Enriched Uranium Metal Criticality Calculations

The effects of varying energy group cross-section sets and the space-angle mesh must be understood to evaluate critical parameters computed by the discrete ordinates S_n method. In this paper, these effects are investigated using critical mass calculations on several enriched uranium metal systems. Based on the variation of critical mass with refinement of the space-angle mesh, mesh specification criteria are deduced. The most significant criteria established for these systems is that S_n (n ≥ 16) calculations are required. Next, the literature on fast (E_n ≥ 3 keV) ²³⁵U and ²³⁸U neutron cross sections is reviewed and a seven-group cross-section set in this energy range is developed. This fast set is found to reproduce to within 0.5% the measured critical masses of several fast uranium systems. Finally, group cross-section sets in the resolved resonance energy ranges of ²³⁵ U and ²³⁸U are developed according to several energy self-shielding models. A recommended set is obtained by interpolation between two of these models. This set yields critical masses in good agreement with observed critical masses for several intermediate energy metal critical systems with several ²³⁵U enrichments.