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Recently, digital instrumentation and control systems have been increasingly installed to the important safety features in nuclear power plants such as the reactor trip system and the actuation system of the engineered safety features. On the other hand, it has been found that it is difficult to model the digital equipment reliability in probabilistic risk assessment (PRA) by the conventional Fault Tree Analysis technique. The OECD/NEA CSNI Working Group of Risk Assessment (WGRisk) set up the task group DIGREL to discuss several issues including quantitative dynamic modeling. This paper shows that, taking account of the relationship among the reactor trip system failures, demand after the initiating event, detection of the reactor trip system fault by self-diagnostic or surveillance tests, repair of the failed reactor trip system components and plant shutdown operation by the plant operators as a stochastic process, the anticipated transient without scram (ATWS) event can be modeled by the event logic fault tree and Markov state-transition diagrams in the case of the 2-out-of-4 digital reactor trip system.

Application of Markov State-Transition Model to Reliability Analysis of 2-out-of-4 Reactor Trip System