In the postulated severe accidents of nuclear power plants, the interaction mode of the molten corium with water happens differently depending on the height of the water level in the reactor cavity. The interaction of the molten corium with the partially filled water in the reactor cavity has been extensively studied. The molten corium in this case was released into the water after free falling to some distance. Meanwhile, some advanced reactors have adapted the in-vessel corium retention concept by cooling the reactor vessel’s outside wall. If a reactor vessel failure happens in this case, the molten corium in the reactor vessel is injected directly into the water without any free fall. Triggered steam explosion experiments were carried out to compare the explosion behavior conditions of the partially flooded cavity and ex-vessel cooling. It was found that the jet breakup process before the explosion appeared differently between the two experiments. These behaviors contributed to the differences in the maximum dynamic pressure and load that express the explosion’s strength. The explosion’s strength under the partially flooded cavity condition was about two times stronger than that under ex-vessel cooling. Accordingly, it is believed that the steam explosions under conditions of ex-vessel cooling are of less concern than the partially flooded cavity condition.