Critical analysis of light-water moderation UO2 and uranium-plutonium mixed oxide lattices by continuous-energy Monte Carlo calculation

Abstract

ABSTRACT Critical analysis of 120 light-water moderation lattices composed of 2.6 wt% UO2 fuel rods or 3.0 wt%-PuO2 uranium-plutonium mixed oxide (MOX) fuel rods was performed using a continuous-energy Monte Carlo calculation code to validate the JENDL-4.0 nuclear data library. As a result, the average neutron multiplication factor (keff ) of the 35 UO2 lattices (hydrogen-to-heavy-metal atomic ratio (H/HM): 4.3–8.7) was 0.99930 with the uncertainty of 0.00005 (1 σ), and that of the 85 MOX lattices (H/HM: 12.0–27.6) was 0.99856 with the uncertainty of 0.00006 (1 σ). The keff values of the UO2 lattices partly showed a decreasing trend with H/HM, while those of the MOX lattices an increasing trend. The keff values of the MOX lattices presented an increasing trend with the measurement dates. Assuming that the trend was relating to the uncertainties in the neutron capture cross-section of 241Am, a simulation to modify the cross-section was performed by increasing the atomic number densities of 241Am in the MOX fuel. It indicated that the neutron-flux-average capture cross-sections of 241Am for the MOX lattices should be increased by 10% to 20% from those calculated by the cross-section in JENDL-4.0.