Reliability Analysis of Bushehr Nuclear Power Plant Shutdown System by Markov Process

The shutdown system (SDS) in a nuclear power plant is the latest expectancy to subcritical the reactor core in accident conditions to avoid core damage. It should be able to bring the reactor to a subcritical state within some specified period of time and to maintain it in that state and must be at the highest level of reliability. The trip system of Bushehr Nuclear Power Plants (BNPP) which is a VVER-1000 PWR type reactor consists of two sets which each set includes three channels to assure high system reliability. The system forwards trip signal when at least two of three channels send signal. Each set is composed of an electrical and a mechanical parts. The shutdown system of BNPP does not activate normally unless there are some anomalies occurred in the plant operation. Therefore, if there is a failure within the SDS, the failure cannot be easily revealed until a demand for action is initiated from a process failure which would be highly undesirable. To avoid such a situation, surveillance tests have to be carried out for assuring that the SDS can indeed operate on demand. By performing surveillance tests, unrevealed failures can be detected early enough and repaired properly. There are many factors such as the preferred availability, the real unavailability, the probability of spurious trips, and the test duration that should be considered in determining the surveillance test interval for the SDS of BNPP. In this study, a Markov process model is developed to study the effect of test interval in the reliability of the shutdown system of BNPP. The Markov process model evaluates the system dynamically according to failure and repair rates and studies all failure states of the system which can transfer to each other by failure or test rates. The analysis shows that by the test frequency increasing the probability of spurious reactor trip from the test increases and the SDS unavailability decreases. The proposed model can provide the quantitative data required for selecting an optimized test interval.

کلیدواژه ها:

Bushehr nuclear power plant (BNPP) ، Shutdown system ، Surveillance test interval ، Markov process