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In the context of research on U/minor actinides for nuclear fuel reprocessing in the transmutation process, developments are first studied with surrogates containing uranium and lanthanides to facilitate testing. The tests consist of precipitating and calcining a hydrazinium uranium/cerium oxalate. The structure of this oxalate had not been previously determined, but was necessary to validate the physicochemical mechanisms involved. The present study, firstly demonstrates the structural similarity of the U/Ce oxalate phase (N 2 H 5 ,H) 2.9 U 1.1 Ce 0.9 (C 2 O 4 ) 5 ·10H 2 O, synthesized using a vortex precipitator for continuous synthesis of actinide oxalates, with previously known oxalates, crystallizing in P 6 3 / mmc symmetry, obtained by more classical methods. This fast precipitation process induces massive nucleation of fine powders. Their structural and microstructural determination confirms that the raw and dried phases belong to the same structural family as (NH 4 ) 2 U 2 (C 2 O 4 ) 5 ·0.7H 2 O whose structure was described by Chapelet-Arab in P 6 3 / mmc symmetry, using single crystal data. However, they present an extended disorder inside the tunnels of the structure, even after drying at 100 °C, between water and hydrazinium ions. This disorder is directly related to the fast vortex method. This structure determination can be used as a basis for further semi-quantitative analysis on the U/minor actinides products formed under various experimental conditions. [ABSTRACT FROM AUTHOR]

In the U–Ce–O system, a solid solution (U,Ce)O2+x of fluorite type containing anionic excess is known in a wide composition range. For high values of x, it transforms to a (U1−yCey)4O9−δ phase deriving from the β-U4O9−δ type [ordered anion-excess fluorite superstructure phase; I-43d space group; a=21.7484(1) A˚ for y=0.10]. The crystal structure of (U0.9Ce0.1)4O9−δ has been refined by the Rietveld method on a powder sample measured on D2B at ILL Grenoble. The structural model, proposed by Bevan et al. for β–U4O9−δ and not fully confirmed till now, has been verified. The structure is based on an ordered distribution of cuboctahedral clusters U6O37 inside a fluorite matrix. A preferential ordering of Ce4+ (and U4+) on the so-called “centaur polyhedra” with 10 coordination is proposed, on the basis of bond valence calculations. The structure so determined has the composition M64O143 (MO2.234) and no traces of excess anions, completing the supposed composition up to M4O9, could be detected. [Copyright &y& Elsevier]