Criticality Characteristics and Sensitivity Analysis of Fukushima Debris Beds Containing MCCI Products

This study investigates the criticality characteristics of debris beds that may have been formed through the molten–core–concrete-interaction (MCCI) at the pedestal floor of the damaged reactors in Fukushima Daiichi Nuclear Power Station. These were modeled as UO₂-concrete systems submerged in water. First, a conservative model was used to evaluate the impact that the presence of concrete has on the neutron multiplication factor (k_eff) of debris beds. The good moderation capacities of concrete were proved, and it was found that recriticality would be possible under the considered conservative assumptions. Second, a more realistic model was used to perform an uncertainty and sensitivity analysis of a wide range of debris parameters (debris porosity, core meltdown grade, debris size, debris composition, concrete erosion factor, etc.). In this case, the results indicate that the probability of a recriticality event is very remote. It was also found that the presence of boron (B₄C) from the control rods within debris has by far the highest influence on k_eff.