Homogeneous Critical Assemblies of 2 and 3% Uranium-235-Enriched Uranium in Paraffin

A series of clean critical experiments has been performed with homogeneous mixtures of finely divided U(2)F₄ or U(3)F₄ dispersed in dispersed in paraffin with H/²³⁵U atomic ratios varying from 133 to 972. The assemblies were constructed in rectangular geometry, and minimum critical masses and volumes in cylindrical and spherical geometries were obtained from buckling conversions. The minimum critical spherical volumes of a U(2)F₄-paraffin mixture when unreflected and when reflected with an effectively infinite solid hydrogenous material are 199 and 138 liters, respectively, and the minimum critical ²³⁵U masses are 7.5 and 5.4 kg, respectively. The experiments with the U(3)F₄-paraffin mixtures were not extensive enough to determine experimentally the values at optimum moderation but transport theory calculations indicate that the unreflected and reflected minimum critical volumes are about 93 and 57 liters, respectively, and that the minimum critical masses are about 4.1 and 2.7 kg of ²³⁵U, respectively. Measurements of the ²³⁸U-to-²³⁵U fission ratio indicated that in some assemblies as much as 6.5% of the fissions occurred in ²³⁸U. Prompt-neutron decay constants, α = β_eff/1 were measured in reflected and unreflected delayed-critical assemblies of several fuel mixtures by the pulsed-neutron technique; the infinite-medium neutron multiplication factor, k_∞, for the various fuel mixtures was inferred from height perturbation measurements.Transport theory calculations of k_∞ and feff have been made using various cross-section sets.