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

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 (k_eff) 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 k_eff values of the UO₂ lattices partly showed a decreasing trend with H/HM, while those of the MOX lattices an increasing trend. The k_eff 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. (author)