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The risk of criticality accidents by fuel debris in the decommissioning of Fukushima Dai-ichi Nuclear Power Station is unlikely to be negligible. Therefore, an estimation of the consequences of such accidents is important. According to the latest study, estimating the energy that would be released if fuel debris became supercritical while falling into water is thought to be important. In falling fuel debris systems, the geometry changes substantially with time, which could cause large changes in neutron generation time. Treating the change in neutron generation time in a kinetics analysis might have a large impact on results such as released energy. Clarifying such phenomena could contribute to conducting a reasonable analysis and avoiding misleading results. Thus, in this study we seek to clarify whether it is necessary to consider changes of neutron generation time when evaluating released energy in a falling fuel debris system at supercritical conditions in water with a kinetics analysis based on a point reactor kinetics model. We found that change of neutron generation time should be considered when evaluating released energy in falling fuel debris systems at supercritical conditions; otherwise, obtained results could be unrealistic.

Impact of neutron generation time on released energy in falling fuel debris at supercritical condition