[en] In a hypothesized severe accident of a light water nuclear reactor, molten fuel may be released from the reactor pressure vessel and mix with a coolant pool in the lower regions of the containment vessel. If a suitable mixture is formed, an explosion may result in large pressure waves that impact and damage containment vessel structures. The purpose of this paper is to analytically study the effect of the molten fuel composition on the premixing phase with the VESUVIUS code. The JRC/Ispra KROTOS experiments, in particular, have shown that prototypic corium fuels are much less likely than alumina to form a pre-mixture that results in a steam explosion. In this paper, first, calculation results of three tests are compared against experimental data, Next, calculated spatial distributions of the fuel and void distributions for alumina and corium fuels and other detailed data that are not available experimentally are presented. These results are used to examine the fuel descent rates through the coolant pool, breakup characteristics, and other aspects of the fuel behaviors. Based on these results, insight is gained on the reasons for the relatively low potential of corium to form a favorable pre-explosion mixture. Finally, directions for future modeling efforts are summarized. (author)