During severe accident of a light water reactor, various thermal hydraulic phenomena including vapor explosion could threaten the integrity of the containment vessel. Thermal detonation model is proposed to describe the vapor explosion. According to the model, several processes should be sequentially satisfied for the trigger phenomena of the vapor explosion. One of the most important processes for the trigger phenomena is the vapor film collapse around high temperature molten material droplets. In the present study, the vapor film collapse behavior around high temperature solid particle submerged into water was experimentally investigated by attacking a pressure pulse to the vapor film on a high temperature sold particle. The interfacial phenomena between vapor and water were measure by using a high-speed video camera of the maximum speed of 40,500 fps. The visual data obtained were processed with visual data processing techniques. That is, the average vapor film thickness was estimated, dynamic behaviors of the interfaces were analyzed with PIV technique and the interface movement was estimated with the digital auto correlation techniques from the visual data obtained. Furthermore, the transients of the temperature and pressure were simultaneously measured. The interfacial temperatures between vapor and water, and between molted liquid and water are analytically estimated by solving the heat conduction equation with the data obtained as the boundary conditions. It is clarified that vapor collapse by pressure pulse occurs homogeneously around the vapor film surface on a high temperature particle. Microscopic information are obtained from the visual data by using visual data processing technique, PIV technique and digital auto-correlation technique. At the time the vapor film surface changes to white, the saturation temperature exceeds the interfacial temperature. The microscopic vapor film collapse behavior indicates the possibility of the phase change at the vapor film collapse.
Decrease of Leidenfrost temperature at quenching in subcooled liquids
