[en] Vapor explosion is one of the most important problems encountered in severe accident management of nuclear power plants. In spite of many effort, a lot of questions still remain about vapor explosion. So, KAERI launched a real material experiment called TROI using 20 kg of UO₂ and ZrO₂ to investigate the vapor explosion. Before running the experiment, a small-scale experiment with molten-tin/water system was performed to quantify the characteristics of vapor explosion and to understand the phenomenology of vapor explosion. Vapor explosion was initiated by dropping a drop of 50 g molten tin into the water pool. A spontaneous vapor explosion was observed while tin temperature and water temperature were systematically varied. The Temperature Interaction Zone (TIZ) for tin/water system within which a spontaneous steam explosion occurred was determined from the experiment. Pressure pulse, the strength of vapor explosion, was also measured as a function of tin temperature and water temperature. In addition, a high speed video filming up to 2000 flame/sec was taken in order to visually investigate the behavior of a spontaneous vapor explosion