Calculations of UH₃ Critical Masses for Various Uranium Enrichments

Monte Carlo neutron transport calculations of critical radii of bare UH₃ spheres as a function of ²³³U and ²³⁵U enrichment in ²³⁸U were carried out using three different calculational methods: 1. the continuous-energy 2. method the multigroup approximation 3. the probability table method. Also calculated were the average neutron removal lifetimes and median fission-producing neutron energies for these critical systems. Results of these calculations were analyzed and compared to quantify and bound self-shielding effects arising from the neutron cross-section resonances in calculating these moderated critical assemblies. Although the critical radii of these assemblies are sensitive to the resonances in ²³⁸U, the removal lifetime seems to be affected much more by the fissile isotope resonances. The probability table method gives results that are in excellent agreement with continuous-energy calculations, thereby eliminating the need for special cross-section processing for each separate enrichment when using multigroup cross sections.