Abstract: In the case of the hypothetical severe accident in a nuclear power plant (NPP), a large amount of hydrogen will be generated and released into the containment, which may be a threat to containment integrity. The hydrogen risk analysis code GASFLOW was adopted to make the research on hydrogen behavior and risk during the hypothetical severe accident initiated by large break LOCA with failure of emergency core cooling system for advanced PWR NPP. It is shown that if the gas source is in the steam generator (SG) cavity, the main route of hydrogen flow is “SG room-dome area -subcompartments below operating deck”. The hydrogen volume concentration in the cavity with break is affected by the hydrogen fraction of mixture source term and configuration of mixture jet, the hydrogen volume concentration stratification occurs in non-break areas and the stratification moves downwards with the effect of diffusion. The flame acceleration (FA) may occur in the SG cavity, while the possibility of deflagration to detonation (DDT) can be practically excluded, and the possibilities of FA and DDT can also be practically excluded for the dome area.