Probabilistic risk criteria and safety goals

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. Generally, all respondents considered introduction of probabilistic safety criteria resulted 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. It seems that there is a strong relation with each country culture and the circumstances. The responses to the questionnaires 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.