Study on depressurization strategy by ADS manually considering hydrogen risk

For advanced passive pressurized water reactor, the automatic depressurization system (ADS) can be applied to depressurize the reactor coolant system (RCS). The main control room can manually open ADS to mitigate the risk of overpressure during severe accidents. However, the design characteristics of ADS may cause hydrogen releasing into the containment. Accumulating in the containment may cause hydrogen hazard in containment. Against this background, the RCS depressurization strategy in severe accidents was analyzed and its negative impact standing from hydrogen risk to make suggestion to severe accident management was evaluated. The station black out accident was selected and analyzed with integral severe accident analysis code. Three different depressurization schemes were discussed on both depressurization effect and hydrogen risk. The hydrogen distribution and risk in different compartments were calculated through severe accident analysis. The results show that all three schemes analyzed in this paper can depressurize the RCS effectively. However, opening ADS stage 1-3 causes immediate increase of hydrogen concentration in in-containment refueling tank compartment if hydrogen igniters are unavailable and can cause hydrogen explosion that threatens containment integration. As a result, the suggestion is made for severe accident management guideline which gives priority to ADS stage 4 when implementing RCS depressurization. (authors)