Uncertainty Quantification of Fuel Inhomogeneity in Low-enriched Uranium Silicide High Flux Isotope Reactor Design

This work evaluates the effect of fuel inhomogeneity uncertainty on Oak Ridge National Laboratory (ORNL)’s HFIR core performance to support the low-enriched uranium (LEU) conversion program. Fuel inhomogeneity is manufacturing-induced non-uniform uranium distribution in the involute fuel element (i.e., deviation from the nominal uranium distribution). Therefore, analyzing the fuel inhomogeneity impact in the HFIR LEU core is essential to assess the HFIR LEU performance and safety sensitivity to the homogeneity distributions. However, inhomogeneity distribution data from LEU U₃O₈-Al fuel element manufacture is still unavailable. Thus, this work leverages proposed conservative highly enriched uranium HEU U₃O₈-Al fuel element inhomogeneity profiles and tolerances to create five representative LEU inhomogeneity profiles. The effect of fuel inhomogeneity is assessed for the proposed low-density U₃Si₃-Al core design, and several key parameters are evaluated, including multiplication factor (i.e., k_eff), cold source moderator vessel cold neutron flux and cold-to-total neutron flux ratio, ²⁵²Cf target thermal neutron flux, flux trap fast neutron flux, reflector fast neutron flux, and fission rate density distribution. This work finds that the uncertainty caused by the fuel inhomogeneity reduces the averaged reactivity and the flux at target by 0.18% and 0.43%, respectively, which can be considered negligible. In addition, the change of the fission density due to the fuel inhomogeneity is still within the assumed safety analyses tolerances.