Treatment of Dependency and Correlation in Multiunit PSA Considering Seismic as External Event

In the wake of Fukushima accident, throughout the world there is a concern about the safety of the nuclear reactor sites from external events such as seismic, flood, tsunami etc. Nuclear reactor sites around the world are mostly twin units or multi units. Hence, these days the concept of Reactor Park is also widely discussed, which brings out the requirement for proper site safety goals. As per International Atomic Energy Agency (IAEA) safety guide lines, the safety goals for single unit Nuclear Power Plants (NPPs) are very well established, but when it comes to multiunit sites the concept of site safety goals is not matured enough, hence, it is very much needed to establish these goals at the site level.Unlike the single unit Probabilistic Safety Assessment (PSA), the procedure for performing the PSA for multi-unit sites is not yet established. However, efforts are being made to standardise the procedure considering the various plants and their releases in a given site under consideration. In the multi unit PSA, special attention should be given while modeling the multi-unit initiating events, multi-unit PSA model, shared resources, inter-unit correlations, Common Cause Failure (CCF), modeling of Human Reliability Analysis (HRA) in the multi-unit parlance and risk aggregation. Among these issues, in the present paper focus is given for treatment of dependency and correlation as seismic core damage frequency is highly dependent on the modelling of dependency among the components. Conservative approach of considering fully dependence will give highly conservative results which may not be realistic. Hence, the main objective of the present study is treatment of dependency and correlation among various components considering the internal and external events possible in the site. The site under consideration is Tarapur site (Maharashtra, India) having two hypothetical advanced reactors. Scope of the analysis includes full power, external event (Seismic), Level 1 PSA considering reactor core as radioactivity source.