Abstract: The risk-oriented accident analysis methodology（ROAAM） is introduced to evaluate the effectiveness of the in-vessel retention（IVR） of the molten corium through external reactor vessel cooling during severe accidents of pressurized water reactors, with a simple mechanistic heat transfer model to simulate the heat transfer from the molten pool to the reactor vessel. By combining the results of Level-1 probabilistic safety assessment, the severe accident sequences calculations, and the uncertainty analysis of heat transfer in corium, the overall success probability of the IVR strategy under severe accident condition can be reasonably predicted. The preliminary application of this approach for ACP1000 nuclear power plant shows that the success probability of the IVR strategy is more than 99%, and the most dangerous position is at the bottom of the top metallic layer due to the focusing effect.