Influence of RCS Depressurization Strategy on Hydrogen Risk

For advanced passive PWR, reactor coolant system (RCS) depressurization through automatic depressurization system (ADS) is an important measurement to avoid high-pressure melt ejection and direct containment heating. It allows injection from passive core cooling system and the implement of in-vessel retention. However, it has negative impact that hydrogen in the RCS can be released to the containment together with coolant, which may lead to hydrogen burning or even explosion in the containment. Therefore, this paper analyzes the RCS depressurization strategy during severe accident, and evaluates its negative impact. Severe accident sequences induced by station black out (SBO) was selected and analyzed with integral severe accident analysis code as a typical high pressure core melt accident scenario. Different depressurization strategies with ADS system were discussed based on Severe Accident Management Guideline (SAMG.) ADS valves were manually opened at a core exit temperature of 923 K with 20min delay for operator reaction. Both depressurization effect and hydrogen risk were evaluated for different strategies. Hydrogen distribution was calculated, which was used to determine the combustion mode in different compartments. Result shows all three strategies analyzed in this paper can depressurize the RCS effectively. And opening the ADS stage 1–3 valves causes rapidly increase of the hydrogen concentration in the in-containment refueling water storage tank (IRWST) compartment and may lead to hydrogen denotation. However, hydrogen can be well dispersed in the loop compartment with intentional open of ADS stage 4 valves to RCS depressurization. Therefore, suggestions are proposed for SAMG: implement RCS depressurization strategy with stage 4 ADS instead of ADS stage 1–3.