Fuel fragmentation and mechanical energy conversion ratio at rapid deposition of high energy in LWR fuels

The fuel fragmentation is one of the important subjects in the field of molten fuel-coolant interaction (MFCI) since it is one of basic processes of the MFCI, and it has not yet been made clear enough. Accordingly, UO2 fuel fragmentation was studied in a postulated reactivity initiated accident (RIA) condition by the Nuclear Safety Research Reactor (NSRR). The distribution of the size of fuel fragments was obtained through the experiments and the mechanism of fuel fragmentation was studied. Also, the relation between the conversion ratio of the mechanical energy to the thermal and the degree of fuel fragmentation was obtained experimentally. It was revealed that the distribution of fuel fragments was well described in the form of logarithmic Rosin-Rammler's distribution law. The fuel fragmentation was found to be explained by the Weber-type hydraulic instability model and the internal pressurization model. It was also shown that the mechanical energy conversion ratio was inversely proportional to the volume-surface mean diameter defined as the ratio of the total volume of fragments to the total surface, and furthermore that it was influenced by the coolant subcooling and the volumetric ratio of fuel to water. (author)