The sodium-cooled fast reactor (SFR) is a potential candidate for the future production of clean sustainable energy. Some SFR designs planned for construction will use high-assay low-enriched uranium (HALEU) metal fuel enriched up to around 19.75%. However, fast reactor technology faces several criticisms, such as its ability to produce weapons-grade plutonium, the possibility of facility diversion from peaceful use to the production of nuclear weapons–usable materials, and the risk of incentivizing the construction of domestic fuel enrichment programs. To compete with traditional light water reactors (LWRs), SFR technology utilizing HALEU fuel should be developed to be proliferation-resistant and sustainable.

In this study, the Serpent 2 Monte Carlo reactor physics code was used to simulate and compare alternative HALEU fuel options that could be implemented to reduce proliferation risk. As part of the comparison, nonpeaceful plutonium diversion pathways were considered. By evaluating its attractiveness for diversion from peaceful use to the production of nuclear weapons–usable material, it was shown that the proliferation resistance of SFR technology utilizing HALEU fuel can be improved.

The operation of a once-through fuel cycle SFR utilizing HALEU fuel was simulated, and the performance and material attractiveness of various U-10Zr fuel options were compared. Currently, hundreds of thousands of tons of spent fuel from the operation of LWRs are stored across the world. Most of the heavy metal in this spent nuclear fuel is comprised of reusable uranium and plutonium. Re-enrichment of reprocessed uranium (RepU), with or without the presence of unseparated 237Np, for the production of SFR fuel is one way to simultaneously improve uranium resource utilization and enhance proliferation resistance due to the co-enrichment of 236U and 237Np.

The presence of these isotopes in fresh HALEU fuel can increase 238Pu production even at low reactor burnup. Due to its high decay heat and spontaneous neutron emission, 238Pu decreases material attractiveness for nuclear weapons construction. Directly doping fresh fuel with preseparated 237Np requires the separate storage of neptunium, introducing its own nuclear security requirements and proliferation risks. Additionally, due to uncertainty in the separation factor between RepU and neptunium, some neptunium impurities will inevitably remain in RepU following separation.

Therefore, this study examined the impact of unseparated 237Np on the production and depletion of U-10Zr fuel. A parametric study comparing fuel options of varying 237Np weight fractions was conducted, and proliferation scenarios considering both clandestine and overt diversion of discharged fuel were considered.