guidelines for accident analysis of wwer nuclear power plants

136 Pages·2005·1.66 MB·English

IAEA-EBP-WWER-01
XA9744138
GUIDELINES FOR
ACCIDENT ANALYSIS
OF WWER
NUCLEAR POWER PLANTS
A PUBLICATION OF THE
EXTRABUDGETARY PROGRAMME ON
THE SAFETY OF WWER AND RBMK
NUCLEAR POWER PLANTS
December 1995
INTERNATIONAL ATOMIC ENERGY AGENCY
VOL 2 8 Ne 1 5
The originating Section of this publication in the IAEA was:
Safety Assessment Section
International Atomic Energy Agency
Wagramerstrasse 5
P.O. Box 100
A-1400 Vienna, Austria
GUIDELINES FOR ACCIDENT ANALYSIS OF WWER NUCLEAR POWER PLANTS
IAEA, VIENNA, 1995
IAEA-EBP-WWER-01
ISSN 1025-2762
© IAEA, 1995
Printed by the IAEA in Austria
December 1995
FOREWORD
The IAEA initiated in 1990 a programme to assist the countries of central and eastern Europe
and the former Soviet Union in evaluating the safety of their first generation WWER-440/230 nuclear
power plants. The main objectives of the Programme were: to identify major design and operational
safety issues; to establish international consensus on priorities for safety improvements; and to provide
assistance in the review of the completeness and adequacy of safety improvement programmes.
The scope of the Programme was extended in 1992 to include RBMK, WWER-440/213 and
WWER-1000 plants in operation and under construction. The Programme is complemented by national
and regional technical co-operation projects.
The Programme is pursued by means of plant specific safety review missions to assess the
adequacy of design and operational practices; Assessment of Safety Significant Events Team (ASSET)
reviews of operational performance; reviews of plant design, including seismic safety studies; and
topical meetings on generic safety issues. Other components are: follow-up safety missions to nuclear
plants to check the status of implementation of IAEA recommendations; assessments of safety
improvements implemented or proposed; peer reviews of safety studies; and training workshops. The
IAEA is also maintaining a database on the technical safety issues identified for each plant and the
status of implementation of safety improvements. An additional important element is the provision of
assistance by the IAEA to strengthen regulatory authorities.
The Programme is extrabudgetary and depends on voluntary contributions from IAEA Member
States. Steering Committees provide co-ordination and guidance to the IAEA on technical matters and
serve as forums for exchange of information with the European Commission and with other
international and financial organizations. The general scope and results of the Programme are reviewed
at Advisory Group meetings.
The Programme, which takes into account the results of other relevant national, bilateral and
multilateral activities, provides a forum to establish international consensus on the technical basis for
upgrading the safety of WWER and RBMK nuclear power plants.
The IAEA further provides technical advice in the co-ordination structure established by the
Group of 24 OECD countries through the European Commission to provide technical assistance on
nuclear safety matters to the countries of central and eastern Europe and the former Soviet Union.
Results, recommendations and conclusions resulting from the IAEA Programme are intended
only to assist national decision makers who have the sole responsibilities for the regulation and safe
operation of their nuclear power plants. Moreover, they do not replace a comprehensive safety
assessment which needs to be performed in the frame of the national licensing process.
EDITORIAL NOTE
In preparing this publication for press, staff of the IAEA have made up the pages from the
original manuscript(s). The views expressed do not necessarily reflect those of the governments of the
nominating Member States or of the nominating organizations.
Throughout the text names of Member States are retained as they were when the text was
compiled.
The use of particular designations of countries or territories does not imply any judgement 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 mention of names of specific companies or products (whether or not indicated as
registered) does not imply any intention to infringe proprietary rights, nor should it be construed as
an endorsement or recommendation on the part of the IAEA.
CONTENTS
SUMMARY 7
1. INTRODUCTION 8
1.1. Objectives of the guidelines 8
1.2. Approach and documents used 8
1.3. Structure of the guidelines 9
2. EVENTS TO BE CONSIDERED 10
2.1. List of initiating events 10
2.2. Initiating events categorization 10
3. ACCEPTANCE CRITERIA 11
3.1. Basic considerations 11
3.2. Safety requirements and related criteria 13
4. ASSUMPTIONS FOR ACCIDENT ANALYSIS 14
4.1. Initial conditions 14
4.2. Availability and functioning of systems and components 15
4.3. Operator actions 16
4.4. Modelling assumptions 16
5. QUALITY ASSURANCE 16
5.1. General requirements 16
5.2. Specific requirements 17
6. COMPUTER CODES 18
6.1. General requirements 18
6.2. Code validation 19
REFERENCES 23
APPENDIX I: LIST OF INITIATING EVENTS TO BE CONSIDERED 24
APPENDIX II: ACCEPTANCE CRITERIA FOR TRANSIENT AND ACCIDENT
ANALYSIS OF WWER NPPs 26
APPENDIX III: RECOMMENDATIONS FOR CONSIDERATION OF INITIAL
CONDITIONS AND ASSUMPTIONS 28
APPENDIX IV: SAMPLE ACCIDENT ANALYSES 29
IV. 1. SPECTRUM OF ROD EJECTION ACCIDENTS FOR NPP TEMELIN . . .. 29
IV.2. LARGE BREAK LOCA FOR WWER-1OOO/32O REACTOR 65
IV.3. STEAM GENERATOR PRIMARY COLLECTOR HEAD RUPTURE
IN A WWER 440/213 REACTOR 74
IV.4. SPECTRUM OF ROD EJECTION ACCIDENTS - FOR BOHUNICE
V-2 (WWER 440/213) 96
IV.5 - PRESSURIZER SURGE LINE BREAK - FOR
BOHUNICE V-2 (WWER 440/213) 115
ABBREVIATIONS 135
CONTRIBUTORS TO DRAFTING AND REVIEW 136
SUMMARY
The need for detailed guidance in the performance and review of accident analysis for WWER
nuclear power plants has been identified as a priority within the IAEA Extrabudgetary Programme on
Safety of WWER and RBMK NPPs.
The present guidelines were developed through the organization of three consultants meetings
in 1994 and 1995. The guidelines deal with the transient and accident analysis required to justify
existing or newly proposed technical solutions such as plant modifications for safety upgrading at
nuclear power plants with WWER type reactors. The guidelines give advice on selection and
categorization of initiating events to be considered, on adequate specification of acceptance criteria,
methods, computer codes, and assumptions to be used as well as on quality assurance procedures for
accident analysis.
The guidelines are complemented with a series of analyses examples of various accident
presented in Appendix IV which illustrate possible applications of these guidelines.
1. INTRODUCTION
1.1. OBJECTIVES OF THE GUIDELINES
These guidelines deal with the transient and accident analysis required to justify existing or
newly proposed technical solutions, such as plant modifications for safety upgrading, at nuclear power
plants with WWER type reactors. The guidelines give advice on selection and categorization of
initiating events to be considered, on adequate specification of acceptance criteria, methods, computer
codes, and assumptions to be used as well as on quality assurance procedures for accident analysis.
The guidelines apply to WWER plants which were designed according to earlier standards,
which at present are in operation and/or under construction. Requirements for new designs might be
in some parts more stringent than expressed in these guidelines.
The objective of the guidelines is to establish a minimum set of requirements for accident
analysis, reflecting current approaches adopted both in eastern or western countries involved in
accident analysis for WWER reactors. Naturally, in some cases national regulations might deviate and
in particular be more restrictive than requirements presented here. On the other hand, the guidelines
go beyond original design requirements, especially for the WWER 440/230 reactor type. However,
application of the guidelines should not be contradictory to original design philosophy and it is subject
to final judgement of competent national authorities.
The guidelines mostly deal with transients and accidents considered in the reactor design and
coped by existing safety systems. To a limited extent, accidents beyond the original design basis are
also addressed.
Generally, accident analysis should consider many different issues related to potential loss of
integrity for all barriers against radioactivity releases into environment. Although no aspect is
explicitly excluded, present guidelines deal for simplicity with considerations related to the integrity
of the fuel and the primary circuit only. Specific guidelines for containment analysis are dealt with
in another relevant IAEA document. Attention was mainly focused on neutronic and thermohydraulic
aspects of reactor accidents.
The analysis under consideration in these guidelines is based on the deterministic assumption
of occurrence of faults and functioning of systems. The so-called "conservative method" of treatment
is characteristic of this analysis, while the level of conservatism may be dependent on frequency of
initiating event analyzed. The guidelines have been elaborated to be used primarily by regulatory
bodies for the review of accident analysis provided by the operating organizations. Of course,
corresponding utility guidelines should follow the same basic principles, being more specific with their
explanations of how to respond properly to more general regulatory requirements.
1.2. APPROACH AND DOCUMENTS USED
The need for detailed guidance in the performance and review of accident analysis for WWERs
has been identified as a priority by the fact that the Safety Review Missions conducted by the IAEA
at several WWER plants have uncovered significant deficiencies in this area [1 ]. The idea was further
supported by the WWER Steering Committee in its meeting of June 1993.
According to the IAEA NUSS Code on Design [2], accident analysis shall be performed "to
ensure that the overall plant design is capable of meeting prescribed and acceptable limits for
radiation doses and releases set by the regulatory body for each plant condition category".
The NUSS Code on Governmental Organization [3] further specifies that the regulatory body
shall review and assess the design of a NPP "to confirm that it can meet acceptable safety
requirements".
Additional NUSS guidance on the requirements and contents for accident analysis is provided
in the Safety Guides 50-SG-D11 [4] and 50-SG-G2 [5], which define the procedures for conducting
accident analysis and reviewing the results, respectively.
More detailed national standards have been developed in several countries. These standards,
especially those from Finland [6], France [7], Germany [8], Russian Federation [9, 10] and USA [11,
12], have been considered for the elaboration of the present guidelines, when appropriate.
It was recognized, however, that further guidance for the performance of accident analysis in
the specific case of WWER NPPs, specially in cases of periodical reassessment or licensing of plant
modifications, was needed.
The development of these guidelines started with the organization of a small consultants group
on 12-14 January, 1994. The consultants reviewed Refs [6-12] and the experiences of WWER users
through the calculations performed under the IAEA Technical Co-operation project RER/9/004 [13].
A preliminary draft document was developed by the consultants and further completed by the IAEA
secretariat.
This document was later circulated among specialists in the field of accident analysis for
comments. A second consultants meeting was convened in August 1994, which reviewed the draft.
A third consultants meeting in April 1995 finalized the guidelines. Relevant IAEA documents have
been considered in the elaboration of these guidelines.
1.3. STRUCTURE OF THE GUIDELINES
The first problem addressed in Section 2 of these guidelines is the proper selection and
categorization of initiating events to be considered for accident analysis. The events are grouped into
categories according to the probability of their occurrence and consequences. Appendix I presents an
extensive list of initiating events to be considered in the analysis of a WWER NPP. Although the list
was developed with the idea of having a list from which a certain number of initiating events would
be selected for analysis of a specific plant, there is still no guarantee that all relevant events have been
included. The list is generally limited to such events which might have an impact on fuel or primary
system integrity (e.g. external events are not included).
Section 3 explains the different types of acceptance criteria to be used when evaluating results
of different accidents. The correlation between the probability of an event and the acceptable
consequences is also discussed. A typical list of acceptance criteria specific for WWERs is presented
in Appendix II.
Section 4 discusses the assumptions to be made when postulating a given event in order to
ensure that the analysis is reasonably conservative, i.e. will lead to a result which will sufficiently and
reliably ensure safety. Adequate selection of assumptions on functioning of systems and operating
actions are discussed. Lists of initial conditions and other assumptions specific for WWERs are
presented in Appendix III.
Section 5 summarizes both general and specific recommendations on quality assurance for
performing accident analysis.
Recommendations for the computer codes used in accident analysis are discussed in Section 6,
with special attention devoted to the problems of code validation.
The guidelines are complemented with a series of analysis examples of various accidents,
presented in Appendix IV, which should be considered as the applications of these guidelines.
It is intended that examples for additional accidents might be included in this appendix in
future revisions.
2. EVENTS TO BE CONSIDERED
2.1. LIST OF INITIATING EVENTS
The aim for setting a list of initiating events is to ensure a complete analysis of the response
of the plant to postulated disturbances in process variables and to postulated malfunctions or failures
of equipment. The analysis should determine the consequences and evaluate the capability built into
the plant to control or accommodate such failures and situations.
Criteria to govern the selection of initiating events and, consequently, the list itself are
continuously evolving due to operational experience feedback and also due to improvement of nuclear
safety knowledge.
The basic criteria for selection can be based on physical effects induced by the event itself
regarding the core neutronic and thermohydraulic field, as for instance:
Increase in heat removal
Decrease in heat removal
Reactivity and power distribution anomalies
Increase in reactor coolant inventory
Decrease in reactor coolant inventory
Decrease in reactor coolant system flowrate
Radioactive releases from subsystems or components
Fuel handling accidents
Anticipated transients without scram (ATWS)
Compilation of the list of initiating events corresponding to each of the above areas is usually
based on engineering judgement and also on probabilistic considerations, taking into account the design
features of a given nuclear power plant.
2.2. INITIATING EVENTS CATEGORIZATION
For events with a high probability of occurrence, more stringent requirements need to be
applied to prevent an excessive equipment damage and to reduce the potential for development of an
initiating event into a more serious accident. So the initiating events may be categorized into two
broad groups:
Anticipated transients which are defined as relatively frequent deviations (probability of
occurrence about > 10 2/year) from normal operating conditions which are caused by
malfunction of a component or operator error. These transients should not have safety related
consequences which prevent the plant operation to be continued.
10