ARCHIVES CSNI Report 136 NUCLEAR SAFETY DIVISION <S)H<S!E> NEA SELECTED SOURCE TERM TOPICS Report to CSNI by an OECD/NEA Group of Experts .J April 1987 COMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONS OECD NUCLEAR ENERGY AGENCY 38, boulevard Suchet, 75016 Paris, France S NUCLEAR SAFETY DiViGSN CSNI REPCHT 13; REPORT TO CSNI ON SELECTED SOURCE TERM TOPICS BY AN OECD (NEA) GROUP OF EXPERTS APRIL 1887 5077 LEGAL NOTICE The Organisation for Economie Co-operation and Development assumes no liability concerning information published in this book. - 1 - CSNI Report 136 summarizes the results of the work performed by the Group of Experts on the Source Term (GREST) of CSNI's Principal Working Group No. 4 on the Source Term and Environmental Consequences (PWG4) during the period extending from 1983 to 1986. It consists of: — an Executive Summary (prepared by the Chairman of the Group); — a section on Conclusions and Recommendations; — five Technical Chapters, a Technical Annex and two Appendices. The Technical Chapters, Technical Annex and Appendices have been prepared by members of GREST and discussed at length at meetings of the Group; this does not mean, however, that every detail of each chapter has been fully endorsed by GREST as a whole. The various chapters provide material supporting the Conclusions and Recommendations of the report. These reflect the unanimous position of the Group. This report 1s complementary to Part 1, "Technical Status of the Source Term" of CSNI Report 135, "Report to CSNI on Source Term Assessment, Containment Atmosphere Control Systems, and Accident Consequences". While Part 1 of CSNI Report 135 was prepared essentially 1n the framework of an evaluation of source term studies available 1n 1985, this report considers 1n detail a number of very specific Issues thought to be Important 1n the source term area. These are not necessarily treated 1n any depth 1n Part 1 of CSNI Report 135. CSNI Report 136 was endorsed for publication by GREST, PWG4 and CSNI at the end of 1986. - 3 - TABLE OF CONTENTS Executive Summary 5 Conclusions and Recommendations 19 Technical chapter 1: Fission Product Chemistry 1n the Primary Circuit of a LWR during Severe Accidents 23 Technical chapter 2: Resuspens1on/Re-Entra1nment of Aerosols In LWRs Following a Meltdown Accident 111 Technical chapter 3: Iodine Chemistry under Severe Accident Conditions 251 Technical chapter 4: Effects of Combustion, Steam Explosions and Pressurized Melt Ejection on Fission Product Behaviour 299 Technical chapter 5: Radionuclide Removal by Pool Scrubbing 321 Technical Annex Fission Product Release from Fuel 1n Severe Accidents 353 Appendix 1 : Containment Leakage Under Post-Meltdown Conditions 371 Appendix 2 : Source Term Uncertainties - Existing Studies of and Plans for Future Work 1n Uncertainty Assessment 379 Annex : List of members of the CSNI-PWG4 Group of Experts on the Source Term (GREST) 387 - 5 - EXECUTIVE SUMMARY - 6 - State-of-the-Art Report of the CSNI/PWG4 Group of Experte on the Source Term on Selected Source Term Topics Executive Summary P N Clough, Chairman Introduction The Group of Experts on the Source Term (GREST), which first met in the autumn of 1982, was established by CSKI to continue the work of the Group of Experts on Nuclear Aerosols in Reactor Safety, but with a broadened remit. A stimulus for the formation of the GREST was the renewed interest and activity in source term research for water-cooled reactors in the aftermath of the TMI-2 accident. There was a consensus view that considerable Improvements could be made in the classic but conservative methodology for source term evaluation developed in WASH-1400. The key technical issues and route to an improved approach as seen at that time were reviewed in NUREG-0772 'Technical Basis for Estimating Fission Product Behaviour During LWR Accidents'. The main function of the GREST, in addition to international information exchange, was to keep under review the most significant areas of source term development in a rapidly evolving situation. Following the successful example of the predecessor Nuclear Aerosols group, the first chairman, Mr F Abbey, set in motion at the April meeting in 1984 the preparation of a State-of-the-Art Report on Selected Source Term Topics. The topics chosen were those agreed by the GREST membership, in consultation with PWG4, as being of high importance in the evaluation of source terms for severe accidents in water-cooled reactors. Nuclear aerosols were intentionally excluded in view of the work of the predecessor group. The Technical Chapters following this summary represent the product of that activity. It should be noted that the recent severe reactor accident at Chernobyl in the USSR is not viewed as affecting the conclusions of this work in any significant respect. There have been many important developments in source term research since the initiation of the GREST exercise. Large quantities of new experimental data have become available, and this process continues apace. In such a rapidly developing situation, no large-scale State-of-the-Art Report can hope to be fully up-to-date. The contents of the Technical Chapters should be seen as providing a position statement relating to early in 1986, when most of the drafts were complete. In addition, 1984/85 saw the publication of several key reviews of the state of source term development; the Report of the American Physical Society Study Group on Radionuclide Release from Severe Accidents at Nuclear Power Plants; the Report of the American Nuclear Society Special Committee on Source Terms; and the Summary Report of the Industry Degraded Core Rule-making Program Study on Nuclear Power Plant Response to Severe Accidents» In response to these and other reviews of source term issues, the CSKI at its November 1984 meeting decided to set up a Special Task Force on Source Terms. This group was very active throughout 1985, and its Executive and Technical Summaries were published in March of this year. Since the activities of the Task Force, both in terms of - 7 - technical coverage and of the experts involved, overlap to some extent those of the GREST, some clarification of the respective roles nay be helpful. The remit of the Task Force was limited to intercomparing the contents of a number of key source term studies, including those mentioned above, and to identifying (a) areas where information appeared to be sufficient, (b) dis­ agreements and the reasons for them, (c) generic source term issues, (d) obstacles to the generic application of results in different plants. A principal purpose was to draw attention to new technical understanding which might have implications for nuclear safety and regulatory issues, for the benefit of the Sub-Committee on Licencing* The aim was not to produce an original technical review of the issues, additional to the source material identified. In the event, however, the Technical Annexes to the Task Force Report do to some extent have the nature of State-of-the-Art reports. The purpose of the GREST activity can be distinguished in several important respects. The aim is to give the independent views of groups of international experts on the state of technical development and importance of key source term topics. Although strongly influenced by the source documents which concerned the Task Force, the products of the GREST activity represent original appraisals in no way confined with respect to source material. Additional technical detail is provided in many areas which can be seen as substantially supplementing the background to the Task Force report. Moreover, the quantitative impact on source terms of the issues reviewed only enters in assessing the relative importance, and it is not the intention of this work to provide judgements on the overall trend in source terms, upwards or downwards. The main body of the work is represented by the six Technical Chapters, one Annex, and two Appendices, which follow this summary. All of this material has been subject to review and comment by the whole of the GREST, but of course represents primarily the views of the authors. The Technical Chapters were agreed by the GREST as providing comprehensive State-of-the-Art reports on their subjects, and are presented in this light. However, on the topic of Fission Product Release from Fuel, there was no agreement by the Group as a whole that the Chapter provided represented a sufficiently wide-ranging review of this topic, being centred on one theoretical model. It was, however, accepted as providing some valuable insights on the subject, and is presented here as an Annex. The first of the Appendices, on leak path characteristics important for aerosol retention, is included to provide some Insights on a neglected aspect of source term analysis. The topic of Source Term Uncertainties is covered in the second Appendix. This does not imply that this topic is viewed as being less important, but reflects the fact that work began on it late relative to the other activities. A study of uncertainties will form a major future activity of the Group. It was originally intended that a Technical Chapter on coupling at the thermal-hydraulic/aerosol behaviour Interface should be included in the work. Unfortunately, this could not be completed in time to include with the present contributions, but the topic Is still viewed as important, and it is hoped that a future supplement to the report will be Issued on it. The conclusions arrived at in this work generally support those of the Task Force, although differences in emphasis may exist in some places. * - 8 - The important role played by the first chairman of the CREST, Mr Frank Abbey, In Initiating this activity and guiding it towards completion through much of its course, oust be fully acknowledged.