Loïc Marchetti

The dissolution of mixed uranium-plutonium oxide (MOx) U1-yPuyO2 fuels in nitric acid is a critical step in the reprocessing of spent nuclear fuel. In this work, we present the first operando dissolution study of (U,Pu)O2 pellets (MOx) with 11 % Pu content in nitric acid, in which dissolution was periodically interrupted to allow SEM monitoring of microstructural changes at the solid/solution interface. Two pellets differing only in grain size but sharing identical density, homogeneity and O/M ratio were characterized by Raman spectroscopy, EPMA, XRD, HERFD-XANES and SEM-EBSD prior to dissolution. Operando dissolutions were conducted in 3.2 mol L-1 HNO3 at 65°C using ICP-MS monitoring of U and Pu concentrations. Specific intragranular dissolution features (”triangular etch pits”, ”square-based pyramids” and ”step edges with tip form”) were directly correlated to grain orientation observed by EBSD prior dissolution. This confirms that orientation-dependent mechanisms evidenced previously on UO2 single crystals also operate in polycrystalline (U,Pu)O2. More specifically, (100) orientation appears to be the most reactive surface while (111) are the most resistant. Plutonium-enriched grains were identified as inclusions refractory to dissolution by SEM-EDS and confirmed by micro-Raman mapping. No preferential attack at grain boundary was observed.

The dissolution of mixed uranium-plutonium oxide (MOx) U1-yPuyO2 fuels in nitric acid is a critical step in the reprocessing of spent nuclear fuel. In this work, we present the first operando dissolution study of (U,Pu)O2 pellets (MOx) with 11 % Pu content in nitric acid, in which dissolution was periodically interrupted to allow SEM monitoring of microstructural changes at the solid/solution interface. Two pellets differing only in grain size but sharing identical density, homogeneity and O/M ratio were characterized by Raman spectroscopy, EPMA, XRD, HERFD-XANES and SEM-EBSD prior to dissolution. Operando dissolutions were conducted in 3.2 mol L-1 HNO3 at 65°C using ICP-MS monitoring of U and Pu concentrations. Specific intragranular dissolution features (”triangular etch pits”, ”square-based pyramids” and ”step edges with tip form”) were directly correlated to grain orientation observed by EBSD prior dissolution. This confirms that orientation-dependent mechanisms evidenced previously on UO2 single crystals also operate in polycrystalline (U,Pu)O2. More specifically, (100) orientation appears to be the most reactive surface while (111) are the most resistant. Plutonium-enriched grains were identified as inclusions refractory to dissolution by SEM-EDS and confirmed by micro-Raman mapping. No preferential attack at grain boundary was observed.

The dissolution mechanisms of UO2 and α-U3O8-z in 4 mol L-1 nitric acid at 40°C were investigated using X-ray photoelectron spectroscopy (XPS). The U 4f core level spectra revealed the coexistence of several uranium oxidation states at the oxide surfaces during dissolution. For UO2, U(IV), U(V), and U(VI) species were initially detected, whereas only U(V) and U(VI) remained observable once dissolution started, indicating rapid oxidation of U(IV) at the solid surface. In contrast, α-U3O8-z surface exhibited only U(V) and U(VI) throughout dissolution. The results support a dissolution mechanism for UO2 involving successive oxidation at the surface of U(IV) to U(V), then to U(VI), and the release of U(VI) into solution. For α-U3O8-z, dissolution follows a similar pathway, excluding the initial U(IV) to U(V) oxidation step. In both systems, the persistent coexistence of U(V) and U(VI) throughout dissolution indicates that both the oxidation of U(V) and the release of U(VI) contribute to the kinetic control of the process. Although both oxides appear to dissolve through related mechanisms governed by same rate-limiting processes, the evolution of U(V)/U(VI) surfaces ratios differs between UO2 and α-U3O8-z, highlighting distinct rates associated with these dissolution processes.

The oxidation of UO ₂ in Ar–O ₂ atmospheres was studied at temperatures between 350 °C and 600 °C and for oxygen partial pressures (pO ₂ ) between 0.20 atm and 0.70 atm. The experiments were carried out on both powder samples and discs cut from sintered pellets. Oxidation kinetics were monitored by TGA and the oxides formed were characterised by XRD, SEM and specific surface area (SSA) measurements. Whatever the temperature or pO ₂ tested, total oxidation of UO ₂ to U ₃ O ₈ was systematically observed. At all temperatures studied, the conversion of UO ₂ to U ₃ O ₈ seemed to involve the formation of an intermediate oxide, which could be U ₃ O ₇ . Experiments carried out on powders with different SSA appeared to show that the effect of solid texture predominates over the effect of temperature in the formation (or non-formation) of intermediate oxides. An increase in pO ₂ systematically led to an increase in the UO ₂ oxidation rate. On the other hand, an increase in the oxidation temperature seemed to cause a decrease in the density of cracks propagating through the samples, probably due to an increase in the plasticity of U ₃ O ₈ . The inert marker experiment also showed an inward growth of U ₃ O ₈ involving anionic sublattice point defects.