Abstract: In-vessel retention of molten core debris（IVR） through the External Reactor Vessel Cooling（ERVC） is an important severe accident management feature employed in the 300MW PWR plant.In the previous IVR analysis,it is usually assumed that there is a two-layer configuration（metal-over oxide） of molten core material in the RPV lower head,however,an alternate three-layer molten pool configuration with a dense bottom metal layer below the oxide layer can be formed due to molten material interactions in the lower plenum,and it may bring a greater threat to the RPV integrity.Based on the established model,IVR analysis is performed for the assumed three-layer molten pool configuration for 300MW PWR plant,and the results show that the dense bottom metal layer will not threaten the RPV integrity,but the thin top metal layer has a small margin-to-failure,which may threaten the RPV integrity.