Experiments on Central Reaction Rate Ratios and Fission Distributions in the FCA-XXII-1 Assembly Simulating Highly Enriched MOX–Fueled Tight Lattice LWR Cores

A series of simulated experiments were conducted at the fast critical assembly (FCA) of the Japan Atomic Energy Agency to simulate a light water reactor core with a tight lattice cell containing highly enriched mixed-oxide fuel with a fissile plutonium (Pu) ratio >15%. The prediction accuracy of the neutron computation codes and nuclear data libraries in a wide range of neutron spectra was evaluated by constructing three experimental configurations of the FCA-XXII-1 assembly with different void fractions (45%, 65%, and 95%) of the moderator material (foamed polystyrene). The hydrogen-to–nuclear fuel atomic ratio was systematically varied from 0.1 to 0.8. In a previous paper, we reported the criticality and reactivity worths measured in these experiments. This technical note provides the experimental results for the central reaction rate ratios and fission distributions as follows. The fission rate ratios of uranium (U) (²³⁸U) and ²³⁹Pu relative to ²³⁵U were measured at the core centers using three calibrated fission chambers, and the ²³⁸U capture reaction rate ratio relative to ²³⁵U fission was measured using depleted U foils. The reaction rate distributions were also obtained by traversing four micro fission chambers of highly enriched U, natural U, Pu, and neptunium through each core region in the radial and axial directions. The experimental analyses were performed using detailed models of the Monte Carlo code MVP3 with the Japanese evaluated nuclear data library of JENDL-4.0. Most calculation results agreed well with the experiments, whereas those for the fission rate ratio of ²³⁹Pu to ²³⁵U were underestimated by up to 6% with the softening neutron spectrum.