Ligas de Urânio são candidatas ao uso como combustível nuclear em reatores avançados, dentre essas ligas se destacam as ligas de Urânio com Nióbio e com Zircônio. Este trabalho investigou como os teores de Nb e Zr, assim como a processamento termomecânico afetam as microestruturas e as propriedades mecânicas de 3 ligas U-XNb-YZr onde X+Y=12. Duas amostras contendo 50g cada, foram fabricadas através de fusão à plasma nos teores U-3Nb-9Zr (liga 39), U- 6Nb-6Zr (liga 66) e U-9Nb-3Zr (liga 93). Uma das amostras de cada liga foi tratada termicamente por 5h a 1000°C para a realização de homogeneização química. A amostra homogeneizada e a amostra bruta de fusão de cada liga foram conjuntamente encapsuladas em aço para a realização de laminação a quente seguida de um recozimento final a 1000°C por 2h. À rota adotada pela amostra bruta de fusão chamou-se \" Rota C\" e a rota adotada pela amostra homogeneizada chamou-se \"Rota H\". A caracterização microestrutural foi feita por microscopia óptica e eletrônica de varredura. Todas as amostras, independente do processamento, apresentaram precipitados ricos em Nióbio e Zircônio em adição a uma matriz rica em Urânio. A caracterização das amostras brutas de fusão mostra que os teores de elementos de liga influenciam diretamente a morfologia das dendritas evidentes na microestrutura assim como das demais fases presentes em cada amostra. A liga 39 apresentou predominantemente fase ?\', a liga 66 a fase ?\" com traços de fase y e a liga 93 a fase y com traços de fase ?\". Após o tratamento térmico de homogeneização, a liga 39 apresentou fase ?\" na forma celular enquanto a liga 66 apresentou as fases y0 e y e a liga 93 apresentou apenas fase y. As propriedades mecânicas das ligas foram avaliadas por ensaios de dureza e de dobramento simples. As amostras da Rota C apresentaram redução de dureza em relação à condição inicial. Todas as microestruturas das amostras laminadas a quente exibiram duas fases ricas em U. A liga 39 apresentou as fases ?\" na forma celular e ?\' após a laminação a quente. Após o recozimento final na rota C, a liga 39 apresentou fase ?\" na forma acicular enquanto as ligas 66 e 93 apresentaram as fases ?\" e y após a laminação e fase y. A fração de área da amostra pobre em U elevou-se nas ligas 39 e 66 e reduziu-se na liga 93. As amostras da Rota H apresentaram redução de dureza em relação à condição bruta de fusão. A liga 39 apresentou fase ?\" na forma celular com orientação e traços da fase ?, a liga 66 exibiu as fases y0 e y e a liga 93 as fases y e y0. Após o recozimento final, a liga 39 mostrou-se novamente na forma ?\" na forma celular, mas sem orientação. A liga 66 apresentou fase y e a liga 93 fase y0. Os testes de dobramento simples mostraram que as ligas da Rota C exibem plasticidade, retendo parte da deformação plástica após a ruptura das amostras testadas. Já as amostras da Rota H mostraram comportamento super elástico, possibilitando maiores deformações mas sem reter deformação plástica após a ruptura das amostras. A melhor relação entre deformação total e residual foi observada na liga 93 fabricada pela Rota C. As análises nos perfis de fratura das amostras da Rota C mostram fraturas transgranulares em todas as amostras. O perfil de fratura de na amostra recozida liga 39 mostra que a fase ?\" na forma acicular tende a deforma-se por deslizamento. As análises dos perfis de fratura nas amostras da Rota H confirmaram a ausência de deformação plástica mesmo em escala microscópica. Para essa condição, a fase ?\" na forma celular com orientação (liga 39) aparenta deformar-se por maclação. As análises de superfície de fratura indicam que a fase pobre em U tem participação durante o processo de crescimento e propagação da fratura na Rota H, atuando como caminho para bifurcação de trincas acelerando o processo de ruptura, enquanto na Rota C, a fase pobre em U deforma-se conjuntamente com a matriz de U. Em uma segunda etapa do trabalho, a estabilidade das microestruturas resultantes na amostras processadas foi investigada por Ensaios de Calorimetria Diferencial Exploratória (DSC) e por calorimetria de queda livre (esta apenas para a amostra 93 da rota H). O teor de Nb e Zr também afeta a estabilidade das fases presentes em cada amostra. foram realizados com as amostras da condição homogeneizada e laminada. A quantidade de transformações assim como o estado final de cada liga diferiu de acordo com a razão Nb/Zr. Após o ciclo de aquecimento e resfriamento da análise térmica, a liga 39 apresentou fase ?\', a liga 66 fase ?\" e a liga 93 fase y. No ensaio de calorimetria por queda livre foi possível observar as diferentes etapas de reação de envelhecimento da matriz g, correspondendo a à transformação y -> y0 (entre 525 e 530 K), a transformação y -> ?\". (entre 623 e 651 K) e à transformação y\' -> y3+? (entre 825 e 925 K). / Uranium alloys are candidates to be used as nuclear fuel in research reactors, among the U alloys, the Nb and Zr containing alloys are promising. This work evaluated how the Nb and Zr content and the thermomechanical processing affects the microstructure and mechanical properties of 3 alloys U-XNb-YZr were X+Y=12. Two 50g slugs of each sample were fabricated using plasma arc melting according to U-3Nb-9Zr (alloy 39), U-6Nb-6Zr (alloy 66) and U-9Nb-3Zr (alloy 93). One slug of each alloy was heat treated for 5h at 1000°C to perform the chemical homogenization. The homogenized sample and the as-cast one were encapsulated in the same steel frame in order to perform hot rolling. After the rolling process, the samples were annealed by 2h at 1000°C. The route that uses only as-cast samples was nominated \"Route C\" and the route that uses the homogenized sample was nominated \"Route H\". The microstructural characterization was performed by optical and scanning electron microscopy. All samples, regardless the processing route, presented Nb and Zr rich precipitates in addition to U rich matrix. The characterization of as-cast samples shows that the content of the alloying element has a direct influence on dendrite morphology as in the phases presented for each alloy. The alloy 39 presented predominantly ?\' phase, the alloy 66 the ?\" phase with a small quantity of ? phase and the alloy 93 presented the ? phase with small quantity of ?\" phase. Afte the homogenization, the alloy 39 presented cellular ?\" phase, the alloy 66 presented ?0 and ?, the alloy 93 presented only ? phase. The mechanical properties were evaluated by hardness measurements and free bending tests. The Route C samples presented hardness reduction in comparison to the initial condition. All microstructures of hot-rolled samples of this route exhibit two U rich phases. The alloy 39 exhibited cellular ?\" and ?\', after the final annealing the alloy 39 presented acicular ?\". The alloys 66 and 93 exhibited ?+?\" after hot rolling and ? phase after the final annealing. The area fraction of poor U phase increased in the alloys 39 and 66, but reduced in alloy 93. The Route H samples presented hardness reduction in comparison to as-cast samples. The alloy 39 presented cellular oriented ?\" phase and a small quantity of ? phase. The alloy 66 exhibited ?0 and ?, the alloy 93 ? and ?0. After the final annealing, the alloy 39 presented the ?\" again, but without orientation. The alloy 66 presented ? phase and the alloy 93 presented ?0. The free bending tests show that Route C samples have real plasticity, retaining part of deformation after rupturing as plastic strain. The Route H samples exhibited superelastic behavior, allowing higher deformations but retaining no plastic strain after sample breaking. The better balance between total and residual strain was observed in alloy 93 fabricated by Route C. The cracking profile analysis of Route C samples shows transgranular fractures in all samples. The Cracking profile of final 39 sample shows that acicular ?\" tends to deform by slipping. The cracking profile analysis of Route H samples confirmed the absence of plasticity even on the microscopic scale. This condition, the oriented cellular ?\" phase (alloy 39) apparently deforms by twinning. The crack surface analysis indicates that the U poor phase has a direct participation in crack growing and propagation, acting as forking points to the fracture and accelerating the fracture process. In the Route C samples, the poor U phase deforms alongside the U matrix. The stability of resulting microstructures of homogenized and hot rolled samples was investigated by Differential Scanning Calorimetry (DSC) and Drop Differential Scanning Calorimetry (only for the homogenized hot rolled 93 sample). The Nb and Zr also affect the stability of present phases in each sample. The number of transformations and the final structure is directly influenced by the Nb/Zr ratio. After the thermal cycle imposed by the DSC analysis, the alloy 39 exhibited ?\' phase, the alloy 66 exhibited ?\" phase and the alloy 93 exhibited ? phase. The Drop-DSC allowed observing the different stages of reaction in ? matrix, corresponding to ? -> ?0 (between 525 and 530 K), ? -> ?\" transformation (between 623 and 651 K) and ?\' -> ?3+? transformation (between 825 and 925 K).

Dans une perspective mondiale de décarbonisation de la production énergétique et de croissance de la production d’électricité d’origine nucléaire, la disponibilité des ressources d’uranium est un enjeu majeur. Les technologies futures qui permettront aux réacteurs nucléaires de s’affranchir de l’uranium naturel mettront du temps à être pleinement déployées. Nous analysons donc les conditions de disponibilité de l’uranium au XXIe siècle. Les deux premières sont liées au coût de production : ce sont l’accessibilité technique et l’intérêt économique. Nous les étudions en modélisant les ressources ultimes d’uranium (quantités découvertes et non découvertes) et leurs coûts. Cette méthode s’appuie sur un découpage régional du monde, la connaissance actuelle des gisements et un filtre économique. Elle permet d’établir une courbe d’offre de long terme où les quantités d’uranium techniquement accessibles sont fonction du coût de production. Les principales incertitudes de ces estimations ont été identifiées et l’on montre qu’en l’absence de découpage régional, les ressources ultimes sont sous-estimées. Les autres conditions de disponibilité de l’uranium prises en compte sont liées aux dynamiques de marché que crée la confrontation de l’offre et de la demande. Nous les étudions en les modélisant sous la forme de contraintes dynamiques dans un modèle de marché en équilibre partiel. Ce modèle est déterministe et les acteurs y sont représentés par région. Il permet de tenir compte, par exemple, de la corrélation à court terme entre le prix et les dépenses d’exploration, qui fait l’objet d’une étude économétrique spécifique. À plus long terme, les contraintes modélisées incluent l’anticipation de la demande par les consommateurs et de la raréfaction progressive des ressources ultimes les moins chères. Par une série de simulations prospectives, nous montrons que le rythme de croissance de la demande d’uranium au XXIe siècle et son anticipation ont une forte influence sur la hausse du prix à long terme. À l’inverse, les incertitudes liées à l’estimation des ressources ultimes ont une influence limitée. Nous soulignons également l’évolution inégale du poids des différentes régions dans la production mondiale. Enfin, certaines variations de l’offre (arrêt de la production d’une région par exemple) ou de la demande (croissance irrégulière ou introduction de nouvelles technologies) ont également une influence significative sur l’évolution du prix à long terme ou sa cyclicité. / From a global perspective, a low-carbon path to development driven by a growth of nuclear power production raises issues about the availability of uranium resources. Future technologies allowing nuclear reactors to overcome the need for natural uranium will take time to fully deploy. To address these issues, we analyze the conditions of availability of uranium in the 21st century.The first two conditions are technical accessibility and economic interest, both related to the cost of production. We study them using a model that estimates the ultimate uranium resources (amounts of both discovered and undiscovered resources) and their costs. This model splits the world into regions and the resource estimate for each region derives from the present knowledge of the deposits and economic filtering. The output is a long-term supply curve that illustrates the quantities of uranium that are technically accessible as a function of their cost of production. We identify the main uncertainties of these estimates and we show that with no regional breakdown, the ultimate resources are underestimated.The other conditions of availability of uranium covered in our study are related to the market dynamics, i.e. they derive from the supply and demand clearing mechanism. To assess their influence, they are introduced as dynamic constraints in a partial equilibrium model. This model of the uranium market is deterministic, and market players are represented by regions. For instance, it takes into account the short-term correlation between price and exploration expenditures, which is the subject of a dedicate econometric study. In the longer term, constraints include anticipation of demand by consumers and a gradual depletion of the cheapest ultimate resources.Through a series of prospective simulations, we demonstrate the strong influence on long term-price trends of both the growth rate of demand during the 21st century and its anticipation. Conversely, the uncertainties related to the estimation of ultimate resources have limited influence. We also underline the uneven evolution of market shares between regions. Finally, particular changes in supply (production shutdown in one of the regions, for example) or in demand (irregular growth or introduction of new technology) also have a significant influence on the evolution of the long-term price or its cyclicity.

Le démantèlement des réacteurs UNGG produit un grand volume de matériaux métalliques contaminés. Parmi ceux-ci, les alliages à base de magnésium, connus comme métaux hautement réactifs, présentent un risque élevé de corrosion et peuvent générer de l'hydrogène gazeux qui peut causer de graves dommages pendant le stockage. Afin de réduire le volume des effluents radioactifs générés et de déclasser les déchets nucléaires, la sonochimie peut être mise en œuvre comme technologie efficace pour la décontamination des surfaces métalliques. La sonochimie traite des effets des ondes ultrasonores sur les réactions chimiques en solution. Les effets observés en sonochimie proviennent du phénomène de cavitation acoustique, qui est la nucléation, la croissance et l'effondrement implosif rapide de microbulles remplies de gaz et de vapeur. Des espèces et des radicaux excités peuvent être générés dans le plasma formé, qui émet de la lumière (sonoluminescence). Lorsque l’effondrement des bulles a lieu à proximité d'une surface solide, il produit des ondes de choc violentes et des microjets dirigés vers la surface. Ces effets physiques contribuent fortement au nettoyage par ultrasons, à la dépassivation de surfaces et à la décontamination.Cette étude se concentre sur : 1) les comportements de cavitation près d'une surface solide ; 2) la structuration ultrasonore des surfaces étendues de magnésium ; 3) la décontamination par ultrasons de surfaces métalliques radioactives. L'activité sonochimique est évaluée en mesurant les rendements en H2O2, la distribution de sonochimiluminescence et les spectres de sonoluminescence. Les effets de la structuration et de la décontamination des surfaces sont suivis au moyen de MEB, EDS, FTIR, Raman, DRX, analyse du comportement de mouillage, spectrométrie de masse et ICP-AES.Ce travail a mis en évidence le fort impact de la fréquence ultrasonore sur l'activité sonochimique, sa distribution spatiale et sur les effets générés sur les échantillons de magnésium. Une répartition spatiale homogène de l'activité sonochimique est observée aux fréquences ≥ 100 kHz. L'effondrement asymétrique des bulles est plus susceptible de se produire près de la surface solide aux ultrasons à haute fréquence. La formation d’une structure allongée semblable à une balle de golf est observée à des fréquences comprises entre 100 et 362 kHz. De telles architectures résultent de la dissolution contrôlée par ultrasons de la surface Mg. Il est probable que la nucléation hétérogène assurée par la création de défauts par les ultrasons et la libération de gaz H2 soient à l'origine de la formation des cratères. La décontamination des surfaces radioactives de Mg et d’alliages de Mg ont d'abord démontré un nettoyage rapide par les ultrasons suivi d’un lent processus de recontamination due à l'adsorption de brucite formée sur les surfaces de Mg ou de ses alliages. / The dismantling of UNGG reactors produces large volumes of contaminated metallic materials. Among these, magnesium-based alloys which are known as highly reactive metals, have a high risk of corrosion and can generate hydrogen gas that can cause serious damages during storage. In order to reduce the volume of generated radioactive effluents and downgrade nuclear wastes, sonochemistry can be applied as an efficient technology for metallic surface decontamination. Sonochemistry deals with the effects of ultrasound waves on chemical reactions in solution. The effects observed in sonochemistry originate from the acoustic cavitation phenomenon, which is the nucleation, growth and rapid implosive collapse of gas and vapor filled microbubbles. Excited species and radicals can be generated in the formed plasma and light (sonoluminescence) is emitted. When the bubble collapse takes place in the vicinity of a solid surface, it produces violent shock waves and microjets directed towards the surface. These physical effects strongly contribute in ultrasonic cleaning, surface depassivation and decontamination.This study focuses on: 1) the cavitation behaviors near a solid surface; 2) the ultrasonic structuration of extended magnesium surfaces; 3) the ultrasonic decontamination of radioactive metal surfaces. The sonochemical activity is evaluated by measuring H2O2 yields, sonochemiluminescence distribution and sonoluminescence spectra. Surface structuration and decontamination effects are followed by means of SEM, EDS, FTIR, Raman, XRD, wetting behavior analysis, mass spectrometry and ICP-AES.The investigations reveal a strong ultrasonic frequency dependency of the cavitation activity and distribution and of the effects generated on the Mg samples. A homogeneous spatial repartition of sonochemical activity is observed at frequencies ≥ 100 kHz. Asymmetrical bubble collapse is found more likely to happen near the solid surface at high frequency ultrasound. A golf-ball like extended structure is observed at frequencies between 100 and 362 kHz. It is shown that such architectures result from the ultrasonically controlled dissolution of the Mg surface. Heterogeneous nucleation provided by the creation of defects by ultrasound and the release of H2 gas are supposed to be at the origin of the crater formation. Decontamination of radioactive surfaces of Mg and Mg alloys demonstrate rapid ultrasonic cleaning followed by a slow recontamination process which is due to the adsorption of brucite formed on the surfaces of Mg or its alloys.

Over the past decade studies into the application of radiometrics for soil and regolith mapping have met with mixed response. While the use of radiometric data for regolith mapping has been generally well received, radiometric methods have not commonly been adopted to assist and improve soil mapping. This thesis contributes to the development of radiometric techniques as soil and regolith mapping tools by examining soil characteristics and radiometric response using non-standard radiometric methods. This is accomplished through the development of new data processing methodologies, which extracts additional information from standard radiometric data that is unattainable using standard processing methods, and development of a new interpretation approach to soil and regolith mapping employing the multispectral processed radiometric data. The new multispectral processing methodology resolves seven gamma ray peaks from standard 256-channel NaI radiometric data to produce new radiometric uranium ternary, thorium ternary and uranium ratio imagery. Changes in the gamma ray relationships, identified through the new imagery, identify changes in soil and/or environmental conditions that are absent or difficult to identify in the standard radiometric imagery. With the isolation of non-standard thorium channels 228 [superscript] Ac (900 keV) and 228 [superscript] Ac (1600 keV), case studies in this thesis demonstrate how the difference of 1.9 years (half-life) between thorium 228 [superscript] Ac and 232 [superscript] Th decay daughter products can be mapped through the interpretation of thorium energy using ternary imagery [red: 208 [superscript] Tl (1764 keV), green: 228 [superscript] Ac (900 keV), blue: 228 [superscript] Ac (1600 keV)]. Energy peak differences may be be linked to local variations in soil chemistry, soil movement, and water movement. / Additionally, through the isolation of non-standard uranium channels 214 [superscript] Bi (1120 keV) and 214 [superscript] Bi (1253 keV), preferential attenuation of lower energy gamma-rays from 214 [superscript] Bi decay events are exploited to map variations in soil density and/or porosity. These variations are illustrated through the interpretation of uranium energy using ternary imagery [red: 214 [superscript] Bi (1764 keV), green: 214 [superscript] Bi (1120 keV), blue: 214 [superscript] Bi (1253 keV)] and uranium peak energy ratio [214 [superscript] Bi 1120 keV / 214 [superscript] Bi 1764 keV] pseudo colour imagery. Case studies examined in this thesis explore the characteristics of 256-channel radiometric spectrum from different resolution datasets from different Western Australian soil types, provide recommendations for acquiring radiometric data for soil mapping in different agricultural environments, demonstrate how high resolution 256-channel radiometric data can be used to model soil properties in three-dimensions, and illustrate how three-dimension soil models can be used to separate surface waterlogging influences from rising groundwater induced waterlogging.

Texture evolution of depleted uranium is investigated using a viscoplastic self-consistent model. Depleted uranium, which has the same structure as alpha-uranium, is difficult to model as it has an orthorhombic symmetry structure, as well as many twin systems which must be addressed in order to properly simulate the textural evolution. The VPSC method allowed for a flexible model which could not only encompass the low symmetry component but also the twinning components of depleted uranium. The model focuses on the viscoplastic regime only, neglecting the elastic regime of deformation, and uses a self-consistent method to solve the model. Different deformation processes, such as torsion, rolling, and swaging, are simulated and the theoretical textures, plotted as pole figures or inverse pole figures, are compared with previous experimental textures found for alpha uranium from previous experimental sources. A specific twin system, the (176)[512] system, is also given special consideration. This twin system is a dominant deformation mode for alpha uranium at high strain rates, but is quite elusive in general. Different deformation processes are compared where this twin system is active and not active. This allows comparison on the effect of this twin on the overall texture of depleted uranium. In addition, a sample of depleted uranium from Y12 which was analyzed for (176)[512] twins is compared to theoretical results from a VPSC simulation where the (176)[512] twin is active.

Uranium and thorium radionuclides were measured on two gravity cores (T17G and T18G) and one box core (T19B) collected from the western South Okinawa Trough (SOT), one gravity core from off shore Southwest Taiwan (N3) and three box cores (C, D and E) from the northern South China Sea (SCS) in order to examine the variations of these radionuclides and their activity ratios in the sediments of the areas and to characterize the source function of the sediments and their geochemical implication based on these nuclides. For long half-life radionuclides such as 238U, 234U, 232Th and 230Th, the activities in the cores of the SOT and Southwest Taiwan areas show no significant vertical or areal variations, implying no significant variation in sediment supply or depositional environment within the past 100 years. The average activity of 238U is 1.65 dpm/g and 1.33 dpm/g in the SOT and Southwest Taiwan areas, respectively, and that of 232Th is 3.57 dpm/g and 3.34 dpm/g, respectively. The average activities of 238U and 232Th are, respectively, 1.37 dpm/g and 2.37 dpm/g in the SCS. The mean 232Th activity is lower in the SCS than in the SOT and Southwest Taiwan. The mean 232Th activity of the sediments in the SOT and Southwest Taiwan is quite comparable to that of the shale, slate and black schist in Taiwan, suggesting that these sediments are the terrigenous detrial materials from Taiwan. The 238U and 234U activities in the cores of these two areas show no significant vertical nor areal variations with activities ranging between 1.3 and 2 dpm/g, and their 234U /238U activity ratios being about 1.1, quite close to that of seawater (1.14). Since 238U and 234U are quite comparable among the three areas, the higher activity of 230Th in excess over 234U in the northern SCS may be due to greater water depth that allows more 234U produced 230Th to be scavenged from the water column. The uranium and thorium radionuclides and their activity ratios in the SOT and Southwest Taiwan sediments suggest that these sediments are the terrigenous detrial materials from Taiwan. The source function of the SCS sediments is more complex than that of the above-mentioned sediments.

Uranium (U) exhibits a high temperature body-centered cubic (b.c.c.) 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. Two separate generalized gradient approximations of the exchange-correlation are used to calculate defect properties and are compared. The bulk modulus, the lattice constant, and the Birch-Murnaghan equation of state for the defect free b.c.c. 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. The results for vacancies agree very well with experimental and previous computational studies. The most probable self-interstitial site in γ-U is the <110> dumbbell and the most probable defect location for dilute Zr in γ-U is the substitutional site. The most likely position for Xe and Kr atoms in uranium is the substitutional site. Helium atoms are likely to be found in a wide variety of defect positions due to the comparable formation energies of all defect configurations analyzed.

Uranium contaminations of the subsurface in the vicinity of nuclear materials processing sites pose a health risk as the uranyl ion in its oxidized state, U(VI), is highly mobile in aquifers. Current remediation strategies such as pump and treat or excavation are invasive and expensive to implement on a large scale. In situ bioremediation represents an alternative strategy that uses the ability of local microbial communities to immobilize contaminants and is actively studied for uranium remediation. The immobilization of U(VI) in groundwater is achieved either by bioreduction to solid uraninite (U(IV)), adsorption to the soil matrix, or non-reductive precipitation of uranium phosphate minerals through the activity of bacterial phosphatases. Bioreduction has been widely studied for remediation of the saturated zone, as anaerobic conditions typically prevail in these environments. This process is only efficient at circumneutral pH, however, and the end product uraninite is unstable under aerobic conditions or in the presence of manganese oxides, nitrite, or even freshly formed iron oxides. Although non-reductive biomineralization of uranium catalyzed by bacterial phosphatase activity successfully removes uranium from the vadose zone, further studies are needed to assess the ability of microbial communities to hydrolyze organophosphate compounds in the saturated zone where oxygen is often depleted and uranium bioreduction may be significant. To investigate this process under anaerobic conditions, low pH soil samples from a uranium contaminated site at the Oak Ridge Field Research Center were incubated anaerobically in flow through reactors in the presence of exogenic organophosphate compounds to stimulate the natural microbial communities in the original soil matrix. Aqueous uranium was injected continuously in the reactors to determine the fraction of uranium removed during these incubations. The reactors amended with organophosphate produced inorganic phosphate in the effluent, suggesting that bacterial phosphatase activity can be stimulated even in anaerobic environments at low pH. Removal of U(VI) in a control amended with organophosphate over a short time period was similar compared to reactors amended with organophosphate for long times suggesting that adsorption may also play a role in U(VI) immobilization. A sequential extraction technique was optimized to differentiate the fraction of uranium loosely adsorbed and the fraction of uranium precipitated as phosphate minerals and batch adsorption experiments were performed to obtain thermodynamic parameters that could be used to predict the fraction of U(VI) adsorbed onto the soil matrix. Results indicated that 100% uranium adsorption was favorable from pH 5 to 10 (without the presence of phosphate), and that most of the solid phase uranium was extracted in the step defined for the strongly adsorbed/uranium phosphate mineral in both long and short-term amended reactors. Overall, these results demonstrate that the biomineralization of uranium phosphate minerals is a viable bioremediation strategy in both the vadose and saturated zones of aquifers at both low and high pH, provided an organophosphate source is available.

The migration behavior of actinides and other radioactive contaminants in the environment is controlled by prominent molecular phenomena such as hydrolysis and complexation reactions in aqueous solutions as well as the diffusion and sorption onto minerals present along groundwater flow paths. These reactions significantly influence the mobility and bioavailability of the metal ions in the environment, in particular at liquid-solid interfaces. Hence, for the assessment of migration processes the knowledge of the mechanisms occurring at interfaces is crucial. The required structural information can be obtained using various spectroscopic techniques. In the present study, the speciation of uranium(VI) and neptunium(V) at environmentally relevant mineral – water interfaces of oxides of titania, alumina, silica, zinc, and alumosilicates has been investigated by the application of attenuated total reflection Fouriertransform infrared (ATR FT-IR) spectroscopy. Moreover, the distribution of the hydrolysis products in micromolar aqueous solutions of U(VI) and Np(V/VI) at ambient atmosphere has been characterized for the first time, by a combination of ATR FT-IR spectroscopy, near infrared (NIR) absorption spectroscopy, and speciation modeling applying updated thermodynamic databases. From the infrared spectra, a significant change of the U(VI) speciation is derived upon lowering the U(VI) concentration from the milli- to the micromolar range, strongly suggesting the dominance of monomeric U(VI) hydrolysis products in the micromolar solutions. In contradiction to the predicted speciation, monomeric hydroxo species are already present at pH ≥ 2.5 and become dominant at pH 3. At higher pH levels (> 6), a complex speciation is evidenced including carbonate containing complexes. For the first time, spectroscopic results of Np(VI) hydrolysis reactions are provided in the submillimolar concentration range and at pH values up to 5.3, and they are comparatively discussed with U(VI). For both actinides, the formation of similar species is suggested at pH ≤ 4, whereas at higher pH, the infrared spectra evidence structurally different species. At pH 5, the formation of a carbonate-containing dimeric complex, that is (NpO2)2CO3(OH)3^-, is strongly suggested, whereas carbonate complexation occurs only under more alkaline conditions in the U(VI) system. The results from the experiments of the sorption processes clearly demonstrate the formation of stable U(VI) surface complexes at all investigated mineral phases. This includes several metal oxides, namely TiO2, Al2O3, and SiO2, serving as model systems for the elucidation of more complex mineral systems, and several alumosilicates, such as kaolinite, muscovite and biotite. From a multiplicity of in situ experiments, the impact of sorbent characteristics and variations in the aqueous U(VI) system on the sorption processes was considered. A preferential formation of an inner-sphere complex is derived from the spectra of the TiO2 and SiO2 phases. In addition, since the in situ FT-IR experiments provide an online monitoring of the absorption changes of the sorption processes, the course of the formation of the U(VI) surface complexes can be observed spectroscopically. It is shown that after prolonged sorption time on TiO2, resulting in a highly covered surface, outer-sphere complexation predominates the sorption processes. The prevailing crystallographic modification, namely anatase and rutile, does not significantly contribute to the spectra, whereas surface specific parameters, e.g. surface area or porosity are important. A significant different surface complexation is observed for Al2O3. The formation of innerspheric species is assumed at low U(VI) surface coverage which is fostered at low pH, high ionic strength and short contact times. At proceeded sorption the surface complexation changes. From the spectra, an outer-spheric coordination followed by surface precipitation or polymerization is deduced. Moreover, in contrast to TiO2, the appearance of ternary U(VI) carbonate complexes on the γ-Al2O3 surface is suggested. The first results of the surface reactions on more complex, naturally occurring minerals (kaolinite, muscovite and biotite) show the formation of U(VI) inner-sphere sorption complexes. These findings are supported by the spectral information of the metal oxide surfaces. In this work, first spectroscopic results from sorption of aqueous Np(V) on solid mineral phases are provided. It is shown that stable inner-sphere surface species of NpO2 ^+ are formed on TiO2. Outer-sphere complexation is found to play a minor role due to the pH independence of the sorption species throughout the pH range 4 – 7.6. The comparative spectroscopic experiments of Np(V) sorption onto TiO2, SiO2, and ZnO indicate structurally similar bidentate surface complexes. The multiplicity of IR spectroscopic experiments carried out within this study yields a profound collection of spectroscopic data which will be used as references for future investigations of more complex sorption systems in aqueous solution. Furthermore, from a methodological point of view, this study comprehensively extends the application of ATR FT-IR spectroscopic experiments to a wide range in the field of radioecology. The results obtained in this work contribute to a better understanding of the geochemical interactions of actinides, in particular U(VI) and Np(V/VI), in the environment. Consequently, more reliable predictions of actinides migration which are essential for the safety assessment of nuclear waste repositories can be performed.

The metallic binary-alloy fuel Uranium-Zirconium is important for use in the new generation of advanced fast reactors. Uranium-Zirconium goes through a phase transition at higher temperatures to a (gamma) Body Centered Cubic (BCC) phase. The BCC high temperature phase is particularly important since it corresponds to the temperature range in which the fast reactors will operate. A semi-empirical Modified Embedded Atom Method (MEAM) potential is presented for Uranium-Zirconium. This is the first interatomic potential created for the U-Zr system. The bulk physical properties of the Uranium-Zirconium binary alloy were reproduced using Molecular Dynamics (MD) and Monte Carlo (MC) simulations with the MEAM potential. The simulation of bulk metallic alloy separation and ordering phenomena on the atomic scale using iterative MD and MC simulations with interatomic potentials has never been done before. These simulations will help the fundamental understanding of complex phenomena in the metallic fuels. This is a large step in making a computationally acceptable fuel performance code, able to replicate and predict fuel behavior.