Uranium Phases Research Articles

Correlative morphological, elemental and chemical phase analyses at the micrometric scale of powdered materials: Application to nuclear forensics

To characterize a mixture of powders using several analytical techniques, it is necessary that successive analyses be carried out on well-identified and localized particles, so that each characterization corresponds to a given powder. In this publication, powdered nuclear materials are characterized at the morphological and elemental levels with a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and at the chemical level with a micro-Raman spectrometer (MRS). However, to avoid a time-consuming and insufficiently accurate microparticle relocation process between SEM/EDS and MRS, micro-Raman analyses are carried out inside the SEM using a coupling device. In this way, all three pieces of information are obtained for exactly the same micrometric spot, without moving the sample or relocating the microparticles analyzed. The information can therefore be combined to characterize each component of the mixture. In this article, we describe in detail the methodology we have developed and optimized for morphological, elemental and chemical analysis of microparticles using a combined SEM/EDS and SRM. This methodology has been applied to powdered nuclear materials in two international nuclear forensics exercises. In the first exercise, named CMX-6, the combined use of the two instruments identified the presence of PuO2 microparticles and several uranium compounds (UO2, U3O8, UO2F2) in both materials. In the second exercise, called CMX-7, the methodology developed enabled us to distinguish two chemical phases of uranium, a uranyl oxy-hydroxide and a uranyl nitrate, each characterized by specific morphologies and the detection or non-detection of a minor elemental constituent (calcium).

Elucidating the Composition and Structure of Uranium Oxide Powders Produced via NO2 Voloxidation.

Voloxidation is a potential alternative reprocessing scheme for spent nuclear fuel that uses gas-solid reactions to minimize aqueous wastes and to separate volatile fission products from the desired actinide phase. The process uses NO2(g) as an oxidant for uranium dioxide (UO2) fuel, ideally producing soluble uranium powders which can then be processed for full recycle. To continue development of the process flowsheet for voloxidation, ongoing examination of the process chemistry and associated process materials is required: discrepancies in the proposed chemical reactions that occur when spent nuclear fuel is exposed to NO2(g) atmospheres must be addressed. The objective of this work is to analyze the intermediate solid phases produced during voloxidation to support verification of the proposed NO2(g) voloxidation reaction mechanisms. This objective was achieved through using (1) powder X-ray diffraction and Raman spectroscopy to identify bulk uranium phases and (2) scanning electron microscopy to describe the morphology and microstructure of the powders at each reaction stage. The initial oxidation of UO2 under NO2(g) reactions produced ε-UO3. Further exposure to NO2(g) did not nitrate the solid to produce uranyl nitrate, as reported in some literature. However, after the powder was hydrated with steam and then further exposed to NO2(g), some traces of uranyl nitrate hexahydrate were found. The results of this study suggest that surface hydration of powders plays a vital role in uranyl nitrate formation under voloxidation conditions and raises questions about the kinetics of the oxide-to-nitrate voloxidation conversion process. Future chemical and engineering design decisions for the voloxidation process may benefit from an improved understanding of these chemical mechanisms.

Chemical Species Change of Radionuclides by Microorganisms: Effects of Exudated Siderophores

Microbial exudates including siderophore, which changes chemical species of actinides and lanthanides. We have investigated effects of desferrioxamine B (DFOB; one of the siderophores) and siderophore-like organic molecules (SLOM) on the adsorption of lanthanides by microbial cells, aluminium oxide (Al2O3), and manganese (Mn) oxides. When DFOB was present, the distribution coefficients of cerium (Ce) were measured to be lower than those of neighboring elements of lanthanum (La) and praseodymium (Pr) (Negative anomaly of Ce adsorption). Even though initial oxidation state of Ce in the solution was III, that was changed to IV after the addition of DFOB, indicating that Ce(III) was oxidized by forming complex with DFOB. When lanthanides were adsorbed by biogenic Mn(IV) oxides, negative anomaly of Ce adsorption was observed in the sorption in alkaline solution. Ce(III) was oxidized to forme the complexes of Ce(IV) with SLOM in the solution. These results show that siderophore possesses high performance of oxidation of Ce(III) to Ce(IV) during association, affectiong the adsorption behavior of Ce. After Fukushima accident, radioactive Cs accumulation by Eleutherococcus sciadophylloides (Koshiabura) caused by the dissolution of Fe from soil around the roots, that was dominated by siderophore releasing microorganisms (SB). These SBs may enhance dissolution of iron (Fe) and uranium (U) phases in the nuclear fuel debris formed in the nuclear reactors in Fukushima Daiichi nuclear power plant. Thus, in the interaction between microorganisms and radionuclides, SLOMs discharged by microorganisms are deeply involved in the chemical state change of radionuclides.

Reactivated fracture-controlled uranium mineralization: An example from NNE-SSW Kamaguttapalle–Kammapalle tract, Kadapa district, Andhra Pradesh, India

The Cuddapah Basin (CB) and its basement can be considered as an appropriate uranium metallogenic province. The basement complex of CB in the Eastern Dharwar Craton (EDC), India, has been the target for structurally controlled uranium exploration since the early 1980s. Most of the brittle-ductile deformation induced reactivated narrow linear fracture zones often exhibit significant surface and sub-surface uranium occurrence. There are about 10 fracture zones associated with uranium mineralization. Amongst them, the Kamaguttapalle–Kammapalle fracture zone (KKF) in the south of the CB is quite interesting from an exploration viewpoint with a suitable geologic setting in space and time frame. Geomorphic, structural and petro-mineralogic exploration guides imply the possibility of economic uranium concentration along the NNE-SSW trend where tectonic reactivation took place over pre-existing ductile shear zone. Progressive exhumation and transition from ductile to brittle deformation regime led to the development of dominant strike slip tectonics with an associated riedel fracture system, which are subsidiary coeval shear fractures used in depicting sense of movement. Intense illitization and hematitization are often observed. Quartz reefs and veins along the NNE-SSW trend cut the preexisting mylonites at places. About 9 stages of fracturing is observed out of which 3 stages are related to uranium mineralization. All the 9 deformations are related to strike slip tectonics which in turn is part of Grenville orogeny. Paragenetic sequence and mode of occurrences of uranium phases deciphers about the importance of low to moderate level epi‑meso type hydrothermal (100°–250 °C) fluids as carrier of U. Reactivated fractures and quartz veination also act as controlling factors of mineralization. Strain analysis points to positive and negative flower structures at different segments of the fracture zone. Based on the litho-structural attributes it is established that maximum compression (σ1) acted along the NNW-SSE direction. Fractured quartz veins provide well defined permeability and trapping facility by restricting transverse fluid movement as a mechanical barrier. The presented model gives an idea on role of strike-slip tectonics in uranium mineralization for narrowing down the target zone for sub-surface exploration.

KINETICS OF HIGH-TEMPERATURE NITRIDIZATION OF Zr-U-BASED ALLOYS

The kinetic patterns of nitride formation have been established and the sequence of structural transformations characterizing high-temperature (at 1900 °C) nitridation of Zr-U alloys containing 2 and 5 wt.% U in the range from 3.5 to 60 minutes is presented. During high-temperature saturation with nitrogen for each composition, the solid solution (Zr,U) decomposes with the formation of composite structures ZrN-(ZrN1-n/UxEy/U)-ZrN (where E is O, N; n, x, y are stoichiometric coefficients ). During the decomposition of the solid solution, zirconium nitride is formed and a phase of metallic uranium is released, which accumulates impurities contained in the initial solid solution in the central part of the sample. Kinetic curves for a temperature of 1900 °C are approximated by an exponential law and correspond to the nitridation of zirconium. The nitridation rate of the (Zr,U) solid solution increases with increasing uranium content. To complete the formation process of a compact nitride solid solution of (Zr,U)N of stoichiometric composition, it is necessary to increase the temperature and increase the reaction duration.

Microstructural and phase changes in alpha uranium investigated via in-situ studies and molecular dynamics

A deeper knowledge of thermally induced microstructural and phase evolution in nuclear metallic fuel can be obtained using novel in-situ microscopic analyses. Such studies can provide information on the dynamics of phase transitions which is not possible with conventional postmortem characterization (post-irradiation examination). In this work, the behavior of alpha uranium (α-U) was investigated via in-situ heating tests in a transmission electron microscope. The main objective is to understand the microstructural and phase changes, such as defect annihilation and β phase formation and retention, observed in reactor in-pile transient studies at the Transient Reactor Test facility. Indeed, defect migration and rearrangement were observed within the α phase starting at 673 K; α→β phase transition was observed between 773 K and 1,073 K during the heating ramp (which is in the temperature window reported for α→β transition temperatures). Recrystallization and formation of nano grains was observed at high temperatures (over 1,073 K). Such recrystallization was possibly related to the formation of the γ phase. Finally, it was indeed observed that the β phase (but not γ phase) was retained at room temperature upon rapid cooling. Molecular dynamics studies support these experimental results and shows that the γ phase of pure uranium cannot be retained at room temperature if not stabilized with the addition of an alloying element.

Grain growth kinetics of the gamma phase metallic uranium

Metallic uranium is a leading fuel form for sodium cooled fast reactors as an enabling technology of future nuclear energy systems. Mechanistic understanding of fuel behaviors and kinetics under thermodynamic equilibrium and highly non-equilibrium conditions are essential for evaluating fuel performance. It is important to understand and predict the grain and pore evolutions of metallic fuels under thermal and irradiation conditions. However, very limited data are available on the grain growth kinetics and mechanisms of pure gamma phase uranium. In this paper, the pure gamma uranium pellets with different grain structures were fabricated by combining high-energy ball milling and spark plasma sintering. Isothermal annealing tests were performed to investigate the grain growth behavior of the pure gamma phase uranium with different initial grain sizes. A parabolic relationship in grain growth with time was identified for the submicron-sized (374 nm) sample. In contrast, for the nano-sized (137 nm) sample, the grain growth shows a linear relationship with time. The activation energies of grain growth were determined as 199.5 KJ/mol and 80.6 KJ/mol for nano-sized and submicron-sized grain structures, respectively. For the nano-sized sample, the rate-control step of grain growth is dominated by the triple-junction migration, in which the grain boundary triple junction drags the grain growth, leading to a higher activation energy than the bulk diffusion. The dominating mechanism for the submicron-sized sample is grain boundary diffusion. The mechanistic understanding and critical data obtained on the kinetics of pure uranium phases will be useful to evaluate fuel behavior under thermodynamic equilibrium conditions and develop a high fidelity model to predict fuel performance.

High-precision ID-TIMS U-Pb geochronology of perovskite (CaTiO3) from the Ice River Complex, Southeastern British Columbia

Uranium‑lead perovskite in situ geochronology has become a cornerstone technique for determining the emplacement timing of alkaline, ultrapotassic, and silica-undersaturated igneous rocks, kimberlites, and carbonatites. Accurate in situ dates are dependent on the availability of matrix matched mineral reference materials which themselves are chemically well characterized and dated accurately to the highest possible precision. When dating perovskite to high precision, such as by isotope dilution thermal ionization mass spectrometry (ID-TIMS), appropriately accounting and correcting for the quantity and isotopic composition of Pb incorporated into the crystal upon crystallization (Pbi) is a large source of uncertainty and potential inaccuracy. Unfortunately, although ultra-high precision perovskite dates are attainable with modern mass spectrometry techniques, the accuracy of applied Pbi compositions, which can be a considerable percentage of total Pb in a crystal, has not kept pace, resulting in percent level inaccuracy on precisely measured isotopic ratios.In an effort to characterize the age and initial Pb isotopic composition of a readily available, relatively pure perovskite endmember (93–98%) which will be useful as a matrix matched age reference material for in situ U-Pb geochronology, we date crystals isolated from three samples from the Ice River Complex (samples 81IR6, I90.3, and I92.30) by ID-TIMS and evaluate their suitability as known-age reference materials. We directly determine the isotopic composition of Pbi in each sample by ID-TIMS measurement of cogenetic low 238U/204Pb (μ < 500) phlogopite, apatite, and/or clinopyroxene. Using these initial common Pb isotopic compositions, which are significantly more radiogenic than those predicted by age appropriate Pb evolution models, we obtain ultra-precise weighted-mean 206Pb/238U perovskite dates of 355.83 ± 0.14, 355.04 ± 0.15, and 357.34 ± 0.12 Ma on the three Ice River samples, respectively. These dates are consistent with the relative emplacement ages previously established for units in the Ice River layered ultramafic series based on field relationships. They also demonstrate the feasibility of attaining accurate perovskite dates in instances when maximum precision is critical by utilizing ID-TIMS analysis of cogenetic low uranium phases for direct measurement of Pbi.

Elucidating mobilization mechanisms of uranium during recharge of river water to contaminated groundwater

The recharge of stream water below the baseflow water table can mobilize groundwater contaminants, particularly redox-sensitive and sorptive metals such as uranium. However, in-situ tracer experiments that simulate the recharge of stream water to uranium-contaminated groundwater are lacking, thus limiting the understanding of the potential mechanisms that control the mobility of uranium at the field scale. In this study, a field tracer test was conducted by injecting 100 gal (379 l) of oxic river water into a nearby suboxic and uranium-contaminated aquifer. The traced river water was monitored for 18 days in the single injection well and in the twelve surrounding observation wells. Mobilization of uranium from the solid to the aqueous phase was not observed during the tracer test despite its pre-test presence being confirmed on the aquifer sediments from lab-based acid leaching. However, strong evidence of oxidative immobilization of iron and manganese was observed during the tracer test and suggested that immobile uranium was likely in its oxidized state as U(VI) on the aquifer sediments; these observations ruled out oxidation of U(IV) to U(VI) as a potential mobilization mechanism. Therefore, desorption of U(VI) appeared to be the predominant potential mobilization mechanism, yet it was clearly not solely dependent on concentration as evident when considering that uranium-poor river water (<0.015 mg/L) was recharged to uranium-rich groundwater (≈1 mg/L). It was possible that uranium desorption was limited by the relatively higher pH and lower alkalinity of the river water as compared to the groundwater; both factors favor immobilization. However, it was likely that the immobile uranium was associated with a mineral phase, as opposed to a sorbed phase, thus desorption may not have been possible. The results of this field tracer study successfully ruled out two common mobilization mechanisms of uranium: (1) oxidative dissolution and (2) concentration-dependent desorption and ruled in the importance of advection, dispersion, and the mineral phase of uranium.

Hydrothermal Uranium Mineralization in Umra, Udaipur District, Rajasthan, India: Inferences from Petrography, Mineral Chemistry and Sulfur Isotopic Studies

Abstract Uranium mineralization at Umra, Udaipur district, Rajasthan is hosted by carbon and siliceous phyllite (with minor calcareous components) of the Paleoproterozoic Aravalli Supergroup. The mineralized zone trends NE-SW to N-S, dipping 45-55° towards SE to E. Uraninite, coffinite and brannerite are the main uranium phases observed that occur in association with sulphides (pyrrhotite, pentlandite, gersdorffite, chalcopyrite and pyrite), magnetite and ilmenite. Uraninite is found to occur as veins and disseminated ultrafine grains. Colloform and botryoidal texture with shrinkage/ synaeresis cracks indicate the presence of pitchblende also, formed at relatively low-temperature. Textural features suggest remobilization of disseminated syngenetic and / or diagenetic uranium minerals during low-grade metamorphism and deformation. Textural features suggest multiple phases of sulphide mineralization also. Mineral chemical data indicate that uraninite and pitchblende are compositionally similar containing 80.09-87.17 wt.% UO2 (av. 84.45 wt.%) and 11.37-11.82 wt.% PbO (av. 11.58 wt.%) with negligible SiO2, CaO, TiO2, FeO, ThO2 and RE2O3. High U/Th ratio (&amp;gt;1000) in uraninite/ pitchblende indicates limited cationic substitution and therefore low temperature (&amp;lt;350°C) hydrothermal origin of mineralization. Chemical age, calculated on the basis of UO2, ThO2 and PbO concentration in uraninite, indicates the minimum age of mineralization is 956±12 Ma. S isotopic data of pyrite suggest syn-sedimentary or diagenetic origin by bacterial reduction of seawater or pore water sulphate under anoxic conditions (-24.7‰ to -16.2‰), as well as hydrothermal (-3.7‰ to +8.8‰) origin. Similarity of S isotopic composition of hydrothermal pyrite with that of pyrite (+5.2‰) from meta-basic rock indicates that a component of S has been derived from the associated basic/meta-basic rocks in the area. Signatures of multiple generations of uranium mineralization, and multiple episodes of deformation, metamorphism and hydrothermal alteration reveal a complex metallogenic history in Umra. Compositional details of uraninite and their occurrence in the form of veinlets in association with hydrothermal pyrite suggest dominantly epigenetic hydrothermal mineralization events coeval with deformations of the Aravalli sediments.

Genesis of Magmatic Albitite Related Hydrothermal Uranium Mineralisation in Geratiyon-ki-Dhani, Sikar District, Rajasthan: Evidences from Fluid Inclusion Studies

Abstract Geratiyon-ki-Dhani (GKD) area is located in Khetri sub-basin of North Delhi Fold Belt and exposes metasediments of Ajabgarh Group. The Ajabgarh Group comprises quartz-biotite schist which is intruded by albitite and granite. The primary uranium phases viz. davidite and brannerite occurring with calcite and /or quartz veins are common in albitite. Quartz and calcite from mineralised albitite have been selected for characterization of the ore-forming fluid, which makes a promising uranium deposit. Fluid inclusion petrography indicated presence of three (03) types of fluid inclusions (FIs) based on phases present at room temperature and mode of occurrence; polyphase (type 1a &amp; 1b) and biphase (type 2) inclusions. Type 1a FIs are primary in nature and are composed of vapour bubble, salt crystals with &amp;lt;50% liquid phase and are irregular or negative crystal in shape varying in size between 12–23μm and occasionally contain hematite grain. The type 1a inclusions homogenised mostly by dissolution of salt crystal between 243-382°C (mode=340°C, n=30) corresponding to salinity of 34.32–46.13 wt% NaCl-eq (mode=42wt% NaCl-eq, n=30). Type 1b FIs have vapour bubble, salt crystal with &amp;gt; 50% liquid phase, occur as trans-granular trails and in clusters, having irregular shapes with size varying from &amp;lt;5–12μm. The inclusions homogenised by disappearance of salt crystals at 209 to 294°C (mode= 280°C, n=35) corresponding to salinity range from 29.03–38.59 wt% NaCl-eq (mode=36wt% NaCl-eq, n=35). A wide range of overlap is noted in salinity (34.32–38.59 wt% NaCl-eq) and homogenization temperature (243 – 294°C), histograms of type 1a &amp; 1b inclusions suggest them to be part of a common fluid activity. Polyphase inclusions show simple cooling and follow halite saturation curve in salinity-temperature diagram. Biphase inclusions are invariably associated with type 1b and also show a cooling trend. On the basis of fluid characteristics, it is concluded that magmatic albitite has provided single mineralising solution resulting in davidite enriched mineralisation. The gradual cooling of high temperature (243- 382°C), high saline (34.32– 46.13 wt% NaC-leq) hydrothermal fluids has ensued U-mineralisation in GKD. The evidences presented here are significant as magmatic albitite hosted mineralisation is found for the first time in North Delhi Fold Belt of Khetri sub-basin of northwest India. It has opened a new concept for exploration which will help more resource addition.

Structure, stability, and actinide leaching of simulated nuclear fuel debris synthesized from UO2, Zr, and stainless-steel

To evaluate the chemical structure and stability of nuclear fuel debris comprising UO2, Zr, and stainless-steel (SUS) generated during the Fukushima Daiichi Nuclear Power Plant accident in Japan in 2011, the simulated debris of a UO2–SUS–Zr system and those of other fundamental-component-containing systems were synthesized and characterized. The simulated debris was synthesized by heat treatment at 1600 °C under inert (Ar) or oxidative (Ar + 2% O2) atmospheres for 1–12 h. 237Np and 241Am tracers were doped for leaching tests from the simulated debris. The simulated debris was characterized using X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, Raman spectroscopy, and Mössbauer spectroscopy, and the results revealed that the major uranium phase of the UO2–SUS–Zr debris comprised a Zr(IV)-and-Fe(II)-containing UIVO2 solid-solution, regardless of the treatment conditions. The results of a long-term immersion test of the simulated debris in pure water and seawater revealed that the macroscale crystal structure of the simulated debris was chemically durable under wet conditions for at least a year. Furthermore, the leaching test results indicated that the leaching ratios of U, Np, and Am from the UO2–SUS–Zr debris were small and lower than 0.08% for all the experiments in this study.

High-pressure structure of praseodymium revisited: In search of a uniform structural phase sequence for the lanthanide elements

Angle-dispersive x-ray powder diffraction experiments have been performed on praseodymium metal to a pressure of 205 GPa. Between 20 and 165 GPa only the $oC4$ ($\ensuremath{\alpha}$-uranium) phase is observed, in agreement with previous studies. At 171(5) GPa we find a transition to a tetragonal $tI2$ phase which is isostructural with the high-pressure post-$oC4$ phase seen in the neighboring lanthanide cerium above 12 GPa, and with the high-pressure phase of the actinide thorium seen above 100 GPa. Electronic structure calculations determine the $oC4\ensuremath{\rightarrow}tI2$ transition to occur at 130 GPa at 0 K, but find another phase, with the $hP1$ (simple hexagonal) structure, to have a lower enthalpy than both the $oC4$ and $tI2$ structures above 20 GPa at 0 K.

Systematic structural study in praseodymium compressed in a neon pressure medium up to 185 GPa

Angle-dispersive x-ray powder diffraction experiments have been performed on praseodymium metal compressed in a soft pressure-transmitting medium at ambient temperature up to 185 GPa. We observe the previously reported high-pressure structural transition sequence up to 20 GPa and the coexistence of body-centered orthorhombic (bco) Pr and \ensuremath{\alpha}-U Pr from $\ensuremath{\sim}20$ up to $\ensuremath{\sim}38\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The \ensuremath{\alpha}-U structure of Pr is stable from 20 to 185 GPa, and no evidence of the proposed transition to a primitive orthorhombic phase $&gt;147\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ was observed. With density functional theory (DFT), we calculated the lattice parameters and $y$ coordinate for \ensuremath{\alpha}-U Pr and found good agreement between our calculations and experimental measurements. The obtained DFT energies of the proposed primitive orthorhombic $(P{2}_{1}{2}_{1}{2}_{1})$ and the \ensuremath{\alpha}-uranium phases at $\ensuremath{\sim}150\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ show that the \ensuremath{\alpha}-uranium phase is lower in energy. Hence, neither our experimental data nor our DFT results support the transition to a primitive orthorhombic phase $&gt;150\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. DFT suggests, however, that Pr may transform to the $P{2}_{1}{2}_{1}{2}_{1}$ phase above $\ensuremath{\sim}220\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. We also compare the axial ratios and lattice parameters of praseodymium to \ensuremath{\alpha}-uranium structured Nd, Ce, and U.

Effect of alkyl chains configurations of tertiary amines on uranium extraction and phase stability – part II: Curvature free energy controlling the ion transfer

We consider a structural and thermodynamic analysis of uranium solvent extraction by tertiary amines for which the alkyl chain configuration has been modified. The first part of this work revealed that tertiary amines with longer or branched alkyl chains allow tuning the phase stability and uranium extraction by modifying the volume of the polar species extracted in the organic phase. A complete fit of SANS data confirms in this second part that this phenomenon is related to the supramolecular self-assembly of the tertiary amines into smaller reverse micelle like aggregates for longer or branched alkyl chains. It moreover shows that these smaller aggregates quench third phase formation thanks to reduced attractive interactions between them. A thermodynamic analysis based on the “ienaic” approach further rationalizes this effect by showing that alkyl chains have a significant effect on the aggregate’s curvature free energy. The latter becomes a predominant inhibitor of uranium free energy of transfer due to the high entropic contribution originated by the extractants branching and packing.

Effect of alkyl chain configuration of tertiary amines on uranium extraction and phase stability – Part I: Evaluation of phase stability, extraction, and aggregation properties

Amine-based extraction processes are broadly exploited for the selective extraction of uranium from sulfuric leaching liquors. The molecular forces (that is, the enthalpy associated with the multi-scale structuration of tertiary amines) are balanced by entropic effects that have not yet been identified. This first part in a series of reports describes the effects of the alkyl chain configuration on the phase stability and aggregation properties of tertiary amines. It is demonstrated that tertiary amines with longer or slightly branched alkyl chains provide better phase stability and enable constant extraction of uranium. In comparison, more branching of the alkyl chains hinders the formation of a highly curved interfacial layer, thus preventing efficient extraction. Combined small-angle scattering determination of the nanostructures in the organic phase and oil–water surface tension measurements revealed that the alkyl chains effects on the efficiency of the extraction process and phase stability are associated with reverse aggregates with smaller polar core volumes and less co-extracted water, suggesting that packing of the alkyl chains has a direct impact on the extraction efficiency of the solvent extraction system.

Density functional theory calculations of the thermodynamic and kinetic properties of point defects in β-U

Density functional theory (DFT) calculations of the thermodynamic and kinetic properties of point defects in the β phase of uranium are reported. Defect energies and entropies were calculated using 2 × 2 × 2 supercells and the Generalized Gradient Approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) for the exchange-correlation potential. Due to computational cost, calculations of the vibrational properties governing entropies were performed by only displacing atoms within (roughly) the 3rd or 4th nearest neighbor shell of the defect, which implicitly assumes that atoms beyond this distance are unaffected by the defect. Migration barriers were estimated by nudged elastic band (NEB) calculations. The low symmetry of the β -U phase (the unit cell is tetragonal and contains 30 atoms) results in many point defect configurations and even more migration pathways. A connectivity map, starting from the most stable point defects, was developed in order to identify the rate-limiting step controlling the net diffusion rate in each crystallographic direction. The uranium self-diffusivity tensor was calculated by combining the defect formation energies , entropies, migration barriers and attempt frequencies. The fastest diffusion rate was determined to be a vacancy mechanism in the z crystallographic direction. The predicted uranium self-diffusivity for this mechanism agrees well with available experimental data. The diffusion mechanisms and rates identified in this study will inform fuel performance models of swelling and gas evolution.

Anisotropy in the elasticity of α-U from first-principles calculations and nanoindentation

The anisotropic behavior of elastic properties of the low-temperature phase of uranium was studied by theoretical and experimental methods in this work. Firstly, based on the density functional theory , the elastic constants and compliance constants of α-U were obtained. Secondly, Young's modulus , bulk modulus , shear modulus, and Poisson's ratio of polycrystalline α-U were calculated using the Voigt-Reuss-Hill model. Then, according to the crystal symmetry of α-U, the spatial distribution characteristics of its Young's modulus were calculated and displayed. The Young's modulus of α-U in some specific crystal directions (including the typical deformation-induced twinning {1 3 0}, {1 7 2}) were also given. Nanoindentation method combined with electron backscatter diffraction (EBSD) technology were used to measure and characterize the grains with different crystal orientations , to obtain experimental values of Young's modulus of α-U in different crystal orientations. Also, the inverse pole figure of Young's modulus of α-U was given.

Cold and hot uranium in DFT calculations: Investigation by the GTH pseudopotential, PAW, and APW + lo methods

The comparison of the three DFT methods: pseudopotential of the Goedecker, Teter, and Hutter form, projected augmented waves, and augmented plane waves with localized orbitals, is presented. Uranium as one of the actinides is considered. One of the main aims of this work is to check a new uranium pseudopotential in the form proposed by Goedecker, Teter, and Hutter. Another significant point is an estimation of the correctness of PAW approach in following number of tasks: structures, energies, and pressure of four uranium phases up to threefold compression, phase transitions, electron densities at different atomic volumes, and thermal contributions (caused by changes of atomic positions) in free energy and forces of bcc uranium at twofold compression and temperatures not higher than 5000 K.

Phreatic uranium mineralisation hosted by Neogene sediments from the Taunsa area, Dera Ghazi Khan, Eastern Sulaiman Range, Pakistan: unique exploration targets in a deformed geological setting

The Taunsa uranium mineralisation is hosted by the upper Miocene–Pliocene Litra Formation, part of the molasse sediments (Siwalik Group) deposited in the Himalayan foreland basin of the eastern Sulaiman Range, Dera Ghazi Khan, Pakistan. The Litra Formation is the only rock unit hosting economic-grade uranium occurrences in the stratigraphic record of Pakistan. Surface radioactivity is mainly associated with cemented layers of the host sandstone, known as hard-bands, whereas weakly cemented sandstone is non-radioactive. The outcropping sandstone is dominantly whitish grey owing to bleaching, which is probably related to hydrocarbon migration from the underlying marine black shales. The main sandstone-type uranium mineralisation is phreatic owing to its parallelism with the water-table, whereas the concordantly oriented tabular (roll-type) ore is subordinate. The host sandstone layers have high dips ranging from 70 to 85°E indicating a deformed geological setting that makes the Taunsa uranium ore unique. Uranium is trapped mostly by scant organic matter, probably related to hydrocarbons, and also by Fe–Ti-oxide phases, phyllosilicates (chlorite and biotite), graphitic schist and black shale clasts in the host sandstone. Uranium ore minerals are mainly coffinite, pitchblende and minor brannerite. Uranium in the ore phases is accompanied by the high contents of Ca, Fe, Ti, Si and Al. Radiometric disequilibrium in the phreatic ore is strongly positive, whereas that in the tabular (roll-front) is moderately positive, which in turn suggests the former is younger than the latter. The coexistence of the two ore-types, characterised by different intensities of radiometric disequilibrium within the same uranium deposit, suggests that the phreatic uranium ore has possibly resulted from remobilisation of the tectonically uplifted tabular (roll-type) ore and its reprecipitation at the horizontally oriented redox interface following the water-table in the host sandstone aquifers. KEY POINTS The Taunsa uranium resource is dominantly phreatic that has resulted from oxidation and remobilisation of the earlier tabular (roll-type) mineralisation. The phreatic uranium mineralisation is very young, which is indicated by low radioactivity and high positive disequilibrium in comparison with that within the tabular (roll-type) ore. Uranium phases are mainly pitchblende, coffinite and brannerite, associated with organic matter of hydrocarbon origin, with phyllosilicates and Fe–Ti-oxide phases. Outcrop of the host sandstone in the Taunsa area is pervasively bleached and is marked by flat topography.