Abstract: With an integral systems analysis computer code, hydrogen risk and the assumed hydrogen control system were analyzed for 600 MWe Pressurized Water Reactor Nuclear Power Plant. The results show that in view of 100% of zirconium clad reacted, the severe accident induced by largebreak lossofcoolant accident (LB-LOCA) with fast core’s melting progression, has more hazards, possibly resulting in hydrogen deflagration, than those induced by smallbreak lossofcoolant accident (SB-LOCA) and station blackout (SBO). With the effect of the assumed hydrogen control system, the average and local hydrogen concentrations in compartments of the containment are less than 10%, which means that hydrogen deflagration or detonation will not occur according with the hydrogen control and risk analysis standard in 10CFR of USA. So the assumed hydrogen control system is feasible and effective.