdom, means virtually everybody; and these effects will be with us for as long as anyone can foresee. The attitude of The Watt Committee on Energy to the civil nuclear power question was stated in a Report entitled Nuclear Energy: a Professional Assessment, published in March 1984. The then Chairman of the Watt Committee, Dr Jack Chesters, in his foreword to

that Report, explained how I had been made Chairman of the Working Group that produced it. Briefly paraphrased, our hope was that we had succinctly described the technology and stated the main issues in such a way that the reader, whether he supported or opposed nuclear power when he began to read, would have a better understanding when he finished. We were therefore in a good position, two years later, to take part in the reassessment of nuclear power that was bound to result from the Chernobyl accident. This did not mean, of course, that we wished to start from scratch in reconsidering every aspect of civil nuclear power, even if our resources had been sufficient. We appointed a new working group, including some members of the previous group, and our instructions to it, narrow in scope but requiring thorough investigation, were to consider the implications of the Chernobyl accident for the United Kingdom. Two years later, a huge volume of information has been made available (some vital, some marginally relevant) and the arguments have been exhaustively ventilated. Our qualification for adding to the mountains of paper (not to mention other media coverage) is that the members of our group, and of the Watt Committee as a whole, include both those who are expert in the many relevant specialist disciplines and those who, having no previous connections with the nuclear industry, are familiar with the needs of public debate. The principal objective of the Watt Committee is to promote the discussion of questions concerning energy for the benefit of the public at large, bringing together, in an impartial forum, those with professional knowledge from a wide variety of backgrounds; and the Chernobyl accident and its implications, considered as a subject for study by one of our specialist groups, presents the need for this approach particularly acutely. The members of the Working Group are listed elsewhere in this Report. On behalf of The Watt Committee on Energy, I am grateful to them all, and can only mention here the outstanding importance of Norman Worley’s role as its Chairman. The study could not have been undertaken (or not on anything like this v

vi

scale) without the financial support of the Central Electricity Generating Board, the South of Scotland Electricity Board, British Nuclear Fuels plc, the United Kingdom Atomic Energy Authority and the National Nuclear Corporation, which was given on the condition, clearly understood, that the Watt Committee was free to conduct the study as it wished and reach whatever conclusions seemed appropriate. I must also acknowledge the willingness of these and other organisations to provide us with any information that we asked for, including the opportunity of visiting nuclear establishments. Our thanks are due to the many individuals in these organisations who helped us.

Foreword

The complexity of life in a modern trading nation is beyond the appreciation of most of its citizens, and the specific individual effects of even such a major event are small by the time they have ramified into every home and work place in the land. Nevertheless, there are comparatively small numbers of people, mainly in limited areas, for whom the direct effects of Chernobyl are still important, and the long-run indirect effects are important for almost everyone. What these effects are, and what decisions are required, is clarified, I hope, in this Report. G.K.C.PARDOE Chairman, The Watt Committee on Energy

Contents List of the Chernobyl accident Working Group members

ii

Foreword G.K.C.PARDOE

v

Background NORMAN WORLEY

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Section

1 1.1 1.2 1.3

Section

Section

Section

Section

Section

Section

Section

Section

Appendices

2

3

4

5

6

7

8

9

Introduction NORMAN WORLEY General description of a nuclear power system Fuel meltdown incidents Energy in the Soviet Union

1 1 4 6

The design of the Chernobyl Unit 4 reactor PETER POTTER

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Description of the Chernobyl accident FRANK ALLEN

19

The radioactive release from Chernobyl and its effects BARRY SMITH and ARTHUR CHARLESBY

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Accident management in the USSR and the United Kingdom GLYNNE LEWIS

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United Kingdom and USSR reactor types JEFFERY LEWINS

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Reactor operation and operator training in the United Kingdom JOHN BINDON

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International dimensions of the implications of the Chernobyl accident for the United Kingdom DAVID COPE

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Comments, recommendations and conclusions

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1. Glossary of terms 2. Units of measurement 3. Summary of significant dates and timing relevant to the Chernobyl accident vii

101 104 105

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Contents

4. Nuclear safety in the Soviet Union From ‘Nuclear Power in the Soviet Union’ by B.A.Semenov—June 1983 International Atomic Energy Agency Bulletin (Vol. 25, No. 2) 5. Chemical reaction aspects of the Chernobyl accident—Gilbert Walton, Jeffery Lewins and Norman Worley 6. The Chernobyl accident trial An article published in The Independent on 30 July 1987—Anthony Barber of Reuters 7. Continuing radiation leakage from Chernobyl An article released by Reuters, Moscow, 4 December 1987 8. International aspects— Norman Worley including: Learned societies European organisations Organisations for economic co-operation and development The International Atomic Energy Agency Recommendations for the IAEA 9. The Nuclear Installations Inspectorate 10. Energy casualties 11. Presentations to the Working Group on the Chernobyl accident A. Presentation on Inherently Safe Reactors Given on 11 February 1987 by Michael Hayns, Head of Nuclear Safety, Technology Branch, Safety and Reliability Directorate, UK Atomic Energy Authority B. Presentation on The Chernobyl Accident and its Consequences’ Given on 1 April 1987 by John Gittus, Director, Safety and Reliability Directorate, UK Atomic Energy Authority 12. Visits made by the Watt Committee Working Group on the Chernobyl accident A. Visit to Dungeness ‘A’ Magnox station —David Cope B. Visit to the CEGB’s Oldbury-on-Severn Nuclear Power Training Centre—John Bindon C. Visit to Hinkley Point ‘B’ AGR station —Glynne Lewis

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112

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116 117

121 123

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127 130

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D.

E.

Visit to the Atomic Energy Establishment Winfrith SGHWR station— Frank Allen Visit to Hunterston ‘A’ nuclear power station to attend an emergency exercise—Glynne Lewis

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135

The Watt Committee on Energy: Objectives, Historical Background and Current Programme

139

Member Institutions of The Watt Committee on Energy

141

Watt Committee Reports

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Index

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Background and UKAEA, as well as reports from the International Atomic Energy Agency (IAEA) in Vienna and the International Nuclear Agency (INA), were made available to the Group. The Watt Co mmittee would also like to express its appreciation of the efforts of the Working Group members who put a lot of hard work into the study. Members sometimes had to accept controversial decisions on the acceptability of items of material or the way it was presented and this they did with good humour. All the members of the Group have been able to comment on and, in some cases, correct the texts prepared by the others, although as Chairman of the Group I have had the final say. With so many authors, the contributions will inevitably be of varying length. The editor has not attempted to change the style of individuals’ work. Appendices cover special aspects of the accident which could not easily be fitted into the text or else would hinder the reading of the main contributions. A glossary of terms and a list of the units of measurement used have been included because some of these are confusing and difficult to interpret. There are many aspects of a disaster such as Chernobyl, including individual experiences, the way the news was released, the international implications and the impact on energy policies, as well as the technical aspects of reactor design and operation. There are many ways of approaching the subject, but this study attempts to review the facts in the light of the implications of the accident for the United Kingdom and its nuclear power. Because it has been written by independent technologists, it is hoped that this report will provide a significant contribution to the rapidly growing literature on the subject.

The disaster of the Chernobyl nuclear power reactor number 4, involving the meltdown of nuclear fuel and a major fire releasing radioactivity, was clearly an event that was likely to have a major impact on energy policy both inside and outside the Soviet Union and on the acceptability of nuclear power throughout the world. To the opponents of nuclear power, the accident was evidence that engineers and scientists were dealing with a source of energy that was just too dangerous. The supporters of nuclear power found their position more difficult to defend publicly, but as information about the accident was made available, it became clear that there were major differences between the Soviet Union and the United Kingdom in terms of reactor design, safety provision and operational safeguards. The Watt Committee on Energy can call on technically informed people from a wide range of backgrounds both within and outside the power industry. It was therefore particularly well qualified to form a Working Group to report on what occurred and make recommendations from an informed but independent position. The Working Group was assembled shortly after the accident happened and, in the early meetings, the material and format of the report were discussed and agreed. To aid their research, the team members were able to make visits to a number of important sites in Britain, and were able to engage in lively and often controversial discussions with many senior staff in the Central Electricity Generating Board, the South of Scotland Electricity Board, the National Nuclear Corporation and the United Kingdom Atomic Energy Authority. The Watt Committee would like to thank these experts both for their help and for the information, vital to the report, that they were able to provide. Internal reports from the NNC, CEGB, SSEB

NORMAN WORLEY Editor, and Chairman of the Chernobyl Working Group

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Section 1

Introduction Norman Worley Chairman of the Working Group on the Chernobyl Accident and its Implications for the United Kingdom, and Deputy Chairman of the Watt Committee on Energy

INTRODUCTION

element uranium, as it occurs naturally, consists of 99·3% of the isotope U238 which does not readily fission when it reacts with neutrons, and 0·7% of U235, which does. By a process known as enrichment, using a diffusion or centrifuge process, the proportion of U235 can be increased and this is often the case with uranium oxide fuels. During the operation of the reactor, U235 is destroyed, forming highly radioactive fission products. At the same time the U238 reacts with neutrons and, as an eventual result of this reaction, forms a new fissionable material (plutonium), mostly as the isotope Pu239. As this isotope builds up, some of this will fission too. In most reactors the amount of plutonium formed will not fully compensate for the loss of U235, and the ability of the fuel to sustain the nuclear reactions (its ‘reactivity’) falls until it is necessary to replace the fuel to keep the plant producing adequate levels of power. The fuel retains much of the radioactive fission products but is wrapped in a sealed can which, under normal operating conditions, forms a further barrier to their release. The nuclear fission reactions operate most readily with neutrons whose energies have been reduced from the high energies at their formation. The material which accomplishes this is a moderator which can be graphite or ordinary or ‘heavy’ water. The role of coolant and moderator can be combined when water is used. The assembly of fuel elements and moderator is called the reactor ‘core’. The fission reactions release a large quantity of heat, and the fuel is cooled by circulating a fluid past the fuel element, either increasing the temperature of the

The Chernobyl nuclear power station disaster started in the early hours of 26 April 1986. It was claimed by the Soviets that this was the first major accident in the 30 years of operation of this type of reactor, which has only been built in the Soviet Union and which is a major component in the rapid increase in electrification of that country. The papers in this Watt Committee report describe the Chernobyl Number 4 Reactor, detail the main components of the accident and its consequences, and then explore what there is of relevance to the United Kingdom at a time when the future of nuclear power here is a major issue between the political parties. To set the scene, and to provide limited background information for the more specialised papers that follow, this introductory section briefly covers three areas: (a) general description of the principal parts of nuclear power plants; (b) an outline of some major incidents involving nuclear fuel meltdown; (c) the Soviet Union’s energy, electrical power and nuclear power situation. 1.1 GENERAL DESCRIPTION OF A NUCLEAR POWER SYSTEM The principal parts of a nuclear power plant are shown in Fig. 1.1. In operation, heat is generated in the fuel, which is usually an oxide of uranium or metallic uranium. The 1

The Chernobyl accident and its implications

2

Fig. 1.1. The principal parts of a nuclear power plant.

fluid or evaporating some of it. The heat is then used directly (if steam is formed in the reactor), or indirectly by making steam in steam generators, to generate electrical power using turbo-generators. The rate of heat generation in the fuel is high. To provide adequate cooling in most reactor systems and to achieve temperatures that give adequate cycle efficiency, the coolant is maintained under pressure so that the whole reactor coolant system, including the steam generator and the pumps or circulators that drive the coolant past the fuel, are contained in a pressure system. This may be a prestressed concrete vessel, pressure vessel and associated circuits of steel, or a series of pressure tubes which pass through the reactor core, with linked piping. With systems where a rapid increase in heat output can lead to a surge in coolant pressure, the whole of the reactor system can be surrounded by a sealed containment system. There can therefore be several barriers to prevent or restrict the escape of radioactive fission products from the fuel to the surroundings: (a) (b) (c) (d)

retention in the fuel the fuel cladding the coolant pressure circuit the containment system.

The fuel, in addition to containing and retaining radioactive fission products, will, in the core area, particularly while the reactor is operating, emit gamma radiation and elementary particles, e.g. electrons, a-particles and neutrons. Shielding surrounding the reactor ensures that these particles

(called collectively ‘radiation’) do not reach the surroundings in damaging quantities. As the fuel becomes less reactive, it requires replacement with new fuel. The equipment to do this, charge and discharge machinery, may operate while the reactor itself is operating (on-load charge and discharge) or shut down (off-load). As a result of the splitting of the ‘fissionable’ U235 or Pu239 by reactions with a low energy neutron, about two high energy neutrons are released which, unlike the other fission products, pass easily through the can. In steady operation there is a balance between neutrons causing fission and those surviving to continue the reaction. Some neutrons leak from the core, and are absorbed by the shielding. Some will be absorbed by the coolant, particularly if this is water, or the moderator, or the fission products, some of which capture a large proportion of neutrons striking them (they have a ‘high cross-section’), or in the can material, in the fuel itself or in special absorbers introduced in the core. The reactivity of the fuel will depend on how long it has been in the reactor. New fuel is highly reactive but this reactivity is lost with burn-up of fuel and with temperature changes, etc., involved in running the reactor up to power. Thus, excess reactivity in the fuel must be provided and this excess must be kept in check with control rods, capturing neutrons. To keep the heat production as uniform as possible, when a core has both new and old fuel at the same time, and to ease control of the plant, materials that can absorb some of the excess neutrons available are introduced into the fuel itself, into the coolant or as separate

Introduction

absorber rods. To start, shut down and regulate the operation of the reactor, rods that absorb neutrons rapidly, i.e. control and shutdown rods, are moved in or out of the core as required. Of the neutrons formed, most are released virtually immediately, but a small proportion which depend on the fissionable isotopic composition of the fuel (U235 0·7%, Pu239 0·4%), are released at an appreciable time after fission—up to a minute or so. It is this special property of delayed fission neutron release that enables the plant to be controlled by manual or relatively straightforward automatic operating systems with control rods that move relatively slowly. In normal conditions of operation, therefore, a nuclear power reactor system can respond to the requirements of the electrical power grid system, and the rate at which power can be increased will be dictated by the mechanical limits of the components, rather than by the reactor physics of the core. However, all reactors have systems by which shutdown rods can be—and in many cases are— automatically forced into the core to avoid circumstances that could endanger the plant (‘Scram’—said to have been formed from the term Safety Control Rod Axe Man at the first man-made reactor, Stagg Field, Chicago). Nuclear reactor power plants are complex systems with three special features which have to be allowed for in design and plant operation. If operation or design, or both, are faulty or inadequate, serious damage to the fuel, the reactor core or, in extreme cases, the reactor circuit, can result. These special features of nuclear power plant can be summarised as follows: (a) Significant amounts of energy can be released if the system is not properly controlled. (b) The fuel elements contain large quantities of highly radioactive material. To keep this from the environment requires containment. (c) After the reactor has operated, the fuel produces heat—a large quantity in operation, smaller but significant quantities when the plant is shut down and after the fuel is removed from the reactor. Cooling of the fuel has to be adequate at all times to avoid excessive temperatures. Complex plant often has problems, and engineers will modify equipment to avoid problems that have occurred in the past. There is therefore an enormous amount of experience built into new and existing nuclear plant which improves both plant reliability and safety.

3

Most reactor plant accidents do not have serious consequences. However, because much of the plant is not easily accessible and is often inside a containment where access is limited by radiation levels, reactor system repairs and modifications can be slow to accomplish and expensive. A characteristic, therefore, of reactor incidents is a long delay in getting the plant back into operation; in a few cases further plant operation is not possible. All remedial operations inside containment are extremely costly. However, with reactor incidents, direct loss of life or casualties are extremely rare. In this respect the nuclear record compares well with other energy-related industries (see Appendix 10). There have been accidents at nuclear plants, leading to injury and in some cases deaths. In these, nuclear plant is similar to all other process and power plant. The special feature of nuclear plant is the awesome potential power in the plant and its highly radioactive contents. Radiation is regarded by many people with special fear. Of course, everyone encounters radiation from many sources throughout life and there is radiation in our own bodies, from outer space and from our surroundings, with which our bodies appear to cope extremely well. The background levels vary considerably from place to place, but the records do not show any correlation between the incidents of radiation-related diseases (cancer and leukaemia) and the level of background radiation. All nuclear plants involve some release of radioactive material, but the effect on the environment corresponds to a very small fraction of the normal background, even near the plant. The effect of radiation depends on its intensity, duration and type and the age, food and living habits of individuals. With relatively low levels of additional radiation, other than that from background sources, it is generally assumed, although the technical data are inconclusive, that a proportion of people receiving this increased radiation will develop cancer over many years (probably 30–40) as a result. These are the effects described in the literature as long term casualties from radiation. These cancers cannot in fact be identified as being due to radiation. Even statistically, always a minefield of varying interpretation, radiation-linked cancers cannot be detected because of the wide scatter of the basic data. In normal operation at power stations, both the staff and the general public do not receive immediately damaging radiation levels. Even during repair and rectification operations, the radiation levels received by workers will be low. There

The Chernobyl accident and its implications

4

are international and national standards on radiation levels and in general the ambient levels are well below these. A serious radiation hazard can exist in a nuclear power plant only if some of the fuel is exposed, particularly if it melts. Even if this occurs, in most cases little or no extra radiation will be released to the environment. Up to the end of 1986 there have been about 100 incidents, some of them deliberate experiments to provide data, at nuclear plants involving some of the fuel melting. These events have been analysed and the results used to improve design and safety. The total number of immediate deaths attributable directly to these incidents over 35 or more years of nuclear reactor operation is less than 35—three at a military prototype reactor in 1981 in the USA and 31 at Chernobyl. However, three subsequent deaths have been reported at Chernobyl (see Appendix 7). Of the fuel meltdown incidents (excluding Chernobyl-4), eight relatively serious incidents have been selected and subjected to some analysis in the following subsection. It is noted that, of these fuel meltdown incidents, only one (Three Mile Island-2) was at an operating, fully developed power plant. All of the other incidents involved research reactors or developmental or prototype plant. Three relatively minor incidents are also reviewed where single channel fuel overheating occurred in graphite-moderated plant. 1.2 FUEL MELTDOWN INCIDENTS The following descriptions of the incidents are deliberately brief. More detailed descriptions are available, for example in the sources listed on p. 8. 1.2.1 NRX—Canada—December 1952 Reactor. Heavy water moderated research reactor. Cause. Inadvertent withdrawal of control rods, leading to core melting, formation of hydrogen and a chemical explosion. Initiation. Communication misunderstanding; instrumentation indication error; ‘Scram’ did not function. Casualties. None; some low level radiation to staff.

Follow-up. Reactor cleaned up and returned to operation. 1.2.2 Experimental breeder reactor (EBR1)—USA—November 1955 Reactor. First reactor to generate nuclear electricity; experimental breeder. Cause and initiation. Safety experiment requiring ‘Scram’ but slow acting rods inserted instead. Result. 50% of core melted. Casualties. None reported. 1.2.3 Windscale No. 1—UK—October 1957 Reactor. Natural uranium metal, graphite moderated, open cycle air-cooled reactor, one of two used for producing military plutonium. Cause and initiation. With graphite used as a moderator at low temperatures (less than about 300 °C), lattice damage to the graphite occurs which is not automatically repaired in service. To avoid operating problems, the graphite was periodically heated under controlled conditions to enable the carbon atoms in the lattice to return to their more normal and stable locations; this operation releases energy. The operation was being carried out but according to the instrumentation, temperatures were falling before completion. In fact, in parts of the graphite away from the thermocouples, energy release was still going on. However, the operator concluded that an extra reactivity boost was necessary to complete the energy release. When the reactor was subsequently returned to production, in one region of the reactor both uranium fuel and graphite were on fire. In a reactor which is air cooled and where the air is filtered and then released to the atmosphere, this was serious. After carbon dioxide was tried without success, the fire was extinguished by water jets. The immediate concern was the release of relatively short lived radioactive iodine and caesium. It was later learnt that polonium had been released. Effects were detectable as far away as France. One hundred and fifty of the fuel channels were affected, with considerable radioactive release from the stack, although filters retained most of the particulate material. District and

Introduction

national monitoring was not adequate enough to ascertain the extent of release or its distribution. Milk over a wide area had to be thrown away. Reactors 1 and 2 were both shut down and ‘entombed’. This, the Windscale accident, was for many years the most serious reactor accident; it had more radiological consequences than the Three Mile Island accident of 1979. Clearly the accident had the salutary effect of making designers and operators safety conscious in the UK. Casualties. None reported, although official figures suggest that up to 30 deaths from cancer may occur in due course. 1.2.4 NRU—Canada—May 195©8 Reactor. Heavy water moderated experimental reactor. Incident. Fuel fire in charge and discharge machine. Cause. Fuel fracture in machine. Casualties. None reported. 1.2.5 SL-1 (Stationary Low Power Reactor 1)—USA—January 1961 Reactor. Pressurised water reactor prototype 3MW(th). Incident. Control rods are disconnected when the vessel head is removed. On this occasion, in reconnecting one of the rods, it was withdrawn too far, causing an extreme reactivity excursion. Considerable damage to fuel, reactor vessel and pile cap. Casualties. Three killed. 1.2.6 E nrico Fermi No. 1—USA—October 1966 Reactor. Prototype/experimental sodium-cooled ‘breeder’ reactor power plant. Incident. While increasing power, erratic neutron monitoring readings eventually led to reactor ‘Scram’. There was a partial core meltdown due to intermittent blocking of reactor coolant flow by a loose baffle in the base of the vessel. Some radiation released.

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Result. Eventually cause located, plant shut down and repaired but later dismantled. Casualties. None reported. 1.2.7 Lucens—Switzerland—January 1969 Reactor. Experimental heavy water moderated, carbon dioxide cooled pressure tube reactor. Incident. Pressure tube burst causing severe fuel damage. Result. No radiation released to environment; reactor shut down and not reused. Casualties. None. 1.2.8 Three Mile Island-2—USA—March 1979 Reactor. Pressurised Water Reactor. Incident. When the steam generator feed pump failed, the turbo-alternator tripped, leading to an increase in reactor steam pressure which ‘scrammed’ the reactor. To remove the heat still being generated in the reactor fuel, emergency boiler feed pumps were activated but could not supply water because valves were closed. The incident developed to an accident when the planned automatic operation of the emergency system was interrupted by operators. Emergency cooling was automatically turned on but was subsequently shut off by operators because, by their interpretation of the situation, the reactor did not require extra cooling. Reactor pressure rose due to fission product heat and lack of cooling, causing a safety valve to lift. This did not reseat as pressure fell again, so the reactor lost water, uncovering the core, leading to core meltdown and to formation of a ‘bubble’ of hydrogen. Result. Core meltdown and considerable release of radiation to containment. A small amount was released to the environment. Reactor written off—operation of TMI-1 suspended. Clean-up and investigations have cost billions of dollars. Since then, there has been extensive reappraisal of safety requirements and no further ordering of nuclear plant by utilities in the USA. Uncertainty of situation and media reaction led to large public concern. Casualties. None.

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1.2.9 Others In addition to these incidents, three relatively minor events involving local channel overheating with graphitemoderated reactors have occurred, at Marcoule, Saint Laurent-des-Eaux and Chapelcross. In each case there was fuel damage and some oxidation of graphite local to the overheated fuel elements. The fuel elements were removed, the affected channels were sealed and the reactors returned to operation. There were no casualties and no release of radiation to the atmosphere. 1.2.10 Discussion The number of serious core meltdown incidents involving operating power reactors is small. Most of the incidents reported involved experimental, research, development or military plant. The causes were operator error or lack of understanding of the state of the plant in most cases; about half were due to instrumentation errors or faults and about half due to equipment failure, a fuel element in a charging machine, a pressure tube failure (surely significant for the future safety of RBMK reactors) and a loose baffle. It is probable that the safety procedures, quality assurance, automatic systems, instrumentation and data presentation improvements and the higher level of operator training which have become general since the Three Mile Island accident would have avoided virtually all of the accidents. The sample is small, but in reviewing these cases, and other problems that have led to extensive plant damage (but no core meltdown), a number of observations can be made: (a) Nuclear plant, like all other complex installations, will have component and operational problems, the vast majority of which can be dealt with by bringing in standby plant. Routine maintenance of all plant, including standby equipment, is essential, as is operator training to deal with unusual situations. (b) The operator interaction with the instrumentation and control is the area which is most likely to lead to accidents. (c) Operator knowledge of the plant and its characteristics should be in depth, and a wide range of incidents should be rehearsed. Here, building a number of plants of virtually identical design enables experience and training to be most

The Chernobyl accident and its implications

effective and staff transfers to be carried out without problems, (d) Plant design to current standards is extremely robust and able to cope with a wide range of incidents without endangering the plant, the operators or the general public. It is interesting to note that the Chernobyl-4 accident would not have occurred if the operators had complied with the operating instructions. At Three Mile Island the operators did initially do what they were trained to do; but the bases of this training were at fault and they did not have satisfactory information on the state of the plant and its components. 1.3 ENERGY IN THE SOVIET UNION The Soviet Union has vast reserves of all of the main fuels, including uranium. There are no published official Soviet statistics and figures are therefore difficult to check, but it appears that about one-quarter of the world reserves of gas and oil and one-sixth of the coal and brown coal lie in Soviet territory. Although the reserves are vast, most resources are remote from centres of population and in difficult terrain. Figure 1.2 shows the main sources of fossil fuel in the Soviet Union. The coal mines in the Donets region are deep mines. As the seams are exhausted deeper seams have to be used and these are generally too thin for the successful use of automatic machinery. Not only is productivity inevitably falling, but also the coal removed from the mines contains an increasing proportion of rock. The main new sources of coal are east of the Urals and here opencast mining is generally used. In the northern areas the long cold winter hampers production, and in the regions where the coal is lignitic the cost of transport is unacceptably high. Here local power generation with long transmission lines to the West is used, but there have been problems with these, particularly in bad weather. Oil around the Caspian Sea has been traditionally the mainstay of Soviet oil production. However, these fields are becoming exhausted. The major production in the years 1950–80 was in the Urals where the deposits were in large fields and recovery was inexpensive. Currently production from this field too has begun to fall off and over half of Soviet oil now comes from the West Siberian field. Conditions here are not so favourable, as the deposits are in relatively small fields and the capital costs associated with opening new areas are extremely high.

Introduction

7

Fig. 1.2. Sketch-map to show the main sources of fossil fuel in the Soviet Union: C, coal; L, lignite; O, oil; G, gas.

The main gas fields lie well to the north of the oil fields and here again each new field costs more to exploit than did the earlier ones. The gas pipelines have been a major industrial success and there is considerable reserve capacity. On fossil fuels the Soviet Union has, contrary to predictions from the USA, been able to meet the targets set. Exploiting the deposits in these remote regions, however, is costly in equipment and capital. The infrastructure of roads, railways, pipelines and grid lines and running and maintenance costs of equipment under extreme weather conditions all contribute to the high costs of energy from these sources. It is also costly to induce the workforce to come and stay in these regions and often productivity is low. Oil and gas exports form a major source of Soviet hard currency. The need of the USSR for hard currency for import requirements is likely to increase as the drive to improve the standard of living gathers momentum. In 1985 oil exports provided 60% of Soviet hard

currency requirements, and gas a further 20%. Both of these figures have been seriously affected by the subsequent drop in world oil prices. Because exploration and capital costs are so high, it is forecast that Soviet oil exports are likely to fall during the next 15 years. Gas exports are likely to expand and there is spare capacity in the pipelines, but not enough to offset the fall in revenue from oil exports. Expansion of coal-fired power generation is very expensive. There will be expansion, but not enough to meet the anticipated surge in electricity demand. The 1986–87 winter was marked in the Soviet Union by power restrictions and staggered shifts. In addition to only half of the Chernobyl plant running, hydro-power suffered from the effects of a summer drought and several new plants were not ready for the winter load. The Soviet power system has virtually no reserve capacity, hence the return of two of the remaining Chernobyl reactors to full power in time for the winter

The Chernobyl accident and its implications

8

peaks. There is an enormous pressure to expand nuclear installations and they are sited relatively close to centres of electricity demand. The Soviet electrical installed capacity is about 300000 MW, about half that of the USA and five times that of the CEGB. The growth was 3·5% per annum, compared to 3·3% in Japan and less than 3% in the USA and the Federal Republic of Germany and 0·1% in Britain over the period 1975–83 (probably an unfortunate selection of dates for Britain, as over the past few years growth has been 3%). In the USSR, three-quarters of the power requirement is in Europe while about the same proportion of energy production is in Asia. Currently 70% of electricity is from fossil plant, mainly coal fired, the rest being shared between hydro-power and nuclear power. The first RBMK-type reactor started up at Obninsk in 1954—two years before Calder Hall. It is still operating. There are plans for 70000 MW of new nuclear plant to be commissioned between 1984 and 1994 but how that target will be affected by the decision to build no more RBMK reactors is not clear. RBMK reactors of the same physical size as Chernobyl, but with improved heat transfer surface on the fuel cans, are operating at 1500 MW(e) at Ignalis in Lithuania. This was the standard for new RBMK plants. The RBMK type of reactor supplied 70% of the nuclear electricity in 1985, the balance being supplied by the Soviet type of PWR (VVER). There have been problems in meeting the target production of heavy pressure vessels for these PWR reactors. However, serious problems have been reported both in organising supplies of equipment and in maintaining the high standards of construction necessary for the safe operation of nuclear plant. A British assessment of RBMK designs in 1976 was critical about a number of design features.

Fast reactor development, too, is rapid in the Soviet Union. There are three developmental fast reactors: a 12 MW(e) unit at Ulyanovsk, a 1000 MW(th) plant in Kazakhstan, used to produce electricity and steam for desalination, and a 600 MW(e) plant in the Urals. An 800 MW (e) plant is under construction. Two district heating low pressure PWR systems are operating, at Gorky and Voronezh. The intention for the next 20 years or so is to use nuclear heat for about one-third of the total heating load in the USSR. In addition, there are three Soviet nuclear-powered ice breakers—this is an important duty, as many ports are ice-bound in winter and oil and gas supplies are close to the Arctic Ocean. There are plans for nuclear oil tankers too. The Soviet nuclear programme is necessary because of the country’s economic and fossil fuel situation. Over half of the new installed capacity is likely to be nuclear, and the aim must be to release as much oil and gas for hard currency exports as can be achieved. Probably, bearing in mind the location of fossil fuel supplies, the total capital cost of nuclear plant, particularly the relatively low technology RBMK, compares favourably with coal-fired installations. Sources The Worst Accident in the World. The Observer, London, 1986. Fremlin, J.H., Power Production, What are the Risks? Adam Hilger, Bristol, 1985. Wyatt, A., Electric Power. The Book Press (Canada), 1986. Stern, J., Soviet Oil & Gas Exports to the West. Gower Press, Swansea, 1987. IAEA Bulletin. International Atomic Energy Agency, Vienna, Autumn 1986. Patterson, W., Nuclear Power. Penguin, Harmondsworth, Middx, UK, 1980.

Section 2

The Design of the Chernobyl Unit 4 Reactor Peter Potter Independent nuclear consultant Northwich, Cheshire

2.1 INTRODUCTION AND GENERAL DESCRIPTION

opportunities to monitor closely individual channels and, if necessary, to repair faulty ones. Until the accident at Chernobyl, another advantage claimed for the design was that the pressure tubes, with their individual connections, eliminated the possibility of a complete loss of coolant from the core.

The Soviet thermal nuclear reactor power programme involves two reactor types which, until now, have been planned in roughly equal numbers. The first is the PWR for which a dedicated manufacturing facility has been built in the south of the Soviet Union at Volgadonsk. Problems in bringing this plant into production have reduced the rate of construction of the planned PWRs and resulted in a greater proportion of the other reactor type, the RBMK (Reactor Bolshoi Moschnosti Kipyashiy—Large Power Boiling Reactor). This is currently being built in two sizes, 1000 and 1500 MW respectively. The RBMK-1000 is a direct cycle, forced circulation, boiling water cooled, graphite moderated reactor fuelled with low-enriched uranium dioxide clad in a zirconium alloy. It consists of a large block of graphite moderator pierced vertically by 1872 ducts fitted with pressure tubes of which 1661 contain fuel elements and 211 are for control rods. The coolant circuit is divided into two loops, each being fed from half the reactor. The steam generated is separated from water in steam drums and is fed directly to a pair of turbinegenerators which generate electricity. Components for this design are relatively small. They thus require no large unique facilities, can be made in a number of small factories, and pose no special problems for transport and installation. As in the case of the British gas-cooled reactors, on-load refuelling enables a high burnup to be obtained from low-enriched fuel. The channel tube design provides

2.2 STATION LAYOUT At the time of the accident, the nuclear power station at Chernobyl comprised four operating units and two under construction. Each unit is made up of one reactor of the RBMK-1000 type and two turbine-generators. The two units 3 and 4 are accommodated in one block as shown in Fig. 2.1. The two reactors are separated by a compartment housing common services. Alongside is the turbine hall with the four turbines in line. The blocks accommodating units 1, 2, 3 and 4 are adjacent (so that all eight turbines are in line). The block for units 5 and 6, not now to be completed, is sited 1.5 km to the south-east. The RBMK-1000 reactors in units 3 and 4, the latter being involved in the accident, are the second generation of this type with design improvements over the earlier units. The information in this section refers specifically to this second generation. An elevation of reactor 4 is shown in Fig. 2.2. 2.3 REACTOR CORE The arrangement of the reactor core toget.her with its biological shield is shown in Fig. 2.3. The active core 9

10

The Chernobyl accident and its implications

Fig. 2.1. Plan of the block housing Chernobyl Units 3 and 4 (from Ref. 2, by permission of the IAEA). Dimensions in metres.

Fig. 2.2. Sectional elevation of Chernobyl Unit 4 (from Ref. 2, by permission of the IAEA). Dimensions in metres.

Design of Chernobyl unit 4 reactor

Fig. 2.3. Sectional elevation through core and biological shield (from Ref. 2, by permission of the IAEA).

of the reactor is roughly octagonal and is composed of stacked, square graphite blocks (250 mm×250 mm×600 mm high) containing pressure tubes in central, axial ducts (114 mm in diameter) on a 250 mm square lattice pitch. It is approximately 12 m in diameter and 7 m high. Reflector blocks are arranged to produce a cylindrical configuration 14m in diameter and 8·0 m high. The core is surrounded by a cylindrical shroud; a bottom support structure consisting of a welded metal base resting on a 2 m thick concrete neutron shield; and an upper metal structure resting on the annular tank of the biological shield which is filled with water. These components together form a light but leaktight cavity. A further annular tank between the inner neutron shield and the outer one of concrete, contains sand. The upper neutron shield consists of 3 m of concrete. Upper and lower metal structures contain ducts for the fuel and control rod channels. The complete reactor structure is situated in a concrete vault with dimensions 21·6 m×21·6 m×25·5 m. Some 5% of the heat from the fission process is released in the moderator. To improve heat transfer

11

from the graphite to the channel tubes and in order to prevent oxidation of the graphite, the reactor space is filled with a mixture of helium and nitrogen. Slow circulation of this permits monitoring of its temperature and moisture content, giving an indication of the integrity of the pressure tubes. The space outside is filled with nitrogen at a pressure 0·5–1·0 MPa greater than that of the helium/nitrogen mixture. If a channel tube should rupture, a pressure suppression system is provided to condense the steam released. Channel tubes (88 mm o.d. and 4 mm thick) are of welded design and contain fuel assemblies which are cooled by boiling light water. The upper and lower parts of the channel are made of stainless steel and the central part, located in the active zone, is made from a zirconium/2½ niobium alloy. The central part is joined to the upper and lower parts by vacuum diffusionwelded stainless steel/zirconium transition joints. The channel tube is attached to the upper duct by a welded joint, and to the lower one by a compensator unit, which is necessary to compensate for the difference in thermal expansion of the channels and ducts without destroying the leak-tightness of the reactor cavity. This type of joint makes it possible to replace a channel during reactor shutdown. Water coolant is fed to each fuel channel from below and the steam-water mixture is removed from the top. In order to compensate for variations in power distribution, the coolant supply to individual channels is regulated by isolating and regulating valves which are installed in the channel feed pipes. Removal of moderator heat from the graphite blocks to the fuel channels is through specially designed sleeves. These sleeves are resilient slotted rings of graphite, 20 mm high, which are disposed along the height of the channel (in the zone of maximum thermal loading) and packed against one another in such a way that alternate ones lie against the channel wall and the others against the surface of the graphite block. This ensures the necessary conditions of heat transfer and compensates for shrinkage of the gap between the block and the channel during the life of the plant. The design maximum temperature in the graphite is 750°C.

2.4 PRIMARY COOLANT CIRCUIT The coolant circuit is shown schematically in Fig. 2.4. It comprises two parallel loops each with two stainless steel-clad carbon steel steam drums. Each steam drum

The Chernobyl accident and its implications

12

Fig. 2.4. Schematic arrangement of main coolant and steam circuits (from Ref. 2, by permission of the IAEA).

has 482 risers (76 mm diameter× 4 mm thick) and 12 downcomers (325 mm× 16 mm). The downcomers connect to a common pump inlet header (752 mm i.d.). Each loop has four electrically driven pumps, with three operating and one on reserve. There is an intermediate system of 22 distribution headers (325 mm×15 mm) per loop between the common pump outlet header (900 mm i.d.) and the channel feed pipes. There is only a rudimentary system of pipe restraints. At full load, water at 270 °C enters the bottom of each fuel channel through individual 57 mm i.d. channel feed pipes. A two-phase mixture of water and 14.5% steam (by weight), generated as the mixture flows over the fuel, passes to the horizontal cylindrical steam separator drums via 76 mm i.d. steam-water lines. Saturated steam at 284°C and 6.9 MPa, with a moisture content of less than 0·1%, passes from the drum through 14 steam discharge pipes (325 mm×19 mm) to two steam headers (426 mm i.d.×24 mm) which connect to a common steam collector (630 mm×25 mm) supplied by all eight steam mains from the four steam drums. This collector feeds the two turbines. Condensate from the turbines is deaerated and returned to the drum separators by electric pumps. A feedwater collector is located at the bottom of each drum separator. From these, the water which has been separated from the steam-water mixture is mixed with feedwater and is taken via the twelve 312 mm downcomers to a 1020 mm pump inlet header. This creates a cooling effect below saturation temperature to give the required cavitation margin at the inlet of

the main circulating pumps. Each pair of steam drums is interconnected on both water and steam sides so that the drum levels remain identical. A common ion exchange unit is provided for the two coolant loops to purify the cooling water, the flow through it amounting to 4% of the boiler system capacity. 2.5 FUEL One fuel element (Fig. 2.5) consisting of two fuel assemblies is installed in a fuel channel. The element is attached to a special suspension, provided with a locking plug, which is set in the neck of the upper duct. This locking plug hermetically seals the duct cavity by means of a ball-valve with a packing gasket. Each fuel assembly consists of 18 fuel pins. A fuel pin is a tube of zirconium/2½ niobium alloy with an external diameter of 13·6 mm and a minimum thickness of 0·825 mm, filled with pellets of uranium dioxide. The fuel pellets are 11·52 mm in diameter and 15 mm high with cavities in the end faces. The inner space of the fuel pin is filled with an argon/ helium gas mixture. Top and bottom terminal grids hold the fuel pins and are positioned above and below the level of the core. Two fuel assemblies are combined in the element giving an active length of 7 m. The uranium feed enrichment is 2·0%. The reactor can be refuelled on load at full power. A refuelling machine is supported on a gantry running along the length of the refuelling hall. As well as

Design of Chernobyl unit 4 reactor

13

Fig. 2.5. Fuel element design (from Ref. 1). Dimensions in mm.

removing spent fuel and replacing it with fresh, the machine can also verify free passage through the fuel channel using a gauge simulating a standard assembly. The semi-automatic machine accurately locates the coordinates of the channel to be refuelled, locks on to it, adjusts its own pressure to slightly above that of the channel, removes the channel plug unit from the top of the channel, withdraws the spent fuel element, checks the channel with a gauge, loads fresh fuel, seals the channel and takes the spent fuel to the storage pond where it is unloaded. While the machine is connected, there is a slight flow of clean water from it to the channel to prevent it becoming contaminated with water from the primary circuit. The refuelling machine can carry out five charge/discharge cycles at full power in 24 h, but in normal operation only one to two channels per reactor are reloaded each day.

2.6 CONTROL AND SAFETY RODS Reactivity is controlled by ‘rods’ consisting of articulated absorber elements formed from hollow cylindrical sections of boron carbide (65 mm diameter ×7·5 mm thick) sheathed in the annulus between two aluminium alloy tubes of 70 mm×2 mm and 50 mm×2 mm respectively. They are inserted or removed from the core at a rate of 0·4 m/s (the 12 local automatic control rods are withdrawn at 0·2 m/s) by individual servomotors installed at the top of the control rod channels. With the exception of the automatic rods, all the rods are fitted with graphite followers so that, as they are withdrawn, they are not replaced by water. The square lattice of 211 control rods and 12 vertical power profile sensors has a pitch of 700 mm and is angled at 45° to the fuel lattice. The channels are made

14

of the same zirconium alloy as the fuel channels and are 88 mm in diameter and 3 mm thick. They have their own cooling water supplied from a special circuit. The control rods are divided into the following groups according to their purpose: 115 manual control (MC), 24 safety (S), 12 local autocontrol (LAC), 12 average power control (APC), 24 local safety (LS) and 24 shortened absorber (SAR) for levelling off the axial power distribution. The SAR rods are inserted from the bottom of the reactor. In order to compensate for the initial reactivity margin, auxiliary absorbers (AAR) are installed in some of the fuel channels instead of fuel assemblies. The absorbing elements in these are inserts of boron and stainless steel. By varying the ratio of the numbers of inserts, the absorption of the auxiliary absorbers can be changed. The entire active zone is divided arbitrarily into groups of cells containing 12 fuel channels with fuel elements, two channels with auxiliary absorber rods and two channels with control and safety rods. This grouping is varied at the reactor periphery by reducing the number of control and safety rods to flatten the radial power distribution. The control rod system provides for automatic control of the required reactor power level and its period; reactor startup; manual regulation of the power level and distribution to compensate for changes in reactivity due to burn-up and refuelling; automatic regulation of the radial-azimuthal power distribution; automatic rapid power reduction to predetermined levels when certain plant parameters exceed preset limits; automatic and manual emergency shutdown under accident conditions. A special unit selects 24 uniformly distributed rods from the total available in the core as safety rods. These are the first rods to be withdrawn to their upper cut-off limit when the reactor is started up. In the event of a loss of power, the control rods are disconnected from their drives and fall into the core under gravity at a speed of about 0·4 m/s, regulated by water flow resistance. The power density distribution is controlled by the 12 LAC and 24 LS rods. The average power control system is used as standby in the 20–100% power range and is switched on automatically when the LAC system malfunctions. The automatic control system holds reactor power to within ±1 % of the required output in the range 20–100% full power and to within ±3% in the range 3·5–20% full power. Removal of the LAC rods is blocked automatically when a power overshoot signal is registered in one of the channels of the local safety zone. If such a signal

The Chernobyl accident and its implications

appears in both channels of an LSZ, two LS rods are motored into this zone until at least one of the signals is corrected. When there is any malfunction of the control system, the withdrawal of more than 8–10 of the MC and S rods is automatically prevented. There is also a built-in limitation on the continuous withdrawal of the LAC rods for more than 8 s. A number of different categories of emergency situation are defined depending on their nature. The highest of these, Level 5, results in the reactor being tripped automatically and shut down completely. The reactor is also tripped at Level 5* but can be started up as soon as the emergency has passed. Lower levels result in reduction of reactor power at different speeds to safe values corresponding to the capacity of plant still in operation. The reactor is tripped automatically when any of the following occur: permissible limits exceeded in power output, water pressure in the steam drum or channel feed pipes, water level in the steam drums, pressure in the leaktight compartments or reactor cavity; permissible limits undershot for reactor period, feedwater flow, water level in the steam drums, flow in the control rod coolant circuit or level in its coolant tank; voltage loss in auxiliary electrical supply; simultaneous tripping of both turbo-generators or tripping of the only operating turbo-generator; simultaneous trip of three out of four main circulating pumps in any pump room. 2.7 CORE MONITORING Neutron flux detectors positioned by hangers are placed in fuel channels, the reflector and the annular water-filled biological shield tank. They are located as follows: (a) Water shield tank: 8 startup range ionisation chambers 16 power range ionisation chambers (b) Reflector; 4 fission chambers (during startup) (c) Core: 24 fission chambers Twenty-four ionisation chambers situated in the water shield tank are divided into groups and used to drive three banks of automatic control rods. The automatic control signal is generated by summing the relative deviations from the set power from three out of four ionisation chamber measurement channels. Twenty-four in-core fission chambers located in the fuel channels monitor neutron flux and are used to

Design of Chernobyl unit 4 reactor

15

drive the local automatic control rods. Their output is displayed in the control room and they are also used in the safety system. In addition to the control and safety systems described above, the reactor is equipped with a number of monitoring systems. The principal systems are designed to provide data as follows: radial (across 130 channels) and vertical (12 channels) power distribution; temperatures of the graphite stack and metal core structures; individual channel water flow control using ball flow meters; the state of the principal components of the main coolant circuit such as steam drums, main circulating pumps and their inlet and outlet headers; burst fuel element can detection using a short-life volatile fission product detecting photomultiplier moving sequentially across the steam—water lines at the outlet to each channel; channel tube integrity using moisture content and temperature of the gas in the reactor space.

All the data are fed to a central computer where they can be displayed and recorded. 2.8 EMERGENCY CORE COOLING SYSTEM (ECCS) The ECCS is shown schematically in Fig. 2.6. It is designed to provide a sufficient supply of water to ensure removal of stored energy in the short term and decay heat for a longer period, following interruption of the normal coolant supply. It is assumed that the emergency system has to supply only one half of the segregated cooling system, and provision is made to identify which half this is. In the event of a loss of coolant, water is first injected from two sub-systems into the distribution headers between the common pump outlet header and the individual channel feeder pipes of the failed circuit from tanks pressurised to 10 MPa which are connected by fast acting valves. At the same time, a third sub-system

Fig. 2.6. Schematic arrangement of ECCS (from Ref. 2, by permission of the IAEA).

16

The Chernobyl accident and its implications

Fig. 2.7. Schematic arrangement of part of the containment system (from Ref. 2, by permission of the IAEA): 1, reactor space; 2, compartments (downcomer pipes and main circulation pump outlet headers); 3, compartments (distribution group header and lower communication lines).

supplies water from the electric feed pump. Each subsystem is capable of supplying at least 50% of the required output. Before these supplies are exhausted, a pumped system takes over, drawing water from the suppression pool beneath the reactor. This system consists of three separate identical loops, each with electrically driven highand low-pressure pumps, and each again capable on its own of providing half the required coolant capacity. At the same time, three parallel loops with electrically driven pumps supply water to the undamaged circuit from a tank containing clean condensate. 2.9 CONTAINMENT The reactor containment system, shown in Fig. 2.7, is designed to cope with the release of steam resulting from a failure of the primary circuit. The circuit passes through a number of leaktight compartments which are designed to withstand different overpressures: 0·08 MPa for the reactor space (1 in Fig. 2.7) and the compartments housing the distribution group header (2 in Fig. 2.7), and 0·45 MPa for the strong compartments (3 in Fig. 2.7) housing the downcomer pipes and pump outlet headers (average diameter 900 mm). These compartments are separated by a system of non-return and release valves and connected by a steam distribution corridor to a two-storey suppression system. In normal operation, the leaktight compartments are all held at a slight underpressure. Should a failure of the primary circuit occur, steam and any other gases released raise the pressure in the

relevant compartment. Non-return valves in the strong compartments open to the steam distribution corridor when the pressure differential rises above 0·002 MPa. For the other leak-tight compartments, this differential has to be over 0·02 MPa to operate the valves. The steam passes into the suppression pool beneath the reactor where it is condensed. Isolation valves are provided on all primary circuit pipes penetrating the leaktight compartment walls with the exception of the risers from the channel tubes to the steam drums. Steam emitted from these can be at least partially condensed by the ventilation system. The suppression pool is supplied with heat exchangers to provide cooling in the event of prolonged operation. 2.10 EMERGENCY POWER SUPPLY The emergency power supply for circuits which are absolutely essential in the event of a reactor trip, consists of a storage battery with static inverter transformers for circuits that cannot tolerate interruptions in power supply or can tolerate interruptions up to several seconds in any regime, and automatic diesel generators for those which can tolerate interruptions of between tenths of a second and tenths of a minute in the same regimes. 2.11 WEAKNESSES IN DESIGN A team from the British nuclear industry led by the British Nuclear Forum visited the first RBMK-1000

Design of Chernobyl unit 4 reactor

reactor power station at Leningrad in 1975. As a result of analyses of the information supplied to them by Soviet engineers, members of the team reached the following conclusions concerning the RBMK reactor: — By UK standards, control rod investment was inadequate to meet possible shutdown/holddown requirements — There was no secondary shutdown system: this is mandatory on British reactors and some even have tertiary systems — The void coefficient of reactivity could be large enough to result in a positive power coefficient under some conditions — There did not appear to be an emergency core cooling system — Venting between channel tube and graphite moderator bricks was probably inadequate to allow the escape of steam from a single channel tube failure, thus facilitating fault propagation to other channels — The core restraint structure was not strong enough to withstand a channel tube failure — The operating temperature of the moderator was considerably higher than would have been accepted in the UK, allowing the possibility of fire when exposed to the atmosphere — The buildings housing the primary circuit were not capable of resisting the pressures likely to result from a LOCA (Loss of Coolant Accident) Possibly as a result of these and the criticisms of similar teams of foreign visitors, the Soviets undertook a number of measures to improve the reactor: — The number of control rod channels was increased at the expense of fewer fuel channels — An emergency core cooling system together with a suppression system was provided — The Accident Localisation System was designed around most of the primary circuit In spite of these changes, the reactor was left with the possibility of a fast positive power coefficient, with a graphite temperature that was too high, and without a secondary shutdown system. In addition, it is now clear that the speed of insertion of the control rods under emergency conditions was totally inadequate. REFERENCES 1. The accident at the Chernobyl nuclear power plant and its consequences, State Committee for the Use of Atomic Energy of the USSR, 1986.

17 2. Summary Report on the Post-accident Review Meeting on the Chernobyl Accident, International Nuclear Safety Advisory Group of the IAEA, Vienna, 1986.

APPENDIX: CHERNOBYL UNIT 4 DESIGN DATA Reactor system Gross fission heat, MW Active core diameter, m Active core height, m Diameter over reflector, m Height over reflector, m No. of graphite columns Duct diameter, mm Mass of graphite stack, t Number of fuel channels Fuel channel pitch (square lattice), mm Number of control and safety channels Control channel pitch (square lattice), mm Mass of uranium in core, t Channel tube material Channel tube outer diameter, mm Channel tube wall thickness, mm Channel tube length, m

3200 11·8 7 14 8·0 2488 114 1700 1661 250 211 700 190 Zr/2½ Nb alloy 88 4 8

Fuel Feed enrichment, % Number of sub-assemblies per fuel element Number of fuel pins per sub-assembly Pin outer diameter, mm Clad thickness (min.), mm Pin overall length, mm Fuel pellet diameter, mm Fuel pellet length, mm Fuel pellet density (min.), g/cm3 Mean mass of uranium dioxide per pin, kg Mean mass of uranium per assembly, kg Spacer/support grids per sub-assembly Active fuel length, mm Plenum length, mm Burnup, MWd/kg Refuelling Isotopic concentration of fuel at discharge, kg/t: uranium-235 uranium-236 plutonium-239 plutonium-240 plutonium-241

2·0 2 18 13·6 0·825 3644 11·5 15 10·4 3·6 114·7 11 2×3428 175 20 on load 4·5 2·4 2·6 1·8 0·5

Power Channel average power, MW

1·92

The Chernobyl accident and its implications

18 Channel maximum power, MW Channel maximum power (design), MW Average fuel rating, MW/t U Radial power factor Vertical power factor Critical power margin (min.)

3·00 3·25 16·8 1·48 1·4 1·25

Number of steam drums Drum diameter (internal), m Drum length, m Drum wall thickness (min.), mm Drum pressure (working), MPa Drum pressure (rated), MPa Weight of steam drum (dry), t

4 2·6 31 110 7·0 7·5 280

Reactivity coefficients Void coefficient at operating point, 10-6% steam Fast power coefficient, a°w, 10-6 Fuel temperature coefficient aT, 10-5/°C Graphite temperature coefficient aG, 10-5/°C Minimum weight of control and safety rods, DK, % Averaged reactivity effect of replacing spent fuel by a fresh fuel assembly, %

Main circulating pumps 2·0 –0·5 –1·2 –6 10·5 0·02

Steam Coolant flow, t/h Coolant flow in max. power channel, t/h Channel inlet temperature, °C Channel inlet pressure, MPa Channel outlet temperature, °C Channel outlet pressure, MPa Mean steam quality at channel outlet, wt % Maximum steam quality at channel outlet, wt % Moisture content of steam at separator outlet, %

37·6×103 28 270 7·8 284·5 7·0 14·5 20·1 <0·1

Coolant circuit Number of parallel loops

2

Number of pumps (including 2 in reserve) Normal pump flow, m3/h Pump head, m water head Pump suction pressure, MPa Pump discharge pressure, MPa Pump speed, rpm Rundown time (1/2 speed), s Pump weight, t Total pump power (normal operation), MW Total pump power (all pumps operating), MW Distributor headers per loop Coolant circuit material

8 8000 200 7·1 8·1 1000 15 100 33 44 22 Austenitic steel

Turbine system Number of steam pipes Steam main diameter, mm Number of turbines Turbine unit power (max.), MWe Turbine unit power (net rated), MWe Turbine frequency, rpm Steam inlet pressure, MPa (abs.) Steam inlet temperature, °C Steam moisture content, % Steam flow, t/h Feedwater temperature, °C

8 400 2 550 510 3000 6·5 280·4 0·5 2902 168

Section 3

Description of the Chernobyl Accident Frank Allen Currently Head of Research and Methods Division, Safety and Reliability Directorate, United Kingdom Atomic Energy Authority, Culcheth, Warrington, Cheshire

3.1 INTRODUCTION

rundown period if the connection to the main electrical distribution grid was lost. If successful, this would enable power for emergency pumps to be provided until the emergency diesel generators on the site started. The results would have been applicable to all of the RBMK reactors. The test was seen as concerning the generator and its electrical systems and so little consideration was given to the safety of the reactor. The test plan did not go into detail on necessary safety precautions and had not been checked and approved by the safety authorities. However, similar tests had been carried out safely in the past and, had the test plan been followed, the accident would probably not have occurred.

At 01.23 hours on 26 April 1986, Unit No. 4 of the Chernobyl Nuclear Power Station was operating at low power prior to a scheduled shutdown when a sudden, very rapid and uncontrolled power increase occurred. This resulted in the destruction of the reactor core and severe damage to the reactor building. There was a release of radioactive material from the core over the subsequent 9 days. The accident is now known to have occurred as a result of a test which was being carried out by the operators. The manner in which the test was conducted, including the disabling of all the safety systems provided to protect the plant, was the immediate cause of the accident. However, aspects of the reactor design resulted in the adverse response of the reactor which caused its destruction. This account is based on the information provided by the USSR to the IAEA post-accident review meeting in Vienna on 25–29 August 1986,1 the report of the IAEA International Nuclear Safety Advisory Group2 and the UKAEA report The Chernobyl Accident and its Consequences.3 Further information has become available more recently through the NEA and IAEA and this has also been incorporated.

3.3 EVENTS PRIOR TO THE ACCIDENT Power reduction for the scheduled shutdown began on 25 April 1986. By 13.05, 50% power was reached and one of the two turbo-generators was switched off. Electrical supplies for the reactor systems were switched to the other. Shortly after, the unit’s emergency core cooling system was switched off. This was in accordance with the (unapproved) plan, but in violation of the unit’s safety principles. However, this is not thought to have significantly affected subsequent events. At this point the test was delayed because of an unexpected request to continue supplying power to the grid at the 50% level. The programmed power reduction was resumed at 23.10 with a target level of 700–1000 MW (thermal). However, due to an operator error in the manipulation of the control system, the power fell to 30 MW

3.2 THE PLANNED TEST Prior to the scheduled shutdown of the No. 4 Unit, a test on the turbo-generator was planned. The test was intended to confirm the operation of a modification made to the generator control circuits. This would enable it to continue to provide power during its 19

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(thermal). Below 700 MW (thermal) this type of reactor was known to be inherently unstable and difficult to control. Therefore, operation below 700 MW (thermal) is not permitted. The operator stabilised the power at 200 MW (thermal) by manual control at 01.00 on 26 April. In regaining this power level, the operator was forced to run the control rods further out from the core than was permitted. This made the reactor even more unstable and at the same time significantly reduced the effectiveness of the shutdown system, since the control rods would have to be driven back into the core before they started to shut down the chain reaction. The operators, according to their own rules, should not have continued to operate the reactor under these conditions; however, there was no automatic system to prevent them. At 01.03 the operator started up additional main reactor cooling pumps as part of the test schedule. Four pumps were connected to the main power grid and four to the remaining turbo-generator. This was intended to ensure adequate reactor cooling after the test when the four connected to the grid would be still operating. However, the water flow in the core was now higher than the permitted value. This caused several problems. Firstly, the water content of the reactor increased because the increased coolant flow resulted in a lower proportion being turned to steam. Normally the steam displaced some of the water in the upper part of the core. The additional water in the core resulted in a reduction in power (because of the positive void coefficient, see below) which was compensated for by running the control rods still further out. Secondly, the water levels in the steam drums started to drop as more water was pumped into the core. The operator attempted to correct this by increasing the flow of feed water from the condenser to the drums. However, he over-reacted and at 01.22 reduced the feedwater flow. To prevent the reactor shutting down from high or low drum water level protection, he had already switched off the reactor trips associated with steam drum level fluctuations. Thirdly, the increased flow rate and lower power meant that the water hardly heated up at all in going through the core and so the whole volume of water in the core was nearly all at the same temperature. Gradually, this temperature rose until it was just below boiling. Finally, the flow conditions and high temperature meant that the pumps were probably operating outside their design conditions and so susceptible to damage or failure. By 01.23 the operator had apparently stabilised the reactor system at least temporarily; however, the

The Chernobyl accident and its implications

reactor was, in fact, in an extremely unstable condition. Not realising this, the operator decided to proceed with the test and, furthermore, he disconnected the reactor trip which should have shut down the reactor when he initiated the test. He did this in order to be able to repeat the test on the turbine, if necessary, but in doing so removed the last line of defence which could have been effective in protecting the reactor. This disconnection of the trip was not called for in the test programme. The trip, if left connected, might have saved the reactor. 3.4 POSITIVE VOID AND POWER COEFFICIENTS A feature of the RBMK reactor design is that, even under normal operating conditions, it has a small positive void coefficient. This means that the nuclear reaction in the core is actually hindered by the presence of the cooling water. If the water turns to steam (which is less dense) then steam-filled ‘voids’ are created in the water and this helps to increase the power of the reactor (it adds reactivity). The increased power will generate more steam which can cause a further increase and so this can lead to a very rapid increase in power. Operating against this effect is the ‘Doppler effect’. This results from a rise in temperature of the fuel as the power rises. As the fuel heats up this reduces the reactivity and so tends to stabilise the power. Under normal operating conditions this outweighs the positive void coefficient and results in the reactor behaving in a stable manner. The reactor still has a rapidly acting negative power coefficient. However, under the conditions which Chernobyl No. 4 unit had reached when the test started, the combined effect of the violations of operating limits, control rods too far out and low power made the void coefficient more positive and at the same time reduced the stabilisation which normally occurred due to the Doppler effect. Overall, a slight increase in steam production would cause an increase in power which would continue to increase since the Doppler effect was now inadequate to control it. This made the reactor difficult to control and extremely unstable because the rapidly acting power coefficient was now positive. Thus the design of the reactor, which allowed the possibility of the overall rapidly acting power coefficient becoming positive under these circumstances, greatly amplified the consequences of the operators’ actions. The positive void coefficient of the RBMK and its rapidly acting positive power coefficient, under certain

Description of the Chernobyl accident

conditions, were the result of choices made at the design stage. Varying the proportions of fuel, absorbers, water and graphite would enable the potential for instability to be removed. However, the designers chose these proportions in order to optimise the fuel utilisation of the reactor, even though optimising fuel utilisation could lead to instability at low power. Since the accident, changes to the reactors are being implemented which will remove this possibility for instability (at the expense of lower fuel economy). In all UK reactors, the proportions of fuel, absorbers, coolant and moderator (where separate) in the core are chosen such that it is not possible for rapid transients due to a positive short-term power coefficient to be induced, either in normal operation or if the operator departs from the normal operating conditions in any way.

3.5 ACCIDENT DEVELOPMENT At 01.23.04 the test was started by isolating the steam supply to the remaining turbo-alternator. The coolant flow reduced because the four main pumps connected to the tripped turbo-generator slowed down. The whole of the cooling water in the core was only just below its boiling point at this stage because of the combined effects of the low power and high flow. Steam started to form in the core as the flow reduced; this added reactivity because of the positive void coefficient. Initially, the automatic control system compensated for this. However, this had only a limited capability which was soon used up and at 01.23.31 the power started to rise. Once this occurred, more steam formed and the power rose faster because under these conditions the reactor was inherently unstable. At 01.23.40 an operator pushed the manual trip button, but it was then too late for the control rods to shut down the reactor since they had to be driven back into the core before having any effect. In fact, it has now been established, in both the Soviet Union and the West,4 that, because of a peculiarity in the design of the control rods and the fact that they were withdrawn so much farther than was permitted, the effect of inserting them may have been to cause the reactor power to increase faster than it would otherwise have done. This is because, under these circumstances, driving in the control rods may initially have added reactivity instead of taking it away—the so-called ‘positive scram’. The control rods have, below them, graphite ‘displacers’ or ‘followers’ which

21

displace cooling water from the control rod channels. When the control rods themselves are withdrawn too far, the ‘followers’ are also withdrawn partially and, because of the positive void coefficient, inserting these actually adds reactivity. This increase in reactivity could have triggered the power increase, or if the power was already rising, it would have made the rise more rapid. By 01.23.44 the reactor power is calculated by the Soviets to have reached 100 times nominal full power in four seconds. Fuel is thought to have fragmented, causing a rapid rise in steam pressure as the water quenched the fuel fragments. This caused the pressure tubes, already weakened by the rise in temperature, to fail over a large region of the core. The explosive release of steam into the reactor vault lifted the reactor top shield, exposing the core debris to the atmosphere. This allowed air ingress. An explosion, possibly chemical, occurred, which destroyed the containment building above the top shield, and a graphite fire ensued, which continued for several days, during which time the radioactive fission products in the core continued to be released. During the course of the rapid power surge, the reactor became ‘prompt critical’. A nuclear chain reaction is maintained within a nuclear reactor because the neutrons produced by fission at one time go on to produce further fissions later on. If the same numbers of fissions are produced, the power remains constant and the reactor is said to be ‘critical’; if more fissions are produced the power increases and if less are produced the power falls. However, not all the neutrons are produced immediately the fission occurs. The ‘prompt’ neutrons (over 99%) do appear immediately but the ‘delayed’ neutrons appear up to a minute later. If the reactor is critical only when the delayed neutrons are taken into account, then the power can change only fairly slowly (over a timescale of a few seconds to a minute) and so is easy to control. However, if enough reactivity is added to make the chain reaction self-sustaining on the prompt neutrons alone, then the power can change much more quickly (over thousandths of seconds). This requires a lot of reactivity to be added, but this is what happened at Chernobyl. The very rapid energy release at Chernobyl is thought to have caused a massive, explosive release of steam as the very hot fuel was quenched by the water. However, the reactor did not explode in a nuclear explosion in the way that an atomic bomb explodes. The power pulse in Chernobyl was terminated because the fuel was dispersed by the very rapid heating. In an

22

atomic bomb, special measures have to be taken to make the rate of power rise very much faster and to delay fuel dispersal so that very much more of the fuel undergoes fission before the nuclear explosion disperses the fuel and stops the reaction.

3.6 CONTAINMENT OF THE DEBRIS Following the damage to the core and reactor building, there was a direct path to the atmosphere through which radioactive material escaped. Fires were burning in various parts of the plant. These were extinguished within a few hours by heroic efforts on the part of the firemen (several of whom subsequently died). The graphite fire in the core continued to burn, however. Materials were dropped on the reactor by helicopter to quench the fire and seal in the radioactivity. This was eventually accomplished after 9 days. There is now a permanent concrete containment surrounding the reactor.

3.7 RELEASE OF RADIOACTIVE MATERIAL The first indication of the accident received in the West was when radioactive material was detected in Scandinavia. Subsequent monitoring in Europe enabled a picture of the release of radioactivity to be built up which represented a gradual release of a significant fraction of the core’s volatile radioactive material over several days. The fuel used in most nuclear reactors consists of uranium dioxide (UO2), a ceramic material. This is in the form of small pellets(less than an inch in diameter) contained within metal tubes, usually of an alloy of zirconium (as in the RBMK) or of stainless steel. These tubes are collected into bundles (fuel elements) and, in the RBMK, are inserted into the pressure tubes to form the core. The vast majority of the radioactive material produced by the fission process is held by the ceramic material itself and what little does escape from the ceramic matrix is retained by the metal tube surrounding the pellets. Fission products can only be released from the fuel elements if these overheat. This is a three-stage process: (1) Failure of the metal cladding tube can occur which will release the very small amount of activity not held in the ceramic pellets. (2) At higher temperature, volatile material which had

The Chernobyl accident and its implications

been held in the ceramic starts to be released. This includes iodine and caesium, the main sources of radiological hazard to man. (3) Finally, at very high temperature, the ceramic fuel itself starts to melt, resulting in much more rapid release of volatile material and also in release of smaller amounts of less volatile material. At the time of the failure of the containment around the core, there would have been a release of radioactive material from the fuel which had failed at that time, through the release path which had been opened by the explosions. Following this, further overheating of fuel seems to have occurred, leading to a continuing release for several days, until the reactor was finally contained and sealed by entombment within a mound of inert material.

3.8 THE ROLE OF THE CONTAINMENT SYSTEM In any water-cooled reactor account must be taken of the remote possibility that a failure of the primary water circuit could occur. The result of this would be a rapid release of steam from the primary circuit. This steam would carry with it radioactive material. In order to prevent this being released to the environment, it is normal practice for water reactors to be surrounded by a ‘containment’ which is a strong structure, designed to prevent the release of radioactive steam to the environment. The RBMK reactor design had a complex containment system. Various parts of the plant had containment structures around them designed to prevent the release of steam to the environment. If a part of the plant failed, the steam would be contained and diverted to the ‘suppression pools’ where the steam would condense and any radioactivity would be trapped in the water. However, the RBMK system included a containment structure around the core itself which was designed only to withstand the failure of one of the 1661 pressure tubes. In the actual accident which occurred, many of these pressure tubes failed and the structure was totally unable to cope with the steam release. The top of the containment was simply blown off by the steam release, effectively allowing the release of radioactivity from the core direct to the environment. The containment was thus so overloaded by the steam release that it was destroyed and played no part in subsequent events.

Description of the Chernobyl accident

3.9 WHAT WENT WRONG? A feature of the subsequent international discussions arranged by the IAEA is that the Russians have been very frank and open in telling the rest of the world what they know of the events at Chernobyl. As a result, we can identify the causes of the accident as follows: (1) The reactor design was inherently unstable under certain conditions. (2) The reactor trip system was easily defeated by the operators and was, in the event, too slow to prevent the rapid power surge when it was finally activated. In fact the peculiarities of the control rods may have actually triggered the accident through the ‘positive scram’ effect. (3) The reactor operators, in their determination to carry out the scheduled test, had taken the reactor into the unstable regime (which was not permitted), had reduced the effectiveness of the control rods and had disabled many of the reactor trip parameters, thus significantly weakening the reactor’s protection. (4) The reactor designers had failed to provide measures to prevent operation in the unstable regime and with the reactor protection seriously weakened, other than by administrative rules. Each of these will be considered in turn. 3.9.1 Reactor stability The RBMK is unique as a power reactor design in having boiling water coolant and graphite moderator. This, together with the designers’ decision to optimise the core design for fuel economy, allowed the possibility of a sudden uncontrollable power surge. Subsequent changes to the other RBMK reactors are believed to have altered this so that the possibility of a short-term positive power coefficient and a resulting rapid power instability has been removed. In UK nuclear power station reactors (described in Section 7) this is already the case. 3.9.2 Defeat of reactor protection system At several stages in the events leading to the accident, the operators defeated a trip signal or other protection system because it seemed likely to interfere with the test. Eventually, a significant part of the system had been disabled. Similarly, the operator ran the control rods much further out from the core than his operating rules permitted. These actions

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resulted in the system being unable to protect the reactor by automatic shutdown and, when the operator manually tripped the system, the rods could not be driven in soon enough to have any effect. In the UK, reactor protection systems are, themselves, protected by locked enclosures and by interlocks with other systems and these would prevent the operator from so seriously degrading the protection provided. It has now been established that, under the circumstances prevailing at the time of the accident with the control rods withdrawn much too far from the core, inserting the control rods would initially have added reactivity to the reactor instead of reducing it. This would have occurred whether the control rods were manually or automatically inserted and could have triggered the power rise or made an existing power rise worse. The designers have now imposed control rod withdrawal limits which would prevent this situation from arising. 3.9.3 Operation in unstable conditions It is clear from what happened that the operators at Chernobyl did not realise the significance of violating the operating rules and taking the reactor to an extremely unstable condition. They seem to have been determined to carry out the planned test to the exclusion of all other considerations. In the UK considerable emphasis is placed on operators being trained to appreciate the factors bearing on safe operation of the plant and to adhere strictly to them. 3.9.4 Failure to prevent maloperation The RBMK design had a number of recognised weaknesses (the possibility of a rapidly acting positive power coefficient and the slow response of the protection system if the control rods were withdrawn too far) which were not protected against other than by administrative rules. It is now recognised as a design weakness by the Russians that positive provision was not made, for example, to shut the reactor down automatically under these conditions. In the UK it is normal procedure to have an automatic shutdown if important safety limits are violated. REFERENCES 1. The accident at the Chernobyl nuclear power plant and its consequences, information compiled for the IAEA Experts

24 Meeting, 25–29 August 1986, Vienna, by the USSR State Committee on the Utilisation of Atomic Energy. 2. Summary Report on the Post-accident Review Meeting on the Chernobyl Accident, International Nuclear Safety Advisory Group, IAEA, Safety Series No. 75—INSAG 1, 1986. 3. GITTUS, J.H., HICKS, D., BULLOCH, R.S. et al., The

The Chernobyl accident and its implications

Chernobyl Accident and its Consequences, UKAEA NOR 4200, 1987 (published by HMSO). 4. Chernobyl and the Safety of Nuclear Reactors in OECD Countries, report by an NEA Group of Experts, NEA, OECD, Paris, 1987.

Section 4

The Radioactive Release from Chernobyl and its Effects Barry Smith Deputy Chief Scientific Officer, Boundary Layer Research Branch, Meteorological Office, Bracknell, Berks

&

Arthur Charlesby Retired Professor of Physics at the Royal Military College, now working as a consultant

I–131:0.17 Te–132:0.15 Cs–134:0.0055 Cs–137:0.011 Total of all active material: 0.83

4.1 THE INITIAL RELEASE At 01.23 local time on 26 April 1986, the Chernobyl No. 4 reactor accelerated from a fraction of full power to 100 times full power in just 4 seconds, causing an explosive generation of steam. The details of what happened internally as a result of this surge are described in Section 3, but the explosion lifted the enormous 1000 tonne roof off the building, tipping it on to its side, and spewed burning debris over the site. Since the reactor had been in full use for well over 2 years, the core content was rich in a wide range of fission products, many of which were highly ‘active’. Two of the most important from a health point of view were iodine-131 and caesium-137. That part of the material which became airborne would have risen due to the intense heat associated with the very large quantities of energy released during the explosion. Nevertheless some would be in the form of large particulates and would have fallen out under gravity to be deposited within a few kilometres of Chernobyl. This local loss makes the assessment of the total release on the first day somewhat tentative. According to the Soviet report to the IAEA,1 the emissions on this first day are as follows, with an uncertainty factor of about 2 (1 EBq=1018Bq):

EBq EBq EBq EBq EBq

4.2 DURATION OF RELEASE Enormous efforts, not to mention courage, were expended to contain the release of active material to the atmosphere and to ground waters. About 5000 tonnes of boron carbide, dolomite, sand, clay and lead were dumped on top of the reactor from helicopters. As a result, by day 5 the emissions had fallen by 90%, but later rose again since the residual power of the fuel heated the debris to over 2000 °C, resulting in partial penetration of the dumped covering material. By 5 May emissions were up to a third of the level of day 1, 26 April. However, by the following day workers had managed to cool the core and emissions virtually ceased. Even so, small detectable traces were sensed in the atmosphere for at least another 3 months in Europe. 4.3 INVENTORY OF TOTAL RELEASE Figure 4.1, reproduced from a paper by Persson et al.,2 displays the daily emissions from Chernobyl-4 25

26

The Chernobyl accident and its implications

at Three Mile Island in the USA both for material released from the reactor core and for the release to the environment. Table 4.2 is from their report and shows that, whilst the releases from the cores are very comparable, the containment building at TMI was highly effective in preventing any significant release to the environment. 4.4 METEOROLOGICAL CONDITIONS

Fig. 4.1. Daily emissions from Chernobyl following the accident on 26 April, according to the Soviet report to the IAEA,1 and reproduced from the paper by Persson et al.2 The broken line indicates the total activity excluding inert gases (left-hand scale) while the solid line indicates the emissions of 137 Cs (right-hand scale). (1 EBq=1018 Bq; 1 PBq=1015 Bq.)

according to the Soviet report1 to IAEA. Table 4.1, using data from the same paper, gives details of some of the major nuclides involved in the release and estimates of their total emissions. The CEGB report on Chernobyl by Collier and Davies3 compares the release at Chernobyl with that

The meteorological situation during the release was fairly complex and variable with winds that carried the activity successively over almost all parts of Europe. At the time of the explosion on 26 April a ridge of high pressure was centred over northwest Russia, whose wind circulation carried the upper part of the plume away towards the Baltic and Scandinavia. Nearer the ground, the clear skies had resulted in cold ground and a temperature inversion reaching up to some 500 m. Within this stable layer there was almost a calm, but above it the winds picked up to 10 m/s and more. After sunrise, the stable layer broke down and the low-level winds increased. Once the plume reached Scandinavia, it split into three ‘fingers’. One moved away to the east across northern parts of the USSR into Japan and China. A second crossed central Norway and out into the Norwegian Sea and moved towards North America. Heavy rain on 28 April resulted in very large depositions of activity in some parts of Scandinavia. A third finger moved south-westwards in response to

Table 4.1 Total emissions of the most significant radionuclides from Chernobyl-4 according to the Soviet report to IAEA1 (1 EBq=1018 Bq)

The nuclides are listed in order of total emission. Analysis of the nuclide content of the plume over Finland suggests that the Soviet estimate of the neptunium emission may be too big by a factor of about 3 (T.Raunemaa, private communication, 1987). There is a suggestion that Np-239 was released slowly over some days and partly in the form of one of the decay products (G.Lewis, private communication, 1988). a b

The radioactive release from Chernobyl and its effects

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Table 4.2 Three Mile Island and Chernobyl releases compared (Collier and Davies3)

a transient ridge of high pressure over the North Sea. A part of this plume crossed central Europe, moved over northern Italy and southern France, and then turned north-westwards as the ridge slipped away to the NE, reaching the French north coast late on Thursday, 1 May. Meanwhile later emissions affected central Europe, Germany, Austria and the western Balkans, giving further widespread heavy depositions whenever the plume was intercepted by rain.

One radioactive plume crossed the English Channel into southern England in the early hours of 2 May, having experienced almost no rain on its tortuous 4100 km track from Chernobyl. Measurements of related activity in the air were made by the Electricity Boards, by British Nuclear Fuels plc, by the UK Atomic Energy Authority and by the National Radiological Protection Board at various sites throughout the UK. The Meteorological Office combined these with airtrajectory analyses to define the subsequent movement

Fig. 4.2. Position of the Chernobyl plume over the UK at 6-hourly intervals. Shaded areas denote approximate regions where outbreaks of rain were occurring. Contour lines of ß-activity in units of Bq/m3 are shown to delineate the plume.

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of the plume, as shown in Fig. 4.2 which covers the period from Friday to Sunday 2–4 May. Friday 2 May was a warm dry sunny day over the southern half of Britain. The plume moved slowly at first up the eastern side of England, but in response to a deepening low to the west of Cornwall and a small subsidiary heat low over the west Midlands, the winds increased over the Pennines and the plume accelerated, reaching Cumbria and north Wales by late afternoon. The winds then decreased again until on Saturday the forward edge of the plume moved northwards towards the Highlands and a section broke off and moved south-westwards across Ireland. Saturday saw the fine spell gone. Heavy rain affected many parts. Thunderstorms, advancing from France, were revitalised as they marched northwards. They caught up the tail of the plume of radioactivity and, drawing great quantities of contaminated low-level air into their systems, caused considerable rainfall and heavy depositions of activity on the underlying terrain. Parts of north Wales, the Pennines, Cumbria, the Isle of Man and southwest Scotland were particularly badly affected. On Sunday the plume was centred over Scotland. Although the size of the plume was diminished by Saturday’s thunderstorms, the concentrations of caesium and iodine were still high enough in parts of the plume to give one of the highest recorded concentrations in rain at Dounreay on the northern Scottish coast. A fragment of the plume broke off and travelled westwards with the brisk winds over the Western Isles, to circulate round the still-active depression and re-cross Britain with concentrations of activity at least one or two orders of magnitude lower later in the following week on Wednesday and Thursday. The main segment of the plume moved NNW so that by midnight on Sunday the main plume had gone from Britain. Thereafter the winds remained from the west for some time and no further segments of the Chernobyl emission directly affected the country. 4.5 NATIONAL RADIOLOGICAL SURVEYS IN THE UK The current network of air-monitoring stations was designed to alert management to accidental releases from sources of radioactivity held on the site of the station. They are therefore rather few in number, being 27 in all (see Fig. 4.3), with some grouped in rather small geographical areas. Nevertheless they are all at

The Chernobyl accident and its implications

Fig. 4.3. Network of monitoring stations at the time of the Chernobyl accident.

centres of expertise, and the measurements they made during the passage of the Chernobyl plume proved to be immensely useful. However, in retrospect it is clear that a denser network of surface monitoring stations capable of making systematic readings of g-radiation, of ß-activity, of the concentration of various radionuclides in the air and in precipitation, as well as the magnitude of deposition to grass and to the ground, would be of tremendous value should another large release ever affect the UK in the future. Some of these stations should be in constant operation to detect any significant plume of activity entering the country from an undeclared accident elsewhere, whilst the remainder are brought on line only on demand. The former should be spaced at little more than about 100 km separation in order to ensure detection of the plume, and presumably mainly along the south and east coasts of the country. The latter may be distributed more freely to ensure good representative coverage of the

The radioactive release from Chernobyl and its effects

UK. Such a network of about 80 sites is currently under installation by the Department of the Environment. An additional potential means of surveying the deposited material following the passage of a radioactive plume is by using an aircraft instrumented with a sodium iodide scintillation counter which can detect g-radiation in different wavebands and hence

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detect emissions from different active species. The aircraft would have to fly many parallel legs back and forth across the country at fairly low altitude, building up maps of deposition. The cost is large, probably around £½m for the UK, and careful calibration with surface-based measurements essential, but the results are more complete than can be achieved by any other

Fig. 4.4. Estimated total (wet+dry) depositions of caesium-137 over the UK arising from the passage of the Chernobyl plume. Units: 1000Bq/m2.

The Chernobyl accident and its implications

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process. A survey of this kind was successfully carried out in Sweden following the Chernobyl release.1 4.6 AFFECTED AREAS OF THE UK The deposition of activity to the ground is the result of two main processes, termed dry deposition and wet deposition. Dry deposition covers the combined effect of sedimentation of larger particles under gravity, of impaction of particulates and aerosols on leaves, etc., and of absorption of reactive gases by the soil and vegetation. The dry deposition depends on the product of the concentration of the material close to the surface and a so-called deposition velocity vd. For gases like iodine-131 and sulphur dioxide, vd is about 1 cm/s in normal turbulent surface layers. Small aerosols and particulates (like the caesium-137 from Chernobyl) have a much smaller vd. Wet deposition is linked with atmospheric water droplets and combines the effects of rain and snow as well as the impaction of wind-blown contaminated cloud or fog droplets on to vegetation when the cloud or fog extends down to the surface. The removal by rain or snow is particularly efficient; just 1 mm of rain can remove more material than can be deposited by dry deposition operating over 24 h. Figure 4.4 shows an estimated map of caesium-137 deposition over the UK arising from Chernobyl. Values over the Scottish Highlands and other remote areas are unfortunately somewhat tentative, mainly because of uncertainties in the rainfall in those areas. The map was derived by using surface rain-gauge measurements at nearly 4000 sites across Britain, in conjunction with estimated concentrations of caesium-137 in air during the time of the rain, to yield an estimate of the wet deposition at each site. This was then combined with an estimate of the total dry deposition to yield the total deposition D. The specific formula used is

where R=site-rainfall affecting the radioactive plume (mm); CR=average concentration of caesium-137 in the air during the rain (Bq/m3); Ca=average concentration in the air during the passage of the plume (Bq/m3); T=time of passage of the plume (hours). The formula has been shown to have reasonable validity by comparing its predictions with spot samples of total caesium-137 deposition in soil carried out by AERE Harwell and by Dr P.G. Appleby of Liverpool University. Furthermore the formula is not very different from one that would be inferred from comparable measurements made in Sweden. As the map shows, depositions in excess of 20000 Bq/m2 are predicted for parts of Galloway in southwest Scotland, and for the northern tip of the Isle of Man. These may be compared with estimated depositions in excess of 85000 Bq/m2 in a small area north of Stockholm in Sweden. Deposition may be underestimated in the more mountainous areas for two reasons: (i) the rain-gauge sites in these areas are often remote and were not read on a daily basis over the weekend; and (ii) the sites are usually in valleys and wet deposition would normally be less there than on the neighbouring mountain tops. The deposition of iodine-131 was broadly similar to that of caesium-137 except for the higher additional dry deposition component, being roughly double in overall magnitude in the wet areas and down to about 400 Bq/m2 in the dry areas of southern England and Wales where dry deposition dominated. 4.7 CONTAMINATION OF FOODSTUFFS In various parts of Europe, including the UK, analyses of the levels of activity in drinking water, milk, vegetables and meat revealed levels approaching or

Table 4.3 UK Derived Emergency Reference Levels

The radioactive release from Chernobyl and its effects

above national emergency levels in one or more ‘species’. Consequently governments imposed restrictions on the sale or use of these species, often causing hardship and loss of revenue to the farmers concerned. In the UK these levels are called ‘Derived Emergency Reference Levels’, or DERLs for short. A selection of these DERLs is given in Table 4.3. Within the UK there were several organisations who were able to make measurements of the levels of activity within food and in air and rain. Measurements of activity in air and rain were made by nuclear power stations of the electricity boards, British Nuclear Fuels plc, the UK Atomic Energy Authority (UKAEA) and the National Radiological Protection Board (NRPB). Surveys of activity in soil have been made by the Harwell Laboratory of the UKAEA and by Liverpool University. Activity on grass was measured at very many sites by NRPB and by the Institute of Terrestrial Ecology (ITE). Similarly extensive measurements in foodstuffs have been organised by the Ministry of Agriculture, Fisheries and Food (MAFF).4 Table 4.4 lists the more important maximum values recorded in the NRPB compilation Levels of Radioactivity in the UK from the Accident at Chernobyl.5 Typical values are very much smaller than these isolated maximum values. In considering these levels in food, it is necessary to remember that naturally-occurring isotopes (for example of potassium) are also present. Thus in West Germany, for example, Chernobyl contamination, whilst at least as large as in the UK, increased the radioactivity in foodstuffs by only some 10% during the first few months. Different countries in Europe have different DERLs, but all of them set limits that are really very small in terms of body dosage when compared to what is

31

received from other sources (natural, medical, etc.). Thus in many EEC countries the DERL for meat is set at 600 Bq/kg, and in Sweden it is 300 Bq/kg, compared with the UK value of 1000 Bq/kg. These disparities largely reflect the uncertainties surrounding the effects of relatively small levels of extra radioactivity on the human body. The UK DERL for iodine in milk was locally exceeded in Poland and Hungary immediately following the accident. Levels in milk in other north European countries were much lower because after a cold spring most of the cattle were still on winter feed and were not grazing the pastures. Further south, in parts of Germany and in Italy and Greece, levels were so high that fresh milk sales were banned during the first three weeks of May. Drinking water levels in Scandinavia and some other European countries also showed levels of activity in some areas that exceeded the DERL set by the International Committee on Radiological Protection (ICRP). Supplies of vegetables, notably spinach, were withdrawn from sale in France, Greece, Italy, Luxembourg, the Netherlands and West Germany, and restrictions were placed on reindeer meat in Lapland and on sheep in some areas of the UK. 4.8 HEALTH EFFECTS Beyond 100 km from Chernobyl, the effects of the additional uptake of radioactivity into the human population are likely to be so small that they will be impossible to detect even by the most careful of medical surveys over the next few decades. This simple truth was largely ignored by the media and has led to considerable anxiety and exaggerated fear of the nuclear

Table 4.4 Peak values of activity in 1986 compared with UK Derived Emergency Reference Levels (DERLs) (from Ref. 4)

The Chernobyl accident and its implications

32

industry. On the other hand, some deaths may well result, although how these should be interpreted is a matter of some debate: for example, many may occur in old people nearing the end of their lives anyway. This is not intended to sound callous: any death is a tragedy to the people concerned; but it is intended to put the problem in perspective. W.K.Sinclair6 has given statistics of deaths per year in the USA from avoidable ‘accidental’ causes. Nuclear power generation, including the very occasional release, is said to cause typically 100 deaths per year. Smoking causes 150000, alcohol 100000, road accidents 50000 and accidents with guns 17500. Chernobyl almost fades into insignificance by comparison. 4.8.1 In the vicinity of Chernobyl Two staff died immediately in the accident, either in the explosion or in the subsequent fire. Twenty-nine others died in the following few days as a result of intense ß-exposure causing extensive radiation burns of the skin. Two hundred and seventy-one other people were admitted to hospital of whom 174 suffered symptoms of acute radiation syndrome, having received wholebody g-irradiation doses between 2 and 16 gray. One hundred and thirty-five thousand people were evacuated from a zone within 30 km of Chernobyl. None of these people showed any clinical symptoms, although it is estimated that up to 1000 of them may develop cancers in the next few decades. 4.8.2 The European Community including the UK The NRPB have produced a report entitled A preliminary assessment of the radiological impact of the Chernobyl reactor accident on the population of the European Community6 which discusses, amongst other issues, the expected incidence of cancers in the European Community during the next 50 years. Their conclusions are summarised in Table 4.5. Iodine-131 is partly gaseous and is concentrated in the body within the thyroid gland. Having a short half-

life (8 days), its effects are relatively rapid. Many other nuclides are particulate and get lodged within the tissue of the lungs when inhaled. Gaseous nuclides are also absorbed. The lungs are particularly sensitive and may then be damaged. W. Burkart7 has given average percentage contributions to the total annual dose from various sources to the inhabitants of Switzerland: 41% from radon and other natural particles, 24% from medical sources, 8% from cosmic rays, less than 1% from the atomic bomb tests of the 1950s and 1960s, and 3·6% from Chernobyl. A much greater risk affects people living in areas with high radon concentrations, for example in Cornwall. Well-sealed modern housing in such areas can, it is believed, give rise to a risk of 1 in 2500 of developing associated lung cancer in each year. This is some 104 times greater than the risk from Chernobyl fall-out. 4.9 EMERGENCY PLANNING IN THE UK To some extent the Government and the nuclear industry at large within the UK found themselves somewhat disorganised by the sheer scale of the Chernobyl incident as it affected the UK. Even so, much good work and many long hours were expended by those concerned. Nevertheless, it has become very clear that a ‘National Plan’ is required, and required quickly. A great deal of activity is currently underway to formulate the Plan and it would be premature at this stage to give many details, since these may change. The monitoring network has already been referred to in Section 4.5. Models of various kinds are being developed; these cover the whole aspect of emission, transport and deposition of released activity, the uptake of deposited material by vegetation and by grazing animals, and how this gets into the food chain and affects human health. Very many organisations are involved in this modelling work. In addition a network of centres interlinked with rapid communication lines are envisaged which could move into action whenever a release occurs, to ensure that proper notification of the consequences is made to other

Table 4.5 Incidence of cancers in the European Community in the next 50 years

The radioactive release from Chernobyl and its effects

33

countries, to the media and to the public wherever they are at risk. Monitoring teams will be concentrated in areas of high deposition to ensure that direct effects to the public are minimised. All this activity will be directed from a central government unit in London.

3. 4.

REFERENCES

6.

1. USSR State Committee on the Utilization of Atomic Energy, The accident at the Chernobyl Nuclear Power Plant and its consequences, IAEA Experts’ Meeting, 25–29 August 1986, Vienna. 2. PERSSON, C., RODHE, H. & DE GEER, L.-E., The Chernobyl Accident—a meteorological analysis of how

7.

5.

radionuclides reached and were deposited in Sweden, Ambio, 16 (1), 1987. COLLIER, J.G. & DAVIES, L.M., Chernobyl, CEGB, 1987. Radionuclide levels in food, animals and agricultural products: Post-Chernobyl monitoring in England and Wales. MAFF report, HMSO, London, 1986. Levels of radioactivity in the U.K. from the accident at Chernobyl NRPB Report, HMSO, July 1986. MORREY, M., BROWN, J., WILLIAMS, J.A., CRICK, M.J., SIMMONDS, J.R. & HILL, M.D., A preliminary assessment of the radiological impact of the Chernobyl reactor accident on the population of the European Community, NRPB Report (CEC Contract 86–398), 1986. BURKART, W., The lessons of Chernobyl. Paper presented 1at Institute of Biology Meeting, Royal College of Surgeons, London, 11 April 1987.

Section 5

Accident Management in the USSR and the United Kingdom Glynne Lewis Retired Superintendent Inspector, Nuclear Installations Inspectorate

5.1 INTRODUCTION

has issued two safety guides on the preparedness of operating organisations and public authorities for emergencies at nuclear power plants 1 for use by member states. The extent to which the USSR adopted these recommendations before the Chernobyl accident and how they are used in the UK are described briefly.

The nuclear power industry is dominated by high safety requirements and low accident probability, but it is essential to have available comprehensive Emergency Arrangements for each station for implementation in the event of an accident. It is difficult to maintain a high degree of preparedness for an event which is unlikely to happen, but the reactor accidents at Three Mile Island and Chernobyl emphasise the need for constant vigilance, not only in preparedness for emergencies, but in all operator activities affecting reactor safety. As far as is reasonably practicable, in the event of a major accident, the release of radioactive fission products from the uranium fuel to the atmosphere is prevented by a series of containment barriers. Should these be penetrated and a release of activity to the environment occur, the Station Emergency Plan is required to be implemented. The immediate response by the operators to this potentially dangerous situation is vitally important if the consequences to station personnel and the local population are to be minimised. The emergency arrangements for nuclear accidents must be integrated with existing civil emergency services for similar man-made or natural disasters. Initially the responsibility for introducing these arrangements rests with the nuclear power station management, but should public safety or concern become a dominating factor, then a Government Appointee should take control of the management of the accident. The International Atomic Energy Agency (IAEA)

5.2 NUCLEAR POWER STATION EMERGENCY ARRANGEMENTS IN THE USSR Details of the emergency arrangements have been obtained from the Soviet reports presented at the Postaccident Review Meeting attended by experts from member states and arranged by the IAEA, which was held in Vienna during 25–29 August 1986, and from the Summary Report issued by the International Nuclear Safety Advisory Group (INSAG).2 The scarcity of information in these reports, particularly with respect to the operators’ response to the accident., has led to some speculation on the sequence of events. 5.2.1 Chernobyl NPS Reactor 4 accident On Saturday 26 April 1986 at 01.23 local time, Reactor 4 and the surrounding plant and building were destroyed by an explosion caused by a prompt critical power excursion which resulted in a massive release of active debris from the reactor core into the atmosphere. The first effects of this active release felt outside the Soviet Union were measured at the Swedish 35

The Chernobyl accident and its implications

36

Nuclear Power Station (NPS) at Forsmark on Monday 28 April 1986. Iodine-131 levels in rainwater samples were ten times those in the reactor coolant and ground deposition measurements were 20 kBq/m2. Following the notification of this information by the Swedish authorities, the Soviet government announced that a reactor accident had occurred at Chernobyl NPS a few days earlier. The Chernobyl NPS is a multi-reactor unit site. Stage 1, consisting of Reactors 1 and 2 with TurboAlternators (T/A) 1–4, was completed in 1977. Stage 2, consisting of Reactors 3 and 4 with T/A 5–8, was completed in 1983 and is an extension of Stage 1. Stage 3, consisting of Reactors 5 and 6 and T/A 9–12, has been under construction since 1981, and is situated in an adjacent area (Figs 5.1 and 5.2). Turbo-alternators 1–8, each of 500 MW(e) capacity, are arranged in line in one long turbine hall. At one end, Reactors 1 and 2 are sited separately, and at the other end of the turbine building Reactors 3 and 4 are arranged in a common building with a central high effluent stack. This is an unusual feature for a NPS, but it may be used to disperse active gaseous effluent, such as tritium, from the direct steam cycle employed. Safety valve vents are directed via the steam suppression pools situated beneath the reactor, before passing up the stack. The 150 m high stack is also used for ventilation system discharges from clean and active facilities in the reactor building, via particulate and iodine filters.

Fig. 5.1. Layout of the Chernobyl nuclear power plant.

Fig. 5.2. Layout of four Chernobyl NPP units.

5.2.2 Response to a nuclear accident by station operators On Friday 25 April 1986, the night shift on the station consisted of 176 operators on duty on Reactors 1–4, together with 268 construction workers on Stage 3 site. Reactors 1–3 were at full power and Reactor 4 was at low power, supplying steam to T/A 8 prior to the turbine rundown test to determine the security of electrical supplies to essential plant if the steam supplies were shut off from the turbine. The sequence of events leading to the destruction of Reactor 4 has already been described in Section 3. The emergency alarm was sounded and operators in the plant withdrew to safe areas. The operators in the Central Control Room (CCR) in each reactor are the only operators to remain at their posts. Reactors 3 and 4 were probably controlled from a common CCR. Notification of the accident was made to the Ukrainian state authorities and assistance requested from local emergency services. Fire-fighting units from the nearby towns of Pripyat (8 km) and Chernobyl (15 km) set out at 01.30 on Saturday morning to assist the station emergency fire teams in their efforts to extinguish the numerous fires started by burning core fragments ejected from the damaged reactor, and from burning oil and other fires started by electrical faults due to damaged cables. It was vital to prevent the fires spreading to the adjacent Reactor 3 which was still at power. The extensive plant damage, severe radiation, and thermal hazards during the hours of darkness complicated the accident situation. No details were given of the radiological precautions taken by the emergency teams to protect themselves from the obvious industrial dangers associated with such a plant, nor of the control

Accident management in the USSR and the UK

exercised by the shift managers, nor whether senior station managers and specialist staff were called in to assist in the emergency arrangements. It was reported that action was taken to stabilise the plant. An attempt was made to flood the damaged reactor with coolant water, but this failed because the water passed through passages into the adjacent Reactor 3. Eventually all the external fires were brought under control by 05.00. At the same time Reactor 3 was shut down. As a result of their actions 300 operators and firemen were admitted to hospitals in Kiev and Moscow during the first days of the accident. Two hundred and three of these casualties were treated for acute radiation syndrome due to whole body irradiation and ß-activity burns to the exposed skin, and from thermally induced burns. There were 31 fatalities, of whom six were firemen. Two operators were fatally injured on the plant, one of whom could not be recovered. Professor Guskova, Medical Consultant at the Moscow Hospital, reported at a recent UK seminar that the estimated radiation exposures received by the operators and firemen were 1–16 gray (100–1600 rad). No information was given concerning the problems of evacuating highly contaminated casualties from the site to the hospitals. No details were given of environmental conditions in the CCR of the reactors, particularly of Reactors 3 and 4. Presumably air-conditioning systems were switched off on receipt of radiation alarms, and respiratory protection worn by the operators. No information on whether the operators had initiated the Emergency Core Cooling System has been supplied, nor have details of plant log sheets or post-accident computer data. No information was given on the arrival of senior station staff, who lived in the vicinity of the station, to assist the shift operators in the first crucial hours of the accident.

37

Commission. Priority was given to safeguarding the local inhabitants, extinguishing the core fire, terminating the release of activity to the environment, containing the damaged reactor, and decontaminating the station so that it could be returned to production as soon as possible. 5.2.4 Health and safety of local inhabitants During Saturday 26 April 1986, air activity and ground deposition measurements in the towns of Pripyat and Chernobyl were such that inhabitants were advised to shelter in their homes with windows and doors shut. Potassium iodate tablets were distributed to protect them from accidental intake of Iodine-131. Later in the day radiation levels in Pripyat reached 10 mSv/h (1 rem/h) in some areas, such that the lower intervention level for whole body dose, 250 mSv (25 rem), would be exceeded in 12 h. It was decided to evacuate the town’s population (45000), and this was completed during Sunday afternoon, using public transport vehicles from the state capital of Kiev, 120 km away. Figure 5.3 shows the area around the Chernobyl nuclear power plant.

5.2.3 Response by central government Following the notification of the Reactor 4 accident to the state authorities and then the central government in Moscow, specialist teams from many establishments in the USSR were mobilised and sent to Chernobyl. A commission of senior ministers and scientists was appointed to manage the accident and coordinate the necessary government emergency resources. Meteorological surveillance of the active discharge, and radiological surveys on and off the site, were initiated and a strategy of action was decided by the

Fig. 5.3. Evacuation area around the Chernobyl NPP.

38

The Chernobyl accident and its implications

During the following week the people in Chernobyl and all the villages in the area within a distance of 30 km from the plant (the ‘30 km zone’—Fig. 5.3) were evacuated from their homes. People from towns in the neighbouring state of Byelorussia were also affected; in all a total of 135000 people were evacuated to relocation centres set up in the principal cities nearest the site. The delayed evacuation of the population in the vicinity of the NPS is understood to have been in accordance with the official Soviet evacuation plans, which closely followed the recommendations of the International Commission on Radiological Protection (ICRP).3 The relocation centres were equipped with medical and other emergency service resources to carry out personal decontamination, compulsory dosimetric monitoring, blood sampling for laboratory testing and replacement of contaminated clothing. Special attention was paid to the monitoring of thyroid glands of children. Whole body doses were generally below 250 mSv (25 rem) and thyroid doses below 300 mSv (30 rem). In addition thousands of farm animals were evacuated from the 30 km zone in hundreds of trucks, and the zone became a restricted area with entry controls enforced by the police. 5.2.5 Extinguishing Reactor 4 core fire Coinciding with the above measures, Reactors 1 and 2 were shut down in the early hours of Sunday 27 April 1986. Dosimetric and plant damage surveys were carried out in Reactor 4 building, as far as this was possible in the severely hazardous conditions prevailing. Most of the installed instrumentation in the reactor had been destroyed. Aerial surveys from helicopters, using infrared imaging equipment and lowering radiation detectors over the core, enabled the Commission to decide a course of action in an attempt to blanket the fire. On Monday 28 April, deposition of selected materials from helicopters commenced: 40 tonnes of boron carbide to keep the core subcritical; 800 tonnes of dolomite to generate CO for a reducing 2 atmosphere; 1800 tonnes of clay and sand to blanket the core; and 2400 tonnes of lead shot to melt, seal and provide shielding. These operations were successful in extinguishing the core fire and terminating the active discharge by 6 May, ten days after the commencement of the accident. Figure 5.4 shows one of the helicopters used for blanketing operations and

Fig. 5.4. Helicopter hovering over the damaged Reactor 4 at Chernobyl (May 1986) (Novosti Press Agency).

for taking radiation measurements above the damaged reactor. 5.2.6 Construction of a heat barrier Initially it was feared that the damaged core might become critical again and burn itself through the reactor bottom biological shield, through the lower coolant pipe gallery, through the suppression pool floor concrete and then into the ground beneath—the ‘China Syndrome’. It was decided to excavate and construct a concrete slab or heat barrier beneath Reactor 4. It is not clear from the reports whether this concrete slab was constructed below the existing foundations or a tunnel was excavated beneath the reactor to provide access to the steam suppression pools, which had been drained of water, to enable these to be filled with a liquid cement mixture. However, the work was carried out by a large force of volunteer miners and was completed by the end of June 1986.

Accident management in the USSR and the UK

5.2.7 Cooling the damaged core

39

During the blanketing operations there was an upsurge of the core temperature and an increase in the release of activity from 1–6 May. Efforts were made to cool the melted down core by pumping water through the damaged coolant pipework beneath the core. Eventually the injection of nitrogen brought the temperatures and activity and release rates down rapidly after 6 May, and it was possible to use air cooling later. Core temperatures were in excess of 2000 °C during the release.

the remainder of the plant in the vast single turbine hall housing turbines 1–8. The concrete roof over the damaged section of the turbine hall was supported on structural steelwork and external columns. This massive concrete mausoleum will be a permanent reminder of man’s fallibility. It has been reported that the entombment was completed in October 1986 and the clean-up and commissioning of Reactor 3 was continuing; this reactor was returned to power at the end of 1987. Figure 5.5 shows the entombment building.

5.2.8 Entombment of Reactor 4

5.2.9 Site decontamination

To enable Reactor 3 to be brought back to power and operated safely, it was necessary to totally enclose the remains of Reactor 4 in a concrete structure of sufficient thickness to reduce external radiation dose rates to below 50 µSv/h (5 mrem/h). This necessitated the construction of massive foundations. Remotely controlled earthmoving equipment was used to clear the area and a prefabricated method of construction was used on the external walls. Heat exchangers were installed to remove the decay heat. An open ventilation system was provided with filters for the building with the necessary control and surveillance equipment to monitor the conditions within the building. Inner shielding and partition walls were also constructed to separate turbo-alternators 7 and 8 from

Polymerising sprays were used to fix loose contamination. Active topsoil on the site was removed and replaced by concrete, etc. Buildings, plant and equipment were decontaminated using washing-down and dry-cleaning techniques. Dose limit controls meant a large replacement work force over a considerable period of time—a formidable task. Figure 5.6 shows two of the remotely controlled earth moving vehicles used in this operation. 5.3 RELEASE OF ACTIVITY Based on the irradiation history of the fuel, radiation measurements and sample analysis from the 30 km zone and throughout the USSR, it is estimated that 1– 2 EBq (30–50 MCi) of activity was discharged to the

Fig. 5.5. Entombment of Reactor 4 completed (November 1986) (Novosti Press Agency).

40

The Chernobyl accident and its implications

Fig. 5.6. Chelyabinsk-built bulldozer tractors, remotely operated by radio, being tested on an improvised testing ground outside the contaminated area before employment near Chernobyl Unit 4 (Novosti Press Agency).

environment, excluding the noble gases Krypton-85 and Xenon-133. The proportions of nuclides released were approximately 30% Iodine-131, 15% Tellurium132, 10% Caesium-134, 13% Caesium-137, 30% mixture of other fission products, and 2% actinides (α−activity). The possible error in release quantities is stated to be about 50%. The complex meteorological conditions during the period of discharge led to a varied pattern of atmospheric transport and ground deposition. About half the fission products were deposited as aerosols and particulates in the 30 km zone. About 0·5% of the actinides, including uranium oxide fuel, was deposited on the power station site. About 10% of the graphite moderator was oxidised in the fire to produce asphyxiating gases in the reactor building. The majority of the oxide fuel and zirconium cladding melted and solidified again in the well of the reactor. Radiation dose-rates near the reactor were in the range 1–10 Sv/h (100–1000 rem/h). Ground deposition dose rates on site were generally about 1 mSv/h (100 mrem/h). In the 30 km zone aerial and ground air sampling and gamma spectrometric analysis with meteorological forecasts enabled radiation hazards to the public to be predicted and countermeasures to be taken. A comprehensive programme of soil, herbage, water and milk sampling and analysis enabled food and milk restrictions to be imposed. Generally, milk products contaminated with I–131 were banned at concentrations above 4000 Bq/l (0·1 mCi/l). Caesium-

134 and caesium-137 are of long-term concern, as sources both of external radiation from ground deposition and of food contamination. Aquatic samples from rivers and reservoirs were taken to measure the ecological effects of fall-out. Dam construction and other civil engineering operations were undertaken to prevent or limit the contamination of sources of drinking water in the region. The artesian wells which were bored to provide drinking water for the people of Kiev (population 2 million) in the event of contamination of reservoirs were not used.3 The Soviet authorities intend to carry out long-term medical surveillance of all population groups, comprehensive ecological studies, and removal or stabilisation of top soils and forests. The time scale in which this massive government assistance was mounted, and the resources of personnel and materials provided to bring this disastrous situation under control, greatly impressed the IAEA member state specialists who attended the postaccident review meeting. 5.4 UNITED KINGDOM EMERGENCY ARRANGEMENTS 5.4.1 Station emergency plan In the UK the Licensees, CEGB, SSEB and British Nuclear Fuels, are responsible for the safety of nuclear

Accident management in the USSR and the UK

power plants. They are required under the conditions attached to the Site Licence to make Emergency Arrangements (EA) for dealing with a reactor accident or other dangerous occurrences. These arrangements include the provision of personnel resources and equipment, such as radiological monitoring equipment, protective clothing, respiratory protection and means of communication by installed equipment and by radio from all plant areas. Mobile emergency maintenance facilities must also be provided to enable damaged plant to be repaired so as to minimise the consequences of a reactor accident. The Licensees are required to produce an Emergency Plan for each NPS and this is approved by the Nuclear Installations Inspectorate (NII). The Plan is implemented by Emergency Instructions which detail the duties and responsibilities of all those personnel involved with the Emergency Arrangements. The Instructions also list the inventory of all plant and equipment which must be available and ready for use in the event of an emergency situation arising. The Licensees are required to coordinate the Emergency Arrangements with those of the local and national emergency services which already exist in the event of other man-made or natural disasters. Basic training under the Emergency Arrangements in the event of a reactor accident, which may involve a release of activity to the environment, is given to all station personnel. This includes personal radiological protection and evacuation procedures; respiratory protection; first aid and rescue techniques; fire fighting; air and ground deposition environmental monitoring; and portable radio communication procedures. A register of emergency training is kept for all operators having specific duties under these arrangements, which are in addition to their normal power station duties. A full complement of all emergency teams must be available on all shifts so as to be able to implement the emergency procedures in the event of an accident. In order to maintain the required degree of preparedness the Emergency Arrangements are demonstrated periodically. Emergency Exercises are held at which a postulated accident situation is simulated and the Emergency Procedures are invoked. The Exercises are held in conjunction with the local emergency services, police, fire and ambulance services and with national authorities having duties under the Emergency Arrangements. These exercises are assessed by observers from the

41

Licensee’s Health and Safety Departments and the NII Additional smaller scale accident scenarios are held so that all shift personnel participate in an exercise at least once per year. Reports on each major exercise together with criticisms from observers enable standards to be maintained and procedures to be improved. The Station Emergency Plan is issued to members of the local Government Liaison Committee and copies are available in local libraries for public information. 5.4.2 Response to a nuclear accident 5.4.2.1 Central Control Room (CCR) The Licensee is responsible for safety on a NPS and this is delegated to the Station Manager. In the event of a reactor accident, which may involve the release of activity to the environment, the Shift Manager, after reporting the details of the accident to the Station Manager or his nominated deputy, if they are on the station, will declare over the public address system that an emergency situation exists. All station staff and other persons on the site withdraw to nominated assembly areas for a roll call. Instructions may be given to issue potassium iodate tablets. Coincidentally with the above activities, the Shift Manager will assume the role of Emergency Controller from the CCR which is the only control centre in the reactor building to remain manned throughout the emergency. The operators’ primary responsibility in the CCR is to stabilise the conditions on the damaged reactor and to take such remedial action as to minimise the effects of the accident and to contain the release of activity as quickly as possible. During the silent hours, when senior management are not on site, the CCR staff will notify appropriate senior staff of the accident and prepare the Emergency Control Room (ECR) for their use on arrival at the station. Other duties include notifying the local Fire Brigade, obtaining meteorological information for health physics control for the radiological monitoring teams, and grid control concerning the security of station electrical supplies. All events are logged together with reactor operating conditions for postaccident analysis. The environmental conditions in the CCR are monitored continuously for activity and CO in air 2 concentrations, and it may be necessary to adjust the

42

The Chernobyl accident and its implications

ventilation plant, wear protective clothing and respiratory protection as conditions require. The Station Security Staff at the gate house are advised to restrict access to the station except for emergency personnel.

emergency instructions and tape recorders, and all actions are logged for post-accident analysis. Use is made of the station public address system to reassure personnel not directly involved in the emergency procedures.

5.4.2.2 Emergency Control Room (ECR) The Station Manager or his nominated deputy, together with his technical advisors, man the ECR and take over the responsibility for the management of the accident from the Shift Manager who resumes his role as plant controller. The Emergency Controller, having satisfied himself that the damaged reactor stabilisation procedures have been satisfactorily introduced and all station personnel have been safeguarded and emergency teams are mobilised, initiates the notification of local and national government departments. These include the Licensee’s headquarters, police, medical and welfare services and government regulatory authorities. Liaison is established with the UKAEA, BNFL and NRPB, who may be required to assist the station in its environmental monitoring activities. The ECR is normally located away from the reactor buildings, but is in communication with all the station emergency control centres and emergency teams by direct telephone or radio. Specialist advice is available on reactor and plant conditions, plant damage repair procedures, chemistry and health physics control. Close liaison is established with the Police Mobile Command Vehicle which is set up on or near the site. Matters concerning public safety of the local inhabitants and access restrictions near the site which may have to be introduced as the extent of the radioactive release is ascertained, are of primary concern. The Station Administrative Officer will have coordinated the results of the station personnel roll call from the nominated assembly areas and any missing personnel should have been identified and their normal place of duty ascertained. Evacuation of nonessential personnel is then arranged via safe routes to rest centres. Industries in the vicinity of the NPS which may be affected by the radioactive plume are advised to introduce pre-planned measures to minimise the effects of radioactive contamination and dosage. Should the active plume envelop the ECR an alternative location similarly equipped can be used. The ECR is essentially a command and communication centre. Use is made of graphic displays, plant drawings,

5.4.2.3 Health Physics Control Room (HPCR) The HPCR forms part of or is located adjacent to the ECR. The Controller is an accredited health physicist whose primary responsibility is to advise the Emergency Controller on all radiological matters. He directs the monitoring teams to the most meteorologically sensitive locations on and off site, to carry out air activity, ground deposition and cloud dose measurements. Up to three off-site and one on-site teams are initially employed using specially equipped vehicles. The aim is rapid deployment, simple activity monitoring procedures and radio communication with base. Equipment and procedures are standardised for all NPS. Radiological samples are transported to the District Survey Laboratory for gamma spectrometric analysis. Advice is given to the Emergency Controller on the radiological risk to the local inhabitants and what action may be necessary to limit doses to the public. It should be noted that where the public living near a NPS may be exposed to radiation arising from a release of activity, the National Radiological Protection Board (NRPB) has issued guidance criteria to be used for limiting doses to the pubic at risk. Emergency Reference Levels (ERL) of dose are prescribed, enabling appropriate counter-measures to be taken in adequate time. These include sheltering from the radioactive cloud, the issuing of stable iodine tablets to protect the thyroid gland, or evacuation to rest centres. The lower intervention levels and actions to be taken are: Sheltering: 5 mSv (whole body dose) Issue of potassium iodate tablets: 50 mSv Evacuation: 100 mSv (whole body dose) 300 mSv (thyroid dose) The upper intervention levels are five times the lower levels of dose. It is desirable to initiate countermeasures at the lower intervention levels. At the upper intervention levels action must already have been taken. Strict controls are exercised to limit the dose commitment of any personnel in the emergency teams from exceeding the specified maximum. Advice is given to the Emergency Controller

Accident management in the USSR and the UK

concerning restrictions to be placed on food, milk and water supplies for action by local inspectors from the Department of the Environment, the Ministry of Agriculture, Fisheries and Food and the local Water Authorities. All the radiological information collected on and off the site is visually displayed on plant layout drawings and Ordnance Survey Maps respectively. Such information is progressively updated as the information is received from the monitoring teams.

5.4.2.4 Incident Control Centre (ICC) The ICC is the location where the site emergency teams assemble and are equipped before entering the damaged reactor building. It is supervised by a senior operational engineer. Information is given to the teams concerning anticipated hazards and location of the plant damage, specific instructions being given on the tasks to be undertaken. The Damage Control Teams’ primary responsibility is to survey the reactor building and report, by radio, the environmental conditions as they proceed into the building. Radiation dose rates, CO2 concentrations and the temperature conditions resulting from the hot gas and steam releases are reported. It may be possible to ascertain and confirm plant damage, locate casualties and request base control to send in assistance: for example, First Aid and Rescue Teams to recover casualties; Damage Repair Teams to isolate damaged plant or remove debris for access; and Fire Teams to extinguish or contain fires in plant areas. With respect to the latter, the local Fire Brigade will take the initiative, assisted by the station Fire teams. Conditions in the damaged reactor building may be so hazardous that progress will be slow and backup teams will be required so as to limit radiation dose commitments to the individual teams. Teams returning to base may be heavily contaminated and will require substantial clean-change facilities near the ICC. It may take many hours or days to gain access and control of the situation and to complete emergency repairs to seal the breach in the primary coolant circuit and terminate the release of activity. Radiation dose restrictions may necessitate the assistance of other NPS personnel who are trained and familiar with these procedures. Cooperation with the plant engineers in the Central Control Room is an essential part of the containment process and the ICC may be the most hazardous and stressful centre in the station during the stabilisation

43

period, involving the control and direction of a large number of personnel, particularly those returning from the reactor building contaminated and exhausted. The Emergency Controller may have to give special attention to this area of activity, especially when plant damage is extensive.

5.4.2.5 Station Medical Centre (SMC) The SMC is manned by the appointed Doctor, Station Nurse and First Aiders. It provides decontamination, first aid and minor treatment to casualties before they are sent by ambulance to the nominated hospitals which have the facilities to handle and treat contaminated and irradiated casualties.

5.4.3 Government assistance The reactor accident at Three Mile Island in the USA led to a reappraisal of the Emergency Arrangements in the UK. One of the more significant results of this review was the introduction of an Operational Support Centre (OSC) to be set up near the affected NPS for the purpose of supporting the station management and informing the Government, media and general public of the progress of the accident. A Government Technical Advisor (GTA) would be appointed by the Secretary of State for Energy to assist in these arrangements. The role of the OSC is to support the Station Emergency Controller by taking over responsibility for all external matters from the Station. These include the evacuation and welfare of the public at risk, arrangements for the restrictions on milk, food and water supplies, public reassurance and media briefing. The OSC would be established as soon as reasonably practicable, after the notification of the nuclear emergency by the Licensee. It would provide facilities for representatives of local and national government departments, police authorities, NII, NRPB, etc. In the event of a significant release of activity to the environment, which might cause widespread public concern, the GTA would take over the management of the accident from the Licensee. Nuclear Emergency Briefing Centres (NEBC) are also set up in appropriate government departments and the Licensee headquarters. These are in direct communication with the OSC for the purpose of advising government ministers and assisting with media briefing. (See Nuclear Emergency Organisation diagram, Fig. 5.7.) The UK Emergency

44

The Chernobyl accident and its implications

Fig. 5.7. Nuclear Emergency Organisation chart. Thick lines between centres denote executive functions, narrow lines denote advisory functions.

Accident management in the USSR and the UK

arrangements follow the procedures given in the Safety Guides issued by the IAEA for dealing with emergencies at NPS. They are designed to enable the Licensee to manage the consequences of a Reference Accident on a gas-cooled reactor in which a major release of activity to the environment has occurred, even though this is extremely unlikely. 5.5 REFERENCE ACCIDENT This is a design-based accident that could lead to the largest off-site release of activity and was originally referred to as the maximum credible accident. The reference accident for the Russian RBMK vertical pressure tube, boiling water, graphite moderated reactor is a loss of coolant accident (LOCA) resulting in the rupture of a header pipe supplying coolant to a number of reactor core channels. The increase in pressure in the containment building housing the headers initiates a reactor trip and the Emergency Core Cooling System (ECCS) to provide coolant to the damaged reactor. The release of high pressure water coolant, at 67 bar and 270°C, is directed into the steam suppression pools beneath the reactor and is vented to the atmosphere via the high central chimney. The partial containment structures within the reactor building are designed to withstand pressures up to 2 bar and 4 bar according to the pipe header location. The fuel temperature transients may be expected to lead to a small number of fuel pin cladding failures, leading to a release of volatile fission products to the environment, whose dispersal from the high stack would not present a serious environmental problem. For the early Magnox reactors the reference accident was considered to be a rupture of one of the large gas ducts connecting the reactor steel pressure vessel to the external boiler vessels, leading to a LOCA. The resulting depressurisation of the reactor in minutes, with the probability of a fuel meltdown in one of the highest rated channels, could lead to the release of activity to the environment, resulting in an Emergency Reference Level of dose, possibly extending 3 km from the station. Such an accident potential required these reactors to be sited remotely, where population density was low. In the early life of these reactors in-service inspection revealed higher than expected steel oxidation rates in the high temperature regions of the primary coolant circuit. Oxidation damage to the core restraint structure could have led to the partial disruption of

45

the graphite brick core structure in the event of a top duct rupture. The power of these reactors was derated by the reduction of the gas outlet temperature to 360°C so as to inhibit further steel oxidation damage. Partial disruption of the core could inhibit the insertion of some control rods under this accident situation, causing a local criticality condition as the core cools down, due to the negative temperature coefficient of reactivity of the fuel. Modifications were made to supplement a number of control rods with a facility to inject boron beads from storage hoppers above the core into in-core thimbles. This secondary shutdown system is automatically triggered by differential pressure sensors; the beads can be recovered from the thimbles and returned to the hoppers in the event of a spurious operation. A tertiary shutdown system is also provided to inject boron dust into the depressurised reactor. The effectiveness of this system can be demonstrated only by rig testing, since its use on a damaged reactor would lead to the permanent shutdown of the reactor and its premature decommissioning. For the later Magnox reactors built into prestressed concrete pressure vessels (PCPV) with integral boilers, the reference accident is considered to be the failure of a small sized pressure vessel penetration, associated with the refuelling standpipes, boilers, and auxiliary gas circuits. This small LOCA would result in the depressurisation of the reactor in about 50 min and could cause cladding failure of a small number of highly irradiated fuel elements. The resulting release of fission product activity would be largely confined to the reactor building, but a potential environmental hazard up to one ERL of dose would be limited to within 1 km of the station. Operators are expected to maintain CO supplies 2 to the reactor so as to exclude air ingress to the core and to seal the breach in the primary coolant circuit by closing isolating valves or plugging the pressure vessel penetration when environmental conditions permit access by the emergency teams. The Advanced Gas-cooled Reactors (AGR) are built into PCPV with internal boilers and gas circulators. The uranium-enriched oxide fuel is clad in stainless steel so that the risk of a channel fire or fuel meltdown under fault conditions inherent in the Magnox design has been greatly reduced. The reference accident for the AGR is a small LOCA due to the failure of a PCPV penetration with the same time to depressurise as above. Failure of the fuel pin cladding may occur, releasing essentially gaseous

The Chernobyl accident and its implications

46

fission products from the oxide fuel. The ERL of dose is expected to be confined to the reactor building and not extend beyond the site boundary. This risk potential allowed relaxation of the siting criteria applicable to the Magnox stations and permitted urban siting of the AGR. In spite of this low risk potential, the NII still required the Licensees to make Emergency Arrangements in the interests of the safety of the operators and the public living near the power station.

through a more open information policy concerning the performance of the nuclear power industry. Reportable incidents from NPS and other nuclear installations which are notified to the NII and reported to Parliament should be given much more urgent and wider media circulation, so that the public are given the facts at first hand rather than the speculative claims of the pressure groups. 5.6.3 International notification

5.6 CONCLUSIONS AND RECOMMENDATIONS 5.6.1 Accident management The Soviet reports on the Chernobyl accident gave little information about the station’s Emergency Arrangements or whether these had been implemented. Representatives from the USSR were on the Senior Advisory Group of specialists in the IAEA who produced the safety guides for these arrangements. The consequences of the accident were so severe that it was beyond the capability of the station management and personnel resources. Little was done to control the situation until the arrival of massive national government assistance. In the UK the most severe accident predicted should be within the Station Management’s capability to bring under control with the assistance of local and national emergency services. Public concern and political pressures suggest that a senior government minister should be appointed to manage the emergency arrangements in the event of a major release of activity to the environment which might involve the short term evacuation of the public in the vicinity of the NPS and lead to restrictions on food, milk and water supplies and the extensive cleanup operations following the accident. This may involve the cooperation and control of a number of government departments together with national and local agencies. 5.6.2 Public confidence and consent Further progress with nuclear power in the UK is not possible without the confidence and consent of the majority of the population. This can be improved and maintained in the present political atmosphere only

Notification of nuclear accidents, whether from NPS or other nuclear installations, especially those resulting in the release of radioactivity to the environment, should be immediately notified not only to the national regulatory authorities, but also to the international advisory authorities such as the IAEA, NEA, OECD, etc. This will enable those countries likely to be affected by the activity released to take appropriate countermeasures so as to reduce the consequences of such an event. 5.6.4 Emergency exercises Emergency exercises should be more realistically planned, especially with respect to the simulation of plant damage and real time scales extending overnight. Reactor simulators, where these are available on NPS, should be used to portray the postulated accident situation to which the operators would be required to respond. All external organisations should be mobilised not only to confirm communication links with the Operational Support Centre, but to transfer the information necessary to reproduce the site situation displays and other information required at the Nuclear Emergency Briefing Centres which may be required for ministerial or media briefing in the event of a real accident. REFERENCES 1. International Atomic Energy Authority, Preparedness of operating organisations for emergencies at nuclear power plants, Safety Guide No. 50-SG-06; Preparedness of public authorities for emergencies at nuclear power plants, Safety Guide No. 50-SG-G6. 2. International Nuclear Safety Advisory Group, Summary Report No. 75-INSAG-1, Post-accident Review Meeting on Chernobyl, Vienna, 25–29 August 1986. 3. WILSON, R. Chernobyl visit, Harvard University, Feb. 1987.

Section 6

United Kingdom and USSR Reactor Types Jeffery Lewins Lecturer in nuclear engineering, University of Cambridge, and Fellow of Magdalene College, Cambridge

6.1 INTRODUCTION

will be recollected, is different in detail from PWRs built elsewhere. We do not include research and test reactors nor the Royal Navy PWRs. Many texts give details of the UK reactor types. The Chernobyl RBMK is described in detail in Section 2 of this report. For our purposes, we make a comparison of salient features. Figures 6.1–6.6, adapted with permission from the UKAEA leaflet1 entitled Nuclear Power Reactors (1987), show the main features. Dimensions, etc., are only typical, and it should be noted that reference in these figures to the FR (sodium-cooled fast reactor) applies to the notional commercial design, not the prototype actually operating. Figure 6.7 refers, however, to the Winfrith SGHWR as built, not the unrealised commercial design.

The purpose of this section is to compare the features of the RBMK reactor operated at Chernobyl with reactor types pertinent to the UK. It will be recollected that the RBMK covers a large number of reactors and the comparisons made are indeed with Chernobyl No. 4. The UK reactors covered are in three classes: the commercial reactors now built and operated or in commission (Magnox and Advanced Gas-cooled Reactor (AGR)); the prototype Steam Generating Heavy Water Reactor (SGHWR) and Prototype Fast Reactor (PFR) that have comparable performance to commercial reactors; and the proposed Pressurised Water Reactor (PWR) or Sizewell ‘B’ design which, it

Table 6.1 Energy characteristics of UK and USSR reactors

Approximate figures. Original-current burnup targets with future targets in parentheses. c Core+blanket/core only. d Four further reactors at commissioning stage. e Proposals for further reactors have now been made (June 1988). a b

47

The Chernobyl accident and its implications

48

Fig. 6.1.

Table 6.1, adapted from the American Nuclear Society’s Nuclear News (1986), compares the installed capacity, typical power density, etc., in both countries. Data for PWR and FR in the USSR are included. Table 6.2 makes a direct comparison of some design features. Here the SGHWR and PFR figures are for the actual prototypes with indication of proposed operating improvements. We continue with a description of major design features and then a more detailed comparison of cooling circuits and reactor stability. We finish with accounts of resonance, Doppler and xenon poisoning effects and some aspects of reactor physics at Chernobyl as more technical appendices.

This section provides a comparison of power reactors built in the UK with the Soviet RBMK. But it is worth recollecting that, elsewhere in the world, other types of power reactors are in use. The most widely built reactor is the Pressurised Water Reactor (PWR) but the second is the Boiling Water Reactor (BWR), a light water reactor in which, like the RBMK, steam is generated in the core and passed to the turbines in a direct cycle. Light (i.e. ordinary) water is used as coolant and moderator. The Canadian industry has developed the CANDU series of reactors, with limited export to India, etc., which have many pressure tubes to retain the coolant, as in the British SGHWR and Soviet RBMK, but are heavy-water-cooled and moderated.

UK and USSR reactor types

49

Fig. 6.2.

6.2 GENERAL DESCRIPTION In general it may be said that the combination of graphite moderation with boiling light water coolant as used in the RBMK design has no equivalent outside the USSR. For comparison with UK reactors, the RBMK uses some 1690 pressure tubes. Only the SGHWR in the UK uses pressure tubes. Graphite moderation as such is used in Magnox and AGRs in the UK; the details of the cooling arrangements for the graphite are different. In the RBMK design, the graphite is run at higher temperatures, about 700°C, to improve the reactor’s physical performance. In UK reactors the graphite runs at around 300°C, with noticeably less potential for burning if exposed to air.

All the reactors under comparison, save Magnox and the PFR, use slightly enriched uranium in the form of a high melting point oxide. Magnox fuel is natural metallic uranium in a Magnox cladding (Magnesiumno-oxidation). The Prototype Fast Reactor at Dounreay uses a mixture of natural uranium oxide and plutonium oxide. AGRs and PFR use stainless steel clad; RBMK, SGHWR and Sizewell ‘B’ use zirconium clad (Zircaloy). Design burnup of fuel is given in Table 6.1, in which 10 MWd/kg corresponds to about 1% burnup of heavy metal atoms. It is to be noted that the reactor’s physical parameters can change appreciably with burnup from all-fresh fuel, through mixed history fuel, to all well-burnt fuel (Table 6.3). Containment in the RBMK design is provided by the

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Table 6.2 Design features of UK and USSR reactors

sequence of oxide fuel integrity, zirconium cladding, pressure tube and the provision of partial secondary containment buildings combined with pressure suppression pools to cool escaping steam. It is not envisaged in the design that the pressure tubes as such would be breached. Secondary cooling circuits are provided to remove decay heat from the fuel in the event of primary coolant failures not involving the pressure tubes themselves. Magnox stations have Magnox cladding of the metal fuel together with a metal or prestressed concrete pressure vessel. AGRs have oxide fuel with stainless steel cladding and a prestressed concrete pressure vessel. AGR buildings provide limited secondary containment, it being assumed that the large excess of replaceable prestressing cabling of the primary pressure vessel obviates catastrophic failure. The SGHWR at Winfrith has oxide fuel, zirconium clad and pressure

tubes as its initial containment barriers. This prototype (although not the envisaged commercial reactor) is further surrounded by a vented concrete containment structure and a low pressure containment building. The Sizewell ‘B’ PWR design has oxide fuel, zirconium clad, a primary steel vessel and a secondary containment building as its containment barriers. The Fast Reactor is operated without pressurisation of the core and has its oxide fuel, cladding and primary vessel as containment; in addition the reactor vessel is surrounded by a vessel providing secondary containment. The RBMK reactor is peculiar compared to UK thermal reactors in being built with a wider than optimum spacing of the fuel elements, the ‘lattice pitch’. This leads to a degree of over-moderation and a consequent effect upon loss of moderator or coolant, described in more detail in the following sub-section.

Table 6.3 Reactivity feedback effects

a

® Change with the proceeding of fuel burnup.

UK and USSR reactor types

51

Fig. 6.3.

6.3 CONTROL RODS Control is exercised in the RBMK through vertical solid control rods. Most of these penetrate from the top, driven in by motors. It is not an automatic feature that these relatively slow moving motors should be disengaged from the absorber head itself on an emergency shutdown. Safety rods in the RBMK have a graphite ‘follower’ hanging beneath the aluminiumclad boron carbide absorbers. This follower displaces water from the sleeve when the rod is withdrawn, and another peculiar feature of the RBMK, that the graphite follower is not the full length of the active core so that not all water is displaced on rod

withdrawal, can be seen as a feature of the accident. All rods are supported by belt cables and moved by electric motors located at the standpipes at the top of the elements. The RBMK has some additional bottom entry shaping control rods which are particularly necessary in a boiling water reactor to compensate for the loss of coolant in the upper channels on boiling. Magnox and AGRs, however, which also have vertically motoring control rods, generally have a ‘disengage-on-trip’ feature which allows free inwards falling of the control head. The PFR has a solid control rod. SGHWR has so-called liquid control rods but its operational control is primarily by liquid moderator level. The SGHWR moderator can also be dumped in an emergency shutdown.

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Fig. 6.4.

The AGR has a nitrogen secondary shutdown circuit. The RBMK helium-nitrogen system, however, is used to adjust graphite temperature while keeping an inert atmosphere and does not have a second defensive shutdown role. Some UK graphite reactors have, in addition, a capacity to drop neutron absorbing beads into the core or inject boron dust into the primary gas circuit. 6.4 COOLANTS Only the SGHWR and RBMK under comparison have a direct cycle of light water coolant to the turbine and therefore the admission of somewhat radioactive steam. Both of these types have additional circulation

of the liquid coolant, over and above that tapped off as steam, to improve the dynamic behaviour, as discussed in the next subsection. Magnox and AGRs employ carbon dioxide as the coolant, in a single phase, i.e. as a dense gas. The PWR employs liquid water, again in one phase, in the operation of the primary coolant circuit; the PFR employs a single phase liquid sodium primary coolant. The RBMK and the SGHWR employ light water coolant which is allowed to boil in normal operation within the reactor core. RBMK, SGHWR and PWRs have emergency cooling systems against the loss of primary water coolant. During normal operation, coolant characteristics

UK and USSR reactor types

53

Fig. 6.5.

would lead to the following. Dense liquids generally have better heat transfer properties than gases, even pressurised and dense gases. For the same fuel temperature limits, therefore, liquid coolants admit higher power densities and corresponding economies of cost for a given output, in the fuel as well as the coolant circuit. Boiling in normal operation still offers good heat transfer, although the resulting steam density lowers the coolant thermal capacity. The lower energy density in gases implies slower thermal transients in operation or accident conditions. The high pressure of the RBMK coolant in operation lessens the liquidgas density change. The use of a liquid coolant, however, raises the question of loss of heat transfer capacity in the event

of a phase change during an accident if there is dryout and the departure from nucleate boiling (DNB or the ‘boiling crisis’). A loss of pressure may lead to a pressure below saturation and the sudden flashing of liquid to vapour. Once the phase change has proceeded to the point of dry-out on areas of the heat transfer surface, the gas (steam) acts essentially as an insulating layer, raising the inner fuel temperature substantially over its previous value. These circumstances may also lead to a rapid rise of pressure (thermal explosion) associated with a return of contact between liquid and the high temperature surface. Thus gas and liquid coolants may be contrasted by saying that gas coolants already have a poor heat

The Chernobyl accident and its implications

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Fig. 6.6.

transfer capability; more gas must be circulated and the volumes and pumping power are correspondingly high. Liquids have good heat transfer capability and thermal capacity, are more compact and require less pumping power; they risk the loss of these qualities in the course of an accident. On the other hand, the presence of water during the course of an accident has been demonstrated to wash out and retain sublimation products of fission such as the iodine isotopes. 6.5 REACTOR STABILITY Reac.tors can be classified as self-stabilising if, without moving controls rods, a power increase produces a

reactivity, and hence a consequent power, decrease. If an increase of power tends to increase the power further, the reactor might be called auto-catalytic or unstable. Such a reactor could be made stable by additional, and preferably automatic, control, but as a general philosophy self-stabilising systems are desirable. Such characteristics are generally manifested through so-called coefficients of reactivity. The significance of a positive feed-back coefficient, associated with unstable systems, is not unconnected with the time scale it operates upon, however. Slow acting systems are more readily controlled. The reactivity is a parameter of the reactor which governs the way the fission rate and power change. If the reactivity is positive, power increases, if negative it

UK and USSR reactor types

55

Fig. 6.7. Winfrith SGHWR.

decreases. If the reactivity is zero the power remains constant. It is essential therefore to be able to vary the reactivity to raise the reactor to power and to shut it down again. This is undertaken by manipulation of the control rods, wasteful neutron absorbers. As the reactor operates, longer-term effects occur which need further movement of the control rods to offset their effect upon reactivity if the power level is to be maintained. These longer-term effects—the rise in temperature, the burnup of fuel, production of neutron-absorbing fission products, etc.—tend to lower the reactivity. Thus considerable positive reactivity must be built in to the fresh, clean and cold reactor. This in turn is contained by absorber rods, to be withdrawn as the effect develops (and perhaps burnable ‘poisons’). One of these effects, the production of xenon, certainly played a part in the Chernobyl accident and is detailed in Appendix 6.1 to this section. The rate of rise of power with positive reactivity is not simply linear with the reactivity. For small

reactivities, the mechanism of delayed neutrons, a slowly released fraction of the neutrons from fission, slows the power change down. If, however, the reactivity exceeds this delayed neutron fraction (about 0·75%) then the natural time scale of the process is much shorter, of the order of milliseconds. It is thus highly significant to restrict the net reactivity to be less than this fraction, referred to as prompt critical; the events of the Chernobyl accident suggest the reactivity was added in excess of prompt criticality. Light water thermal reactors have a coolant that can be an appreciable moderator. Indeed, in the PWR and the conventional BWR, the two roles are intimately mixed. To promote inherent stability, the LWR is designed to be under-moderated, with a lattice pitch closer than optimum. In the event of a loss of moderator/coolant, by leakage or by turning from liquid to steam, the reactivity is lowered; the reactor is self-stabilising if this voiding was caused by a rise in power. The water coolant in the RBMK has only a

The Chernobyl accident and its implications

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minor role as moderator compared to the large amount of graphite present. The primary neutronic effect of the water is as an absorbing annulus around the fuel, reducing the flow of thermal neutrons from the graphite moderator into the fuel. Depending on the burnup of the fuel and the presence of control rods, water removal then allows free access of neutrons to the fuel and raises the reactivity; the void coefficient in the RBMK cooling water is positive under these conditions, unlike the conventional LWR. Correspondingly, the conventional BWR tends to shut itself down when a demand for more power is experienced. The opening of the turbine throttle allows a drop in pressure in the coolant which leads to more steam voids. This in turn lowers the reactivity (the reactivity coefficient is negative) and the power, just when more power is being demanded. The simple boiling water reactor is not load-following. This defect is met in commercial BWR designs by additional circulation of the coolant around the reactor core, over and above the coolant removed as steam through the turbines. Controllers link the flow rate to the power demand as felt on the turbine, so that the increased coolant input sweeps out the steam bubbles and compensates for the void effect. The RBMK has a similar additional coolant circulation which is necessary, perhaps, for its satisfactory operation with a negative water void coefficient but would exacerbate the disadvantage of a positive coefficient. This again was a feature of the Chernobyl accident. Further reactivity coefficients are important, particularly the power coefficient, the change of reactivity with power. The size and indeed sign of this coefficient vary with the power level at which the reactor operates. These power coefficients are thus a function of temperature and express the effect of a temperature or power change. Power coefficients have components from thermal expansion, which will vary leakage and the balance of neutrons; by production of steam voids, or at any rate, less dense regions (as in the void coefficients discussed); by a change in moderator temperature which changes the effective temperature of the neutrons; and perhaps most significantly at Chernobyl, by the Doppler effect. The last two mechanisms operate through so-called resonance absorptions, whose theory is outlined in Appendix 6.1. Suffice it here that the Doppler effect in the fuel requires a substantial fuel temperature to operate and then generally leads to a negative power coefficient. If the reactor is being run at low power, however, the Doppler effect may not be felt. Then other phenomena may provide a positive

power coefficient. Although ultimately an excursion raising the power can be expected to shut itself down via the Doppler effect, the peak power and total energy released in the excursion may well, as at Chernobyl, be too much for the system to absorb. It should be noted that as fuel is exposed in the reactor, U-235 densities are burnt down and both plutonium and fission products are produced. This isotopic change can lead directly to some variation in the behaviour of both void and power coefficients. There can also be an important indirect effect: when the reactivity is high (e.g. with fresh fuel) and fixed absorbing rods are inserted to compensate, a large thermal reactor tends to behave as a number of small, admittedly linked, reactors with a different balance of capture, leakage and production of neutrons. This indirectly affects the various reactivity coefficients. In the graphite moderated reactors, Magnox, AGR and RBMK, the graphite reactivity coefficient with temperature rises through the fuel burnup period and tends to become positive. Whether this is significant in relation to other and negative coefficients depends also on the time constants of the process—generally slow in graphite—and on the extent to which the reactor power and fuel temperature are closely coupled to the graphite temperature. In the Magnox and AGR, this coupling is relatively weak since there is direct carbon dioxide cooling of the graphite. In the RBMK, the graphite is not separately cooled but is in thermal contact with the main coolant stream. This close-coupling will narrow the region in which a positive moderator coefficient can operate satisfactorily,2 balanced by a negative fuel (perhaps Doppler) coefficient. Table 6.3 illustrates the magnitudes3 of various reactivity feedback coefficients and their variation with fuel burnup. REFERENCES 1. Nuclear Power Reactors, UKAEA, 1987. 2. LEWINS, J., Introduction to Nuclear Reactor Kinetics and Control, Pergamon, Oxford, 1968. 3. YOUNG, J.D., personal communication, 1986.

APPENDIX 6.1: RESONANCES, DOPPLER AND XENON EFFECTS A6.1.1 Resonance and Doppler effects The tendency of an atom to have its nucleus react with a neutron is called its reaction cross-section. These cross-

UK and USSR reactor types

sections vary with the speed, v, and energy of the neutron. Simple substances have cross-sections inversely proportional to neutron speed, i.e. proportional to 1/ v. But a faster moving neutron sees more nuclei as it travels, proportional to its speed v. The upshot is that for simple substances, the reaction rate overall is independent of the neutron speed and hence of the temperature of the moderating material releasing thermal neutrons from the slowing down process. More complicated materials, especially uranium in this context, display ‘non 1/v’ behaviour. This may be described as showing a marked preference or resonance for neutrons close to certain resonance energies. Such resonances may be fairly sharp so that only neutrons close to the resonance speed or energy react. Some resonances are associated with fission and produce more neutrons; others simply lead to capture, removing neutrons. On the whole, the resonance effects tend to lead to a net loss of neutrons and thus a negative reactivity effect. If the moderator temperature is now raised, changes in overall reaction rate can be expected because the neutron speeds are changed and resonances do not preserve the overall reaction rate. The high RBMK graphite temperature is imposed to secure favourable resonance effects. Furthermore, if the reacting atoms themselves are made highly mobile, again by raising the temperature, they move in all directions. Then the important feature is the relative speed between neutron and atomic nucleus and more neutrons have an opportunity to be experienced as having resonance energies. The result is called the Doppler effect and is comparable to the spread of frequencies heard as a moving source of sound approaches and leaves the listener. Consequently, once the fuel temperature is sufficiently high, the Doppler effect can broaden the resonance absorptions and can be designed to lead to a negative power coefficient. The effect does not operate if the fuel temperature is too low; the operating temperatures in oxide fuels at full power are such that the Doppler effect can be a significant feature. A6.1.2 Xenon effects Some 6% of all products from fission in uranium are in the form of the iodine-135 isotope. This is radioactive and decays with a characteristic half-life of 6.7 h into the xenon-135 isotope. Xenon-135 is remarkable for having the largest known absorption cross-section, a resonance effect close to thermal

57

neutron energies. As a consequence, this fission product competes for neutrons and its production causes a substantial loss of reactivity. The xenon-135 isotope is itself radioactive and decays at a slower rate with a 9.2 h half-life. While the reactor is operating at power, the xenon isotope is also removed by neutron capture; the resulting xenon136 isotope has an insignificant cross-section. The first effect of this phenomenon, therefore, is to require some 2–3% additional reactivity to maintain criticality at power as the xenon builds up. This amount is appreciably larger than the reactivity corresponding to prompt criticality. The non-linear nature of the neutron-xenon reaction and the time delays in this second-order system are such as to promote a tendency for fluctuations of reactor power. Generally this is easily dealt with by an overall control system adjusting absorbing rods, since the time scale is of tens of hours. If the reactor is shut down, however, a second aspect occurs. The xenon is now not being removed by neutron interaction but is still being produced, faster than it decays, by the decay of the previously produced iodine. Thus the xenon concentration rises sharply to several times its previous value (depending on the operating power level) before it subsequently decays with the drop in iodine production after such a shutdown. If the reactor is to be restarted in this interval, of 20–40 h, yet further reactivity will be needed. If the reactor is partially shut down, a partial xenon transient can occur, calling for addition and subsequent removal of reactivity to compensate if the reactor is to continue to be operated. Finally, a third aspect of the xenon effect can be noted. In large graphite moderated reactors, there is a tendency for parts of the reactor to operate independently of each other. The linkage of the iodine and xenon production and removal then promotes a further type of instability showing itself as spatial oscillations of power. If the power locally rises, then more xenon is burnt out; there is a tendency for the local power to rise further until, after some hours, the higher power produces more xenon via the iodine decay chain and swings the power down. Meanwhile, in another region of the reactor, a lowering of power will burn up less xenon; the increase in local capture cross-section will tend to depress the power further until, in time, the decreased production of iodine leads to a reduction in neutron capture. Thus a spatial oscillation of power, temperature and local reactivity effect can occur.

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A6.2.1 Control rod effects

Fig. A6.1.1. Xenon reactivity transient following power reduction to 7%.

Again the time scale of these spatial oscillations is slow so that there is no great difficulty in controlling the fluctuations, whose temperature peaks might be damaging, so long as instrumentation to measure the fluctuations and control rods to compensate for them are sufficiently widely distributed in the reactor. Such zone control is therefore essential in graphite reactors (and to a lesser extent in the more compact water reactors). The sequence of pre-accident operations at Chernobyl suggests that considerable xenon poisoning had occurred which would manifest itself most strongly in the centre of the reactor where the power had been high. Correspondingly, the upper and lower reactor regions would tend to operate independently. Control rods had been withdrawn to meet the overall loss of reactivity from xenon and were not positioned to control the lower half of the reactor, nor to suppress the tendency to a positive void coefficient. Figure A6.1.1 illustrates a typical xenon bulk transient after partial shutdown from power. APPENDIX 6.2: CHERNOBYL: REACTOR PHYSICS There are three essential points to understand in the development of the accident: (1) Nearly all the control rods were out and in a position to be driven in, reducing reactivity. The obvious worry would be to have no rods available to drive in, so why was this not safe? (2) Why should the coolant have a positive void coefficient under certain circumstances? (3) Under what circumstances does the fuel temperature coefficient with its negative value not stabilise the system? The answers are as follows.

The previous history of fuel burnup in the reactor had led to relatively low reactivity in the centre and high reactivity at the top and bottom of the core. The immediate history of operating at partial power for some 24 h had exacerbated this effect by releasing xenon-135 in the mid-plane. The reactivity was generally low, requiring withdrawal of control rods. The resulting reactor configuration was that of two loosely coupled reactors, at the top and bottom, a ‘double humped’ flux distribution. In addition, the abnormally high rate of circulation of coolant had produced abnormally low voidage; with control rods already removed the voidage coefficient was positive so that a lack of voids called for even further removal of control rods and a reinforcement of the operation in the forbidden region of positive void coefficient. When control rods were ordered into the reactor, therefore, the slow motion (0·4 m/s) meant that the absorbing heads would not affect the lower part of the core for some tens of seconds, long enough for the lower core to be independently super-critical. The effect was further exacerbated by the peculiarity of the rod design, displacing about 1 m of water in the control rod shaft tube, top and bottom of the graphite ‘follower’ hanging below the control rod. With the control rod fully out, the two water gaps line up with the top and bottom of the core which have been described as being the most reactive sections. If there is a positive void coefficient, the initial penetration of the absorbing section at the top simultaneously removes (voids) water at the bottom and adds reactivity in this area. It is possible that this peculiarity of rod insertion was the final mechanism for the prompt critical accident. The calculation of the size of the effect depends upon assumptions about the previous operating history of the reactor, but estimates made at the Berkeley Nuclear Laboratory3 suggest that this final water displacement may have added between ½% and 1½% reactivity, enough indeed to bring about the prompt critical excursion. The Soviets now accept that ‘positive scram’ could have played a part in the accident. Cavitation in the pumps may have restricted cooling water flow, causing saturated water to flash to steam. This is a possible alternative initiating mechanism. A6.2.2 Coolant voidage If coolant is removed (voided) it will reduce the neutron

UK and USSR reactor types

absorption properties and hence tend to raise the reactivity. It also allows easier diffusion of neutrons and increased leakage, which would lower the reactivity. This leakage can be from the whole reactor or into any absorbing or control rods in the reactor. With the well-burnt fuel, there were no semipermanent absorbing rods in position and the control rods were withdrawn. It is in these circumstances that the net effect of water voidage gives a positive void coefficient. A6.2.3 Doppler effect The Doppler effect in the fuel would be expected to stabilise the void coefficient with a negative effect. But the Doppler coefficient only operates if the fuel temperature is high. Operated below 30% power, the normally high temperature rise in the ceramic fuel is much decreased and the Doppler effect inoperative until, during the course of the accident, the fuel temperature rose markedly. Indeed it is possible that this was the mechanism that terminated the nuclear excursion before break-up. It was for these three reasons that the accident occurred: a bringing together of the worst possible operating regimes. Indeed the dangers of these regimes were known and operators had been forbidden to operate within them. The effect of the positive void coefficient when leakage is small is significant to the USSR’s planned action to prevent repetition of an accident in RBMKs. During the initial loading with fresh fuel, the reactivity would be high and is held down with fixed absorbing rods. These provide for the higher leakage and the negative void coefficient. Only when the fuel is well burnt and these rods withdrawn does the coefficient become positive. Then the solution is to keep absorbing rods permanently in the reactor, at the expense of raising the initial uranium enrichment to compensate for the reactivity loss. It can then be expected that the void coefficient remains negative. APPENDIX 6.3: POSITIVE VOID COEFFICIENTS It has been said that as fuel burnup progresses, moderator and water void coefficients tend to become positive. Furthermore, a step that is proposed to be taken in the RBMKs since Chernobyl is to force a

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negative coefficient by the addition of fixed absorbing rods with the necessary compensation of higher enrichment. It would be useful to develop a simple understanding of why the presence of absorbing rods (but not, it seems, uniform burnable poisons) should restore a negative coefficient. It will be noted that the first loading of ‘clean’ fuel in these reactors is offset by similar fixed absorbing rods and leads to the desired negative coefficient. The key to understanding this question is to regard such rods as tending to separate segments of the reactor into a number of smaller, isolated reactors. Such small reactors have an increased neutron leakage into the fixed rods. This leakage can be characterised through the geometric buckling, B2, which is large for small reactors. In the following we take a much simplified model, sufficient to indicate the physical trends, where the scattering and moderating material is represented only through the diffusion process in a single energy group representation. In such a model, the reactivity, r, may be represented as

where v=yield of neutrons from fission; åf=fuel fission cross-section; D=1/3åsdiffusion coefficient; ås=moderator (and coolant) scattering cross-section; åa=neutron absorption cross-section for both fuel and moderator/coolant. Let b be the ratio of the moderator scattering crosssection to its absorption cross-section. Suppose now we change the moderator/coolant loading by means of a void, with a negative change of absorption crossThe consequent change of reactivity section, is given by

It is seen that the reactivity change on voidage is negative for sufficiently large B2 but can be positive for smaller B2, i.e. larger reactors. Thus the fixed absorbing rods, which can be thought of as the equivalent of several small reactors, can be expected to promote a negative void coefficient.

Section 7

Reactor Operation and Operator Training in the United Kingdom John Bindon Independent nuclear consultant, Bangor, Gwynedd

describes fundamental and general safety principles. The objectives are ‘to avoid accidents and to limit the radiological consequences of the operation of the nuclear power plant during all operational states and accident conditions’. It postulates as a safety principle, among others, that ‘the personnel must be adequately qualified and trained to perform the required tasks’. These safety principles apply equally to Gas-cooled Reactors. There are various ways of reaching the necessary qualification standard, depending on reactor type and design. Differing training objectives may be necessary. In some countries differences concerning the acquisition and preservation of the qualifying standard are to be expected, but this Section examines only the UK position. Since the middle 1950s, both national and international procedures have imposed strict control upon the nuclear industry in the United Kingdom, to ensure that the dangers from ionising radiation to both the general public and workers in nuclear power plants are at the lowest reasonably achievable levels (ALARA—‘As Low As Reasonably Achievable’). This control has been embodied in several Acts of Parliament. There is no single Act in the UK which covers all aspects of control. All nuclear sites must comply not only with the legislation affecting nuclear matters, but also with normal industrial legislation laid down by the Health and Safety Executive, as defined in the Health and Safety at Work Act (1974). The Nuclear Installations Acts of 1965 and 1969 control the safety and the building and operation of nuclear reactors. They impose on the licensee an

7.1 INTRODUCTION There were many factors which led to the Chernobyl disaster, ranging from the initial decisions on the reactor type, design details and system of control and containment, to the Soviet policy on making decisions and the execution of such plans. All of these contributed to the accident, but it would not have occurred if the plant operators had proceeded to carry through correctly the special instructions necessary for the operation of the test and with the full knowledge of the reactor characteristics. Operating Instructions were overruled, contravening reactor safety, and other station operating procedures were also not complied with. It is therefore important to review the system of reactor operation and operator training in the UK in the light of the Chernobyl accident, as in the last resort the operators’ actions at Chernobyl were clearly responsible for what occurred there. This Section reviews the present position in the UK regarding nuclear power station operation and maintenance, and the policy for appointing and training staff. 7.2 PRINCIPLES One of the prerequisites for the safe operation of nuclear power plants is the combination of qualifications and experience of the operational staff. The publication of the Commission of the European Communities (CEC) entitled Safety principles for light water reactor nuclear power plants 1 (CEC 81a) 61

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absolute liability for injury or damage caused by ionising radiations. Thus, the CEGB and the SSEB have to obtain a licence before a nuclear station is installed or operated. The station is examined at regular periods and an approval for start of the reactor is issued by the Nuclear Installations Inspectorate (NII). 7.3 THE SITE LICENCE AND OTHER REGULATIONS All nuclear power stations in the UK are built and operated to the conditions of a nuclear site licence in which the definition of the site and description of the plant are unique to its location. The responsibility for the licensing and inspection of all nuclear installations lies with the Health and Safety Executive, these functions being performed by the Nuclear Installations Inspectorate (NII). The Site Licence is a simple document containing a brief identification of the site, the type and size of the reactors, and a plan from an Ordnance Survey map delineating the site boundary. The Licence refers to schedules which require operating rule and maintenance schedules to be written and formally submitted to the NII for approval. These are legally binding upon all station personnel and such documents are unique to a particular power station site. All appointed staff are therefore required to have an understanding of the Site Licence. The engineers appointed to Operating and Maintenance positions need to have an in-depth understanding and appreciation of Station Operating Instructions (SOIs), Plant Item Operating Instructions (PIOIs), Plant Maintenance Instructions (PIMIs) and other technical and safety related matters. These are described below. Thus, there are a number of main features attached to a Nuclear Power Station’s Site Licence affecting the operational aspects of the station over its lifetime. Apart from the licensing procedures from preconstruction to full operation, the site licence contains major sections governing Operating Rules, Maintenance Procedures and Emergency Plans. The station Operating Rules are the instructions from the Board (CEGB and SSEB) to the Station Manager. They can be altered only after the endorsement of the Nuclear Safety Committee and the formal approval of the NII The Safety Committee, which is again a requirement of the Site Licence,

The Chernobyl accident and its implications

consists of representatives from other organisations, such as the UKAEA and independent assessors. The Operating Rules are formulated and approved before the initial fuel loading and are designed to be relatively simple, short, and above all unambiguous. These Rules provide the detailed requirements for safe operation. The nuclear station must be operated within the boundaries set by the Operating Rules and other regulations called Station Operating Instructions. These are formulated to ensure that all operations are carried out within specified limits of the plant and the general requirements of the Operating Rules. Station Operating Instructions are supported and implemented by detailed Plant Item Operating Instructions and other documentation. 7.4 MAINTENANCE Maintenance of nuclear power stations has a similar pattern of safety control as operation. All nuclear power stations are required by the conditions of the Site Licence to have approved maintenance schedules. These schedules require the examination, inspection, maintenance and testing of all items of plant which are important to nuclear safety. The documentation details the inspection and maintenance procedures and specifies the plant’s subsequent testing after maintenance. The inspection and maintenance requirements are taken into account in the design and layout of the nuclear plant. The Site Licence specifies that each reactor should be shut down every two years for major maintenance, and that the licensee must obtain the consent of the NII before operations can restart and must send a full and detailed report to the NII describing the maintenance carried out. Because of some essential safety or performance need, modifications can be undertaken, following a detailed formal procedure involving the Nuclear Safety Committee. Final approval is necessary from the NII for the highest category modifications. 7.5 SAFETY RELATED MATTERS In the CEGB and SSEB, there are safety systems for all of the electrical, mechanical and radiological work undertaken. The radiological rules which apply to nuclear stations follow the same principles as the

Reactor operation and operator training in the UK

electrical and mechanical safety rules which have been in use in UK power stations for many years. These safety rules aim to ensure the safety of personnel who undertake operational or maintenance duties on the plant. The Safety Rules (Radiological) are designed to ensure that specified radiation dose levels are not exceeded and that all radiation doses are kept as low as reasonably achievable (ALARA principle). All persons concerned with the control, preparation and carrying out of work involving radiological hazards, must make themselves thoroughly familiar with the Safety Rules and the supporting documents. Each station has Operating Engineers who are designated ‘Senior Authorised Persons’ (SAP), who can, within the definitions of the Safety Rules, prepare, issue and cancel Safety Documents. These Safety Documents are specific to the electrical, mechanical or radiological aspects of the power station, and can be a combination of all three disciplines. The Safety Documents can be a ‘Permit for Work’, a ‘Limited Work Certificate’ and/or a ‘Sanction for Test’. These SAPs are responsible for ensuring that the precautions laid down in the Safety Rules are taken before access or work is permitted. This includes making sure all appropriate electrical supplies, mechanical systems and other hazards are isolated, so that the work can proceed safely. The SAP must determine that the recipient of the Safety Documentation understands all aspects of the work or tests. With radiological hazards, the SAP can seek the advice of specialist staff, Accredited Health Physicists at the station, who can advise on hazards, measurements and the precautions necessary. The engineers appointed as SAPs will, however, have had training in this field, thus enabling them to determine the aspects surrounding the safety of the work. The maximum allowable levels for radiation and contamination are set out in the Ionising Radiation Regulations 1985 and the Safety Rules (Radiological) already mentioned. Any breach of these Rules and Regulations is an offence in law. Formal patterns of training for all duties at nuclear power stations are given in Standard Specifications and some of these are outlined below. A training register must be maintained and be available for inspection. In addition to their normal duties, those with roles in the Emergency Arrangements, such as fire fighting, first aid, etc., must be trained and instructed in these duties, with records kept for inspection.

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There are both organisational and plant differences between the nuclear power stations, and so each will produce its own training documents, although all must conform with the general policy laid down by the CEGB or SSEB. 7.6 SHIFT STAFFING AT NUCLEAR POWER STATIONS For the two-unit plants in the UK (Magnox or AGR) with one control room, the normal complement of staff for one shift is illustrated in Fig. 7.1. In addition, there are industrial staff, e.g. two Operations Foremen and one Health Physics Monitor Foreman, who supervise about 20 persons of a non-academic qualification level, defined as ‘skilled plant operators’ and ‘semi-skilled plant attendants’. At some power stations, maintenance is carried out on a shift basis. This requires an additional number of shift staff of the appropriate craft skill to undertake the work, with foremen and engineers. This system of Shift Maintenance offers advantages and

Fig. 7.1. Typical shift staffing in a nuclear power station (AGR).

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disadvantages, but the majority of British nuclear power stations today do not employ shift maintenance, because shift maintenance does not offer a good return in terms of manpower deployment and productivity. All Shift Engineers and industrial staff work on a five-shift cycle, which allows for some additional shift staff to provide cover for sickness, leave, or training. For the Shift Engineers assigned to the different positions mentioned above, the training is identical. However, the higher positions on shift are filled with engineers with extra experience and technical ability. Because of their low operating costs, nuclear power stations, whenever possible, operate on a continuous basis at the highest practicable output. The Shift Manager has full technical and administrative functions, and he is responsible for the safety and operation (and shift maintenance, where applicable) of the station site, within the duties specified. On completion of his shift (nominally 8 hours) he hands over his duties to the on-coming Shift Manager and another shift team. The Shift Manager’s duties are considerable and can be briefly defined below: (a) Supervision and control of the station. (b) Operation of the plant, safely and efficiently in accordance with the Site Licence and the Station Operating Instructions and Procedures. (c) Control of the work, responding to breakdowns or incidents at the plant during the shift, and their rectification. (d) To ensure that the operating staff observe operating procedures, Safety Rules and radiation protection procedures. (e) To be responsible for his staff issuing Permits for Work and other safety documentation, in accordance with the Safety Rules. (f) To be conversant with the operational and maintenance workload carried out by the shift staff under his control, also checking on their performance. (g) To ensure that his staff are trained to meet all demands on them in those tasks necessary for the Station’s safe and efficient operation. (h) To apply his technical expertise and advise his staff on problems in plant operation and abnormalities which occur. (i) To take charge of any emergency by virtue of his role as Emergency Controller. This position remains with the Shift Manager until he is replaced

The Chernobyl accident and its implications

by a more senior officer, as described under the Station Emergency Plan (see Section 5). The essential requirement of the shift team, under the Shift Manager, operating a complex nuclear plant is their reliability when performing their tasks. That they should be able to supervise and operate the plant in the specified normal condition, and be able to identify operational abnormalities and incidents quickly, is all-important. They must at the same time be able to take the correct action swiftly to solve any plant problems that arise. The tasks and responsibilities have therefore to be defined and allocated to all members of the shift team. Any inadequacy must be rectified by training or other suitable means. The shift team must always be sufficient in numbers to guarantee smooth and safe operation, allowing for predictable and unpredictable absence. The shift teams at all nuclear stations are supported by technical experts. The Shift Manager is therefore able to call for specialist advice outside his own team. This may be in the Mechanical, Electrical, Control and Instrumentation, Chemistry, Operational Performance (physics) or Health Physics fields. A system exists whereby the Shift Manager can gain advice during the ‘out-of-hours’ period, thus covering the full 24-hour period of operation. 7.7 CRITERIA FOR SELECTION OF PERSONNEL In UK power stations the practice is to engage only persons with professional engineer or technician status for positions of Shift Engineer or Reactor Desk Operators. The basic qualification corresponds to that of a graduate in engineering or physics. The minimum qualification is at least a Higher National Diploma or equivalent. In addition, all staff are given suitable training for their duties, which are described briefly below. Selection tests are not required by any regulations, but staff are appointed as a result of an interview. The staff are then required to undergo medical examinations to ensure that they are medically fit enough for their job, which may involve the wearing of respirators and protective clothing. 7.8 TRAINING OF SHIFT PERSONNEL The basic training is much the same for all shift staff engineers. It starts with a 4-week ‘Introduction to

Reactor operation and operator training in the UK

Nuclear Power’ course on nuclear and reactor physics, followed by a 6–8 week course of detailed information on the plant for familiarisation. Distinctions are made between Magnox and the AGR stations in this course. The courses are run at a special training establishment. Information on the design concepts are given and safety is much emphasised. The engineer has more than 40 hours of practical training with simulators, to acquaint him with the running of the plant under normal operation, startup, shutdown, etc. The training covers safety systems, transients and abnormal situations, and shows the interrelationship between the parameters and operational conditions which will be experienced. These are demonstrated on the simulator, which is able to display the relevant parameters of the plant. In many cases, the simulator is a replica of the nuclear plant control room. The great value of simulation is that it affords the engineers the practicalities of demonstrating their skill in tackling the transients and unsafe conditions that they could face on the operating plant. Often incidents which have occurred at a nuclear station are set up on the simulator to illustrate practical situations. In addition to this off-site training, engineers have extensive training at their own station to familiarise themselves with the plant. The engineer will be given an interview before taking up his duties. Every two years all shift staff have to attend refresher courses, which will include some further simulation work and which emphasise safety related topics at their station. Only in the formal courses at the beginning of an engineer’s appointment do written examinations take place. The results are then sent back to the newly appointed engineer’s Station Manager. Written examinations are not considered suitable for the refresher courses, because the objective is team participation, as well as individual performance. However, the Shift Manager must make judgements on each individual’s performance at these refreshers and then additional training can be given if it is thought that this is required in a particular case. Most nuclear stations are now incorporating training exercises which examine closely abnormal conditions of plant operation by a system of selfappraisal. They spend a specified time looking at particular transients or abnormalities. The shift teams evolve work exercise sheets which take a particular plant abnormality which might lead to a more difficult state of plant condition. These are studied, either by

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individuals or by shift teams working in groups, and the correct responses to the conditions set in the exercise are analysed. The final assessment is undertaken by a Senior Engineer. All shift teams are involved and they set up more work sheets themselves on a range of emergencies for study by others, thereby extending the programme of this form of training. Overall, this training system assists shift engineers to be ready to deal with factors that could lead to a dangerous plant state. Although much of this type of work is better served on simulation exercises, these situations can only be fully appreciated by those working at the plants. The Magnox reactors do not have a simulator that can cover the whole range of station operating parameters, but employ a ‘point’ or generic type of simulator model. 7.9 NORMAL SHIFT DUTIES The main task of the shift team at a nuclear power station is to operate the plant in the most safe and efficient way and to ensure the output is always to the standard required, in accordance with the normal parameters of the plant. When there is a need to take plant out of service by reason of a yearly or two-yearly maintenance, or other essential planned maintenance, every effort is made to keep the out-of-service time to a minimum. Normally with each shift, the Central Control Room team’s main task is to maintain the station at its normal full load. Their duty is to monitor, control and survey the plant and its parameters and keep these within the safe operating limits. All UK nuclear power stations are designed for ‘on-load’ refuelling, and safety during operation is important if the station is to continue to generate at full load over the days and weeks ahead. During operations close liaison between the Central Control Room (CCR), engineers and industrial staff is essential. When the plant is not at full load, the shift team’s directions are generally aimed at progressing the plant towards return to the optimum condition. There is additional documentation for operation staff on local management procedures, which includes the important aspect of the hand-over of the plant to the oncoming shift. It is important that details of the plant’s condition are known to the plant operators at all times, and this

The Chernobyl accident and its implications

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information has to be recorded in logbooks and passed on again verbally to the oncoming shift. This is necessary for continued safe operation and covers the normal operation, as well as any abnormalities which may be occurring during the changeover of shifts. Communications up and down a management chain are always important for safe and efficient operation of any organisation, but in shift operation it is essential. Logbooks for groups of operational staff are maintained and formally initialled by the appropriate Engineer or Operator before leaving shift. These logbooks are kept as a record of the shift operations and constitute a permanent history for the station. In addition, they are useful for the plant management, so they can ensure that any variations from the norm are kept under review. 7.10 REACTIONS TO ABNORMAL EVENTS Perhaps the most important part of the operation of a power station is in the Control Room, and so the Control Room staff’s duties and reactions to the responsibilities they undertake in coping with both normal and abnormal conditions are now covered (Fig. 7.2). With nuclear stations operating in a base load mode, the staff in a Control Room have to spend many hours

of duty acting as plant monitors rather than as controllers. This is a situation with long periods of stable operation. However, the Control Room Desk Operator has always to be prepared to respond to plant abnormalities or transients, requiring the immediate application of his training and experience. Such situations demand quick, decisive and, of course, correct action. Qualified and able assistants must at the same time be ready to react to the situation out on the plant, as the Control Room staff only operate in the Control Room. With modern instrumentation and automation of control equipment, and ever improving methods of data presentation, visual and audible alarms, etc., most plant operations are now controlled from the Central Control point. The reactor is designed to be capable of shutting itself down, inside preset limits of the plant conditions of the transient, by multiple lines of automatic protection. The Operator’s role is therefore not critical in this respect. However, he must ensure that the plant is operated safely, efficiently and economically. Vigilance is essential. Personnel in control rooms must be able to act swiftly, both mentally and physically, in any departure from the routine stable plant operation and to respond correctly for a transient. This is achieved through experience and training. In the course of their

Fig. 7.2. Photograph of a control desk at a nuclear power station showing visual displays of plant information.

Reactor operation and operator training in the UK

training, Operators rehearse what actions should be taken in any circumstances. Most reactor accidents that occur are not of the catastrophic type. The Three Mile Island (TMI) accident in the USA in 1979 was a very great financial loss to the electrical power utility running that plant, but saw no radioactivity released to the environment, other than in very minor and insignificant amounts. The accident at Chernobyl would be called catastrophic, as loss of life did occur, large amounts of radioactivity were released to the environment, and a large evacuation of the population had to be undertaken. Some minor accidents are always possible, but good design and training will ensure that such events become less and less likely. In the UK nuclear power stations have one reactor to supply one turbo-generator, or are designed for one reactor to supply steam to two or more generators. The double turbo-generator plant is particularly demanding for the Control Room Operator to ensure availability of plant. In the event of a single turbine loss at these latter stations, the Operator needs to reduce reactor power to match the remaining turbogenerator’s load. With one turbine per reactor, the loss of the turbine means an automatic reactor shutdown. A reactor shutdown for any reason will result in a delay of 24–36 hours before the reactor can be restarted and restored to the full-load condition. If the operator is unable to control a transient, the automatic protection is designed to take the reactor into the safe shutdown state automatically. In the event of a turbine fault, the operator, while performing correct and safe actions, should prevent reactor shutdown if the reactor is supplying another turbine. While the reactor desk operator is not alone in the Control Room, his initial reaction to any abnormality is his responsibility and he must rely on his training and experience. This ‘isolation’ may be for less than one minute, but his immediate and correct action will dictate the subsequent course of events. As soon as possible assistance will be given by the Control Room Supervisor and others. As the incident progresses, such corporate action becomes essential to bring about restoration of normal operation, or the safe shutdown and isolation of the defective unit or piece of equipment. Actions will be based on the data fed into the Control Room on the changing parameters of both healthy and faulty plant. There will be a need for checking the correct procedures, covering the Operating Rules, Station Operating Instructions and

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other documentation already referred to. The Control Room staff and other shift engineers must therefore be aware of recent changes to such documentation, and any temporary instructions in force. Shift staff returning from absence or a new shift have to place the highest priority on document checking, including the previous shift’s logbooks. Finally, it must be noted that most transients and abnormalities in the plant parameters do not lead to serious problems. 7.11 CAUSES OF ABNORMALITIES Unplanned events arise from human error, malfunction or breakdown of plant equipment, usually within the station. However, grid faults will have different effects, which depend on the type of fault. All of these faults have to be dealt with by the Control Room staff. The Control Room houses all the main operating controls for the reactor, its boilers, the turbogenerators, and station electrical supplies for auxiliary and essential plant, as well as outgoing grid connections. Coupled with the array of controls and equipment for the overall station control is a large quantity of plant instrumentation, displayed on visual display screens, as well as over 1000 alarm states, which have to be monitored by the operator and actioned. The response to any human error on the plant, or any equipment malfunction, is determined by knowledge of the activities of other operational and maintenance staff on the plant. Control Room staff need to know at all times the extent of maintenance work, testing or plant adjustments, so that they are always ready and able to make the correct response swiftly. Actions resulting from human error are usually handled by direct intervention, rather than waiting for automatic correction. In most cases, where plant instabilities are not causing the reactor to trip, the operator’s knowledge and experience will enable him to retrieve the position and bring the plant back to the stable condition. At no time is the Central Control Room manned by less than three persons when two reactors are operating. Plant faults can be caused by deterioration with age or by faulty maintenance. On newly commissioned stations difficulties sometimes occur in the early days as staff familiarise themselves with the plant. The safety record to date in UK nuclear power plants is impressive, but this must not lead to complacency nor to any relaxation of safety standards.

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7.12 GENERAL PHILOSOPHY OF REACTOR PROTECTION It is the policy of electricity supply utilities in the UK that all plant should be provided with automatic protection to: (a) ensure the safety of the public; (b) ensure the safety of the operators; (c) ensure the plant is protected from damage. To give this far-reaching degree of protection, decisions have to be taken as to the type and magnitude of some credible faults. Safety costs money, and there has to be always some balance between protection and the spending of money out of all reason in attempting to protect against the virtually incredible incident. In general, four factors are used to ensure that a protective system is reliable, and these form the basis of UK nuclear power plant design philosophy: (a) All equipment should be designed so that its components are rated well below their ultimate limit. (b) The ‘fail-safe’ factor, i.e. the equipment is designed so that in the event of a component failure, the protection operates to make the system safe. (c) Redundancy, i.e. if an item of protective equipment fails, it should have a back-up from other protective devices to protect personnel and the plant. (d) Diversity of redundant components, e.g. power supplies, manufacturers and maintenance teams. Reactor faults can act rapidly, allowing no time for operator action should the protection fail. In these circumstances it is vital to have a number of lines of identically protecting systems (redundancy) to maintain safety. ‘Defence in depth’ is provided. For example, containment of fission products is ensured by a combination of fuel integrity, cladding, pressure vessel integrity and containment structures. All protective devices are tested on a routine basis. The demonstration that the protection is in full working order not only allows the operator a feeling of confidence but in certain cases allows a realistic time delay for him to correct a fault. The normal protection philosophy, however, does not rely on operator action. It has to be assumed that the protection has been correctly maintained and set at its correct ‘trip’ value, either by reactor maintenance at shutdown every one or two years, or on a regular weekly or monthly routine basis. Once set, it must not be touched. Vital and essential protective devices

The Chernobyl accident and its implications

must be safeguarded against inadvertent or unauthorised interference. There is a sophisticated system of key interlocks combining electrical, mechanical and administrative features which play a vital role in preventing unauthorised interference. If it is operationally essential for some protective device to be adjusted, the system is provided with additional protection which will trip the reactor if the correct procedure is not followed explicitly. Current safety philosophy can be classified as follows: (a) Postulated faults prevented by specific plant design. (b) Fault conditions prevented by interlocks. (c) Exceeding limiting parameters prevented by automatic plant condition monitoring. (d) Reliance on operator action (given adequate instrumentation and data presentation) to correct developing faults. This scheme covers the following: (a) The reactor and its associated equipment will shut down safely under all ‘credible’ fault conditions, while maintaining shut down auxiliary plant in an operational state. (b) A single operator error will not jeopardise the safety of the reactor. (c) Interlocks are not a protection against a fault, but serve as an inhibition against incorrect or unauthorised usage. (d) When protective devices are required to be maintained under operational conditions, a maintenance ‘veto’ device should only be used when no other means are possible. (e) Operator action must never be claimed as a line of protection. A system of reactor protection that is reliable needs to be demonstrably so. This reliability is demonstrated in three ways: (a) by direct testing under fault conditions realistically simulated on the plant; (b) by showing that its protective action under simulated fault conditions is less onerous than the worst conditions met with in practice; (c) by calculation. ACKNOWLEDGEMENTS The author wishes to thank many of his friends within the CEGB and other nuclear organisations for their

Reactor operation and operator training in the UK

help and assistance in the preparation of this Section. He also wishes to thank the Novosti Press Agency (London) for information supplied, and his colleagues on the Working Party and the Secretariat of the Watt Committee. REFERENCES AND BIBLIOGRAPHY 1. CEC Report 81a, Safety principles for LWR nuclear power plants, Commission of the European Communities. 2. CEC Report EUR 8174 EN, Simulators for nuclear power stations, 1984.

69 3. CEC Report EUR 10118 EN, Qualification, training, licensing, and retraining of operating shift personnel in nuclear power plants, 1985. 4. CEC Report EUR 10981 EN, Qualification, training, licensing, authorisation, and retraining of operating personnel, 1987. 5. Central Electricity Generating Board, Modern Power Station Practice—Section ‘H’—Nuclear Power Generation. 6. BINDON, F.J. L., The role of the power station operator, invited paper to the International Conference on Training for Nuclear Power Plant Operators, Bristol, 1982 (Inst. Nuclear Engineers). 7. BINDON, F.J. L., Promoting excellence in nuclear power station operation in the UK. Paper presented at Conferencia Internacional Sobre Entrenaniento de Operadores de Instalaciones Nucleares, Madrid, 1983.

Section 8

International Dimensions of the Implications of the Chernobyl Accident for the United Kingdom David Cope Executive Director of the UK Centre for Economic and Environmental Development, London

8.1 INTRODUCTION

responses to releases of radioactivity to the environment will be relevant to all potential sources of such a release, no matter from where they originate in the nuclear power system. Since the 26 April 1986 Soviet accident much has been made of the international consequences, with assertions on the lines that an accident anywhere is an accident everywhere and that policies on nuclear power adopted in individual countries are diluted or invalidated by differing policies adopted in other countries (e.g. by Sir Eldon Griffiths, MP, during the parliamentary debate on the Sizewell PWR reactor, 23 February 1987). It has also been observed that nuclear reactors tend to be located at sites which increase the likelihood of incidents having direct international repercussions. While such observations have sometimes implied a conscious decision by nuclear plant regulators to transfer risks to exogenous populations, there are good reasons why nuclear reactors are frequently located in marchland regions. In continental Europe, rivers often form boundaries, while coasts are by definition limits to the nation state. However, both locations are attractive for power stations of all types because of the availability of cooling water. Boundary and coastal regions also, for a variety of historical and economic reasons, often have sparse populations and therefore also have merits as sites for nuclear power stations. Paradoxically, Chernobyl-4 itself was not located in a peripheral region of the Soviet Union, but its international impacts merely served to reinforce

Given the level of public concern about nuclear power issues, it is inevitable that any significant incident at any nuclear station, especially one which involves offsite effects, will have some international consequences, if news of its occurrence reaches the public domain. Comparisons will be made with similar or different systems operating in other countries and attempts will be made to draw lessons from the incident. If, as in the Chernobyl case, the incident has effects which reach out across international frontiers, then the international consequences will be considerably greater. Prior to the 26 April 1986 event in the Soviet Union, transboundary environmental effects were already a subject high on the European agenda and Chernobyl provided new impetus to those who were seeking to raise public awareness of, and political responses to, international environmental impacts. Furthermore, any nuclear incident tends to result in renewed examination of all aspects of the nuclear power and nuclear fuel systems, from fuel supply through reactor operation and transport of irradiated materials to radioactive waste management. Although this has certainly been the case with the Chernobyl accident, this review of international responses mainly restricts itself to issues arising from the possibility of an incident at an operating reactor and does not cover areas such as radioactive waste management or transport of materials. However, the discussion of 71

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existing apprehensions about sites such as Cattenom, Barsebaeck, la Hague, Gravelines or Sellafield. 8.1.1 The international consequences of the Chernobyl accident The Chernobyl event focused attention on the international regulatory system applying to nuclear power and on ways in which events in one country affect the situation in others. In particular, Chernobyl concentrated attention on: — the international reporting system for incidents, especially those likely to have transfrontier effects — international cooperative effort in response to incidents to minimise their effects — arrangements for liability for effects from an incident in one country which affect others — differing national liability arrangements for nuclear incidents — differing national and international standards on the radioactive content of commodities, especially foodstuffs and trade in them — public awareness of, and responses to, nuclear incidents happening in other countries, in both countries directly affected and those not directly affected by the incident — the sensitivity of countries’ plans for developing nuclear power to events happening elsewhere — the internal ability of countries to respond to nuclear events occurring outside their territories. In the remainder of this Section, these aspects are discussed either through an examination of the leading international agency with responsibility for them, or as subjects in their own right. In all cases, particular attention is given to the UK context of the subject. 8.2 INTERNATIONAL NUCLEAR AGENCIES The role of international agencies in the fields of nuclear and environmental protection was highlighted by the incident. The main agencies are: — The International Atomic Energy Agency (IAEA) in Vienna, an associated agency of the United Nations Organisation — The Nuclear Energy Agency (NEA) of the Organisation for Economic Cooperation and Development (OECD) in Paris

The Chernobyl accident and its implications

— The Directorate for Environment, Consumer Affairs and Nuclear Safety (DG XI) of the European Commission in Brussels and other Directorates. These agencies and the main fields of their activities relevant to the Chernobyl incident are discussed below. However, a second tier of international agencies is also involved in aspects of the response to the accident: — The World Health Organisation (WHO) of the United Nations in Geneva which, jointly with the IAEA, is coordinating the analysis of a long-term health study being carried out in the Soviet Union of populations affected by the Chernobyl accident — The Food and Agriculture Organisation (FAO) of the United Nations, which is concerned with radionuclide contamination of foodstuffs, especially those entering international trade — The United Nations Scientific Committee on the Effects of Atomic Radiation, created in 1955 to coordinate radiological information on radiation levels and their effects on man and the environment, now part of the United Nations Environment Programme — The International Commission on Radiological Protection, founded in 1928 to provide technical guidance and promote international cooperation in the field of radiation protection — The World Meteorological Organisation, whose network of atmospheric monitoring and information exchange is relevant to evaluating the dispersion of radioactivity from the site of an accident. 8.3 THE INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA) The International Atomic Energy Agency was created in July 1957 as an independent intergovernmental organisation within the United Nations system to advance the contribution of atomic energy to peace, health and prosperity. The United Kingdom has been a member since its foundation. The IAEA has been primarily concerned with measures to prevent the proliferation of weapons manufacture associated with civil nuclear power, although a modest programme concerned with reactor safety and incident response was in place prior to Chernobyl, based on a duty in its statutes to ‘establish or adopt…standards of safety for

International dimensions of Chernobyl implications for UK

protection of health and minimization of danger to life and property’. A Nuclear Safety Division had been set up at the inception of the Agency. In its early years this mainly considered matters of a self-evidently international nature such as the transport of radioactive materials across international boundaries. In the early 1970s the IAEA began developing a Nuclear Safety Standards Programme to create internationally accepted standards on subjects such as siting, design, operation and quality assurance. In 1983 the Agency began an international Incident Reporting System and developed an Operational Safety Review Team programme, involving the despatch of a team of experts for a period of 2–3 weeks to visit individual plant and review operating procedures. In 1985 the Agency created an International Nuclear Safety Advisory Group. In all these areas the IAEA was primarily oriented towards helping and supervising ‘newly nuclear’ states rather than closely monitoring developments in countries with long-established nuclear programmes. As an agency under UN auspices, there were strong ‘technology transfer aid’ emphases behind its programmes. However, with the occurrence of the Chernobyl accident the Agency found itself uniquely placed to handle the East-West dimensions of the incident, being the leading international nuclear organisation with common membership. It has assumed a key role in determining and coordinating international level responses. Although other organisations such as the EEC have developed independent initiatives, these have wherever possible been formulated to harmonise with IAEA activities and with the intention of facilitating wider agreements under IAEA auspices.1 A delegation from the IAEA visited the Soviet Union from 5 to 9 May 1986, including the Chernobyl site. A joint communiqué was issued after this visit, agreeing on provision of detailed information and to the release to the IAEA of information on radiation levels measured at one site about 30km from Chernobyl and six others on the western borders of the Soviet Union. A second IAEA visit to the Soviet Union, including Chernobyl, was made in January 1987. This followed up agreements reached in the immediate aftermath of the Chernobyl accident and reviewed the upgrading of safety provisions at the 13 operational and 8 RBMK reactors under construction. Proposals for implementing long term studies of radiation-affected individuals were also considered.

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At a special board meeting held on 21 May 1986 there was a unanimous agreement that the Agency’s nuclear safety programme be strengthened and during 1986 Agency spending in this area was increased by 33%. The 1987 budget for this area was further expanded by $2 million at the end of 1986, but by March 1987 the IAEA’s Director-General was expressing concern about the financial ability of the Agency to continue its work at the newly requested levels.2 The IAEA held a post-accident review meeting on the Chernobyl event in August 1986 with over 600 delegates. At this meeting the Soviet Union provided detailed information on the accident. Two conventions were drawn up by member government representatives in August 1986—on Early Notification and on Emergency Assistance in the Case of a Nuclear Accident or Radiological Emergency.3 Adopted by the General Conference in September 1986, these Conventions were signed by 60 states. Most have agreed to observe them even prior to official ratification in their national legislatures. 8.3.1 Notification of international nuclear incidents and provision of emergency assistance The Early Notification Convention requires signatories to provide immediate information to the IAEA following a civil nuclear incident and voluntarily directly to potentially affected outside states. It specifies the type of information required and further requires signatories to agree to requests for additional information from affected states. The IAEA will also provide monitoring assistance to non-nuclear member states bordering on non-member states with a nuclear power programme. The Emergency Assistance Convention requires signatories to register with the IAEA the nature and level of technical and logistical assistance they could provide for external emergency assistance. States requesting assistance would take responsibility for overall direction and would provide legal indemnity for those providing assistance from outside. All signatories undertake to facilitate the transit of assistance across their territories in response to a request from another signatory. The IAEA is also accorded a role in developing the capability of member states to respond to external incidents.3 8.3.2 Conclusions The International Atomic Energy Agency has enhanced

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its reputation by the way in which it swiftly and efficiently achieved agreement on the two Conventions discussed above. Prior to this, its effectiveness as an international organisation had been weakened by political disputes during the early 1980s. As the premier nuclear organisation straddling East-West divides, it found itself uniquely placed to handle the delicate international political difficulties which the Chernobyl accident created and has generally been successful in doing so. However, questions must still remain over the effectiveness of the Conventions and these can only really be answered by the occurrence of an event which triggers their application. The provision of emergency assistance to the Soviet Union, once details of the Chernobyl accident had become established, proceeded fairly smoothly in the absence of any mutual assistance convention—the IAEA agreement will simply codify and marginally extend what has been tested and shown capable already. The effectiveness of the early notification convention is a far more significant consideration, given the experience in the earliest days after the Chernobyl accident. It is probably likely that immediately after an incident, states will hesitate from responding to the provisions of the convention for fear of over-reacting. Once the true nature of an incident has been established and the notification procedures initiated, affected external states may quite possibly feel that they could have received earlier notification. The effectiveness of the convention will depend on the interpretation placed by different agents in the chain of responsibility, from reactor operators to national energy ministries, on whether a release of radioactive material ‘is likely to occur’, and whether it ‘may result’ in transboundary contamination that ‘could be’ of radiological significance (Article 1 (1)). Experience in the UK over the past decade has shown that such terms can be interpreted inclusively or exclusively. 8.4 THE NUCLEAR ENERGY AGENCY OF THE OECD The OECD’s Nuclear Energy Agency (NEA) was created in 1972, although an earlier European Nuclear Energy Agency has existed since 1958. The United Kingdom was a founder member of this earlier agency. The aim of the agency is to coordinate the policies of the Western industrial countries in the field of peaceful uses of nuclear energy. Over 80% of the current world

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nuclear reactor capacity exists within member countries of the NEA. The NEA has a cooperative agreement with the IAEA. It has particularly concerned itself with radioactive waste management and administers the London Dumping Convention on marine disposal of radioactive waste. The other main area of its work, more directly relevant to the Chernobyl accident, is in coordinating international and national liability arrangements for incidents at nuclear power stations among its member nations. 8.4.1 International liability for nuclear incidents Legal provisions for liability from nuclear incidents, covering both events with purely domestic and those with international consequences, are special cases of the more general field of environmental protection law. This field is, generally speaking, not highly developed, especially when considering international impacts and effects which are suffered indirectly rather than directly.4 For example, within Europe, it is the United Nations Economic Commission for Europe 1979 Geneva Protocol on Long Range Transfrontier Pollution which sets the framework for international cooperation to reduce such pollution. This agreement, however, contains no provisions for compensating alleged damage and has, anyway, mainly been focused on ‘conventional’ sources of pollution. Where international compensation agreements have been reached, they involve specific and comparatively localised events such as oil spills. Consequently, it is not surprising that the Chernobyl accident has provoked considerable disquiet about the adequacy and scope of international liability arrangements for third-party compensation after nuclear incidents. The foundation of present international liability arrrangements for nuclear incidents is the Paris Convention, drawn up in the early days of the ENA (the European predecessor organisation of the NEA) and modelled on earlier liability provisions (without any international component) developed in the US Price-Anderson Act. Adopted in 1960, the provisions of the Paris Convention came into force in 1968 and set out minimum and maximum levels of compensation to be provided by the operator of the guilty facility, based on an early elucidation of the ‘polluter pays principle’. Signatory states were able to introduce higher maximum liabilities in national legislation, and in practice most have done so.

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Table 8.1 International nuclear liability arrangements, 1987

Including additional government-provided cover. na Country is not member of convention organisation. c ×Signatory of convention of organisation of which country is a member. d —Information not available. e c Non-signatory of convention of organisation of which country is a member. For further details of conventions, see text. Source: from data supplied in Shapar and Reyners.5 a b

The Convention also required that plant operators provide financial security for their liability in a form approved by the state and that each signatory state identify a single ‘competent court’ to which all actions under the convention be brought, to ensure harmony of judgements. The Paris Convention was drawn up as much to provide favourable economic circumstances to nurture the development of nuclear power as to ensure fair compensation for damages—hence the maximum liability ceiling. Furthermore, the Paris Convention set a time limit of 10 years on the liability of an operator for an incident, a limit which could be significant given the delay in revelation of some forms of nuclear damage. Signatory states of the Paris Convention at January 1987 are identified in Table 8.1. Paralleling the NEA Paris Convention initiative, in 1963 the IAEA introduced its own convention, known

as the Vienna Convention, with essentially the same provisions as the Paris Convention. This did not come into force until 1977 and has been ratified by a group of countries which, apart from Yugoslavia, have insignificant nuclear capability and even less significance for Europe (see Table 8.1). No members of the Eastern Bloc countries are signatories. Even in 1960 it was soon realised that the provisions of the Paris Convention were inadequate to deal with potential claims for damage compensation. Accordingly, in 1963 an extension to the Convention was agreed by 11 of the original Paris signatories, including the United Kingdom, known as the Brussels Convention. This provided for a threetier compensation system. Above the level of maximum operator liability and to a figure of 70 million Special Drawing Rights (SDR) of the International Monetary Fund (1 SDR=1·2336

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(1987)US$) the government of the reactor operator’s state would assume responsibility. Beyond 70 million SDR up to 120 million SDR, liability would be assumed collectively by the signatories, apportioned by a formula based on the signatory states’ Gross National Products and the megawattage of nuclear reactors within each state. By 1982 it was recognised that inflation was eroding the value of the compensation provisions of the Paris and Brussels Conventions and Protocols were agreed raising the ceiling of individual government responsibility to 175 million SDRs and that of signatory collective liability to 300 million SDRs. However, at present the Protocols have not been ratified by all the signatory states and the NEA can only indicate that it hopes they ‘will come into force in the near future’.5 It is important to note that the provisions of the Paris and Brussels Conventions apply to reactor incidents happening only within the signatory states and for compensation only to nationals of the signatory states, unless individual governments decide to extend them. In most cases, member states have made provision within their national legislation to handle default by liable operators but only the Federal Republic of Germany has provision within its legislation for the state to assume compensation responsibility in lieu of an operator when damage occurs for which no operator is liable under the law. Furthermore, the IAEA Vienna Convention has never been extended in the way that the Brussels Convention extended the original Paris Convention. Nor are any updating protocols to the Vienna Convention currently proposed. A good summary of the comprehensiveness of current international nuclear liability arrangements is that out of the approximately 380 reactors existing worldwide, only about 120 are covered by the provisions of the Paris and Brussels Conventions and only three, of which but one is in Europe, by the Vienna Convention. These figures probably illustrate better than any others the inadequacy of current arrangements. Furthermore, even the existing arrangements were framed in consideration of direct damage to persons or goods by nuclear operations and the legislators adopted very restrictive definitions. For example, the Paris Convention states: ‘The operator of a nuclear installation shall be liable, in accordance with this Convention, for:

(i) damage or loss of life of any person: and (ii) damage to or loss of any property… upon proof that such damage or loss…was caused by a nuclear incident involving either nuclear fuel or radioactive products or waste in, or nuclear substances coming from such installation…’ (Article 3a) The Vienna Convention adopts a similar definition. Generally, the implementing legislation in individual states has not extended this definition. Accordingly, it is unclear whether compensation for the forms of economic hardship suffered after the Chernobyl incident—which resulted more from precautionary measures introduced by individual states than from direct nuclear damage—are in fact covered by the Conventions’ provisions, even had the incident happened within a signatory state. Similarly, it is not clear whether signatory states could claim against another signatory state for the administrative costs of introducing precautionary measures. This uncertainty is exacerbated by the varying standards which different countries have adopted in determining intervention levels, as discussed in detail in sub-section 8.5 on the European Economic Community and illustrated by some examples in Table 8.2. Thus, although the international regime on compensation provisions is not directly linked to harmonisation of intervention standards, ‘it is nevertheless important to the functioning of this regime that a solution be found’.5 Considering specifically the impacts of the Chernobyl accident, the preceding discussion of compensation provisions may seem somewhat academic, since the Soviet Union is not a signatory of any international liability agreements and does not even have specific provision within its domestic law for overall nuclear liability. Thus, any action against the Soviet Union in pursuit of damages would have to be based on customary principles of international law requiring states not to carry on activities which cause significant damages in territories of other states.7 However, as already noted, much of the economic disruption suffered by individuals and governments from the Chernobyl incident resulted from the destabilising effects of preventative measures adopted by states responding to perceived threats—opening up the prospect of unending disagreement over liability and the ‘reasonableness’ of the measures adopted which led to the economic hardship claimed. Consequently, although there was discussion in several countries soon after the Chernobyl incident

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Table 8.2 Examples of post-Chernobyl ‘action levels’ applied by different countries as at December 1986 for radionuclides in imported foods

a Different terms used in different countries, e.g. ‘Levels of concern’ (USA), ‘Screening limits’ (Canada), ‘Maximum permitted levels for import from third countries’ (EC countries). b Belgium, Denmark, France, Federal Republic of Germany, Greece, Republic of Ireland, Italy, Luxembourg, Netherlands, Portugal, Spain, United Kingdom. c Recommended values by EC in May 1986. Source: Food and Agriculture Organisation.6

about action to pursue damages from the Soviet Union, intentions have invariably been tempered by a recognition of the hopelessness of a case against the Ukrainian Peoples’ Republic, in which the reactor is situated. This was reinforced by statements made at the September 1986 conference of the IAEA by Boris Shcherbina, Deputy Chairman of the USSR Council of Ministers, questioning the validity of measures introduced in western countries and quoting the nonexistence of international agreements in this area. Governments of affected countries have had themselves to step in and make compensation for their measures, either under one-off provisions or, in the case of the Federal Republic of Germany, under preexisting legislation. In May 1986, an Austrian individual did bring a

civil case against the Soviet Union, claiming damages, and the Austrian court held that since the Soviet Union owned property in Austria, it was competent to try the case. A criminal case against six individuals identified by the Soviet Union as responsible for the Chernobyl incident was also brought in Vienna. However, Austrian law required the defendants to be present for the case to be heard.8 The civil case is still pending at the time of writing this report. In the UK, some representations about compensation were made to a visiting Soviet agriculture minister in July 1986 by both the Foreign Secretary and the Secretary of State for Agriculture, Fisheries and Food. However, the latter minister declined a Parliamentary suggestion that, during the UK’s Presidency of the European Community’s Council

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of Ministers, he coordinate European claims for compensation (Hansard, 11 and 22 July 1986). 8.4.2 Conclusions The entire experience of the Chernobyl accident has underlined the urgency of establishing a more realistic and universal system of international compensation provision. The Soviet Union has indicated at the September 1986 IAEA conference a willingness to consider future arrangements for national (rather than operator) liability, and the possibility of extending the Vienna Convention and establishing a protocol linking it to the Paris and Brussels Conventions has been discussed by the IAEA and NEA. Of course, the enormous potential costs of meeting compensation claims (the economic costs of the Chernobyl incident outside the USSR have been estimated9 at $1·5–4 billion (109)) will be a strong disincentive to rapid progress on agreement. It has been argued that this is particularly true of states with their own nuclear industries, even though their citizens may have suffered economic damage from the Chernobyl incident, because of fear of creating precedents which may later be used against them.4 Within the operation of agencies such as the NEA it is difficult to avoid the impression that compensation agreements have not been given a high priority. For example, a volume published by the NEA10 to celebrate its 20th anniversary makes no mention of compensation agreements among the achievements of the Agency. The assertion that reactor loss of core incidents were extremely unlikely to happen, an assertion which, as numerous commentators have noted, was made so frequently in the period prior to Chernobyl, seems to have deflected attention from addressing the difficult problems of compensation provision were such an accident to occur. 8.5 THE EUROPEAN ECONOMIC COMMUNITY Alone among the international organisations reviewed in this section, the European Economic Community is able to pass legislation and introduce regulations which become binding on member states. This power has been both a strength and a weakness for the Community in its response to the Chernobyl accident. No part of the territory of the Community lies closer than 1000 km to Chernobyl but, in common with other

The Chernobyl accident and its implications

non-member states, it experienced varying levels of contamination from the passage of the radioactive plume across Europe. Levels differed according to distance from Chernobyl, wind directions and precipitation activity. Over 33% of Community electricity is currently generated by nuclear power, giving it a critical role in overall economic security. However, this average ranges from 0% in countries such as Denmark and Ireland to 60% or more in Belgium and France, so that individual members of the Community find themselves with differing perceptions of the importance and benefit/ cost trade-offs of nuclear power. Community activity in the nuclear field is based on the provisions of the Euratom Treaty of 1958. This treaty and developments from it have been primarily concerned with promoting nuclear research and development. Although there has also been activity in the field of design safety criteria, worker and public dose limits and responses to incidents, after Chernobyl the Community found, for example, that it had no provisions in any existing Directives specifying limits to the radioactive content of foodstuffs. Furthermore, the Commission’s attempts to coordinate a Community response after Chernobyl showed that there were major differences in sampling practices, reporting of results, etc., between member countries which made harmonisation of responses difficult. In fact, a Commission communication prepared after the incident argued that Community competence in the field of radiation protection had actually declined compared with the first provisions of the Euratom Treaty in 1959.11 The same paper advanced the inability of the Commission to access adequate data as the reason why it did not use the provisions of Article 38 of the Euratom treaty, allowing for Directives in cases of urgency, after the Chernobyl accident. In response to the accident, the Commission proposed Community action in a range of areas of which the most important were: — determination of design safety criteria for nuclear installations — development of an emergency planning and response system — consideration of the establishment of a Community inspectorate for safety and radiation protection — introduction of agreed permanent intervention levels for controlling the market in foodstuffs and animal feeds produced within or imported into the Community.

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By one year after the Chernobyl accident, there had been action on only the last point, and then on a temporary basis, largely because of disagreements among member states on the desirability or details of Community involvement in the other areas. 8.5.1 European Community action controlling trade in foodstuffs Unlike the other international agencies, the EEC was able swiftly to implement certain responses to the Chernobyl accident because of its ability to regulate internal and external trade. On 6 May 1986 the Commission issued recommendations designed to handle the hazards posed immediately by detected levels of the short-lived isotope I-131. The recommendations were not observed in all countries and on 12 May the Commission implemented a ban on imports of milk, dairy products, fresh fruit and vegetables, meat and fish, from Eastern European countries. During May 1986 there was considerable jpublic confusion as individual states implemented their own foodstuff control programmes involving banning marketing of produce, precautionary stabling of livestock, etc. The confusion was especially rife in border areas where cross-border trade flows were disrupted by national intervention differences. On 30 May a Council Regulation laid down restrictions on the specific activity of milk, cheese and other foodstuffs imported into the Community, of 370 Bq/kg for milk, rising to 600 Bq/kg for cheese and other foodstuffs. This regulation was intended to deal specifically with foodstuff contamination by Cs-134 and Cs-137 and was stricter than recommendations to the European Commission by a Group of Experts set up under the provisions of the Euratom Treaty (the ‘Article 31 Group’) (see Table 8.3). This was because the Council felt it should harmonise its regulations with those of countries outside the Community such as the USA, which had set similar limits. By agreement, member states accepted that trade in foodstuffs within the Community would abide by the same levels and that no state would set lower national levels. These provisions were successively extended to the end of February 1987 when the Commission proposed they be continued for a further year in the light of expectations that 1987 harvests and foods traded in 1987 from stored products of the 1986 harvest might require intervention. The UK, together with France, initially objected to this proposal but, in the absence

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of agreement on permanent intervention levels, finally accepted their continuation to the end of October 1987. The ‘Article 31 Group’ had meanwhile been working on proposals for permanent intervention levels to apply to commodities traded within and across the Community border. It recommended identification of two reference levels for radiation exposure protection—a lower level below which action is unlikely to be justified and an upper level at which it is ‘almost certain’ that action should be attempted. For the first year after an incident, the Group recommended adoption of ICRP lower and upper limits of 5 mSv and 50 mSv and levels of 10 mSv and 10 mSv for subsequent years. These subsequent year figures were proposed because after one year ‘there will have been time to organise effective and economic means of control should these still be necessary’.12 The Group noted the problems in translating these radiation protection limits into intervention levels based on the specific activity of foodstuffs but in September 1986 recommended new limits, listed in Table 8.3. The Group proposed intervention limits for I and Sr isotopic contamination and for Pu, as well as Cs. These proposals did not find favour with the Commission which discussed internally its own recommendations in autumn 1986 (see Table 8.3). The Commission never came forward with specific recommendations to the Council of Ministers’ meeting in early 1987. Instead, it decided to organise an international scientific discussion meeting on the subject in April 1987. At the same time, the ‘Article 31 Expert Group’ reconsidered their recommendations and came forward with simplified limits. The Commission decided that even these were pitched at an unacceptably high level and advanced in support the disagreement on appropriate intervention levels which had become apparent at the April 1987 scientific discussion meeting. In May 1987 the Commission came forward with its own limits to be submitted to the Council of Ministers in June 1987. These are also listed in Table 8.3 and are considerably more stringent for Cs-134 and Cs-137 contamination than the recommendations of the ‘Article 31 Expert Group’. The Commission justified its proposals on the ground that these isotopes are long-lived, on the previously mentioned existence of stricter limits outside the Community (which raised the possibility of continuing disruption of trade flows), and on the ground that ‘the levels

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Table 8.3 Development of European Community Intervention Limits for foodstuffs in the wake of the Chernobyl accident

—No intervention limit set. COM (87) 28, Appendix 1. c For later determination. Source: Commission of the European Communities.11–13 a b

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chosen should command public confidence’.13 Some idea of the range of limits in other countries can be gauged from Table 8.2. 8.5.2 Community activity in other areas The Commission has also been developing, postChernobyl, a draft directive on rapid exchange of information on nuclear incidents or unusually high levels of radioactivity occurring within member states. This would be integrated with Community initiatives in the foodstuffs area while being compatible with the wider IAEA proposals. The Commission has been working in this area partly in response to a request from the European Parliament to ensure that there are common standards in the various bilateral information exchange agreements between member states. However, at May 1987, progress on this proposal seemed uncertain because of doubts over its legal basis and concern expressed by member states about revelation of confidential information. The Commission has also had preliminary discussions on the desirability of introducing emission standards on nuclear plant during routine operation under the 4th Environmental Action Programme and with the apparent intention of harmonising policy with Commission policy on control of emissions from fossilfuelled plants. However, it seems unlikely that a proposal for such standards will be made by the Commission because of the difficulties of reconciling emission standards with the dynamic control levels implied by the As Low As Reasonably Achievable (ALARA) concept, which lays itself open to varying interpretations by different member states. Prior to the Chernobyl accident, the European Commission had never favoured proposals, originating from the European Parliament, for a Community level Nuclear Inspection Force to complement national inspectorates. However, the accident encouraged a reevaluation of this proposal, especially its possible contribution to harmonising ALARA principles. It is probable that some member states would resist a Commission initiative in this area as strongly as they have in other areas, as outlined above. 8.5.3 Conclusion The effectiveness of European Community activity in the nuclear incident field can perhaps best be summarised in the words of the Commission itself:

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‘The transfrontier impact of nuclear installations has been too readily dismissed in the past and it is now necessary to reinforce environmental monitoring and to provide the possibility of independent verification. Chernobyl has revealed the Community’s inability to respond in a coordinated way to a major accident…’11 The action taken over foodstuff intervention levels was taken as much in an attempt to prevent economic chaos from the disruption of markets as it was to establish human health protection. Concerted action over intervention levels has fallen foul of uncertainty about dose-response patterns at low levels of radiation exposure and the different ‘philosophies’ adopted in member countries about how to handle environmental pollution, in particular whether to take immediate action on a ‘reasonable’ basis of presumption about effects or to delay action until ‘certainty’ has been scientifically established. The adoption of one or other of these ‘philosophies’ could have profound implications for international disagreements in other areas of pollution control, and for this reason the impasse in framing a concerted response to nuclear contamination is likely to continue. 8.6 UK PREPAREDNESS FOR A NUCLEAR INCIDENT OCCURRING OUTSIDE THE UK In response to generally expressed concern in the UK, including Parliamentary questioning, about the consequences of the Chernobyl accident, two review studies were set up involving the Departments of the Environment and Energy and the Cabinet Office. The two reviews were of national responses to events originating both within and outside the UK and drew upon the expertise and responsibilities of a wide range of agencies, including the Department of Health and Social Security, the Department of Trade and Industry, the Ministry of Agriculture, Fisheries and Food, the Foreign and Commonwealth Office, the Ministry of Defence, the Department of Transport, the National Radiological Protection Board (NRPB) and the Nuclear Installations Inspectorate. A review of emergency response procedures to events originating within the UK had earlier been carried out after the 1979 accident at Three Mile Island. As a result of this earlier review, it was felt that response procedures for UK domestic incidents were adequate but some of the specific details known about the Chernobyl accident

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were considered for their relevance to UK circumstances. Details of emergency procedures at UK reactor sites were given in Section 5. A Prime Ministerial Parliamentary statement made on 18 December 1986 acknowledged a need to improve arrangements for responses to events outside the UK. It was also stated that the lead role in coordination of responses to all nuclear incidents occurring outside the UK would be with the Department of the Environment, whereas the Department of Energy is responsible for domestic civil incidents (the Scottish and Northern Ireland Offices have responsibilities within their respective countries). On the completion of the review, which was coordinated by the Cabinet Office as a special application of its more general civil emergency responsibilities, a further statement was promised. By the time of the anniversary of the Chernobyl accident at the end of April 1987, some concern was expressed in the press that this statement had yet to be made. A List of possible sites Aberporth (Met) Aldergrove (Met) Aldermaston (MOD) Amersham (AI) Aultbea (Met) Aviemore (Met) Ballinrees (DENI) Benbecula (Met) Berkeley (CEGB) Binbrook (Met) Boscombe Down (Met) Boulmer (Met) Bradwell (CEGB) Brawdy (Met) Brize Norton (Met) Capenhurst (BNF) Cardiff (AI) Carlisle (Met) Castle Archdale (DANI) Chapelcross (BNF) Chilton (NRPB) Coltishall (Met) Coningsby (Met) Culdrose (MOD) Dalcross (Met) Devonport (MOD) Dounreay (UKAEA) Dungeness (CEGB) Dyce (Met)

Elmdon (Met) Eskdalemuir (Met) Exeter (Met) Faslane (MOD) Finningley (Met) Fort William (Met) Gatwick (Met) Glasgow (NRPB) Gravesend (CEGB) Hartlepool (CEGB) Harwell (UKAEA) Heathrow (Met) Herstmonceux (Met) Heysham (CEGB) Hinkley (CEGB) Hunterston (SSEB) Jersey (Met, Jersey) Kirkwall (Met) Lake Vyrnwy (Met) Leeds (NRPB) Leeming (Met) Lerwick (Met) Leuchars (Met) Llandovery (Met) Lochinver (DAFS) London (DOE) Lossiemouth (Met) Lowestoft (MAFF) Machrihanish (Met) Mallaig (DAFS) Manchester (Met)

statement was finally made by the Prime Minister on 30 June 1987 announcing that the coordination review was complete, that individual implementing agencies were preparing detailed plans, and that when these were in place the adequacy of the system would be tested by a series of exercises. The full details of the proposed Radioactive Incident Monitoring Network (RIMNET) were announced on 21 January 1988 (see below). Of course, once the UK nuclear emergency system has become aware of an external event, response to it will be similar to that to an event occurring within the UK, except that the incidence of potential contamination will not be arising from a point source within the control of the UK emergency services. For this reason, the most important part of the review is that concerned with ensuring that the UK has an adequate monitoring system for radioactive contamination which is not solely focused on the possibility of this contamination arising from a source within the UK, although UK monitoring will provide Manston (Met) Marham (Met) Montrose (DAFS) Mull of Galloway (Met) Mumbles Head (Met) Newcastle (Met) Oban (Met) Oldbury (CEGB) Portsmouth (MOD) Ronaldsway (Met, IOM) Rosyth (MOD) Sellafield (BNF) Shawbury (Met) Silent Valley (DENI) Shobdon (Met) Sizewell (CEGB) St Mawgan (Met) Stansted (Met) Stornaway (Met) Springfields (BNF) Tiree (Met) Torness (SSEB) Trawsfynydd (CEGB) Tummel Bridge (Met) Valley (Met) Watnall (Met) Weybridge (MAFF) Wick (Met) Winfrith (UKAEA) Wylfa (CEGB) Wyton (Met)

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Fig. 8.1. Initial proposals for RIMNET monitoring sites (subject to confirmation). (Reproduced, with permission, from The National Response Plan and Radioactive Incident Monitoring Network (RIMNET): A Statement of Proposals, published by Her Majesty’s Inspectorate of Pollution, Department of the Environment, January 1988.)

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an alert to an event only after its radioactive consequences are detected in the UK. Despite the provisions of bilateral and multilateral agreements on early notification of nuclear incidents, such as those discussed elsewhere in this Section, it is intended that the monitoring system will give the earliest warning of an incident independently of the receipt of any information through such agreements. The system is based on coordination and enhancement of about 80 existing monitoring services maintained not only by nuclear reactor operators but also by additional central government departments such as the Ministry of Agriculture, Fisheries and Food and the Ministry of Defence and other agencies such as the NRPB and British Nuclear Fuels plc. The location of the proposed sites is shown in Fig. 8.1, although the final set of sites still has to be chosen. The main monitoring activity at each site will be continuous gamma-ray dose rate measurement, but radioactivity in air, rainwater and primary water supplies will also be monitored. Additional monitoring will follow the detection of an incident, such as crop, foodstuff and livestock screening, and supplementary portable devices will also be available. Data from all the monitoring sites will be transmitted to a Central Database Facility (CDF) at the Department of the Environment in London, on which will also be stored assessment of the data and advice based upon them. There will also be a back-up facility elsewhere. Continuous radiation monitoring systems in the UK have hitherto been concentrated in the immediate vicinity of UK nuclear sites. These are not necessarily located to give a complete coverage providing notification of radiation reaching the UK from outside. A central concern of the Monitoring and Assessment Network Group created as part of the review was therefore to establish whether any significant ‘gaps’ existed in the monitoring sites, especially along the south and east coasts. Radiation incidents from marine reactor sources could of course impinge on the UK from any direction, while the complex meteorological patterns which were well illustrated by the Chernobyl accident could mean that the first impact of a nuclear incident on the continent of Europe would not strike the UK from a ‘crow’s flight’ direction. Also, Chernobyl showed that nuclear incidents could have impacts over distances equivalent to those separating the UK from reactors in the eastern USA and Canada. However, the main focus of concern is likely to be the UK’s southern and eastern coasts. There

The Chernobyl accident and its implications

is, for example, a distance of over 200 km between the Dungeness and Winfrith reactors and over 100km between Dungeness and Bradwell. These gaps are proposed to be filled by monitoring stations at Portsmouth, Herstmonceux (East Sussex), Manston (Kent) and Gravesend. There is also a monitoring station in Jersey. The Chernobyl accident also showed up some problems within the UK in the rapid dissemination of accurate and consistent information from the various sources which the public and emergency services would need to access to determine their response to an incident. Again, in principle the requirements are identical whether the incident has originated within or outside the UK. However, it is probable that even if the various provisions for early notification should work perfectly, the level and quality of information on an ‘overseas’ event would emphasise the need for accurate advice to public and specialist services. The government proposals are for an Information Centre to draw on assessments of the data in the CDF made by a group of technical experts assembled into a Technical Coordination Centre. This assessment and any plan of action will be provided by the existing staffing of the Department of the Environment, drawing in particular on the expertise of Her Majesty’s Inspectorate of Pollution, enhanced by specific expertise from other agencies such as the NRPB. The entire emphasis of the response plan is on minimising the need for additional agencies and expenditures. Various recent developments in televisual and telecommunication technology, such as teletext broadcasts, will be used to provide links between those requiring information and the CDF. Their effectiveness will depend on the number of households able to gain direct information from such sources. Only 19% of UK households currently have teletext receivers. The proposals are aimed at reducing the demands for information made by the general public on individual government departments, as occurred in the days after the Chernobyl accident. Whether this would be fulfilled could only be tested by experience of a real event. It is not proposed to incorporate local authorities into the data collection and information dissemination system. This has led to some criticism of the proposals by organisations such as the Association of Metropolitan Authorities which has proposed a local authorities’ Radiation Collation Centre which would act independently of the central government Central Database Facility, Technical Coordination Centre and Information Centre. The local government agencies

International dimensions of Chernobyl implications for UK

are concerned that public requests for information are often directed at them rather than central government and also that the RIMNET scheme would not have the flexibility to respond to localised incidents in the way that a local authority based system could. There has also been criticism of the proposed scheme from independent sources such as the National Society for Clean Air, who have argued that the public has a low level of confidence in central government statements about risk in such circumstances and that the monitoring, assessment and information dissemination activities would be better carried out independently by the National Radiological Protection Board. 8.6.1 The UK’s nuclear relations with neighbouring countries The UK has a number of bilateral agreements with neighbouring countries on the sharing of information and logistical help in the event of a nuclear incident. Of these, the most important is that with France, given that 12 French PWR nuclear establishments are located along the Channel coast (Flamanville 1 and 2, Paluel 1–4 and Gravelines 1–6), with two others (Penly 1 and 2) under construction. The total megawattage of these stations surpasses that of all civil nuclear stations in the UK. There are also nuclear stations in Belgium (Doel 1–4, 2700 MW(e)) and the Netherlands (Borssele, 450 MW(e)), in close proximity to the Channel coast. The Anglo-French agreement is based on an exchange of notes in 1983 14 concerning civil emergencies in either state which could have radiological consequences for the other and is solely concerned with exchange of information. It sets out the agencies and mechanisms through which information will be provided and details the information to be transmitted, including the date, time, place and nature of the occurrence, the character and quantity of the emissions, and meteorological and hydrological data which will aid forecasting of the dispersion pattern of the emissions. Anglo-Irish relations have also had a nuclear power component to them. These have mainly focused on the question of radioactive discharges to the Irish Sea from the Sellafield reprocessing plant and therefore do not relate to operational reactor safety. However, the Irish government also made representations to the UK government about the decision to proceed with

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the Sizewell ‘B’ PWR on the grounds that this presaged plans to locate further nuclear power stations in closer proximity to the Irish Republic. In fact the CEGB announced plans for a PWR at Hinkley Point in August 1987. Concern was also expressed about the continued operation of Magnox reactors. In February 1987 the Commission of the European Communities published an Opinion on the operation of the Heysham 2 reactor. This included a recommendation that discussions take place ‘as a matter of urgency’ between the two governments about arrangements in the event of an accident at a UK nuclear installation. There is an Ireland/UK Contact Group on Nuclear Matters, through which the Irish Republic is informed of all nuclear incidents in the UK which are reported to the Department of the Environment. The UK, Belgium, France, Ireland and the Netherlands are all signatories of the IAEA Conventions on Early Notification and Mutual Assistance, as well as being members of the European Economic Community. However, prior to the Chernobyl accident, several European countries with land borders, such as the Netherlands, Belgium and the Federal Republic of Germany, had set up bilateral committees to handle specifically the question of location of nuclear facilities at sites adjacent to each other’s territories. The UK’s sea boundaries have probably led to such arrangements being perceived as less important for the UK, but the consequences of the Chernobyl accident could be argued to suggest that public demonstration of the existence of formal, continuing contact between the UK and its nuclear and non-nuclear neighbours may be advantageous. 8.7 SENSITIVITY OF COUNTRIES’ NUCLEAR POWER PLANS TO THE CHERNOBYL ACCIDENT Several countries, including the UK, were at critical points in the development of their nuclear power programmes at the time of the Chernobyl accident. In the Netherlands, after a protracted national debate on national energy policy and in particular nuclear power, the government was preparing to introduce, against strong opposition, a plan for two new reactors. In response to the accident, this plan was postponed indefinitely, until a review of the significance of Chernobyl for the Dutch proposals was completed. In Sweden, where an earlier national referendum

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had supported a nuclear power phase-out by 2010, there had been some discussion of the validity of this policy. In response to the Chernobyl accident, a review of all aspects of nuclear power policy was initiated, including in particular the possibility of an early closure of the Barsebaeck plant, located about 20 km east of Copenhagen, and more generally a virtually immediate phase-out, and an accelerated phase-out by 1997, of all nuclear plants.15 After receipt of this report the Swedish government reconfirmed its commitment to the phase-out programme. In Italy, divisions over nuclear power undermined the formation of political coalitions throughout 1986. As the only way of resolving the intensification of this dispute following the Chernobyl accident, the government embarked on a large public and professional opinion-gathering exercise prior to a ‘national conference’ on energy in February 1987. There is likely to be a moratorium on nuclear station construction, at least in the short term. In Austria, the Chernobyl accident finally sealed the fate of the single nuclear reactor at Zwentendorf, which had never been put into operation. A decision

was taken to dismantle the plant. A major inquiry into the consequences of Chernobyl was begun, which reported in October 1986.16 In Switzerland, a dispute between the federal and cantonal governments over the future of nuclear power, already in existence before the Chernobyl accident, continued with renewed intensity. A study group was set up to advise the federal government on the feasibility of phasing out nuclear power in Switzerland, while in mid-1987 the cantonal government of Bern voted to phase out nuclear power and cancelled a planned reactor. Nuclear power plants were also cancelled or postponed in countries such as Yugoslavia and the Philippines. However, in countries outside Europe, in particular the USA and Japan, Chernobyl had comparatively little impact on the context of nuclear power planning. The design differences between the Soviet RBMK and the reactor types deployed and planned in these countries seem to have been sufficient to allay public opinion to the extent that governments did not feel under pressure to review their existing programmes. A full analysis of the possible impact of

Table 8.4 Changing attitudes to nuclear power in Britain

N/A=Not asked. Source: Annual questionnaire surveys of British Social Attitudes Survey, Social and Community Planning Research.18 a

International dimensions of Chernobyl implications for UK

the Chernobyl accident on the future development of nuclear power outside the Eastern Bloc countries is given in a report by Evans and Bullen.17 The accident undoubtedly had an immediate negative impact on public attitudes to nuclear power in the UK. This is probably best illustrated by the results of the Social and Community Planning Research annual ‘British Social Attitudes’ survey. 18 By a coincidence, the 1986 field interviews, which included questions on attitudes to nuclear power, were being conducted at the time of the Chernobyl accident. Some main results from the survey are given in Table 8.4. There is some evidence to suggest that public opinion was becoming less favourable to nuclear power in the UK even before the accident and the figures undoubtedly show a peak effect due to the timing of the field interviews. The Chernobyl accident occurred after the end of the Public Inquiry into the proposals for construction of the country’s first pressurised water reactor at Sizewell in Suffolk but before the Inquiry Inspector had submitted his report to the Secretary of State for Energy. After 26 April there were calls for the Inspector to take into account the consequences of Chernobyl in framing his report, which went to the Energy Department in December 1986, but for legal reasons he was not able to do so without reopening the Public Inquiry. These calls continued up to the time of the Parliamentary debate on the Sizewell PWR proposal on 23 February 1987. In introducing this debate, the Secretary of State for Energy made a brief statement which made reference to the differences in design between the Soviet RBMK and the proposed PWR station at Sizewell ‘B’. Opposition speakers placed particular emphasis on the need to recalculate estimates of the frequency of incidents at nuclear plant of all types, extrapolating from allegedly erroneous estimates made for the RBMK reactor by the Soviet authorities.19 The Parliamentary debate on Sizewell ‘B’ was followed by a ministerial statement on 12 March 1987, giving approval to the construction of the Sizewell ‘B’ reactor. In reaching his decision the Secretary of State for Energy was able to take into consideration the implications of the Chernobyl accident and reported that he had specifically sought the advice of the Nuclear Installations Inspectorate on this matter. He reported that he had been advised that the accident did not require any reconsideration of the conclusions of the Sizewell ‘B’ Inquiry Inspector because UK nuclear power stations had engineered control systems

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lacking in the Soviet-designed systems and because there was ‘a superior safety culture’ in the UK.20 8.8 CONCLUSIONS The accident at the Chernobyl No. 4 reactor was undoubtedly the most serious single international environmental pollution incident that has occurred in the world to date, although it could be argued that the continuous emission of acidic gases from fossil fuel combustion and other sources constitutes a greater overall international environmental problem. This review has shown that the international mechanisms for handling the accident and its consequences were deficient and were hampered by a range of factors. Above all, the political divisions between East and West, resulting in the absence of any participation by Eastern Bloc countries in international liability compensation arrangements (in themselves currently inadequate), and the confusions over differing national foodstuff intervention standards prevented effective resolution of the secondary and longer term consequences of the accident. The conclusion of the early notification and mutual assistance agreements of the IAEA is an obvious advance on the pre-existing situation, and it is likely that the international dispersion monitoring and modelling system is now more developed as a result of the increased attention devoted to it. However, many of the international agreements necessary for an effective incident control and response system are still wanting nearly two years after the accident at Chernobyl. It remains a fact that even if these were to be put into place in the near future, the efficacy of international arrangements for nuclear incidents would only be established when tested by occurrence of a real world incident. ACKNOWLEDGEMENTS The author would like to acknowledge the assistance of the following organisations and individuals: Dr Hans Blix and colleagues, International Atomic Energy Agency, Vienna. Mr P. Reyners, Nuclear Energy Agency, Paris. Mr A.Brenton, DG XI, European Commission, Brussels. HE the Austrian Ambassador for the United Kingdom. Dr W.Hoffman, Science Counsellor, West German Embassy, London. Mrs A.Dismorr, Swedish Embassy, London. Mr P.Sands, Law Department, Cambridge University. Brig.Budd, Cabinet Office. Staff of the UKAEA library.

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REFERENCES 12. 1. International Atomic Energy Agency, One Year After Chernobyl Vienna, June 1987. 2. International Atomic Energy Agency, IAEA Newsbriefs, 2, 3, 5 March 1987. 3. International Atomic Energy Agency, Final Document Resolutions and conventions adopted by the first special session of the General Conference, GC(SPL.1)/ RESOLUTIONS, Vienna, 24–26 September 1986. 4. SANDS, P., The Chernobyl accident and public international law, paper presented at a conference on International Disaster Management with Special Reference to Chernobyl and Three Mile Island, sponsored by the Annenberg School of Communication, University of Pennsylvania and the University of Southern California, Washington, DC, October 1986. 5. SHAPAR, H.K. & REYNERS, P., The nuclear third party liability regime in western Europe: the test of Chernobyl, paper presented at the Atomic Information Forum conference on Nuclear Insurance and the Indemnity Issue, San Diego, California, February 1987. 6. Food and Agriculture Organisation, Recommended limits for radionuclide contamination of foods, ESN/MISC/87/ 1, Rome, 1987. 7. International Atomic Energy Agency, The question of international liability for damage arising from a nuclear accident (GOV/INF/509), Vienna, January 1987. 8. After Chernobyl things are different, Austria Today, 3, 1986, Vienna. 9. FoE (Friends of the Earth) International, April 1987. 10. Nuclear Energy Agency, Symposium on International Cooperation in the Nuclear Field: perspectives and prospects, Paris, OECD, 1978. 11. Commission of the European Communities, The Development of Community Measures for the Application of Chapter III of the Euratom Treaty ‘Health and Safety’,

13.

14.

15.

16. 17. 18. 19. 20.

Commission Communication to the Council, COM(86)434, Brussels, 20 August 1986. Commission of the European Communities, Communication from the Commission to the Council on a permanent system for establishing limits for the radioactive contamination of drinking water and agricultural products in the case of a nuclear accident, COM(87)28, Brussels, 23 January 1987. Commission of the European Communities, Proposal for a Council Regulation (Euratom) laying down maximum permitted radioactivity levels for foodstuffs, feedingstuffs and drinking water in the case of abnormal levels of radioactivity or of a nuclear incident, COM(87) 281, Brussels, 16 June 1987. HMSO, Exchange of Notes between the Government of the United Kingdom of Great Britain and Northern Ireland and the Government of the French Republic concerning Exchanges of Information in the Event of Emergencies Occurring in One of the Two States which Could Have Radiological Consequences for the Other State, London, October 1983. Ministry of Industry, After Chernobyl: Consequences for Energy Policy, Nuclear Safety, Radiological Protection and Environmental Protection, Report of the Expert Group for Nuclear Safety and the Environment, Stockholm, Sweden, 1987. Umweltbundesamt, Tschernobyl und die Folgen fuer Oesterreich, Bundesministerium fuer Gesundheit und Umweltschutz, Vienna, November 1986. EVANS, N. & BULLEN, W., Nuclear Power in the Western World—post Chernobyl, Cambridge Energy Research, Cambridge, UK, 1987. Social and Community Planning Research, British Social Attitudes Survey, The 1987 Report, Gower, London, 1987. Sizewell report debated in Commons, Atom, 366, 20–36, April 1987. Ministerial statement: Sizewell B Nuclear power station, Atom, 367, 36–38, May 1987.

Section 9

Comments, Recommendations and Conclusions This Section summarises the background to the Chernobyl accident, the accident itself, its consequences and implications, steps taken in the USSR following the event, and the recommendations of the International Atomic Energy Agency. Finally, it gives the Watt Committee Working Group’s recommendations. 9.1 BACKGROUND TO THE CHERNOBYL ACCIDENT The first nuclear power reactor in the USSR was a graphite moderated pressure tube design cooled by pressurised water. It started generating electricity in 1954. The RBMK system was developed from this. It has the basic advantages of large scale manufacture without the need for advanced fabrication facilities and, without major development, expansion to large outputs. Currently 1500 MW(e) RBMK reactors are in operation and under construction using a reactor the same size as the Chernobyl reactors which have outputs of 1000 MW(e). Limitations in the Soviet manufacturing capacity for specialist equipment such as electronics and control, and the lack of a comprehensive quality assurance system, linked to the pressure to design and build plant that could be manufactured, constructed and commissioned quickly, led to a design which, in both concept and detail, has features which impaired ultimate plant safety. Among the features of the design which appear to be questionable, some of which contributed to the initiation and development of the accident at the Chernobyl 4 reactor, the following merit noting:

(2)

(3)

(4)

(5)

(6) (7)

(1) The reactor was considerably over-moderated to allow the use of low enrichment levels for the fuel elements. The water in the fuel channels

(8)

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therefore acts predominantly as an absorber and displacement by steam from evaporation can lead to increases in reactivity (i.e. positive void coefficient). Under normal conditions, neutron capture in absorbers in the core and in the fuel, and the Doppler effect, more than compensate for this. However, the Doppler effect depends on the fuel temperature and at low operating loads, where fuel temperatures are also low, this effect is small and the positive void coefficient predominates. When combined with the withdrawal of absorber rods from the core, this can lead to an unstable condition where an increase in steam production leads to an increase in heat generation, increasing steam production further and escalating heat output, i.e. a rapidly acting positive power coefficient. The rate of movement of the control rods, even when tripped to shut down the reactor quickly, was slow—0·4 m/s with a travel of 6·25 m. There were no ‘stops’ to prevent the operators withdrawing control rods to their full extent, but there were station rules which were intended to prevent this. The details of control rod design with an absorber and graphite ‘follower’ can, under some conditions, lead to an increase in reactivity as the rods enter the core. This is known as ‘positive scram’. Graphite core cooling was by conduction across tiles in direct contact with the pressure tubes which contain pressurised boiling water. The automatic reactor trips could apparently be easily deactivated by the operators. Instrumentation and alarms to indicate unsafe operation, or operation outside laid down limits, were either inadequate or ignored. Start-up of a standby diesel generator was slow and a delay of between 25 and 50 seconds has

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been quoted as typical for Soviet equipment. With a quicker response, the test which led to the accident would probably have been unnecessary. (9) The containment system was designed for a single channel or pipe failure and therefore was inadequate for a situation when many channels failed. (10) There was no independent secondary shutdown system. It is notable, however, that the reactor accident occurred not in normal operation but as a result of action taken to carry out an experiment. 9.2 EVENTS LEADING TO THE ACCIDENT The reactor was in the course of being shut down for scheduled maintenance. A test was planned to check the electricity generation from the alternator during run-down after it had been tripped. The test requirement had been specified by the Power Ministry in Moscow as a ‘Working Programme for Experiments on Turbo-generator No. 8 of the Chernobyl Nuclear Plant’. Similar tests had been carried out at Chernobyl and another RBMK reactor some years before without mishap, but the alternator run-down generating characteristic was unsatisfactory. Modifications to the turbo-generator magnetic field regulator were the subject of this experiment, and consequently the test at Chernobyl was probably under the control of a power engineer. Unlike the earlier tests, the reactor was not immediately tripped on completion because the operators felt that it was necessary to be able to repeat the test, perhaps with changes to the wiring, without the long delays associated with restarting the reactor. The schedule for the test appears to have been specified without adequate safety analysis or safety instructions. Because another opportunity would not occur for at least a year, there was pressure to complete the test, but the tests were delayed until the early hours of Friday/Saturday night, 25/26 April 1986, because the reactor was required to meet grid demands during the afternoon. However, there were station safety rules and provisions, which have not been published but have been referred to in an ‘official’ statement. The operators on this occasion appear not to have complied with some of these, but it is not clear whether some of these breaches of the safety rules were necessary to carry out the test.

The Chernobyl accident and its implications

9.3 BREACHES OF SAFETY RULES Because of the delay in starting the test, owing to the grid requirements, the test was carried out when core reactivity was low due to xenon poisoning. The low reactivity as a result of this and the ‘poisoning’ due to the high water content in the channels, led to the need for withdrawal of control and absorber rods further and in greater numbers than specified in station operating instructions. A minimum steady operating load of a quarter of full power was laid down for RBMK reactors. The operators of the reactor setting up the plant for the turbo-alternator test operated the reactor at a thermal power as low as 30 MW(th), i.e. 1% of full power. By withdrawing more control rods, the operators managed to increase power to 200 MW(th), about 7% of full power but still only 30% of the specified minimum. In getting the reactor to a condition suitable for the test, safety provisions were removed as follows: (1) Emergency core cooling. This had been isolated earlier before the grid requirement for half power and not reinstated, but in fact this did not affect the initiation of the accident, although it might have been able to provide some short term cooling. (2) Several control rod trips were by-passed. (3) Reactor cooling pumps were in full operation, four of which were necessary to provide an electrical load during the test. This meant that the pumps were operating close to cavitation with an unstable characteristic, and the reactor channels were overcooled and contained mainly saturated water with a small quantity of steam near the outlet, requiring further control rod withdrawal. Under these conditions, a minor perturbation could lead to a rapid increase in steam generation and escalation in power output as the control rods could not compensate fast enough for the effects of water turning to steam. The operators deliberately avoided tripping the reactor, as it had been in the earlier alternator tests, when the emergency stop valve to the turbine was shut to start the test. Automatic insertion of some control rods in fact commenced. When it was realised that reactor power was rising, the operators tripped the remaining control rods, but it was then too late to save the reactor.

Comments, recommendations and conclusions

Recent calculations indicate that given the conditions in the core at the time that the rods were dropping, the initial location of the rods in their withdrawn position, and the design of the control rod assemblies involving an absorber section and a nonabsorbing graphite follower, the control rod insertions actually added reactivity and contributed to the initiation of the accident. 9.4 COURSE OF THE ACCIDENT The situation in the core was one in which a small change in the proportion of steam could lead to an increase in core reactivity which generated enough neutrons for the chain reaction to expand without waiting for the delayed neutrons—a prompt critical situation. The rapid escalation in heat generation in the reactor core reached about 100 times full power in a few seconds, leading to fuel melting and a rapid rise in pressure, shattering pressure tubes. This was the first explosion. A second explosion was reported which may have been another nuclear excursion or other parts of the pressure circuit failing, perhaps combined with a chemical explosion. The immediate damage was the overturning of the top shield, fracturing all of the coolant pressure tube connections and allowing air to come into contact with the hot fuel and graphite, leading to a graphite fire. There were two immediate fatalities from the explosion. Firefighters, and other staff at the station received high radiation doses and this proved fatal for 29 of them. They were successful in avoiding damage to Reactor no. 3 which kept running until 5a.m., 3½ hours after the accident. Reactors 1 and 2 operated for 24 hours after the accident. High radiation doses were suffered by another 200 people on site, leading to their retention in hospital. At the time of the accident, there were about 500 people on the Chernobyl site. The graphite fire and explosions spread radioactive material from the reactor over a large area. However, no-one outside the reactor site received radiation doses sufficiently high to justify their having to stay in hospital. Over several days after the accident, 135000 local people, and their livestock, were evacuated. There was concern about the contamination of water supplies, but up to May 1987, radiation levels in public supplies were below internationally acceptable limits. Artesian wells drilled quickly after

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the accident to provide an alternative safe supply have not been used. To stem the release of radioactive material from the reactor and to ensure that reactivity did not rise again to dangerous levels, sand, lead, dolomite and limestone (to release carbon dioxide to extinguish graphite fires), clay and boron compounds, were dropped from Soviet army helicopters on to the reactor core. The rate of radiation release fell but after about a week started to rise again. The core was still generating significant quantities of heat and the insulating effects of the material dropped on to the core to reduce radiation release also reduced the amount of heat removed, causing the core temperature to rise sharply to about 1900°C. Nitrogen was injected under the core and, either from the cooling circulation induced by this or from some other mechanism, after reaching a peak 3 days later, both temperature and the rate of release fell rapidly. Shielding was then built around the damaged reactor and Soviet engineers built a concrete isolating screen underneath it. Walls were built around the reactor area to avoid contamination of water supplies. The Soviet authorities appeared to be in a dilemma on how to release information about the accident and its consequences both to Soviet citizens and to the outside world. Official information in the press and on radio or television was terse and given little prominence, even in the Ukraine. The first reasonably full report was in Pravda about 10 days after the accident. Subsequently, reporting was more informative. The Soviet leader’s address on television was on 14 May, three weeks after the accident. 9.5 ACTIONS IN THE SOVIET UNION The immediate actions in addition to those related to the reactor were: (a) checks on the health of people in the neighbourhood; (b) decontamination of buildings and land around the plant; (c) checks and restrictions on the distribution and consumption of food from the contaminated area; (d) shutdown of the other Chernobyl reactors, two of which have since been restarted.

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9.6 RBMK REACTOR DECISIONS SINCE THE ACCIDENT There will be no further orders for RBMK plants and Chernobyl reactors 5 and 6, which were under construction, may not be completed. Reactors 1 and 2 at Chernobyl were operating again in autumn 1986, and Chernobyl 3 at the end of 1987. Because there are problems in the building of Soviet PWRs due to the poor rate of production of reactor pressure vessels, the repercussions for the Soviet nuclear and electricity generating capacity will be serious for several years. There is little spare generating capacity in the Soviet Union. Among other decisions taken in the Soviet Union, several appear important. Improvements will be made in respect of the safety of existing RBMK reactors. In the short term: (1) Control rod limit switches are being installed to ensure a minimum insertion of 1·2 metres. (Information released late in 1987 indicates that this minimum insertion figure has been reduced to avoid unacceptable axial flux peaking.) During the test with Chernobyl reactor 4, control rods were fully withdrawn. Calculations have indicated that, with a core overcooled with saturated water and the most reactive fuel being in the lower part of the core, the early stages of slow insertion of control rods from these extreme positions can increase rather than decrease core output. This probably contributed to the initiation of the accident. Limiting the withdrawal of control rods is therefore important. (2) Arrangements are being made to avoid the operation of the reactor below quarter power except during start-up and shutdown. (3) The equivalent of 70–80 rods will be kept within the core at all times. This can be compared with the specified operating minimum at Chernobyl of the equivalent of 30 rods in the core and the 6–8 equivalent rods at the time of the accident. This extra provision will greatly reduce the value of the void coefficient of reactivity. (4) An automatic system for computing reactivity reserve is to be provided and will trip the reactor if this is exceeded. In the longer term, fixed absorbers will be installed which will reduce the positive void coefficient, and consequently fuel enrichment will have to be increased from the present level of 2% to 2·4%. Work has been

The Chernobyl accident and its implications

initiated to study and eventually install faster shutdown control rods. Pump cavitation indicators are also being installed. In addition, operator training is being overhauled and simulators will be more widely used. There will be increased security of the reactor automatic protection against unauthorised interference by operators and improved administrative arrangements to ensure that rules and regulations will be complied with. Research and development of the reactor physics and fuel element cooling of RBMK reactors will be increased. 9.7 INTERNATIONAL CONSEQUENCES Following the meeting of international experts with Soviet engineers in August 1986, the International Atomic Energy Agency set up a safety advisory group to consider the implications for reactor design, operation and safety training. The report is one of the Safety Series, namely No. 75 INSAG-1, and contains a wide range of recommendations. Their conclusions represent the results of detailed knowledge and study by experts. Such measures, if fully implemented, would improve nuclear safety against possible reactor accidents. Those related to reactor safety are summarised as follows: (1) In the Chernobyl accident, no new physical phenomena were identified. However, nuclear power plant operators should review their safety analyses critically and conduct specific risk studies. (2) The ‘human element’ was at least in part responsible for the Chernobyl accident, and there were three recommendations aimed at those supervising and running nuclear plant: (a) training with emphasis on understanding the reactor, particularly in accident situations and employing simulators; (b) auditing to avoid complacency from routine operation; (c) awareness of the safety implications of departures from agreed procedures. Authority for plant safety needs to be given to a senior member of the operational staff. (3) Design should implement defence in depth: (a) inherent reactor stability; (b) automatic systems to operate when the safety of the plant is threatened; (c) mandatory provision of a barrier to contain most radio nuclides if the above two lines of defence fail.

Comments, recommendations and conclusions

(4) Satisfactory man-machine interfaces are important, requiring: (a) clear display to operators of essential data, tailored to ensure optimum use. Built-in diagnostic capability should be included and real-time data display and interpretation are important; (b) safety of plant should not depend primarily on operator intervention and there must be reliable automatic devices with back-up that ensure the safety of plant. Their action must be rapid enough to respond to, and control, dangerous situations, and devices must be difficult for operators to defeat or bypass. In addition, new and extended investigations have been initiated on a number of fields by both the International Atomic Energy Agency and the International Nuclear Agency. These are summarised in Appendix 4. 9.8 IMPLICATIONS FOR THE UNITED KINGDOM The levels of radiation over the UK and Western Europe have been reviewed by the National Radiological Protection Board (NRPB). In the UK, the levels were generally a few per cent above normal background levels, but rainfall led locally to considerably higher levels of radiation in rainwater and pastures. There were restrictions on the import of some foodstuffs from Eastern Europe, on drinking rainwater and marketing lamb that had been on pastures where there had been rainfall during the period when the radioactive plume was over the country (Cumbria, North Wales and southwest Scotland). Restrictions on lamb were still in force in 1988. However, there is little doubt that the way official data were gathered and information following the Chernobyl accident passed to the public, by both official ‘experts’ and government ministers in the UK, was confusing and uncoordinated. The situation in other countries does not appear to have been better. The ‘intervention’ levels used in different countries, even within the EEC, often allowed accusations of danger to the public health to have greater credence than the figures justified. There seems little doubt that the whole area of monitoring and publishing of data needs review and it is encouraging to note that discussions have started on this within the EEC.

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An aspect of the subject which has added to the confusion is the different units used in various publications. The effect on health in EEC countries was reviewed by the NRPB in a report to the European Commission and summarised in Nuclear Europe in May 1987. With natural background dose of 1000–2000 µSv per year, the average additional dose in the UK due to Chernobyl was 50 µSv. The European figures for the Chernobyl consequence ranged from 0·3 µSv in Portugal to about 500 µSv in Italy, the German Federal Republic and Greece. Of this dose, 60% occurred during the 12 months following the accident and the rest will be received over the next 10 years. Locally and individually there were higher exposures of up to about 4000 µSv or more in West Germany and 1000 µSv in parts of North Wales, Cumbria and western Scotland. Food restrictions probably halved the exposure of individuals in some countries. With a dearth of factual information and the official predictions on the effects of large nuclear accidents which had been published, the press in the West tended to publish speculative material with reports of large-scale devastation and casualties. Most of the television reporting, in the UK anyway, tended to be factual. The effect of the accident and the confusing information on the seriousness of the levels of radiation in the UK increased the support for policies phasing out nuclear power; but a year after the event, public opinion, as reported by opinion surveys, appears to have returned to the pre-Chernobyl figures. Although the Watt Committee Working Group has not been able to review in detail UK reactor design and operational safety, they were impressed by the discussions with station staff and training managers on these topics. These discussions covered topics such as the operating characteristics of UK reactors where, it was asserted, minor perturbations cannot lead to a power excursion and automatic control and safety trips are hard-wired so that they cannot be made inoperative by plant operators. The United Kingdom Atomic Energy Authority’s report on the Chernobyl accident, published by HMSO in 1987 as The Chernobyl Accident and its Consequences details many ways in which Soviet design, operation and safety standards would not meet Nuclear Installations Inspectorate requirements in the UK. The Electricity Boards and the UKAEA are convinced that UK designs and safety standards are as safe as practicable and that an accident approaching the severity of Chernobyl (or Three Mile

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Island) is almost impossible under UK conditions. The Working Group found their arguments and reports convincing. One aspect of plant safety in the UK is the diversity of plant in operation. Most of the UK’s operating reactors have no counterparts elsewhere, and differences between individual station designs made direct application of some of the safety studies between stations and transfers of staff more difficult than in countries, including the Soviet Union, where there is greater replication. With PWR plants, experience from many similar power stations abroad will be available. For safety, replication of designs is obviously desirable and should receive a high rating in evaluation of improvements and design changes. 9.9 RECOMMENDATIONS The following recommendations may, in some cases, be the subject of actions already taken or in progress, as the Watt Committee on Energy has no means of checking all the activities of official bodies in the United Kingdom or Europe. (1) Both the International Atomic Energy Agency and Nuclear Energy Agency have published programmes and recommendations which are given in the Appendices. The Watt Committee Working Group endorses these initiatives. In particular, the IAEA recommendations, summarised in this section of the report, appear to cover the most important areas of design, operation and training for nuclear safety. While it is the Working Group’s impression that UK designs and practice follow these recommendations, it is important that a formal report should be prepared by the Electricity Boards or the Nuclear Installations Inspectorate dealing with the points and detailing how UK practice measures up to these standards, and if there are any areas of doubt as to what is being done, how to improve the situation. This report should be published. (2) The international programmes listed by the International Atomic Energy Agency and the Nuclear Energy Agency should receive full support and financing. The United Kingdom should play a leading role in paying for and providing technical support and expertise for these programmes, and for stimulating support from other countries. If

The Chernobyl accident and its implications

extra funding is required, the United Kingdom should meet its share. The United Kingdom should also arrange to implement agreements and support international bodies that stem from these investigations. Where there is obvious overlap between UK and international activities, consideration should be given to linking national bodies with the international organisation. Publication and public discussion of the conclusions of the investigations by the International Atomic Energy Agency and the Nuclear Energy Agency should be encouraged. (3) There are emergency plans which are regularly rehearsed with the involvement of local organisations and these appear to the Working Group to be adequate. An accident involving widespread release of radiation from a nuclear plant in the UK or a neighbouring country is extremely unlikely. There does, however, appear to be a case for a central plan to ensure that all necessary actions are covered and coordinated. Such a plan could be extended to cover industrial and chemical installations where there could be a widespread public hazard. Particularly important is the release of accurate and consistent information which is in a form that is useful to the public. To manage this requires a formal organisational structure with a senior Minister in charge to ensure that all instructions to government departments, police, military and local government come from a single source. It is understood that some proposals along these lines are being prepared, and these are necessary regardless of the future of nuclear power in the UK since they would be invoked in the event of an accident to a French plant or to one elsewhere in Europe. (4) Each nuclear establishment has extensive fire fighting equipment and its own fire fighting personnel and carries out exercises with local fire stations. However, the Chernobyl accident demonstrated, in the extreme form, the hazards to plant and personnel that can arise. The task at Chernobyl was made hazardous by the high levels of radiation involved and the need to prevent the fire spreading to Reactor 3. In view of the casualties among fire fighters at Chernobyl, a review of the situations leading to reactor fires and the scope for improved protective clothing and shielding and the use of remote fire fighting methods, perhaps with robotics, can be justified.

Comments, recommendations and conclusions

Some of these developments could also have application in non-nuclear plant accidents. (5) The Nuclear Installations Inspectorate is understaffed, although it is understood that a recently improved salary structure and a recruiting drive is producing results. It is essential that the inspectors should be highly qualified and receive comprehensive training with retraining as necessary. In the future it is unacceptable, in the opinion of the Working Group, that the Inspectorate should be allowed to fall below strength, so that safety reviews are delayed or not carried out in depth. A build-up in the strength of the Nuclear Inspectorate is particularly important for adequate coverage of a PWR programme, reappraisal and decommissioning of Magnox plant, and the implications of the privatisation of the electricity industry, involving an emphasis on profitability. (The decommissioning of older stations, the Magnox stations, has to be funded.) Safety considerations do not suggest that privatisation necessarily affects the organisation of nuclear power, since countries such as the Federal Republic of Germany and Japan have successful nuclear power programmes. Staffing shortages have led in some cases to long term attachments of inspectors to individual nuclear stations, but it is not obvious to the Working Group that this is consistent with achieving the highest practicable safety standards. (6) In ‘efficiency’ improvements and economy reviews in the Central Electricity Generating Board and the South of Scotland Electricity Board (or their successors), the United Kingdom Atomic Energy Authority and British Nuclear Fuels, safety and training activities should not be subjected to budget and manpower cuts that could impair their effectiveness. The ‘in house’ organisations responsible for these have a key role to play in maintaining the safe operation of UK nuclear plant. The proposed privatisation must secure proper funding for the timely decommissioning of old nuclear stations. (7) Instrumentation, control and automation are fields where developments are rapid. While it is appropriate that plant simulators reproducing the systems at individual stations are used mainly for training and safety exercises, it is important to use

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them also to develop and test new ideas in control and operation and, of great importance, operatorplant interaction. Manufacturers could use the simulator to demonstrate new equipment where this is practicable. (8) Universities can have a major role to play in research and in providing independent views on nuclear issues. The difficulties of specialist departments of Nuclear Engineering are considerable, e.g. Queen Mary College, London, is closing its nuclear department and several years ago ceased operating its research reactor. The utilities and UKAEA should help build up independent expertise in universities and consider including nuclear training in the job specifications for key appointments. The availability of independent and expert opinion based on universities, which operate outside the large institutions, can be useful in providing balanced and acceptable information on nuclear topics for the media and public. University research in safety studies, plant-operator interaction, chemical interactions, plant modelling and other nuclear fields should be generously sponsored. Nuclear experts from the industry should be encouraged to provide specialist lectures to graduates and undergraduates as well as, perhaps, to schools. Areas where university as well as government research and cooperation between teams in the UK and overseas should be encouraged include: (a) deposition of significant isotopes from atmosphere to soils and vegetation, especially the effect of precipitation; (b) take-up of radioactive elements into food chains and run-off into water courses; the effect of these on the human body; (c) decontamination procedures—soil and water; (d) low level radiation research to provide information for setting internationally acceptable intervention levels in human and animal foodstuffs. (e) The rates of chemical reactions, and heat generation, between the materials of reactor construction at high temperatures (see Appendix 5). (9) Close collaboration should be established and built up with overseas plant operators in neighbouring countries—France, Belgium, the Netherlands and

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the Federal Republic of Germany as well as the USA, Canada, Sweden and the Eastern Bloc—to provide: (a) exchanges of information on nuclear plant incidents and remedial measures; (b) establishment of clearly defined links to provided rapid transfer of information in the event of a serious leakage of radiation which could require action by the other country; (c) frequent exchanges between experts on nuclear operational, design and safety matters. Some of these objectives can probably be achieved through existing international committees, working parties and other arrangements of this kind, but the details of the arrangements should be published and public discussion encouraged. (10) For some years there have been ‘paper’ studies for ‘safe’ designs of small water, gas and sodiumcooled reactors. A careful study and economic comparison should be made of these new generation small reactor designs with the Pressurised Water Reactor (PWR). If, in fact, they have inherent safety features, these could lead to a simplification of safety arrangements, more straightforward operational procedures, and greater public confidence in safe nuclear power. It is not obvious that these reactors would be uneconomic. There may be scope for international collaboration in this reactor assessment through the IAEA or NEA. This should be investigated. (11) There are British-designed Magnox reactors in Italy (Latina) and Japan (Tokai Mura). Exchange of information on safety provisions and operator training—perhaps with staff attachments and exchanges, including the safety training centres— can be beneficial to all of the operators. There may also be benefit in exchanges of this type between the SGHWR and the Japanese Fugen plant which is similar. Exchanges with the Soviets and East Europeans on PWR operation, safety and training could be useful. Some technical exchanges already occur but further developments may be useful in the light of the Chernobyl accident. (12) There is scope for a number of international conferences under either an ‘official’ body, like the International Atomic Energy Agency, or the

European Nuclear Society (but involving the American Nuclear Society, the USSR and the Eastern Bloc). Suitable topics arising from the Chernobyl accident include: (a) containment philosophy and design. It is not obvious to the Working Group that a Western-style containment would have been fully effective for an incident on the scale of Chernobyl; (b) emergency core cooling and its role in major accidents; (c) radioactive decontamination, the impact of major accidents on food and agriculture and water supplies (Soviet participation is essential here); (d) state of knowledge on performance of teams operating complex plant under pressure and in emergencies. Factors such as the effect of delays, fatigue, over confidence, unorthodox operation, work at night, etc., should be included. Such a conference should involve human behaviour experts (can military experience be of value here?) as well as managers and could identify areas for research; (e) modelling of nuclear accidents; (f) release of radioactive material from nuclear accidents, their deposition and dispersal. (13) Developments on simulation and staff training in the UK and elsewhere in the West appear to be superior to those available in the USSR and the Eastern Bloc. Although there may be problems, there is scope for interchanges and perhaps sale of equipment in this field. This may also be the case with nuclear installations in countries outside the OECD. There is some scope for further international conferences on the subject. Other areas for potential sales are fastacting control rods and systems to avoid operators by-passing essential safety trips. (14) While accepting the extremely high standards of safety training in the UK, there does appear to the Working Group to be a case for establishing a system of independent qualification and regular requalification of operating and other key staff. Suitable qualifying bodies could include universities or engineering institutions. The establishment of international qualifying standards should be encouraged and the procedures in the UK should be compatible with or more extensive than these.

Comments, recommendations and conclusions

(15) Comparison of the extreme conditions at Chernobyl with a simulation of a reactor incident exercise in the UK suggests that the changing and decontamination facilities and the monitoring associated with these at UK stations require reassessment to ensure that they are adequate for real situations. (16) The information from the Soviets, although extensive, does not provide adequate background information on a number of topics vital to the understanding of the accident. A selection of these questions are summarised as follows: (a) What was the technical reason for the turboalternator test? Was it related to the power failure incident at Kursk and its extrapolation to a full power case? (b) Was there a full safety analysis of the earlier turbo-alternator run-down trials? Can the operator instructions be published? Were these to be used at Chernobyl-4? Were the team or the team leaders the same as those employed for the earlier tests? (c) How many of the publicised breaches of station safety instructions were necessary for the successful completion of the test? It may be that at least some of the steps taken by the operators would have been necessary for a test of this type. (17) There also seems to be scope for follow-up discussions on other questions with the Soviet authorities: (a) The test was considered essential for RBMK reactor system safety. Did the information obtained from the turbo-alternator provide sufficient test data? If not, how is the information going to be obtained now? How do the Soviets ensure plant safety during diesel generator run-up? (b) The Soviet authorities have initiated further work on reactor physics and core cooling. These are fields of interest both internationally and to the United Kingdom. Details of the proposed work and exchanges of papers would be useful. (c) Follow-up information on radiation levels and health monitoring would be useful and presumably this will be published. (d) Two hundred and three people were retained in hospital after the accident. Information on their medical history and record over the years to come would be valuable.

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(e) Changes have presumably been made to operational procedures and ways of defining and establishing non-routine operation at Soviet nuclear plant. Information about these should be published. (f) Fire fighting was a problem area, causing many casualties at Chernobyl. What changes in equipment and protective clothing have been made? (g) What modifications are being made to Sovietdesigned PWR plant and containment in the light of the Chernobyl experience? (h) There have been continuing reports of extra casualties and exposure of staff at Chernobyl to hazardous conditions, including high radiation levels, even after the damaged No. 4 reactor plant had been shielded and made ‘safe’. This situation implies poor control, organisation and monitoring. If this is general in Soviet nuclear installations, it could have serious implications for the safety of existing and new Soviet nuclear plants. (i) It is now clear that the control rod design probably contributed to the Chernobyl incident by a ‘positive scram’ effect. This suggests that the Soviet plant design analyses are far from exhaustive and do not cover all possible operational situations. This too has serious implications for Soviet nuclear plant, in both the USSR and Eastern Europe. The emergence of a situation with the Chernobyl control rod configuration where rod insertion increased rather than decreased reactivity draws attention to the importance of plant analysis in depth, even for situations which are rated as improbable or even impossible. This analysis is required at the plant design stage and requires careful planning to ensure that all eventualities are covered. It is costly, tedious, but essential for all nuclear installations.

9.10 CONCLUSIONS Finally, the Working Group discussed the question: could such an accident occur in the United Kingdom? At a trivial level perhaps the answer is ‘no’, because the UK has no RBMK reactors. But at a deeper level the Group asks: (a) Are there analogous design weaknesses that could,

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if mal-operation occurred, lead to an equivalent accident? (b) Are there organisational or structural weaknesses that could allow such mis-operation? To the first question, the answer seems to be again ‘no’. Two mechanisms that might lead to a sudden gain of reactivity of the order of the Chernobyl accident in circumstances implying a positive power coefficient have become visible to us. The first is a positive graphite temperature coefficient. This effect in AGRs is well known and long publicised in open literature. It differs with respect to the Soviet design by virtue of a slow time constant and a weaker coupling of the graphite thermally to the fuel. It is difficult to see that this mechanism could operate to cause a major accident. The second mechanism involves the relatively large absorption coefficient of steel used as the fuel cladding in AGRs. If some mechanism that melted away the steel could be involved, then reactivity and power could be conceived to increase rapidly. This would have to occur, however, in a range of temperature that was high enough to melt steel but not yet the uranium dioxide fuel. In view of the pellet nature of the fuel, it is not readily credible that the neutron-absorbing steel would fall to the bottom of the core without an equal fuel slumping. The Group concludes that no similar mechanism has been identified in UK reactors and remarks again that the RBMK weaknesses were acknowledged and known to the designers. The Soviet design also led to a situation where, under unusual circumstances, control rod insertion could add to, rather than reduce, reactivity. Exhaustive

The Chernobyl accident and its implications

computer analysis is required to identify situations like this, which the Soviets appear not to have done in the plant design stage. There would appear to be no circumstances with the AGR or PWR where ‘positive scram’ could occur. To the second question, the failure of the human system, it is harder to give a definite answer, this being a matter more for the social sciences than the natural sciences. The matter was pursued over a wide range of operators, designers, safety experts, etc. The reader may feel able to reach the same conclusion as the Working Group: that no inherent weakness has been identified but that the price of safety is continuous vigilance. The installation in UK nuclear power stations of extensive automatic equipment to protect the reactor plant from faulty actions by operators, and also the provision of plant protection under abnormal situations, is therefore to be welcomed. ACKNOWLEDGEMENTS This review has only been possible as a result of help and information freely provided by all of the bodies with whom the Working Group has had contact. The Group found few problems in pursuing, in some detail, controversial questions on technical or training and organisational matters. The Group was impressed, both by the technical competence of those they met and by the depth of the analysis in which aspects of safety and organisation had been investigated. However, both the conclusions and recommendations presented here are those of The Watt Committee on Energy Working Group.

APPENDICES

Appendix 1

Glossary of Terms Absorbed dose. The quantity of radiation energy absorbed by a given material. Measured in the SI unit the gray (q.v.). Formerly measured in rads (q.v.). Also known as radiation absorbed dose. Activity. The rate at which atomic disintegrations are taking place in a sample of material. Measured in the SI unit the becquerel (q.v.). Formerly measured in curies (q.v.). AGR. Advanced Gas-cooled Reactor. Alpha radiation. Radiation consisting of fast-moving particles made up of 2 protons and 2 neutrons combined together. Emitted by a radionuclide (q.v.). Nucleus of helium. Atom. The smallest particle into which an element can be divided whilst still retaining the properties of the element. Becquerel. The SI unit of activity. 1 becquerel is one atomic decay per second. Replaces the old unit the . curie (Ci).

Control rod. A rod made of a strongly neutron-absorbing material used to suppress the chain reaction. Critical. The state in which a reactor achieves a selfsustaining chain reaction. Cross-section. A measure of the interaction between a nucleus and either a neutron or a gamma ray. Unit: barn (10-24cm2). Curie. A unit of radioactivity. Replaced by the SI unit the becquerel (q.v.). Depleted uranium. Uranium in which the proportion of U 235 has been decreased below the natural proportion of 0·7% by weight. Deuterium. An isotope of the element hydrogen having one neutron in addition to one proton in the nucleus. See also hydrogen, isotope, tritium, heavy water. Dose equivalent. A measure of the biological consequences of energy deposition by ionising radiation. The absorbed dose (q.v.) is multiplied by quality factors, particularly the relative biological effect of different radiations. Beta, gamma and X-ray radiations are weighted by one, alpha radiation by 20 as recommended by ICRP. Neutrons (although not directly ionising) are weighted by 2·3 if thermal and 10 if fast or of unknown energy. Measured in sieverts (q.v.), formerly in rems (q.v.). Electron. Elementary negatively charged particle. Element. Member of the group of about 105 basic substances, including man-made ones, which cannot be further simplified by chemical means. Enrichment. Process whereby the proportion of a desired isotope (q.v.) is increased as in the enrichment processes for uranium or heavy water. Enriched uranium. Uranium in which the proportion of U235 has been increased above the natural level of 0·7% by weight. Fast reactor. A reactor which uses fast neutrons to maintain the chain reaction, thus avoiding the need for a moderator.

Beta radiation. Radiation consisting of fast-moving electrons or positrons emitted by a radionuclide (q.v.). BNF. British Nuclear Fuels plc. Boron. A metallic element used as a control rod material. It is strongly neutron-absorbing. BWR. Boiling Water Reactor. Cadmium. A metallic element used as a control rod material. Caesium. A metallic element. It has radioactive isotopes which are products of the fission of uranium. Calder Hall. Site of the first British Magnox reactor. Carbon dioxide. A gas used as a reactor coolant in gas-cooled reactors. CEGB. Central Electricity Generating Board. Chain reaction. The process whereby a disintegrating nucleus releases neutrons which go on to trigger the disintegration of one or more further nuclei. Chernobyl. Site of the Russian RBMK reactor which exploded in April 1986. 101

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FBR. Fast Breeder Reactor. A fast reactor in which the core is surrounded by a blanket of U238 which is converted to the fissile material plutonium by neutron bombardment. An FBR can generate more fissile material than it consumes. Fertile. Describes material which can be made fissile by bombardment with neutrons in a reactor, particularly U238 in a fast reactor. Fissile material. Material which is capable of undergoing fission by thermal neutrons. Normally used to describe atomic reactor fuel. Fission. The process whereby an atomic nucleus splits into two or more parts. Fossil fuels. Organic fuels such as coal and oil composed of decayed organic matter and laid down in geological formations of great age. Gamma radiation. Very high frequency electromagnetic radiation which produces an ionising effect in matter. Emitted by a radionuclide. Graphite. One of the two crystalline forms of the element carbon, the other being diamond. It is used as a moderator material in thermal reactors. Reactor grade graphite is highly purified and has been fired to give a hard, structural material. Gray. The SI unit of radiation absorbed dose. 1 gray is 1 joule of absorbed energy per kg of matter. Replaces the old unit the rad. 1 gray=100rad. Half-life. The time taken for half the atoms of a radioactive element to undergo spontaneous disintegration. Heavy water. Deuterium oxide. See also light water. High level waste. Highly radioactive fission products present in concentrations of 1 to 3% in spent atomic fuel. High level waste must be carefully contained and not released into the environment. HSE. Health and Safety Executive (UK). Hydrogen. An element whose atoms have one proton in the nucleus. See deuterium and tritium. IAEA. International Atomic Energy Agency. ICRP. International Commission on Radiological Protection. INA. International Nuclear Agency. Ionisation. The process by which a neutral atom or molecule acquires an electric charge. Ionising radiation. Radiation that produces ionisation in matter. Examples are alpha and beta particles, gamma and X-rays, and, indirectly, neutrons. Isotope. Isotopes of an element have the same number of protons in the atomic nucleus, and therefore have the same chemical properties. They have different numbers of neutrons and therefore have different

The Chernobyl accident and its implications

atomic weights and physical properties. For example U235 and U238 are isotopes of uranium. Joule. A unit of energy. One joule is defined as the energy represented by a force of one newton moving through a distance of one metre. Light water. Ordinary water, i.e. H2O—hydrogen oxide. See also heavy water. Magnox. Early British gas-cooled reactor type so called after its magnesium alloy fuel cans (Magnox: Magnesium-no-oxidation). Moderator. A material used in an atomic reactor to reduce the speed of fast neutrons to slow (thermal) velocities. MW(e). Megawatt (electrical)—the unit of electrical power output of a power source. MW(th). Megawatt (thermal)—the unit of heat output of a turbo-generator. Neutron. Atomic particle carrying no electrical charge, of similar mass to proton (q.v.). NII. Nuclear Installations Inspectorate (UK). NRPB. National Radiological Protection Board (UK). Nucleus. Central part of an atom composed of at least one proton but normally having a number of protons and neutrons around which electrons orbit. Obninsk. Site of the first Russian RBMK reactor. Pile. Atomic pile was a term used in the early days to describe the graphite and uranium matrix which comprised the reactor. Plutonium. A fissile metallic element, not naturally occurring. It is produced following the capture of a neutron by a U238 nucleus. Positive feedback. An amplifying condition in which the output of a process positively enhances the input, thereby further increasing the output. Positron. A positively charged atomic particle with the same mass as an electron. Proton. Positively charged atomic particle. Present in all atomic nuclei. Nucleus of (ordinary) hydrogen. PWR. Pressurised Water Reactor. Rad. A unit of radiation absorbed dose. Replaced by the SI unit the gray (q.v.). Radiation. Radiation (implying ionising radiation) describes both electromagnetic emission (X-rays and gamma rays) and particulate emission (alphas, betas and neutrons). Radioactive. A material which undergoes spontaneous disintegration and emits radiation in the process. Radionuclide. An unstable isotope that emits ionising radiation in the process of decay. Radon. Naturally radioactive element (Rn222), the heaviest of the inert gases. It is formed by the

Appendix

disintegration of radium. Thoron (Rn220) is an isotope of radon. RBMK. A Russian reactor design which uses a graphite moderator and natural water coolant. Reactor. Nuclear reactor. A device within which the nuclear fission process is contained and controlled. Rem. A unit of radiation absorbed dose equivalent. Replaced by the SI unit, the sievert (q.v.). Sievert. The SI unit of radiation absorbed dose equivalent for body tissue. 1 sievert is 1 gray multiplied by a weighting factor which takes account of the different effects on body tissue of different forms of radiation. Replaces the old unit the rem. 1 Sv=100 rem=1 J/kg. Sodium. A metallic element with low melting point used in liquid form as a coolant in a fast reactor. TCE. Tonne coal equivalent. Having the energy release equivalent of 1 tonne of coal—about 250 GJ/t, but coals very widely. Thermal efficiency. The efficiency with which the heat produced in an atomic reactor is converted in the generator to work (electricity). Thermal reactor. Atomic reactor which slows emitted neutrons to thermal velocities by using a moderator (q.v.). Three Mile Island (TMI). Site in Pennsylvania where a pressurised water reactor core melted in March 1979. Tritium. An isotope of the element hydrogen having

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two neutrons in addition to one proton in the nucleus. See also deuterium. Turbo-generator. Steam turbine driving an electrical generator. Uranium. A heavy metallic element with 92 protons in the nucleus. The two most common isotopes of uranium, U235 and U238, also have 143 and 146 neutrons in the nucleus respectively. Uranium hexafluoride. A gaseous uranium compound used in an enrichment process. Watt. A measure of power, or the rate of energy flow. One watt is that power level represented by a force of one newton moving at one metre per second, i.e. 1 joule per second. Xenon. One of the inert gases. Xe135, produced as a result of the fission process, has an exceptionally high absorption cross-section for neutrons. Yellow cake. Uranium ore concentrate (UOC). Zirconium alloy (Zircaloy). A metallic alloy containing the element zirconium. Acknowledgement This glossary is reproduced by kind permission of the Institution of Electrical Engineers and first appeared in the IEE fact sheet Nuclear Power in the UK (December 1986), with minor modifications.

Appendix 2

Units of Measurement The text has employed, as far as possible, the internationally accepted units of the Système International d’Unités (SI Units). Some of the source material, however, was expressed in other units and has been quoted as such. This appendix shows the connections and lists the standard prefixes of size.

Temperature:

Radioactivity:

1 Equivalents Radiation—Dose and dose equivalent:

Length and volume:

The effective dose in sievert is the physical dose in gray multiplied by modifying factors, the most important of which is the quality factor expressing the relative biological effectiveness compared to gamma radiation (see Appendix 1).

Mass:

Force: 2 Prefix scale factors in the SI canon Symbols in powers of ten, i.e. 18 means 1018,—18 means 10-18: Pressure:

exa peta tera giga mega kilo hecto deca

Energy and power:

E P T G M k h da

18 15 12 9 6 3 2 1

atto femto pico nano micro milli centi deci

a f P n µ m c d

-18 -15 -12 -9 -6 -3 -2 -1

Reference Foo-Sun Lau, A Dictionary of Nuclear Power and Waste Management, Research Studies Press (Wiley), 1987.

104

Appendix 3

Summary of Significant Dates and Timing Relevant to the Chernobyl Accident 1 Selected events prior to the Chernobyl-4 accident Dec. 1942

CPI West Stands, Stagg field, Chicago, USA

First man-made self-sustaining chain reaction. Graphite-natural uranium (uncooled)

‘X’ Reactor, Oak Ridge, Tennessee, USA

First plutonium breeding reactor. Graphite-natural uranium

July 1945

Alamogardo, New Mexico, USA

First nuclear explosion

Aug. 1945

Hiroshima and Nagasaki, Japan

First use of atomic bombs

Sept. 1945

Soviet Union

First reactor in USSR

1943

1947

Harwell, UK

First British nuclear reactor—Gleep

Aug. 1947

USSR

First Soviet nuclear bomb test

1949

USA

Start of research on nuclear powered submarines

1950

USSR

Decision to build a power reactor

105

1948–1952

Harwell/Risley, UK

Evaluation work on graphite moderated, carbon dioxide-cooled plutonium and power reactor

June 1952

Groton, Connecticut, USA

Keel of first nuclear submarine laid

Dec. 1952

Canada

Reactor accident (NRX)

Mar. 1953

Idaho, USA

Land based PWR, ‘first power reactor’

Apr. 1953

Risley, UK

Work started on Calder Hall—graphite moderated, gas-cooled

1953 onwards

USA

Work relevant to technology of in-core boiling

Oct. 1953

Shippingport, Pennsylvania, USA

Building of PWR nuclear plant announced

June 1954

Obninsk, USSR

First nuclear power station operated— graphite moderated, pressurised water cooled

Aug. 1955

Geneva, Switzerland

First International Atomic Energy Conference

The Chernobyl accident and its implications

106 Nov. 1955

USA

Core meltdown accident, FBR 1

Oct. 1956

Calder Hall, UK

Official opening of Calder Hall Nuclear Station

Oct. 1957

Vallecitos, California, USA

Boiling water experimental reactor operating

Oct. 1957

Windscale, UK

Graphite fire in military plutonium production reactor

Dec. 1957

Shippingport, USA

May 1958

NRX, Canada

Fuel fire in charge machine

Aug. 1960

Dresden, USA

Commercial BWR operational

Jan. 1961

1963

USA

Winfrith, UK

PWR plant started operating

Oct. 1975

Leningrad, USSR and elsewhere

Visit of UK engineers to RBMK and other plant. Subsequent report identifies a number of reservations about the design of RBMK Leningrad. These reservations passed to USSR but not published in UK

Mar. 1979

Three Mile Island, Pennsylvania, USA

Major core meltdown incident (TMI-2)

Jan. 1980

Kursk, USSR

Loss of power incident on an RBMK plant

1981

Chernobyl, Ukraine, USSR

Reactors 1 and 2 started up

1982

USSR

First ‘loss of power’ alternator tests on RBMKs

1983

Chernobyl

Reactor 3 commissioned

1984

Chernobyl

Reactor 4 commissioned

1984

Chernobyl

Test for ‘loss of power’ alternator characteristics

SL1 reactivity excursion 3 casualties

Building of steam generating heavy water reactor (SGHWR) started. Heavy water moderated, boiling water channel reactor

First Russian PWR in operation

Note: Dates for the Soviet power stations are taken from a Russian paper published in the International Atomic Energy Agency Bulletin, June 1983, except for Chernobyl-4 which had not been commissioned then. The dates are the year of reaching nominal power. Other earlier dates, presumably of commissioning, have been quoted for some of these elsewhere.

2 Sequence for Chernobyl-4 accident, April 1986

Dec. 1964

Novo-Voronezh, USSR

Oct. 1966

Detroit (Fermi), USA

Sodium cooled fast reactor accident

01·00

1967

Beloyarsk, USSR

Channel graphite moderated boiling water reactor operating

13·05

Power at 500MW(e), 50% full load. Turbo generator 7 shutdown and electric power systems switched to turbo generator 8

1967

Winfrith, UK

Start-up of SGHWR plant

14·00

Jan. 1969

Lucens, Switzerland

Pressure tube failure

Emergency core cooling system (ECCS) disconnected. Apparently this is not necessary for the turbo-alternator power rundown test as planned

Dec. 1973

Leningrad, USSR

Start-up of first 1000 MW RBMK

14·10 (approx.)

Power reduction stopped to meet grid power requirements from Kiev Grid Controller. ECCS not reconnected*

(Note: * indicates safety provision violation)

25 Apr.

Preparation for tests, power reduced slowly to 50%, i.e. 500MW(e)

Appendix

107

23·10

00·28

Kiev Grid Controller cleared reactor power requirement. Maintenance schedule and preparation for turbo-alternator power rundown test continued. Technicians switched off automatic regulation for 12 control rods which would normally stabilise output between set limits

26 Apr.

Technicians unable to control reactor output and thermal power dropped to 30 MW(th), considerably below minimum steady limit of 800MW(th)

01·00

Reactor stabilised at 200 MW(th)* with difficulty—no spare reactivity (mainly due to xenon poisoning) to increase power further. Even so, to achieve this required withdrawing more rods than was allowed in the station rules*

01·03

All 8 circulating pumps brought into operation. Four of these provided the electrical load for the turbo generator. Increased water content in core reduced reactivity, necessitating more control rod withdrawal.* Pumps close to cavitation as cool feedwater quantity low.* Instability in steam pressure and drum levels—water level and steam pressure reactor trips disconnected.* Difficulty in maintaining core output

01·19

01·20

01·22

01·23

As reactor pressure increased and voidage reduced, automatic control rods started withdrawing. Then as voidage increased because of reduced feed flow, rod reinsertion starts again. Clearly automatic system unable to cope. Normally at this stage the reactor should have been tripped, but operators realised that if the turbo-alternator rundown test was unsuccessful, a repeat test would be delayed by 3 days to get the reactor up to power again. Test started at 01·23·04. Power to pumps fell, causing increases in pressure but also increased boiling—net effect was increase in heat output. Things then moved quickly. The operators did now trip the reactor, but there were not enough rods in effective positions (6 out of 30) and they moved slowly. The immediate effect appears to have been to increase rather than decrease power output. Core heat output rose quickly —over 3–4 seconds—to 100 times full power. First explosion—pressure system failure. Second, more powerful, explosion destroyed much of the reactor core, the reactor hall and building. Refuelling machine fell into core. Upper shield lifted and fragmented, fracturing all channel pressure and tube connections. Considerable fires started—2 men immediately killed

Operator increased feed flow. Automatic rods withdrawn to maximum extent. Further withdrawal of manual rods to compensate for loss of reactivity due to void collapse with channel inlet temperature reduction. Drum level started to rise again. Steam pressure fell

3 Selected events and actions following the accident

01·30

26 Apr. 1986

Firemen and ambulances set out from Pripyat. Main objective to stop fires spreading to Reactor 3 which was still running. Control Room Supervisor and colleague inspected damage, receiving lethal doses of radiation

In an attempt to bring steam pressure up, steam by-pass valve shut

As feed flow considered excessive, operator decreased flow again—steam pressure still falling and combined with increase in water temperature to channels induced more voidage in core. Automatic rods began to move in to compensate for increased reactivity. Print-out of reactor flux distribution and reactivity as forerunner to test. Indicated reactivity reserve margin too low and this should have brought about an immediate shutdown of the reactor by the operators.* Power distribution distorted—some channels producing much more steam than others

All turbo-alternator protective warning circuits disconnected. Emergency stop-valve closed and steam valve to turbine shut. Turbo-alternator and reactor now disconnected.

02·30

Generating hall roof fires extinguished

05·00

Most other fires extinguished. 36 firefighters and technicians rushed to hospital in Kiev. All retained—6 died within a few days

07·30

Team of experts from administration, civil and military defence started arriving on site. Set up in an underground bunker 60m from the reactor

11·00

Military and civil telecommunication links set up with Moscow—military, medical and civilian

15·00

First helicopter flights over reactor to photograph core—crews wore special clothing. Machine and crew ‘decontaminated’ on return. Measurements taken above and around reactor

The Chernobyl accident and its implications

108 02·13

27 Apr. 1986

13·30

Chernobyl Reactors 1 and 2 shut down

Evacuation of 57 500 people from Pripyat and Chernobyl started, supervised by medical, army and radiation monitoring teams

13·40

a.m.

Polish authorities recommended restrictions in milk and issued tablets containing iodine

30 Apr. 1986

p.m. 14·00

First measurements on recorders in Sweden of increased radiation levels. (Not read until 28 April)

16·30

Evacuation from Pripyat and Chernobyl complete

During day

Foreign Embassy staff noted high radiation levels in Kiev. Citizens officially informed of no need to panic or take precautions. Note that Embassy figures are up to 10 times the official Soviet figures for the City

During day

09·00

28 Apr. 1986

09·30

77 500 other residents and farm animals evacuated. Helicopter crews started flying over burning reactor to drop boron carbide, dolomite, lead, clay and sand to suppress fire and reduce core reactivity. 93 sorties during day

Staff at Forsmark Nuclear Power Station in Sweden detected radioactive contamination on clothes of staff. Nonessential staff evacuated and emergency services alerted. Reactor shutdown. Swedes noted figures from automatic recorders and other sources. Decided line of contamination lay through Lithuania in direction of the Ukraine

19·00

Swedish announcement and note to USSR. Moscow Radio: terse statement in news about accident

During day

700-fold increase in radioactivity measured in Poland, 100-fold in West Germany. 100 radiation-affected people arrived in Moscow hospitals. 186 helicopter sorties

a.m.

p.m.

During day

29 Apr. 1986

Moscow Radio statement printed in Kiev but not Moscow newspapers. Not given any prominence

Soviet television released photograph of Chernobyl which suggested that the plant was not severely damaged. Radio Kiev made some more reports, quoting 2 casualties to counter Western figures of high casualties. Radiation release about one-fifth of the peak during first day of the accident

1 May 1986

21·00

During day

First announcement in Pravda and Izvestia about accident

May Day celebrations as usual even in Kiev. Radio reports via Radio Kiev that more people in grave condition. Also report of fall in radiation levels. Radiation release from Chernobyl-4 started to increase again. Izvestia published some more details of the accident. Statement by Soviet delegates at United Nations and on American television. During programmed lull in helicopter activity, underground inspection of area below reactor, limited by radiation levels, to determine further action

First report (brief5) on Soviet television

2 May 1986

Start of pumping water out of reactor basement. Fall-out reaches Britain

3 May 1986

Draining basement complete—entrances and exits sealed. Nitrogen pumped in and circulated to reduce core temperature

5 May 1986

Radiation release from reactor reached a second peak, about half initial release

6 May 1986

Pravda published detailed report on accident and impact on surroundings. From now on information released officially. Radiation release levels now very low

7 May 1986

Core temperature reduced to 300 °C

9 May 1986 TASS: somewhat more detailed statement. Moscow Radio world news gave conflicting statements, first making political capital and later giving a moderate description of the accident

Underground bunker at Pripyat to direct operations

11 May 1986

Excavations started under reactor preparatory to fitting a concrete platform under reactor

Many people left or attempted to leave Kiev

13 May 1986

Decision to entomb reactor

Appendix

109 14 May 1986

Mr Gorbachov’s first television address

Oct. 1986

Entombment complete—Reactors 1 and 2 recommissioned

19 May 1986

Remote bulldozer started clearing debris in nuclear plant

Spring 1987

Decision to stop building Chernobyl 5 and 6 or any further RBMKs

Soil close to reactor overturned and sealed by spray from helicopters

July 1987

Trial of senior staff—proceedings in camera with public announcement of verdict of guilty and sentences

Underpinning complete. Encasing reactor and isolation from Reactor 3 started

Nov. 1987

Chernobyl-3 back in operation following

20 May 1986

End of June

extensive decontamination and refitting Jun–Aug.

Isolation of reactor area from River Pripyat to avoid runoff to water supplies

22 Aug. 1986

Official report by Soviet government to International Atomic Energy Agency. Remedial measures reported

Appendix 4

Nuclear Safety in the Soviet Union from ‘Nuclear Power in the Soviet Union’ by B.A.Semenov—June 1983 International Atomic Energy Agency Bulletin (Vol. 25, No. 2)

The safety of nuclear power plants in the Soviet Union is assured by a very wide spectrum of measures, the most important of which are:

to ensure safety at all stages of construction and operation of nuclear power plants; Regulation of technical and organisational aspects in securing safety; and Introduction of a system of state safety control and regulation.

Securing high quality manufacture and installation of components; Checking of components at all stages; Development and realisation of effective technical safety measures to prevent accidents, to compensate for possible malfunctions, and to decrease the consequences of possible accidents; Development and realisation of ways of localising radioactivity released in case of an accident; Realisation of technical and organisational measures

The regulation of safety by official documents is one of the main tools for ensuring the safety of nuclear power plants in the USSR. The state supervision of nuclear power plant safety is accomplished by:

a

110

The State Committee on Supervision of Safe Operations in Industry and Mining under supervision of the Council of Ministers of the USSR

Appendix

(Gosgortekhnadzor of the USSR), which supervises compliance with regulations and standards of engineering safety in design, construction, and operation of nuclear power plants;

a

a

The State Nuclear Safety Inspectorate (Gosatomnadzor of the USSR) which supervises compliance with rules and standards of nuclear safety in design, construction, and operation of nuclear power plants; The State Sanitary Inspectorate of the USSR under the Ministry of Public Health which supervises compliance with rules and standards of radiation safety in design, construction, and operation of nuclear power plants.

The established system of three supervisory bodies has largely determined the structure of the whole complex of regulatory documents on nuclear power plant safety. The main regulatory document on nuclear power plant safety in the USSR, General regulations to ensure the safety of nuclear power plants in design, construction, and operation, was enforced in 1973. This document covers all types of commercial reactors used and to be used in the USSR in the nearest future (WWER, RBMK, BN, and district-heating reactors). In this approach, requirements are presented in a general way, without concrete details. In most cases the General regulations only prescribe tasks which have to be solved to ensure safety (what must be done); they do not determine the solutions (how it should be done). Other normative documents (codes, guides, rules, procedures) develop further and specify more concretely the General regulations establishing thus the basis for activities of designers and corresponding

111

supervisory bodies. One of the main documents in the field of engineering safety is Regulations for design and safe operation of components for nuclear power plants, test and research reactors, and installations. The basic document in Gosatomnadzor’s activity, Nuclear safety regulations for nuclear power plants, was introduced in 1975. It regulates nuclear safety, governing not only criticality problems in reactor operation, but also refuelling, transportation and storage of fuel assemblies. It contains the main technical and organisational requirements to ensure nuclear safety in the design, construction, and operation of nuclear power plants, and the training requirements for personnel associated with reactor operation. In the field of radiation safety, the basic document by which the health and inspection protection bodies are guided is Radiation safety standards (RSS-76). These standards were worked out on the basis of recommendations of the International Commission on Radiological Protection (ICRP) and establish the system of dose-limits and principles of their application. The health regulations for design and operation of nuclear power plants, issued in 1978, further develop and specify the basic RSS-76 document to include siting, monitoring, and inspection problems. The system of regulatory documents on nuclear power plant safety is complemented by the system of state standards developed and established by the State Committee on Standards (Gosstandart of the USSR). The system of standards extends the system of regulatory documents by ensuring nuclear plant safety through establishing requirements for many components, materials, processes, etc. The above documents play a significant role in nuclear power plant quality assurance.

Appendix 5

Chemical Reaction Aspects of the Chernobyl Accident Gilbert Walton Professor Emeritus of Nuclear Technology, Imperial College, University of London

Jeffery Lewins & Norman Worley

One of the lasting impressions that the public must have of the Chernobyl accident is that of flames and bright, red-hot graphite burning in the reactor and sending radioactive material into the atmosphere. With the Three Mile Island accident, much publicity was given to the potential explosion hazard associated with a hydrogen ‘bubble’ which had formed above the nuclear core. Both of these examples are the consequences of chemical reactions which can occur in reactor systems when the fuel elements overheat and disintegrate. The purpose of this appendix is to identify some of the major chemical reaction lessons from Chernobyl and link them with United Kingdom reactors. The Chernobyl accident has been analysed in the Soviet Union. Two explosions were reported, although there is some doubt as to whether the second occurred. The first was a nuclear excursion leading to an enormous increase in heat production in the core. The Soviet modelling also supports the occurrence of a second nuclear excursion. The reported maximum rate of heat release has been given as about 100 times normal full power. At the very high temperatures produced by the nuclear excursions, chemical reactions between Zircaloy and steam/water would be rapid and chemical reactions are likely to have contributed to the second explosion and the subsequent course of the accident. Most water-cooled reactors, including RBMK and PWR, have fuel elements with zirconium alloy cladding. Under normal operating regimes, there is

virtually no detectable reaction between the zirconium and the pressurised water coolant. However, the reaction between zirconium and water which gives zirconium dioxide and hydrogen as products, generates heat (i.e. is exothermic) and so, if initiated, is likely to proceed rapidly. The rate of reaction will increase as the temperature of the components increases. An exothermic reaction, once started, will tend to raise the system temperature and increase the rate of the reaction. Such reactions can lead to instabilities of a positive feedback nature. Endothermic reactions, however, which require energy to be supplied—the reverse of exothermic—are therefore of a negative feed-back and inherently stable type. The rates of chemical reactions generally follow an Arrhenius type of rate equation with a solution of the form , where E is the activation energy for the reaction A and T is the absolute temperature. In view of the exponential form, rates increase rapidly with temperature. At normal operating temperatures for water reactors (300°C), the rate of reaction is low because a passive oxide coating on the surface suppresses any reaction. The oxide coating would cease to be protective with a rapid rise in temperature of the cladding and the reaction would then proceed vigorously at temperatures above 1200°C. A sequence of events at Chernobyl can be described. The first nuclear excursion led to elevated temperatures which rapidly breached several, possibly many, of the clad fuel elements. Hot fuel particles would then be 112

Appendix

dispersed in the coolant channels, leading to a ‘thermal explosion’ and a local pressure surge. The elevated temperatures would in themselves rapidly thin the Zircaloy pressure tubes which could be expected to fail under the influence of the thermal explosion and weakening; the continued attack on the zirconium of the cladding and pressure tubes would release energy. The combined pressure rise from the heat generation and chemical reactions was sufficient to lift and disrupt the reactor head. This might have been the second ‘explosion’. The reactor containment design was designed to accommodate the failure of a single pressure tube only. More chemical reactions followed. Steam liberated on to the hot graphite would react according to the well-known endothermic water-gas reaction, providing hydrogen and carbon monoxide/dioxide. But once the reactor vault was breached, the ingress of air would allow a hydrogen-oxygen reaction: at best a rapid deflagration, at worst an explosion. This was then followed by the graphite-air burning which released substantial heat and elevated the plume of radioactive contaminants to about 1 km. In the Three Mile Island core meltdown, it is known that there was also a Zircaloy-water chemical reaction leading to a considerable release of hydrogen. However, the reactor circuit pressure relief and containment system there prevented damage with no significant fission product release to the atmosphere. There was no free oxygen in the vessel so that hydrogen did not react chemically. The Zircaloy-water reaction induced by uncontrolled energy release in reactor fuel can therefore be regarded as a major potential hazard with water-cooled reactors. In this respect, a thick steel pressure vessel circuit has some advantage over the pressure tube system where the pressure membrane is thin and close to the fuel elements.

113

The Chernobyl accident, as well as that at Three Mile Island, therefore suggests that the circumstances under which a rapid rise in cladding temperature can occur should receive special study, both experimentally and analytically, for water-cooled reactor systems. The other major chemical reaction which was significant at Chernobyl was the combustion of graphite. Here the low temperature of operation of the graphite in UK reactors (300–400 °C compared with about 700°C in the RBMK) would ensure that the rate of reaction is likely to be negligibly low. The reactions between carbon dioxide or steam (from failed boiler tubes) and graphite both absorb energy and are, therefore, not self-sustaining. The austenitic steel fuel element cladding used in AGRs reacts with carbon dioxide only at considerably higher temperatures than those of normal operation, and as the temperature rises the reaction tends to absorb rather than generate heat. The potential for chemical reactions adding to the hazards following a reactor accident requires serious assessment. The safety systems of reactors need to be designed to ensure that the probability of temperatures leading to dangerous levels of chemical reaction is extremely low. In the selection of materials for reactor components, potential chemical reactions should receive special attention. Sources Brown, M.L. and Walton, G.N., Polarisation of zirconium and its alloys in high temperature water. J. Nucl. Materials, 66, 44 (1977). Walton, G.N., Hot atoms and the safety of nuclear power, Inaugural Lecture, Imperial College, London, January 1973.

Appendix 6

The Chernobyl Accident Trial An article published in The Independent on 30 July 1987, by Anthony Barber of Reuters

The former head of the Chernobyl nuclear plant power station, and two aides, were sentenced today to 10 years in a labour camp, the maximum term they faced for their part in the world’s worst nuclear accident. The Ukranian plant’s former director Viktor Bryukhanov, former chief engineer, Nikolai Fomin, and his deputy, Anatoly Dyatlov, were found guilty of gross violation of safety regulations which led to an explosion. Three other officials received terms of up to five years in a labour camp at the end of a three week trial, most of which was held behind closed doors in the town of Chernobyl, 18 km (11 miles) southeast of the atomic station. The explosion and a fire at the plant’s fourth reactor on April 26 last year killed 31 people, forced the evacuation of 135,000 residents from high radiation zones around Chernobyl, and spread radiation across much of England and Europe. In a 90 minute summing-up of the case in an improvised courtroom in the Chernobyl House of Culture, Judge Raimond Brize criticised the way Bryukhanov handled the evacuation of personnel after the accident and said the plant was poorly run. He said Bryukhanov should bear most blame for the accident and gave him a five year sentence for abuse of power to run concurrently with the 10 year term. “There was an atmosphere of lack of control and lack of responsibility at the plant,” the judge said. People had played cards and dominoes and wrote letters while at work, he said. Bryukhanov, 51, and the five other defendants showed no visible emotion and sat with bowed heads as their sentences were announced by Brize, but several of their relatives in a packed courtroom wept.

Fomin, dressed like the others in an open-necked shirt and jacket, occasionally took off his spectacles under the glare of television cameras and mopped his brow with a handkerchief. Boris Rogozhkin, shift chief at the plant’s fourth reactor, was sentenced to five years in a labour camp for violating safety rules and a two-year sentence to run concurrently for negligence and unfaithful execution of his duties. Yuri Laushkin, a senior engineer, was sentenced to two years in a camp on the latter charges. Alexander Kovalenko, overall chief of the reactor, was sentenced to three years in a camp for violating safety regulations. Bryukhanov, Fomin, and Dyatlov, had accepted professional responsibility for the accident but denied criminal liability. Kovalenko, Rogozhkin and Laushkin had pleaded not guilty. All six were convicted on all charges they faced except Fomin, who had also been charged with abuse of power. Chernobyl Information Director, Anatoly Kovalenko, said the trial, which had been closed to foreign correspondents since the opening session on July 7, had passed without emotional outbursts. “There were no hysterics. It was normal,” he told several Moscow based foreign reporters allowed to the last day of the proceedings. Kovalenko said three more trials would take place to establish who was responsible for technical failures in design and construction of the plant, for failings in evacuation and medical procedures, and for security errors after the accident. He did not say when or where the trials would take place or who would face judgement. Evacuation of the town of Pripyat, where plant staff lived, did not take place for 36 hours after the disaster. 114

Appendix

A government enquiry blamed the disaster on tests to see how long stable power could be maintained after switching to a diesel generator. Brize said Laushkin had given permission for the tests to take place without knowing fully what they entailed. Kovalenko said 40 witnesses, including at least nine Chernobyl victims, had given evidence. The defendants delivered their final speeches on Monday, he said. The trial had been scheduled to start earlier but was delayed because Fomin was ill in hospital with radiation sickness until late April, he said.

115

Watt Committee on Energy comment on the UK situation on individual responsibility It is The Watt Committee on Energy’s understanding that the powers of the Nuclear Installations Inspectorate under the Nuclear Installations Act, are very similar to those of the Factory Inspectorate under the Factories Act/Health and Safety at Work Act. In the very unlikely event that a major nuclear incident took place in the UK, it seems that the NII would be able to prosecute the corporate body responsible for failures in statutory duty, and individuals for negligence. Presumably, this would follow a full technical enquiry by the NII.

Appendix 7

Continuing Radiation Leakage from Chernobyl An article released by Reuters, Moscow, 4 December 1987

A Communist Party official revealed on Friday [4 December 1987] that radiation exposure was still a problem at the damaged Chernobyl nuclear power plant in the Ukraine and said three fatal accidents had occurred there this year. V.Lukyanenko, head of the party in the new town of Slavutich built to house Chernobyl staff, said directors of the power plant had been disciplined for security violations by plant personnel during the extraction of nuclear fuel. “In the past 10 months in our organisations there have been 36 accidents, including three with fatal consequences,” Lukyanenko said in a party report published in the newspaper Sotsialisticheskaya Industria. “Despite the measures which have been taken, incidents of (radiation) overdoses have not been excluded up to the present time,” he said. The published sections of Lukyanenko’s report did not make clear whether the fatalities were linked to radiation exposure. The world’s worst nuclear accident occurred at Chernobyl on 26 April, 1986, when the plant’s fourth reactor exploded and caught fire, spewing radiation over a huge area. At least 31 people were killed, according to Soviet officials. The fourth reactor was entombed in reinforced

concrete, and Soviet officials announced last December that radiation leakage had halted. Repairs on the adjacent third reactor continue, while the first two units resumed energy production last year. Lukyanenko said three plant officials had been disciplined by the party for an incident last June involving the extraction of nuclear fuel from the plant’s second reactor. They were Yevgeny Ignatenko, chief of the Kombinat group set up in October 1986 to oversee the Chernobyl clean-up, plant director Mikhail Umanets and the station’s party chief, Y.Borodavko. Details of the incident were not provided. Lukyanenko said the plant’s directors and even the workers themselves had failed to make security a priority. Sotsialisticheskaya Industria included Lukyanenko’s comments in a critical report on a recent plenum of the party organisation for the Kiev region, which includes Chernobyl. The newspaper said the plenum devoted only one paragraph of a 78 page report to the power plant and commented that it could not share the opinion of Kiev regional party leader, N.Donchenko, who said everything was fine at Chernobyl.

116

Appendix 8

International Aspects Norman Worley

1 Learned societies

distributed to all members of the national nuclear institutes (70000 copies). It also organises major international conferences. It has close links with the American Nuclear Society (ANS) which also has European members. Turning to the commercial front, the main companies and bodies in the UK that supply nuclear equipment have formed the British Nuclear Forum, which has links with similar European bodies, its objectives being to promote nuclear exhibitions and conferences, often associated with conferences of the European Nuclear Forum. In the United States of America, the American Nuclear Society and American Nuclear Forum have considerable political influence. Both help to draw up standards of design, safety and operation. Recently (September 1987) the Atomic Industrial Forum and other nuclear organisations have been restructured under the direction of the Nuclear Power Oversight Committee (NPOC) with three organisations reporting to it:

Contrary to the public conception of the nuclear industry as secretive, nuclear engineers organise public conferences covering virtually all of the technical design, safety and operating aspects of nuclear plant. Most of these conferences are international ones with delegates and papers from all over the world. In the UK, for example, there are two bodies that represent nuclear experts—The Institution of Nuclear Engineers and The British Nuclear Energy Society, which is run with membership from the major engineering and scientific societies with nuclear interests. In the UK these societies collaborate for many meetings and the diary for 1987 had a full programme, including conferences on ‘Nuclear containment’, ‘Health effects of low dose ionising radiation’ and ‘Materials for nuclear reactor core applications’. This is a typical programme for a year. In October 1986 there was a one-day symposium on the Chernobyl accident. In addition, conferences on nuclear topics are organised in the UK by other bodies and there are lectures on special subjects organised in London and elsewhere in the United Kingdom by both the Institution of Nuclear Engineers and British Nuclear Energy Society. The public concept of the nuclear industry is one of secrecy. In fact, the UK programme is typical of the range of nuclear conferences and meetings in most western countries. Nuclear experts publish enormous quantities of technical material. Dealing still with technical and learned societies, the European Nuclear Society, which has member societies from all Western European countries, publishes a monthly journal Nuclear Europe which is

NUMARC—Nuclear Management and Resources Council USCEA—US Council for Energy Awareness ANEC—American Nuclear Energy Council 2 European organisations The electrical utilities in Europe collaborate and exchange experience through an organisation called Union Internationale des Producteurs et Distributeurs d’Énergie Électrique (UNIPEDE). Most of the Western European countries are represented. The headquarters are in Brussels. In the nuclear field, there is a special grouping called Nuclesur, and one of its current tasks 117

118

The Chernobyl accident and its implications

is the comparison of nuclear safety criteria in the utilities in western Europe. The European Commission has nuclear organisations that have never appeared to achieve the regulatory significance or influence of their coal and steel activities. Euratom issues directives on radiation protection, and an independent committee coordinate codes and standards. In addition, there are safety working groups on fast reactor technology.

represents about 80% of the world nuclear generating capacity, generating about 20% of the electricity in member countries. These include 18 Western European countries, the USA, Australia, Japan and New Zealand. Yugoslavia has special status. After the Chernobyl accident, the senior group of experts on Severe Accidents was activated by the NEA. A number of areas were identified for increased international collaboration:

3 Organisations for economic co-operation and development

(a) Research efforts in the study of severe accidents. (b) A system for rapid exchange of information and assistance in emergencies. (c) Improvement in techniques for radiation monitoring. (d) ‘Intervention levels’ of radiation in member countries and associated emergency pressures of harmonisation. (e) Studies to re-examine insurance, third party liability compensation. (f) Improvement of dissemination of information to the public. (g) Coordination of regulatory policies for implementing radiation protection. Linked to the Chernobyl accident, there are a number of NEA initiatives:

Centred on Paris, the Organisation for Economic Cooperation and Development (OECD) developed out of the Organisation for European Economic Cooperation (OEEC), which was set up in 1948 as part of the Marshall Plan to promote a sound European economy. The countries of the Eastern Bloc refused to join, and set up their own organisation—CMEA. By the late 1950s the role of OEEC was taken over by the EEC and the main objectives had in fact been achieved. However, the collaboration between governments—not only members of the EEC—was continued to form a link allowing economic policy, environment issues, development assistance to the Third World’ and exchanges of data. Energy was regarded as important, and two agencies were established: first, the International Atomic Energy Agency and, second, the Nuclear Energy Agency. The Nuclear Energy Agency promotes international nuclear research and development, accumulates data on uranium resources and has committees and working groups on: — Nuclear Fuel Cycles — Licensing — Safety Research and Development Relative to Pressurised Water Reactors (PWR), this being the most common system in member states, there are working groups on: — — — —

Operational experience and human factors Transients Primary circuit integrity ‘Source terms’ associated with nuclear incidents and environmental consequences — Risk assessment The Nuclear Energy Agency also holds conferences. Most of the work is by personnel seconded from member countries for short periods. The Agency

(a) A ‘Severe Accident’ review report has been issued with material which will form the basis of new studies and research, particularly on containment. (b) Checks and developments of accident computer codes using information from Chernobyl. (c) Operator training and the use of simulators. (d) International joint projects integrating and adapting national research to meet the needs revealed by the Chernobyl accident. (e) Upgrading the Reactor Incident Reporting System (IRS). (f) On radiation protection, in addition to current studies, there are new studies to investigate the bases for intervention levels to enable more uniform levels to be derived. After the Chernobyl accident, the acceptance levels of various isotopes in different countries varied by factors of up to 20. Data reporting methods and measurement techniques and standards will be prepared. (g) Accident scenarios and emergency planning, (h) Application of the Chernobyl experience to standards for workers at nuclear plants. In many of these initiatives, it is planned that there will be collaboration and links with work by the

Appendix

International Atomic Energy Agency IAEA and the countries of the Eastern Bloc (CEC). 4 The International Atomic Energy Agency

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(c) awareness of the safety implications of departures from agreed procedures. Authority for plant safety needs to be given to a senior member of operational staff. (3) Design should implement defence in depth:

The IAEA was set up by the United Nations after the successful International Nuclear Conference in 1957. Its headquarters are in Vienna. It has a budget of about $100 m per year, of which 25% is from the USA, 12% from the USSR and 5% from the United Kingdom. Its main areas of activity are:

—First, inherent stability —Second, automatic systems to operate when the safety of the plant is threatened —Finally, there must be a barrier to contain most radionuclides if the first two lines of defence fail.

(a) Non-proliferation of nuclear weapons. (b) Development of the use of radiation techniques and tracers in agriculture, medicine, preservation of food, new varieties of crops, pest control and irrigation. (c) Group training and preparation of guides on nuclear matters for member states. (d) Environmental and radioactive waste reports. (e) Technical assistance to member states—this is a major and growing item in the budget.

(4) Satisfactory man-machine interfaces are important. This requires:

As both Western and Eastern Bloc countries are members of IAEA, it was clearly the ideal body to organise an international conference on the Chenobyl accident. In fact, most of the technical information about the accident in these papers stems from the report by the USSR State Committee on the Utilization of Atomic Energy to the special meeting of IAEA experts and other invited delegates, held in Vienna during 25–29 August, 1986. Since then, a report has been issued by the International Nuclear Safety Group (INSAG-75). This Group’s report makes a large number of recommendations and lists a number of conclusions. A selection of these is summarised below. 4·1 Nuclear safety conclusions (1) No new physical phenomena were identified. However, nuclear power plant operators should review their safety analyses critically. (2) The ‘human element’, which was basically responsible for the Chernobyl accident, also implied three needs: (a) training and use of simulators, with emphasis on understanding the reactor; (b) auditing to avoid complacency from routine operation;

—First: clear display to operators of essential data, tailored to ensure optimum use. Built-in diagnostic capability should be included, and real-time data display and interpretation are important —Second: there must be reliable safety backups by automatic devices that ensure the safety of the plant. Their action must be rapid enough to respond to and control dangerous situations and must be difficult to bypass. 5 Recommendations for the IAEA (1) Promotion of international cooperation in simulation of the Chernobyl accident. (2) Coordination, analyses and distribution of information on all serious nuclear accidents. (3) Promotion of Probabilistic Safety Assessment. (4) Promotion of exchanges of experience on prevention of accidents and operator qualification; accreditation of operator training programmes is suggested. (5) Promotion of exchanges on man-machine interfaces and organisation of a conference on this subject. (6) Organise work and conference on Quality Assurance of Nuclear Power Plant Operation. (7) IAEA should finance work on documentation on the basic safety principles of reactor types. (8) Review existing international standards to incorporate lessons from accidents. (9) Upgrade the Incident Reporting System (IRS). (10) Member States should review procedures for safe operation of plant during non-routine tests. (11) IAEA should organise a symposium on fires and protection of personnel.

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5·1 Radiation protection The IAEA should accumulate and evaluate information on high irradiation doses, on the levels of doses from the Chernobyl accident and on the sheltering and evacuation experience obtained. 5·2 Other activities (1) Promotion and establishment of international values of radiation doses; provision of guidance on ‘Intervention Levels’, radiological sampling and monitoring in emergencies, reporting and compiling of data after the accident. (2) Establishment of criteria and guidelines for entry after an incident and for recovery operations; protective clothing specifications. (3) Technical guidance on contamination and subsequent decontamination operations. (4) Coordination of meteorological models for transport and deposition of radionuclides; establishment of databases for model validation. (5) Training of medical practitioners on advice to public on the health aspects of radiation exposure. There is a need for expansion of the IAEA programme on nuclear plant operator training, with priority given to accident prevention. Additional provision may be

required to provide assistance to countries with limited resources. 6 General There appears to be some overlap between these organisations, and it is not clear at present whether the additional resources to carry out the work specified by NEA and IAEA are being defined and what the procedure is for providing both the trained personnel and the funds. There is also a need for collaboration and rationalisation with other bodies such as the World Health and World Meteorological Organisations. It is important that the momentum of work and exchanges in the summer of 1986 is not allowed to falter in consequence of the inertia of governments and international bureaucracy. Sources IAEA, Safety series No. 75-INSAG, Vienna, 1986. Statement of Director-General of OECD at the Council of Europe, 9 January 1987, with supporting reports. Personal communication with J.Harrison, CEGB. Other IAEA reports and publications.

Appendix 9

The Nuclear Installations Inspectorate The legislative basis for nuclear safety in the United Kingdom lies in the Health and Safety at Work, etc., Act 1974 and the included relevant statutory provisions of the Nuclear Installations (NI) Act 1965. These statutes require that, before any commercial nuclear plant may be built or operated in the UK, a licence must be obtained from the Health and Safety Executive (HSE). The division responsible for this licensing work is HM Nuclear Installations Inspectorate (NII). Under the Acts the basic responsibility for nuclear safety rests with the licensee. The NII’s task is to assure the HSE, Ministers, and hence the public, that the licensee is suitable and competent and is exercising this responsibility satisfactorily through all stages from design and construction to operation and eventual decommissioning. The major licensees are the Central Electricity Generating Board, the South of Scotland Electricity Board and British Nuclear Fuels. Other licensees include universities (for research reactors) and Amersham International. The Inspectorate is a comparatively small body with an allowable complement of 120 at present. It has fallen well short of this number due to a relatively poor salary structure and the effects of moving the bulk of its staff from London to Bootle in Lancashire. A new and considerably improved salary structure has recently been introduced, and combined with a recruiting drive, the Inspectorate appears now to be able to attract suitable applicants. The CEGB’s projected PWR programme will, apart from other considerations, justify a larger complement for the Inspectorate. The NII is at present divided into five branches with the following general responsibilities:

— Nuclear chemical plant construction and operation — Reactor systems new to the UK — Policy coordination and administration A reorganisation of the above responsibilities into two assessment branches, two inspection branches and one policy branch is planned. The Inspectorate attempts to recruit highly qualified engineers and scientists with good degrees with practical experience (generally but not necessarily nuclear) from various sources including the generating boards. Applicants for Inspector posts are usually expected to be over 30 years of age and for Principal Inspector posts, over 35. Training allied to career development is carried out via a wide range of courses together with ‘on the job training’. There are, for example, Induction Courses covering the activities and responsibilities of the Nuclear Installations Inspectorate and the Health and Safety Executive. Law Courses on management, risk assessment, etc., are also available within the HSE. Outside courses are used to broaden the Inspectors’ technical knowledge where necessary and here such organisations as the UKAEA, universities and technical conferences are used. By these means it is ensured that Inspectors keep in touch with advances in technology. The Inspectorate has a relatively small ‘research and support’ budget of £1·5m per annum, used mainly for special investigations related to licensing and safety matters. It is also used to supplement the expertise within the Inspectorate. The work is contracted out and often specialist outside consultancies are used for specific analysis work. This enables further access to powerful computing facilities apart from those already within house. The Inspectorate maintains close relations with the UKAEA and with its equivalents in other major countries. There are formal arrangements for exchanging information with other countries, e.g. France, West Germany, the USA and Italy, where there

— General gas-cooled reactor assessment and construction — Inspection — Operating reactors 121

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are many years of experience, especially with respect to the operation of water-cooled plants. In particular, cooperation with the French in matters affecting PWR safety has proved of great value. During the period of understaffing, the NII has retained some inspectors beyond the normal retirement age of 60. The priority of NII activity has been in maintaining supervision of operating plant. This has meant that some other areas have fallen behind, including, to a certain extent, staff training. Even with a full complement of staff, it will be some time before the new recruits can be trained and able to play a full role in the Inspectorate. The financial control of the Inspectorate, being part of the Health and Safety Executive, is subject to the rigours of Treasury control and efficiency

The Chernobyl accident and its implications

improvements required by the Government. With the planned privatisation of the electricity industry, the effectiveness of the Nuclear Installations Inspectorate, and the influence it has on the safety activities of the Electricity Boards, must be of increasing significance. The Watt Committee recommends that at the time of electricity privatisation, the Nuclear Installations Inspectorate’s activities should be maintained and increased, as the independent role that it exercises on nuclear safety will be even more important than it is today. The Chernobyl accident, as well as experience in the USA with private utilities operating nuclear plant, illustrates the importance of active and independent supervision by inspectors who can, if the situation warrants it, order plant to shut down if safety is at risk.

Appendix 10

Energy Casualties From the point of view of casualties, hydroelectric power has a bad record. In the United States of America alone, 100 dams have failed since 1930—mostly small irrigation plants. Among the large incidents have been Gujarat in India, where 15000 died, Vaiont in Italy, killing 2000, Guavio in Colombia with 150 killed and, in Britain, 16 killed at Dolgarog in Wales in 1925. Liquefied gas is another major hazard. In the recent (1984) Mexico City explosion, 500 or more people were killed. A road tanker accident in Spain in 1978 killed 150. A tank failure in Ohio, USA, in 1944 killed 130. Oil firing can also be hazardous. In 1982 an explosion at Tacao in Venezuela killed more than 98 and caused immediate injuries to 200; it also caused the evacuation of 20000 local residents. Chlorine is a hazard of oil refineries and power stations and there have been a number of chlorine store

explosions; 60 people were injured in Romania and there have been casualties in Western Europe and Canada, involving wide scale evacuation. Coal mining in the UK, where more than half of the output is used to generate electricity, has a current accident death rate for miners of about 30 per annum. Substantially more deaths may be attributed to occupational diseases. Nearly all plants for winning energy resources and converting and distributing energy involve construction and transport so that an overall figure would include the high risks in the building industry and on the roads. It has been suggested that the biggest risk to life in the Three Mile Island Reactor 2 accident arose on the roads in the precipitate evacuation of the local population. This is certainly a factor for consideration before ordering evacuation in an emergency.

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Appendix 11

Presentations to the Working Group on the Chernobyl Accident Appendix 11 A: Presentation on Inherently Safe Reactors Given on 11 February 1987 by Michael Hayns, Head of Nuclear Safety, Technology Branch, Safety and Reliability Directorate, UK Atomic Energy Authority

Dr Hayns made his presentation to the Watt Committee’s Working Group on the Chernobyl accident and its implications and also to a selection of guests nominated by the UK Centre for Economic and Environmental Development. The meeting was held at Savoy Hill House, London. Dr Cope, a member of the Chernobyl Working Group and Director of UK CEED, welcomed Dr Hayns and said that it seemed to him that in the past there had been a lack of discussion on inherently safe reactors in the UK compared with other countries. He was therefore encouraged by the research undertaken by the UKAEA.

During his presentation, Dr Hayns discussed the criteria needed for ‘inherent safety’ in reactor design and compared three ‘inherent safety’ systems, namely: (a) HTR—High Temperature Reactor (helium cooled/ graphite moderated) (b) PIUS—Process Inherent Ultimate Safety (water cooled/water moderated) (c) IFR—Integral Fast Reactor (sodium cooled/metal fuel).

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Appendix 11 B: Presentation on ‘The Chernobyl Accident and its Consequences’ Given on 1 April 1987 by John Gittus, Director, Safety and Reliability Directorate, UK Atomic Energy Authority

Dr Gittus spoke to the Working Group on the Chernobyl accident at Savoy Hill House, London. The group found Dr Gittus’s talk, and the discussion following, of great interest. Dr Gittus was one of the authors of an HMSO report entitled The Chernobyl Accident and its Consequences, which was published around the time of the presentation, and his talk was based on the report’s findings.

In August 1986 there was an IAEA meeting in Vienna to discuss the accident at Chernobyl and Dr Gittus was one of the British representatives at that meeting. Subsequently, Dr Gittus has carried out a critical study of the accident and made a comparison of design features and emergency measures between Britain and the USSR. As a result of these studies, Dr Gittus was able to clear up a number of points for the Working Group.

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Appendix 12

Visits Made by the Watt Committee Working Group on the Chernobyl Accident Appendix 12A: Visit to Dungeness ‘A’ Magnox Station

David Cope

Two visits were made to the Dungeness site. On 21 November 1986, a familiarisation visit was made to the Magnox Dungeness ‘A’ station, where there were particular discussions on the procedures for monitoring of accidental releases and on the planned Magnox Dissolution Plant for handling spent fuel element cladding. A second visit was made on 5 December 1986 to attend a meeting of the Local Liaison Committee (since renamed Community Liaison Committee) for both the ‘A’ and ‘B’ stations. The Committee has representation from the county and district local authorities and other interest groups in the area, such as representatives of MAFF and the fire and police

services, although one local council refuses to participate. Six-monthly meetings are held at which the managers of both stations present reports on the stations’ operations, with particular emphasis on operational incidents. Separate Health Physics reports are made for each of the stations. Abnormal incidents are also reported to members of the Local Liaison Committee at the time of occurrence, although at Dungeness, county council representatives recommended extensions to the notification procedure beyond advising Liaison Committee members alone of small events, however minor, so that questions from the public could be more effectively answered.

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Appendix 12B: Visit to the CEGB’s Oldbury-on-Severn Nuclear Power Training Centre John Bindon

Several members of the Working Group examining nuclear power safety in the UK since Chernobyl visited the CEGB’s Nuclear Power Training Centre on 19 December 1986. The purpose was to gain first-hand knowledge of the procedures and the practice of the formal training given to nuclear power station staff. This was thought to be an important part of assessing the approach of such staff towards safety. It would also provide an input to the Working Group in formulating its Final Report which would ultimately be published. The visiting team consisted of six members and they were received warmly by the Principal of the Centre, Mr V. J.Madden, who gave an introductory talk. Although the CEGB had established a Training School in the early days of the commercial operation of Berkeley and Bradwell nuclear power stations, (circa 1962), the decision to finance a purpose-built training establishment was not taken until 1971 (Fig. 1). The first course began in February 1973. At that time the Centre was equipped with only an analogue simulator, a single-point generic model. Nevertheless, it must be said that it served its purpose very well, enabling operating staff to involve themselves in a number of important exercises concerned with startup as well as practising the control of differing types of transients.

· · · · · · · · ·

The expansion to other, far more sophisticated, simulators did not occur until the mid-1970s, brought about by the need to upgrade the degree of training necessary. This was mainly for the new AGRs being commissioned. The first use of simulation specific to the AGRs was for training of staff for the Hinkley Point ‘B’ AGR in May 1980. This was followed over the next two years by simulators designed to reflect the plant parameters of specific stations, like Dungeness ‘B’, Hartlepool and Heysham. Since that time considerable expansion has taken place with the training starting on the Heysham 2 simulator in April 1986. Engineers from all the CEGB’s AGR stations will be able to undergo full training programmes, with all exercises in line with the real parameters of the plant and its behaviour. The design of the simulators was seen as most representative of the control rooms of the stations, being a very close replica of the station’s own individual design. (An article describing the ‘History and Development of Nuclear Power Station Training in the CEGB’ appears in The Nuclear Engineer, No. 3, May/June 1987.) This allows the staff, during their initial training for such stations, to become very conversant with the plant before the station is commissioned. It further affords the means to provide excellent retraining

Decision to build 1971 Date of first course February 1973 Sanction of AGR simulators 1976 First training use of Hinkley Point B Simulator May 1980 First training use of Dungeness & Hartlepool/Heysham 1 Simulators 1982 Sanction for Heysham 2 Simulator August 1982 First training use of Heysham 2 Simulator April 1986 Planned completion of Heysham 2 Simulator March 1988 Planned completion of New Extension January 1988 Fig. 1. Key dates in the history of Oldbury Nuclear Power Training Centre.

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The Chernobyl accident and its implications

128

Fig. 2. Typical organisational structure of a nuclear power station.

facilities which are being conducted for the AGR stations on a 12-month basis. It is perhaps worth mentioning that, since the visit of the Working Group to Oldbury, the Layfield Inquiry, in one of its many recommendations, requires that permission for full loading at Sizewell ‘B’ will not be given until 12 months after the specially built simulator for the PWR becomes available for operator training. The staff at the Training Centre are divided into five groups. Specialist Technology, Operations, PWR, Development and Administration. The PWR training team were appointed in 1983 and they have all undergone a very comprehensive training programme at overseas utilities with PWR operating stations. The CEGB have formal documents for all nuclear power plant training with a standard specification covering details of the courses which must be undertaken by all nuclear power station engineering

staff. Figure 2 shows a station’s organisational tree which is representative of most nuclear power stations of the CEGB and SSEB. Figure 3 shows a brief outline of the training courses, the details of which are very extensive in content. A number of points need to be mentioned about these courses. One is that there is always provision made for refresher courses for staff. These take place at yearly intervals for AGR staff and at twoyearly intervals for Magnox station staff. As the refresher courses can only be of a few days’ duration, they have to be carefully planned to match the individual requirements of the particular station and indeed the particular group of engineers. These complement courses on site. Again, as much use as possible is made of simulation techniques, but at present this is very limited for those coming from the Magnox stations. This is because they

Fig. 3. Outline of training courses.

Appendix

have to use the old type generic single-point model for simulation. However, the Group members were given to understand that this is to be shortly rectified by the replacement of the old machine by a newly designed simulator. For the newly appointed nuclear power station staff coming to the initial courses at the Centre, there is an assessment undertaken of the engineer’s performance while on the course and a final written examination does take place. The records of performance are fed back to the engineer’s own Station Manager. On aspects of safety, the CEGB provide lectures from their own Health and Safety Branch, as well as some external lectures where appropriate. The SSEB are catered for at Oldbury for these initial courses, although they have their own simulator machines available, at, for example, Hunterston, to take care of the practical training requirements at SSEB stations. In pure statistics, the output of Oldbury as a training establishment is high. A total of 1660 persons undertook courses there in 1986/87, 179 courses being organised. This is equivalent to some 2137 student-weeks.

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With the completion of Heysham II and Torness, the UK’s nuclear capacity will have reached nearly 13000 MW. It is worth noting that this is being achieved while the CEGB’s original nuclear power stations at Berkeley and Bradwell are still in operation (commissioned circa 1962). The safety record of British nuclear power stations has been excellent and much of the credit for this must rest with the fine training standards which have been achieved through the use of the Training Centre at Oldbury. This must also be a compliment to the dedication of the staff at the Centre who clearly create an environment in which personnel can undertake such courses with a considerable degree of technical support and advice. The Watt Committee’s Working Group members were impressed with what they observed during their short visit. The Committee wishes to record its thanks for all the help and assistance given and for the hospitality received from the Principal and all his staff.

Appendix 12C: Visit to Hinkley Point ‘B’ AGR Station Glynne Lewis

Introduction

spindle passing through the closure had blown out during maintenance and the reactor had suffered a slow depressurisation fault. A Site Incident Alert had been declared by the Emergency Controller, and emergency repair teams wearing breathing apparatus had effected a temporary repair to stop the release of gas. The reactor was shut down at the time. There were neither casualties nor contamination. The incident had been reported under reportable incidents’ procedures and appropriate modifications made.

Two members of the Working Group visited Hinkley Point ‘B’ on 12–13 February 1987 to study some of the normal and emergency operational procedures on an Advanced Gas-cooled Reactor station. The principal personnel that the Watt Committee representatives met during their visit were Mr J Outram, Station Manager, Mr J Moares, Resources Manager, and a fuel handling engineer. The plant areas were visited, including the Central Control Room (CCR), the Gas Circulator Hall (GCH), the Irradiated Fuel Discharge (IFD) facilities, the pile cap and the fuelling machine. Operational and emergency procedures and operational training were discussed with the station staff.

Fuelling machine In November 1978, on-load refuelling at full power was suspended when one fuel assembly gave difficulty during its discharge from the reactor. Inspection of the fuel assembly during discharge using remote viewing equipment and with the reactor shutdown revealed that the graphite sleeves of three of the eight fuel elements which make up the fuel stringer had been severely damaged. Examination of the graphite sleeve debris recovered from the reactor indicated that one sleeve had been broken by the differential pressure which exists across the pressure dome and which is imposed upon the fuel assembly during initial loading. This damaged sleeve allowed a partial bypass of the coolant during its subsequent irradiation and probably caused overheating of the lower fuel elements which eventually led to premature fuel pin failure. The fission product gas leakage into the coolant was detected by the gas activity monitoring equipment and action was taken to discharge the fuel assembly which led to this incident. After major modifications to the fuel machine, hoist controls to provide load sensing devices so as to monitor the progress of the fuel assembly during loading and discharge from the reactor, the provision

Control and instrumentation The Data Processing System displays all operating parameters and the state of the plant. Alarms are displayed separately. Operator action is taken immediately to clear operational faults. Plant defects relating to safety are investigated and cleared if possible in 1 day. Lesser defects are cleared within 3 days. Reasons for all standing alarms are recorded for the information of all operational staff. Major faults may result in the immediate shutdown of the reactor or wait for a planned outage. Safety related plant is covered by alternative conventional instrumentation as backup to the computer-derived data. The Central Control Room is compact and is manned continuously by a supervisor and two desk operators. Gas Circulator Hall The eight gas circulators are housed within the concrete pressure vessel. The primary closure area of one gas circulator was visited, where previously an operating 130

Appendix

of alternative emergency cooling to the fuel machine, improved quality assurance standards for graphite sleeve integrity and a comprehensive fault analysis of fuel handling procedures, the Board’s nuclear safety committee agreed, and the Nuclear Installations Inspectorate approved, that on-load refuelling at 30% power could be permitted. Operational experience during the past year or so, together with a revised safety submission, is currently being considered which, it is hoped, will allow onload refuelling operational at 50% power. After the new fuel assembly has been loaded and the fuel machine removed from the reactor standpipe, closure locking devices are installed, the gag motor drive for channel gas flow adjusted and channel gas outlet thermocouples and gas activity monitoring facilities are connected. Adjustments are then made by the Central Control Room operators to bring the operating conditions for the new fuel assembly to those specified. The fuel machine cycle—new fuel into machine, visit reactor standpipe, fuel exchange and discharge of irradiated fuel to breakdown facility—takes about 6 hours. The reactor power is reduced to 30% and conditions allowed to stabilise before refuelling operations at the reactor commence. About eight fuel stringers are changed at each power reduction and 120–140 stringers would be processed per year. There are 308 fuel channels and 81 control rod channels per reactor. Post-irradiation examination of the fuel pins from the damaged fuel stringer has not yet been completed because of potential difficulties which may be experienced in the only irradiated fuel handling cell available, which is used to handle fuel from both reactors. Reactor Shutdown Sequence Equipment (RSSE) This automatically provides essential cooling to the reactor to remove decay heat from the fuel following a reactor trip. All the necessary auxiliary plant operations are sequentially initiated by this control equipment and are completed in a period of about 10 minutes. In the event of the coincident loss of normal station electrical supplies, emergency generators are automatically brought on load to supply essential auxiliary plant. Of course, uninterrupted electrical supplies are provided from batteries via motor generator sets. These include control and instrumentation supplies and emergency lighting, etc.

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Appropriate sequencing of the essential auxiliaries is designed to limit temperature transients to the internal structures of the reactor. After about 30 minutes, when the fuel temperatures have subsided, the small capacity independent decay heat removal plant is introduced and the boilers are allowed to dry out and are isolated. Following the reactor trip the operators monitor the progress of the RSSE from a display panel in the Central Control Room. They may intervene if any particular item of auxiliary plant fails to respond in its correct time sequence. However, sufficient redundancy of essential plant is provided, making such intervention unnecessary on safety grounds. Finally, all discrepancies are noted and defects rectified and functionally tested before reactor startup. Emergency arrangements The Design Basis Accident (DBA) (which was originally referred to as the maximum credible accident) for the AGR is considered to be a loss of coolant accident (LOCA) resulting from the failure of one of the many concrete pressure vessel penetrations. These are provided for refuelling standpipes, for connections to be made to steam boilers, auxiliary gas circuit pipe-work and pressure relief valve headers. Failure of any of these penetrations, or rupture to the external pipe-work between the pressure vessel and the pipe-work isolating valves, would cause the reactor to depressurise. In the event of a worse LOCA, the reactor will depressurise in approximately 50 minutes. A continuous supply of CO and N to the 2 2 reactor, by an alternative route, in the event of auxiliary gas circuit failure, is designed to prevent air ingress to the hot core so as to limit fuel and graphite temperature transients and to remove decay heat from the fuel. Fuel cladding temperature transients are not expected to reach the melting point for stainless steel (1350°C). The fault analysis pessimistically assumes that some of the cladding failures occur in the highest rated fuel pins, due to the build-up of fission product gas pressure, and that radioactivity escapes to the environment. Such a release is not expected to result in Emergency Reference Levels of dose to be exceeded beyond the station boundary. Nevertheless, the licensee (CEGB) is required to maintain Emergency Arrangements so as to protect the station staff and local residents within 3 km of the station.

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Emergency arrangements and training methods were discussed with the operators with particular reference to the following aspects: (a) On and off site air and deposition activity monitoring, and determination of Emergency Reference Levels. (b) Action to safeguard the damaged and operating plant. (c) Damage location and repair teams. (d) Fire fighting, first aid and rescue teams. (e) Emergency Control Room, incident control points; Health Physics Control Room procedures.

The Chernobyl accident and its implications

(f) Notification and communications. (g) Assistance by local, regional and national organisations. (h) Setting up of the Operational Support Centre (OSC) and appointment of the Government Technical Advisor (GTA) to provide facilities for media briefing and public reassurance. These emergency facilities were implemented following the Three Mile Island experience; their need was emphasised again by the Chernobyl accident.

Appendix 12D: Visit to the Atomic Energy Establishment Winfrith SGHWR Station Frank Allen

The Chairman and seven members of the Watt Committee Working Group visited the Atomic Energy Establishment Winfrith on 23 March 1987. They were accompanied during their visit by Winfrith staff, including Dr D.Pooley (Deputy Director), B.Negus (Steam Generating Heavy Water Reactor Operations Manager) and K.J. McLean (SGHWR Safety Manager), and by Dr R.S.Peckover of the UKAEA Safety and Reliability Directorate. Dr Holmes, Director of Winfrith, welcomed the Group. The morning was devoted to the Winfrith reactor—the prototype SGHWR—including

presentations about the reactor and its safety systems, and a tour of the reactor itself. In the afternoon, the Group were shown some of the experimental reactor safety research going on at Winfrith. The Group were interested in visiting SGHWR because it is a direct cycle pressure tube reactor, with some superficially similar features to the Chernobyl reactor. The moderator is, however, ‘heavy’ water rather than graphite as at Chernobyl. Control is by pumping boric acid solution into liquid shutdown tubes and ‘dumping’ heavy water moderator.

Table 1 Points of contrast between the Chernobyl-4 RBMK and the Winfrith reactor

a

LSD=Liquid shutdown.

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The design and operation of the reactor were explained by Mr Negus. The safety procedures and how they related to reactor operations were explained by Mr McLean, who also went into the automatic safety provisions and the safeguards associated with by-passing. (The Chernobyl accident would not have occurred if the reactor operators there had not disabled the automatic protection system.) The reactor physics and power coefficients were explained by Mr Hinton. Unlike Chernobyl, the SGHWR has a negative void coefficient and a negative power coefficient at all power levels. The system for operator training which was outlined by Mr McLean, includes the use of the Winfrith simulator. The UKAEA reactors are not formally licensed by the NII, although they are required to maintain a standard of safety equivalent to that of licensed sites. Dr Peckover of SRD explained how SRD monitored the safety of operations and reported independently to the Chairman of the Authority on this. Nevertheless, as for all industrial plant in the UK, the responsibility for safety lay with the line management on the site concerned. A convenient table contrasting important characteristics of the SGHWR and the Chernobyl

The Chernobyl accident and its implications

reactor was made available by Winfrith and is included here. Mr Gratton gave a brief overview of some of the reactor safety research at Winfrith. Dr Wood discussed the results produced in the Molten Fuel Test Facility (MFTF) which is devoted to steam explosion experiments involving kilogram quantities of fuel. Dr Bird showed the newly commissioned Achilles Rig which investigates fuel pin behaviour. The group also visited the Horizontal Impact Facility which investigates the damage caused by massive but relatively slowly moving missiles. All the work discussed will help to further reduce uncertainty in safety cases as they relate to the initiation and development of hypothesised severe accidents. There followed a discussion of a number of points raised in the course of the visit. In conclusion, the Working Group gained a clear picture of how SGHWR functions as a result of the tour and the presentations. The differences in design and in ‘safety culture’ between SGHWR and what is believed to have been the case at Chernobyl were elucidated. It is clear that a Chernobyl-type accident could not occur at SGHWR.

Appendix 12E: Visit to Hunterston ‘A’ Nuclear Power Station to attend an Emergency Exercise Glynne Lewis

Introduction

simulator from the desk and the plant panels, to simulate the generation of electricity up to full power of the real plant of 660 MW(e). At any time during the training the instructor can change plant conditions by means of a variety of special facilities such as switches and keyboards, and can inject a single or multiple fault sequence one after another, or all at once. The operator would hope to counteract the simulated fault conditions and, if unsuccessful, the reactor would trip like the real plant, to prevent any safety parameter exceeding its set limit. The operator can communicate with the instructor and call for actions such as closing or opening valves. This would be carried out by the instructor. The instructors can analyse the results of the operator’s actions as these affect the reactor plant, which are recorded in analogue and digital form. Communications can be connected in parallel to the real station systems, allowing the simulator to be used in place of the real control room during emergency exercises. Other training programmes on the simulator include reactor shutdown, on-load refuelling and turbine run-up, as well as abnormal or fault situations, including the loss of auxiliary plant with the reactor at power, such as:

A visit to Hunterston was made on 11–12 May 1987 by Norman Worley and Glynne Lewis on behalf of the Watt Committee Working Group on the Chernobyl Accident and its Implications. The attendance at the emergency exercise on 12 May was by the kind permission of the South of Scotland Electricity Board. To become familiar with the plant and the postulated accident details in advance of the exercise, a visit was made to the plant the day before (11 May), and the two Watt Committee representatives were escorted around the ‘A’ Station by one of the Assistant Shift Charge Engineers. Included in the tour were the Central Control Room, the Turbine Hall, the Reactor Building and the Emergency and Health Physics Control Rooms. Later in the afternoon the Deputy Station Manager of the ‘B’ Station took the Watt Committee representatives to the AGR simulator building where the Simulator Manager demonstrated a reactor trip. The simulator consists of a large number of microcomputers which model the separate plant functions: reactor, turbine, feed heating, boiler, safety and protection circuits, auto controls and alarms. The models can be used totally integrated or in split mode, e.g. the reactor can be operated independently from other plant items in real time. All the desks and control panels are the same as for the real reactor. The simulator is used for basic training of operators on normal reactor operating procedures, and under abnormal or fault conditions. Plant programmes are stored in computer memories and can be further programmed for different training requirements. The computer can be programmed for reactor startup with various reactor boiler and turbine temperatures and pressures. The operator controls the

— — — — — —

Loss of main boiler feed pump Loss of standby boiler feed pumps Loss of coolant accident Loss of station electrical supplies Loss of reactor auxiliary cooling water system Loss of reactor pressure vessel cooling water system — Loss of main cooling water system — Reactor gas baffle failure A demonstration was given of a reactor trip from full power using the emergency trip button with the automatic initiation of the Reactor Shutdown Sequence 135

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Equipment which required no operator intervention. The following events were noted: the insertion of the control rods; the tripping of the turbine and gas circulators; the venting of the boilers to a lower steam pressure; the starting of four gas circulators sequentially from the 11 kV Station Board; and the starting of the standby boiler feed pumps. All the essential auxiliary plant functions were completed in a timed sequence to limit plant temperature transients to a minimum. Alarms and flashing indications had to be accepted by the operator, and reactor conditions gradually subsided in real time, finally to allow the boilers to be dried out and the low power decay heat loops to be introduced manually. A visit was also made to the ‘B’ station which consists of 2×1500 MW(th) AGRs with 2×660 MW(e) turbo-alternators, giving a design net station output of 1235 MW(e). The tour of the station included a visit to the Central Control Room, Gas Circulator House and Turbine Hall. The pile cap was viewed from the visitors’ observation gallery. At the time of the visit, Reactor 3 was at 30% power for on-load refuelling operations and Reactor 4 was at full power. ‘A’ Station emergency exercise (12 May 1987) Information on the extent to which the emergency plan would be involved had already been given to the two Watt Committee representatives. The exercise commenced at 0900 hours. The simulated loss of coolant accident (LOCA) occurred on Reactor 1 (Magnox) when a weld on the large gas ducts between the reactor pressure vessel and the external boiler vessel was deemed to have failed. The gas pressure dropped from 160psi to atmospheric in about 80 s. The reactor tripped on the rate of change of pressure protection. The gas circulators tripped on electrical disturbance due to plant damage. The depressurisation resulted in a single channel fuel fire, releasing about 40TBq I-131 (about 1000Ci to the environment in the general direction of Great Cumbrae island, about 3 km distant across the Firth of Clyde. Those attending the accident simulation were able to see, from various vantage points, the action of the Station Emergency Teams dealing with the accident situation.

The Chernobyl accident and its implications

Central Control Room Operators on the spare shift were given simulated information concerning the accident. They carried out the actions that would have been necessary to stabilise the damaged reactor. These included attempts to isolate the damaged plant by closing duct valves, supply of carbon dioxide reactor coolant to prevent air ingress to the damaged core, and start-up of gas circulators to provide forced convective cooling of the fuel. The CCR with its normal duty staff, the spare shift staff, umpires and observers, was overcrowded and this made it very difficult for the participants to react realistically to the changing situation. The use of a simulator, as described previously, will assist considerably in providing a more real accident environment. Reactor building The observers saw the emergency teams being equipped at the Emergency Store, and move to the Forward Base set up on the upwind side of the reactor building. Basic health physics controls and change facilities were quickly established so as to allow the emergency teams to enter into the hazardous interior of the building. Damage Location Teams demonstrated the effectiveness of equipment and communications. Damage Control Teams carried out emergency repairs and demonstrated plant isolating techniques. Rescue and First Aid Teams recovered three casualties back to the Medical Centre for decontamination and subsequent transmission to hospital. Although the Station Fire Teams assembled and the Strathclyde Fire Brigade reported to the station, there were no fires to extinguish. The Fire Brigade assisted in the rescue of the casualties. Emergency Control Room The Emergency Control Room and the adjacent Health Physics Control Room, permanently established to manage all on-site activities, were both visited. Maps of the area displayed meteorological and activity information received from monitoring teams by radio. Engineering Managers were giving advice on plant damage limiting requirements. Reactor physicists were studying the developing fuel and graphite temperature transients. The Emergency Controller was directing the necessary actions to safeguard the plant, limit the releases and protect the public at risk. Involvement from the Strathclyde Police Force was in the form of security duties associated with the

Appendix

reactor accident and the release of activity to the environment. These duties included access road entry controls, evacuation of employees of the fish farms located near the site boundary to the West Kilbride Reception Centre, and issue of potassium iodate tablets to members of the public at risk. Communications with site control centres and with local and national organisations were observed. The Operational Support Centre (OSC) was set up at Cathcart HQ SSEB to receive information concerning the Reactor 1 accident. It did not carry out its normal function of assisting the station by taking over all its external communication responsibilities. A press centre was set up in the Barfield Pavilion, Largs, the nearest town, for media briefing. A full register was carried out on the site. All staff and contractors reported to assembly areas and roll call results telephoned to the Emergency Control Room. Comments The emergency exercise representing activities which would be expected to cover several days, took place on an accelerated time scale lasting between 3 and 4 hours, and so activities were carried out in parallel where normally they would be sequential. Groups consisting of emergency team members, umpires (to give information), assessors and observers led to overcrowding. There was no physical simulation of plant damage. While the Watt Committee representatives attended as observers and did not attend the formal discussion of the exercise, there appeared to be a number of areas

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where the exercise could perhaps have been more representative of a real situation. Access to damaged plant area could have been made more difficult by temporary barriers and prohibition notices to test the teams’ knowledge of alternative routes. Noise generators could have been used to test the effectiveness of radio communications whilst using breathing apparatus. Radiation and contamination hazards in the vicinity of the damaged duct, and the release of hot gas that would have occurred in the reactor building, were not appreciated by the participants under these simulated conditions. The change and decontamination control facilities set up at the Forward Base on the ground floor level of Reactor 1 would have been inadequate to handle members of the emergency teams returning from the highly contaminated damaged plant areas, in a real accident situation on the scale postulated for the exercise. Facilities for the removal and storage of heavily contaminated clothing and equipment, the personal decontamination of operators, the availability of hot showers, provision of clean clothing and radiological dose supervision also appeared to be less than adequate for a major incident. Lessons can be learned from the Chernobyl accident in this respect and facilities and procedures provided to suit the worst situation resulting from a design basis accident. However, it is difficult to suggest how accident conditions can be portrayed more realistically. Tests and emergency exercises are carried out regularly at all nuclear stations throughout the United Kingdom. The Watt Committee on Energy attended the exercise at Hunterston in order to see how such operations are conducted.

THE WATT COMMITTEE ON ENERGY Objectives, Historical Background and Current Programme 1. The objectives of The Watt Committee on Energy are: (a) to promote and assist research and development and other scientific or technological work concerning all aspects of energy; (b) to disseminate knowledge generally concerning energy; (c) to promote the formation of informed opinion on matters concerned with energy; (d) to encourage constructive analysis of questions concerning energy as an aid to strategic planning for the benefit of the public at large. 2. The concept of the Watt Committee as a channel for discussion of questions concerning energy in the professional institutions was suggested by Sir William Hawthorne in response to the energy price ‘shocks’ of 1973/74. The Watt Committee’s first meeting was held in 1976, it became a company limited by guarantee in 1978 and a registered charity in 1980. The name ‘Watt Committee’ commemorates James Watt (1735– 1819), the great pioneer of the steam engine and of the conversion of heat to power. 3. The members of the Watt Committee are 61 British professional institutions. It is run by an Executive on which all member institutions are represented on a rota basis. It is an independent voluntary body, and through its member institutions it represents some 500000 professionally qualified people in a wide range of disciplines. 4. The following are the main aims of the Watt Committee: (a) to make practical use of the skills and knowledge available in the member institutions for the improvement of the human condition by means of the rational use of energy; (b) to study the winning, conversion, transmission and utilisation of energy, concentrating on the United Kingdom but recognising overseas implications; (c) to contribute to the formulation of national energy policies; (d) to identify particular topics for study and to appoint qualified persons to conduct such studies; (e) to organise conferences and meetings for discussion of matters concerning energy as a means of encouraging discussion by the member institutions and the public at large; (f) to publish reports on matters concerning energy; (g) to state the considered views of the Watt Committee on aspects of energy from time to time for promulgation to the member institutions, central and local government, commerce, industry and the general public as contributions to public debate; (h) to collaborate with member institutions and other bodies for the foregoing purposes both to avoid overlapping and to maximise cooperation. 5. Reports have been published on a number of topics of public interest. Notable among these are The Rational Use of Energy (an expression which the Watt Committee has always preferred to ‘energy conservation’ or ‘energy efficiency’), Energy Development and Land in the United Kingdom, Energy Education Requirements and Availability, Nuclear Energy and Acid Rain. Others are in preparation. 6. Those who serve on the Executive, working groups and sub-committees or who contribute in any way to the Watt Committee’s activities do so in their independent personal capacities without remuneration to assist with these objectives. 139

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The Chernobyl accident and its implications

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Enquiries to: The Information Officer, The Watt Committee on Energy, Savoy Hill House, Savoy Hill, London WC2R 0BU Telephone: 01–379 6875

Member Institutions of The Watt Committee on Energy

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British Association for the Advancement of Science British Nuclear Energy Society British Wind Energy Association Chartered Institute of Building Chartered Institute of Building Services Engineers Chartered Institute of Management Accountants Chartered Institute of Transport Combustion Institute (British Section) Geological Society of London Hotel Catering and Institutional Management Association Institute of Biology Institute of British Foundrymen Institute of Ceramics Institute of Chartered Foresters Institute of Energy Institute of Home Economics Institute of Hospital Engineering Institute of Internal Auditors (United Kingdom Chapter) Institute of Management Services Institute of Marine Engineers Institute of Mathematics and its Applications Institute of Metals Institute of Petroleum Institute of Physics Institute of Purchasing and Supply Institute of Refrigeration Institute of Wastes Management Institution of Agricultural Engineers Institution of Chemical Engineers Institution of Civil Engineers

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Institution of Electrical and Electronics Incorporated Engineers Institution of Electrical Engineers Institution of Electronic and Radio Engineers Institution of Engineering Designers Institution of Gas Engineers Institution of Geologists Institution of Mechanical Engineers Institution of Mining and Metallurgy Institution of Mining Engineers Institution of Nuclear Engineers Institution of Plant Engineers Institution of Production Engineers Institution of Structural Engineers International Solar Energy Society—UK Section Operation Research Society Plastics and Rubber Institute Royal Aeronautical Society Royal Geographical Society Royal Institute of British Architects Royal Institution Royal Institution of Chartered Surveyors Royal Institution of Naval Architects Royal Meteorological Society Royal Society of Arts Royal Society of Chemistry Royal Society of Health Royal Town Planning Institute Society of Business Economists Society of Chemical Industry Society of Dyers and Colourists Textile Institute

Watt Committee Reports 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 13. 14. 15. 17. 18. 19.

Deployment of National Resources in the Provision of Energy in the UK The Rational Use of Energy Energy Development and Land in the United Kingdom Energy from the Biomass Evaluation of Energy Use Towards an Energy Policy for Transport Energy Education Requirements and Availability Assessment of Energy Resources Factors Determining Energy Costs and an Introduction to the Influence of Electronics The European Energy Scene Nuclear Energy: a Professional Assessment Acid Rain Small-Scale Hydro-Power Passive Solar Energy in Buildings Air Pollution, Acid Rain and the Environment The Chernobyl Accident and its Implications for the United Kingdom

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Index Abnormalities causes of, 67 reactions to, 66–7 Accident management United Kingdom, 32–3, 40–5 USSR, 35–40 Watt Committee recommendations on, 46 Accidents casualties due to, 3, 4, 123 general consequences of, 3 AGR (Advanced Gas-cooled Reactor) containment system, 50 control rods, 51 coolant used, 52 described, 52 design features, 50 energy characteristics, 47 reactivity coefficients, 56 reactivity feedback effects, 50 reference accident, 45–6 training of staff for, 65, 127, 28 visit by Watt Committee, 130–2 ALARA (As Low As Reasonably Achievable) principle, 61, 63, 81 Alcohol, health risks of, 32 Animal feedstuffs, EEC limits on, 80 Arrhenius equation, 112 Article 31 Expert Group, 79, 80 Atomic bomb, 21–2 Austria, nuclear power plans affected, 86 Automatic shutdown procedures, 14, 23 disconnected at Chernobyl, 20, 90 Barium isotopes, emissions from Chernobyl, 26 Berkeley (UK) Magnox Station, 51 monitoring point, 83 Boiling crisis, 20, 53 Boiling water reactor, 55, 56 Borodavko, Y., 116 Bradwell (UK) Magnox Station, 51 monitoring point, 83 British Nuclear Energy Society, 117 British Nuclear Forum, 117 British Nuclear Fuels plc, monitoring by, 27, 31, 82, 83, 84 Brussels Convention, 75–6

Bryukhanov, Viktor, 114 Bulldozer tractors, radio-controlled, 40 Caesium isotopes deposition in UK, 27, 29 emissions from Chernobyl 4 reactor, 25, 26, 40 Three Mile Island 2 reactor, 27 foodstuffs contaminated with, 31, 40 reference values, 30, 31 UK levels, 31 intervention levels quoted, 77 Calder Hall (UK), Magnox Station, 51, 106 Canada, fuel meltdown incidents, 4, 5 Cancers, radiation-linked, 3, 32 CANDU reactors, 48 Casualties energy industry generally, 123 nuclear power plant, 3, 4, 32, 37, 91 Catastrophic accidents, 67 CCR (Central Control Room), 136 Soviet system, 36 UK system, 41–2, 67 CDF (Central Data Facility), 84 CEGB (Central Electricity Generating Board) power stations visited by Watt Committee, 126, 130–2 training centre, 127–9 Cerium isotopes emissions from Chernobyl 4 reactor, 26 Three Mile Island 2 reactor, 27 Chapelcross (UK) Magnox Station, 51 monitoring point, 83 Chemical reactions, 112–13 Chernobyl Unit 4 reactor accident, 35–6, 91, 106–7 background, 89–90 casualties, 32, 37, 91 causes, 23, 90–1 chemical reactions in, 112–13 chronology, 106–7 debris from, 22 development, 21–2 economic costs outside USSR, 78 effects on UK, 27–31, 93–4 emergency services involved, 36, 107

143

events prior to, 19–20, 90 international consequences, 72, 92–3 radiation levels, 37 response by central government, 37, 91, 107–8 response of station operators, 36–7 role of the containment system, 22 technical information required about, 97 circulating pumps used, 18 compared with Winfrith SGHWR, 133–4 containment system, 16 role in accident, 22 continuing radiation leakage, 116 control rod effects, 13–14, 58, 89, 91 coolant circuit design, 11–12, 18 coolant voidage, 58–9 cooling of damaged core, 39, 91, 108 core monitoring systems used, 14–15 decontamination of site, 39, 109 design data, 17–18 Doppler effect, 59, 89 emergency core cooling system, 15–16, 19, 37, 90 emergency power supply, 16 emergency procedures, 14 entombment of, 39, 108–9, 116 extinguishing of core fire, 38, 91, 108 fuel element design, 12, 13, 17, 22 geographical location, 37 heat barrier constructed for damaged core, 38 layout of plant, 9, 10, 36 managers named, 114 materials deposited on core fire, 38, 91, 108 maximum design temperature, 11 power data, 17–18 protection system defeated by operators, 20, 23 radioactive material released from, 22, 25–6, 39–40 compared with Three Mile Island reactor, 27 duration of release, 25 first emissions, 25 inventory of release, 25–6

Index

144 meteorological factors affecting dispersal, 26–8 reactivity coefficients, 18 reactor core arrangement, 9, 11, 17 reactor physics, 58–9 refuelling system, 11–12 safety rods, 14 safety rules breached, 20, 90 sectional elevation, 10 shift personnel, 36 stability characteristics, 19–20, 23 steam circuit arrangement, 12 steam generation system, 12, 18 test planned, 19, 90 trial of responsible officials, 114–15 turbine system, 18 turbo-alternator capacity, 18, 36 xenon poisoning in, 58, 90 China Syndrome, 38 Chlorine, accidents involving, 123 Civil court actions, 77 CMEA, 118 Coal mining accidents, 123 USSR reserves, 6, 7 Communications, public media, 84 Conferences, 96 Containment system, 49–50 Chernobyl Unit 4 reactor, 16 role in accident, 22 term defined, 2 Control rods, 51–2, 58, 89 Chernobyl Unit 4 reactor, 13–14, 58, 89, 91 increase in reactivity caused by, 21 term defined, 3, 101 withdrawal beyond safety limits, 20 Conversion factors, 104 Coolant system Chernobyl Unit 4 reactor, 11–12, 18 general description of, 1–2 Coolant voidage, Chernobyl Unit 4 reactor, 58–9 Coolants, 52–4 gas and liquid compared, 53–4 Critical condition, 21, 101 Dairy produce, EEC limits on, 79, 80 Delayed neutron release, 3, 21, 55 DERL (Derived Emergency Reference Levels) term defined, 31 values quoted for UK, 30 District heating systems, 8 Doppler effect, 20, 56, 57, 59, 89 Dounreay, Prototype Fast Reactor, 50 Drinking water EEC limits on, 80 measures taken to prevent contamination, 40, 91 radionuclides in, 30, 31 Dungeness (UK) AGR Station, 52 Community Liaison Committee, 126 Magnox Station, 51, 126 monitoring site, 83 Dyatlov, Anatoly, 114

Early Notification Convention, 73, 74, 85 ECCS (Emergency Core Cooling System) Chernobyl Unit 4 reactor, 15–16, 19, 37,90 UK AGR stations, 131 Economic costs, Chernobyl accident, 78 ECR (Emergency Control Room), 42, 136–7 EEC (European Economic Community) foodstuffs controlled by, 79 other actions by, 81 response of, 78–81 Emergency arrangements Central Control Room response, 36, 41–2 Emergency Control Room manned, 42 exercises held, 41, 65 recommendations for, 46, 137 Health Physics Control Room manned, 42–3 Hunterson ‘A’ Station, 136–7 Incident Control Centre manned, 43 National Plan, 32, 43–5 need for, 35 Station Emergency Plan, 35, 40–1 Station Medical Centre manned, 43 training given, 41, 65 United Kingdom, 32–3, 40–5, 94 USSR, 35–40 Emergency Assistance Convention, 73, 74, 85 Emergency Exercise, Watt Committee on Energy criticism of, 137 Emergency power supply, Chernobyl Unit 4 reactor, 16 Emission standards, 81 Energy industry casualties, 123 England caesium isotrope deposition in 27, 29 see also UK Enrichment, term defined, 1, 101 Euratom Treaty, 78 European Nuclear Society, 117 European organisations, 117–18 Evacuation of population Chernobyl area, 37–8, 91, 108 intervention level for 37, 42 Three Mile Island area, 123 Fail-safe factor, 68 FAO (Food and Agriculture Organization), 72 Farm animals, evacuation of, 38 Fast reactor containment system, 50 design features, 50 development in USSR, 8 energy characteristics, 47 general description, 54 Fire-fighting, 36–7, 94, 97, 107 Fission products, containment/release of 2, 22 Fomin, Nikolai, 114 Foodstuffs contamination of, 30–1

EEC limits on, 79–81, 93 intervention levels for, 77, 79, 80 Fossil fuels accidents involving, 123 emission standards for, 81 USSR reserves, 6, 7 France fuel meltdown incidents, 6 UK contact with, 85 Fruit, radionuclides in, 30 Fuel element design, 12, 13, 17, 22 meltdown incidents, 4–6 factors affecting, 6 listed, 4–6 reactivity of, 1, 2 replenishment system, 2 Chernobyl Unit 4 reactor, 11–12 Fuelling systems, 2 Gas accidents involving, 123 USSR reserves, 6, 7 Gittus, John, 125 Glossary of terms, 101–3 Government assistance Chernobyl Unit 4 reactor, 37 UK procedures, 43–5 Graphite, chemical reactions of, 113 Graphite moderated reactors reactivity coefficients for, 56 xenon effects in, 57, 58 see also AGR…; Magnox…; RBMK reactor Gray, unit defined, 104 GTA (Government Technical Advisor), 43 Gun accidents, casualties due to, 32 Hartlepool (UK) AGR Station, 52 monitoring point, 83 Hayns, Michael, 124 Health effects, 3–4, 31–2 Chernobyl area, 32 Europe, 32 radiation compared with ‘normal’ risks, 32 Health precautions Chernobyl Unit 4 reactor, 37–8 UK procedures, 42–3 Helicopters, materials dropped from, 38, 91, 108 Heysham (UK), AGR Station, 52 Hinkley Point (UK) AGR Station, 52, 130–2 control and instrumentation, 130 emergency arrangements, 131–2 emergency shutdown equipment, 131 fuelling problems, 130–1 gas circulator hall incident, 130 Magnox Station, 51 monitoring site, 83 Historical background, 105–6

Index HPCR (Health Physics Control Room), 42–3 Hunterston (UK) AGR Station, 52 Magnox Station, 51, 135–7 emergency exercise at, 136–7 monitoring site, 83 Hydroelectric power, casualties, 123 IAEA (International Atomic Energy Agency), 72–4 areas of activity, 119 budget, 119 delegation to USSR, 73 emergency assistance convention, 73, 74, 85 guides on emergency arrangements, 35,45 notification convention, 73, 74, 85 Nuclear Safety Division, 73 Watt Committee recommendations for, 119–20 ICC (Incident Control Centre), 43 Ice breakers, 8 ICRP (International Commission on Radiological Protection) recommendations, 38, 111 Ignatenko, Yevgeny, 116 Inherently safe reactors, 124 INSAG (International Nuclear Safety Advisory Group) reports, 19, 35, 92, 119 Institution of Nuclear Engineers, 117 Interlocks, reactor protection system, 23, 68 International agencies, 72 see-also FAO; IAEA; NEA; OECD: WHO International Commission on Radiological Protection, 72 International liability agreements, 74–8 International notification, 46 Intervention levels, 37, 42, 80 different levels, 77, 93 foodstuffs, 77, 79, 80 Introductory section, 1–8 Iodine isotopes deposition in UK, 30 emissions from Chernobyl 4 reactor, 25, 26, 40 Three Mile Island 2 reactor, 27 foodstuffs contaminated with, 31, 40, 77 European levels, 31 reference values, 30, 31 health effects of, 32 intervention levels quoted, 77 measures to prevent absorption of, 31, 37 Ion exchange unit, cooling water, 12 Ionising Radiation Regulations (1985), 63 Ireland, UK contact with, 85 IRS (Incident Reporting System), 118, 120 Italy, nuclear power plans affected, 86

145 Kovalenko, Alexander, 114 Kursk (USSR), power failure incident, 97, 106 Laushkin, Yuri, 114, 115 Layfield Inquiry, 130 Learned societies, 117 Licensees, 121 LOCA (Loss of Coolant Accident), 45 Local government agencies, 84–5 Local inhabitants advice given to, 37, 42 evacuation of, 38, 42 protection of, 37–8, 42 Logbooks, 66 Lukyanenko, V., 116 MAFF (Ministry of Agriculture, Fisheries and Food), monitoring by, 31, 82, 83, 84 Magnox (magnesium-no-oxidation) reactor containment system, 50 control rods, 51 coolant used, 51, 52 decommissioning of, 95 described, 51 design features, 50 emergency control of, 45 energy characteristics, 47 fuel used, 49 reactivity coefficients for, 56 reactivity feedback effects, 50 reference accident, 45 training of staff for, 65, 128–9 visits by Watt Committee, 126, 135–7 Maintenance requirements in UK, 62 shift system, 63–4 Maloperation, prevention of, 23 Meat, radionuclides in, 30, 31 Milk EEC limits on, 79, 80 radionuclides in, 30, 31 Moderator, term defined, 1, 102 Molybdenum isotopes, emissions from Chernobyl, 26 Monitoring and Assessment Network Group, 84 National Society for Clean Air, 85 NEA (Nuclear Energy Agency—OECD), 74–8, 118 international liability agreements, 74–8 working groups, 118 NEBC (Nuclear Emergency Briefing Centres), 43, 46 Neptunium isotopes, emissions from Chernobyl, 26 Netherlands, nuclear power plans affected, 85 Neutron flux detectors, location in Chernobyl Unit 4 reactor, 14–15

NPOC (Nuclear Power Oversight Committee), 117 NRPB (National Radiological Protection Board), 27, 31, 32, 42 Nuclear fission reactions, 1 Nuclear Installations Acts (1965, 1969), 61–2, 121 Nuclear Installations Inspectorate, 41, 46, 121–2 staffing of, 95, 121, 122 training within, 121, 122 Watt Committee recommendations on, 122 Nuclear power plant described, 1–4 health risks, 32 special features, 3 OECD (Organization for Economic Cooperation and Development), 118 Nuclear Energy Agency, 74–8, 118 see main entry: NEA Oil accidents involving, 123 USSR reserves, 6, 7 Oldbury-on-Severn (UK) Magnox Station, 51 Nuclear Power Training Centre, 127–9 number of students, 129 outline of courses, 128 Operatives selection of, 64 training of, 23, 64–5 Operator errors, 6, 20, 23 OSC (Operational Support Centre), 43, 46, 137 Paris Convention, 74–5, 76 Personnel selection criteria for, 64 training of, 64–5 PFR (Prototype Fast Reactor), 49 control rods, 51 coolant, 52, 54 see also Fast reactor Plutonium, 1, 102 Positive graphite temperature coefficient, 56, 98 Positive scram, 21, 23, 58, 89, 97 not possible with UK reactors, 98 Positive void coefficients, 56, 59, 89 reduction of 59, 92 Power coefficients, 56 SGHWR, 134 Prefix scale factors, 104 Pressure tubes, 49 Prompt critical condition, 21 Protection of reactors defeated in Chernobyl accident, 19, 37, 90, 106–7 general philosophy of, 68 Protection systems, 14 Public confidence, 46 Public opinion survey, 86, 87 Publicity, USSR, 91, 108, 109

Index

146 Pump cavitation, 58, 90 PWR (Pressurised Water Reactor) containment system, 50 coolant used, 52, 53 described, 53 design features, 50 energy characteristics, 47 NEA working groups on, 118 reactivity feedback effects, 50 training of staff for, 130 Radiation dose-rates on Chernobyl site, 37, 40 in surrounding area, 37, 38 units used, 104 health affected by 3–4, 31–2 term defined, 2, 102 Radionuclides emissions from Chernobyl Unit 4 reactor, 22, 25–6, 39–40 Three Mile Island 2 reactor, 27 intervention levels quoted, 77 Radon, effects of, 32 Rainwater, radionuclides in, 31, 36 RBMK (Reactor Bolshoi Moschnosti Kipyashiy) reactor advantages, 89 compared with SGHWR, 49, 51, 52, 133–4 UK reactors, 47–56 containment system, 49–50 control rods, 13–14, 51–2 coolant system, 11–12, 18, 48, 52, 53 described, 9, 48–50 design decisions for, 59, 92 design features, 50 energy characteristics, 47 first built, 8, 48 graphite temperatures, 49 reason for, 57 modifications made to improve, 17, 23 protection system defeated by operators, 20, 23 reactivity coefficients, 56 reactivity feedback effects, 50 reference accident, 45 stability characteristics, 19–20, 23 total capacity installed, 89 void coefficient, 20 weaknesses in design, 16–17, 23 Reactivity coefficients, 54–6 Chernobyl 4 reactor, 18 Reactivity feedback effects, 50 Redundancy, 68 Reference accident, 45–6 AGR, 45–6 Magnox reactor 45 RBMK reactor, 45 Regulations UK, 61–2, 121 USSR, 111 Resonance absorptions, 56–7

RIMNET (Radioactive Incident Monitoring Network) monitoring sites, 82–3 Road accidents, casualties due to, 32 Rogozhkin, Boris, 114 RSSE (Reactor Shutdown Sequence Equipment), 131 Russia. See USSR Ruthenium isotopes emissions from Chernobyl Unit 4 reactor, 26 TMI-2 (Three Mile Island—USA) reactor, 27 Safety Rules (Radiological), 63 UK power stations, 62–3 SAP (Senior Authorised Person), 63 Scotland caesium isotope deposition in, 27, 28, 29, 30 see also UK Scram system failures of, 4, 5 term explained, 3 see also Positive scram SGHWR (Steam Generating Heavy Water Reactor) compared with RBMK reactor, 49, 51, 52, 133–4 containment system, 50 control rods, 51 coolant, 49, 52 design features, 50 energy characteristics, 47 general description, 49 safety aspects, 134 visit by Watt Committee, 133–4 Shift duties (in UK), 65–6 Engineers, 64 Manager duties of, 64 emergency procedure, 41, 64 SI units, 104 Sievert, unit defined, 104 Simulation exercises, 65, 127 Simulators, 96, 127, 135 Site Licence, 41, 62 Sizewell (UK) Magnox Station, 51 monitoring point, 83 proposed PWR station, 50, 53, 87 Smoking, health risks of, 32 SOI (Station Operating Instructions), 62 Soviet Union. See USSR SSEB (South of Scotland Electricity Board), 121 power stations, 129, 135–7 Stability characteristics, 19–20, 23 Stability of reactors, 54–6 Staff qualification of, 96 refresher courses for, 128 training, 96 UK nuclear power station, 128 Staffing

Chernobyl site, 36 UK nuclear power stations, 63–4 Standardisation of plant designs, 94 Station emergency plan, 40–1, 64 Steam generation, Chernobyl Unit 4 reactor, 12 Steel absorption coefficient of, 98 chemical reactions of, 113 Strontium isotopes, emissions from Chernobyl, 26 Super Phenix reactor, 54 Suppression pools, 16, 22 Sweden DERL values, 31 nuclear power plans affected, 85–6 radionuclide deposition in, 30, 108 Switzerland nuclear power plans affected, 86 sources of radiation in, 32 Tellurium isotopes, emissions from Chernobyl, 26, 40 TMI-2 (Three Mile Island—USA) reactor cost of accident, 5, 67 fuel meltdown incident, 5, 106 hydrogen bubble formed, 5, 112, 113 radioactive material release compared with Chernobyl Unit 4 reactor, 27 UK emergency response procedures reviewed after, 81–2 Zircaloy-water reaction in, 113 Torness (UK), AGR Station, 52 Training emergency procedures, 41, 65 simulators used, 65, 127 Trawsfynydd (UK) Magnox Station, 51 monitoring point, 83 Turbo-generators turbine loss effect, 67 UK reactors, 136 UK areas affected, 29, 30–1 caesium isotope deposition, 27, 29 emergency arrangements, 40–5 emergency planning, 32–3 foodstuff contamination actual values quoted, 31 reference levels, 30, 31 fuel meltdown incidents, 4–5, 6, 106 implications of Chernobyl accident, 93–4 legislation, 61–2 monitoring stations, 27, 28–30, 82–4 national radiological surveys, 28–30 see also NRPB (National Radiological Protection Board) Nuclear Emergency Organisation, 44 nuclear power reactors listed, 49, 51, 52, 53, 54 operation, 61–4, 65–8 protection systems, 23

Index stability, 23 total capacity, 51, 52, 129 types listed, 47, 50 operative training, 23, 64–5 preparedness for incident outside UK, 81–5 public attitudes to nuclear power, 86, 87 relations with neighbouring countries, 85, 95–6 UKAEA (United Kingdom Atomic Energy Authority) monitoring by, 27, 31, 83 report on Chernobyl accident, 19, 125 Umanets, Mikhail, 116 UNIPEDE (Union Internationale des Producteurs et Distributeurs d’Énergie Électrique), 117 United Nations Scientific Committee on the Effects of Atomic Radiation, 72 Units of measurement, 104 University research, 95 Unstable conditions, operation in, 20, 23 Uranium dioxide (UO2) pellets, 22 natural composition of, 1 USA fuel meltdown incidents, 4, 5, 106

147 learned societies, 117 USSR electrical capacity of, 7–8 energy reserves in, 6–8 exports of fossil fuels, 7 fossil fuel reserves in, 6, 7 nuclear power plant in, 8, 9 nuclear safety in, 110–11 reactor types listed, 47, 50 regulations for nuclear power plants, 111 Vegetables, radionuclides in, 30, 31 Vienna Convention, 75, 76 Void coefficient, 56, 59, 89 Chernobyl Unit 4 reactor, 18 RBMK reactors, 20–1 SGHWR, 134 Wales caesium isotope deposition in, 27, 29 see also UK Watt Committee on Energy address, 140 concept behind, 139 conclusions, 97–8 main aims, 139

member institutions, 140, 142 objectives, 139 recommendations, 94–7, 122 reports published, 139, 142 telephone number, 140 visits to Dungeness ‘A’ Station, 126 Hinkley Point ‘B’ Station, 130–2 Hunterston ‘A’ Station, 135–7 Oldbury Training Centre, 127–9 Winfrith SGHWR, 133–4 WHO (World Health Organization), 72 Windscale (UK), accident, 4–5, 106 Winfrith (UK) monitoring point, 83 SGHWR, 49, 55, 106, 133–4 World Meteorological Organization, 72 Wylfa (UK) Magnox Station, 51 monitoring point, 83 Xenon effects, 55, 57, 90 Zirconium and alloys, chemical reactions of, 112, 113 Zirconium isotopes, emissions from Chernobyl, 26