High Flux Isotope Reactor (HFIR) highly enriched uranium (HEU) to low-enriched uranium (LEU) conversion activities are ongoing as part of the Department of Energy (DOE) National Nuclear Security Administration (NNSA)'s nuclear nonproliferation mission. Design activities studying the conversion of HFIR from HEU to LEU fuel explored different fuel design features and shapes with a low density uranium-silicide dispersion (U 3 Si 2 -Al) fuel, which has a uranium density of 4.8 gU/cm 3 . The goal of these studies is to generate several HFIR LEU fuel designs of varying fuel fabrication complexity that meet the current HEU performance metrics and safety requirements. The documented designs will serve as references for fuel fabrication and qualification activities.Recent advancements in modeling and simulation tools enable quick prototyping of fuel designs. Shift, a Monte Carlo neutron transport and depletion tool optimized for high-performance computing (HPC) architectures, is used for efficient fuel cycle and performance metrics calculations. The HFIR Steady State Heat Transfer Code (HSSHTC) is used to vet the thermal safety margin. Also, a new automation tool that connects all fuel design analysis steps, named Python HFIR Analysis and Measurement Engine (PHAME), has been developed to expedite the design study in an efficient and reproducible manner. Leveraging these tools, several candidate fuel designs were selected for varying fabrication complexity.This report provides design feature details for four selected HFIR LEU low density U 3 Si 2 -Al fuel designs and their corresponding performance and safety metrics. Nominal, best-estimate design parameters and irradiation conditions, including fission rate densities, power densities, heat fluxes, and cumulative fission densities are provided for candidate fuel designs relevant to framing irradiation experiments to support fuel qualification efforts. Simulations show that the low density U 3 Si 2 -Al, with design features to enhance safety, can meet HEU core performance metrics and safety requirements if the reactor power is increased from 85 MW (HEU) to 95 MW (LEU) and if the active fuel length is increased from 50.80 cm (HEU) to 55.88 cm (LEU).* This Q value of 200.7 Mev/fission is a 'typical' Q value used in HFIR analyses close to the approximate cycle-averaged value that was recently calculated for the optimized silicide design. The BOC and EOC Q values were estimated to be 200. 49 and 201.46 MeV/fission, respectively, givin the average Q value of 200.98 MeV/fission [9].

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HEU highly enriched uranium HFIR High Flux Isotope Reactor HPC high-performance computing HSSHTC HFIR Steady State Heat Transfer Code IFE inner fuel element LEU low-enriched uranium MCNP Monte Carlo N-Particle code MOC middle-of-cycle NNSA National Nuclear Security Administration OFE outer fuel element ORNL Oak Ridge National Laboratory PHAME Python HFIR Analysis and Measurement Engine SL safety limit TSR technical safety requirement U-10Mo uranium-molybdenum

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