2010:36 Guidance for the Definition and Application of Probabilistic Safety Criteria

Safety goals are defined in different ways in different countries and also used differently. Many countries are presently developing them in connection to the transfer to risk-informed regulation of both operating nuclear power plants (NPP) and new designs. However, it is far from self-evident how probabilistic safety criteria should be defined and used. On one hand, experience indicates that safety goals are valuable tools for the interpretation of results from a probabilistic safety assessment (PSA), and they tend to enhance the quality and realism of a risk assessment. On the other hand, strict use of probabilistic criteria is usually avoided, due to the large number of different uncertainties in a PSA model.

This report aims at providing general guidance concerning the formulation, application and interpretation of probabilistic criteria. The Nordic project “The Validity of Safety Goals” that was initiated in 2006 and finalised in 2010, had the aim to provided a general description of the issue of probabilistic safety goals for nuclear power plants, of important concepts related to the definition and application of safety goals, as well as of experiences in Finland and Sweden. The project has also aimed at providing guidance related to the resolution of some of the problems identified, such as the problem of consistency in judgement, comparability of safety goals used in different industries, the relationship between criteria on different levels, and relations between criteria for level 2 and 3 PSA. In parallel, a wide international overview was achieved by contributing to and benefiting from a survey on PSA safety criteria which was initiated in 2006 within the OECD/NEA Working Group Risk.