Contribution of Energetically Reactive Surface Features to the Dissolution of CeO2 and ThO2 Analogues for Spent Nuclear Fuel Microstructures

In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater. There remain several uncertainties associated with understanding spent fuel dissolution, including the contribution of energetically reactive surface sites to the dissolution rate. In this study, we investigate how surface features influence the dissolution rate of synthetic CeO2 and ThO2, spent nuclear fuel analogues that approximate as closely as possible the microstructure characteristics of fuel-grade UO2 but are not sensitive to changes in oxidation state of the cation. The morphology of grain boundaries (natural features) and surface facets (specimen preparation-induced features) was investigated during dissolution. The effects of surface polishing on dissolution rate were also investigated. We show that preferential dissolution occurs at grain boundaries, resulting in grain boundary decohesion and enhanced dissolution rates. A strong crystallographic control was exerted, with high misorientation angle grain boundaries retreating more rapidly than those with low misorientation angles, which may be due to the accommodation of defects in the grain boundary structure. The data from these simplified analogue systems support the hypothesis that grain boundaries play a role in the so-called “instant release fraction” of spent fuel, and should be carefully considered, in conjunction with other chemical effects, in safety performance assessements for the geological disposal of spent fuel. Surface facets formed during the sample annealing process also exhibited a strong crystallographic control and were found to dissolve rapidly on initial contact with dissolution medium. Defects and strain induced during sample polishing caused an overestimation of the dissolution rate, by up to 3 orders of magnitude

La résistance à la sécheresse du soja II. - Etude des réactions variétales à un déficit hydrique

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La résistance à la sécheresse du soja I. - Influence d'un déficit hydrique sur la croissance et la production

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Dissolution of (U,Th)O2_2 heterogeneous mixed oxides

International audienceHeterogeneous U$_{1-x}$Th$_x$O$_2$ polished pellets were prepared through sintering of oxides obtained from hydroxide precursors. Mixing several homogeneous U$_{1-x}$Th$_x$O$_2$ powders with various stoichiometries allowed the preparation of a panel of heterogeneous sintered pellets with a matrix-inclusions type microstructure. The characterization of the polished pellets was performed by X-EDS. From image analysis of large cartographies of the pellets, the distribution and amount of each type of heterogeneity was evaluated. To follow the chemical durability of the prepared materials, the pellets were submitted to dissolution tests in 2 and 4 mol.L$^{-1}$ HNO$_3$ at 60 and 90 °C. The obtained dissolution rates of the materials highlighted the need to access a more detailed microscopic study due to the presence of enriched dissolution residues at the end of the experiments. The heterogeneities were simulated by using powders heated at 1600 °C to isolate their specific dissolution behaviour. The sintered powders surrogates dissolved extremely slowly in solution. However, the normalized dissolution rates increased when HNO$_2$ catalytic species were introduced in the solution. Another set of experiments obtained by varying the incorporation rates in heterogeneities showed a decrease of the dissolution rate with increasing incorporation rate of Th-enriched heterogeneities. This effect is counterintuitive as Th-enriched heterogeneities are not supposed to dissolve during the first instants of the dissolution. Microscopic study based on operando monitoring of the solid/liquid interface highlighted preferential dissolution zones with a dissolution front in the matrix and the refractory character of the heterogeneities. Finally, the collected residues of dissolution were also characterized. They showed slight dissolution progress with the formation of dissolution pits, preferential attacks of the grain boundaries and a mechanism resembling the cracking core model of dissolution. This work offers a first study of the dissolution of heterogeneous U$_{1-x}$Th$_x$O$_2$ mixed oxides with a matrix-inclusion type microstructure

Effect of the fission products on the kinetics of dissolution of uranium dioxide

International audienceStudies dedicated to the chemical durability of spent nuclear fuel are usually based on the overall inventory of elements present in solution under various dissolution conditions. However, these studies do not allow identifying and quantifying the role of the different phases present in the solid on the dissolution rates. Therefore, it is necessary to evidence separately the effect of the main fission products incorporated in the uranium dioxide structure or precipitated at grain boundaries on the evolution of the microstructure and the dissolution rates. First of all, different model compounds containing fission products were synthesized from oxalic and hydroxide precursors [1]. Uranium dioxide based compounds with lanthanides molar ratio in the range 1 to 10 mol. % were prepared (with 13 % La; 7.4 % Y; 25.6 % Ce; 12 % Pr; 42 % Nd). Divalent (Ba, Sr) or monovalent (Cs, Rb) cations were also incorporated separately in the UO2 matrix. Finally, samples of uranium dioxide containing 0.6 to 3 mol. % of precipitated platinoids (with 55 % Ru; 9.6 % Rh; 35.4 % Pd) were also obtained. Then, the starting precursors were converted into oxides, pelletized and sintered. The aim of these syntheses and heat treatments was to obtain pellets with fission products amounts, structure and microstructure representative of spent nuclear fuel. Different characterizations have been done. On the one hand, concerning Uranium dioxide based compounds with lanthanides, XRD analyzes allowed to identify the crystal lattice as the CaF2 fluorite structure Pm3 et773;m, characteristic of (+IV) uranium dioxide. On the other hand, concerning the other kinds of fission products, an additional phase was present in addition of the CaF2 fluorite structure Pm3 et773;m. Furthermore to control the total incorporated amount of fission products in the UO2 matrix, X-EDS analyzes on oxides and ICP-AES analyzes of oxides total dissolution were done. A multiparametric study of the dissolution was achieved in concentrate nitric acid in order to simulate the reprocessing conditions and to highlight the effect of the fission products on the overall dissolution rates. Furthermore, the microstructural evolution of the pellets during the dissolution was monitored by ESEM. The aim of the study was to identify the preferential dissolution zones and to allow a better understanding of the impact of the fission products on the dissolution mechanisms