| Abstract: | The unit sphere concept was developed to predict the triggering stage of vapor explosions for coarse mixtures composed of hot liquid droplets, cold liquid and its vapor. With an assumption that hot liquid droplets are arranged with a uniform spatial interval to construct a hexagonal cell structure, a unit sphere with thirteen droplets is formed in the coarse mixture. A droplet and adjacent twelve droplets were placed at the center and on the surface of the unit sphere, respectively. Two indices for triggering were introduced in the unit sphere concept. The first index is the ratio of mechanical energy generated at the center droplet to one required for the mechanical collapse of vapor film on an adjacent droplet. Another shows the probability that the mechanical energy at the center droplet impacts onto the minimum number of molten adjacent droplets. The present concept predicted that the triggering could occur at smaller water subcooling for a coarse mixture with alumina droplets and water than corium droplet case, and that vapor explosions were suppressed when the ambient pressure was elevated up to approximately 0.5 MPa in both cases. The evaluation of KROTOS experiments indicated that the latter triggering index was smaller for corium droplets than alumina case due to the increase in the fraction of solidified droplets in the coarse mixture, implying less triggerability for corium droplets. Those findings showed a consistency with the results of vapor explosion experiments using corium and alumina. It was qualitatively confirmed in the experiments where a molten tin jet penetrated into a water pool that the latter index is applicable to the evaluation of the triggerability. |
