Analysis of RCS injection strategy with RNS on severe accident mitigation

According to the severe accident management guidelines (SAMG), the active non-safety systems can be considered for severe accidents mitigation for 1000MWe advanced passive pressurized water reactor (PWR). The normal residual heat removal system (RNS) which is a non-safety system can be adopted to implement the reactor coolant system (RCS) injection strategy during severe accidents. Hence its effectiveness needs to be evaluated. Two typical severe accident sequences: the typical low-pressure core melt accident induced by Large-break Loss of Coolant Accident with double-ended guillotine break in the cold leg (LLOCA) and typical high-pressure core melt accident induced by Loss of Feed Water (LOFW) are selected and analyzed with an integral severe accident analysis code. The mitigation effect and negative impact of the RNS injection strategy are both evaluated. The result shows that RNS injection can effectively terminate core melt progression and keep long-term core cooling in these two typical severe accident sequences. However, more hydrogen will be generated during the reflooding which may lead to hydrogen hazard in the containment. The RCS needs to be depressurized for RNS injection. Therefore, for LOFW two RCS depressurizing strategies are discussed: opening the automatic depressurization system (ADS) stage 1-3 and opening the ADS stage 4. The result indicates that opening ADS stage 1-3 valves can cause rapidly increase of the hydrogen concentration in the in-containment refueling water storage tank (IRWST) compartment and lead to hydrogen denotation if hydrogen igniters unavailable. However, hydrogen can be well dispersed in the loop compartment with intentional open of ADS stage 4 valves. Therefore, suggestions are made: opening ADS stage 4 should be preferred for RCS depressurization before implementing RNS injection strategy. (author)