Reliability Analysis of Active Reactor Pit Flooding Function of EHR DFT System of a NPP Using an Enhanced Component-Connection Method

Redundancy design is often used to enhance the reliability of complex systems which often run as safety-critical systems of nuclear power plant (NPP). Due to the redundancy design and management strategies, safety-critical systems of a NPP often have complex sequential failure behaviors. For a safety-critical system of a NPP with sequential failure behaviors, it is very important to evaluate its reliability for a given mission time. In this paper, the active reactor pit flooding function of EHR (containment heat removal system) of a nuclear power plant (NPP) is modeled by dynamic fault tree (DFT) and analyzed by BDD. To build the BDD, an enhanced component-connection-based method is proposed. The failure probability of the active reactor pit flooding function is analyzed. Besides, the FV, RAW and RRW importance of the EHR equipment are also evaluated. The results show the cumulative failure probability at mission time 24 hour is 1.81E−05, which is very low. The cumulative failure probability of train A (train B) is 4.25E−03. It can be concluded that the active reactor pit flooding function of EHR has a very high reliability given a mission time 24h.