Abstract: As the in-vessel environment under the reactor severe accident condition is complicated and the experiment on the behavior of corium in the lower plenum is limited, some hypothetical corium configuration models are usually adopted to perform the accident assessment. In this paper, three kinds of corium configuration models built-in ASTEC code were used to assess the effects of different corium configurations on the heat transfer of the lower head and the integrity of the pressure vessel. The calculation results show that in case of adiabatic outer wall of lower head and only the temperature limit used as the failure criterion, the two-layer corium configuration leads to the shortest failure time of the lower head, and the failure point is located at the upper part because of the heat focus effect of top metal layer. For three-layer corium configuration, the temperature of the lower part of lower head continues to rise due to the presence of the bottom metal layer. The failure point is located at the lower part of the lower head, but the failure time is longer than that of the two-layer corium configuration.