Abstract: In-vessel retention-external reactor vessel cooling (IVR-ERVC) is a key severe accident management strategy, and the present assessment method on the effectiveness of IVR is mainly based upon the lumped parameter model. With the enlargement of core mass and the RPV size, the increase of Rayleigh number was found in the calculation of nature convection heat transfer for advanced large size PWR. The applicability of several natural convection heat transfer correlations was studied and compared, and natural convection heat transfer under the high Rayleigh number was simulated by using lumped parameter analysis code. A two-layered stratified molten pool configuration was assumed to be developed in the reactor vessel lower head. The heat flow distribution at the outer boundary of RPV wall was simulated, and the code was applied to assess the feasibility of the IVR-ERVC strategy under a selected severe accident for advanced large size PWR. Recommendation for the designs of reactor core internals was proposed.