It is the important feature of passive system and the basic difference from the active system that nuclear plant can be driven to safe state or shutdown by inherent safety characters of the reactor and physical principles, independent of human interfere or the operation of outside equipments, when the reactor is in abnormal condition. So passive system is widely used in new generation nuclear power plant (NPP) such as high-temperature gas-cooled reactors and AP1000 NPPs. While physical process failure become one of the important contributors to the system operation failure since system operation is depending on natural force but not on outside power and both the driven force and resistance are influenced by many uncertain factors. Then finding the key factors for the system operation, analyzing the development of the passive system combining with the accident scenarios are the main steps of the analysis of the passive system reliability, and the important content of the probability safety assessment (PSA) of nuclear plant with the passive design. In this paper, a model for analyzing the passive system reliability is described, in which variance decomposition and analytic hierarchy process (AHP) methods are used to select the key factors for the system operation, and Monte Carlo simulation and dynamic event tree methods are used to evaluate the system reliability according to the accident scenarios. Finally, Passive Residual Heat Removal System in the High Temperature Gas-Cooled Reactor (HTGR) is analyzed as an example.

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