Semi-Markov reliability analysis of three test/repair policies for standby safety systems in a nuclear power plant

Abstract

Abstract During the course of efforts in re-examining the technical specifications for nuclear power plants, particular attention has been paid to the ‘additional test’ requirement on the redundant components in a standby safety system. When a failure of one part is detected by the scheduled test, the following three test/ repair policies are considered on the other redundant parts during the allowed outage time of the first failed component: (1) Policy 1—a prompt additional test, (2) Policy 2—no additional test, and (3) Policy 3—an additional test after repairing the first failed one. For the analysis of the three test/repair policies, a computer program MARADD ( Mar kov R eliability Analysis for the Add itional Test Requirement) was developed in the present study, based on semi-Markov reliability analysis. The allowed outage time (AOT) is usually larger than the calculation time step required by acceptable accuracy. A component can change its state from repair to operable state during the AOT according to the assumed Poisson repair process. However, some system states make forced transitions to other system states due to the additional test requirement, and to the plant shutdown state on exceeding the AOT that is a deterministic parameter. Thus, the three test/repair policies have semi-Markovian characteristics. The methodology was applied to the diesel generator system on which Policy 1 is currently performed. For the three test/repair policies, the system unavailability, the core damage frequency, and the plant shutdown probability in 1 year of plant operation were calculated for several AOTs and STIs using the nominal input data and also using various input data for sensitivity studies. The current test/repair procedure, i.e. Policy 1, has no advantage over Policies 2 and 3 in a nuclear power plant with relatively large conditional core damage probability given a reactor trip and with a diesel generator system that has short repair time, and low failure rate. Furthermore, the allowed outage time could be extended to improve the core damage frequency when the plant has large conditional core damage probability given a reactor trip. However, there is significant benefit in plant availability resulting from extending the allowed outage time, independently of the magnitude of the conditional core damage probability given a reactor trip.