Investigation of triggering phenomenon in the vapor explosion

Based on high-speed video imaging of the vapor explosion observation that occurs when a water droplet impinges on a molten lead-bismuth alloy surface, the following investigations were made: (1) The following process induces vapor explosion: a ring of bubbles generated by spontaneous nucleation forms and spreads over the bottom surface of a water hemisphere at a speed of 35 m/s. The water hemisphere then fragments finely from the bottom. Subsequently, the pool surface of the molten alloy is deformed in a crown shape with a height of more than twice the droplet diameter and is also fragmented into small particles (less than 0.5 mm in diameter) and dispersed. (2) On an oxidation layer, a water droplet becomes entrapped in a dome of deformed molten alloy, since the oxidation layer restricts the coherent vaporization of the water droplet. (3) Vapor explosion occurs when droplet subcooling is higher and molten alloy temperature is lower than a certain value. Increasing impingement velocity enlarges the vapor explosion region due to the destabilizing effect one the vapor layer. (author)