Probabilistic safety assessment of multi-unit nuclear power plant sites - An integrated approach

Multi-unit safety assessment has gained global importance after the Fukushima disaster in 2011. Most of the nuclear sites in the world have more than one reactor and hence it is important to evolve a methodology to systematically assess the safety of the multi-unit site. In this paper, unique features to be addressed in multi-unit safety assessment are discussed and an integrated approach is developed to assess the risk contribution of multiple nuclear plants at the site. The paper highlights the importance of risks for multi-unit sites arising from shared system, common cause failures, failure correlations, cliff-edge effects, etc. from different hazards. Though the main emphasis on multi-unit safety is on external hazards, the proposed approach also includes risk from random internal events. The approach developed not only quantifies the frequency of multiple core damage for a multi-unit site but also evaluates site core damage frequency which is the frequency of at least single core damage per site per year.