Author’s proposed risk monitor system of Nuclear Power Plant (NPP) is based on the idea of Plant Defense-in-Depth (DiD) risk monitor and reliability monitor to monitor what degree of safety functions incorporated in the plant system is maintained by multiple barriers of Defense-in-Depth (DiD). In the risk monitor system, the range of risk state is not limited in core damage accident but includes all kinds of dangerous states brought by severe accident. In present study, method of the reliability monitor of a risk monitor system is applied to the PWR safety system in order to evaluate the risk state numerically by pursuing all conditions of reliability evaluation given by plant DiD risk monitor. Large break LOCA is taken as an initiating accident event and the implementation of method of the reliability monitor is discussed in detail for single loop PWR safety system by considering the Multilevel Flow Model (MFM), Failure Mode and Effect Analysis (FMEA), and the qualitative reliability evaluation by Fault Tree Analysis (FTA) and the dynamic reliability evaluation by GO-FLOW. The summary of reliability results of PWR safety subsystems are also presented.

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