[en] Several CeO₂-based mixed oxides with general composition Ce_1-xLn_xO_2-x/2 (for 0 ≤ x ≤ 1 and Ln = La, Nd, Sm, Eu, Gd, Dy, Er, or Yb) were prepared using an initial oxalic precipitation leading to a homogeneous distribution of cations in the oxides. After characterization of the Ce/Nd oxalate precursors and then thermal conversion to oxides at T 1000 degrees C, investigation of the crystalline structure of these oxides was carried out by XRD μ-Raman spectroscopy. Typical fluorite Fm(3-bar)m structure was obtained for relatively low Ln(III) contents, while a cubic Ia(3-bar) superstructure was evidenced above x ≅ to 0.4. Moreover, since Nd₂O₃ does not crystallize with the Ia(3-bar)-type structure, two-phase systems composed with additional hexagonal Nd₂O₃ were obtained for x(Nd) ≥ 0.73 in the Ce_1-xNdO_2-x/2 series. The effect of heat treatment temperature on these limits was explored through μ-Raman spectroscopy, which allowed determining the presence of small amounts of the different crystal structures observed. In addition, the variation of the Ce_1-xLn_xO_2-x/2 unit cell parameter was found to follow a quadratic relation as a result of the combination between increasing cationic radius, modifications of cation coordination, and decreasing O-O repulsion caused by oxygen vacancies. (authors)

[en] To underline the potential links between the crystallization state and the microstructure of powdered cerium neodymium oxides and their chemical durability, several (Ce_1-xNd_xO_2-x/2) (Nd(IV), Ce(III)) mixed dioxides were prepared in various operating conditions from oxalate precursors and then leached. The powdered samples were first examined through several physicochemical properties (crystallization state and associated crystallite size, reactive surface area, porosity ...). The dependence of the normalized dissolution rates on various parameters (including temperature, nitric acid concentration, crystallization state) was examined for pure CeO₂ and Ce_1-xNd_xO_2-x/2 solid solutions (with x = 0.09 and 0.16). For CeO₂, either the partial order related to the proton activity (n = 0.63) or the activation energy (E_A = 37 kJ . mol^-1) suggested that the dissolution was mainly driven by surface reactions occurring at the solid liquid interface. The chemical durability of the cerium neodymium oxides was also strongly affected by chemical composition. The initial normalized dissolution rates were also found to slightly depend on the crystallization state of the powders, suggesting the role played by the crystal defects in the dissolution mechanisms. On the contrary, the crystallite size had no important effect on the chemical durability. Finally, the normalized dissolution rates measured near the establishment of saturation conditions were less affected, which may be due to the formation of a gelatinous protective layer at the solid/liquid interface. (authors)

[en] The dissolution of Th_1-xCe_xO₂ solid solutions samples prepared by thermal conversion of oxalate precursors was studied by varying independently several parameters (such as chemical composition, leachate acidity, leaching temperature, firing temperature, and crystallization state). The relative effects of these parameters on the normalized dissolution rate were examined. Either the obtained partial order related to the roton activity (n = 0.50 ± 0.01) or the activation energy (E_A = 57 ± 6 kJ.mol^-1) suggested that the dissolution was mainly driven by surface reactions occurring at the solid/liquid interface. Conversely to that observed for Th_1-xU_xO₂ and Ce_1-xNd_xO_2-x/2) solid solutions, the chemical composition did not induce strong modifications of the chemical durability of the leached samples. While the initial normalized dissolution rate slightly depended on the elimination of crystal defects for firing temperatures below 800 degrees C, it was mainly independent of the crystallite size (T ≥ 900 degrees C). The role of crystal defects (then that of preparation conditions) appeared thus important to consider since they contributed to modifications of the normalized dissolution rates of the same order of magnitude than that of the leachate acidity. (authors)

[en] Thoria (ThO₂) presents a great interest for the nuclear fuel cycle. Indeed„ the preparation of Pu-and U- bearing ThO₂ samples was already described in the literature and proposed as advanced fuel materials. Also, ThO₂-based materials incorporating various actinides and/or lanthanides elements are frequently used as redox-free model compounds for UO_x fuels, with the possibility to avoid phenomena linked to the oxidation of uranium (IV) into U (VI). With this aim, we developed several studies aiming to build a transverse approach to the life cycle of ThO₂-based ceramics by bringing together : i) The synthesis of homogenous solid solutions through the development of original wet chemistry routes based on the initial precipitation of low-temperature precursors; ii) The study of ThO₂ sintering at various length scales using in situ observation thanks to Environmental Scanning Electron Microscopy (ESEM); iii) The evaluation of the chemical durability through dissolution tests, with a special focus on the impact of microstructural parameters. (author)

[en] The microstructural evolution of several Ce_1_-_xLn_xO_2_-_x_/_2 (Ln = Nd, Er, x = 0.28; 0.59 and 0.76) sintered pellets was studied during their dissolution in 4 M HNO_3 at 90 C or 60 C. Environmental Scanning Electron Microscopy (ESEM) experiments were developed to follow the changes in the sample microstructure and their consequences on the dissolution evolution by monitoring a constant zone of the material throughout the alteration process. The Ln(III) content was the parameter affecting most of the dissolution kinetics of such solid solutions. From a microstructural point of view, grain boundaries dissolve preferentially during the first dissolution step due to their relative weakness compared to the well-crystallized bulk material. Crystal defects, namely screw and edge dislocations, constitute the second kind of preferential dissolution zone and induce the formation and growth of corrosion pits, mainly of pyramidal shape on the whole surface. Both microstructural parameters are responsible for significant and non-linear increase of the reactive surface developed at the solid/liquid interface and thus important modifications of the topology. On sintered samples and even more clearly on the powdered Ce_0_._7_2Nd_0_._2_8O_1_._8_6 sample, the formation of a gelatinous layer resulting from processes occurring in the back-end of the dissolution process (e.g. local saturation of the leaching solution), was evidenced through ESEM observations. This gel is also presumably responsible for the progressive decrease of the normalized dissolution rate. From these observations, it appears that the study of material dissolution kinetics can clearly benefit from the monitoring of the microstructure evolution and particularly that of the reactive surface area during dissolution. (authors)

[en] This work gives some examples of a transverse approach aiming to describe the life cycle of ThO₂-based ceramics, including synthesis, sintering and dissolution (reprocessing) aspects. First, the synthesis of morphology-controlled thorium oxides (possibly incorporating uranium or lanthanides elements) through original wet chemistry routes will be detailed. The study of ThO₂ sintering at various length scales (typically from the grain to the pellet) using in situ Environmental Scanning Electron Microscopy (ESEM) observations will then be discussed. Finally, the evaluation of the chemical durability of ThO₂-based samples through dissolution tests will be presented, with a special focus on the impact of microstructural parameters. (author)

[en] An accurate method for the characterization of the topographic evolution of materials during dissolution at the submicrometric scale is reported. This method is based on the recording of Environmental SEM tilted images of a selected zone at the surface of a sample for various dissolution times. After each observation, a 3D surface reconstruction was made leading to a series of height maps of the selected zone of interest. One 3D reconstruction was compared to an AFM image of the same zone in order to estimate the accuracy of the heights determined using stereoscopic images. The maximum achievable resolution along the z-axis on the 3D reconstructions was found to be 38 nm. Several micro-structural parameters were extracted from the 3D-image series and their evolutions were related to the dissolution process. This method offers new insights for the monitoring of the changes occurring at the surface of a sample at the micrometer scale during dissolution or corrosion of the material. (authors)

[en] UO₂ pellets were prepared by densification of oxides obtained from the conversion of the oxalate precursor. Then characterized in order to perform a multiparametric study of the dissolution in nitric acid medium. In this frame, for each sample, the densification rate, the grain size and the specific surface area of the prepared pellets were determined prior to the final dissolution experiments. By varying the concentration of the nitric acid solution and temperature, three different and successive steps were identified during the dissolution. Under the less aggressive conditions considered, a first transient step corresponding to the dissolution of the most reactive phases was observed at the solid/solution interface. Then, for all the tested conditions, a steady state step was established during which the normalised dissolution rate was found to be constant. It was followed by a third step characterized by a strong and continuous increase of the normalised dissolution rate. The duration of the steady state, also called 'induction period', was found to vary drastically as a function of the HNO₃ concentration and temperature. However, independently of the conditions, this steady state step stopped at almost similar dissolved material weight loss and dissolved uranium concentration. During the induction period, no important evolution of the topology of the solid/liquid interface was evidenced authorizing the use of the starting reactive specific surface area to evaluate the normalised dissolution rates thus the chemical durability of the sintered pellets. From the multiparametric study of UO₂ dissolution proposed, oxidation of U(IV) to U(VI) by nitrate ions at the solid/liquid interface constitutes the limiting step in the overall dissolution mechanism associated to this induction period. (authors)

[en] Uranium dioxide containing 3 mol% platinum group metals (PGMs) (Ru, Rh, Pd) was synthesized by hydroxide precipitation. The powders were converted to oxides, pelletized and sintered to prepare dense pellets of UO₂ incorporating PGMs particles. The characterization techniques performed revealed a microstructure similar to that of spent nuclear fuel (SNF). Dissolution tests in nitric acid demonstrated that in the presence of PGMs, the uranium dissolution rate was increased and the induction period was shortened. We used a new method based on the acquisition of Environmental Scanning Electron Microscopy (ESEM) micrographs recorded at three tilt angles and at different dissolution times. This method allowed the reconstruction of the topography of the solid/liquid interface. By monitoring the evolution of the solid/liquid interface during dissolution by means of 3D reconstructions, we were able to observe preferential dissolution zones in the vicinity of the PGMs particles and to determine microscopic dissolution rates for several regions of interest. PGMs particles were found mainly at the grain boundaries. In 0.1 M HNO₃ solution at 60 degrees C, the normalized dissolution rate for uranium at the grain boundaries reached R_L(U) = (7 ±1) x 10^-2 gm^-2.d^-1, a value similar to the normalized dissolution rate determined for the whole image over the first 30 days of the experiment. This result showed that the dissolution occurred mainly at the UO₂ grain boundaries in the vicinity of PGMs particles. Furthermore, the 3D reconstructions of the solid/liquid interface were used to determine the evolution of the surface area of the pellet. By combining the weight losses determined at the macroscopic scale using the uranium concentrations in solution with the reactive surface area values, it was possible to estimate an effective normalized dissolution rate for the whole pellet. (authors)

[en] Millimetre UO₂ single crystals were cut and oriented at JRC Karlsruhe. The orientation of each face of the parallelepiped single crystals was determined with Laue diffraction and the corresponding surface area by geometric measurements. Then, the (111) (100) (110) faces of each single crystal were polished to optical grade and characterized by XRD in order to confirm the surface orientation. The dissolution of the three single crystals was achieved in nitric acid media under dynamic conditions, at room temperature. Two dissolution regimes were observed for all samples. The normalized dissolution rate measured in the first step was not influenced by the crystallographic orientation of the faces. However, during the second step (110) oriented faces were found to dissolve 4 times faster than the (100) faces. One explanation could involve the atomic composition of each oriented surface in the fluorite-type structure. (authors)