Final Report Summary - ASAMPSA2 (Advanced Safety Assessment Methodologies : level 2 PSA (European Best Practices L2 PSA guidelines)) Executive summary: In the context of nuclear power plant (NPP) safety, level-2 probabilistic safety assessment (L2-PSA) is a structured methodology aiming at assessing the risk of radioactive release in environment in the case of an accident on a NPP. L2-PSA completes L1-PSA which quantifies all accident scenarios due to human and equipment failures that could lead to a degradation of nuclear fuel. L2-PSA results are generally expressed in terms of frequency (per year and per reactor) of radioactive release (amplitude and kinetics) in environment. The objective of the ASAMPSA2 coordination action (2008 to 2011), was to develop best practice guidelines for L2-PSA, with a view to harmonisation at European level and allowing the development of meaningful and practical uncertainty evaluation. This project was coordinated by IRSN and involved 21 European partners from 12 European countries: IRSN, GRS, NUBUKI, TRACTEBEL, IBERINCO, UJV, VTT, RSE SpA, AREVA NP GmbH, AMEC NNC, CEA, FKA, CCA, ENEA, NRG, VGB, PSI, FORTUM, STUK, AREVA NP SAS, SCANDPOWER. In addition to the development of European L2 PSA guidelines, the ASAMPSA2 project offered a framework to share experience and develop competences of technical teams. It provides the opportunity to discuss the link between the research area activities on severe accident and NPP risk assessment. Some follow-up actions are now considered in the European framework to complete these guidelines, for example on the assessment of specific risks induced by beyond-design extreme events, in relation with the Fukushima accident lessons, or on the verification of the efficiency of NPP severe accident management measures. Project context and objectives: Within the European community responsible for fission reactor safety, a need to develop best practice guidelines for the L2-PSA methodology has been repeatedly expressed, with the aim of both fulfilling the requirements of safety authorities in an efficient way, and also promoting harmonisation of practices in European countries in order to use results from L2-PSAs with a greater confidence. Existing guidelines, like those developed by the IAEA, mainly propose a general stepwise procedural methodology, but no details on the technical applicable methodologies. Before the ASAMPSA2 project, in Europe, integration of probabilistic findings (for severe accident) and insights into the overall safety assessment of NPPs was quite differently understood and implemented. Within this general context, the ASAMPSA2 project objectives were to highlight some common best practices, to develop the appropriate scope and criteria for different level-2 PSA applications, and to promote optimal use of the available resources. The ASAMPSA2 guidelines scope was defined to cover generation II and III PWRs and BWRs but also application for future generation IV reactors. One important feature of the ASAMPSA2 project was to bring together stakeholders of nuclear industry (plant operators, plant designers, technical safety organisations (TSOs), safety authorities, service providers, research organisations), with different roles in the safety demonstration, analysis and regulation. The partners of the ASAMPSA2 consortium have been chosen on the basis of their high experience in the development and application of L2 PSA or severe accident analysis. They had to propose some best practices guidelines for a limited-scope and a full-scope L2-PSA from their experience and international cooperation (especially SARNET). Such a common assessment framework was intended to support harmonised views on the role of probabilistic safety assessment on nuclear safety. Another objective of the project was to establish specific relationships with the community in charge of nuclear reactor safety (organisations that were not involved in ASAMPSA2) in order to define the existing needs in terms of guidelines for L2-PSA development and applications, and at the end of the project to organise an external review of the ASAMPSA2 guidelines. In terms of dissemination and / or exploitation of the results, the main objectives of the project were: - to establish some public European best-practices guidelines for L2 PSA, acceptable and useful for the European end-users, whatever their position (safety authorities, TSO, utilities, vendors); - to identify specific issues where research activities are still needed to allow a meaningful quantification of risks. ASAMPSA2 was supposed to contribute to the preparation of future research activities in connection with SARNET. Project results: General considerations a) Scope of the ASAMPSA2 guidelines The aim of the ASAMPSA2 coordination action was to build a consensus on the L2-PSA scope and on methods deemed to be acceptable, according to the different potential applications. It was clear from the beginning that, depending of the issues, there is a range of outcomes that can be considered acceptable. To represent this range, the partners initially tried to distinguish between a limited-scope methodology and a full-scope one, based on what is currently technically achievable in the performance of a L2-PSA. The notion of limited-scope methodology may correspond to the case where the study is performed to answer some precise question (for example the quantification of large early release frequency (LERF)), allowing simplification of some parts of the analysis, and limitation of the needed resources. The distinction between limited-scope and full-scope methodologies has been widely discussed in the initial phase of the ASAMPSA2 project and the possibility to establish two separated guidelines has been examined. But from a practical point of view, it appeared that many variations in the definition of what is a limited-scope study exist in relation with the different applications. Consequently, the partners have decided to build a single document including all issues related to L2-PSA development and applications. For each technical issue, the different possible levels of details and acceptable methods are described with some recommendations. It was also stated that the guidance on applications of L2-PSA was a crucial part but also very difficult to establish. It has conducted the project partners to develop a specific volume of the guidelines on this topic. b) Relationship with the L2-PSA end-users In the definition of the ASAMPSA2 project, the relationships with the L2-PSA end-users (establishing the needs of the end-users for the performance of a L2-PSA as well as assuring the acceptance of the guidelines to be prepared at the end of the project by a majority of the end-users) were considered as a key point. A dedicated working group, coordinated by PSI, has been established to help in formalising these relationships. At the beginning of the project, a survey was conducted to establish more precisely the needs of the end-users community regarding many aspects of performing a L2-PSA. The results of the survey were discussed during a dedicated workshop, hosted by Vattenfall in Hamburg (Germany) in October 2008. At the end of the project, an external review of the guidelines has been organised to receive the response from the end-users community. The review has been discussed during a workshop hosted in March 2011 by Fortum in Helsinki (Finland). This final review, like the initial survey, has associated European stakeholders and also organisations from other countries, especially those members of the working group risk of the nuclear energy agency (OECD). ASAMPSA2 partners have taken into consideration the positions provided by end-users irrespective of their role (plant operators, plant designers, TSOs, safety authorities). c) Link with the international scientific research activities related to severe accidents A L2-PSA is mainly based on a set of deterministic studies on the different phenomena related to severe accident progression. A large part of the guidelines concerns the way of quantifying each part of the accident progression. The first draft of the different chapters has gathered the methodologies currently used by the partners PSA experts and describe some rationale. To improve their final quality regarding the state-of-art for each topic, the guidelines have been open for review by specialists involved in the 'Severe accident Network of Excellence' (SARNET) or in the working group 'Accident management' of the Nuclear Energy Agency (OECD). d) Link with other existing standards Others countries, outside the European Union (EU), or international organisations have developed some L2-PSA guidance. The ASAMPSA2 guidelines provide a number of key references from OCDE, IAEA or countries that can be useful. In many cases, examples of good practices have been extracted from these references. Some outcomes of the initial end-users survey Feedback on the 2008 end-users survey helped in the identification of some technical issues where harmonisation or best-practices are particularly needed, for example: - L1-PSA - L2-PSA Interface: advantages and disadvantages of the integrated and non integrated studies, use of L1- PSA probabilistic tools or dedicated tools for L2-PSA; - methods for uncertainty assessment depending of the considered issues and L2-PSA objectives; - the closure of issues in accident progression regarding research activities: in that context, an issue is 'closed' when L2-PSA developers find enough knowledge or validated codes for the assessment of risks (it can be dependent on the plant design); - the assessment of initial containment leakage, use of historic data (tests), assessment of containment isolation failure. The end-users survey also showed that there was a lack of uniformity between the countries in the objectives and applications of L2-PSAs. Only a few EU safety authorities had precise safety goals regarding severe accidents, and in general the legislation or rules, when they existed, were not strictly applied. Very few utilities had a voluntary approach for 'risk-informed' application of L2-PSA (Finish utilities as mandated in legislation, EDF which has recently developed application for periodic safety review). Some utilities still had an unclear view on how and why to develop a L2-PSA. It was expected that the ASAMPSA2 project will help in harmonisation of technical issues by providing a global (but practical) vision of how the different risks can be assessed within a L2-PSA taking into account the existing knowledge and codes. It should also help in harmonisation on application of L2-PSA, in particular to help to identify some plant 'risk reduction options'. It seems clear today, in the context of the post-Fukushima accident, but also in the context of development of the WENRA safety reference levels, that positions of safety authorities in Europe regarding risk of severe accident are evolving in the direction of more demanding requirements. Structure of the guideline At the end of the project, the ASAMPSA2 guidelines are composed of three volumes. The first volume includes a general description of L2-PSA content and structure and discusses the different possibilities to present the L2-PSA results depending on applications. The project has used (as much as possible) information available on public domain, mainly from other international collaboration initiative, for example on the description of safety criteria. The second volume of the document contains all technical recommendations gained from the experience of the ASAMPSA2 partners and external sources, for generation II and generation III NPPs. It covers the methodological topics (L1-L2 PSA interface, human reliability assessment, event tree structure, uncertainties assessment), the quantification of severe accident progression and containment loading, the containment performance (tightness), the plant system behaviour in severe accident conditions and the source term assessment. This volume shows the very large number of issues that must be examined in a L2-PSA and discusses topic by topic the assessment of uncertainties. The different topics were distributed among the partners, depending on their experience, and based on resources available for this project. The development of these two volumes was coordinated by IRSN. The last volume of the guidelines concerns the applications for generation IV reactors, with the objective to describe how far the existing recommendations for generation II and III reactors L2-PSA may apply for the generation IV reactors concepts. It raises some key issues for very-high temperature reactors (VHTR), sodium-cooled fast reactors (SFR), the gas-cooled fast reactors (GFR) and lead-cooled fast reactors (LFR). This volume can be used a starting point for L2-PSA development for these reactors. The development of this volume was coordinated by CEA. Outcome of the final end-users survey and further needs The end-users survey has conducted to the following conclusions: a) Relevance of the ASAMPSA2 guidelines regarding recent research and development (R&D) results It is apparent that respondents appreciated the precision and level of details of the ASAMPSA2 guidelines, which can be considered as a precious handbook for the L2-PSA developers. The scientific community, and in particular SARNET representatives, have indicated that for severe accident phenomenology, no major point was missing or wrongly addressed in the guideline. They have also provided some complements for some issues. The guidelines will be also used as one of the technical bases for the preparation of next plan of activities of SARNET. b) Expectations from L2-PSA developers on additional R&D activities The ASAMPSA2 guidelines show that knowledge and simulation tools are in place for most severe accident phenomena but some expectations for R&D progress have also been identified, for example: method for introduction of recovery actions in L2-PSA, core degradation for shutdown states with open reactor vessel (air ingress), in-vessel and ex-vessel degraded core coolability, positive and negative impact of in-vessel water make-up, kinetics and amplitude of release in case of basemat penetration, reduction of uncertainties of source term prediction, steam explosion (corium explosibility, real status of corium in the vessel which may be too cold for explosion, capabilities of codes to accurately predict the behaviour of structures), accurate reactivity accident modelling. c) General comments The ASAMPSA2 guidelines rarely proposed a single solution, but instead often proposed several possibilities to build a L2-PSA. This status is in fact consistent with the direction taken by the ASAMPSA2 partners: to identify a panel of acceptable methods but not to impose a unique set of procedures. Nevertheless, the perspective to derive a shorter document from the existing ASAMPSA2 guidelines that could be endorsed by both regulators group (ENSREG or WENRA) and utilities group (ENISS) has been identified as a useful follow-up action. Some reviewers have noted that the ASAMPSA2 guidelines do not cover the accident initiated by external events. This was not in the objectives of the project as defined with the partners and the European Commission (EC). This topic was identified as a possible useful follow-up activity in Europe. The Fukushima accident has of course confirmed this conclusion. d) Further needs A set of possible follow up activities has been identified. It includes, for example: - the improvement of tools able to incorporate deterministic simplified model to simulate accident progression in an event tree, to calculate source term assessment; such tools should include Monte-Carlo simulations and offer possibilities for dynamic reliability methods; - the improvement of tools and methodologies to interface L1 and L2-PSA; - the extension of the ASAMPSA2 guidelines to external events, and some complement for shutdown states even if partially addressed; - the elaboration of L2-PSA standardised risk metrics that could be applied for all plants; - the elaboration of database on experimental results useful for L2-PSA developers; - the use L2-PSA for validation of severe accident management options; - the training and review services. e) A proposal for a new European project: ASAMPSA_Extended As mentioned above, after the ASAMPSA2 project, some follow-up projects that could be useful for European nuclear stakeholder were identified. After the Fukushima accident, the EC has submitted a call of tender to promote a project making the link between PSA and extreme external events: 'The nuclear accident in Japan resulted from the combination of two correlated extreme external events (earthquake and tsunami). The consequences (flooding in particular) went beyond what was considered in the initial NPP design. Such situations can be identified using PSA methodology that complements the deterministic approach for beyond design accidents. If the performance of a L1-L2 PSA concludes that such a low probability event can lead to extreme consequences, the industry (system suppliers and utilities) or the safety authorities may take appropriate decisions to reinforce the defence in depth of the plant. The present topic aims at providing best practice guidelines for the identification of such situations with the help of level 1-level 2 PSA and for the definition of appropriate criteria for decision making in the European context. Involvement of regulatory authorities in the foreseen action is a must. Cooperation with Japan is welcome.' 28 European organisations (from 18 countries) have submitted a 3 years project, called ASAMPSA_Extended, with a format close to ASAMPSA2, including the following activities: - guidance for practicable and meaningful methods to characterise and introduce external events in L1-L2 PSAs; - guidance for a decision making process based on extended PSAs; - additional guidance for L2-PSA (SAM validation, complements for shutdown states of reactors). The outcomes of this project should help European stakeholders to verify, by means of probabilistic approaches (with PSA extended to cover risk contribution from all sources of radioactivity, all operating states for all important sources of radioelements and all types of initiating events (internal events, internal hazards and external hazards)), the NPPs safety robustness, after having included the additional improvements defined in response to the Fukushima Daïchi accident and the stress test process conducted in all European countries. The precise content of the project has to be discussed with EC. It should provide a useful framework for further improvements of L1-L2 PSA quality and impact of NPP safety. Potential impact: The ASAMPSA2 project provided opportunity to gathers European experts, from a large panel of organisations, to examine how to perform meaningful risk assessment for severe accident on a NPP. It is recognised that phenomena related to severe accident are highly complex and the application of all results gained from research activities developed in Europe or outside since the TMI accident may be extremely difficult without specific guidance. The guidelines developed by the ASAMPSA2 project offer many keys to develop such probabilistic risk assessment based on up-to-date methodologies and research results. The safety of NPPs is of prime importance for all European countries and probabilistic risk assessment is a major tool for the continuous safety enhancement process. Harmonisation of practices in Europe, at a technical level, should contribute to this process. The ASAMPSA2 project shows that such a harmonisation at a technical level is possible in Europe. The interest of other countries (United States (US), China, Japan, Korea, Ukraine) for the outcomes of ASAMPSA2, even before the Fukushima accident, proves the relevance of this project. The impact of the ASAMPSA2 project in the international community of nuclear safety is significant due to the involvement different types of stakeholders (but also to formalised relationships established among more than 100 organisations in Europe and the members of the nuclear energy agency working groups of OECD). Number of references used in the ASAMPSA2 guidelines refers to the experience of European organisations and the public dissemination of the ASAMPSA2 guidelines should contribute to promote this experience. List of websites: http://www.asampsa2.eu Coordinator: Emmanuel Raimond IRSN BP 17 92262 Fontenay-aux-Roses Cedex France Email: emmanuel.raimond@irsn.fr Tel : +33-158-357870 Fax : +33-158-358559 Other documents Final Report Volume 1 - General Volume 3 - Extension to Gen IV reactors Volume 2 - Best practices for the Gen II PWR, Gen II BWR L2PSAs. Extension to Gen III reactors
