Reliability assessment of the passive heat removal system of the VVER-1000 reactor at Kudankulam NPP

For analyzing the failure probability of the system, a model of the system was developed in best estimate T-H code RELAP5/Mod3.3. The parameters affecting the system performance were identified, their range and distribution was determined. Feasibility of various methods for calculating functional reliability was analyzed. The response surface method was selected. A response surface was constructed for the entire range of parameters for the model developed using Monte Carlo method. A large number of samples of the input parameters was generated, taking into account their individual distributions. They were classified as success/failure based on their position with respect to the failure surface. The ratio of the failed samples to the successful samples gives us the failure probability. An attempt was also made to integrate the functional failure probability with the component failure probability of the system to get an overall failure probability of the system.