Criticality of Plutonium-Uranium Nitrate Solutions

A series of experiments was performed providing new criticality data on plutonium-uranium nitrate solutions in cylindrical and spherical geometry. For the experiments in cylindrical geometry, the plutonium content of the total uranium plus plutonium was ∼30 wt%; whereas, in the case of the water-reflected spheres, measurements were performed with both 15 and 30 wt% plutonium. The uranium in the mixture was slightly depleted, containing 0.66 wt% ²³⁵U. The plutonium concentration covered by these experiments ranged between 12.4 to 97.3 g Pu/ℓ (uranium plus plutonium concentrations between 30 to 310 g/ℓ. The ²⁴⁰Pu content of the plutonium was 5.6 wt% in the first case and 4. 7 wt% in the second. The experiments were analyzed using ENDF/B-III cross-section data, and criticality factors were computed in each case. Some comparative calculations also were made, showing the differences obtained with ENDF/B-II, ENDF/B-III, and GAMTEC cross sections. The KENO code, with ENDF/B-III cross sections, as well as the HFN code, provide conservative results on the criticality factors for these systems. The average value of the computed k_eff for the cylinders, using KENO, was 1.022, and for the spheres, 1.024 using HFN. Thus, using these methods and cross-section data, the computed critical masses and volumes would be expected to be smaller than those measured by ∼2% in terms of k_eff.