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Although uranium-cerium dioxides are frequently used as a surrogate material for (U,Pu)O2-δ nuclear fuels, there is currently no reliable data regarding the oxygen stoichiometry and redox speciation of the cations in such samples. In order to fill this gap, this manuscript details a synchrotron study of highly homogeneous (U,Ce)O2±Î´ sintered samples prepared by a wet-chemistry route. HERFD-XANES spectroscopy led to determining accurately the O/M ratios (with M = U + Ce). Under a reducing atmosphere (pO2 ≈ 6 × 10-29 atm at 650 °C), the oxides were found to be close to O/M = 2.00, while the O/M ratio varied with the sintering conditions under argon (pO2 ≈ 3 × 10-6 atm at 650 °C). They globally appeared to be hyperstoichiometric (i.e., O/M > 2.00) with the departure from the dioxide stoichiometry decreasing with both the cerium content in the sample and the sintering temperature. Nevertheless, such a deviation from the ideal O/M = 2.00 ratio was found to generate only moderate structural disorder from EXAFS data at the U-L3 edge as all the samples retained the fluorite-type structure of the UO2 and CeO2 parent compounds. The determination of accurate lattice parameters owing to S-PXRD measurements led to complementing the data reported in the literature by various authors. These data were consistent with an empirical relation linking the unit cell parameter, the chemical composition, and the O/M stoichiometry, showing that the latter can be evaluated simply within a ± 0.02 uncertainty.
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We report here a detailed experimental and theoretical investigation of hexavalent uranium in various local configurations with a high-energy-resolution fluorescence-detected X-ray absorption near-edge structure at the U M4 edge. We show the pronounced sensitivity of the technique to the arrangement of atoms around the absorber and provide a detailed theoretical interpretation revealing the nature of spectral features. Calculations based on density functional theory and on crystal field multiplet theory indicate that for all local configurations analyzed, the main peak corresponds to nonbonding 5f orbitals, and the highest energy peak corresponds to antibonding 5f orbitals. Our findings agree with the accepted interpretation of uranyl spectral features and embed the latter in a broader field of view, which interprets the spectra of a large variety of U6+-containing samples on a common theoretical ground.
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In view of safe management of the nuclear wastes, a sound knowledge of the atomic-scale properties of U1-xMxO2+y nanoparticles is essential. In particular, their cation valences and oxygen stoichiometries are of great interest as these properties drive their diffusion and migration behaviors into the environment. Here, we present an in-depth study of U1-xCexO2+y, over the full compositional domain, by combining X-ray diffraction and high-energy resolution fluorescence detection X-ray absorption near-edge structure. We show, on one hand, the coexistence of UIV, UV, and UVI and, on the other hand, that the fluorite structure is maintained despite this charge distribution.
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Neutron diffraction, X-ray absorption spectroscopy (XAS), and Raman spectroscopy measurements of the quaternary perovskite phase Ba2NaMoO5.5 have been performed in this work. The cubic crystal structure in space group Fm3Ì m has been refined using the Rietveld method. X-ray absorption near-edge structure spectroscopy (XANES) measurements at the Mo K-edge have confirmed the hexavalent state of molybdenum. The local structure of the molybdenum octahedra has been studied in detail using extended X-ray absorption fine structure (EXAFS) spectroscopy. The Mo-O and Mo-Ba distances have been compared to the neutron diffraction data with good agreement. The coefficient of thermal expansion measured in the temperature range of 303-923 K, using high temperature X-ray diffraction (HT-XRD) (αV = 55.8 × 10-6 K), has been determined to be â¼2 times higher than that of the barium molybdates BaMoO3 and BaMoO4. Moreover, no phase transition nor melting have been observed, neither by HT-XRD nor Raman spectroscopy nor differential scanning calorimetry, up to 1473 K. Furthermore, the standard enthalpy of formation (ΔfHm°) for Ba2NaMoO5.5(cr) has been determined to be -(2524.75 ± 4.15) kJ mol-1 at 298.15 K, using solution calorimetry. Finally, the margin for safe operation of sodium-cooled fast reactors (SFRs) has been assessed by calculating the threshold oxygen potential needed, in liquid sodium, to form the quaternary compound, following an interaction between irradiated mixed oxide (U,Pu)O2 fuel and sodium coolant.
RESUMEN
Hydrothermal conversion of thorium oxalate, Th(C2O4)2·nH2O, into thorium dioxide was explored through a multiparametric study, leading to some guidelines for the preparation of crystallized samples with the minimum amount of impurities. As the formation of the oxide appeared to be operated through the hydrolysis of Th4+ after decomposition of oxalate fractions, pH values typically above 1 must be considered to recover a solid phase. Also, because of the high stability of the thorium oxalate precursor, hydrothermal treatments of more than 5 h at a temperature above 220 °C were required. All the ThO2·nH2O samples prepared presented amounts of residual carbon and water in the range 0.2-0.3 wt % and n ≈ 0.5, respectively. A combined FTIR, PXRD, and EXAFS study showed that these impurities mainly consisted of carbonates trapped between elementary nanosized crystallites, rather than substituted directly in the lattice, which generated a tensile effect over the crystal lattice. The presence of carbonates at the surface of the elementary crystallites could also explain their tendency to self-assembly, leading to the formation of spherical aggregates. Hydrothermal conversion of oxalates could then find its place in different processes of the nuclear fuel cycle, where it will provide an interesting opportunity to set up dustless routes leading from ions in solution to dioxide powders in a limited number of steps.
RESUMEN
The formation of a thin layer, the so-called Joint Oxyde-Gaine (JOG), between the (U,Pu)O2 fuel pellets and the cladding has been observed in fast neutron reactors, due to the accumulation of volatile fission products. Cs2MoO4 is known to be one of the major components of the JOG, but other elements are also present, in particular tellurium and palladium. In this work, an investigation of the structural and thermodynamic properties of Cs2TeO4 and Cs2Mo1-xTexO4 solid solution is reported. The existence of a complete solubility between Cs2MoO4 and Cs2TeO4 is demonstrated, combining X-ray diffraction (XRD), neutron diffraction (ND), and X-ray absorption spectroscopy (XAS) results. High-temperature XRD measurements were moreover performed on Cs2TeO4, which revealed the existence of a α-ß phase transition around 712 K. Thermal expansion coefficients were also obtained from these data. Finally, phase equilibra points in the Cs2MoO4-Cs2TeO4 pseudobinary phase diagram were collected using differential scanning calorimetry and used to develop a thermodynamic model for this system using a regular solution formalism.
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The thermal decomposition of actinide oxalates is greatly dependent on the oxidation state of the cation, the gas involved, and the physical characteristics of the precursor. In the actinides series, uranium(IV) oxalate U(C2O4)2·6H2O can be viewed as a peculiar case, as its sensibility toward oxidation leads to a specific series of reactions when heating under an oxygen atmosphere. In order to clarify the disagreements existing in the literature, particularly concerning potential carbonate intermediates and the possible transitory existence of UO3, we show here an extended characterization of the different intermediates through a combination of X-ray diffraction, vibrational spectroscopies and X-ray absorption near-edge spectroscopy. In this frame, uranium oxidation was found to occur at low temperature (200 °C) concomitantly to the onset of oxalate groups decomposition, leading to an amorphous oxo-oxalato compound. Pursuing the thermal conversion up to 350 °C led to complete oxidation of U(IV) into U(VI), then to the formation of amorphous UO3 still bearing adsorbed carbonates. The first pure oxide formed during the thermal conversion was further identified to substoichiometric UO3-δ after heating at 550 °C. Finally, U3O8 was obtained as the final stable phase after heating above 660 °C. The mechanism of thermal conversion of uranium(IV) oxalate into oxide under oxygen is then driven by a complex interplay between redox reactions and decomposition of the organic fractions. Such chemical reactions were also found to significantly modify the morphology of the powder through high-temperature environmental scanning electron microscopy observations: decomposition led to a 20% reduction in the size of the aggregates, while uranium oxidation clearly promoted growth within the agglomerates.
RESUMEN
Within the development of future nuclear reactors, wet chemistry routes have been investigated for the fabrication of advanced oxide fuels. In this frame, a multiparametric study focused on the hydrothermal conversion of uranium(IV) oxalate U(C2O4)2·nH2O into uranium oxides was undertaken in order to unravel the effects of temperature, pH, and kinetics. For pH ≤ 1, the lowest temperatures explored (typically from 180 to 200 °C) led to stabilized UO2+x/U4O9 mixtures exhibiting a global O/U ratio evaluated as 2.38 ± 0.10 from U M4-edge HERFD-XANES experiments. Higher temperatures (220-250 °C) led the oxide stoichiometry to decrease down to 2.13 ± 0.04 which corresponds to a lower fraction of U4O9 in the mixture. Additionally, increasing the temperature of the hydrothermal treatment efficiently improved the elimination of residual carbon species and water. Hydrothermal conversion of U(C2O4)2·nH2O also led to a drastic modification of the powders morphology. With this aim, pH tuning could be used to shift from bipyramidal aggregates (up to pH 1) to microspheres (2 ≤ pH ≤ 5) and then to nanometric powders (pH > 5). Finally, a kinetics study showed that uranium oxides can be obtained from the hydrothermal decomposition of oxalate within only few hours. If the samples collected early during the treatment always presented the characteristic XRD lines of UO2+x/U4O9 fluorite-type structure, then they were found to be strongly oxidized (O/U = 2.65 ± 0.14) which suggested the existence of a U(VI)-bearing amorphous secondary phase. The latter further tended to reduce through time. Hydrothermal conversion then probably proceeds as a two-step mechanism composed by the oxidative decomposition of uranium(IV) oxalate followed by the reduction of uranium by organic moieties and its hydrolysis. It appears as an easy and efficient way to yield highly pure uranium oxide samples in solution.
RESUMEN
For nuclear fuel related applications, the oxygen stoichiometry of mixed oxides U1-yMyO2±x is an essential property as it affects fuel properties and may endanger the safe operation of nuclear reactors. A careful review of the open literature indicates that this parameter is difficult to assess properly and that the nature of the defects, i.e., oxygen vacancies or UV, in aliovalent cation-doped UO2 is still subject to controversy. To confirm the formation of UV, we have investigated the room-temperature stable U1-yLayO2±x phase using several experimental methods (e.g., XRD, XANES, and NMR) confirmed by theoretical calculations. This paper presents the experimental proof of UV and its effect we identified in both electronic and local structure. We observe that UV is formed in quasi-equimolar proportion as LaIII in U1-yLayO2±x (y = 0.06, 0.11, and 0.22) solid solutions. The fluorite structure is maintained despite the cationic substitution, but the local structure is affected as variations of the interatomic distances are found. Therefore, we provide here the definitive proof that the substitution of UIV with LaIII is not accommodated by the creation of O vacancies as has often been assumed. The UO2 fluorite structure compensates the incorporation of an aliovalent cation by the formation of UV in quasi-equimolar proportions.
RESUMEN
Most materials expand with temperature because of the anharmonicity of lattice vibration, and only a few shrink with increasing temperature. UO2, whose thermal properties are of significant importance for the safe use of nuclear energy, was considered for a long time to belong to the first group. This view was challenged by recent in situ synchrotron X-ray diffraction measurements, showing an unusual thermal decrease of the U-O distances. This thermal shrinkage was interpreted as a consequence of the splitting of the U-O distances due to a change in the U local order from Fm3Ì m to Pa3Ì . In contrast to these previous investigations and using an element-specific synchrotron-based spectroscopic method, we show here that the U sublattice remains locally of the fluorite type from 50 to 1265 K, and that the decrease of the first U-O bond lengths is associated with an increase of the disorder.
RESUMEN
In the frame of minor actinide transmutation, americium can be diluted in UO2 and (U, Pu)O2 fuels burned in fast neutron reactors. The first mandatory step to foresee the influence of Am on the in-reactor behavior of transmutation targets or fuel is to have fundamental knowledge of the Am-O binary system and, in particular, of the AmO2-x phase. In this study, we coupled HT-XRD (high-temperature X-ray diffraction) experiments with CALPHAD thermodynamic modeling to provide new insights into the structural properties and phase equilibria in the AmO2-x-AmO1.61+x-Am2O3 domain. Because of this approach, we were able for the first time to assess the relationships between temperature, lattice parameter, and hypostoichiometry for fcc AmO2-x. We showed the presence of a hyperstoichiometric existence domain for the bcc AmO1.61+x phase and the absence of a miscibility gap in the fcc AmO2-x phase, contrary to previous representations of the phase diagram. Finally, with the new experimental data, a new CALPHAD thermodynamic model of the Am-O system was developed, and an improved version of the phase diagram is presented.
RESUMEN
The electronic structure of U(V) - and U(VI) -containing uranates NaUO3 and Pb3 UO6 was studied by using an advanced technique, namely X-ray absorption spectroscopy (XAS) in high-energy-resolution fluorescence-detection (HERFD) mode. Due to a significant reduction in core-hole lifetime broadening, the crystal-field splittings of the 5f shell were probed directly in HERFD-XAS spectra collected at the U 3d edge, which is not possible by using conventional XAS. In addition, the charge-transfer satellites that result from U 5f-O 2p hybridization were clearly resolved. The crystal-field parameters, 5f occupancy, and degree of covalency of the chemical bonding in these uranates were estimated by using the Anderson impurity model by calculating the U 3d HERFD-XAS, conventional XAS, core-to-core (U 4f-3d transitions) resonant inelastic X-ray scattering (RIXS), and U 4f X-ray photoelectron spectra. The crystal field was found to be strong in these systems and the 5f occupancy was determined to be 1.32 and 0.84 electrons in the ground state for NaUO3 and Pb3 UO6 , respectively, which indicates a significant covalent character for these compounds.
RESUMEN
We report and discuss here the unambiguous uranium valence state determination on the complex compound [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] by using high-energy-resolution fluorescence detection-X-ray absorption near-edge structure spectroscopy (HERFD-XANES). The spectra at both U L3- and M4-edges confirm that all five nonequivalent U atoms are solely in the hexavalent form in this compound, as previously suggested by bond-valence-sum analysis and X-ray diffraction pattern refinement. Moreover, the presence of the preedge feature, due to the 2p3/2-5f quadrupole transition, has been observed in the U L3-edge HERFD-XANES spectrum, in agreement with theoretical and experimental observations of other uranium-based compounds. Recently, this feature has been proposed as a possible tool to determine the uranium oxidation state in a manner similar to that of 3d and 4d metals. Nevertheless, this feature is also very sensitive to the uranium local environment, as revealed by our theoretical calculations, and consequently could not be used to attribute without ambiguity the uranium valence state. In contrast, U M4-edge HERFD-XANES appears to be the most straightforward and reliable way to assess the uranium valence state in very complex materials such as [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] or a mixture of compounds.
RESUMEN
Innovative americium-bearing uranium-plutonium mixed oxides U1-yPuyO2-x are envisioned as nuclear fuel for sodium-cooled fast neutron reactors (SFRs). The oxygen-to-metal (O/M) ratio, directly related to the oxidation state of cations, affects many of the fuel properties. Thus, a thorough knowledge of its variation with the sintering conditions is essential. The aim of this work is to follow the oxidation state of uranium, plutonium, and americium, and so the O/M ratio, in U0.750Pu0.246Am0.004O2-x samples sintered for 4 h at 2023 K in various Ar + 5% H2 + z vpm H2O (z = â¼ 15, â¼ 90, and â¼ 200) gas mixtures. The O/M ratios were determined by gravimetry, XAS, and XRD and evidenced a partial oxidation of the samples at room temperature. Finally, by comparing XANES and EXAFS results to that of a previous study, we demonstrate that the presence of uranium does not influence the interactions between americium and plutonium and that the differences in the O/M ratio between the investigated conditions is controlled by the reduction of plutonium. We also discuss the role of the homogeneity of cation distribution, as determined by EPMA, on the mechanisms involved in the reduction process.
RESUMEN
Valence-to-core resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detection (HERFD) X-ray absorption measurements were performed at the U L3 edges of UO2 and UO2(NO3)2(H2O)6. The results are compared with model calculations based on the local-density-approximation formalism, taking into account Coulomb interaction U (LDA + U). We show that despite strong 5f-5f electronic correlations in the studied systems and the use of core-level excitations in the intermediate stage of the spectroscopic process, the RIXS technique probes a convolution of the single-particle densities of states in the valence and conduction bands. For UO2, the detected crystal-field splitting between the U 6d eg and t2g orbitals from the RIXS spectra (â¼3.5 eV) is larger than that previously derived from optical spectroscopy. Furthermore, by using an example of the U0.75Pu0.25O2 mixed oxide, we show that the RIXS technique at the U L3 edges is sensitive to the substitution of U with other actinide, in contrast to conventional X-ray absorption methods. That is, due to changes in the occupied part rather than in the unoccupied part of the U 6d states caused by the substitution.
RESUMEN
Uranium-plutonium mixed oxide containing 30% of plutonium is a candidate fuel for several fast neutron and accelerator driven reactor systems. In this work, a detailed structural investigation on sol-gel synthesized stoichiometric U0.7Pu0.3O2.00 and substoichiometric U0.7Pu0.3O2-x, using X-ray diffraction (XRD), oxygen 17 magic angle spinning nuclear magnetic resonance ((17)O MAS NMR) and X-ray absorption spectroscopy is described. As observed by XRD, the stoichiometric U0.7Pu0.3O2.00 is monophasic with a lattice parameter in good agreement with Vegard's law, while the substoichiometric U0.7Pu0.3O2-x material is biphasic. Solid solution ideality in terms of a random distribution of metal atoms is proven for U0.7Pu0.3O2.00 with (17)O MAS NMR. X-ray absorption near-edge structure (XANES) spectroscopy shows the presence of plutonium(III) in U0.7Pu0.3O2-x. Extended X-ray absorption fine-structure (EXAFS) spectroscopy indicates a similar local structure around both cations, and comparison with XRD indicates a close similarity between uranium and plutonium local structures and the long-range ordering.
RESUMEN
In U(1-x)Am(x)O(2±Î´) compounds with low americium content (x ≤ 20 atom %) and oxygen-to-metal (O/M) ratios close to 2.0, Am(III+) cations are charge-balanced by an equivalent amount of U(V+) cations while the fluorite structure of pure U(IV+)O2 is maintained. Up to now, it is unknown whether this observation also holds for higher americium contents. In this study, we combined X-ray diffraction with Raman and X-ray absorption spectroscopies to investigate a U(0.5)Am(0.5)O(2±Î´) compound. Our results indicate that americium is again only present as Am(III+), while U(V+) remains below the amount required for charge balance. Unlike lower americium contents, this leads to an overall oxygen hypostoichiometry with an average O/M ratio of 1.92(2). The cationic sublattice is only slightly affected by the coexistence of large amounts of reduced (Am(III+)) and oxidized (U(V+)) cations, whereas significant deviations from the fluorite structure are evidenced by both extended X-ray absorption fine structure and Raman spectroscopies in the oxygen sublattice, with the observation of both vacancies and interstitials, the latter being apparently consistent with the insertion of U6O12 cuboctahedral-type clusters (as observed in the U4O9 or U3O7 phases). These results thus highlight the specificities of uranium-americium mixed oxides, which behave more like trivalent lanthanide-doped UO2 than U(1-x)Pu(x)O(2±Î´) MOX fuels.
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Uranium-americium mixed oxides could be used as transmutation targets to lower Am inventory in spent nuclear fuels. Due to (241)Am activity, these materials are subjected to α-self-irradiation which provokes crystallographic disorder. Previous studies on U-Am mixed oxides gave first insight into α-radiation tolerance of these compounds, but have never been carried out for more than a year, whereas these compounds might be stored up to a few years between fabrication and irradiation. In this work, we study effects of self-irradiation on the structure of U(1-x)Am(x)O(2±Î´) solid solutions (x = 0.15 and 0.20) aged 3 to 4 years. Especially, X-ray diffraction and X-ray absorption spectroscopy are combined to observe these effects from both long-range and local perspectives. Results show that the fluorite-type structure of U-Am mixed oxides withstands (241)Am α-irradiation without major damage. Despite the increase of interatomic distances and crystallographic disorder observed during the first months of storage, the present results show that a steady state is then reached. Thus, no detrimental factors have been identified in this study in terms of structural damage for several-year storage of U(1-x)Am(x)O(2±Î´) pellets before irradiation. Furthermore, comparison between long-range and local evolution suggests that α-self-irradiation-induced defects are mainly located in low-ordered domains. Based on literature data and present results, the steady state appears related to the equilibrium between radioinduced defect formation and material self-healing.
RESUMEN
The oxygen to metal ratio (O/M) is directly related to oxygen potential, which strongly influences the sintering and irradiation performance of nuclear fuels. A better understanding of these two parameters is therefore of major interest. To further ascertain the correlation between O/M ratio and oxygen potential in Am-bearing MOX, several thermodynamic descriptions are being developed. Despite their differences, they all involve the valence of actinide cations (e.g., U, Pu, and Am) as essential parameters. However, as no experimental data on their valence are available, these models rely on assumptions. In the present work, we coupled X-ray diffraction and X-ray absorption spectroscopy to follow the behavior of Pu and Am in three hypo-stoichiometric, U-free Pu(1-y)Am(y)O(2-x) compounds. We provide for the first time a quantitative determination of Pu and Am valences, demonstrating that plutonium reduction from Pu(4+) to Pu(3+) starts only when americium reduction from Am(4+) to Am(3+) is completed. This result fills in an important gap in experimental data, thereby improving the thermodynamic description of nuclear fuels. At last, we suggest that the O/M ratio may evolve at room temperature, especially for high Am content, which is of main concern for the fabrication of Am-loaded MOX and their storage prior to irradiation.
RESUMEN
Partitioning and transmutation (P&T) of minor actinides (MA) is currently studied to reduce the nuclear waste inventory. In this context, the fabrication of MA bearing materials is of great interest to achieve an effective recycling of these highly radioactive elements. To ensure the in-pile behavior, nuclear oxide fuels have to respect several criteria including preservation of the fluorite structure and defined oxygen to metal ratio (O/M). In the case of Am bearing materials, such as U(1-y)Am(y)O(2±x) (y = 0.10, 0.15, 0.20), the O/M determination is quite challenging using conventional methods (TGA, XRD) because of the particular thermodynamic properties of Am. Despite the lack of experimental data in the U-Am-O system, thermodynamical models are currently developed to effectively assess the O/M ratio. In this work, the O/M ratios were calculated for various oxygen potentials using the cation molar fraction determined by XAS measurements. These results are an important addition to the experimental data available for the U-Am-O system. Moreover, XRD and XAS indicated that the fabrication of fluorite U(1-y)Am(y)O(2±x) solid solution was achieved for all Am content and oxygen potentials investigated. On the basis of the molar fraction, a description of the solid solution was proposed depending on the considered sintering conditions. Finally, the occurrence of an unexpected charge compensation mechanism was pointed out.