Reprocessing of spent high assay low enriched uranium nitride and uranium dioxide fuel

Sammanfattning

Spent nuclear fuel characteristics relevant to the reprocessing of high-assay low-enriched uranium nitride and uranium dioxide fuels intended for a lead-cooled fast reactor concept were evaluated. Burnup calculations were performed using the Serpent 2 Monte Carlo code for 12 at.% U-235-enriched UN and UO2 fuels irradiated to 75 MWd/kgHM, and the spentfuel isotopic inventories, activity, and decay heat were used to assess criticality and thermal constraints under representative aqueous and pyrochemical reprocessing conditions. Aqueous and pyrochemical reprocessing were modeled as spent nuclear fuel homogeneously dissolved in nitric acid and molten LiCl–KCl salt, respectively. The criticality calculations showed that aqueous systems were sensitive to fuel loading and vessel size, with keff demonstrating converging behavior for increasing vessel size. For 400 g/L fuel mass per initial acid volume, the diameter limit for maintaining keff < 0.8 was 66 cm for both fuel types. In pyrochemical systems, no criticality constraint was observed within the studied range. The thermal analysis showed that pyrochemical reprocessing did not impose vessel-size limits incompatible with practical electrorefiner operation, whereas the thermal analysis of aqueous reprocessing was inconclusive. The exothermic dissolution reaction dominated the aqueous thermal balance, with decay heat contributing only secondarily. Overall, the results did not rule out aqueous or pyrochemical reprocessing of HALEU UN and UO2 for the conditions studied. Further work should focus on experimental validation of solution properties and dissolution heat, more detailed process modeling, and broader feasibility considerations including engineering, regulatory, safeguards, and economic constraints.