UO2 Surface Research Articles

Ab initio study of the adsorption of O, O2, H2O and H2O2 on UO2 surfaces using DFT+U and non-collinear magnetism

In order to model correctly the corrosion of spent nuclear fuel under disposal conditions, it is important to understand its behavior in the presence of oxidants. To advance in this direction, we consider the oxidation of UO2. We investigate computationally the adsorption of various species on its three most stable surfaces: (111), (110), and (100), with emphasis on incorporating a full non-collinear PBE+U approach. Various species, namely O, O2, H2O and H2O2 are considered due to their relevance for the oxidation of UO2. The dissociation energy and an estimate for the dissociation barrier for O2 were obtained, using the preferred adsorption configurations of O and O2. The adsorption configurations for H2O in our study compare well with previous studies that used collinear approximations, both in terms of relative stability of configurations and bond lengths. Differences in adsorption energies were found, which may be important for reaction kinetics. Dissociative reactions in which the water molecule splits in hydrogen and hydroxyl occur only on one of the three surfaces. The hydrogen further reacts with a surface oxygen to also form a hydroxyl group. Not surprisingly, we find that H2O2 binds more strongly to the three surfaces than water (lower formation energy), and similar to H2O adsorption, dissociative reactions may occur. The dissociated hydrogen reacts with a surface oxygen to form a hydroxyl group and the hydroperoxyl molecule binds with a surface uranium. Our study, which includes a detailed study of electron transfer, magnetic structure and the preferred adsorption configurations, gives insight into the uranium oxidation states and the influence of surface geometry on adsorption. The findings contribute to a more comprehensive understanding of the early stages of UO2 oxidation.

UO2 dissolution in aqueous halide solutions exposed to ionizing radiation

In this work, we have experimentally studied UO2 dissolution in pure water and in 1 M aqueous solutions of either Cl- or Br- exposed to γ-radiation. It has previously been found that high ionic strength can facilitate adsorption of dissolved UO22+ on UO2 surfaces. The adsorption is also affected by the solution pH relative to the point of zero charge of UO2. In our experiments, Br3- was observed in 1 M Br- solution exposed to γ-radiation. Experiments confirmed that Br3- can quantitively oxidize UO2. XPS and UPS were used to characterize potential surface modifications after exposure. The XPS results show that the UO2 surfaces after exposure to γ-radiation in pure water and in 1 M aqueous solutions of either Cl- or Br- were significantly oxidized with U(V) as the dominating state. U 4f7/2 and O 1 s spectra of the UO2 surface after exposure to γ-radiation in pure water demonstrates the formation of uranyl peroxide secondary phases. UPS results indicate that there is a large percentage of U(VI) on the ultra-thin outer layer of UO2 after exposure to γ-radiation in 1 M aqueous solutions of Br- and Cl-, and 100 % of U(VI) in the pure water case.

A Raman spectroscopy study of bicarbonate effects on UO2+x

ABSTRACT Direct disposal of spent nuclear fuel in underground repositories in stable geological formations is an alternative option for the reprocessing of spent fuel in Japan. Upon breakdown of the barriers between waste storage containers and the environment in a deep geological repository, release of radioactive nuclei from spent nuclear fuel into the environment will occur via groundwater. The release of radioactive nuclei will be dependent on the dissolution rate at the spent fuel/groundwater interface. The surface of UO2 in the spent fuel will be in the oxidized form UO2+x, and the extent of hyper-stoichiometry has implications for U dissolution. A key component of typical groundwater is bicarbonate which can range in concentration depending on repository location. Therefore, to understand the effect of bicarbonate on the UO2+x surface, UO2 pellets were immersed in bicarbonate solution for 298 days, and the UO2+x oxide surface was analysed by Raman spectroscopy. Detailed peak analysis of the Raman spectra showed increased defects after immersion attributed to the preferential dissolution of UV/VI from UO2+x. Differences in the defect concentration with and without bicarbonate were attributed to a change in U speciation (U(OH)4 to UO2(CO3)3 4-) as determined by speciation calculations. Raman depth profiles showed a constant value of x through the oxide surface.

Exploring the Change in Redox Reactivity of UO2 Induced by Exposure to Oxidants in HCO3– Solution

Understanding the possible change in UO2 surface reactivity after exposure to oxidants is of key importance when assessing the impact of spent nuclear fuel dissolution on the safety of a repository for spent nuclear fuel. In this work, we have experimentally studied the change in UO2 reactivity after consecutive exposures to O2 or γ-radiation in aqueous solutions containing 10 mM HCO3-. The experiments show that the reactivity of UO2 toward O2 decreases significantly with time in a single exposure. In consecutive exposures, the reactivity also decreases from exposure to exposure. In γ-radiation exposures, the system reaches a steady state and the rate of uranium dissolution becomes governed by the radiolytic production of oxidants. Changes in surface reactivity can therefore not be observed in the irradiated system. The potential surface modification responsible for the change in UO2 reactivity was studied by XPS and UPS after consecutive exposures to either O2, H2O2, or γ-radiation in 10 mM HCO3- solution. The results show that the surfaces were significantly oxidized to a stoichiometric ratio of O/U of UO2.3 under all the three exposure conditions. XPS results also show that the surfaces were dominated by U(V) with no observed U(VI). The experiments also show that U(V) is slowly removed from the surface when exposed to anoxic aqueous solutions containing 10 mM HCO3-. The UPS results show that the outer ultrathin layer of the surfaces most probably contains a significant amount of U(VI). U(VI) may form upon exposure to air during the rinsing process with water prior to XPS and UPS measurements.

Modelling radiation-induced oxidative dissolution of UO2-based spent nuclear fuel on the basis of the hydroxyl radical mediated surface mechanism: Exploring the impact of surface reaction mechanism and spatial and temporal resolution

A combined kinetic and diffusion model with an accurate α-dose rate profile was used to model radiation induced dissolution of UO2. Previous experimental data were used to fit the surface site reaction system involving the surface bound hydroxyl radical as an intermediate species for both UO2 oxidation and surface catalysed decomposition of H2O2. The performance of the model was explored in terms of sensitivity to spatial and temporal resolution as well as simplifications in the models describing the surface reactions and the reactions in solution. As a result, optimal conditions for running the numerical simulations were identified.

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.

Front Cover: Modeling Heterogeneity in UO2 Nanoparticles Using X‐ray Absorption Spectroscopy (Eur. J. Inorg. Chem. 7/2023)

The Front Cover shows UO2 nanoparticles with a pictorial representation of the atoms (uranium in blue and oxygen in red). In this work, user-defined X-ray absorption spectroscopy models informed by surface relaxation effects and inherent surface disorder were generated and preferentially fit to experimentally derived data. The developed method enables representative local surface structure attributes to be extracted from bulk X-ray absorption spectroscopy data. Results show that the UO2 surface is highly disordered compared with the bulk structure, and this cannot be explained through surface relaxation effects alone. Additionally, this trend is notably not observed in 8 nm nanoparticles (which resemble the bulk), as it is in 1.4 and 4.7 nm particles. This suggests that there may be a critical domain size for ordering within UO2 nanoparticle surfaces. More information can be found in the Research Article by L. M. Moreau and co-workers.

UO2 dissolution in bicarbonate solution with H2O2: the effect of temperature.

Upon nuclear waste canister failure and contact of spent nuclear fuel with groundwater, the UO2 matrix of spent fuel will interact with oxidants in the groundwater generated by water radiolysis. Bicarbonate (HCO3-) is often found in groundwater, and the H2O2 induced oxidative dissolution of UO2 in bicarbonate solution has previously been studied under various conditions. Temperatures in the repository at the time of canister failure will differ depending on the location, yet the effect of temperature on oxidative dissolution is unknown. To investigate, the decomposition rate of H2O2 at the UO2 surface and dissolution of UVI in bicarbonate solution (0.1, 1, 10 and 50 mM) was analysed at various temperatures (10, 25, 45 and 60 °C). At [HCO3-] ≥ 1 mM, the concentration of dissolved UVI decreased with increasing temperature. This was attributed to the formation of UVI-bicarbonate species at the surface and a change in the mechanism of H2O2 decomposition from oxidative to catalytic. At 0.1 mM, no obvious correlation between temperature and U dissolution was observed, and thermodynamic calculations indicated this was due to a change in the surface species. A pathway to explain the observed dissolution behaviour of UO2 in bicarbonate solution as a function of temperature was proposed.

Intrinsic oxygen defects in UO2 (1 1 1) and PuO2 (1 1 1) surfaces

Density functional theory (DFT) + U combined with the first principle atomistic thermodynamic method was employed to study the physical properties of intrinsic oxygen defects in (111) surfaces of UO2 and PuO2. The calculated formation energy indicates that the oxygen interstitial (Oi) defects are dominant in UO2, while the oxygen vacancy (Ov) and Oi defects can coexist in PuO2. Such a difference can be attributed to the distinct electronic properties of U and Pu elements. Moreover, the formation energy of Ov is found to be sensitive to the surface depth, and Ov prefer to form on the sub-surface layer for both UO2 and PuO2. Comparatively, the formation of Oi is insensitive to the incorporation depth. The strain effect of formation energy is mainly contributed by the local structural distortion, rather than the electronic hybridization. Through thermodynamic calculation, the strain-modulated formation energy phase diagrams of the oxygen defects have been established over a wide range of temperature and pressure, providing the potential strategy for controlling the type and concentration of intrinsic oxygen defects in UO2 and PuO2. Our results shed some light on the defect behaviors of actinide dioxide, and thereby the oxidation properties of actinide metals.

Outstanding stability of Gd-doped UO2 against surface oxidation: First-principles study

An accurate understanding of structural integrity and chemical reactivity of UO2 disposed in deep underground sites is of importance. Owing to the specific condition of the site location, UO2 may have substantially different properties from the conventional prediction. In this study, we demonstrate that the oxidation resistivity of UO2 is considerably modified by gadolinium (Gd), which is the element of neutron absorber and a byproduct of nuclear decay of radioactive U-235. Using density functional theory calculations, we investigate how the oxidation mechanism of UO2 changes with Gd incorporation in U lattice. Our study indicates that Gd remarkably enhances the thermodynamic stability of pristine UO2 against surface oxidation via three underlying mechanisms: (i) weakens the chemical bonding of adsorbed oxygen atom (O) with U, (ii) reduces active sites (U) for oxygen adsorption, and (iii) suppresses the subsurface diffusion of adsorbed O delaying the growth of the oxide layers on the UO2. Electronic and lattice structure analyses for Gd-doped UO2 indicate that amount of charge transfer from U to O is critically reduced and the lattice of the UO2 surface is contracted. Our results provide useful information for understanding long-term stability and improving the structural integrity of UO2 through the chemical doping process.

A general picture for water dissociation on uranium dioxide surfaces

Despite extensive theoretical and experimental studies, there is still no universal picture for water dissociation on actinide dioxide surfaces. Based on ab initio molecular dynamics studies, we systematically investigated the influences of surface morphology and stoichiometry of the UO2 (001) surface on the adsorption behavior of a water molecule. We find that the critical factor corresponding to water dissociation is the existence of surface actinide atoms. On the normal UO2 (001) surface, surface U atoms are ready to bond with dissociated hydroxyl groups and the adsorbed water molecules dissociate spontaneously upon approaching. Comparatively, the surface U atoms on the oxygen-over surface are fully bonded with oxygen atoms, and the approaching water molecule do not dissociate. On a more realistic trench UO2 surface, we find that approaching water molecules can steadily adsorb in the trench area after weakly bonding with two oxygen atoms in the trench. Our work reveals the vital role of surface actinide atoms in dissociating approaching water molecules, which is meaningful for macroscopic modellings of actinide corrosion.

Monitoring the gradual change in oxidation state during surface oxidation or reduction of uranium oxides by photoemission spectroscopy of the 5f states

Photoelectron spectroscopy study of the U5f emission gives valuable insight into the surface oxidation mechanism of uranium oxides. Its intensity is directly related to the electron count nf, which decreases with increasing oxidation number (U(IV): nf=2; U(V): nf=1; U(VI): nf=0). n5f can be quantified by analysing the U5f/U4f intensity ratio and using a standard of known composition. In addition, the 5f emission has a characteristic multiplet shape, directly related to n5f, which can be used to distinguish the 5f2 and 5f1 configuration of U(IV) and U(V), respectively. Three independent methods are used to determine the surface oxidation state: the U5f/U4f intensity ratio, the relative intensities of the U4f oxide shifted peaks, and the O1s/U4f intensity ratio. The first two reveal the concentration of the U in each oxide, the third indicates the total concentration of oxygen. These methods are applied to follow the surface modification of UO2 films when exposed to various oxidative conditions: molecular and atomic oxygen and water plasma at 400°C. In addition, the reduction of UO3 by atomic H is studied. Molecular oxygen oxidizes UO2 to UO2+x(x = 0.22), containing both U(IV) and U(V). Atomic oxygen also oxidizes U(IV) to U(V) at low dosages, but then continues oxidizing U(V) to U(VI) (UO3) at high dosages. Conversely, atomic hydrogen reduces UO3. In the early phase of reduction U(V) forms exclusively – no U(IV) is observed. Water plasma first transforms almost all UO2 (surface and subsurface) into U(V). With further plasma exposure the surface is oxidized to about 80% U(VI) and 20% U(V). Up to this point, a small fraction of U(IV) remains at the surface. Once it disappears, the surface oxidation stops and further water plasma exposure now leads to surface reduction into U(V) (the 5f1 peak increases again). Despite the reduction at high dosage, the O1s/U4f intensity ratio keeps increasing.

Proton irradiation-induced blistering in UO2

Proton (H+) irradiation effects in polycrystalline UO2 have been studied. The irradiation was carried out using three ion energies and two different ion fluxes at 600 °C. Scanning electron microscopy (SEM) investigations showed that significant surface flaking took place. Focused ion beam (FIB) milling in SEM was successfully applied for extracting lamellas from uneven blistered surfaces for transmission electron microscopy (TEM) investigations allowing detailed investigations for the degradation mechanisms. High-resolution TEM for the flaked UO2 surfaces revealed that the implanted H+ formed sharp two-dimensional cavities at the peak ion-stopping region instead of diffusing to the matrix. The resulting lateral stress likely caused UO2 surface deterioration in good agreement with previous blistering and flaking studies on crystalline materials.Graphical abstract

A mathematical model to describe the alpha dose rate from a UO2 surface

A model to determine the dose rate of a planar alpha-emitting surface has been developed. The approach presented is a computationally efficient mathematical model using stopping range data from the Stopping Ranges of Ions in Matter (SRIM) software. The alpha dose rates as a function of distance from irradiated UO2 spent fuel surfaces were produced for benchmarking with previous modelling attempts. This method is able to replicate a Monte Carlo (MCNPX) study of an irradiated UO2 fuel surface within 0.6% of the resulting total dose rate and displays a similar dose profile.

Mixed H2O/H2 plasma-induced redox reactions of thin uranium oxide films under UHV conditions.

X-ray photoelectron spectroscopy (XPS) has been used to study the effect of mixed H2O/H2 gas plasma on the surfaces of UO2, U2O5 and UO3 thin films at 400 °C. The experiments were performed in situ under ultra-high vacuum conditions. Deconvolution of the U4f7/2 peaks into U(IV), U(V) and U(VI) components revealed the surface composition of the films after 10 min plasma exposure as a function of H2 concentration in the feed gas of the plasma. Some selected films (unexposed and exposed) were also analysed using grazing-incidence X-ray diffraction (GIXRD). The XPS results show that U(V) is formed as a major product upon 10 minutes exposure of UO3 by a mixed H2O/H2 plasma in a fairly wide H2 concentration range. When starting with U(V) (U2O5), rather high H2 concentrations are needed to reduce U(V) to U(IV) in 10 minutes. In the plasma induced oxidation of UO2, U(V) is never observed as a major product after 10 minutes and it would seem that once U(V) is formed in the oxidation of UO2 it is rapidly oxidized further to U(VI). The grazing incidence X-ray diffraction analysis shows that there is a considerable impact of the plasma and heating conditions on the crystal structure of the films in line with the change of the oxidation state. This structural difference is proposed to be the main kinetic barrier for plasma induced transfer between U(IV) and U(V) in both directions.

Structure and properties of amorphous uranium dioxide

Amorphous uranium dioxide (UO2) has been modelled on the atomic scale using a combination of quantum mechanical (density functional theory) and classical forcefield methods (molecular dynamics and reverse Monte-Carlo). The atomic scale structure of the amorphous state has been predicted and is presented in the form of simulated X-ray diffraction patterns, pair correlation functions and bond angle distributions. These are shown to be consistent with the experimental patterns previously reported. To enable accurate calculation of the energies and magnetic properties, using quantum mechanics, reverse Monte-Carlo was used to generate reduced cells from larger molecular dynamic melt-quenches. This allowed density functional theory energy minimisation for structures consistent with the amorphous state. Building on this, the material's propensity to deviate from stoichiometry, the magnetic structure and amorphous UO2 surface energy were computed. Non-stoichiometry is accommodated more readily in the amorphous system than in crystalline UO2. This indicates that deviations from stoichiometry in fuel (for example as a result of operation) will be accommodated at amorphous phases, if present, leaving a more stoichiometric crystalline phase – impacting processes including fission gas mobility, melting points and a number of other safety relevant properties. The magnetic structure of bulk amorphous UO2 is predicted to be a spin-glass, unlike crystalline UO2 which has anti-ferromagnetic ordering at 0 K. The surface energy of amorphous UO2 is computed to be 0.79 Jm−2, which is similar to the experimentally observed surface energy of grain boundary bubbles in UO2 and comparable to the surface energies reported for crystalline UO2. Results are pertinent to ongoing efforts to understand the nature of grain boundaries in nuclear fuels, to model fission gas release from them, radiation induced amorphization and the impact of dopants and impurities on nuclear fuel's manufacture and in-reactor behaviour.

XPS study of external α-radiolytic oxidation of UO2 in the presence of argon or hydrogen

UO2 pellets exposed to 1.85 MBq and 3.30 MBq Am-241 sources with a 30 μm separation in solution were studied under Ar and H2 atmospheres. The 1.85 MBq Am-241 source was shown to have a too low radiolytic production to cause any measurable change in UO2 surface oxidation state. However, a significant oxidation was observed after exposure to the 3.30 MBq source under Ar atmosphere through the production of U(V), as seen from the U4f7/2-peak deconvolution and valence band peak analysis. The H2 atmosphere was shown to suppress the oxidation of the UO2 pellet surface as well as the dissolution in both Milli-Q and NaHCO3 solutions. The dissolved uranium concentration was reduced by a factor 10 under H2 atmosphere as compared to the Ar atmosphere experiments.

An Atomic-Scale Understanding of UO2 Surface Evolution during Anoxic Dissolution.

Our present understanding of surface dissolution of nuclear fuels such as uranium dioxide (UO2) is limited by the use of nonlocal characterization techniques. Here we discuss the use of state-of-the-art scanning transmission electron microscopy (STEM) to reveal atomic-scale changes occurring to a UO2 thin film subjected to anoxic dissolution in deionized water. No amorphization of the UO2 film surface during dissolution is observed, and dissolution occurs preferentially at surface reactive sites that present as surface pits which increase in size as the dissolution proceeds. Using a combination of STEM imaging modes, energy-dispersive X-ray spectroscopy (STEM-EDS), and electron energy loss spectroscopy (STEM-EELS), we investigate structural defects and oxygen passivation of the surface that originates from the filling of the octahedral interstitial site in the center of the unit cells and its associated lattice contraction. Taken together, our results reveal complex pathways for both the dissolution and infiltration of solutions into UO2 surfaces.

Coffinite formation from UO2+x

Most of the highly radioactive spent nuclear fuel (SNF) around the world is destined for final disposal in deep-mined geological repositories. At the end of the fuel’s useful life in a reactor, about 96% of the SNF is still UO2. Thus, the behaviour of UO2 in SNF must be understood and evaluated under the weathering conditions of geologic disposal, which extend to periods of hundreds of thousands of years. There is ample evidence from nature that many uranium deposits have experienced conditions for which the formation of coffinite, USiO4, has been favoured over uraninite, UO2+x, during subsequent alteration events. Thus, coffinite is an important alteration product of the UO2 in SNF. Here, we present the first evidence of the formation of coffinite on the surface of UO2 at the time scale of laboratory experiments in a solution saturated with respect to amorphous silica at pH = 9, room temperature and under anoxic conditions.

Interaction of hydrogen with actinide dioxide (011) surfaces.

The corrosion and oxidation of actinide metals, leading to the formation of metal-oxide surface layers with the catalytic evolution of hydrogen, impacts the management of nuclear materials. Here, the interaction of hydrogen with actinide dioxide (AnO2, An = U, Np, or Pu) (011) surfaces by Hubbard corrected density functional theory (PBEsol+U) has been studied, including spin-orbit interactions and non-collinear 3k anti-ferromagnetic behavior. The actinide dioxides crystalize in the fluorite-type structure, and although the (111) surface dominates the crystal morphology, the (011) surface energetics may lead to more significant interaction with hydrogen. The dissociative adsorption of hydrogen on the UO2 (0.44 eV), NpO2 (-0.47 eV), and PuO2 (-1.71 eV) (011) surfaces has been calculated. It is found that hydrogen dissociates on the PuO2 (011) surface; however, UO2 (011) and NpO2 (011) surfaces are relatively inert. Recombination of hydrogen ions is likely to occur on the UO2 (011) and NpO2 (011) surfaces, whereas hydroxide formation is shown to occur on the PuO2 (011) surface, which distorts the surface structure.