Steam explosion in nuclear reactors: Droplets of molten steel vs core melt droplets

Abstract

Relative contributions of molten steel and core melt droplets to heat transfer from the droplets to ambient water at typical regimes of the droplet cooling and solidification are considered. It is shown that fine fragmentation of steel droplets overheated significantly with respect to the steel solidification temperature makes these droplets more dangerous for the steam explosion triggering, and this effect cannot be neglected in nuclear safety analysis. On the contrary, the solid crust on the surface of core melt droplets is formed before the first pressure drop in the steam envelope. As a result, the probability of fragmentation of these droplets is relatively small and there is no increase in the overall heat transfer due to the expected large total surface area of fine particles. The predicted low explosivity of core melt droplets is confirmed by the published data of laboratory experiments. It is recommended to revise the existing theoretical and computational models of steam explosion, paying more attention to the role of fine fragmentation of steel droplets.