QUALIFICATION METHOD TO MODERNIZE STRATEGIES FOR SCHEDULED REPAIRS OF ACTIVE SAFETY SYSTEMS OF NUCLEAR POWER PLANTS WITH WWER

Abstract

The strategies for scheduled tests and repairs of active safety systems (with pumps) established by the Technological Regulations for the safe operation of NPPs with WWER do not take into account many years of operating experience and the results of numerous tests of active safety systems, current reliability indicators and the residual life of the main equipment and other factors. Modernization/improvement of design strategies for scheduled repairs and tests of active safety systems determines the need to develop appropriate methods for qualification (substantiation) of modernized strategies. There is presented a risk-informed method for qualifying modernizations of strategies for scheduled repairs of active safety systems, taking into account experience in operation, scheduled repairs and tests, as well as the current reliability of the main equipment. The developed method is based on the following provisions. The determining reliability indicator when qualifying strategies for scheduled repairs of active safety systems is the safety unavailability factor, which determines the probability of a critical failure at the current time depending on the critical failure rate established in scheduled tests/repairs over the expired operation life. A critical failure here means a failure to perform safety functions (for example, failure to turn on a pump, forced shutdown of a pump due to a violation of operability conditions, leaks in the circuits of active safety systems, etc.). The qualification criterion to modernize strategies for scheduled repairs of active safety systems is a risk function that determines the ratio of the safety unavailability factors of the design and modernized strategies for scheduled repairs of active safety systems. Condition for successful qualification is the safety unavailability factor of the modernized strategy for scheduled repairs is not greater than the safety unavailability factor of the design strategy for scheduled repairs. Based on the developed method, modernized strategies for scheduled repairs of active safety systems of the WWER-1000 reactor were substantiated and the conditions for the applicability of the modernized strategies were determined.