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（LP） model. With related regulations considered, four severe accident scenarios were selected and calculated using MAAP4 codes to determine the probability distribution functions of input parameters needed in the assessment process. And the feasibility of the IVR-ERVC strategy for large size PWRs is determined to be potentially higher than 98% by using a lumped parameter analysis code. In the end, considering the impact of the input parameters, recommendation for the designs of reactor core internals has been proposed.