Neutronic and thermal-hydraulic feasibility studies for high flux isotope reactor conversion to low-enriched uranium U₃SI₂-al fuel

Oak Ridge National Laboratory (ORNL) operates the High Flux Isotope Reactor (HFIR) for the US Department of Energy Office of Science to perform neutron scattering experiments, produce isotopes, and conduct materials irradiation research. ORNL is funded by the National Nuclear Security Administration (NNSA) Office of Materials Management and Minimization to evaluate the conversion of HFIR's fuel from high-enriched uranium (HEU) to low-enriched uranium (LEU). Due to potential fabrication issues with the complex UMo fuel design required to convert HFIR, NNSA requested ORNL to evaluate an alternate LEU fuel form - U3Si2-Al dispersion fuel. Neutronic and thermal-hydraulic feasibility analyses were performed with Shift and HSSHTC, respectively, to predict reactor performance and thermal safety margins. A number of designs were proposed and evaluated using an iterative approach in an effort to show that reactor performance could match that obtained using HEU fuel and that thermal safety margins were adequate. This study concludes that conversion of HFIR with U3S12-AI LEU (19.75 wt.%) fuel with 4.8 gU/cm3 is feasible if, among other requirements, the fuel meat region is centered and symmetric about the fuel plate thickness centerline, the active fuel zone length is increased from 50.80 cm to 55.88 cm, the proposed fabrication tolerances can be met, and the fuel can be qualified for HFIR conditions.