Uranium (U) exhibits a high temperature body-centered cubic (bcc) allotrope that is often stabilized by alloying with transition metals such as Zr, Mo, and Nb for technological applications. One such application involves U–Zr as nuclear fuel, where radiation damage and diffusion (processes heavily dependent on point defects) are of vital importance. Metallic nuclear fuels swell under fission conditions, creating fission product gases such as helium, xenon and krypton. Several systems of U are examined within a density functional theory framework utilizing projector augmented wave pseudopotentials. The bulk modulus, the lattice constant, and the Birch–Murnaghan equation of state for the defect free bcc uranium allotrope are calculated. Defect parameters calculated include energies of formation of vacancies in the α and γ allotropes, as well as self-interstitials, Zr, He, Xe and Kr interstitial and substitutional defects. This work is utilized in the construction of modified Embedded-Atom Method interatomic potentials for the bcc phase of uranium as well as the binary systems of U-Xe, U-Kr and U-He. Using this potential, equilibrium volume and elastic constants are calculated at 0 K and found to be in close agreement with previous first principles calculations. Further, the melting point, heat capacity, enthalpy of fusion, thermal expansion and volume change upon melting are calculated and found to be in reasonable agreement with experiment. Calculations of dilute fission gas defects show reasonable agreement with first principles calculations. Finally, void and xenon bubble energetics are analyzed as a function of temperature.

This project aims to quantify natural gamma radiation in gold tailings disposal facilities (TDFs) relative to uranium concentration data in order to use natural gamma detection methods as alternative methods for uranium resource estimation modelling in gold tailings. Uranium migration within the New Machavie TDF was also investigated as migration affects both the grade of the TDF as a uranium resource and poses a threat to the environment. In order to determine the most appropriate radiometric testing procedure, various methods were employed for natural gamma detection, including surface natural gamma spectrometry, borehole natural gamma spectrometry and scintillometry, as well as incremental sampling. These measurements were then statistically compared to ICP-MS analyses to find the best method, and then modelled to apply volumetric resource estimation procedures. The oxidation reduction potential was also tested as uranium geochemistry is dependent on oxidation for mobilisation. Furthermore, leaching tests were employed to relate specific anions as a mode of transportation in solution. Results indicated that down-hole natural gamma spectrometry performed the best and that 2376.87 kg of uranium is present in the TDF. Migration modelling indicated that uranium is mobilised away from the oxidized top area of the TDF and that accumulation occurs in the saturated zone of the TDF under a reducing environment. Sulphate anions as the result of pyrite oxidation are primarily responsible for the mobilisation as radionuclides in New Machavie. The results of this project can be applied to the resource estimation of all uranium bearing tailings facilities prior to re-mining as a means to decrease exploration costs and to accurately model the distribution of uranium. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2014

Extreme environments created through natural and anthropogenic processes harbor microbes with diverse physiologies capable of catalyzing chemical reactions which are environmentally beneficial on local and global scales. This work focused on two unique environments, the Gulf of Mexico (GoM) submarine mud volcano systems and the subsurface soils at the Department of Energy s (DOE) Field Research Center (FRC) located in the Oak Ridge National Laboratory Reservation (Oak Ridge, TN). In addition to the physical and chemical extremes present within mud volcano sediments and FRC subsurface soils, these environments are sources of greenhouse gases as well as metal/radionuclide contaminants, respectively. Within the previously uncharacterized mud volcano cold seep sediments, culture-independent analyses of microbial community structure via DNA and RNA clone libraries indicated Gammaproteobacteria and anaerobic methane oxidizing Archaea as the dominant methane oxidizing taxa. Culture-dependent studies of FRC subsurface Arthrobacter and Bacillus isolates demonstrated extensive lateral gene transfer of the PIB-type ATPase metal resistance genes. Additionally, FRC Bacillus and Rahnella isolates demonstrated U(VI) sequestration capabilities as up to 95% soluble U(VI) was immobilization via biogenic phosphate mineral production resulting from constitutive nonspecific phosphohydrolase activity. Findings from these studies identify the prokaryotic diversity within aquatic and terrestrial sediments that contribute to the geochemical cycling of carbon, metals, and radionuclides.

Uranium has been released into the environment due to improper practices associated with mining and refinement for energy and weapons production. Soluble U(VI) species such as uranyl carbonate can be reduced to form the insoluble U(IV) mineral uraninite (UO2) via microbial respiratory processes. Formation of UO2 diminishes uranium mobility and prevents uranium-laden groundwater from being discharged into surface water; however, oxygen and other oxidants re-solubilize UO2. Many organisms have been shown to reduce uranium, but variations in microbial physiology change the dynamics of microbial uranium reduction in situ and affect uraninite stability. Anaeromyxobacter dehalogenans is a metal-reducing delta-Proteobacterium in the myxobacteria family that displays remarkable respiratory versatility and efficiently reduces U(VI). The approach of this research was to enhance characterization of A. dehalogenans by identifying unique genetic traits, describing variability within the species, and examining the environmental distribution of A. dehalogenans strains. Genome analysis revealed that A. dehalogenans shares many traits with the myxobacteria including type IV pilus-based motility and an aerobic-like electron transport chain. In addition, the genome revealed genes that share sequence similarity with strict anaerobes and other metal-reducing organisms. Physiological examination of microaerophilism in A. dehalogenans strain 2CP-C revealed growth at sub-atmospheric oxygen partial pressure. Physiological characterization of novel isolates demonstrated that strain-level variation in the 16S rRNA gene coincides with metabolic changes that can be linked to the loss of specific gene homologs. Anaeromyxobacter spp. were present at the Oak Ridge Integrated Field-scale Subsurface Research Challenge (IFC) site and multiplex qPCR tools designed using a minor-groove binding probe gave insights into strain and species differences in the community. Finally, 16S rRNA gene sequences were identified which suggest a novel Anaeromyxobacter species that is responsible for uranium reduction at the Oak Ridge IFC site. This research contributes new knowledge of the ecophysiology of a widely distributed, metal-reducing bacterial group capable of uranium immobilization. The characterization of Anaeromyxobacter spp. helps to elucidate the dynamics of biological cycling of metals at oxic-anoxic interfaces, like those at the Oak Ridge IFC, and contributes to the broader study of microbial ecology in groundwater and sediment environments.

Uranium contamination of soils and groundwater at Department of Energy facilities across the United States is a primary environmental concern and the development of effective remediation strategies is a major challenge. Bioremediation, or the use of microbial enzymatic activity to facilitate the remediation of a contaminant, offers a promising in situ approach that may be less invasive than traditional methods, such as pump and treat or excavation. This study demonstrates for the first time the successful biomineralization of uranium phosphate minerals as a result of microbial phosphatase activity at low pH in both aerobic and anaerobic conditions using pure cultures and soils from a contaminated waste site. Pure cultures of microorganisms isolated from soils of a low pH, high uranium- and nitrate-contaminated waste site, expressed constitutive phosphatase activity in response to an organophosphate addition in aerobic and anaerobic incubations. Sufficient phosphate was hydrolyzed to precipitate 73 to 95% total uranium as chernikovite identified by synchrotron X-ray absorption spectroscopy and X-ray diffraction. Highest rates of uranium precipitation and phosphatase activity were observed between pH 5.0 and 7.0. Indigenous microorganisms were also stimulated by organophosphate amendment in soils from a contaminated waste site using flow-through reactors. High phosphate concentrations (0.5 to 3 mmol L-1) in pore water effluents were observed within days of organophosphate addition. Highest rates of phosphatase activity occurred at pH 5.5 in naturally low pH soils in the presence of high uranium and nitrate concentrations. The precipitation of uranium phosphate was identified by a combination of pore water measurements, solid phase extractions, synchrotron-based X-ray spectroscopy, and a reactive transport model. The results of this study demonstrate that uranium is biomineralized to a highly insoluble uranyl phosphate mineral as a result of enzymatic hydrolysis of an organophosphate compound over a wide range of pH, in both aerobic and anaerobic conditions, and in the presence of high uranium and nitrate concentrations. The nonreductive biomineralization of U(VI) provides a promising new approach for in situ uranium bioremediation in low pH, high nitrate, and aerobic conditions that could be complementary to U(VI) bioreduction in high pH, low nitrate, and reducing environments.

Mémoire (M. Sc. A. (geologie))- Université du Québec à Chicoutimi, 1982. / "Mémoire présenté en vue de l'obtention d'un diplôme de M.Sc.A. (géologie)" CaQCU Document électronique également accessible en format PDF. CaQCU

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La précipitation est largement utilisée dans la chimie, l’hydrométallurgie, l’industrie pharmaceutique, le nucléaire ou de nombreuses autres activités industrielles. Les réactions de précipitation sont très sensibles à de nombreux paramètres de fonctionnement, tels que les concentrations de réactifs, la température, la sursaturation, l’hydrodynamique ou l'intensité du mélange. Ces paramètres jouent un rôle majeur pour contrôler la qualité physique du précipité. Dans l'industrie nucléaire, la précipitation est une opération très répandue, que ce soient dans l'extraction de l'uranium à partir de minerais d'uranium, dans le retraitement du combustible nucléaire irradié ou pour la gestion des effluents radioactifs. Dans cette étude, nous nous sommes intéressés à la précipitation de l'oxalate d’uranium IV. Seules quelques références bibliographiques liées à la précipitation de l'oxalate d'uranium tétravalent sont disponibles dans la littérature, elles concernent les fortes sursaturations (jusqu’à 2000). La présente étude a permis de proposer un modèle thermodynamique et des lois des cinétiques de précipitation à basse sursaturation à partir d’acquisitions expérimentales. De nouvelles mesures de solubilité de l’oxalate d’uranium à différentes acidités et différents excès oxaliques ont conduit à l’identification des constantes de complexation et du produit de solubilité. Les sursaturations sont calculées à partir des concentrations des ions libres. La comparaison avec l’approche basée sur les ions constitutifs montre que les deux concepts conduisent à des valeurs de sursaturations équivalentes. En ce qui concerne l’identification des cinétiques, de nombreux essais de précipitation de l’oxalate d’uranium IV en continu ont permis d’analyser l’influence des paramètres opératoires à la fois sur le rendement et les distributions de taille de particules. Les paramètres cinétiques sont obtenus par résolution du bilan de population selon la méthode des moments. Les lois de nucléation et d’agglomération sont exprimées à partir de la croissance cristalline. Le mécanisme de nucléation est de type secondaire et dépend de la puissance dissipée ainsi que de la concentration des cristaux. Quant au noyau d’agglomération, il a été identifié de type orthocinétique / Precipitation is widely used in many other industrial activities. Precipitation reactions are very sensitive to many operating parameters that play a major role in controlling the precipitate quality. In the nuclear industry, precipitation is a common operation, either in uranium extraction from uranium ore, nuclear fuel reprocessing or radioactive waste management. In this study we are interested in uranium IV oxalate precipitation. This study proposes a thermodynamic model and precipitation kinetics at low supersaturations on the basis of experimental acquisitions. New measures of uranium IV oxalate solubility at different acidities and oxalic excess led to the identification of the complexation constants and the solubility product. The supersaturation is calculated from free ions concentrations. The comparison with a constituent ions approach shows that the two concepts lead to equivalent supersaturation values. Regarding the identification of kinetics, many uranium IV oxalate precipitation continuous tests allowed the analysis of the influence of the operating parameters on efficiency and particle size distribution. Kinetic parameters are obtained by solving the population balance equation by the method of moments. Nucleation kinetics and agglomeration kernel are expressed from crystal growth kinetics. The nucleation mechanism corresponds to a secondary type and depends on the power dissipation and crystal concentration. As for the agglomeration kernel, it is identified of orthokinetic type

La sorption et les réactions redox des radionucléides (RN) sont des processus critiqués pour une évaluation de la sécurité des dépôts de déchets nucléaires. Dans les dépôts géologiques, ces procédés peuvent se produire dans (i) une couche de corrosion (acier), (ii) un béton armé, par exemple, sur le ciment hydraté et (iii) l'argilite, sur la pyrite et les argiles ou le granit. Les produits de corrosion de l'acier et la pyrite agissent comme des tampons de réduction locaux, contrôlant le potentiel redox (Eh) et donc le comportement de sorption des RN sensibles au rédox. En revanche, la sorption de RN n'impliquant pas de processus redox peut se produire sur des argiles, des oxydes de fer et des produits d'hydratation de ciment et impliquent souvent des processus d'adsorption de surface, d'échange d'ions ou de co-précipitations. Dans cette thèse de doctorat, des phases d'AFm cimentaires mineures, mais hautement réactives (acides gras AFm-Cl2 ou AFm-SO4, appartenant aux LDH CaAl) ont été utilisées pour adsorber MoO42- et SeO32- à diverses charges de surface. Une combinaison de la modélisation de l'équilibre chimique PHREEQC et des techniques de rayons X à base de synchrotron (par exemple, XRD, PDF et XAFS résolus dans le temps) révèle que les sites de sorption multiples, y compris deux types de sites de bord, des sites d'échange d'ions intercalaires et une précipitation de phase riche en Ca, sont des processus actifs dans la rétention des RN sur les phases AFm. Une relation linéaire permet de lier l'espacement basal AFm et le rayon d'anion intercalé hydraté. L'adsorption macroscopique MoO42 a été évaluée sur le ciment hydraté renforcé d'acier et de ses composants individuels (p. Ex. Fe0, CSH, ettringite, phase AFm, portlandite, gypse, pyrite, mackinawite) à pH 13,5 et le signal EXAFS ne pouvait être obtenu que pour Mo sorbed sur les phases AFm et les produits d'oxydation Fe0, en montrant qu'ils sont les absorbants les plus efficaces. La co-sorption de U et Mo sur le ciment-ciment hydraté renforcé par Fe0 a également été étudiée par cartographie micro-sonde, montrant que U doit être immobilisé instantanément par des matériaux de ciment tandis que Mo est préférentiellement sorbé sur des produits de réaction de Fe. La valeur Eh prédominant dans le béton est difficile à déterminer. Ici, les RN sensibles à la réduction rénale (par exemple, UVI, SeIV, MoVI et SbV) sont utilisées comme sondes, pour mesurer les valeurs Eh in-situ, en calculant l'équation de Nernst de la manière suivante. La concentration des espèces réduites a été mesurée en fonction de la concentration totale de RN précipitée par réduction et de la spéciation parmi ces espèces réduites, tel qu'obtenu par l'analyse LCF des données XANES. La concentration de l'espèce oxydée unique a été prise égale à la concentration chimique aqueuse totale, car toutes les espèces réduites identifiées sont extrêmement insolubles. Les valeurs Eh déterminées expérimentalement obtenues de cette façon étaient remarquablement fermées pour toutes les RN avec des valeurs centrées de -368 à -524 mV pour l'eau de pore de ciment (CPW) équilibrée avec Fe0 et des valeurs de -346 à -509 mV pour CPW équilibrées avec des produits de corrosion Fe - couples d'oxydes (magnetite / hématite ou magnetite / goethite) à pH ~ 13,5. Ni la valeur Eh calculée pour ces couples ni pour Fe0 / Fe (OH)2 correspond à ces données. Au lieu de cela, le potentiel redox semble être contrôlé par le couple Fe (OH)3 / Fe (OH)2 prédominant au début de la corrosion Fe0. Enfin, dans le domaine de l'argile ou du granit, plusieurs facteurs peuvent affecter de manière critique l'Eh imposé par la minérale mine de pyrite, à savoir les impuretés élémentaires dans le réseau de pyrite et les fractures résultant du broyage et de la présence de Fe3+et S2- à la surface de la pyrite. Les impuretés des éléments et la présence de S2- sur la surface de la pyrite ont largement accéléré la réduction des U (VI). / Sorption and redox reactions of radionuclides (RNs) are critical processes for a nuclear waste disposal repository safety assessment. In geological repositories, these process may occur in (i) canister (steel) corrosion layer, (ii) reinforced concrete, e.g. on hydrated cement and (iii) argillite, e.g. on pyrite and clays or granite. Both steel corrosion products and pyrite act as local reducing buffers, controlling the redox potential (Eh) and thus the sorption behavior of redox-sensitive RNs. In contrast, sorption of RNs not involving redox processes may occur on clays, iron oxides and cement hydration products, and often involve surface adsorption, ion exchange, or co-precipitations processes. In this PhD thesis, minor but highly reactive cementitious AFm phases (AFm-Cl2 or AFm-SO4 solids, belonging to CaAl LDHs) were employed to adsorb MoO42- and SeO32- at various surface loadings. A combination of PHREEQC chemical equilibrium modelling and synchrotron-based X-ray techniques (e.g., in-situ time-resolved XRD, PDF, and XAFS) reveals that multiple sorption sites, including two types of edge sites, interlayer ion exchange sites, and a Ca-rich phase precipitation, are active processes in the RNs retention on AFm phases. A linear relationship is shown to link AFm basal spacing and hydrated intercalated anion radius. MoO42- macroscopic adsorption was evaluated on steel-reinforced hydrated cement and its individual components (e.g., Fe0, C-S-H, ettringite, AFm phase, portlandite, gypsum, pyrite, mackinawite) at pH 13.5, and EXAFS signal could only be obtained for Mo sorbed on AFm phases and Fe0 oxidation products, showing they are the most effective absorbents. Co-sorption of U and Mo on Fe0-reinforced hydrated cement-core has also been investigated by micro-probe mapping, showing U to be instantly immobilized by cement materials while Mo is preferentially sorbed on Fe reaction products.The Eh value prevailing in concrete is hard to be determined. Here, redox-sensitive RNs (e.g., UVI, SeIV, MoVI, and SbV) are employed as probes, to measure in-situ Eh values, by computing the Nernst equation in the following way. Reduced species concentration were measured based on the total concentration of reductively precipitated RN and on speciation among these reduced species as obtained by LCF analysis of XANES data. The single oxidized species concentration was taken equal to the total aqueous chemical concentration, as all identified reduced species are extremely insoluble. The experimentally determined Eh values obtained that way were remarkably closed for all RNs with centered values of -368 to -524 mV for cement pore water (CPW) equilibrated with Fe0 and values of -346 to -509 mV for CPW equilibrated with corrosion products Fe-oxides couples (magnetite/hematite or magnetite/goethite) at pH ~13.5. Neither the Eh value computed for these couples or for Fe0/Fe(OH)2 match these data. Instead, the redox potential appear to be controlled by the Fe(OH)3/Fe(OH)2 couple predominating at the beginning of Fe0 corrosion. Finally, within clay or granite far field, several factors may critically affect the Eh imposed by pyrite minor mineral, namely element impurities in pyrite lattice and fractures resulting from grinding and presence of Fe3+ and S2- at the pyrite surface. Element impurities and presence of S2- on the pyrite surface were shown to largely speed up U(VI) reduction. The experimental results obtained above could provide fundamental data for the safety assessment of nuclear waste disposal.

Thorium(IV) and uranium(IV) coordination complexes have been studied for the last 60 years. They have shown interesting reactivity that is often divergent from that of transition metal complexes, and that also provides an insight into some unanticipated differences between thorium(IV) and uranium(IV). An introduction to thorium(IV) and uranium(IV) organometallic chemistry supported by carbocyclic and N-donor ligands is given in Chapter One. The reactivity of actinide alkyl, amide and alkynyl complexes towards small molecules is discussed and select examples provided. The redox chemistry of thorium and uranium is also introduced. Chapter Two describes the alkylation and amination chemistry of uranium(IV) and thorium(IV) trans-calix[2]benzene[2]pyrrolide ((L)2-) complexes, [(L)AnCl2], yielding new actinide(IV) complexes of the type [M(L-2H)An(R)] (M = Li or K, R = Me, CH2SiMe3, CH2Ph, N(SiMe3)2), where (L)2- undergoes further deprotonation to (L-2H)4-. Additionally, the lability of the [M(L-2H)An(R)] “ate”-complexes towards M+ ion exchange is addressed. Further, the selective ligand reprotonation of (L-2H)4- to (L)2- using HSiR'3 (R' = Me, iPr) and [Et3NH][BPh4] yielding [(L)An(C≡CSiR'3)2] and [(L)An(R)][BPh4] respectively, is explained. The reactivity of these complexes towards amines, silanes, alkenes, tin hydrides, silicone grease, tBuNC, H2, CO, CO2 or CS2 is described. Crystallographic characterisation shows that [(L)Th(N(SiMe3)2)][BPh4] contains an unusual example of a thorium(IV) bis-arene coordination mode. The reactivity of [(L)Th(C≡CSiMe3)2] towards a number of substrates including alkenes, [Ni(COD)2], [Pt(norbornene)3], P4, CO2 or H2 is also discussed. Activation of CO2 by [(L)Th(C≡CSiMe3)2] at 80 °C results in (L)2- functionalisation and abstraction to yield a new tricyclic organic molecule with the general formula LCO. The addition of [Ni(COD)2] to [(L)Th(C≡CSiMe3)2] and PR''3 (R'' = phenyl, cyclohexyl) yields heterobimetallic complexes [(L)Th(C≡CSiMe3)2·Ni(PR''3)]; these products display both dipyrrolic and bis-arene coordination. The changes in ligand coordination mode are discussed alongside DFT computational analyses that have been carried out by collaborators. The substitution reactions of [(L)AnCl2] with NaBH4 to form actinide(IV) borohydride complexes [(L)An(BH4)2] and subsequent attempted abstractions of BH3 from [(L)Th(BH4)2] are presented. Conclusions are provided at the end of the chapter. Chapter Three focusses on the oxidation chemistry of uranium(IV) within the (L)2- and (L-2H)4- ligand framework, prompted by the isolation of a uranium(V) complex [Li[(L)UO2]·LiI] from the oxidation of the uranium(IV) complex [Li(L-2H)U(Me)]. Conclusions are provided at the end of the chapter. Experimental methods and characterising data are given in Chapter Four.