The development of Nordic guidance in level 3 PSA

Probabilistic Off-site Consequences Analysis, commonly referred to as Level 3 Probabilistic Safety Assessment (Level 3 PSA), has been infrequently performed and is generally poorly represented in the literature in comparison with Level 1 and Level 2 PSA. Due to new nuclear construction projects and the multi-unit accident at the Fukushima Daiichi site, there is a renewed interest in Level 3 in general and what risk insights that can be gained from Level 3 PSA compared to Level 1 and 2. Based on an inquiry from the Nordic Nuclear Safety Research (NKS) and Nordic PSA Group (NPSAG), a consortium of Swedish nuclear risk consultancies (Lloyd's Register Consulting, ÅF, Risk Pilot and Vattenfall) and the Finnish research institute VTT have performed a multi-year study of Level 3 PSA. The objective of the project has been to further develop understanding within the Nordic countries in the field of Level 3 PSA, the scope of its application, its limitations, appropriate risk metrics, and the overall need and requirements for Level 3 PSA. This paper describes the project and its results. During the project, targeted discussions between consultancies, utilities, regulators, and insurance companies on the subject of Level 3 PSA have taken place. Part of the project has also been to participate in ongoing international activities with the International Atomic Energy Agency (IAEA) and the American Nuclear Society (ANS). Other important parts of the project have been to perform an international industrial survey, a study of appropriate risk metrics, and two pilot studies. The main objective of the pilot studies was to gain practical experience that, together with insights from the other tasks included in the project, could be transferred to recommendations in a final guidance document directed to satisfy the needs for the Nordic nuclear power industry. The first pilot study was made by using IDPSA methodology (Integrated Deterministic and Probabilistic Analysis). In this pilot the atmospheric release from the Fukushima accident was studied using weather conditions typical of Japan. The other pilot was performed using a more traditional approach by studying the potential health effects of a release from a generic Nordic plant site considering atmospheric release below, near and above Level 2 PSA regulatory limits, using source terms for the UK EPR (which are publicly available). The end result of the project is a guidance document that aims to provide clear and applied guidance on Level 3 PSA studies toward regulators, utilities, and practitioners based on the conclusions made over the course of the work. During the course of the project a deeper understanding has been developed within the Nordic countries in the field of Level 3 PSA, the scope of its application, its limitations, appropriate risk metrics, and the overall need and requirements for performing a Level 3 PSA. In the short term this experience will be valuable for increasing quality to Level 1 and 2 PSA (e.g. reduced conservatism). In the longer term, the work will set the foundation for performing state-of-the-art Level 3 PSA.