Numerical analysis of criticality of fuel debris falling in water

Abstract

The dynamic behavior of fuel debris in water raises the possibility of criticality accidents during debris removal. Hence, accurate evaluations of criticality that consider the dynamic behavior of fuel debris are an essential part of the decommissioning process. In this study, we develop a calculation system that combines the moving particle semi-implicit method and Monte Carlo neutron transport calculation code. We then clarify that it is possible to evaluate criticality safety using the actual dynamic behavior of fuel debris in water with this calculation system. We first carry out validation experiments to compare the experimental and simulation results of solid-fluid multiphase flows calculation. We demonstrate that the MPS method could calculate the dynamic behavior of fuel debris in water, mostly within the 95% confidence interval of experimental results. We then show it could also evaluate criticality during the sedimentation of cubic fuel debris. Moreover, in the case of fundamental conditions such as a large amount of cubic debris falling in water, it was possible that the effective multiplication factor was maximized in the state of falling in water rather than with complete sedimentation.