Effect of Soluble Neutron Absorbers on the Criticality of Low-Uranium-235 Enriched UO₂ Lattices

A series of experiments was performed to determine the effect of soluble neutron absorbers on the criticality of water-moderated latticed assemblies. The soluble absorbers used were boron, gadolinium, and cadmium. These materials were mixed with the moderator and reflector of lattices of 4.3-wt%-²³⁵U-enriched UO₂ fuel pins with stainless-steel cladding. Lattice spacings of 22.9, 27.9, and 33.0 mm, center-to-center, were measured to determine the critical number of rods. The value of k_eff was computed for each assembly on which criticality was determined. The computed k_eff value were up to ∼2% above unity in some cases, with the average value for the entire series being 1.007.