Status and experience with the technical basis and use of probabilistic risk criteria for nuclear power plants

Probabilistic safety criteria, including safety goals, have been progressively introduced by regulatory bodies and utilities. They range from high level qualitative statements to technical criteria. They have been published in different ways, from legal documents to internal guides. They can be applied as legal limits down to “orientation values”. The OECD/NEA Working Group on Risk (WGRISK) prepared a questionnaire on the probabilistic risk criteria for nuclear power plants. Answers were received from 13 nuclear safety organizations and 6 utilities. The reported probabilistic risk criteria can be grouped into 4 categories, in relation with the tools to be used for assessing compliance: core damage frequency, releases frequency, frequency of doses and criteria on containment failure. Introduction of probabilistic safety criteria is generally considered to result in safety improvements. Opinion is widespread on the benefits of using probabilistic safety criteria for communication with the public, ranging from bad to good experiences. The responses to the questionnaire suggested that more work should be considered in the definition of releases frequencies: some regulators include a time range (generally 24 hours) in the criterion while others do not limit the time to be considered. It is suggested that, in the first case, the existing PSAs should be revisited to assess if long development accident sequences were considered.