Hydrothermal Conversion of Uranium(IV) Oxalate into Oxides: A Comprehensive Study

Research progress on the analysis and application of radioactive hot particle

Radioactive hot particle is the particulate form of nuclear material that exists in the environment. The U, Pu, Am, Cs, and other radionuclides isotope in the hot particle contain abundant and accurate fingerprint information, such as the origin and age of the nuclear material. The acquisition and analysis of the key information in the hot particle can be equivalent to the analysis of bulk nuclear material, which could directly reflect the real situation of nuclear activities. Therefore, the single particle analysis of hot particles has become an irreplaceable key technology in nuclear safeguards inspection. The rapid identification, screening, locating, and accurate isotope analysis of hot particles from a large number of particles dispersed in environmental media or on the surface of other materials are one of the most important research field in nuclear emergency. In this review, the research process of the analytical methods for hot particles in the last decade was summarized, including the physical character of hot particles, and the techniques of localization, screening, and extraction of hot particles. Furthermore, we also focused on the mass spectrometry technology for the analysis of hot particle. The advantages and disadvantages of the most used mass spectrometry were summarized. Finally, the research trend for hot particle analysis methods was proposed.

Microscale Electrochemical Corrosion of Uranium Oxide Particles

Understanding the corrosion of spent nuclear fuel is important for the development of long-term storage solutions. However, the risk of radiation contamination presents challenges for experimental analysis. Adapted from the system for analysis at the liquid-vacuum interface (SALVI), we developed a miniaturized uranium oxide (UO2)-attached working electrode (WE) to reduce contamination risk. To protect UO2 particles in a miniatured electrochemical cell, a thin layer of Nafion was formed on the surface. Atomic force microscopy (AFM) shows a dense layer of UO2 particles and indicates their participation in electrochemical reactions. Particles remain intact on the electrode surface with slight redistribution. X-ray photoelectron spectroscopy (XPS) reveals a difference in the distribution of U(IV), U(V), and U(VI) between pristine and corroded UO2 electrodes. The presence of U(V)/U(VI) on the corroded electrode surface demonstrates that electrochemically driven UO2 oxidation can be studied using these cells. Our observations of U(V) in the micro-electrode due to the selective semi-permeability of Nafion suggest that interfacial water plays a key role, potentially simulating a water-lean scenario in fuel storage conditions. This novel approach offers analytical reproducibility, design flexibility, a small footprint, and a low irradiation dose, while separating the α-effect. This approach provides a valuable microscale electrochemical platform for spent fuel corrosion studies with minimal radiological materials and the potential for diverse configurations.

Production of mixed element actinide reference particulates to support nuclear safeguards using THESEUS, an aerosol-based particulate synthetic methodology

The THermally Evaporated Spray for Engineered Uniform particulateS (THESEUS) production platform was developed to generate highly uniform mixed actinide oxide particles. The particulate synthesis platform builds on previous efforts and utilizes an aerosol-based technology to generate, calcine, characterize, and aggregate a monodisperse oxide phase particle product. In this study, particles comprised of uranium oxide, incorporated with varying compositions of thorium, were produced. Th/U test materials with ²³²Th concentrations between 1 ppm and 10%, ratioed to ²³⁸U, were successfully generated with in situ calcination at 600 °C and characterized by in situ aerodynamic particle size spectrometry and ex situ microanalytical methods. Populations of monodisperse particulates (geometric standard deviation - GSD < 1.15) with an average diameter near 1 μm were generatated and micro-Raman spectroscopy of individual particles identified U₃O₈ as the primary material phase for the range of Th/U samples analyzed. Single particle measurements and automated particle analyses by secondary ion mass spectrometry (SIMS) were performed. Uniform inter-particle elemental and isotopic homogeneity for uranium and thorium isotopes was characterized by SIMS, and a ²³²Th/²³⁸U relative sensitivity factor of 0.53 was determined. SIMS results demonstrated differences in the ²³²Th/²³⁸U profiling behavior for Th/U particulates with increased Th content (>1%). Despite the observed profiling behavior, single particle measurements of the 10% Th sample indicate inter-particle homogeneity. This work represents the first systematic study of Th/U microparticulate reference materials generated and intended for nuclear safeguards applications and serves as a demonstration of THESEUS to support a sustained capability for the production mixed-element particulate reference materials.

Influence of Sintering Conditions on the Structure and Redox Speciation of Homogeneous (U,Ce)O2+δ Ceramics: A Synchrotron Study

Although uranium-cerium dioxides are frequently used as a surrogate material for (U,Pu)O_2-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)O_2±δ 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.

Grain growth of NpO2 and UO2 nanocrystals

We report on the crystallite growth of nanometric NpO₂ and UO₂ powders. The AnO₂ nanoparticles (An = U and Np) were synthesized by hydrothermal decomposition of the corresponding actinide(iv) oxalates. NpO₂ powder was isothermally annealed between 950 °C and 1150 °C and UO₂ between 650 °C and 1000 °C. The crystallite growth was then followed by high-temperature X-ray diffraction (HT-XRD). The activation energies for the growth of crystallites of UO₂ and NpO₂ were determined to be 264(26) kJ mol^-1 and 442(32) kJ mol^-1, respectively, with a growth exponent n = 4. The value of the exponent n and the low activation energy suggest that the crystalline growth is rate-controlled by the mobility of the pores, which migrate by atomic diffusion along the pore surfaces. We could thus estimate the cation self-diffusion coefficient along the surface in UO₂, NpO₂ and PuO₂. While data for surface diffusion coefficients for NpO₂ and PuO₂ are lacking in the literature, the comparison with literature data for UO₂ supports further the hypothesis of a surface diffusion controlled growth mechanism.

Ultrasonically controlled synthesis of UO2+x colloidal nanoparticles

Actinide colloids and nanoparticles (NPs) currently constitute a topic of strong interest due to their potential role in advanced nuclear energetics and the environmental migration of radioactivity. A better understanding of the physico-chemical properties of nanoscale actinide oxides requires robust synthesis approaches. In this work, UO_2+x NPs were successfully prepared by sonochemistry from U(IV) solutions previously stabilised in a hydrochloric medium (20 kHz, 65 °C, Ar/(10%)CO). Colloidal suspensions were found to be composed of crystalline and spherical NPs showing a UO₂-like structure and measuring 18.0 ± 0.1 nm (SAXS, HR-TEM and PXRD techniques). In comparison with the controlled hydrolysis approach used as a reference, sonochemistry appears to be a simple and original synthesis route providing larger, better defined and more crystalline UO_2+x NPs with a narrower size distribution. These well-defined NPs offer new opportunities for the preparation of reference actinide materials devoted to fundamental, technological and environmental studies.

Synthesis of Nanocrystalline PuO2 by Hydrothermal and Thermal Decomposition of Pu(IV) Oxalate: A Comparative Study

In recent years, the hydrothermal conversion of actinide (IV) oxalates into nanometric actinide dioxides (AnO₂) has begun to be investigated as an alternative to the widely implemented thermal decomposition method. We present here a comparison between the hydrothermal and the conventional thermal decomposition of Pu(IV) oxalate in terms of particle size, morphology and residual carbon content. A parametric study was carried out in order to define the temperature and time applied in the hydrothermal conversion of tetravalent Pu-oxalate into PuO₂ and to optimize the reaction conditions.

Room temperature synthesis of UO2+x nanocrystals and thin films via hydrolysis of uranium(iv) complexes

Methods for the straightforward, room temperature synthesis of UO2+x nanoparticles and thin films using solution processable, molecular uranium(iv) compounds is described.

Influence of additives on low-temperature hydrothermal synthesis of UO2+x and ThO2

UO2+x and ThO2 were synthesized through a hydrothermal reaction by adding aldehydes.

Hyperstoichiometric Uranium Dioxides: Rapid Synthesis and Irradiation-Induced Structural Changes

Uranium dioxide (UO₂), the primary fuel for commercial nuclear reactors, incorporates excess oxygen forming a series of hyperstoichiometric oxides. Thin layers of these oxides, such as UO_2.12, form readily on the fuel surface and influence its properties, performance, and potentially geologic disposal. This work reports a rapid and straightforward combustion process in uranyl nitrate-glycine-water solutions to prepare UO_2.12 nanomaterials and thin films. We also report on the investigation of the structural changes induced in the material by irradiation. Despite the simple processing aspects, the combustion synthesis of UO_2.12 has a sophisticated chemical mechanism involving several exothermic steps. Raman spectroscopy and single-crystal X-ray diffraction (XRD) measurements reveal the formation of a complex compound containing the uranyl moiety, glycine, H₂O, and NO₃^- groups in reactive solutions and dried combustion precursors. Combustion diagnostic methods, gas-phase mass spectroscopy, differential scanning calorimetry (DSC), and extracted activation energies from DSC measurements show that the rate-limiting step of the process is the reaction of ammonia with nitrogen oxides formed from the decomposition of glycine and uranyl nitrate, respectively. However, the exothermic decomposition of the complex compound determines the maximum temperature of the process. In situ transmission electron microscopy (TEM) imaging and electron diffraction measurements show that the decomposition of the complex compound directly produces UO₂. The incorporation of oxygen at the cooling stage of the combustion process is responsible for the formation of UO_2.12. Spin coating of the solutions and brief annealing at 670 K allow the deposition of uniform films of UO_2.12 with thicknesses up to 300 nm on an aluminum substrate. Irradiation of films with Ar²⁺ ions (1.7 MeV energy, a fluence of up to 1 × 10¹⁷ ions/cm²) shows unusual defect-simulated grain growth and enhanced chemical mixing of UO_2.12 with the substrate due to the high uranium ion diffusion in films. The method described in this work allows the preparation of actinide oxide targets for fundamental nuclear science research and studies associated with stockpile stewardship.

Charge Distribution in U1-xCexO2+y Nanoparticles

In view of safe management of the nuclear wastes, a sound knowledge of the atomic-scale properties of U_1-xM_xO_2+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 U_1-xCe_xO_2+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 U^IV, U^V, and U^VI and, on the other hand, that the fluorite structure is maintained despite this charge distribution.

Phase Analysis of Australian Uranium Ore Concentrates Determined by Variable Temperature Synchrotron Powder X-ray Diffraction

The chemical speciation of uranium oxides is sensitive to the provenance of the samples and their storage conditions. Here, we use diffraction methods to characterize the phases found in three aged (>10 years) uranium ore concentrates of different origins as well as in situ analysis of the thermally induced structural transitions of these materials. The structures of the crystalline phases found in the three samples have been refined, using high-resolution synchrotron X-ray diffraction data. Rietveld analysis of the samples from the Olympic Dam and Ranger uranium mines has revealed the presence of crystalline α-UO₂(OH)₂, together with metaschoepite (UO₂)₄O(OH)₆·5H₂O, in the aged U₃O₈ samples, and it is speculated that this forms as a consequence of the corrosion of U₃O₈ in the presence of metaschoepite. The third sample, from the Beverley uranium mine, contains the peroxide [UO₂(η²-O₂)(H₂O)₂] (metastudtite) together with α-UO₂(OH)₂ and metaschoepite. A core-shell model is proposed to account for the broadening of the diffraction peaks of the U₃O₈ evident in the samples.

Supercritical hydrothermal synthesis of UO2+x: stoichiometry, crystal shape and size, and homogeneity observed using 23Na-NMR spectroscopy of (U, Na)O2+x

The hydrothermal synthesis of pure uranium dioxide under supercritical water (SCW) conditions was investigated using a starting material composed of a uranyl(vi) nitrate solution at 450 °C.

Hydrothermal Conversion of Thorium Oxalate into ThO2·nH2O Oxide

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.