Probabilistic safety assessments

Probabilistic safety assessment (PSA) has been widely used as a risk based tool to support the decision making process more than just for the benefit of risk informed regulation. PSA has been used to determine a plant safety level, for backfitting and to reduce the unnecessary regulatory burden caused by decisions based on deterministic requirements that lead to insignificant changes on risk monitor parameters (e.g. core damage frequency (CDF)). There are some cases where deterministic and probabilistic approaches are in conflict and it is indispensable to reconcile them. Some countries solve these conflicts using a case-by-case analysis, while others use conservative solutions. In the early days nuclear safety was defined by deterministic methods, together with operational experience feedback. Probabilistic ideas and methods were used increasingly against very stiff resistance. With time, however, probabilistic safety criteria (PSC) turned out to be indispensable. However, few countries have included PSC within their formal licensing process or standards. Performing the PSA of a plant is a difficult process. First, the starting point for an international agreement is the definition of a standard PSA that might vary according to the reactor types considered. In particular, it is necessary to obtain a clear understanding of the meaning and extent of risk comparative parameters (or monitors), such as CDF, large early release frequency or individual dose probability versus CDF. Without a common understanding of these parameters, it will be complicated to define PSC. Second, the establishment of an adequate quality assurance programme for conducting PSA projects on policy and objectives, management control and work implementation is necessary to achieve the desired quality. Third, it is essential to make explicit what impact an uncertainty will lead to in the analysis when PSC are used to decide what is acceptable. The experience of using PSA considering the three aspects mentioned above would lead to a real agreement on the definition of PSC for different PSA applications, as has happened historically in many technical disciplines. A sufficient legal basis would be established when a standard PSA comes in force in a given country. On the other hand, nowadays the use of PSA represents a clear understanding between the utilities and the regulatory bodies in many countries. The definition of PSC would improve such an understanding by providing a suitable goal for each PSA application or a better assessment of both the need for or the proposed changes to a plant