To investigate the fundamental mechanism behind the recent experimental observation that the composition of a material considerably affects the strength of a steam explosion, physical and chemical analyses for the fast-quenched particles of a prototypic corium were performed. Six cases including fully oxidized and partially oxidized corium were selected for the study, in which the melt composition was changed, while the other initial and boundary conditions of the molten fuel and water interaction tests, such as the melt temperature, amount of water, and free fall height, were maintained the same. The proposition that the inner structure and solidification behavior of particles due to the existence of a mushy phase are responsible for the strong influence of the material composition on the strength of a steam explosion was examined from the results of physical and chemical analysis. The results of the present analysis are supportive of the proposed argument.