Résumé
The development of advanced nuclear reactors requires innovative methods for producing mixed oxide (MOx) fuels, particularly (U,Pu)O2±δ. The current MELOX process results in a heterogeneous microstructure with uneven U and Pu distribution, which might be inadequate for future scenarios like multi-recycling of plutonium in PWRs or deployment of FNRs1. Traditional wet chemistry processes, such as oxalic precipitation, often induce unsuitable morphologies and can introduce carbon impurities, impacting shaping and sintering steps. Hence, hydrothermal conversion of oxalate compounds into oxides has recently emerged as a promising method, eliminating carbon impurities and improving powder morphology. This process involves precipitating actinide cations in oxalic acid, followed by hydrothermal treatment at 180-250°C for 24-72 hours, yielding actinide oxides with good cationic homogeneity. If these works mainly focused on An(III) and An(IV), insights from geochemistry suggest that the reduction of uranyl ions can be achieved under hydrothermal conditions, particularly through interactions with organic compounds, such as carboxylic acids2 .Our study then investigated the possible conversion of a mixture of uranyl and oxalate ions into uranium(IV) oxide, through a multi-parametric study (time, pH, U/oxalate ratio). pH was found to be the most crucial conditions. At 250°C and pH 1.2, pure UO2 was obtained. However, higher pH values resulted in a mix of UO2, α-U3O8, and meta-schoepite. Optimal hydrothermal conversion conditions leading to UO₂₊ₓ with a precipitation yield of 97% were achieved after 24 hours at an initial pH of 1. In parallel, in situ XANES analyses were performed on the FAME beamline at ESRF and revealed an almost complete reduction of uranium(VI) in solution and changes in uranium complexation due to oxalate decomposition3. A preliminary mechanism for the reductive hydrothermal conversion of uranium(VI) to UO2+x was proposed. Future research will focus on extending investigations to mixed systems incorporating thorium, as a surrogate for Pu(IV). 1 Haas et al., Prog Nucl Energ 49 (2007) 574-5822 S. Nakashima et al., Geochimica et Cosmochimica Acta, vol. 48, no 11, p. 2321‑2329, 19843 S. Benarib et al., Dalton Trans., vol. 53, no 33, p. 13982‑13995, 2024