International audienceThe direct electrochemical reduction of UO2 solid pellets was carried out in LiF-CaF2 (+ 2 mass. % Li2O) at 850°C. An inert gold anode was used instead of the usual reactive sacrificial carbon anode. In this case, oxidation of oxide ions present in the melt yields O2 gas evolution on the anode. Electrochemical characterisations of UO2 pellets were performed by linear sweep voltammetry at 10mV/s and reduction waves associated to oxide direct reduction were observed at a potential 150mV more positive in comparison to the solvent reduction. Subsequent, galvanostatic electrolyses runs were carried out and products were characterised by SEM-EDX, EPMA/WDS and XRD. In one of the runs, uranium oxide was partially reduced and three phases were observed: non reduced UO2 in the centre, pure metallic uranium on the external layer and an intermediate phase representing the initial stage of reduction taking place at the grain boundaries. In another run, the UO2 sample was fully reduced. Due to oxygen removal, the U matrix had a typical coral-like structure which is characteristic of the pattern observed after the electroreduction of solid oxides

Pierre Chamelot, et al.. Electrochemical behavior of plutonium fluoride species in LiF-CaF2 eutectic melt. Electrochimica Acta, Elsevier, 2019, 301, pp.

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The CaCl 2 -CaF 2 -CaO phase diagram was investigated in the CaO low region (<40 mol.%). CaCl 2 -CaF 2 and CaCl 2 -CaO binary diagrams, constituting the ternary system, were first studied by Differential Scanning Calorimetry (DSC) measurements and X-Ray Diffraction (XRD) characterization; a good agreement was obtained between the phase diagram models calculated with Fact-Sage ® software (FTsalt database) and present experimental data. As the CaF 2 -CaO liquidus could not be measured by DSC due to the high melting temperature, this diagram was calculated using FTsalt database combined with FToxid database of FactSage ® software. The ternary phase diagram was obtained by calculations and exhibits an eutectic at the composition CaCl 2 -CaF 2 -CaO (78.2-15.7-6.1 mol.%) melting at 637˚C, and five peritectic points. Measurements of relevant vertical cross-sections for three CaCl 2 -CaF 2 compositions (50-50, 40-60 and 30-70 mol.%) up to 18 mol.% CaO are in agreement with the ternary phase diagram model. For each section, the liquidus temperature is constant up to around 11 mol.% CaO and then sharply increases. Moreover, an increase of CaF 2 content in CaCl 2 -CaF 2 melt leads to a decrease of the CaO solubility in isothermal condition.

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