Reactor Physics Characteristics of High Flux Isotope Reactor Core Designs with Low-Enriched Uranium Fuels

Ongoing efforts to convert the High Flux Isotope Reactor from highly enriched uranium (HEU) to low-enriched uranium (LEU) fuel are focused on refining the design characteristics and reactor physics analyses of selected candidate fuel designs with a silicide dispersion fuel. In design optimization studies, designs are assessed for key performance and safety metrics. As the designs are further refined, reactor physics metrics are generated for additional supporting analyses such as transient simulations. Current high-fidelity tool sets are extended to support the rapid generation of these characteristics in such a way that allows candidate designs to progress further along in the design process. Development of this workflow is critical to ensuring a quick response to future experimental outcomes on potential fuel designs. Relative to the HEU design, the increase in ²³⁸U in candidate LEU fuel designs hardens the neutron spectrum, driving a shorter neutron generation time and reducing the reactivity impact from coolant density changes. However, increased resonance absorption in the fuel drives a more negative fuel temperature feedback coefficient as compared to the HEU design. The addition of molybdenum in the uranium-molybdenum candidate LEU fuel hardens the spectrum further; the silicide-fueled candidate design represents a more incremental change from the current HEU fuel design. Despite the changes in these reactor physics parameters, the fuel designs are still expected to perform safely throughout additional reactor analyses.