MODERNIZATION OF SCHEDULED MAINTENANCE STRATEGIES FOR ACTIVE SAFETY SYSTEMS OF NUCLEAR POWER PLANTS WITH VVER

The strategies for scheduled tests and repairs of active safety systems (with pumps) established by the technological regulations for safe operation of VVER NPP do not take into account many years of operating experience and the results of numerous tests of active safety systems, current reliability indicators and residual life of the main equipment, and other factors. The modernization/improvement of design strategies for scheduled maintenance and testing of active safety systems determines the need to develop appropriate methods for qualifying (justifying) modernized strategies. The paper presents a risk-based method for qualifying modernization of strategies for scheduled maintenance of active safety systems, which takes into account the experience of operation, scheduled maintenance and testing, as well as the current reliability of the main equipment. The developed method is based on the following provisions. The determining reliability indicator for qualifying strategies for scheduled maintenance of active safety systems is the safety function unavailability factor, which determines the probability of a critical failure at the current time depending on the parameter of the intensity of the critical failure flow established in scheduled tests/repairs over the past service life. A critical failure here means a failure to perform safety functions (e.g., failure to turn on a pump, forced pump shutdown due to a violation of performance conditions, leaks in the circuits of active safety systems, etc.) The qualification criterion for the modernization of active safety systems outage strategies is a target risk function that determines the ratio of the coefficients of unreadiness to perform safety functions of the design and modernized active safety systems outage strategies. The conditions for successful qualification are that the unavailability rate of the safety functions of the modernized scheduled maintenance strategy is not greater than the unavailability rate of the design scheduled maintenance strategy. Based on the developed method, the modernized strategy of scheduled repairs of the active part of the emergency core cooling systems of the VVER-1000 reactor is substantiated, and the limitations of the applicability of the modernized strategy are determined.