Self-Triggering Mechanism of Vapor Explosions for the Large-Scale Experiments Involving Fuel Simulant Melt

Abstract

Vapor explosions are a major safety concern in the severe accident management of nuclear power plants. Here, we investigated the self-triggering mechanism and the occurrence conditions for spontaneous vapor explosions analytically. A spontaneous vapor explosion is assumed to occur when a vapor film between a hot and a cold liquid naturally collapse. We modeled a simple system consisting of a hot liquid droplet in a pool of cold liquid. The stability of perturbed oscillations in a vapor film was then investigated analytically. The model included the effect of thermal radiation on this stability for high temperature melts. The analytical results show that the stability of a vapor film is significantly affected by the emissivity of thermal radiation and by the heat transfer coefficient for the condensation of the vapor to the subcooled cold liquid. Furthermore, as the hot liquid temperature increases, thermal radiation causes the temperature region in which the vapor film is stable to increase in size. Comparisons were then made between our analytical results and those from large-scale experiments in which vapor explosions were studied for water and molten core materials in light-water reactors. They showed that our model explains the tendency of the occurrence of vapor explosions in those experiments.