Abstract: Based on the analysis by MELCOR for hydrogen concentration distributions in compartments of NPP containment during severe accidents which is induced by station blackout, the hydrogen combustion risk was investigated. According to the hydrogen control and risk analysis standard of US, the results show that the average hydrogen concentration will not bring on global deflagration, but local deflagration may occur. By application of CFD code, further simulation of hydrogen release and flow process in pressurizer relief tank compartment was performed. More details of hydrogen distribution and hydrogen accumulation zone were showed. Through the results, some insights were given as references for severe accident mitigation measures and hydrogen recombines arrangement design.