Abstract: The reference plant is a typical three-loop PWR-NPP. The progress of core damage under low pressure and high pressure was studied respectively during a station blackout accident．The results show that the high pressure core melt accident is eliminated when pressurizer surge line creep failure is considered, but core damage is more serious and much hydrogen is generated under low pressure transient．The reason was analyzed and the mitigation measure was suggested when core outlet temperature reached 923 K. Calculation results show that core melt progress is greatly delayed, thus the operators have more time available to recover AC electrical source and take other mitigation measures.