Semi-markov reliability analysis of three test/repair policies in a standby safety system

Abstract

During the course of efforts in reexamining the technical specifications for nuclear power plants, a 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 : Policy 1 : An additional test is carried out promptly. Policy 2 : No additional test on the other parts is performed. The test is carried out according to the periodic test scheme as originally scheduled. Policy 3 : An additional test will be performed after the first failed part is repaired. For the analysis of the three test/repair policies, a computer program MARADD (Markov Reliability Analysis for the Additional Test Requirement) was developed in the present study, based on semi-Markov reliability analysis. The allowed outage time (AOT) is usually larger than the calculational 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 transitions 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. The following results are obtained in the present study. Policy 1 may induce higher risks than Policy 2 or 3 in a nuclear power plant with relatively large shutdown risks and with a diesel generator system that has poor test overriding capability and high failure rate. Furthermore, the plant unavailability (i.e., plant downtime) in Policy 1 is much greater than that in Policy 2 or 3 since Policy 1 invokes too many inadvertent reactor trips due to the requirement of the plant shutdown in ca...