Abstract: In this paper MELCOR code was applied for modeling of AP1000 NPP primary and secondary circuits, passive safety system and containment compartment. And by applying the code, a severe accident scenario of small loss of coolant accident (LOCA) at hot leg concurrent with failure of internal refueling water storage tank (IRWST) function was selected for simulation and calculation mainly during the invessel phase, the passive mitigation measures were analyzed and evaluated. The analysis results show that the execution of ADS (automatic depressurization system)1-4, will lead to quick and effective injection of cold water from core make-up tank (CMT) and accumulators into the core, before they are exhausted, the core is kept flooded all the time. With broken of ADS4 valve, primary pressure is fast balanced to containment pressure. The passive containment cooling system (PCS) can be adopted as a counter measure to slow pressurization of containment by airborne decay heat and non-condensable gases. The ignition of H2 by igniters before the concentration of H2 are high enough to deflagrate automatically, and can alleviate the risk of containment failure from overpressure due to global deflagration, while continuously ignition may cause temperature in some compartments rise to a value that exceed much more than its design temperature. Under such condition, other system supports to severe accident mitigation are difficult to survival.