A steam explosion can occur when molten corium falls from the reactor vessel into the water in the reactor cavity. While various research studies have been conducted on steam explosions following the free fall of molten corium in air before entering the water, steam explosions following submerged corium discharge under the ex-vessel cooling condition have received relatively little analysis. The aim of this paper is to compare the progress and consequences of a steam explosion in experiments and simulations for the partially flooded cavity and ex-vessel cooling conditions. Three steam explosion tests carried out in the TROI (Test for Real cOrium Interaction with water) experimental facility were simulated by the TEXAS-V code. Experimental tests were first modeled, followed by a comparison of the experimental and simulation results. The effect of the molten corium mass involved in the steam explosion under water at the moment of triggering on the strength of the explosion was higher than that of the corium composition in the tests and simulations for the condition of a partially flooded cavity. In the test and simulations of different corium injection modes to the water, the maximum pressure and impulse of the steam explosion appeared in the partially flooded cavity condition. In the simulations for the partially flooded cavity condition, the mass of the molten corium fragmented by Rayleigh-Taylor instability (RTI) was higher than that fragmented by Kelvin-Helmholtz instability (KHI). Modeling of KHI fragmentation caused solidification of the fragmented corium particles, and the impulses reduced accordingly. In the simulations for the ex-vessel cooling condition, as melt jet breakup did not occur before the triggering time, simulations with only RTI fragmentation underestimated the impulse of the steam explosion. Otherwise, modeling of KHI fragmentation increased the impulse of the steam explosion due to fragmentation on the side of the corium jet. Steam explosion simulations in the ex-vessel cooling condition require more detailed modeling of the melt jet and premixing area, as well as variable adjustment for the fragmentation by KHI.