The Optimum Operation Strategy of Hybrid SIT with PAFS following a Station Blackout Accident

A coolant storage tank of PAFS can provide coolant for reactor cooling more than 8 hours and a dedicated battery system of PAFS can provide electricity for I-C more than 72 hours. PAFS is 2-train system, that is, PAFS has two water tanks, two battery systems and two heat exchangers. PAFS provides feedwater to steam generator more than 8 hours, even if single train was unavailable, AC power was not provided and water tank is not refilled. Following Fukushima Daiichi Accident, we have made many improvements and challenging research to prevent and mitigate accidents which can be caused by earthquake, tsunami or station blackout. It includes the Hybrid SIT to deliver cooling water into core even if RCS pressure is high. To prevent a waste of SIT water and maintain core cooling more long time, an optimum operation strategy of Hybrid SIT has been developed. It considers the operation of PAFS and the optimum coolability of SIT water. For the optimum coolability of Hybrid SIT with PAFS, some operation methods were considered. It shows that the coolant injected before the swelling of RCS water is released during the first POSRV opening and has very little effect on core cooling. The core cooling period is longest when the Hybrid SIT is actuated one by one after a exhaustion of PAFS and POSRV opening