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/sub eff/ was computed for each assembly on which criticality was determined. The computed k/sub eff/ value were up to approx. 2% above unity in some cases, with the average value for the entire series being 1.007