UO2 Particles Research Articles

A novel class of ATF fuels with large grain size, enhanced thermophysical properties and oxidation resistance

Two strategies have been extensively employed to develop advanced accident tolerant fuels (ATF): improving thermal conductivity and producing large grain sized pellets. However, there are few reports on the simultaneous utilization of both strategies. In this work, we fabricated Mo–Cr alloy reinforced UO2 (UMC) composite pellet with both high thermal conductivity and large grain size by a simple in-situ alloying method for the first time. The average grain size of UO2 increased from 6 μm to 113 μm. Thermal conductivity of the UMC pellet (with 2 vol% Mo) at 1200 °C increased by 36.46% compared with pure UO2. The increase rate of thermal conductivity per 1 vol% dopant reached about 18%. The Mo–Cr alloy formed continuous micro-cell layer around UO2 particles. Such continuous micro-cell Mo–Cr alloy layer and increase of UO2 grain size both helped improve the thermal conductivity of UMC composite pellets. Thermal expansion coefficient of the UMC under the operating temperature decreased by 11.90% compared with pure UO2. The oxidation resistance of the UMC pellet under high temperature steam was also improved. This work provides a new strategy towards fabricating ATF with both high thermal conductivity and large grain size.

Electrodeposition of UO2 nanoparticles in molten salt with microplasma gaseous cathode

Nano-sized UO2 particles have great application potential in the field of nuclear fuel fabrication and catalysis, while most of the current synthesis methods either cannot obtain products with high stability or need to introduce organic agents which would cause organic residue pollution. Herein, we have developed a new synthesis method by combining the microplasma gas electrode with molten salt electrochemistry. Uranyl ions (UO22+) in molten salt can be directly reduced to UO2 nanoparticles at the interface between plasma and molten salt. These particles have uniform size distribution, strong crystallization and high purity, with no organic reagent contamination or volatilization concerns, and show good long-term oxidation resistance in water, without any change for more than 30 days. The effects of electrolytic parameters such as electrolytic time, melt temperature, initial UO22+ concentration, and electrolytic current on the composition and size of the products have been studied. It is found that these electrolytic parameters have little effect on the composition of the products, which are all UO2, but strongly affect the size. The average particle size increased with electrolysis time, temperature, and the initial UO22+ concentration increased. According to these phenomena, the size of the product can be regulated at the range of 20–200 nm.

High-gradient magnetic separation of colloidal uranium oxide particles from soil components in aqueous suspensions

The separation of uranium oxide (UO2) particles from soil-surrogate particles in aqueous suspensions was achieved using filtration enhanced by a magnetic field. Enhanced attraction of paramagnetic UO2 colloids to a ferromagnetic stainless-steel filter placed in a strong magnetic field arises because of the positive magnetic susceptibility of the particles and the high-gradient field generated near ferromagnetic fibers. Enhanced uptake of smaller particles over larger ones occurs through Brownian motion that promotes the collision of particles with the ferromagnetic fibers of the filter. Hence, this work focused on UO2 particles in the colloidal size range. Experiments used a water-cooled electromagnet and an array of permanent magnets. Chemical analysis showed that the magnetic field increased the capture efficiency of uranium particles from a range of 27–53% with the magnet off up to 98% with the magnet on after a single pass of the suspension through the filter. The recovery of the UO2 particles from the filter, however, was more difficult to achieve. Small amounts of UO2, together with significant amounts of background SiO2 particles, were removed from the filter during a first flush with the magnetic field on. A much larger recovery of UO2 was not observed until a second out-of-field flush was performed, which also released some SiO2. The degree to which particle separation was enhanced through the use of multi-stage filtration compared to single pass-through filtration was also examined. A design was suggested that could be used to optimize the separation efficiency for a continuous process.

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.

Toward Elucidating the Influence of Hydrostatic Pressure Dependent Swelling Behavior in the CERCER Composite

A ceramic-ceramic (CERCER) fuel with minor actinide-enriched ceramic fuel particles dispersed in a MgO ceramic matrix is chosen as a promising composite target for accelerator-driven systems (ADS). Fission swelling is a complex irradiation-induced phenomenon that involves recrystallization, resolution, and hydrostatic pressure under extreme conditions of high temperature and significant fission flux. In this study, a multiscale computational framework was developed to integrate simulations of continuum-scale thermo-mechanical behavior in the CERCER composite with a grain-scale hydrostatic pressure-dependent fission gas swelling model. Hydrostatic pressure-dependent fission welling is taken into account in the stress update algorithms for UO2 particles. Accordingly, we programmed the user subroutines to define the thermo-mechanical constitutive relations in the finite element simulations. The obtained results indicate that (1) the proposed method accurately predicts the swelling deformation at various burnup levels while taking into account hydrostatic pressure and (2) prior to recrystallization, the particle swelling is primarily influenced by temperature variation, whereas after recrystallization, the presence of hydrostatic pressure favorably suppresses the swelling deformation. This work effectively captures the swelling behavior influenced by hydrostatic pressure within the dispersed-type CERCER composite fuel in ADSs.

Thermal air oxidation of UO2: Joint effect of precursor's nature and particle size distribution

The rate and extent of the air oxidation of UO2 are of paramount importance due to its relevance in the management of spent nuclear fuel at dry storage facilities. In particular, the formation of U3O8 requires special interest because it could compromise the integrity of the fuel cladding. In an attempt to reproduce the behavior of used fuel (in which the presence of fission products and Pu will also influence the oxidation), unirradiated UO2 was studied in this work. UO2 oxidation is strongly dependent on physical parameters of the starting material, although some parameters are not fully-studied. In the present study, the nature of the UO2 starting material and the particle size distribution on thermal oxidation temperature were evaluated in detail. Since UO2 oxidation follows a complex kinetic mechanism comprising various crystalline phases, which composition and quantity change influenced by temperature and oxidant gas phase, oxidation conditions were set to favor the formation of U3O8, due to its practical importance, as well as to study the impact of the physico-chemical properties of raw material in UO2 oxidation. Various powdered sample substrates ranging from synthetic UO2 (prepared via ADU route) to grinded and sieved fragments of fresh UO2 pellets were oxidized in the same conditions. The average O/U atomic ratios of the samples were calculated by assuming total conversion to U3O8 from data of thermogravimetric analysis (TGA). Each weight variation curve was characterized and discussed in terms of the Oxidation Onset Temperature (OOT), the Maximum Reaction Temperature (MRT), and the Reaction Rate at the maximum reaction temperature (RRMRT). Scanning Electron Microscopy (SEM) was employed to evaluate the surface morphology. All thermally oxidized samples exhibit the presence of oxides with spallation. Surface morphology of starting materials confirms the different nature of the samples, while the oxidized substrates present the typical pop-corn-like particles. In order to accomplish a better description of UO2 oxidation, the results are discussed relative to the nature of different UO2 precursors, surface area and particle size. The nature of the starting materials affects the rate of U3O8 formation, being faster in synthetic powders than in powder coming from milled pellets. By comparing materials with the same origin, the effect of the width of the particle size distribution is proved for the first time, being the oxidation to U3O8 slower in the substrate with a wider distribution (labelled material P3). Since P3 is more similar to a real irradiated fuel pellet, from a mechanistic point of view, this work present valuable input to security studies of dry storage facilities for spent fuel.

Alteration of fuel debris simulants by Bacillus subtilis

ABSTRACT This study aims to reveal the potential bacterial alteration of nuclear fuel debris. A melt simulant composed of U0.5Zr0.5O2 and Fe(0), and three powder simulants (UO2, Fe(0), and their mixed powder) were prepared and exposed to Bacillus subtilis in a Fe-deficient medium under aerobic conditions. B. subtilis could not dissolve Zr from the melt simulant but enhanced the dissolution of U and Fe, although the change of the melt simulant was too small to evaluate. Experiments using powder simulants were performed to investigate the bacterial influence in detail. B. subtilis enhanced the dissolution of U from the UO2 powder and Fe from the Fe(0) powder, although the dissolution of U was low. Most of the aqueous U species were probably U(VI), while the U that remained as a solid phase was mostly U(IV). The Fe(0) powder was oxidized by this bacterium, which transformed it into aqueous and solid Fe species. The solid species consisted of amorphous nano-sized particles. A fraction of the dissolved Fe was adsorbed or precipitated onto the undissolved UO2 particles. These results strongly suggest that bacteria can degrade nuclear fuel debris, although this study did not elucidate the mechanism of oxidative dissolution.

UO2-liquid metal suspension fuel concept for enhanced passive safety of LWRs: A heat pipe case study

This work proposes a novel fuel concept for use in light-water type reactors (LWRs): UO2 particles suspended in a low-melting-point liquid metal (LM). This new fuel form offers lower energy density and higher thermal conductivity, thus enhancing the fuel’s thermal performance compared to sintered UO2 pellets. Two candidate alloys for the LM are proposed based on previous studies into accident-tolerant fuels: Bi-Pb-Sn and Pb-Sn. Ten research questions are provided to guide future development of this fuel.A simple heat pipe model was constructed by coupling a reactor physics code to a heat transport code and other physics modules. Scoping calculations were performed on this model core as a preliminary investigation into five of the research questions. A fuel with 13 wt% UO2 was shown to maintain a steady-state peak coolant channel wall temperature below 400 °C, and maintain criticality via self-regulating reactivity feedback.Shortcomings of the present model are discussed herein, along with proposed future model improvements and fuel development. The appendix provides an example of a reactor concept that could be developed to utilize this fuel, but more modeling and experimental work are needed before a full reactor concept can be developed.

Geometrical aspects of alpha dose rates from UO2 based fuels

Models for calculating dose rates of spherical particles as well as in fuel cracks are important for radionuclide source term estimations. Dose rates from UO2 based fuels were calculated for planar, spherical, and crack geometries. The escape probability for α-particles in spherical UO2 particles was derived as a continuous equation. The dose rate increased with increasing spherical radius due to the decreasing relative volume of the surrounding water layer. The model produced escape probabilities that were closely predicted by the theoretical derivation. It was shown that the dose rate in water filled fuel cracks with width smaller than the range of an α-particle led to dissolution rates that were lower per unit surface area.

Submicrometer spectromicroscopy of UO2 aged under high humidity conditions

The oxidation of uranium dioxide is a complicated process, depending on factors including humidity, temperature, and microstructure. To further determine the characteristics of this process, UO2 particles were allowed to age and agglomerate under 98% relative humidity at room temperature for 378 days. A focused ion beam (FIB) section of this agglomeration was then measured at the O K-edge, U N5-edge, and C K-edge using the scanning transmission x-ray microscope (STXM) at the Advanced Light Source. O K-edge and U N5-edge x-ray absorption measurements allowed for the elemental and chemical species mapping of the agglomerates and indicated the formation of schoepite at the submicrometer scale in specific locations. Non-negative matrix factorization was employed to elucidate the main components at the O K-edge, which were uranyl (schoepite) formed primarily at the interface of the sample with controlled atmosphere, a UO2-like bulk component present in the majority of the sample, and an oxygen species present at the surface of the FIB section, which is likely adsorbed water. STXM spectromicroscopy measurements at the U N5-edge measurements also confirmed the location of oxidized uranium. This analysis is a valuable insight into the formation of schoepite on UO2 and shows the sensitivity to and utility of STXM spectromicroscopy for uranium speciation.

Impact of ruthenium metallic particles on the dissolution of UO2 in nitric acid

UO2 pellets incorporating 3 mol.% of Ru was prepared by using a wet chemistry route and then characterised. The speciation, morphology, as well as spatial distribution of Ru in the sintered samples, were determined. The synthesised samples were submitted to dissolution tests in 0.1 M nitric acid at 60 °C and the dissolution of pure UO2 pellets was also studied with and without the presence of Ru metallic particles in the solution. The evolution of the U, Ru, and nitrous acid concentrations in solution was measured and the residues of dissolution were further characterised. The obtained results unambiguously demonstrated the catalytic activity of Ru-metal particles during UO2 dissolution in nitric acid provided that a solid/solid interface existed between UO2 and Ru-metal particles. This positive impact on the dissolution kinetics of UO2 was supported by redox reactions taking place at both nitric acid solution/Ru-metal particles and at Ru-metal particles/UO2 interfaces.

Stable isotope signatures of hydration water in secondary mineralization on UO2

Hydrated secondary mineralization readily forms on the surface of UO2 particles exposed to humidity in an oxidizing environment. The oxygen stable isotope composition of the secondary uranium oxide may reflect that of the water vapor, as well as the hydrogen and oxygen stable isotopic composition of the mineral hydration water. The geospatial organization of δ2H and δ18O values of atmospheric humidity and precipitation is increasingly well understood, which suggests that the hydrogen and oxygen stable isotopes in secondary mineral hydration water may yield information on the environment in which the mineralization formed. UO2 powders were exposed to air with constant 30%, 61%, and 91% relative humidity, and constant H and O stable isotope composition. Aliquots were sampled from the UO2 materials at intervals of 1–10 days through the total humidity exposure duration of 180 days. Scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analysis of the humidity-exposed UO2 indicates that schoepite/metaschoepite [(UO3)•2H2O] secondary phases had formed on the underlying UO2. The δ2H and δ18O values of mineral hydration waters were determined by thermogravimetry-enabled isotope ratio infrared spectroscopy (TGA-IRIS). Results indicate that hydrogen in the surface sorbed and mineral hydration waters is exchangeable and thus their δ2H values are difficult to interpret. However, oxygen in these waters is less exchangeable, and thus the oxygen stable isotope composition of the schoepite/metaschoepite hydration water is likely to be related to that of the exposure water vapor. After formation of schoepite/metaschoepite, the δ18O values of the hydration water in schoepite/metaschoepite does not change in response to changes in exposure vapor δ18O values, which suggests that the δ18O values of the hydration water is relatively durable. These findings suggest that information about the origin and storage history of a UO2 sample may be discernable from δ18O values of schoepite/metaschoepite hydration water.

Li2O Entrainment during UO2 Reduction in Molten LiCl-Li2O: Part II. Effect of Cathodic Reduction Mechanism

A study was performed to assess the effect of controlling the reduction mechanism on Li2O entrainment in an electrolytic UO2 reduction process. The reduction mechanism was controlled by isolating the UO2 particles from the lead, eliminating the direct reduction mechanism. Cathode design made it possible to eliminate the direct UO2 reduction mechanism, as evidenced by the cathode potential, the result being indirect reduction via electrolytically produced Li metal. Characterization of the reduced product was achieved via acid-base titration to measure Li2O entrainment and thermogravimetry to measure reduction extent. Significantly increased entrainment was observed when the electrolytic UO2 reduction portion was eliminated compared to normal operation. In addition to the entrainment the representativeness of dip sampling, compared to cup sampling, was investigated. A statistically significant difference between Li2O concentration measurements from dip and cup samples was observed, dip samples measured 8 ± 3% less Li2O than cup samples.

Modeling of oxidation of UO2 pellets

In this study, a mathematical model has been developed for oxidation of pellets of uranium dioxide in the air at the bulk temperature range between 605 and 727 K. Oxidation of this pellet causes an increment in the size of the UO2 particles, decrement of the pellet porosity and breaking-down of this pellet, subsequently. Therefore, the presented model has two stages for this oxidation process, before and after the breaking-down of the pallet. These stages have different physical properties of the pellet and different mechanisms of the oxidation reaction. The predictions of the presented model is validated using the existed experimental data. It is resulted that increasing the bulk temperature increases the rate of the oxidation reaction and decreases the required times for the breaking-down and complete conversion of the pellet. In addition, increasing the size of the pellet increases the required time for breaking-down and decreases the required time complete conversion of the pellet.

UO2 Spheres Produce by External Gelation Process

UO2 kernels, a key component of fuel elements for high temperature gas cooled reactors, have usually been prepared by sol-gel methods. Sol-gel processes have a number of advantages, such as simple processes and facilities, and higher sphericity and density. In this study, to produce 900 μm-sized UO2 particles using an external gelation process, contact length extension of the NH3 gas of the broth droplets pass and the improvement of the gelation device capable of spraying 14 MNH4OH solution are used to form 3,000 μm-sized liquid droplets. To produce high-sphericity and high-density UO2 particles, HMTA, which promotes the gelation reaction in the uranium broth solution, is added to diffuse ammonium ions from the outside of the gelation solution during the aging process and generate ammonium ions from the inside of the ADU gel particles. Sufficient gelation inside of ADU gel particles is achieved, and the density of the UO2 spheres that undergo the subsequent treatment is 10.78 g/cm3; the sphericity is analyzed and found to be 0.948, indicating good experimental results.

The formation of stoichiometric uranium brannerite (UTi2O6) glass-ceramic composites from the component oxides in a one-pot synthesis

Brannerite glass-ceramic composites have been suggested as suitable wasteform materials for high-actinide content wastes, but the formation of glass-ceramic composites containing stoichiometric uranium brannerite (UTi2O6) has not been well-studied. Uranium brannerite glass-ceramic composites were synthesised at by a one-pot cold-press and sinter route from the component oxides. As a comparison, two further samples were produced using an alkoxide-nitrate route. A range of compositions with varying molar ratios of uranium and titanium oxides (from 1:2 to 1:3.20) were synthesised, with a range of different heat treatments (1200 °C for 12–48 h, and 1250 °C for 12 h). All compositions were analysed by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray near-edge spectroscopy, and found to contain UTi2O6 as the majority crystalline phase forming within a glass matrix of nominal stoichiometry Na2AlBSi6O16. In compositions with UO2:TiO2 ratios of 1:2 and 1:2.28, particles of UO2 were observed in the glass matrix, likely due to dissolution of TiO2 in the glass phase; this was prevented by the addition of excess TiO2. This work demonstrates the suitability of this system to produce highly durable wasteforms with excellent actinide waste loading, even with a simple one-pot process. Some grains of brannerite consist of a UO2 particle encapsulated in a shell of UTi2O6, suggesting that brannerite crystallises around particles of UO2 until either the UO2 is fully depleted, or the kinetic barrier becomes too large for further diffusion to occur. We propose that the formation of brannerite within glass-ceramic composites at lower temperatures than that for pure ceramic brannerite is caused by an increase in the rate of diffusion of the reactants within the glass.

Study of mineral grains extracted from the Chernobyl “lava”

Results of a spectroscopic and microstructural investigation of mineral grains extracted from lava-like fuel-containing materials – Chernobyl “lava” – are presented. Raman, photoluminescence and infrared spectra reveal the presence of abundant U4+, traces of Sm3+ and Cr3+ in grains of U-containing zircon and ZrO2. No oxidation of UO2 particles is observed. The ZrO2 grains consist predominantly of the monoclinic polymorph and are often twinned. Raman spectra indicate the presence of the tetragonal polymorph, which is likely stabilized by the presence of U. It is suggested that spontaneous transformation of partially stabilized tetragonal (Zr,U)O2 to the monoclinic phase, and a corresponding volume expansion, are important factors affecting the rate of mechanical degradation of the “lava”. Transmission electron microscopy of U-rich zircon suggests the absence of precipitation of any independent U-containing phases, implying that U is fully incorporated within the zircon lattice as an impurity. Grains of rutile, corundum, quartz and natural zircon originating from construction materials of the reactor are described. Results obtained here are important for the validation of computer codes describing severe nuclear accidents.

A method for the correction of size effects in microparticles using a peak-to-background approach in electron-probe microanalysis

We describe a correction method in electron probe microanalysis (EPMA) for size effects in microparticles. Accounting for size effects is important in quantitative EPMA of microparticles, where bulk standards are usually used for quantification. In this work, the correction was applied using peak-to-background ratios PB that are quasi-independent of absorption path lengths, which enables the difference in absorption between a microparticle and a bulk sample to be taken into account. For microparticles smaller than the X-ray generation volume, the X-ray emission inside the particle can be significantly affected by the transmission of electrons through the particle, thus requiring a correction in the measured PB ratios. Moreover, transmitted electrons induce Bremsstrahlung from the substrate that can also affect the PB ratios. This raises the necessity to include a correction of the substrate emission in the PB methods applied for microparticles. The present method enables to correct for the effects due to electron transmission as well as the parasitic emission of the substrate. In order to assess the validity of our method, the corrected PB of small particles were compared with those obtained for a bulk sample or a pseudo-bulk particle where size effects are negligible. Corrected PB were further used in a ZAF PB method [J. Lábár, S. Török, X-Ray Spectrom. 21 (1992) 183–190] to evaluate the correction model in terms of quantitative analysis. The study was performed on homogeneous spherical microparticles of K411 glass and microparticles of UO2. Both substrate and transmission effects were taken into account in the K411 glass particles, which led to significant improvements in terms of quantitative results. For UO2 particles, size effects did not have much impact on the PB ratios and accurate quantification was obtained from both uncorrected and corrected ratios.

Study of entrainment of Li2O in product from direct electrolytic reduction of UO2 in molten LiCl–Li2O. Part 1: Post-processing and analysis techniques

This paper describes experimental and characterization methods for gram-scale uranium oxide electrolytic reduction tests in molten LiCl–Li2O. Post-test analysis of the cathodically reduced UO2 involves acid-base titration and thermogravimetric analysis under flowing air. The titration was used to measure Li2O entrained inside reduced UO2 particles. Centrifugal salt removal was demonstrated to eliminate Li2O from the basket, but the Li2O entrained in the reduced uranium particles remained and was reacted over a period of several days in an autotitrator. Measuring the mass increase by thermogravimetry while heating to 800 °C and flowing air was found to be an effective way to measure the reduction extent, which was verified with pure U metal. The error of this measurement was estimated to be 1.65%. In addition, a LECO oxygen compositional analyzer was used to independently verify the oxygen content in the product from the thermogravimetric analysis.

Silicate stabilisation of colloidal UO2 produced by uranium metal corrosion

U(IV) mobility can be significantly enhanced by colloids in both engineered and natural environments. This is particularly relevant in decommissioning and clean-up of nuclear facilities, such as legacy fuel ponds and silos at the Sellafield site, UK, and in long-term radioactive waste geodisposal. In this study, the product of metallic uranium (U) corrosion under anaerobic, alkaline conditions was characterised, and the interaction of this product with silicate solutions was investigated. The U metal corrosion product consisted of crystalline UO2 nanoparticles (5–10 nm) that aggregated to form clusters larger than 20 nm. Sequential ultrafiltration indicated that a small fraction of the U metal corrosion product was colloidal. When the uranium corrosion product was reacted with silicate solutions under anaerobic conditions, ultrafiltration indicated a stable colloidal uranium fraction was formed. Extended X-ray absorption fine structure (EXAFS) spectroscopy and high resolution TEM confirmed that the majority of U was still present as UO2 after several months of exposure to silicate solutions, but an amorphous silica coating was present on the UO2 surface. This silica coating is believed to be responsible for formation of the UO2 colloid fraction. Atomic-resolution scanning TEM (STEM) indicated some migration of U into the silica-coating of the UO2 particles as non-crystalline U(IV)-silicate, suggesting alteration of UO2 at the UO2-silica interface had occurred. This alteration at the UO2-silica interface is a potential pathway to the formation of U-silicates (e.g. coffinite, USiO4).