Risk informed decision making

To date, probabilistic safety assessments (PSAs) have been performed for more than 200 nuclear power plants (NPPs) worldwide and are under various stages of development for most of the remaining NPPs. The state-of-the-art is to have a full scope Level 2 PSA (including external events and low power and shutdown) which is maintained as a 'living PSA' with regular updating. Modern computer technology allows frequent recalculations of the PSA to evaluate the impact of changes in operation or design and allows use of the PSA in the form of safety or risk monitors. There is a general agreement, as documented in various IAEA Safety Standards, that the deterministic approach to nuclear safety should be complemented by a probabilistic approach. There is now a recent development, led by the USA and followed by several other countries, to move to a much expanded use of PSA in what is termed risk informed decision making'. The main driving force behind this movement is the expectation that the use of risk insights can result in both improved safety and a reduction in unnecessary regulatory requirements, hence leading to a more efficient use of resources for NPP operators and the regulatory authority. One of the key challenges in truly risk informed decision making is the reconciliation of PSA results and insights with traditional deterministic analysis. This is particularly true when it comes to defence in depth and safety margins. A prerequisite for an expanded use is the availability of a high quality 'living PSA' which supports the various applications. The PSA quality should be commensurate with its intended application. This means that there is not one standard for judging the adequacy of the PSA, but that the quality of the PSA should be judged in relation to each specific use or application. Many efforts have been devoted to achieving consistency and quality of PSAs. They include peer reviews (e.g. the IAEA's IPERS/IPSART programme), PSA standardization efforts (e.g. United States Nuclear Regulatory Commission (NRC) PRA Procedures Guide, IAEA PSA Guidelines, the recent draft American Society of Mechanical Engineers (ASME) PSA Standard, IAEA,OECD NEA guidance for regulatory review of PSAs) and compilation and comparisons of PSAs for similar types of NPPs, including comparisons of the success criteria and failure rates used. Another concept being pursued is to provide a quality grading of the major PSA elements and sub-elements required to support a specific application and to assess the quality of the PSA in these required areas. This paper draws on some information compiled in IAEA-TECDOC-1200, Applications of Probabilistic Safety Assessment (PSA) for Nuclear Power Plants. This document has been drafted as a result of several meetings