Weapons-grade plutonium disposition in thermal-spectrum molten salt reactor

The disposition of surplus weapons-grade plutonium (WPu) remains an unsolved challenge that could be addressed via conversion to advanced reactor fuel, as supported by a recent executive order. Additionally, WPu may provide an alternative fueling pathway in the scenario that high-assay low-enriched uranium (HALEU) demands exceed available supply. This work investigates the feasibility of disposing of WPu in a thermal-spectrum molten salt reactor (MSR) through continuous refueling. Neutronics, depletion, refueling, and redox control behavior over 25 years were evaluated using Serpent and the open-source NERTHUS reactor concept, inspired by a ThorCon design. Criticality was sustained via continuous WPu refueling while constraining plutonium concentrations to below solubility limits. The core exhibited a consistently negative fuel temperature coefficient of reactivity, ranging from −3 to −5 pcm/K. Simplified corrosion mitigation was implemented through continuous beryllium additions, targeting electroneutrality of the salt estimated by excess fluorine. Isotopic analysis showed the weapons-useable 239 Pu was depleted over time. Most other actinides built up to a near-equilibrium state, making the salt unattractive from a proliferation standpoint without significantly increasing the mass of the eventual spent fuel. However, the accumulation of 249 Bk leads to the accumulation of 249 Cf and subsequently 245 Cm, which was identified as a primary long-term radiological concern that should be considered in future work. Compared to HALEU, the WPu-fueled configuration required less fissile mass to maintain criticality because of the larger thermal fission cross-section. Compared to solid-fueled configurations proposed for WPu disposition in the past, liquid-fueled MSRs do not require complex and expensive gallium removal from WPu. These findings demonstrate that thermal-spectrum MSRs represent a viable and underexplored option for surplus plutonium disposition.