Abstract: Melt pool configuration in the lower plenum of reactor pressure vessel (RPV) is the most controversial issue in the assessment methodology for severe accident strategy in-vessel retention (IVR). The generally accepted two-layer and three-layer melt pool configurations and a four-layer configuration which may form during the procedure of stable melt pool configuration formation were described. Heat transfer models of the three configurations were established, and effects of the three models to RPV integrity of reactors with different power levels were analyzed. The results show that RPV safety margin decreases as power level increases, and RPV failure is most likely to occur under the condition of a four-layer configuration.