Interactions of Severe Accident Researchand Regulatory Positions

166 Pages·2001·0.53 MB·English

by  B. R. Sehgal

SKI Report 01:23

Research

Interactions of Severe Accident Research
and Regulatory Positions (ISARRP)

Prof. B. R. Sehgal

December 2001

ISSN 1104–1374
ISRN SKI-R-01/23-SE

FINAL REPORT

INTERACTIONS OF SEVERE ACCIDENT RESEARCH
AND
THE REGULATORY POSITIONS

(ISARRP)

CO-ORDINATOR

Prof. B. R. SEHGAL
Nuclear Power Safety
Royal Institute of Technology
S-100 44 Stockholm
Sweden
Telephone: + 46-8-790 9252
Fax: + 46-8-790 9197
E-mail: [email protected]

LIST OF PARTNERS

1. S. Chakraborty, Swiss Federal Nuclear Safety Inspectorate (CH)
2. J. Peltier, Institut de Protéction et Sûreté Nucléaire (FR)
3. J. Martinez, Consejo de Seguridad Nuclear (ESP)
4. J. Rohde, Gesellschaft für Anlagen und Reaktorsicherheit (DE)
5. M. El-Shanawany, Health and Safety Executive/NII (GB)

CONTRACT No. FI4S-CT98-0058

EC Contribution 100,000 €
SWISS Contribution 60,000 CHF
Starting Date April 1, 1999
Duration 18 months

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CONTENTS

Page.No:
EXECUTIVE SUMMARY 3

A. OBJECTIVES and SCOPE 4
A.1 Objectives 4
A.2 Scope and work procedure 5
B. WORK PROGRAMME 6
B.1 Task Description 6
C. WORK PERFORMED and RESULTS 7
C.1 A critical review of severe accident phenomenological 7
research
C.2 Relevance of severe accident research to severe accident 29
management requirements and implementation
C.3 Relevance of severe accident research to probabilistic 44
safety assessment and risk informed regulatory approaches
C.4 Questionnaire and the evaluation of the responses to 55
questions
C.5 Relevance of example Level 2 probabilistic safety 62
assessment results to severe accident research
C.6 State of resolution of severe accident issues with respect to 70
regulator needs
C.7 Regulatory use of severe accident research 79
C.8 Remaining issues and concerns 80
C.9 Conclusions and Recommendations on future directions 83
of severe accident research

APPENDIX. 1 Questionnaire and the Copies of the Responses from Various
Regulatory Organisations:
A:1 Responses from Belgium 88
A:2 Responses from Czech Republic 91
A:3 Responses from Finland 94
A:4 Responses from France 98
A:5 Responses from Germany 104
A:6 Responses from Hungary 108
A:7 Responses from Japan 112
A:8 Responses from Netherlands 116
A:9 Responses from Slovakia 120
A:10 Responses from Slovenia 124
A:11 Responses from Spain 127
A:12 Responses from Sweden 133
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A:13 Responses from Switzerland 138
A:14 Responses from U.K. 144
A:15 Responses from U.S.A. 149

EXECUTIVE SUMMARY

This project, started on April 1, 1999, had the specific objectives of determining whether; (i) the
focus of the severe accidents (SA) research is consistent with that of the regulatory authorities,
(ii) the results obtained so far by SA research satisfy the regulatory concerns, (iii) the future
programs, envisaged will address the potential regulatory needs into next century, and (iv) how
much weight in the future SA research should be placed on preventive versus mitigative accident
management measures.

The project work consisted of Workshops to which the partners contributed. The partners
represented their respective regulatory organizations or their technical support organizations. A
Questionnaire based on the objectives, listed above, was prepared and sent to several
European regulatory authorities. The Questionnaire was also sent to United States Nuclear
Regulatory Commission, Japanese Nuclear Safety Commission and the regulatory authorities of
Hungary, Slovakia, Slovenia and Czech Republic. Responses have been received from nine
European organizations, four Eastern European organizations, Japanese Safety Commission and
the United States Nuclear Regulatory Commission. The responses showed differences between
the attitudes of the various regulatory organizations towards SA research accomplishment and
needs. Clearly, the responses obtained have statistical value since a wide spectrum of regulatory
organizations have contributed, although no statistical analysis was performed. Insights obtained
from their responses have been combined and are reported here.

In addition to the analysis of the responses to the Questionnaire, a critical review of the severe
accident phenomenological research conducted in the World for the past 20 years was
performed. The accomplishments made by this research activity were examined and related to
the needs of the regulatory organisations as evidenced by the responses to the Questionnaire,
referred to above. The research accomplishments were also related to the requirements of the
severe accident management guidance and its implementation. The impact of the more recent
approaches e.g. the probabilistic safety assessment (PSA) and the risk informed regulations was
examined and their needs related to the accomplishments of the severe accident research
performed so far. In this context the accomplishments of the SA research, sponsored by the
European Commission in the fourth framework program, were reviewed. This has lead us to
summarize the state of resolution of the SA issues with respect to the needs of the regulatory
organisations. It was found that most regulatory organisations state that they have employed the
results obtained, and the insights gained, from the SA research for regulatory decision making.
They have also used the same for establishing greater confidence in either the regulations they
have proposed or in reviewing the actions that the licensees (plants) have proposed for
preventing or mitigating the consequences of severe accidents. The specific needs for further
research, that the majority of the regulatory organisations [or their technical support
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organisations (TSO´s)] have indicated, have been classified in the report and recommendations
have been made on the future directions of the SA research. The highest priority for future SA
research has been assigned to the resolution of the issue of ex-vessel melt/debris coolability to
achieve stabilization and termination of the postulated severe accident.

The recommendations provided in the report, although, quite general, may not apply to all
countries and plants. Some of the SA issues and recommendations, particularly with respect to
the application of the severe accident guidance (SAMG), may require plant specific
modifications.

A. OBJECTIVES and SCOPE
A.1. Objectives

The European Commission has been sponsoring research into the phenomenology of postulated
severe accidents in reactor plants, and, in severe accident management measures, for several
years. The Third Framework Program, primarily, collected information on the national research
programs in several specific areas and produced state-of-the-art reports. The Fourth
Framework Program dedicated substantial funds towards specific cost-shared research projects
on severe accidents, and on accident management. Some European countries, e.g., France and
Germany are pursuing large severe accident research programs with their own funds.
Substantially similar and/or complementary research on severe accidents has been pursued in
USA, Japan, Canada and a few other countries.

Concurrent with these efforts world-wide, the regulatory positions (concerns) have also been
evolving. As an example, the requirements on containment integrity, and the environmental
release of radioactivity, have been strengthened. The USNRC has stated the position that
containment integrity should be maintained for at least 24 hours, however, the emergency
planning includes evacuation of the population from the vicinity of a postulated accident. In
Europe, the French-German safety approach goes even a step further and requires that for new
reactor designs, after a postulated severe accident, there shall be no need for permanent
relocation and evacuation from the immediate vicinity of the plant, and long restrictions in food
consumption. In Germany, this requirement has been turned into an extension of the existing law
(Atomgesetz). Differences in the U.S. and European regulatory approach are also apparent in
the backfit considerations for an existing nuclear plant: the USNRC´s backfit rule requires a
cost/benefit analysis, while the European regulatory authorities do not prescribe such a rule.

Clearly, the nuclear regulatory positions and the findings from the research programs influence
each other. The former determine the research directions and topics, while the latter influence
the regulatory thinking, positions and concerns. It should be pointed out that the severe accident
research programs are to a large extent a function of the positions of the regulatory authorities
regarding the safety design of the nuclear power plants. Thus, there is mutual interaction
between the regulatory positions and severe accident research programs, that varies from
country to country in Europe.

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The specific measurable objectives of the ISARRP Project are to determine:

(a) whether the focus of the present directions in the severe accident research is consistent with
that of the regulatory authorities,
(b) whether the results (findings) obtained in the severe accident research programs have
provided the information necessary to satisfy the regulatory positions (concerns) e.g., with
respect to SAMG and,
(c) whether the future research programs, envisaged, address the potential regulatory needs.

A.2. Scope and Procedure

The ISARRP is a Concerted Action (CA) Project requiring a series of working meeting
(workshops) between the partners to develop the information to satisfy the objectives of the
Project. It turned out that the scope of the project envisioned at the time of the submission to the
E.C., and at the start of the Project work was increased considerably. Many more regulatory
organisations were contacted than originally contemplated. The work expanded to a review of
the SA research that has been performed so far and that which is scheduled to be performed in
the near future. Most of the previous and current important research programs pursued in the
World were reviewed and their accomplishments considered in the evaluations. The research
results obtained and the regulatory needs, as expressed by the responding regulatory
organisations and the TSO´s were compared. Intercomparisons were made between the needs
expressed by the various organisations to arrive at the findings that have been described in the
report.

The procedure for the work in the Project was to develop a Questionnaire of twenty-five
questions after due discussions between the partners. The Questionnaire was sent to the
regulatory and TSO organisations in Western Europe. Later, the Questionnaire was also sent to
the USNRC, Japanese Safety commission and the regulatory organisations in Eastern Europe.
The responses obtained were discussed and analysed by the partners in the workshops.

Each of the partners was assigned the responsibility for developing a short report on one or two
of the work packages in the Project. These individual reports were reviewed by all the partners
and served as the base-documents for this final report.

A trip was made by the Coordinator and the Swiss partner to Washington D.C. to discuss with
the USNRC their current positions on severe accidents, in general, and on accident
management, in particular. Discussions were also held on their views on the benefits of the SA
research that they had derived and on the remaining SA issues for further resolution. These
discussions helped to clarify the responses they had provided to the Questionnaire.

The partner group for the ISARRP project is much smaller than those for the other E.C. funded
Concerted Action projects, however, the main nuclear countries in Europe are represented. We
believe, the small size has helped in generating frank and lively discussions and afforded deeper
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analyses of the SA issues, the research results, their usefulness in general and their utilization by
the regulatory and technical support organisations. In addition, the small group could openly
analyse the responses of the various regulatory organisations and comprehend the different
national philosophies and attitudes.

B. WORK PROGRAMME

The work Programme of the ISARRP Project was divided into several work packages. The
work was conducted in the form of presentations and discussions, held during several meetings
whose character was that of workshops. Short reports were prepared by the partners assigned
to each task.

Work Package 1: Critical review of the SA phenomenological research
The objective of this work package was to consider the progress made world-wide in research
on the resolution of the outstanding phenomenological issues posed by severe accidents.

Work Package 2: Relevance of severe accident research to SAMG requirements
and implementation
The objective of this work package was to relate the progress made in the resolution of the SA
issues to the practical matter of what results are required or have been used for the management
of severe accidents. Clearly, the SAMG is the most important avenue employed by the
regulatory organizations to assure themselves of the safe (from public perspective) performance
of a nuclear plant in a postulated severe accident event.

Work Package 3: Relevance of severe accident research to PSA and the risk informed
regulatory approach
The objectives of this work package is to relate the results obtained by the severe accident
research to the requirements of a PSA and of the new trend of employing the risk informed
approach in promulgating regulations. Clearly a PSA identifies vulnerabilities in the knowledge
base, however, their importance is decidedly plant specific. Nevertheless the uncertainties in the
phenomenology or in resolution of issues lead to uncertainties in the PSA conclusions and in the
adoption of the risk informed approach.

Work Package 4: Questionnaire and the evaluation of responses to the questions
The purpose of this work package is to solicit the views of the regulatory organizations towards
the results of the SA research and the benefits they have derived from it in terms of regulatory
actions, or in the confidence they have gained in assessment of plant safety. This work package
was also designed to distinguish the differences between the attitudes and approaches followed
by the various regulatory organisations in Europe, Eastern Europe, U.S.A. and Japan.

Work Package 5: Relevance of example PSA results to SA research
The objective of their work package was to employ the results of some recent PSAs (preferably
for a PWR and a BWR) and relate their findings to the results obtained in SA research, and to
the effectiveness of the SAM measures already taken or contemplated.

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