Study on criticality uncertainties of MCCI products due to concrete compositions

The criticality properties of fuel debris from Molten Core-Concrete Interaction (MCCI) are considered to depend on the composition of the concrete aggregate. Various concrete aggregate compositions are known, and experiments have been conducted to produce MCCI using concretes with different aggregates, but few studies have addressed their criticality characteristics. From these backgrounds, critical experiments at the modified Static Experiment Critical Facility (STACY) were conducted to clarify the criticality characteristics of fuel debris by making structural simulant rods of mortar composition that did not contain aggregate to simulate concrete. In this study, to evaluate the representativeness of the samples used in the STACY experiment, we clarified differences in criticality characteristics, such as effective multiplication factors, under varying neutron moderation and moisture conditions for concretes with typical compositions and for the experimental mortar composition. In addition, the critical radius and the possible range of critical mass, which are important for handling fuel debris, were clarified. We also evaluated the impact on effective multiplication due to uncertainties in composition. As a result, it can be concluded that differences in criticality characteristics due to aggregate type, including compositional uncertainties, are generally small.