Criticality of Low Enrichment Uranium in Hydrogen

In this paper we present calculated critical masses of homogeneous water-moderated assemblies containing low enrichment uranium. The calculations were made using the multigroup DSN code with eighteen energy groups. Effective absorption cross sections for U²³⁸ were computed with the “infinite mass” and “narrow resonance” approximations. The calculations have been compared with various experiments and rather good agreement was found. The results are presented as a parametric survey for U²³⁵/U atom ratios from 0.014 to 0.300 and for all H/U²³⁵ ratios for which criticality is possible. The decrease in critical radius with an infinite water reflector is also shown. We find that a bare homogeneous system with U²³⁵/U < 0.010 cannot be made critical at any H/U²³⁵ ratio.