The Chernobyl Accident and its Implications for the United Kingdom

Watt Committee: report no 19

N. Worley, J. Lewins

• Publisher: Routledge
• Year: 2003
• Print ISBN: 9781851662197
• eBook ISBN: 9781135382919
• Topic: Architecture
• Subtopic: Architecture Methods & Materials

Published on behalf of The Watt Committee on Energy

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

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:

• general description of the principal parts of nuclear power plants;
• an outline of some major incidents involving nuclear fuel meltdown;
• 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 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 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.

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

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:

• 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 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:

• Significant amounts of energy can be released if the system is not properly controlled.
• The fuel elements contain large quantities of highly radioactive material. To keep this from the environment requires containment.
• 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.

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 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 ...

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• FOREWORD
• BACKGROUND
• SECTION 1 INTRODUCTION
• SECTION 2 THE DESIGN OF THE CHERNOBYL UNIT 4 REACTOR
• SECTION 3 DESCRIPTION OF THE CHERNOBYL ACCIDENT
• SECTION 4 THE RADIOACTIVE RELEASE FROM CHERNOBYL AND ITS EFFECTS
• SECTION 5 ACCIDENT MANAGEMENT IN THE USSR AND THE UNITED KINGDOM
• SECTION 6 UNITED KINGDOM AND USSR REACTOR TYPES
• SECTION 7 REACTOR OPERATION AND OPERATOR TRAINING IN THE UNITED KINGDOM
• SECTION 8 INTERNATIONAL DIMENSIONS OF THE IMPLICATIONS OF THE CHERNOBYL ACCIDENT FOR THE UNITED KINGDOM
• SECTION 9 COMMENTS, RECOMMENDATIONS AND CONCLUSIONS
• APPENDICES
• APPENDIX 1
• APPENDIX 2
• APPENDIX 3 SUMMARY OF SIGNIFICANT DATES AND TIMING RELEVANT TO THE CHERNOBYL ACCIDENT
• APPENDIX 4
• APPENDIX 5 CHEMICAL REACTION ASPECTS OF THE CHERNOBYL ACCIDENT
• APPENDIX 6
• APPENDIX 7
• APPENDIX 8 INTERNATIONAL ASPECTS
• APPENDIX 9
• APPENDIX 10
• APPENDIX 11
• APPENDIX 11 B: PRESENTATION ON ‘THE CHERNOBYL ACCIDENT AND ITS CONSEQUENCES’
• APPENDIX 12
• APPENDIX 12B: VISIT TO THE CEGB’s OLDBURY-ON-SEVERN NUCLEAR POWER TRAINING CENTRE
• APPENDIX 12C: VISIT TO HINKLEY POINT ‘B’ AGR STATION
• APPENDIX 12D: VISIT TO THE ATOMIC ENERGY ESTABLISHMENT WINFRITH SGHWR STATION
• APPENDIX 12E: VISIT TO HUNTERSTON ‘A’ NUCLEAR POWER STATION TO ATTEND AN EMERGENCY EXERCISE
• THE WATT COMMITTEE ON ENERGY
• MEMBER INSTITUTIONS OF THE WATT COMMITTEE ON ENERGY
• WATT COMMITTEE REPORTS