Abstract: During steam generator tube rupture (SGTR) accident for China LEAd based Reactor (CLEAR), highly pressurized water in secondary loop may directly interact with the molten lead-bismuth eutectic (LBE) in the primary loop and then induce rapid boiling of water/steam mixture or even vapor explosion, which would damage the safety of the reactor. To investigate the phenomena of boiling heat transfer and vapor explosion in the contact interface of water/LBE, an observable experimental apparatus was set up to carry out the direct contact of LBE/water, and the visualized information was obtained using high-speed video camera technology. In the experiment, the effects of LBE temperature (250-500℃) and water temperature (25-80℃) on fragmentation behavior of molten LBE were investigated. The experimental results indicate that the fragmentation behavior is enhanced and the median mass diameter of fragment decreases with the increase of molten LBE temperature and water temperature. A significant phenomenon of vapor explosion occurs when interface temperature between droplet and water is higher than homogenous nucleation temperature of water.