Mitigation coverage evaluation of passive systems based on causality estimation using multi-level flow model

After the Fukushima accident, the application of passive safety features (PSF) has been importantly suggested for mitigating severe accidents in addition to the conventional active safety features. Nowadays, substantial number of studies related to passive system have been performed and each system has different impact on a nuclear power plant (NPP). For this reason, it is important to use appropriate passive features based on their impacts. Mitigation coverage of PSFs can be importantly used to estimate impact of PSFs. It can be estimated based on the number of accident scenarios, which can be mitigated by applying additional PSFs. In this study, the accident scenarios, which can cause core damage, are firstly defined using fault tree (FT) analysis. Multilevel flow modelling (MFM) is also used to verify completeness of FT. In this study, MFM model is also used to define additional PSFs, which need to mitigate accidents and to determine the number of scenarios that can be mitigated by applying each PSF. As a result, we estimate mitigation coverage of each passive feature as follow: (1) 62.2% of passive secondary cooling system (2) 42.2% of passive high-pressure injection system, (3) 14.6% of passive condenser cooling system.