LAUSR

Abstract In nuclear power plants, core melt accidents are often accompanied with fuel–coolant interactions (FCI), which may escalate to steam explosion destroying the integrity of structural components and even the containment under certain conditions. The vapor film and two-phase region formed by boiling around the molten jet at high temperature have a great influence on the fragmentation of the jet and the possibility of steam explosion. In this study, by changing the initial temperature of coolant, a series of experiments on the interactions of molten tin at high temperature with water have been done and several distinct interactions under different boiling conditions including stable film boiling, unstable film boiling and nucleate or transition boiling were observed. These interactions were distinguished by the high-speed photography, liquid level swell, dynamic pressure, water temperature variation and jet breakup length during interactions as well as the morphology and size of debris after interactions synthetically. It was found that energetic jet-water interactions were only possible under unstable film boiling, and nucleate or transition boiling conditions. While under the stable film boiling, the jet was surrounded by a very stable vapor film, so only mild interactions occur. With the decrease of coolant temperature, the boiling mode transferred from stable film boiling to nucleate or transition boiling. Significant differences in jet breakup length as well as size and morphology of the debris were observed. Because less steam was generated, the melt was more likely to action with water than steam, and water was more conducive to the fragmentation caused by interfacial instability due to higher density. In addition, the thermodynamic fragmentation caused by violent boiling gradually played a more important role. Furthermore, the results were compared with existing theories which ensured the effects of boiling during FCI.