Experiments on central reaction rate ratios and fission distributions in the FCA-XXII-1 assembly simulating highly enriched MOX fueled tight lattice LWR cores

福島 昌宏   ; 安藤 真樹  ; 長家 康展  

Fukushima, Masahiro; Ando, Masaki; Nagaya, Yasunobu

A series of simulated experiments were conducted at the FCA to simulate a light water reactor core with a tight lattice cell containing highly enriched MOX fuel with a fissile 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. In a previous paper, we reported the criticality and reactivity worths measured in these experiments. This paper provides the experimental results for the central reaction rate ratios and fission distributions as follows. The fission rate ratios of U and Pu relative to U were measured at the core centers using three calibrated fission chambers, and the U capture reaction rate ratio relative to U fission was measured using depleted uranium foils. Reaction rate distributions were also obtained by traversing four micro fission chambers of HEU, NU, Pu, and Np 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 JENDL-4.0. Most calculation results agreed well with the experiments, whereas those for the fission rate ratio of Pu to U were underestimated by up to 6% with the softening neutron spectrum.