N. Dacheux, N. Clavier
Marcoule Institute for Separation Chemistry (ICSM), The French Alternative Energies and Atomic Energy Commission (CEA).
For many years, the development of new generations of nuclear reactors led to envisage innovative methods for the fabrication of oxide fuels. Wet chemistry routes have been studied for the fabrication of mixed oxide fuels such as (U,Pu)O2 MOx, notably to improve the cations distribution homogeneity and to enhance the resistance towards proliferation. Such processes are mainly based on the initial precipitation of low-temperatures precursors[1,2] which are further converted into the final compounds through a heat treatment at high temperature. Nevertheless, the resulting oxide powders can still contain residual carbon while the powder morphology is inherited from the starting precursor.
In this context, several authors explored the hydrothermal conversion of An(IV) oxalates (An(IV) = Th, U, Np, Pu)[4,5] which allowed them to directly achieve the precipitation of hydrated oxides from solution. Yet, the hydrothermal processes reported in the literature have almost never addressed complex systems with actinides and/or lanthanides cations exhibiting different redox states. In this context, we focused our work on the direct preparation of (U,Ce)O2 solid solutions through hydrothermal conversion of U(IV)-Ce(III) mixed oxalates. Herein, cerium was used as a surrogate for plutonium owing to several similar properties among which cationic radius and stabilized oxidation states.
The effect of the duration of mild hydrothermal conversion (T = 250°C, t = 24 – 48h) of oxalate precursors was first investigated. From 24 hours, the typical fluorite-type structure of AnO2 was observed while detailed XRD analysis highlighted the single-phase characteristic of the obtained samples. Moreover, the powder morphology was found to be mostly spherical, with particle of about 1 µm in size, and so widely different from the oxalate precursor. Preliminary work on the conversion of Nd-oxalates was also carried out to avoid the complex redox behaviour of cerium. Nevertheless, neodymium carboxylate hydroxide was systematically obtained in the studied conditions. Therefore, Nd does not seem to be a correct Ce surrogate, thus underlining that the -> oxidation of Ce(III) into Ce(IV) plays an important role in the hydrothermal conversion. The forthcoming steps of this study will focus on the sintering of the powders prepared by the hydrothermal route.
 B. Arab-chapelet et al., “Synthesis of new mixed actinides oxalates as precursors of actinides oxide solid solutions,” vol. 445, pp. 387–390, 2007
 J. Martinez et al., « From uranium (IV) oxalate to sintered UO2 : consequences of the powders’ thermal history on the microstructure », J. Europ. Ceram. Soc. 35 (2015) 4535-4546.
 Manaud et al, « Hydrothermal conversion of uranium(IV) oxalate into oxides: a comprehensive study », Inorg. Chem. 2020, 59, 3260-3273
 Popa et al., « A low-temperature synthesis method for AnO2 nanocrystals and associate solid solutions », CrystEngComm, 2018
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Tuesday – 14th September 2021