Probabilistic safety analysis of station blackout event in VVER-1000 NPP following implementation of FLEX strategies

The nuclear power plants have been requested to perform a comprehensive safety and risk assessment to cope with the station blackout (SBO) accident following Fukushima Daiichi accident. It is assumed that both the normal power supply sources and the emergency diesel generators are lost due to extended natural event. The diverse and flexible coping strategies (FLEX) shall be utilized in order to prevent the severe core damages and maintain the safety function during the SBO accident as a lesson learned from Fukushiam accident. PSA reference model of VVER-1000 shows that this SBO is the dominant contributor to the total core damage frequency. In this study the portable equipment including diesel generators and diesel pumps are deployed for implementation of FLEX strategies in VVER-1000 NPP .The event tree of SBO is developed by consideration of FLEX strategies and portable equipment. The objective of this paper is the safety investigation and improvement of a VVER-1000 reactor by implementation of portable equipment against the consequence of SBO event. The core damage frequency caused by SBO is analyzed by SAPHIRE code as a reliability and risk assessment software. The results show substantial reduction in the core damage frequency could be achieved by using the FLEX strategies based on portable equipment.