ASAMPSA2 Project

ASAMPSA2 partners
France: IRSN, CEA, AREVA NP SAS • Germany: GRS, VGB, AREVA NP GMBH • Belgium: TRACTEBEL • Spain: IBERINCO • Finland: VTT, FORTUM, STUK • Hungary: NUBIKI • Italy: RSE SpA, ENEA • Netherlands: NRG • Czech Republic : UJV • United Kingdom: AMEC NNC Ltd • Sweden: FKA, SCANPOWER • Switzerland: PSI, Cazzoli Consulting
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1. Concept and Project Objectives Print E-mail

Within the European community responsible for fission reactor safety (plant operators, plant designers, Technical Safety Organizations (TSO), Safety Authorities) a need to develop best practice guidelines for the level 2 PSA methodology was repeatedly expressed, with the aim of both fulfilling the requirements of safety authorities in an efficient way, and also promoting harmonization of practices in European countries in order to use results from level 2 PSAs with a greater confidence.

Existing guidelines, like those developed by the IAEA, proposed a general stepwise procedural methodology, mainly based on US NUREG 1150 and high level requirements (for example on assessment of uncertainties). While it was clear that such a framework was necessary, comparisons of existing level 2 PSA, performed and discussed in SARNET L2 PSA work packages, showed that the detailed criteria and methodologies of level 2 PSAs at the time strongly differed from each other in some respects. In Europe integration of probabilistic findings or insights into the overall safety assessment of Nuclear Power Plants (NPPs) was quite differently understood and implemented.

 

Within that general context, the project objectives were to highlight common best practices, develop the appropriate scope and criteria for different level 2 PSA applications, and promote optimal use of the available resources. Such a common assessment framework supported a harmonized view on nuclear safety, and helped formalize the role of probabilistic safety assessment.
A common assessment framework required that some underlying issues were clearly understood and well developed. Some important issues were:

  • the PSA tool should be fit for purpose in terms of quality of models and input data,
  • the scope should be appropriate to the life stage (e.g. preliminary safety report, pre-operational safety report, living PSA) and plant states (e.g. full power, shutdown, maintenance) considered,
  • the objectives, assessment criteria and presentation of results should facilitate the regulatory decision making process.

The main characteristic of this coordination action was to bring together the different stakeholders (plant operators, plant designers, TSO, Safety Authorities, PSA developers), irrespective of their role in the safety demonstration and analysis: this promoted a common definition of the different types of L2 PSA and helped develop common views.

The aim of the coordination action was to build a consensus on the L2 PSA scope and on detailed methods deemed to be acceptable, according to different potential applications. In any methodology, and especially one developed from a wide range of contributing perspectives, there is a range of outcomes considered acceptable. To represent this range, the project considered a ‘limited-scope' methodology and a ‘full-scope' one, based on what was technically achievable in the performance of a level 2 PSA. In this respect it should be noted that what is technically achievable may not be cost effective, but for the purpose of this project was taken to represent the upper bound of what may be considered ‘reasonable'.

  • ‘Limited-scope' methodology
a limited description of the main reactor systems associated with standard data on the reactor materials and on the severe accident phenomenology and human actions reliability led to a simplified L2 PSA that included some indication of the main accident sequences that contribute to the risk of atmospheric releases due to a severe accident. For example, ‘limited-scope' methods could apply to a L2 PSA performed with a limited number of top events in the event-tree and mainly dedicated to identification of accident sequences contributing to the Large Early Release Frequency (LERF). Such a L2 PSA can, nevertheless, include very detailed and complex supporting studies for the quantification of these top events. Engineering judgment may also help in the quantification of the top events of limited scope L2 PSA but the justification of this engineering judgement is considered as a key issue.
  • ‘Full-scope' methodology
sophisticated methods that consider the full range of reactor initial states and possible accidents together with detailed physical phenomena modeling and a uncertainty analysis: as a consequence these L2 PSAs allow not only identification of the most sensible sequences with their occurrence probability and the related fission product release to the environment, but also the uncertainty range of the results, the weak points in the reactor system and operation, and also to identification of the accident phenomena which need further assessment to improve the result's relevance.  In such a wide ranging L2 PSA, quantification of sequences leading to large early release is clearly not the only objective.

In reality, most level 2 PSAs were at an intermediate level between these two approaches, but this representation was foreseen as a pragmatic way to organize the coordination action, allowing discussion both on simple and elaborated methodologies. It was assumed that the need for application of an advanced method is established from the results obtained by an earlier simplified study in regard to specific requirements of the national safety authorities.

Evidently, the second type of approach is time consuming and supposes a qualified dedicated team. Some applications do not warrant this level of detail and, additionally, some small stakeholders (especially utilities) cannot afford this level of commitment. Clearly, the scope should be appropriate to the application and life stage under consideration and the detailed methods should represent an acceptable balance between best practice and available resources. Level 2 PSAs results obtained using differing approaches or for differing scopes should not be directly compared.

 

The project focused on the definition of the objectives and the scope of both methodologies by making the best use of the End-User needs for level 2 PSA (Plant operators, Plant designers and Safety Authorities), know-how and practices at the time.

 

The main objectives of the ASAMPSA2 project were:

  • to clarify the current need of level 2 PSA "End-Users" in terms of guidelines beyond the existing guidelines (the project will clearly depart from existing guideline to avoid non useful activities),
  • to define precisely the differences between ‘limited-scope' L2 PSA and ‘full-scope' L2 PSA in terms of structure, contents and applications,
  • to formalize best-practices for L2 PSA applications and developments, in complement of existing guidelines, either for ‘limited-scope' L2 PSA and ‘full-scope' L2 PSA,
  • to disseminate these best-practices towards L2 PSA End-Users community.

 

For the ASAMPSA2 project the L2 PSA ‘End-Users' community was larger than ‘End-Users' partners of the ASAMPSA2 Consortium. This very important point is clarified in §3.1.

 

Deliverables and milestones of the project are defined in the following chapter.