SAFETY SERIES No. 75-INSAG-7 INSAG-7 The Chernobyl Accident: in Updating of INSAG-1 LL A REPORT BY THE CO INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1992 CATEGORIES IN THE IAEA SAFETY SERIES A new hierarchical categorization scheme has been introduced, according to which the publications in the IAEA Safety Series are grouped as follows: Safety Fundamentals (silver cover) Basic objectives, concepts and principles to ensure safety. Safety Standards (red cover) Basic requirements which must be satisfied to ensure safety for particular activities or application areas. Safety Guides (green cover) Recommendations, on the basis of international experience, relating to the ful- filment of basic requirements. Safety Practices (blue cover) Practical examples and detailed methods which can be used for the application of Safety Standards or Safety Guides. Safety Fundamentals and Safety Standards are issued with the approval of the IAEA Board of Governors; Safety Guides and Safety Practices are issued under the authority of the Director General of the IAEA. An additional category, Safety Reports (purple cover), comprises independent reports of expert groups on safety matters, including the development of new princi- ples, advanced concepts and major issues and events. These reports are issued under the authority of the Director General of the IAEA. There are other publications of the IAEA which also contain information important to safety, in particular in the Proceedings Series (papers presented at symposia and conferences), the Technical Reports Series (emphasis on technological aspects) and the IAEA-TECDOC Series (information usually in a preliminary form). THE CHERNOBYL ACCIDENT: UPDATING OF INSAG-1 INSAG-7 A report by the International Nuclear Safety Advisory Group The following States are Members of the International Atomic Energy Agency: AFGHANISTAN HOLY SEE PANAMA ALBANIA HUNGARY PARAGUAY ALGERIA ICELAND PERU ARGENTINA INDIA PHILIPPINES AUSTRALIA INDONESIA POLAND AUSTRIA IRAN, ISLAMIC REPUBLIC OF PORTUGAL BANGLADESH IRAQ QATAR BELARUS IRELAND ROMANIA BELGIUM ISRAEL RUSSIAN FEDERATION BOLIVIA ITALY SAUDI ARABIA BRAZIL JAMAICA SENEGAL BULGARIA JAPAN SIERRA LEONE CAMBODIA JORDAN SINGAPORE CAMEROON KENYA SLOVENIA CANADA KOREA, REPUBLIC OF SOUTH AFRICA CHILE KUWAIT SPAIN CHINA LEBANON SRI LANKA COLOMBIA LIBERIA SUDAN COSTA RICA LIBYAN ARAB JAMAHIRIYA SWEDEN COTE D'lVOIRE LIECHTENSTEIN SWITZERLAND CUBA LUXEMBOURG SYRIAN ARAB REPUBLIC CYPRUS MADAGASCAR THAILAND CZECHOSLOVAKIA MALAYSIA TUNISIA DEMOCRATIC PEOPLE'S MALI TURKEY REPUBLIC OF KOREA MAURITIUS UGANDA DENMARK MEXICO UKRAINE DOMINICAN REPUBLIC MONACO UNITED ARAB EMIRATES ECUADOR MONGOLIA UNITED KINGDOM OF GREAT EGYPT MOROCCO BRITAIN AND NORTHERN EL SALVADOR MYANMAR IRELAND ESTONIA NAMIBIA UNITED REPUBLIC OF ETHIOPIA NETHERLANDS TANZANIA FINLAND NEW ZEALAND UNITED STATES OF AMERICA FRANCE NICARAGUA URUGUAY GABON NIGER VENEZUELA GERMANY NIGERIA VIET NAM GHANA NORWAY YUGOSLAVIA GREECE PAKISTAN ZAIRE GUATEMALA ZAMBIA HAITI ZIMBABWE The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Head- quarters of the Agency are situated in Vienna. Its principal objective is "to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world". © IAEA, 1992 Permission to reproduce or translate the information contained in this publication may be obtained by writing to the International Atomic Energy Agency, Wagramerstrasse 5, P.O. Box 100. A-1400 Vienna, Austria. Printed by the IAEA in Austria November 1992 SAFETY SERIES No. 75-INSAG-7 THE CHERNOBYL ACCIDENT: UPDATING OF INSAG-1 INSAG-7 A report by the International Nuclear Safety Advisory Group INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1992 The International Nuclear Safety Advisory Group (INSAG) is an advisory group to the Director General of the International Atomic Energy Agency, whose main functions are: (1) To provide a forum for the exchange of information on generic nuclear safety issues of international significance; (2) To identify important current nuclear safety issues and to draw conclusions on the basis of the results of nuclear safety activities within the IAEA and of other information; (3) To give advice on nuclear safety issues in which an exchange of information and/or additional efforts may be required; (4) To formulate, where possible, commonly shared safety concepts. THIS SAFETY SERIES IS ALSO PUBLISHED IN FRENCH, RUSSIAN AND SPANISH VIC Library Cataloguing in Publication Data The Chernobyl accident : updating of INSAG-1 : INSAG-7 : a report by the International Nuclear Safety Advisory Group. — Vienna : International Atomic Energy Agency, 1992. p. ; 24 cm. — (Safety series, ISSN 0074-1892 ; 75-INSAG-7) STI/PUB/913 ISBN 92-0-104692-8 Includes bibliographical references. 1. Chernobyl Nuclear Accident, Chernobyl, Ukraine, 1986. I. Inter- national Atomic Energy Agency. II. International Nuclear Safety Advisory Group, n. Series. VICL 92-00042 FOREWORD by the Director General New information that has come to light since the Post-Accident Review Meet- ing on the Chernobyl Accident (held in Vienna from 25 to 29 August 1986) bears on general issues of the operational safety of nuclear power plants in the then Union of Soviet Socialist Republics as well as on specific issues relating to the design of the Chernobyl type light water cooled, graphite moderated RBMK reactors. As regards general issues, the new information demonstrates the lack of feed- back of operating experience and the inadequacy of communication between designers, engineers, manufacturers, constructors, operators and regulators. These deficiencies, coupled with a lack of clear lines of responsibility, were critical factors in the events leading up to the Chernobyl accident. These shortcomings have been tackled at the national level and some improvements have been made. Valuable lessons can be learned from incidents and accidents, as was demon- strated after the accident at Three Mile Island nuclear power plant in the United States of America in 1979, when far reaching follow-up actions were taken to minimize the risk of a recurrence and to improve procedures for accident manage- ment. The accident at Chernobyl demonstrated that the lessons from the Three Mile Island accident had not been acted upon in the USSR: in particular, the importance of systematic evaluation of operating experience; the need to strengthen the on-site technical and management capability, including improved operator training; and the importance of the man-machine interface. Specific issues relating to the design of the RBMK reactors were the subject of intensive studies in the USSR after 1986, and some modifications have since been made to these reactors and to their operating regimes. More recently, discussions have been held at the international level on the safety of nuclear power plants with RBMK reactors. International efforts to assist in the safety assessment of the RBMK reactors have been intensified in recent months. Efforts to enhance the safety of RBMK reactors will continue; however, inter- national assistance can only achieve a fraction of what has to be done at the national level. In addition, the general issues mentioned earlier require sustained efforts to upgrade the national nuclear safety regime before a safety culture can be inculcated at all levels and in all the organizations concerned. The emergence of the new information prompted the IAEA's International Nuclear Safety Advisory Group (INSAG) to review its former conclusions about the causes of the Chernobyl accident of April 1986. The present INSAG-7 report updates the Summary Report on the Post-Accident Review Meeting on the Chernobyl Acci- dent (IAEA Safety Series No. 75-INSAG-l), published in September 1986. The con- clusions formulated in the report will broaden the basis for international consultations to enhance the safety of RBMK reactors. With this report, INSAG has made a valuable contribution to the task that lies ahead. EDITORIAL NOTE Although great care has been taken to maintain the accuracy of information contained in this publication, neither the IAEA nor its Member States assume any responsibility for consequences which may arise from its use. The use of particular designations of countries or territories does not imply any judge- ment by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries. The authors are responsible for having obtained the necessary permission for the IAEA to reproduce, translate or use material from sources already protected by copyrights. Material prepared by authors who are in contractual relation with governments is copy righted by the IAEA, as publisher, only to the extent permitted by the appropriate national regulations. CONTENTS 1. INTRODUCTION 1 2. FEATURES OF THE REACTOR 3 2.1. Void coefficient of reactivity 3 2.2. Design of control and safety rods 4 2.3. Speed of insertion of the emergency protection rods 4 2.4. Control of power 4 2.5. Instrumentation indicating the reactivity margin 6 2.6. Size of the reactor core 6 2.7. Capability to alter safety systems, plant trips and alarms 6 2.8. Subcooling of the inlet water 7 2.9. Primary coolant system 7 2.10. Containment 8 3. THE ACCIDENT 10 4. MORE RECENT ANALYSES OF THE FAULT SCENARIO 13 4.1. The scenario 13 4.2. Operating reactivity margin 14 5. VIEWS OF INSAG 16 5.1. Design 16 5.2. Operator actions 17 5.3. Safety framework 20 5.4. Implications of ignoring deficiencies 20 5.5. Importance of competent safety analysis 20 5.6. Deficiencies in the regulatory regime 21 5.7. General remarks on the lack of safety culture 21 5.8. Summary assessment 22 6. CONCLUSIONS ON FACTORS CONTRIBUTORY TO THE ACCIDENT 23 APPENDIX: MEASURES TO IMPROVE THE SAFETY OF RBMK PLANTS 27 ANNEX I: REPORT BY A COMMISSION TO THE USSR STATE COMMITTEE FOR THE SUPERVISION OF SAFETY IN INDUSTRY AND NUCLEAR POWER 29 1-1. Introduction 29 1-2. Brief description of the design of Chernobyl Unit 4 32 1-3.