Synthesis and Direct Sintering of Nanosized (MIV ,MIII )O2-x Hydrated Oxides as Electrolyte Ceramics.

Highly reactive and nanosized Th1-x Yx O2-x/2 or Ce0.8 Ln0.2 O1.9 mixed oxides were prepared through the initial precipitation of hydroxide precursors which were further dried under vacuum. Whatever the chemical system investigated, the characterization of the powdered samples evidenced a rapid aging process leading to hydrated oxides. The thermal behavior of these samples was further investigated and first showed a two-step dehydration process, with the successive departure of adsorbed and constitutive water, both yielding a drastic drop of the powders' reactivity (i.e. decrease of the specific surface area). Sintering experiments were then undertaken by starting directly from raw powders and revealed very rapid densification kinetics. Highly densified pellets (above 95 %TD) with a fine grain microstructure were obtained after only 1 hour of heat treatment at 1600 °C. This easy and versatile process of precipitation, that can be followed by direct densification of the powders, then appears as a promising option for the elaboration of homogenous ceramic electrolytes.

Oxidative dissolution of Cr-doped UO2 nuclear fuel.

Alternative UO2 nuclear fuels, incorporating Cr as a dopant, are currently in use in light-water reactors. Dissolution experiments using Cr-doped UO2, performed as a function of Cr content in a simplified groundwater solution and under oxic conditions, established that the addition of Cr to the UO2 matrix systematically reduced the normalised dissolution rate of U at 25 and 40 °C. This effect was most notable under dilute solution conditions, and is the result of galvanic coupling between Cr and U, resulting from the presence of Cr2+ in the UO2 matrix, as corroborated by activation energy determination. Under conditions of solution saturation, where schoepite ((UO2)8O2(OH)12·(H2O)12) and Na2U2O7·6H2O were identified as secondary phases, the rate of U dissolution was invariant with Cr content. Moreover, at 60 °C, the trend was reversed and the rate of U dissolution increased with increasing Cr content. Under these conditions, other factors, including U solubility or bicarbonate-surface interactions, exert a stronger influence on the U dissolution kinetics than Cr. Increased grain size, a feature of Cr-doped UO2 fuel, was also found to reduce the normalised dissolution rate of U. In establishing the mechanisms by which Cr dopants influence UO2 fuel dissolution, it can be concluded that, overall, Cr-doped UO2 nuclear fuel possesses similar dissolution kinetics to undoped UO2 fuel, giving confidence for its eventual disposal in a geological facility.

Cr2+ solid solution in UO2 evidenced by advanced spectroscopy.

Advanced Cr-doped UO2 fuels are essential for driving safe and efficient generation of nuclear energy. Although widely deployed, little is known about their fundamental chemistry, which is a critical gap for development of new fuel materials and radioactive waste management strategies. Utilising an original approach, we directly evidence the chemistry of Cr(3+)2O3-doped U(4+)O2. Advanced high-flux, high-spectral purity X-ray absorption spectroscopy (XAS), corroborated by diffraction, Raman spectroscopy and high energy resolved fluorescence detection-XAS, is used to establish that Cr2+ directly substitutes for U4+, accompanied by U5+ and oxygen vacancy charge compensation. Extension of the analysis to heat-treated simulant nuclear fuel reveals a mixed Cr2+/3+ oxidation state, with Cr in more than one physical form, explaining the substantial discrepancies that exist in the literature. Successful demonstration of this analytical advance, and the scientific underpinning it provides, opens opportunities for an expansion in the range of dopants utilised in advanced UO2 fuels.