Delayed critical ORNL unreflected uranium (93.20) metal sphere and the pure unreflected uranium (93.80) sphere critical mass

A spherical critical assembly of unreflected and unmoderated uranium (93.20 wt.% 235U) metal was assembled at the Oak Ridge Critical Experiments Facility in 1971. All the characteristics of this assembly are very accurately known; dimensions to ± 0.0001 in, masses to ± 0.01 g, isotopics, impurities, etc. Two spheres were assembled. The first was above delayed criticality and the second, after machining of the first, was slightly below delayed criticality. A variety of reactivity effects such as the influence of support structure, imperfections in the sphere, etc. were evaluated experimentally, where practical. The very accurate description of this sphere, as assembled, establishes it as an ideal benchmark for calculational methods and cross-section data files. In this regard, the accurate specification of one of these spheres for a Monte Carlo code input is included as an Appendix. In order to provide data for verification of one-dimensional, transport-theory calculations, these two spherical uranium metal assemblies were corrected to idealized spherical geometries. Either all imperfections in the sphere were evaluated experimentally or corrections were made based on previous measurements with GODIVA I at Los Alamos National Laboratory. The experimental configurations first were corrected to idealized spheres of pure uranium metal of the density and enrichment of these measurements. In order to compare the results of these experiments to the earlier idealized delayed critical sphere obtained from the GODIVA I experiments, these spheres were also corrected to an enrichment of 93.80 wt.% 235U and density of 18.750 g U/cm3 with no impurities. The estimates of the purely spherical critical masses of 93.20-wt.%-enriched uranium were 52,911±66 g (ρ=18.765 g U/cm3) from the slightly supercritical configuration, and 52,956±64 g (ρ=18.754 g U/cm3) from the slightly subcritical configuration. Correction to a pure uranium sphere of enrichment of 93.80 wt.% 235U and density of 18.750 g U/cm3 with no impurities yielded a critical mass of 52.42±0.07 kg which is in agreement with the value from the GODIVA I experiments, which is 52.30 ± 0.16 kg. The uncertainty in the pure uranium sphere mass from the Oak Ridge experiments is considerably less than that from the GODIVA I experiments because the Oak Ridge spheres were more spherical with less structural material in close proximity to the sphere.