Development and Application of Level 1 Probabilistic Safety Assessment for Nuclear Power Plants
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Development and Application of Level 1 Probabilistic Safety Assessment for Nuclear Power Plants - IAEA
DEVELOPMENT AND
APPLICATION OF
LEVEL 1 PROBABILISTIC
SAFETY ASSESSMENT FOR
NUCLEAR POWER PLANTS
IAEA SAFETY STANDARDS SERIES No. SSG-3 (Rev. 1)
DEVELOPMENT AND
APPLICATION OF
LEVEL 1 PROBABILISTIC
SAFETY ASSESSMENT FOR
NUCLEAR POWER PLANTS
SPECIFIC SAFETY GUIDE
INTERNATIONAL ATOMIC ENERGY AGENCY
VIENNA, 2024
COPYRIGHT NOTICE
All IAEA scientific and technical publications are protected by the terms of the Universal Copyright Convention as adopted in 1952 (Berne) and as revised in 1972 (Paris). The copyright has since been extended by the World Intellectual Property Organization (Geneva) to include electronic and virtual intellectual property. Permission to use whole or parts of texts contained in IAEA publications in printed or electronic form must be obtained and is usually subject to royalty agreements. Proposals for non-commercial reproductions and translations are welcomed and considered on a case-by-case basis. Enquiries should be addressed to the IAEA Publishing Section at:
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International Atomic Energy Agency
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PO Box 100
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fax: +43 1 26007 22529
tel.: +43 1 2600 22417
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www.iaea.org/publications
© IAEA, 2024
Printed by the IAEA in Austria
March 2024
STI/PUB/2056
https://doi.org/10.61092/iaea.3ezv-lp49
IAEA Library Cataloguing in Publication Data
Names: International Atomic Energy Agency.
Title: Development and application of level 1 probabilistic safety assessment for nuclear power plants / International Atomic Energy Agency.
Description: Vienna : International Atomic Energy Agency, 2024. | Series: IAEA safety standards series, ISSN 1020–525X ; no. SSG-3 (rev. 1) | Includes bibliographical references.
Identifiers: IAEAL 24-01658 | ISBN 978-92-0-130623-4 (paperback : alk. paper) | ISBN 978-92-0-130723-1 (pdf) | ISBN 978-92-0-130823-8 (epub)
Subjects: LCSH: Nuclear power plants — Safety measures. | Nuclear power plants — Risk assessment
Classification: UDC 621.039.58 | STI/PUB/2056
FOREWORD
by Rafael Mariano Grossi
Director General
The IAEA’s Statute authorizes it to establish…standards of safety for protection of health and minimization of danger to life and property
. These are standards that the IAEA must apply to its own operations, and that States can apply through their national regulations.
The IAEA started its safety standards programme in 1958 and there have been many developments since. As Director General, I am committed to ensuring that the IAEA maintains and improves upon this integrated, comprehensive and consistent set of up to date, user friendly and fit for purpose safety standards of high quality. Their proper application in the use of nuclear science and technology should offer a high level of protection for people and the environment across the world and provide the confidence necessary to allow for the ongoing use of nuclear technology for the benefit of all.
Safety is a national responsibility underpinned by a number of international conventions. The IAEA safety standards form a basis for these legal instruments and serve as a global reference to help parties meet their obligations. While safety standards are not legally binding on Member States, they are widely applied. They have become an indispensable reference point and a common denominator for the vast majority of Member States that have adopted these standards for use in national regulations to enhance safety in nuclear power generation, research reactors and fuel cycle facilities as well as in nuclear applications in medicine, industry, agriculture and research.
The IAEA safety standards are based on the practical experience of its Member States and produced through international consensus. The involvement of the members of the Safety Standards Committees, the Nuclear Security Guidance Committee and the Commission on Safety Standards is particularly important, and I am grateful to all those who contribute their knowledge and expertise to this endeavour.
The IAEA also uses these safety standards when it assists Member States through its review missions and advisory services. This helps Member States in the application of the standards and enables valuable experience and insight to be shared. Feedback from these missions and services, and lessons identified from events and experience in the use and application of the safety standards, are taken into account during their periodic revision.
I believe the IAEA safety standards and their application make an invaluable contribution to ensuring a high level of safety in the use of nuclear technology. I encourage all Member States to promote and apply these standards, and to work with the IAEA to uphold their quality now and in the future.
The authoritative versions of the publications are the hard copies issued and available as PDFs on www.iaea.org/publications.To create the versions for e-readers, certain changes have been made, including the movement of some figures and tables.
THE IAEA SAFETY STANDARDS
BACKGROUND
Radioactivity is a natural phenomenon and natural sources of radiation are features of the environment. Radiation and radioactive substances have many beneficial applications, ranging from power generation to uses in medicine, industry and agriculture. The radiation risks to workers and the public and to the environment that may arise from these applications have to be assessed and, if necessary, controlled.
Activities such as the medical uses of radiation, the operation of nuclear installations, the production, transport and use of radioactive material, and the management of radioactive waste must therefore be subject to standards of safety.
Regulating safety is a national responsibility. However, radiation risks may transcend national borders, and international cooperation serves to promote and enhance safety globally by exchanging experience and by improving capabilities to control hazards, to prevent accidents, to respond to emergencies and to mitigate any harmful consequences.
States have an obligation of diligence and duty of care, and are expected to fulfil their national and international undertakings and obligations.
International safety standards provide support for States in meeting their obligations under general principles of international law, such as those relating to environmental protection. International safety standards also promote and assure confidence in safety and facilitate international commerce and trade.
A global nuclear safety regime is in place and is being continuously improved. IAEA safety standards, which support the implementation of binding international instruments and national safety infrastructures, are a cornerstone of this global regime. The IAEA safety standards constitute a useful tool for contracting parties to assess their performance under these international conventions.
THE IAEA SAFETY STANDARDS
The status of the IAEA safety standards derives from the IAEA’s Statute, which authorizes the IAEA to establish or adopt, in consultation and, where appropriate, in collaboration with the competent organs of the United Nations and with the specialized agencies concerned, standards of safety for protection of health and minimization of danger to life and property, and to provide for their application.
With a view to ensuring the protection of people and the environment from harmful effects of ionizing radiation, the IAEA safety standards establish fundamental safety principles, requirements and measures to control the radiation exposure of people and the release of radioactive material to the environment, to restrict the likelihood of events that might lead to a loss of control over a nuclear reactor core, nuclear chain reaction, radioactive source or any other source of radiation, and to mitigate the consequences of such events if they were to occur. The standards apply to facilities and activities that give rise to radiation risks, including nuclear installations, the use of radiation and radioactive sources, the transport of radioactive material and the management of radioactive waste.
Safety measures and security measures¹ have in common the aim of protecting human life and health and the environment. Safety measures and security measures must be designed and implemented in an integrated manner so that security measures do not compromise safety and safety measures do not compromise security.
The IAEA safety standards reflect an international consensus on what constitutes a high level of safety for protecting people and the environment from harmful effects of ionizing radiation. They are issued in the IAEA Safety Standards Series, which has three categories (see Fig. 1).
Safety Fundamentals
Safety Fundamentals present the fundamental safety objective and principles of protection and safety, and provide the basis for the safety requirements.
Safety Requirements
An integrated and consistent set of Safety Requirements establishes the requirements that must be met to ensure the protection of people and the environment, both now and in the future. The requirements are governed by the objective and principles of the Safety Fundamentals. If the requirements are not met, measures must be taken to reach or restore the required level of safety. The format and style of the requirements facilitate their use for the establishment, in a harmonized manner, of a national regulatory framework. Requirements, including numbered ‘overarching’ requirements, are expressed as ‘shall’ statements. Many requirements are not addressed to a specific party, the implication being that the appropriate parties are responsible for fulfilling them.
Safety Guides
Safety Guides provide recommendations and guidance on how to comply with the safety requirements, indicating an international consensus that it is necessary to take the measures recommended (or equivalent alternative measures). The Safety Guides present international good practices, and increasingly they reflect best practices, to help users striving to achieve high levels of safety. The recommendations provided in Safety Guides are expressed as ‘should’ statements.
APPLICATION OF THE IAEA SAFETY STANDARDS
The principal users of safety standards in IAEA Member States are regulatory bodies and other relevant national authorities. The IAEA safety standards are also used by co-sponsoring organizations and by many organizations that design, construct and operate nuclear facilities, as well as organizations involved in the use of radiation and radioactive sources.
The IAEA safety standards are applicable, as relevant, throughout the entire lifetime of all facilities and activities — existing and new — utilized for peaceful purposes and to protective actions to reduce existing radiation risks. They can be used by States as a reference for their national regulations in respect of facilities and activities.
The IAEA’s Statute makes the safety standards binding on the IAEA in relation to its own operations and also on States in relation to IAEA assisted operations.
The IAEA safety standards also form the basis for the IAEA’s safety review services, and they are used by the IAEA in support of competence building, including the development of educational curricula and training courses.
International conventions contain requirements similar to those in the IAEA safety standards and make them binding on contracting parties. The IAEA safety standards, supplemented by international conventions, industry standards and detailed national requirements, establish a consistent basis for protecting people and the environment. There will also be some special aspects of safety that need to be assessed at the national level. For example, many of the IAEA safety standards, in particular those addressing aspects of safety in planning or design, are intended to apply primarily to new facilities and activities. The requirements established in the IAEA safety standards might not be fully met at some existing facilities that were built to earlier standards. The way in which IAEA safety standards are to be applied to such facilities is a decision for individual States.
The scientific considerations underlying the IAEA safety standards provide an objective basis for decisions concerning safety; however, decision makers must also make informed judgements and must determine how best to balance the benefits of an action or an activity against the associated radiation risks and any other detrimental impacts to which it gives rise.
DEVELOPMENT PROCESS FOR THE IAEA SAFETY STANDARDS
The preparation and review of the safety standards involves the IAEA Secretariat and five Safety Standards Committees, for emergency preparedness and response (EPReSC) (as of 2016), nuclear safety (NUSSC), radiation safety (RASSC), the safety of radioactive waste (WASSC) and the safe transport of radioactive material (TRANSSC), and a Commission on Safety Standards (CSS) which oversees the IAEA safety standards programme (see Fig. 2).
All IAEA Member States may nominate experts for the Safety Standards Committees and may provide comments on draft standards. The membership of the Commission on Safety Standards is appointed by the Director General and includes senior governmental officials having responsibility for establishing national standards.
A management system has been established for the processes of planning, developing, reviewing, revising and establishing the IAEA safety standards. It articulates the mandate of the IAEA, the vision for the future application of the safety standards, policies and strategies, and corresponding functions and responsibilities.
INTERACTION WITH OTHER INTERNATIONAL ORGANIZATIONS
The findings of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and the recommendations of international expert bodies, notably the International Commission on Radiological Protection (ICRP), are taken into account in developing the IAEA safety standards. Some safety standards are developed in cooperation with other bodies in the United Nations system or other specialized agencies, including the Food and Agriculture Organization of the United Nations, the United Nations Environment Programme, the International Labour Organization, the OECD Nuclear Energy Agency, the Pan American Health Organization and the World Health Organization.
INTERPRETATION OF THE TEXT
Safety related terms are to be understood as defined in the IAEA Nuclear Safety and Security Glossary (see https://www.iaea.org/resources/publications/iaea-nuclear-safety-and-security-glossary). Otherwise, words are used with the spellings and meanings assigned to them in the latest edition of The Concise Oxford Dictionary. For Safety Guides, the English version of the text is the authoritative version.
The background and context of each standard in the IAEA Safety Standards Series and its objective, scope and structure are explained in Section 1, Introduction, of each publication.
Material for which there is no appropriate place in the body text (e.g. material that is subsidiary to or separate from the body text, is included in support of statements in the body text, or describes methods of calculation, procedures or limits and conditions) may be presented in appendices or annexes.
An appendix, if included, is considered to form an integral part of the safety standard. Material in an appendix has the same status as the body text, and the IAEA assumes authorship of it. Annexes and footnotes to the main text, if included, are used to provide practical examples or additional information or explanation. Annexes and footnotes are not integral parts of the main text. Annex material published by the IAEA is not necessarily issued under its authorship; material under other authorship may be presented in annexes to the safety standards. Extraneous material presented in annexes is excerpted and adapted as necessary to be generally useful.
¹ See also publications issued in the IAEA Nuclear Security Series.
The authoritative versions of the publications are the hard copies issued and available as PDFs on www.iaea.org/publications.To create the versions for e-readers, certain changes have been made, including the movement of some figures and tables.
CONTENTS
1. INTRODUCTION
Background
Objective
Scope
Structure
2. GENERAL CONSIDERATIONS RELATING TO THE PERFORMANCE AND USE OF PSA
Scope of the PSA
Validation and review of the PSA
Living PSA
Probabilistic safety goals or criteria
Use of PSA in decision making
3. PROJECT MANAGEMENT AND ORGANIZATION FOR PSA
Definition of objectives and scope of the PSA project
Project management for PSA
Selection of methods and establishment of procedures
Team selection and organization
Establishing a quality assurance programme for PSA
General aspects of PSA documentation
4. FAMILIARIZATION WITH THE PLANT AND COLLECTION OF INFORMATION
5. LEVEL 1 PSA FOR INTERNAL INITIATING EVENTS FOR POWER OPERATION
General aspects of Level 1 PSA methodology
Initiating event analysis
Accident sequence analysis
Systems analysis
Analysis of dependent failures
Analysis of common cause failures
Human reliability analysis
Other modelling issues
Data for a Level 1 PSA
Quantification of the analysis
Importance analysis, sensitivity studies and uncertainty analysis
6. GENERAL METHODOLOGY FOR LEVEL 1 PSA FOR INTERNAL HAZARDS AND EXTERNAL HAZARDS
Introduction
Analysis process
Collection of initial information
Identification of hazards
Screening of single and combined hazards
7. SPECIFIC ASPECTS OF LEVEL 1 PSA FOR INTERNAL HAZARDS
Introduction
Bounding assessment for Level 1 PSA for internal hazards
Analysis of internal fire
Parameterization of external hazards
Analysis of internal flooding
Analysis of other internal hazards
8. SPECIFIC ASPECTS OF LEVEL 1 PSA FOR EXTERNAL HAZARDS
Introduction
Bounding assessment for Level 1 PSA for external hazards
Detailed analysis of external hazards
Frequency assessment for external hazards
Fragility analysis for structures, systems and components
Integration of external hazards in the Level 1 PSA model
Documentation and presentation of results
9. LEVEL 1 PSA FOR SHUTDOWN STATES
General aspects of Level 1 PSA for shutdown states
Specification of outage types and plant operational states
Initiating events analysis
Accident sequence analysis
Systems analysis
Analysis of dependent failures
Human reliability analysis
Data assessment
Quantification of accident sequences
Uncertainty analysis, importance analysis and sensitivity studies
Documentation and presentation of results
10. SPECIFIC ASPECTS OF LEVEL 1 PSA FOR THE SPENT FUEL POOL
Undesired end states
Plant operational states
Initiating events
Accident sequence analysis
Human reliability analysis
Quantification of the analysis
Interpretation of the results
11. LEVEL 1 MULTI-UNIT PSA
Scope of multi-unit PSA
Risk metrics for multi-unit PSA
Plant operational states
Initiating events analysis
Systems analysis
Human reliability analysis
Common cause failure and hazard fragility correlations
Quantification of the multi-unit PSA risk profile
12. USE AND APPLICATIONS OF LEVEL 1 PSA
General aspects of PSA applications
Scope of Level 1 PSA applications
Risk informed approach
Use of PSA for design evaluation
Use of PSA for optimization of inspections, testing and maintenance
Risk informed classification of structures, systems and components
Monitoring and managing risk configuration
Risk based safety performance indicators
PSA based event analysis (precursor analysis)
Risk informed regulations
Risk informed oversight and enforcement
Use of PSA insights to develop or enhance emergency operating procedures
Use of PSA insights for risk informed training of plant personnel
Use of PSA to address emerging issues
REFERENCES
Annex I: EXAMPLE OF A GENERIC LIST OF EXTERNAL HAZARDS
Annex II: EXAMPLES OF FIRE EVENT TREES AND SEISMIC EVENT TREES
Annex III: SUPPORTING INFORMATION ON PSA FOR SHUTDOWN STATES
CONTRIBUTORS TO DRAFTING AND REVIEW
1. INTRODUCTION
Background
1.1. IAEA Safety Standards Series No. SF-1, Fundamental Safety Principles [1], establishes principles to ensure the protection of workers, the public and the environment, now and in the future, from harmful effects of ionizing radiation. These principles emphasize the need to assess and control the inherent risk. In particular, para. 3.22 of SF-1 [1] on optimization of protection states:
To determine whether radiation risks are as low as reasonably achievable, all such risks, whether arising from normal operations or from abnormal or accident conditions, must be assessed (using a graded approach) a priori and periodically reassessed throughout the lifetime of facilities and activities.
1.2. Several IAEA Safety Requirements publications establish more specific requirements on risk assessment for nuclear power plants. Requirement 42 of IAEA Safety Standards Series No. SSR-2/1 (Rev. 1), Safety of Nuclear Power Plants: Design [2], states:
"A safety analysis of the design for the nuclear power plant shall be conducted in which methods of both deterministic analysis and probabilistic analysis shall be applied to enable the challenges to safety in the various categories of plant states to be evaluated and assessed."
Furthermore, para. 5.76 of SSR-2/1 (Rev. 1) [2] states (footnote omitted):
"The design shall take due account of the probabilistic safety analysis of the plant for all modes of operation and for all plant states, including shutdown, with particular reference to:
(a) Establishing that a balanced design has been achieved such that no particular feature or postulated initiating event makes a disproportionately large or significantly uncertain contribution to the overall risks, and that, to the extent practicable, the levels of defence in depth are independent;
(b) Providing assurance that situations in which small deviations in plant parameters could give rise to large variations in plant conditions (cliff edge effects) will be prevented;
(c) Comparing the results of the analysis with the acceptance criteria for risk where these have been specified."
Thus, probabilistic safety assessment (PSA) is considered to be an important tool for analysis to ensure the safety of a nuclear power plant in relation to potential initiating events that might be caused by random component failure or human error, as well as by internal and/or external hazards.
1.3. Paragraph 4.13 of IAEA Safety Standards Series No. GSR Part 4 (Rev. 1), Safety Assessment for Facilities and Activities [3], states:
The safety assessment shall include a safety analysis, which consists of a set of different quantitative analyses for evaluating and assessing challenges to safety by means of deterministic and also probabilistic methods.
Paragraph 4.55 of GSR Part 4 (Rev. 1) [3] states:
The objectives of a probabilistic safety analysis are to determine all significant contributing factors to the radiation risks arising from a facility or activity, and to evaluate the extent to which the overall design is well balanced and meets probabilistic safety criteria where these have been defined.
Thus, a comprehensive PSA is required to investigate the safety of a nuclear power plant thoroughly.
1.4. PSA has been shown to provide important safety insights in addition to those provided by deterministic analysis. PSA provides a methodological approach to identifying accident sequences that can follow from