Guidance for the definition and application of probabilistic safety criteria

A guidance document has been developed as part of a four-year Nordic project dealing with the use of probabilistic safety criteria for nuclear power plants. The Guidance sums up, on the basis of the work performed throughout the project, issues to consider when defining and applying probabilistic safety criteria. The Guidance describes the terminology and concepts involved, levels of probabilistic safety criteria and relations between these, how to define a criterion, how to apply a criterion, on what to apply the criterion, and how to interpret the result of the application. It specifically deals with what makes up a probabilistic safety criterion, i.e., the risk metric, the frequency criterion, the PSA used for assessing compliance, and the application procedure for the criterion. It will also discuss the concept of subsidiary criteria, i.e., different levels of safety goals, their relation to defense in depth and to a primary safety goal in terms of health effects or other off-site consequences.