Critical Experiments with Low-Moderated Homogeneous Mixtures of Plutonium and Uranium Oxides Containing 8, 15, and 30 wt% Plutonium

The results from a series of criticality experiments with three different mixtures of oxides of plutonium and uranium are presented. The fuel mixtures consisted of ²³⁵U-depleted uranium homogenized with ∼8, 15, and 30 wt% plutonium and blended, homogeneously, with polystyrene to achieve H:(Pu + U) atomic ratios of ∼7, 3, and 3, respectively. Critical sizes are given for rectangular parallelepipeds of each of the fuels fully reflected with a methacrylate plastic (Plexiglas). Critical sizes are also given for unreflected parallelepipeds of the 30-wt% plutonium-enriched fuel mixture. For the 30-wt% plutonium-enriched mixture, sufficient fuel was available to permit determining that the critical thickness of a fully reflected slab of this material, infinite in two dimensions, was 12.93 + 0.14 cm. Comparisons were made between the critical assemblies and calculational results using ENDF/B-III cross sections and the KENO and DTF-IV computer codes. Wherever comparisons could be made, the DTF-IV and KENO results were within 1% of each other; however, some of the comparisons between calculations and experiments differed by 2 to 3% in k_eff.