An initial period of surface solidification of large opaque particles of molten metal oxides is considered. It is shown that low overheating and very high melting temperature of corium lead to very fast formation of solid crust on the particle surface. The transient problem solution showed that maximal tensile stress in this crust due to pressure drop in expanding steam bubble around the particle is much less than the stress in relatively thin crust layer on the surface of alumina particle. The solidifying corium particle seems to be more stable as compared with the alumina particle of the same size. The latter is treated as one of the reason of the experimental results on relatively low explosivity of corium. (c) 2007 Published by Elsevier Ltd.