Direct electrochemical reduction of solid uranium oxide in molten fluoride salts

Abstract : The 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.
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Mathieu Gibilaro, Laurent Cassayre, Olivier Lemoine, Laurent Massot, Olivier Dugne, et al.. Direct electrochemical reduction of solid uranium oxide in molten fluoride salts. Journal of Nuclear Materials, Elsevier, 2011, vol. 414, pp. 169-173. ⟨10.1016/j.jnucmat.2011.02.053⟩. ⟨hal-00815297⟩

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