A site risk assessment for internal events: A case study

Abstract

Site risk assessment (SRA) is to estimate the risk of accidents that are probable to occur on the site. The group of accidents consists of not only the initiating events inducing impact on a single unit, but also the events causing simultaneous malfunctions, such as reactor trip on more than 2 units by common causes, called common-cause initiators (CCIs). These events need to be distinguished at the initiating event analysis and the accident progress should be analyzed separately to conduct the SRA. Furthermore, inter-unit dependencies such as inter-unit common-cause failure (CCF) and unavailability of shared components should be treated appropriately to model the accident sequences with CCI events. In this study, in order to perform the SRA of a reference site, the accident history of nuclear power plants in Korea is investigated to identify single-unit initiators (SUIs) and CCIs for SRA and to calculate the frequency of each initiator. The Level 1 SRA model is developed implementing inter-unit dependencies, unavailability of alternate AC diesel generator and inter-unit CCF. In addition, re-evaluation of failure probability of off-site power recovery is performed. Using these models, site core damage frequency (SCDF) and the site risk indicators identified in this study are calculated to deduce the impact of the multi-unit factors considered in SRA. It is expected that the SRA model developed in this study can be applied to perform the site risk assessment for the other site and contributes to improve the safety from the site risk perspective.