Fig. 4-1 Conceptual diagram of steam explosion experiments High-temperature molten materials simulating molten core have been dropped into a pool of water to investigate the conditions of the occurrence of a steam explosion, the energy released, and possible measures to suppress the steam explosion. This is one of the largest experimental facilities for steam explosions in the world and is capable of conducting high-pressure tests.

t = 0 ms t = 1.0 ms t = 5.0 ms

Fig. 4-2 Steam explosion behavior recorded by high-speed camera (4,000 frames/second) High-temperature melt is covered with a vapor film (t = 0 ms). If the vapor film collapses at one location, the vapor film also collapses in other locations due to rapid vapor generation. This pressure pulse results in the spattering of molten materials and the rapid heat transfer from the melt to the water (t = 1.0 ms). The large amount of vapor generated expands vigorously due to the extremely high internal pressure (t = 5.0 ms).

Several in-containment severe accident phenomena have been investigated in the Assessment of Loads and Performance of containment in Hypothetical Accident (ALPHA) Program. Especially, the effect of steam explosion from contact of the molten core with the coolant on containment integrity has been studied in these experiments. Molten material at about 2,500 degrees cent. was dropped into a pool of water, and the melt propagation before the explosion and steam explosion behavior were recorded with a high-speed camera. In parallel with these experiments, the JAERI Simulator for Multiphase Interaction and Explosion (JASMINE) code has been developed for the prediction of steam explosion behavior and is assessed with these experiments. Water being introduced onto the molten core in the containment has also been investigated from the viewpoint of terminating a severe accident. It was found that this measure is effective since the probability of a steam explosion is low and the energy released by such a steam explosion is very small.

Reference J. Sugimoto, Thermal - Hydraulics in Severe Accident of LWRs, Dennetsu Kenkyu, 34 (133), 52 (1995).

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