Benchmark Calculations of the Saxton Plutonium Program Critical Experiments

Abstract

The Saxton critical experiments, which used mixed-oxide (MOX) fuel of 6.6 wt% PuO₂ in natural UO₂ and UO₂ fuel of 5.74 wt% ²³⁵U, are analyzed with MCNP-4B and continuous-energy cross-section libraries ENDF/B-V and ENDF/B-VI. An excellent agreement of calculated and experimental effective multiplication factors for the entire set of 1.3208-cm MOX lattices and 1.4224-cm MOX and UO₂ lattices was obtained. The analysis of criticality calculations for the five different lattice pitches show a bias with lattice pitch, which led to an increase of ~0.8% when doubling the lattice pitch. Good agreement between calculated and measured data was obtained for some of the relative power distribution experiments for MOX single-region cores and MOX/UO₂ multiregion cores; however, for others the agreement was less satisfactory. No significant difference in the results for relative power with the two libraries was observed.