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

International audienc

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

International audienc

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