Evaluation of critical experimental core configurations to simulate non-uniform fuel debris

The fuel debris generated by the accident at TEPCO's Fukushima Daiichi Nuclear Power Plant is expected to have not only heterogeneous but also non-uniform compositions. Therefore, the calculation method used in their criticality management is required to be validated experimentally. For this purpose, in this study, several core configurations of a new critical assembly STACY of JAEA with non-uniform arrangements of uranium oxide fuel rods, concrete rods and stainless steel rods, which are components of the fuel debris, were studied. In this paper, four cases of the New STACY core configurations with a 15 x 15 central test region with different components are shown. In addition, 100 patterns of the test regions were created based on random numbers in each case, and their effective multiplication factors were evaluated. In each case, the median value was larger than the mean effective multiplication factor. It was also confirmed that there are differences in the effective multiplication factor of more than one dollar by the pattern changing, and that the neutron spectra can change significantly by changing the local neutron moderation conditions. In particular, the effective multiplication factor became smaller when over-moderated regions with large water-to-fuel ratios were formed in the centre of the core configurations due to increases in thermal neutron absorption. Such criticality experiments with non-uniform arrangements of multiple compositions will be useful not only to evaluate the validity of the calculation code but also to develop methods to measure the subcriticality. (authors)