Uranium Surface Research Articles

The role of CO on initial oxidation behavior of α-U(001) surface: A first principles study

A first-principles approach based on density-functional theory has been used to investigate the corrosion resistance of alpha-U in the CO environment. Calculations show that O2 molecules spontaneously dissociate on the uranium surface, and the two O atoms formed by dissociation tend to adsorb on the hole sites and bind to the surface in the form of a U–O bond to emit a large amount of heat. The CO molecules occur on the surface of uranium as a non-dissociative chemical. The mechanism of CO inhibiting the adsorption of O2 molecules stems from the fact that CO molecules occupy the optimal adsorption sites. Another inhibition mechanism, the combination of C atoms and O atoms to form bonds and consume oxygen atoms, has little effect on uranium corrosion.

The dissociation of H2O on the γ-U (110) and (100) surfaces: an ab initio study

The dissociation of H2O on γ-U (110) and γ-U (100) surfaces has been studied by using ab initio molecular dynamics simulations at an elevated temperature of 800 K. The simulation results show the dissociation of H2O into the OH group and H atom, which are finally adsorbed on the uranium surface. The dissociation results from electronic interactions between surface uranium 6d/5 f states and the s orbitals of H and the 2p orbitals of O. Additionally, the hybridization between the 6d orbital of surface uranium and the 2p orbital of oxygen plays a dominant role in dissociative adsorption. A significant charge transfer from the uranium surface to the O and H atoms is observed, indicating the formation of U–O and U–H chemical bonds. Specifically, for γ-U (110) surface, the most preferred site for OH is the 3-fold hollow site and H occupies the bridge site or the 3-fold hollow site. On the other hand, for γ-U (100) surface, OH prefers to adsorb on the bridge site and H occupies the 3-fold hollow site or the bridge site. Furthermore, when H2O is placed on the TOP site, its initial dissociation on the γ-U (110) surface is easier compared to the γ-U (100) surface.

In Situ Electronic Structure Analysis of Water-Corroded UNx in High Vacuum

Nitriding technologies are promising surface modification techniques of uranium based on pulsed laser irradiating and glow plasma treatment. Nitrided layers with different nitrogen contents (UN0.35, UN0.75, UN1.08 and UN1.5) were prepared on the surface of uranium. The present study aims to investigate the microstructure and corrosion properties of the reaction of the UNx layers with ultra-low water vapor at room temperature. The electronic structures were analyzed in situ by X-ray photoelectron spectroscopy in high vacuum. The results showed that the UN0.35, UN0.75, and UN1.08 samples were mainly composed of uranium nitride (UN) and metallic uranium, while the surface microstructure of the UN1.5 sample was U2N3. The dense and uniform nitride layer with a grain size of 20 to 50 nm was obtained on the uranium surface, which acted as a barrier and prevented the further diffusion of anions into the matrix. The corrosion products of the UN0.35, UN0.75, and UN1.08 samples were mainly UO2-xNy and UO2 after reaction with the water vapor. The contents of UO2-xNy increased with increasing nitrogen contents, and the corrosion rate decreased significantly. The intermediate compounds UO2-xNy reacted slowly with the water vapor, and eventually converted to UO2. Meanwhile, the corrosion products of the UN1.5 sample were mainly U2N3+xOy and UO2-xNy after reaction with the water vapor. The percentage of U2N3+xOy and UO2-xNy remained almost stable over a long period of time, which indicated that the high contents of U2N3+xOy and UO2-xNy prolonged the time for complete conversion to UO2. It can be concluded that the U-N-O ternary compounds retarded the corrosion process and the UNx layers with high nitrogen contents showed excellent corrosion resistance.

Preparation of the gradient nanostructured layer in uranium by surface shot peening treatment

The gradient nanostructured layer in uranium prepared by surface shot peening treatment has been systematically characterized. The evolution of the ultrafine grains within the surface layer of uranium was derived. The results show that the gradient microstructures from the surface to the matrix could be divided into four zones including ultrafine grains, high density deformation twins, low density deformation twins, and coarse grain matrix. The deformed microstructures in the four zones were characterized in detail. Twinning and dislocation activities were found to be the main plastic deformation mechanisms for grain refinement in uranium. Four types of twinning and dislocation slip modes were distinguished. The formation of fine equiaxed grains on uranium surface should be attributed to the combined action of the twinning, dislocation activities, and the subgrain rotation driven by mechanical-thermal coupling. The surface microhardness and the natural oxidation corrosion performance were improved in uranium prepared by shot peening treatment.

A reusable graphene oxide aerogel for pre-concentrating and recovering uranium from water and valorisation of spent material in a cementitious material

Graphene oxide is an excellent substrate for pre-concentrating and recovering uranium in water. However, the need for scalable and affordable methods for producing graphene oxide has hindered its field applications. Besides, a sustainable strategy is required to recycle spent adsorbents and prevent them from entering the waste stream. In these contexts, this paper describes the application of graphene oxide aerogel (GOA) produced from graphite through a single-step combustion route for pre-concentrating and recovering uranium in water. The method allows GOA production in bulk quantity at 8–10 times less cost than prevalent Modified Hummer's graphene oxide (HGO). The paper also demonstrates using spent GOA as an active admixture in mortar to improve its mechanical properties. The physicochemical properties of the GOA and mechanistic details of uranium surface bindings were studied thoroughly. GOA showed enhanced uranium adsorption capacity (278 mg/g) compared to HGO due to abundant hydroxyl, carboxyl, and epoxy groups on GOA. However, the presence of HCO3− in water reduced uranium uptake due to highly water-soluble uranium-carbonate complexes such as (UO2)2CO3(OH)3−,UO2(CO3)34−, and UO2(CO3)22−. The addition of the spent GOA improved the cement mortar's compressive strength by 15 % compared to the pristine cube and arrested the leaching of residual uranium. The GOA addition and associated enhancement in compressive strength help to save cement and lower CO2 emissions. In short, the present work addresses the critical challenges of using GOA in pre-concentrating and recovering aqueous uranium, such as bulk production, production cost, and spent materials disposal.

The initial oxidation behavior of a UCxN1-x layer prepared by the pulsed laser ablation in the 1:1 N2+CH4 atmosphere

The uranium surface is CN-co-doped by the pulsed laser ablation technique in the 1:1 mixed atmosphere of CH4 and N2. The results of X-ray diffraction, Auger electron spectroscopy and X-ray photoelectron spectroscopy show that the modified layer prepared is a UCxN1-x solid solution with trace metallic U. When contacted with O2, the NaCl-like structure of the outmost surface layer is soon transferred to a UO2-x-like structured UO2-x(C/N)y (y < x), which can be subsequently oxidized to a CaF2-like structured UO2-x(C/N)y (y ≈ x), in which the C, N and O atoms share the tetrahedral interstitials of the U lattice. After the formation of the CaF2-like structure, the surface C and N atoms are gradually replaced by O atoms during the following oxidation process. During the oxidation of the outmost surface layer, C is more preferentially oxidized than N to produce a product of UO2-x(C/N)y, in which the ratio of C/N is lower than that in the as-prepared layer. Besides, C is easily oxidized to CO and adsorbed on the layer surface while there is no obvious evidence for the escape and adsorption of N2 or NO, implying that the migration of N may be different from that of C during the oxidation of the UCxN1-x solid solution. After oxidation, a tri-layered structure, composing of UO2 located on the outmost surface, UO2-x(C/N)y and UCxN1-x distributed in the superficial layer, and a UCxN1-x solid solution with trace metal U in the inner layer, is formed. This structure of layer can effectively prevent the further migration of O atoms into the layer from the surface and maintain the existence of C and N in the inner layer, and thus provide an excellent passivation layer for U metal against O2.

Uranium surface processes with sandstone and volcanic rocks in acidic and alkaline solutions

Understanding the behaviour of uranium waste, for disposal purposes, is crucial due to the correlation between pH values and the disposal of distinct types of waste, with low level waste typically associated with acidic pH values, and higher and intermediate level waste commonly related to alkaline pH values. We studied the adsorption of U(VI) on sandstone and volcanic rock surfaces at pH 5.5 and 11.5 in aqueous solutions with and without bicarbonate (2 mM HCO3–) using XAS and FTIR. In the sandstone system, U(VI) adsorbs as a bidentate complex to Si at pH 5.5 without bicarbonate and as uranyl carbonate species with bicarbonate. At pH 11.5 without bicarbonate, U(VI) adsorbs as monodentate complexes to Si and precipitates as uranophane. With bicarbonate at pH 11.5, U(VI) precipitated as a Na-clarkeite mineral or remained as a uranyl carbonate surface species. In the volcanic rock system, U(VI) adsorbed to Si as an outer sphere complex at pH 5.5, regardless of the presence of bicarbonate. At pH 11.5 without bicarbonate, U(VI) adsorbed as a monodentate complex to one Si atom and precipitated as a Na-clarkeite mineral. With bicarbonate at pH 11.5, U(VI) sorbed as a bidentate carbonate complex to one Si atom. These results provide insight into the behaviour of U(VI) in heterogeneous, real-world systems related to the disposal of radioactive waste.

Effect of low concentration of impurity gases on the hydrogen absorption performance of uranium

The performance of uranium as a hydrogen storage material has attracted much attention. Herein, the hydrogen absorption properties of depleted uranium in impure hydrogen containing He, Ar, CH4, N2, CO, CO2 and O2 were studied by PVT method and XPS analysis. When these impurity gases were mixed in H2 at low concentrations (0.1%∼1.5%), their behavior can be classified into three categories. The He, CH4 and Ar were chemically inert to the activated uranium powder under room condition. These three gases inhibited the absorption kinetics during the whole stage by hindering the diffusion of H2 molecules, showing a blanketing effect. The N2 and O2 did not affect the absorption kinetics but reduced the capacity by forming nitrides and oxides. The poisoning effect of N2 was weaker than that of O2. The CO and CO2 not only affected the hydrogen absorption capacity, but also strongly inhibited the absorption kinetics. These two gases are chemically adsorbed on the uranium surface to form passivation layers, thus inhibiting the adsorption and dissociation of H2 molecules and the diffusion of H atoms. The poisoning and retardation effect of CO2 were much stronger than that of CO. The above conclusions are important to further study the reactivity of mixed gas with uranium, and can also be used as a reference for other hydrogen storage systems.

Low-Melting Phosphate Glass Coatings for Structural Parts Composed of Depleted Uranium

The applications of depleted uranium in mechanical engineering are limited by its high susceptibility to corrosion. Among various methods of corrosion protection, painting is usually considered a fast and cost-efficient method; however, organic polymer paints are sensitive to ionizing radiation, which is a limiting factor, e.g., for the fabrication of shielding containers or structural parts. The solution presented in this work is the creation of a glassy inorganic layer on top of the depleted uranium surface. Zinc lead phosphate low-melting glass was investigated for this purpose. Glass frit was obtained as an amorphous solid, as confirmed by differential scanning calorimetry and X-ray diffraction. The frit was easily ground in liquid media down to sizes suitable for spraying onto the surface of depleted uranium. When the glass powder is sprayed onto the surface of a substrate and fired at 440 °C, a partially crystallized continuous film with a complex morphology is formed, which significantly inhibits corrosion. The coating material shows resistance against high doses of γ-irradiation.

Nickel electrodeposition in LEU metal foil annular targets to produce Mo-99

The most used production route of Mo-99 is through the fission of U-235 in irradiation targets that are irradiated in research reactors. The annular target is a promisor design since it can incorporate high U-235 quantities, thus increasing the production yield of Mo-99. This target type uses a foil of uranium metal enveloped by a thin nickel foil that acts as a diffusion barrier. The process of uranium enveloping with nickel foil is today done manually. This operation risks the nickel foil breaking up during target assembling. In the present work, we studied the nickel electrodeposition over uranium metal foil surfaces to replace nickel foils. A pre-forming procedure of the uranium metal foil by calendering was developed to facilitate the assembling operation. The electrodeposition was done over the uranium foil pre-conformed in a tubular shape. An automated apparatus for electrodeposition of nickel in uranium tubular-shaped foil was developed. The results showed that the high nickel adherence to uranium metal depends on the proper activation of the uranium surface. Among the activation processes studied, the mechanical activation showed good adhesion of the nickel layer, with a loss of only 0.16% of uranium mass. Homogeneous and regular 12 μm thickness electrodeposited layers over the uranium metal were obtained. This work showed that the process could be used in continuous production technology, such as the production of irradiation targets.

The modified wear resistance of uranium induced by ultrasonic surface rolling process

With the development of the nuclear industry, uranium materials with better wear resistance are demanded. This paper aims to demonstrate the effects of ultrasonic surface rolling process (USRP) on the tribological properties of uranium. Surface characteristics, i.e., surface topography, residual stress, microstructure, hardness and wear behavior, were examined. The results indicated that nanostructure, deformation twins and texture were generated on the uranium surface after USRP treatment. Surface roughness Ra, Rp and Rv showed a remarkable decline by 85.6%, 85.5% and 74.8% compared to the untreated specimen, respectively. Moreover, increased microhardness and enhanced residual compressive stress were obtained by USRP treatment. According to the wear tests against silicon carbide counterball in dry sliding conditions, the friction coefficient was decreased to 0.66 and the wear volume was also drastically reduced, which prolonged the friction and wear life of uranium materials. Additionally, it was concluded that abrasion wear, adhesion wear and oxidation wear played the dominant roles in the USRP treated specimens and the silicon carbide counterball showed the characteristics of abrasion wear.

Middle Jurassic - Early Cretaceous drastic paleoenvironmental changes in the Ordos Basin: Constraints on sandstone-type uranium mineralization

Middle Jurassic - Early Cretaceous paleoenvironmental evolution is fundamental to U (Uranium) migration and U-enrichment in the Ordos Basin. However, the detailed long-term terrestrial climate evolution across the Jurassic/Cretaceous (J/K) transition is still lacking. This study presents a comprehensive geochemical element analysis of 88 fine-grained sediments from Well Pengyang-SDI to verify the major paleoenvironmental changes including the changes in paleoclimate, paleoweathering, paleosalinity, and paleo-redox conditions. Various paleoweathering proxies (e.g., CIA) suggest that the sediments in the source area experienced a stronger chemical weathering process during the Jurassic than during the Early Cretaceous. The paleoclimate proxies (e.g., C–value, Rb/Sr, and Sr/Cu ratios) and the sharp increase in the salinity of the Luohandong Formation (LHDF) in the Ordos Basin may provide strong evidence for the continuous aridification since the Early Cretaceous. In the Lower Cretaceous non-uranium mineralization section in the basin, large U primary enrichments (high U/Th ratio) may be related to the rise in atmospheric oxygen levels during the Cretaceous. This remarkable paleoenvironmental transition controls the spatial–temporal collocation of the coal and oil in deep strata and the uranium in upper host rocks. The large-scale hydrocarbon migration from the deep strata transformed the Cretaceous aeolian sandstones into favorable uranium reservoirs. Core alteration observation and iron content testing shows that the large-scale oxygen-containing uranium surface fluid came from the western margin of the Tianhuan Depression. Finally, based on the analysis of the genetic relationships among uranium, coal, hydrocarbons, and alteration this study proposes that regions adjacent to the western margin of the Changqing oilfield with large faded alteration in the Lower Cretaceous strata are new ideal target areas for further uranium exploration.

Soil and Aquifer Properties Combine as Predictors of Groundwater Uranium Concentrations within the Central Valley, California.

With the increasing global need for groundwater resources to fulfill domestic, agricultural, and industrial demands, we face the threat of increasing concentrations of naturally occurring contaminants in water sources and a consequential need to improve our predictive capacity. Here, we combine machine learning and geochemical modeling to reveal the biogeochemical controls on regional groundwater uranium contamination within the Central Valley, California. We use 23 environmental parameters from a statewide groundwater geochemical database and publicly available maps of soil and aquifer physicochemical properties to predict groundwater uranium concentrations by random forest regression. We find that groundwater calcium, nitrate, and sulfate concentrations, soil pH, and clay content (weighted average between 0 and 2 m depths) are the most important predictors of groundwater uranium concentrations. By pairing multivariate partial dependence and accumulated local effect plots with modeled aqueous uranium speciation and surface complexation outputs, we show that regional groundwater uranium exceedances of drinking water standards, 30 μg L-1, are dependent on the formation of uranyl-calcium-carbonato species. The geochemical conditions leading to ternary uranyl complexes within the aquifer are, in part, created by infiltration through the vadose zone, illustrating the critical dependence of groundwater quality on recharge conditions.

Atomic uranium modified graphdiyne as catalytic material for hydrogen evolution reaction: An interfacial descriptor led mechanistic study

Catalysis operated on supported single metal atom is considered as one of the vital efforts for chemical and energy conversion. Despite the enormous amount of research on single transition metal (TM) enhanced graphdiyne (GDY) material, its modification with slightly depleted uranium remains unexplored. Herein, we conducted relativistic density functional theory (DFT) calculations for the accumulation of atomic uranium inside size-suitable GDY pore. The stability of graphdiyne-uranium (GDY-U) is corroborated in terms of short U–C bond distances (2.34–2.44 Å), with local depletion of charge and greater U(5f)-C(p) molecular orbital overlap. The magnificent structural and electronic properties of GDY-U system qualify it for the investigation of hydrogen evolution reaction (HER). The HER performance of all exposed sites, including central metal atom and four dissimilarly coordinated acetylenic carbons has been examined and compared with that of pure GDY. As the overall HER interfacial descriptor, free energy change of the intermediate state (ΔGH∗) at the central metal surface of GDY-U was found to be most favorable (0.153 eV) amongst all sites and far superior to those of pure GDY. In addition to the uranium surface, the coordinated carbons also show improved HER activity, indicating the enhancement of the system upon metal insertion. The calculated ΔGH∗ of the GDY-U system here is comparable to some of the recently reported GDY-TM materials, suggesting that atomic uranium could be an exceptional alternative for applied common catalysts.

Hydride Mapping in Uranium Using MLLS Fitting of Electron Energy-Loss Spectra

Previous studies of the reaction of hydrogen gas with uranium metal have been mainly limited to microscale or higher observations of uranium surfaces. We have characterized subsurface features of uranium hydride structures formed in a depleted uranium pellet aged for over 9 years under a controlled atmosphere with an initial H2 partial pressure of 0.667 kPa (5 torr) using electron energy-loss spectroscopy (EELS), giving information at lower length scales than previously reported. We demonstrate that multiple linear least-squares fitting of the O4,5 edge of the uranium EEL spectrum, with input spectra generated from the dataset being analyzed, can be used to map hydride and metal phases in aged uranium metal at the nanometer scale. While the interface between the bulk metal and the subsurface hydride structures is sharp and well defined, the interior of the reacted region is heterogeneous, containing a mix of polycrystalline UH3 and stranded, apparently unreacted, U metal.

The preparation and properties of the C-doped modified layers on uranium surfaces by pulsed laser irradiating

The C-doped modified layer on uranium surface was prepared by pulsed laser irradiating in methane atmosphere. The microstructure of the obtained layer was characterised by X-ray diffraction (XRD), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The results show that the obtained layer is a composite layer with a gradual transition structure from UO2/free carbon to UO2-x/UCxOy/free carbon to UC1-xOy/UC, and eventually to metallic U. To investigate the oxidation mechanism of the C-doped modified layer, initial oxidation behavior of O-rich surface and O-poor surface obtained after sputtering for 7 min and 60 min were studied by XPS. And the results illustrate that UCxOy with U 4f7/2 binding energy at 379.5 eV in the O-rich region is more resistant to oxidation than UC1-xOy/UC under oxygen exposure. Free carbon is distributed in the surface layer of C-doped modified layer prepared by pulsed laser irradiating, and the free carbon is also formed after oxidation of UC1-xOy, UC and UCxOy at room temperature. We speculate that the aggregation of free carbon increases the ability to prevent the inward diffusion of O atoms. Moreover, the results of the potentiodynamic polarization tests show that the C-doped modified layer improved the corrosion protective properties of metallic U. The self-corrosive potential and self-corrosive current of the C-doped modified layer are −1.47 × 10−1 V vs. SCE and 9.46 × 10−8 A/cm2, respectively, while the values of metallic U are −5.67 × 10−1 V vs. SCE and 8.08 × 10−7 A/cm2, respectively.

Preparation and analysis of strain-free uranium surfaces for electron and x-ray diffraction analysis

This work describes a methodology for producing high quality metallic surfaces from uranium primarily for characterisation and investigations involving electron backscatter diffraction. Electrochemical measurements have been conducted to inform ideal polishing conditions to produce surfaces free from strain, induced by mechanical polishing. A commonly used solution for the electropolishing of uranium, consisting in part of phosphoric acid, was used to conduct the electrochemical experiments and polishing. X-ray diffraction techniques focusing on the surface show low stresses and strains are exhibited within the material. This is mirrored in good quality electron backscatter diffraction.

The microstructure and oxidation behavior of the N-doped modified layers on uranium surface prepared by pulsed laser irradiating

The N-doped modified layer on uranium surface was prepared by pulsed laser irradiating in nitrogen atmosphere. The results of microstructure characterised by XRD, AES and XPS show that it is a composite layer with a gradual transition structure from UO2-xNy and uranium oxy-nitride (UNxOy) to UN and eventually to uranium from surface to interior. Initial oxidation behavior after sputtering for different time illustrates that UNxOy with U 4f7/2 BE at 379.6 eV is inert under oxygen exposure. Further, the thermal stability study shows that the modified layer exhibits excellent ability to prevent the O atoms from diffusing into the interior at 423 K. Moreover, the corrosion protective properties of the N-doped modified layers before and after thermal treatment were evaluated through potentiodynamic polarization tests. The results show that the N-doped modified layer improved the corrosion protective properties of metallic uranium. The self-corrosive potential and self-corrosive current of the N-doped modified layer are −5.75 × 10−2 V vs. SCE and 1.37 × 10−7 A/cm2, while the values of metallic uranium were −5.67 × 10−1 V vs. SCE and 8.08 × 10−7 A/cm2. After thermal treatment, its values are −8.57 × 10−2 V vs. SCE and 4.83 × 10−8 A/cm2, indicating that the corrosion resistance of the N-doped modified layer was further improved.

Monitoring the in-situ oxide growth on uranium by x-ray photoelectron spectroscopy

In the present work, we report the in situ monitoring of oxide growth on metallic uranium surface at room temperature for oxygen pressures from 1 × 10−7 to 5 × 10−7 Pa by means of x-ray photoelectron spectroscopy. The oxidation products were analyzed briefly and a quantitative relationship between the oxide film thickness and oxidation time was established and the rate constant was obtained. Results show that the sub-stoichiometric uranium oxides (UO2−x) were formed, and further transformed to those with smaller x values. We found that oxygen pressure has a positive effect on the rate constant as well as the oxide film thickness, and instead of oxygen pressure, oxygen exposure has a vital impact on determination of the thickness of the oxide layer.

Abiotic reduction of uranium(VI) with humic acid at mineral surfaces: Competing mechanisms, ligand and substituent effects, and electronic structure and vibrational properties

Abiotic reduction represents an attractive technology to control U(VI) contamination. In this work, an abiotic route of U(VI) reduction with humic acid at mineral surfaces is proposed and reaction mechanisms are addressed by periodic density functional theory calculations. Different influencing factors such as ligand effect, content of CO32− ligands and substituent effect are inspected. The coordination chemistry of uranyl(VI) surface complexes relies strongly on substrates and ligands, and the calculated results are in good agreements with experimental observations available. For the OH− ligand, two competitive mechanisms co-exist that respectively produce the U(IV) and U(V) species, and the former is significantly preferred because of lower energy barriers. Instead, the NO3− ligand leads to the formation of U(V) while for the Cl− ligand, the U(VI) surface complex remains very stable and is not likely to be reduced because of very high energy barriers. The U(V) and U(IV) complexes are the predominant products for low and high CO32− contents, respectively. Accordingly, the abiotic reduction processes with humic acid are efficient to manage U(VI) contamination and become preferred under basic conditions or at higher CO32− contents. The U(VI) reduction is further promoted by introduction of electron-donating rather than electron-withdrawing substituents to humic acid. Electronic structure analyses and vibrational frequency assignments are calculated for the various uranium surface complexes of the reduction processes, serving as a guide for future experimental and engineered studies. The molecular-level understanding given in this work offers an abiotic route for efficient reduction of U(VI) and remediation of U(VI)-contaminated sites at ambient conditions.