Reliability Evaluation of Emergency Diesel Generator Systems (EDGS) for VVER-1000/V446 Reactor by Markov Model

The Emergency Diesel Generator Systems (EDGS) are the emergency electrical source of the nuclear power plants (NPP). These systems work as standby systems in the normal operation of the nuclear power plant (NPP). In the maintenance of standby system, Surveillance Test Interval (STI) plays as a key role in the reliability evaluation of these systems, living PSA (LPSA) and risk monitoring (RM). Optimum test interval and test duration can reduce the unavailability of the EDGS and the risk and cost of testing procedure. In this paper, using Markov approach, all the possible states of EDGS are considered and according to the transition between them, the optimum test interval and test duration are selected. To do this, first, the chosen states are classified in some groups to analyze the Markov model. The groups are as follow: at least one train work, four trains failed, normal trains in test and failed trains in test. The desired state is the state with no fail and no test. This study concludes that the best test interval and test duration are 7 days (168 hours) and 25 days (600 hours) respectively, which are almost the same as the considered STI and duration test in the reference NPP.