A level 2 site risk assessment for internal event and seismic event: a case study

Current probabilistic safety assessment (PSA) methodologies are performed from the perspective of analyzing single-unit accidents. After the Fukushima Daiichi Nuclear Power Plant accident, interest and concern for simultaneous accidents involving multiple units at a site rather than a single-unit accident have increased, and safety assessment based on a site where multiple units are operating has become an issue. In this study, methodology of level 2 multi-unit probabilistic safety assessment (MUPSA) including uncertainty analysis was developed and conducted for the nuclear power plants in a reference site. A statistical propagation method using Monte Carlo sampling was applied to propagate the uncertainty of the factors associated with the reactor core damage to the severe accident scenarios and to reflect the uncertainty factors associated with the containment building failure. Point estimation and uncertainty analysis of the level 2 MUPSA for internal event was conducted. Also, level 2 multi-unit probabilistic safety assessment (MUPSA) for internal event and seismic event was performed to the reference site. The risk associated with the containment failure of multi-units assessed by categorizing the initiating event into single-unit initiator (SUI), which can only cause an accident at a specific reactor type from a site perspective, and common cause initiator (CCI), which can affect more than one reactor at a site. Finally, the site risk was evaluated by adding the risk due to SUI and the risk due to CCI.