Supercritical transient analysis in hypothetical fuel-debris systems by multi-region approach based on integral kinetic model

Criticality safety during fuel debris retrieval from the damaged cores of the Fukushima Daiichi nuclear power station (1FNPS) Units 1-3 is an important topic for the overall activity of the plant's decommissioning. Analyses of possible criticality accidents in the fuel debris and its consequences are crucial for safe criticality control. In order to analyze a possible criticality accident, the condition of fuel debris and also potential changes in the fuel debris in terms of, for example, its geometry and the amount of water during fuel debris retrieval must first be known in detail; however, the condition of fuel debris at the 1FNPS is currently unknown. Second, an appropriate method for use in such analyses is crucial. In this study, hypothetical fuel-debris systems, i.e., systems that are to some extent possible at the 1FNPS, were considered. It is reasonable that fuel debris at the 1FNPS is coupled, whereby physically or compositionally distinct debris regions interact with one another via neutron transport. In such coupled systems, a multi-region approach is appropriate as it models a system in a time- and space/region-dependent manner. Accordingly, in the current work, we used the Multi-region Integral Kinetic (MIK) code, a code with a multi-region approach that we had previously developed. The purpose of this study was to confirm the applicability of the multi-region approach to supercritical transient in hypothetical fuel-debris systems with more complicated geometry and composition than those of our previous studies. This study also aimed to investigate the relative impact of several fuel-debris parameters on the supercritical transient. A total of fourteen hypothetical systems were made by varying the three considered parameters. As a result of this study, a region-wise power profile, temperature profile, and energy release were obtained for each system. For the systems studied herein the parameter with the largest impact on the supercritical transient was found to be the enrichment of 235U, followed in most cases by the angle of repose of the conical fragmented debris region, while the height of the consolidated debris region generally had the least impact. Our study showed that the multi-region approach can be a useful tool for analysis of supercritical transient in fuel-debris systems with rather complicated geometry and composition.