Hydrodynamic fine fragmentation of partly solidified melt droplets during a vapour explosion

In this paper we investigate the fine fragmentation criterion for partly solidified droplets during the explosion phase of a vapour explosion. First, a modified Weber number is proposed for the determination of the critical conditions for the fine fragmentation. Next, this introduced dimensionless number, which considers the crust stiffness as a stabilizing force acting to retain the crust in the presence of hydrodynamic forces, is compared with experimental data. It is further investigated to see whether the limitation of the fine fragmentation by the crusts can help explain the strongly reduced explosion strength observed in experiments with corium as compared to alumina. Finally, the conclusions are used to assess the limitation due to solidification on the strength of the explosions that could occur during severe accidents in nuclear reactors.