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.

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.

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.

A multiscale in situ high temperature high resolution transmission electron microscopy study of ThO2 sintering

Two-grain model systems formed by ThO₂ nanospheres have been used to experimentally study for the first time the initial stage of sintering from room temperature to 1050 °C using high temperature high resolution transmission electron microscopy. In each grain, oriented attachment drove the reorganization and growth of the crystallites up to 300 °C to form a pseudo single crystal. Crystallite size kept growing up to 950 °C. At this temperature, a fast transformation probably corresponding to the elimination of stacking faults or dislocation walls led to the formation of single-crystals. The contact formed at room temperature between the two grains was stabilized during heat treatment by a slight reorientation of the crystallographic planes (T≈ 400 °C), leading the neck to be formed by numerous boundaries between the crystallites. At higher temperatures, the neck evolved and stabilized in the form of a plane of crystallographic orientation mismatch between the grains, which corresponds to the usual definition of the grain boundary. The growth of the neck by the addition of atomic columns was further observed in real time and quantified. At T = 950 °C, the evolution of the microscopic sintering parameter λ was obtained from HT-HRTEM images and indicated that the neck formation mostly proceeded through volume diffusion.

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.

Linker-assisted structuration of tunable uranium-based hybrid lamellar nanomaterials

Linker-assisted formation of tunable uranium-based hybrid lamellar nano-sheets through a non-hydrolytic condensation process.

Determination of the isotopic composition of single sub-micrometer-sized uranium particles by laser ablation coupled with multi-collector inductively coupled plasma mass spectrometry

RATIONALE

A multi-collector inductively coupled plasma (MC-ICP) mass spectrometer coupled to a UV ns-laser ablation (LA) system was used to measure uranium isotopic ratios (²³⁴ U/²³⁸ U, ²³⁵ U/²³⁸ U and ²³⁶ U/²³⁸ U) in single uranium particles of various sizes and isotopic compositions, including home-made sub-micrometric natural uranium particles of narrow size distribution (415 ± 60 nm).

METHODS

The LA-ICP mass spectrometer was operated in wet plasma conditions thanks to simultaneous injection of the laser aerosol and water vapor through a desolvating nebulizer. The isotopic ratios were corrected for mass bias and gain factors between detectors. The ²³⁶ U/²³⁸ U ratios were also corrected for the presence of ²³⁵ U hydrides and tailing of the ²³⁸ U⁺ peak.

RESULTS

²³⁶ U/²³⁸ U ratios were successfully measured in micrometer-sized particles from the NBS U050 certified standard material with a ²³⁶ U/²³⁸ U ratio of ~5 × 10^-4 . The analysis of 77 natural uranium sub-μm-sized particles yielded a very good trueness with respect to the expected ²³⁴ U/²³⁸ U and ²³⁵ U/²³⁸ U ratios, while the measurement errors for single particles ranged from -2.7% to +2.1% for ²³⁵ U/²³⁸ U and from -17% to +33% for the ²³⁴ U/²³⁸ U ratios. Their relative combined standard uncertainties ranged from 3.3% to 32.8% and from 0.4% to 4.0% for ²³⁴ U/²³⁸ U and ²³⁵ U/²³⁸ U ratios, respectively. In addition, extremely low detection limits, in the attogram range, were achieved.

CONCLUSIONS

This study demonstrates that coupling of a ns-laser ablation system with a MC-ICP mass spectrometer allows measurements of the isotopic composition in natural uranium particles of a few hundreds of nm with very good trueness, average combined standard uncertainties of ~1% for ²³⁵ U/²³⁸ U ratios and 12% for ²³⁴ U/²³⁸ U ratios, and detections limits of a few ag for minor isotopes.