Application of probabilistic risk assessment methodology to VVER reactors in Czechoslovakia

Probabilistic risk assessment (PRA) methods have been developed and their application to VVER-440 reactors is being prepared. It is considered useful to confine PRA studies at initial stages to Level 1 -accident sequences analysis - with the aim of estimating the overall frequency of core melt or severe degradation and of evaluating dominant contributions to this kind of risk. Initiating events of different LOCA sizes were specified for VVER from which event trees can be developed. From these event trees, requirements can be specified both on thermohydraulic calculations of processes in reactor systems leading to core melt and on reliability analyses of safety systems. Issues and intentions of our PRAs are mentioned. The interrelation between PRAs and the evaluation of safety-related events from the actual operation of nuclear power plants is underlined. As an example, comparison of the reliability of the VVER containment spray system with that for PWR is presented. Owing to greater redundancy in the VVER spray system, its reliability and availability was calculated better. It is shown on a random example that the distribution of pump repair times from out plant operation is in good agreement with the data given in the RSS. Some generic conclusions of PRAs from abroad can be applied to our plants as well. (author)