No description available.

Nitride fuels have always been considered a good candidate for GENIV reactors, as well as space reactors, due to their high fissile density, highthermal conductivity and high melting point. In these concepts, not beingcompatible with water is not a significant problem. However, in recent years,nitride fuels started to raise an interest for application in thermal reactors,as accident tolerant or high performance fuels. However, oxide fuels havebenefited from decades of intensive research, and thousands of reactor-years.As such, a large effort has to be made on qualifying the fuel and developingtools to help assess their performances.In this thesis, the modeling side of this task is chosen. The effort istwo-fold: determining fundamental properties using atomistic models andputting together all the properties to predict the performances under irradi-ation using a fuel performance code. The first part is done combining manyframeworks. The density functional theory is the basis to compute the elec-tronic structure of the materials, to which a Hubbard correction is added tohandle the strong correlation effects. Negative side effects of the Hubbardcorrection are tackled using the so-called occupation matrix control method.This combined framework is first tested, and then used to find electronic andmechanic properties of the bulk material as well as the thermomechanicalbehavior of foreign atoms. Then, another method, the self-consistent meanfield (SCMF) one, is used to reach the dynamics properties of these foreignatoms. In the SCMF theory, the data that were obtained performing the abinitio simulations are treated to provide diffusion and kinetic flux couplingproperties.In the second step of the work, the fuel performance code TRANSURA-NUS is used to model complete fuel pins. An athermal fission gas releasemodel based on the open porosity is developed and tested on oxide fuels.A model for nitride fuels is introduced, and some correlations are bench-marked. Major issues remaining are pointed out and recommendations asto how to solve them are made. / <p>QC 20170227</p>

La région de Ladoga est une des plus intéressantes du bouclier Baltique pour la métallogenèse de l’uranium. Des minéralisations uranifères sont connues dans tous les niveaux stratigraphiques de la région. Les principaux gisements sont : celui de Karku associé à une discordance localisé dans les grès Mésoprotérozoïques (Riphéen) du bassin Pasha Ladoga, ceux de type grès et de type schistes noirs dans les dépôts de platforme paléozoïques inférieurs, les minéralisations localisées dans les granites anatectique et dans des roches carbonatées riches en phosphore metamorphisées appartenant au socle cristallin Archéen-Paléoprotérozoïque de la zone de Raahe-Ladoga; des minéralisations de type filonien dans le socle et les formations riphéennes. Une croissance graduelle des teneurs en U des formations géologiques et des resources minières en U est mise en évidence pendant de l’évolution géologique depuis l’Archéen jusqu'au Paléozoïque. Trois stades principaux de la métallogenèse de l’uranium ont été déterminés : le Svécofennien (1.97-1.77 Ga), le Mésoprotérozoïque (1.6-1.2 Ga) et de l’Ordovicien au Devonien (0.45-0.38 Ga). Il est montré que les gisements de type discordance, grès et filonien sont les plus prospectifs pour de découverte de gisements d’U de dimension économique dans la région de Ladoga. La plupart d’entre eux sont localisés le long de la zone Raahe-Ladoga. / The Ladoga region is one of the most interesting areas of the Baltic Shield with respect to U metallogenesis. U mineralization occurs at all stratigraphic levels of the geological succession in the region. The main U occurrences are: the unconformity-related Karku deposit in the Mesoproterozoic (Riphean) sandstones of the Pasha Ladoga basin, those of sandstone and blackshale type in the Lower Paleozoic platform sediments, the mineralization located in anatectic granites and metamorphosed phosphorites in the Archaean-Palaeoproterozoic crystalline basement of the Raahe-Ladoga zone, vein type U mineralization hosted in the basement and in the Riphean. U content in the geological formations and U ore resources gradually increase during the geological evolution of the Ladoga region from Archaean to Paleozoic. Three main stages of U metallogenesis have been identified: Svecofennian (1.97-1.77 Ga), Mesoproterozoic (1.6-1.2 Ga) and Ordovician-Devonian (0.45-0.38 Ga). Unconformity-related, sandstone and vein types represent the most prospective ones for the discovery of economic-grade U deposits in the Ladoga region. Most of the promising areas are located along the Raahe-Ladoga zone.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand in fulfilment of the requirements for the award of Master of Science degree, 2017. / Mining is an integral sector of most developing countries and it is a highly lucrative industry that has been in existence for centuries, and assumes an essential part in their economies. However, the legacy of mining in these countries has posed a threat to underground and surface water as a result of contamination arising from Acid Mine Drainage (AMD). Bearing in mind the environmental and ecological impairment posed by AMD there is a need for innovation in the treatment of AMD, to enable financially savvy treatment of the contaminated waters. This research is focused on the extraction of U(VI), As(III) and As(V) from synthetic metal solutions as well as field removal of these metal ions by application of iron hydroxide/oxide-modified zeolite. Batch experiments were performed to evaluate the effectiveness of iron hydroxide/oxide-modified zeolite as a potential low-cost sorbent for extracting As(III), U(VI) and As(V) from AMD. The research approach was based on the possible changes that can occur to a zeolite surface that has been in contact with an iron-laden solution. Zeolite is a commonly used adsorbent, but fewer studies have explored changes that it undergoes as an adsorbent on contact with iron solutions. Thus, the study involved modifying zeolite with iron hydroxide/oxide, which are the main precipitates of iron in the environment and which can possibly alter the adsorption properties of zeolite. Batch extraction studies were performed using the modified zeolite. In paper I, the synthesis of iron (hydr) oxide modified zeolite was achieved through precipitation of iron on the zeolite. The kinetic data for As(V) adsorption by iron (hydr) oxide-modified zeolite model fit well into pseudo second-order and the adsorption capacity was obtained as 0.080 mg g-1. The application of iron (hydr) oxide modified zeolite on AMD for As(V) recovery showed that > 99% of As(V) was extracted from the solution. The high removal efficiency of oxyanionic arsenic species was attributed to arsenic forming complexes with iron oxyhydroxide surface on the surface of the sorbent. Paper II dealt with adsorption of U(VI) from aqueous solution by application of iron hydro (oxide)-modified zeolite in a single-component system. Parameters such as: solution pH, contact time, adsorbent dosage, initial concentration and temperature were optimized before field application to real acid mine drainage. The optimum parameters for U(VI) adsorption were: adsorbent dosage (3.0 g), solution pH (6 ±0.1) and contact time (30 min). Optimum parameters where then applied to acid mine drainage were the effluent was found to be cleaner than the influent. In Paper III, iron oxide-coated zeolite (IOCZ) nanocomposite was prepared and fully characterized. This sorbent was then used for extraction of U(VI) and As(III) from aqueous solutions by application of batch techniques. Batch study results were modelled best by the pseudo second-order kinetic model and Freundlich isotherm. The adsorption capacity of both U(VI) and As(II) was dependent on the temperature. The presence of Cd2+, Co2+ and Cr3+ ions enhance the adsorption of As(III) whereas the opposite trend was observed for U(VI) sorption onto IOCZ nanocomposite. / XL2018

by Tsang Kee Kung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1978. / Includes bibliographical references (leaves 99-100).

Thèse doctorat : Sciences de la Terre et de l'Univers : Nancy 1 : 2005. / Titre provenant de l'écran-titre.

The Deilmann Tailings Management Facility (DTMF) at Key Lake in northern Saskatchewan, Canada, is an active deposition site for uranium tailings and it has been in operation since 1996. In terms of geochemical stability of the tailings, a ferrihydrite secondary phase is utilized for the sequestration of contaminants, such as As, Ni, Mo, and Se, under alkaline and highly oxic conditions. Arsenic is highly abundant in the DTMF tailings and the principal environmental concern is the possibility for leaching of ferrihydrite-attached As into the surrounding environment. Microorganisms can proliferate in a broad range of habitats and their activities are key factors in determining fate and transport of contaminants in various environments. This thesis attempts to obtain insights into the biogeochemical processes that may occur during the early phase of the DTMF’s history that could potentially become significant over extended periods of time that run from 100’s to 1000’s of years. Hence, a primary focus was to characterize microbial diversity and extrapolate their potential functional roles as well as their potential to chemically alter the Eh and ferrihydrite, which are the primary controlling conditions within the DTMF tailings and in the mineral secondary phase, respectively. To achieve these goals, two molecular techniques (clone library construction and Ion Torrent sequencing), a range of conventional culture-based techniques, metabolic assays addressing metabolic transformation and resistance to metals/metalloids, microscopic technique (Confocal Laser Scanning Microscope), spectroscopic analyses (Scanning Transmission X-ray Microscope) and bench-scale microcosm assays were carried out. Culture-dependent and -independent methods revealed that the most prevalent microbial groups in the water column, tailings mass and at the tailings-water interface affiliated into phyla (e.g., Proteobacteria, Actinobacteria, Firmicutes and Bacteriodetes) that have previously been detected at uranium-, heavy metal- and complex hydrocarbon-contaminated sites. Phylotypes closely related to well-characterized sulfate-, thiosulfate- and iron-reducing bacteria (e.g., Desulfosporosinus, Dethiobacter, Geoalkalibacter, Ralstonia, Georgfuchsia) were also detected at low frequency, with the exception of the tailings-water interface where sequences closely related to Desulfosporosinus were abundant. The readily culturable heterotrophs (e.g., Pseudomonas, Arthrobacter, Massilia, Hydrogenophaga, Polaromonas, Bacillus) retrieved from the tailings exhibited reducing/oxidizing capabilities as well as high tolerance to metal/metalloids. Bench scale microcosm assays showed that heterotrophs native to the DTMF site could not only reduce ferrihydrite but could also create highly reducing (< -300 mV) conditions within the tailings amenable to strict anaerobic bacteria such as Desulfosporosinus. STXM image analyses confirmed the presence of reduced iron in close proximity to bacterial cells in biofilm grown in situ and in microcosm tailings, strongly suggesting that ferrihydrite served as electron acceptor during microbial processes. Reduced iron detected in situ also indicated that microscale iron reduction could occur even though macroscale DTMF chemistry remained oxidizing. Overall, the nature of microbial community present in the DMTF system strongly indicated that complex hydrocarbons (e.g., kerosene) discharged into the tailings during processing could potentially support microbial processes that involve Fe and S cycling and that this process could become significant over extended period of times, contributing to arsenic escape into the environment.

No description available.

The behavior of U in near surface sedimentary environments has changed throughout geologic time. There is a marked shift in U (IV) mineral stability and deposit style at approximately 2200 Ma due to changes in the concentration of O in the atmosphere. For example, prior to 2200 Ma, fluvial U deposits could form and U(IV) were stable in surface and near-surface environments. In modern, anthropogenic sedimentary systems such as U tailings, U (IV) minerals are not stable and readily oxidize to U (VI) minerals. In addition, U is much more mobile in modern sediments relative sediments that were deposited prior to 2200 Ma.

No description available.