Abstract: The effect of depressurization on hydrogen generation during a typical high pressure severe accident sequence in a 1 000 MWe pressurized water reactor (PWR) nuclear power plant was analyzed. Analyses results indicate that the hydrogen generation rate is obviously increased by the reactor coolant system depressurization of opening one, two or three power operated relief valves (PORVs) at three core damage states. The first is peak core temperature from 1 500 K to 2 100 K. The second is peak core temperature from 2 500 K to 2 800 K. The third is from formation of molten pool supported by crust to slumping of molten materials into reactor pressure vessel lower head. The more PORVs are opened the more increment of hydrogen generation rate.