Module level reliability performance evaluation of digital reactor protection system considering the repair and common cause failure

The Reactor Protection System (RPS) is designed and installed in the nuclear power plants (NPPs) to ensure both safety and economy. Nowadays the RPS adopts the digital techniques which consist of different digital modules. Therefore, this paper focuses on evaluating the reliability performance of the digital RPS using the Colored Petri Net (CPN) considering the module repair time whenever it fails and the Common Cause Failure (CCF). The module repair is considered as it takes some time to repair or replace the failed module and during the repair duration the digital RPS is operated in the degraded configuration and the common cause failure would severely impact the system in the event of occurrence. By studying the failure phenomenon and mechanism, the random probability shock model is adopted for CCF. Using the proposed model, the Monte Carlo simulation is carried out. Consequently, the indicators such as Mean Time To Repair (MTTR), Mean Time Between Failures (MTBF), Probability of Failure on Demand (PFD) and Probability of Spurious Trip (PST) are calculated. Following main conclusions are drawn i.e., i) the CCF is the main contribution to the PFD and PST. So the countermeasure for the CCF must be designed for the digital RPS; ii) the CCF has no effect on the MTTFF, MTBF, MTTR and subsystem unavailability; iii) the failure detection time has adverse effect on the system. Therefore, the digital system should shorten the detection time or decrease the coverage for the failures that take long time to be detected.