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U-Pb isotope systems have been used to constrain the timing of formation, alteration, and oxidation of U minerals from the meta-granitic bedrock at Forsmark, eastern Sweden. Secondary ion mass spectrometry (SIMS) has been used to collect U-Pb data from uraninite. Discordant data suggest a ~1.8 Ga emplacement of uraninite-bearing pegmatites and an event of uraninite alteration at ~1.6 Ga. The latter age is contemporaneous with the Gothian orogeny in Scandinavia, which was associated with hydrothermal fluid circulation in the Fennoscandian Shield. Ca-uranyl silicates haiweeite and uranophane predominately formed 1.3–1.2 Ga, contemporaneous with the emplacement of the Satakunta complex of the Central Scandinavian Dolerite Group. A Palaeozoic group of Ca-U(VI)-silicates is also present, which indicates that the geochemical composition of geologic fluids was heterogeneous throughout the fracture network during this time. Low Pb concentrations in the U(VI) silicates of several samples are compatible with a recent (

The Younger Granites of Yahmid-Um Adawi area, located in the southeastern part of Sinai Peninsula, comprise two coeval Late Neoproterozoic post-collisional alkaline (hypersolvous alkali-feldspar granites; 608–580 Ma) and calc-alkaline (transsolvous monzo- and syenogranites; 635–590 Ma) suites. The calc-alkaline suite granitoids are magnesian and peraluminous to metaluminous, whereas the alkaline ones are magnesian to ferroan alkaline to slightly metaluminous. Both granitoid suites exhibit many of the typical geochemical features of A-type granites such as enrichment in Nb (>20 ppm), Zr (>250 ppm), Zn (>100 ppm) and Ce (>100 ppm) and high 10000*Ga/Al2O3 ratios (>2.6) and Zr + Nb + Y + Ce (>350 ppm). Accessory mineral saturation thermometers demonstrated former crystallization of apatite at high temperatures prior to zircon and monazite separation from the magma for both granitoid suites. The mild zircon saturation temperatures of the studied Younger Granites (around 800 °C) imply low-temperature crustal fusion and incomplete melting of the largely refractory zircon. The two Younger Granite suites were semi-synchronously evolved during the post-collisional stage of the Arabian-Nubian Shield subsequent to the collision between the juvenile shield crust and the older pre-Neoproterozoic continental blocks of west Gondwana. Their parental magmas has been generated by melting of crustal source rocks with minor involvement from mantle, which might participated chiefly as a source of heat necessary for fusion of the crustal precursor. Extensive in-situ gamma-ray spectrometry revealed anomalously high radioactivity of some Younger Granite exposures along Wadi Um Adawi (eU; 388–746 ppm and eTh; 1857–2527 ppm) and pegmatitic pockets pertaining to the calc-alkaline suite (equivalent U and Th; 212–252 ppm and 750–1757 ppm, respectively). The radioactivity of the syngenetic pegmatites arises from the primary radioactive minerals uranothorite and thorite together with the U- and/or Th-bearing minerals zircon, columbite, samarskite and monazite. The anomalously high radioactivity of some Younger Granite exposures in Wadi Um Adawi stem from their appreciable enclosure of the epigenetic uranium minerals metatorbenite and uranophane.

The Gabal Nasb El Atshan Upper Carboniferous-Lower Permian altered trachytes include uranium up to 3165 ppm. The paleostress and resolved shear stress analyses of the deformation systems in Gabal Nasb El Atshan area indicate that the trachyte was subjected to WNW-ESE to E-W tensile shear stress directed extensional regimes. The low-stress regions in the vicinity of extensional faults and their associated joints were favorable locations for fluid flow and the consequence alteration and U-mineralization. This occurred more extensively along the contacts between the sills of trachyte and the Hammamat sedimentary rocks; where the latter acted as a physical barrier for the alteration fluids migration outward. Alteration styles include albitization , aegirinization, arfvedsonization, chloritization and ferruginisation. The albitization is the most common sodic metasomatism , giving sanidine from Or 98.8Ab0.7 to Or62.3Ab37.6, anorthoclase from Or51.4Ab48.0 to Or12.2Ab87.6 and albite from Or 11.0Ab89.0 to Or0.8Ab99.2 . Aegirine and arfvedsonite formed due to decreasing sodium activity in the metasomatic fluids. Sodic metasomatism may be the source of uranium-enrichment, taking place during the late magmatic to deuteric processes. This was followed by a retrograde alteration of chloritization between 175 and 42 °C toward precipitation of Fe-oxides and alteration of primary uranium. Surficial low-temperature alteration remobilized and redistributed the produced uranylhydroxides and ferruginisation caused the reduction and adsorption of U forming betafite, uranophane, soddyite, umohoite, uranotile and uranopilite.

A cation exchange chromatography method employing sulfonated polysterene cation resin (DOWEX AG50-X8) was developed in order to separate rare-earth elements (REEs) from uranium-rich materials. The chemical separation scheme is designed to reduce matrix effects and consequently yield enhanced ionization efficiencies for concentration determinations of REEs without significant fractionation using solution mode-inductively coupled plasma mass spectrometry (ICP-MS) analysis. The method was applied to determine REE abundances in four uraninite (ideally UO2) samples and their associated U(VI) alteration minerals. In three of the samples analyzed, the concentration of REEs for primary uraninite are higher than those for their corresponding secondary uranium alteration phases. The results for U(VI) alteration minerals of two samples indicate enrichment of the light REEs (LREEs) over the heavy REEs (HREEs). This differential mobilization is attributed to differences in the mineralogical composition of the U(VI) alteration. There is a lack of fractionation of the LREEs in the uraninite alteration rind that is composed of U(VI) minerals containing Ca2+ as the interlayer cation (uranophane and bequerelite); contrarily, U(VI) alteration minerals containing K+ and Pb2+ as interlayer cations (fourmarierite, dumontite) indicate fractionation (enrichment) of the LREEs. Our results have implications for nuclear forensic analyses since a comparison is reported between the REE abundances for the CUP-2 (processed uranium ore) certified reference material and previously determined values for uranium ore concentrate (UOC) produced from the same U deposit (Blind River/Elliott Lake, Canada). UOCs represent the most common form of interdicted nuclear material and consequently is material frequently targeted for forensic analysis. The comparison reveals similar chondrite normalized REE signatures but variable absolute abundances. Based on the results reported here, the latter may be attributed to the differing REE abundances between primary ore and associated alteration phases, and/or is related to varying fabrication processes adopted during production of UOC.

Uranium deposits form in a variety of settings. They are partially controlled by the secular evolution of Earth processes, including deposits in extension-related settings such as the intra-cratonic Rio Grande rift. Plio-Quaternary volcanism, mineral deposits, and hydrothermal spots occur along the Chihuahua Central Graben. The age of the Sierra de Gomez U-deposit is 1.8 Ma (based on LA-MC-ICP-MS dating on a uranophane monocrystal), which is contemporaneous with the late mineralization event of the Pena Blanca U-deposit, as well as Rio Grande Rift (RGR)-type deposits in Chihuahua and intraplate volcanism. Studies of fluid inclusions in fluorite and late calcite indicate the presence of hydrocarbons and CH4-rich brine. Homogenization temperatures range from 87 to 112 °C, and the mean composition (2.0 mol NaCl and 0.3 mol CaCl with CH4) is comparable to mineralizing brines in MVT deposits and carbonated hydrocarbon reservoirs. Evolution of C and O stable isotopic values for the calcite cement in the Sierra de Gomez Limestone-hosted U deposit illustrates that two separate calcite precipitation events occurred: (1) travertine filling karst structures in the presence of meteoric water and (2) U mineralization during deep hydrothermal fluid circulation that included interactions with a heat source and basement leaching. In a regional context, a metallogenic model suggests that the Chihuahua Trough area is deep enough to generate fluid migration by hydrothermal and/or compaction processes through RGR extensional faults until a favorable trapping horizon is reached. This causes uranium precipitation because water/rock interaction processes generate a local redox barrier.

Eastern Brazilian Pegmatite Province includes many topaz-bearing pegmatitic bodies. Residual melts from the Fe–K-rich alkaline Medina granite (ca. 500 Ma) formed the Serrinha pegmatite—a system of branched thin pegmatite veins hosted by pink facies of the parent granite. The colourless topaz from Serrinha pegmatite contains both mineral and fluid inclusions. Microcline (513, 476, 456 cm−1), albite (507, 479, 457 cm−1), topaz (926, 858, 267, 239 cm−1), quartz (463 cm−1), rutile (610, 444 cm−1), wolframite (884 cm−1) and uranophane (968, 788 cm−1) represent solid inclusions formed by fluid-induced processes from the pneumatolytic (∼600–400 °C) to hydrothermal (

The mineralogical studies on Black mica raw material revealed the presence of notable high radioactive minerals such as uranium minerals (Soddyite, Uranophane, Uranothorite, and Ishikawaite), niobium - tantalum minerals (Columbite and Plumbopyrochlore), and rare earth elements minerals (Allanite, Xenotime, and monazite). From mineralogical and chemical studies make it possible to determine optimal processing conditions, the preferred lixiviate option, and the required costs. Black mica sample was found to assay 1680 mg/Kg U, 5000 mg/Kg REEs, and 50,750 mg/Kg Nb while Ta not exceeds 5000 mg/Kg. Through applying sulfuric acid agitation leaching, 92% of uranium was dissolved under optimum leaching conditions of: 8% concentration of H2SO4 acid, 1/4 solid/liquid ratio at room temperature for 4 h reaction time, while RE2O3, Nb2O5 and Ta2O5 remained in the unattacked residue. A phosphate mixture was used for rapid dissolution of refractory minerals, REEs, Nb and Ta minerals, after fusion, which give 97.8%, 99%, and 97%, respectively, leaching efficiencies. Ion exchange, solvent extraction and precipitation techniques were used for separating and recovery of U, REEs, Nb, and Ta species from their leach liquor.

The southern Central Zone (sCZ) of the Neoproterozoic to early Palaeozoic Damara orogenic belt hosts significant uranium deposits in Namibia. Rossing South-West, a leucogranite-hosted uranium deposit in the sCZ is partitioned into two sections; north-easterly and south-westerly sections with the latter (termed mining licence 120, i.e. ML120) forming the locus of this investigation. The deposit lies within the vicinity and to the west of the Welwitschia lineament which served as a conduit for mineralized fluids along with the Rossing and Husab deposits. Mineralogy and major and trace element geochemistry of the uraniferous leucogranites in ML120 suggest they are S-type granites derived from a sedimentary source as evidenced by the presence of igneous phases of garnet and cordierite together. Microscope and electron microprobe results from 133 powdered, drill core and hand samples prove that the deposits host similar primary uranium mineralization (chiefly uraninite which mainly exist as independent uranium minerals) and similar secondary U mineralization. At Rossing-SW U minerals include uraninite, coffinite, pitchblende, betafite, uranophane, betauranophane, carnotite, gummite and might partially exist as Th isomorphs. Mineralization at all locales is associated with post-tectonic D and E-type sheeted leucogranites containing quartz, alkali-feldspar and variable amounts of plagioclase producing rocks of variable compositions from tonalite to alkali-feldspar granite. Total reserves calculated at a cut-off grade of 100 ppm were reported as 171 Mt of ore at 376 ppm for Rossing and 205 Mt of ore at 497 ppm for Husab (probable). The Rossing-SW orebody is open-ended however total inferred resources stand at 304.4 Mt of ore at 235 ppm while measured resources stand at 8830t ore at circa 231 ppm. A comparison of the geological parameters (uranium mineralogy, leucogranite petrology and geochemistry) suggests similar ore composition. Consequently, ore from Rossing-SW's ML120 area could be processed at existing metallurgical plants (such as at Rossing or Husab) with a probable increase in U3O8 yield.

223Ra,224Ra,226Ra, and228Ra isotopes have been measured in groundwaters from depths ranging 50–900m in fractured crystalline bedrock (Forsmark, Sweden) to understand the reason for elevated (up to 150μg/L) aqueous uranium (Uaq) at 400–650m depth. Ra isotope data is interpreted alongside previously reported222Rn,234U, and238U data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [223Ra/226Ra]GW-based approach (where brackets and “GW” subscript refer to expression of an activity ratio measured from groundwater) to groundwater residence time estimation shows that elevated [Uaq] is most common in Holocene-age groundwaters of marine origin. Although these groundwaters are geochemically reducing, the [223Ra/228Ra]corr(where “corr” subscript refers to a correction applied to compare [223Ra/228Ra]GWto the more commonly reported [226Ra/228Ra]GW) suggest that they interact with U-rich pegmatites containing Proterozoic- and Palaeozoic-age Ca-U(VI)-silicate minerals, which are undersaturated in the present groundwaters. Local aqueous U(VI) can be stabilized in Ca2UO2CO30complexes at pe-values as low as −4.5 but is susceptible to reduction after a modest decrease in pe-value, alkalinity, or Ca concentration. The [223Ra/228Ra]corrand [224Ra/228Ra]GWalso suggest that U(VI)aqprecipitates as UO2+Xat the interface between marine and non-marine groundwaters. From these data, local [Uaq] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals.

Development of unconventional shale gas wells can generate significant quantities of drilling waste, including trace metal-rich black shale from the lateral portion of the drillhole. We carried out sequential extractions on 15 samples of dry-drilled cuttings and core material from the gas-producing Middle Devonian Marcellus Shale and surrounding units to identify the host phases and evaluate the mobility of selected trace elements during cuttings disposal. Maximum whole rock concentrations of uranium (U), arsenic (As), and barium (Ba) were 47, 90, and 3333 mg kg−1, respectively. Sequential chemical extractions suggest that although silicate minerals are the primary host for U, as much as 20% can be present in carbonate minerals. Up to 74% of the Ba in shale was extracted from exchangeable sites in the shale, while As is primarily associated with organic matter and sulfide minerals that could be mobilized by oxidation. For comparison, U and As concentrations were also measured in 43 produced water samples returned from Marcellus Shale gas wells. Low U concentrations in produced water ( Geochemical modeling to determine mobility under surface storage and disposal conditions indicates that oxidation and/or dissolution of U-bearing minerals in drill cuttings would likely be followed by immobilization of U in secondary minerals such as schoepite, uranophane, and soddyite, or uraninite as conditions become more reducing. Oxidative dissolution of arsenic containing sulfides could release soluble As in arsenate form under oxic acidic conditions. The degree to which the As is subsequently immobilized depends on the redox conditions along the landfill flow path. The results suggest that proper management of drill cuttings can minimize mobilization of these metals by monitoring and controlling Eh, pH and dissolved constituents in landfill leachates.

Uraninite is a mineral considered as an analogue of the spent fuel, and the study of its alteration products has been used to predict the secondary phases produced during the fuel storage under specific environmental conditions. In this work, we study by Raman spectroscopy the alteration by weathering of the primary uraninite from the uranium deposit of Sierra Albarrana. The identification of the different secondary phases is based on the analysis of the symmetrical stretching vibration of the uranyl group (UO 2 2+ ), which allows the identification of individual uranyl phases and can be used as a fingerprint. Additionally, we show in this work a new approach to perform a semi-quantitative analysis of these uranium minerals by means of Raman spectroscopy. From this analysis we found the next sequence of alteration products: rutherfordine in contact with the uraninite core, then a mixture of uranyl silicates: soddyite, uranophane alpha and kasolite. Soddyite prevails in the inner part while uranophane alpha predominates in the outer part of the sample, and kasolite appears intermittently (1.0–3.3 mm; 4.6–7.1 mm and 8.8–10 mm).

Retention or release of radionuclides in a deep geological repository for radioactive wastes strongly depends on the geochemical environment and on the interaction with near-field components, e.g. waste packages and backfill materials. Deep geological disposal in rock salt is one of the concepts considered for cemented low- and intermediate-level wastes. Long-term experiments were performed to observe the evolution of full-scale cemented waste simulates (doped with (NH 4 ) 2 U 2 O 7 ) upon reaction with relevant salt brines, e.g. MgCl 2 -rich and saturated NaCl solutions, and to examine the binding mechanisms of uranium. Throughout the experiments, concentrations of major solution components, uranium and pH values were monitored regularly and compared to thermodynamic equilibrium calculations, which indicate that close-to-equilibrium conditions have been achieved after 13–14 years duration of the leaching experiments. Two of the full-scale cemented waste simulates were recovered from the solutions after 17–18 years and studied by different analytical methods to characterize the solids, especially with respect to uranium incorporation. In drill core fragments of various lateral and horizontal positions of the corroded monoliths, U-rich aggregates were detected and analyzed by means of space-resolved techniques. Raman, μ-XANES and μ-XRD analyses of several aggregates demonstrate that they consist of an amorphous diuranate-type solid. Within error, calculated U solubilities controlled by Na-diuranate (Na 2 U 2 O 7 ·H 2 O) are consistent with measured U concentrations in both, the NaCl and the MgCl 2 -system. Since uranophane occurs also in the corroded monoliths, it is proposed that a transition towards the thermodynamic equilibrium U(VI) phase is kinetically hindered.

Uranophane-beta of supergene origin formed in the Borborema Pegmatite Mineral. Province, which is situated in the north-easternmost part of Brazil. The sampling site lies in the topmost parts of the Quintos Pegmatite, about ten kilometers north of the town of Equador. The uranyl silicate was investigated for its age and physical chemical regime of formation. Age dating yielded a U/Pb age of 6.77 ± 0.61 Ma. Uranophane has been derived together with autunite from weathering of brannerite in a tropical climate under alternating wet and dry seasons, when the pH was below 8. This canary-yellow well-crystallized uranyl silicate can be used as a physical chemical marker as well as a clock for supergene alteration.

Boltwoodite and uranophane are uranyl silicates common in oxidized zones of uranium ore deposits. An understanding of processes that impact uranium transport in the environment, especially pertaining to the distribution of uranium between solid phases and aqueous solutions, ultimately requires determination of thermodynamic parameters for such crystalline materials. We measured formation enthalpies of synthetic boltwoodites, K(UO 2 )(HSiO 4 )·H 2 O and Na(UO 2 )(HSiO 4 )·H 2 O, and uranophane, Ca(UO 2 ) 2 (HSiO 4 ) 2 ·5H 2 O, by high temperature oxide melt solution calorimetry. We also studied the aqueous solubility of these phases from both saturated and undersaturated conditions at a variety of pH. The combined data permit the determination of standard enthalpies, entropies and Gibbs free energies of formation for each phase and analysis of its potential geological impact from a thermodynamic point of view.

Sodium boltwoodite (Na[(UO 2 )(SiO 3 OH)] · x H 2 O), uranophane (Ca[(UO 2 )(SiO 3 OH)] 2 · y H 2 O), and sklodowskite (Mg[(UO 2 )(SiO 3 OH)] 2 · z H 2 O) were synthesized in our laboratory using an optimized procedure described herein. Mineral identities were confirmed using powder X-ray diffraction. We also characterized our uranophane using Raman spectroscopy. The surface area of each material we produced was determined by two methods. We used the standard BET approach (gas phase absorption, N 2 ), and we also estimated surface areas in solution using sorption of methylene blue. These two techniques provided markedly different results, possibly due to the collapse of the mineral structure under the vacuum necessary for the BET analysis. In aqueous suspensions, the surface areas are 83 ± 10, 72 ± 7, and 47 ± 4 m 2 /g, for sodium boltwoodite, sklodowskite, and uranophane, respectively, using the methylene blue sorption method. The amount of water associated to each mineral was determined by thermogravimetric analysis. Results showed that minerals that had been dried in a dessicator contained less structural water than samples that had been allowed to set in aqueous suspension. The numbers of water for dry sodium boltwoodite, uranophane, and sklodowskite were found to be 1.5 (± 0.1), 2.8 (± 0.6), and 6.0 (± 0.6), respectively. The zeta potentials in suspension of 0.1 M NaClO 4 were similar for all three minerals, decreasing from − 20 mV to − 45 mV from pH 5 to 12, and remaining stable over a 10 day time period. The combination of a large surface area and negative zeta potential implies that these solids will behave much like clay minerals, serving as important sinks for other dissolved radioactive cations present in radioactive waste repositories.

The concentration and distribution of uranium (U) in sediment samples from three boreholes recovered near radioactive waste storage tanks at Hanford, Washington, USA, were determined in detail using bulk and micro-analytical techniques. The source of contamination was a plume that contained an estimated 7000 kg of dissolved U that seeped into the subsurface as a result of an accident that occurred during filling of tank BX-102. The desorption character and kinetics of U were also determined by experiment in order to assess the mobility of U in the vadose zone. Most samples contained too little moisture to obtain quantitative information on pore water compositions. Concentrations of U (and contaminant phosphate—P) in pore waters were therefore estimated by performing 1:1 sediment-to-water extractions and the data indicated concentrations of these elements were above that of uncontaminated “background” sediments. Further extraction of U by 8 N nitric acid indicated that a significant fraction of the total U is relatively immobile and may be sequestered in mobilization-resistant phases. Fine- and coarse-grained samples in sharp contact with one another were sub-sampled for further scrutiny and identification of U reservoirs. Segregation of the samples into their constituent size fractions coupled with microwave-assisted digestion of bulk samples showed that most of the U contamination was sequestered within the fine-grained fraction. Isotope exchange (233U) tests revealed that ∼51% to 63% of the U is labile, indicating that the remaining fund of U is locked up in mobilization-resistant phases. Analysis by Micro-X-ray Fluorescence and Micro-X-ray Absorption Near-Edge Spectroscopy (μ-XRF and μ-XANES) showed that U is primarily associated with Ca and is predominately U(VI). The spectra obtained on U-enriched “hot spots” using Time-Resolved Laser-Induced Fluorescence Spectroscopy (TRLIFS) provide strong evidence for uranophane-type [Ca(UO2)2(SiO3OH)2(H2O)5] and uranyl phosphate [Ca(UO2)2(PO4)2(H2O)10–12] phases. These data show that disseminated micro-precipitates can form in narrow pore spaces within the finer-grained matrix and that these objects are likely not restricted to lithic fragment environments. Uranium mobility may therefore be curtailed by precipitation of uranyl silicate and phosphate phases, with additional possible influence exerted by capillary barriers. Consequently, equilibrium-based desorption models that predict the concentrations and mobility of U in the subsurface matrix at Hanford are unnecessarily conservative.

This paper examines the problems associated with analysis of low levels of neptunium in a uranium matrix with electron energy-loss spectroscopy (EELS) on the transmission electron microscope (TEM). The detection of neptunium in a matrix of uranium can be impeded by the occurrence of a plural scattering event from uranium (U-M(5)+U-O(4,5)) that results in severe overlap on the Np-M(5) edge at 3665 eV. Low levels of Np (1600-6300 ppm) can be detected in a uranium solid, uranophane [Ca(UO(2))(2)(SiO(3)OH)(2)(H(2)O)(5)], by confirming that the energy gap between the Np-M(5) and Np-M(4) edges is at 184 eV and showing that the M(4)/M(5) ratio for the neptunium is smaller than that for uranium. The Richardson-Lucy deconvolution method was applied to energy-loss spectral images and was shown to increase the signal to noise ratio.

The Nopal I uranium deposit of the Sierra Pena Blanca, Mexico, has been the focus of numerous studies because of its economic importance and its use as a natural analog for nuclear-waste disposal in volcanic tuff. Secondary uranyl minerals such as uranophane, Ca[(UO2)(SiO3OH)]2(H2O)5, and weeksite, (K,Na)2[(UO2)2(Si5O13)](H2O)3, occur in the vadose zone of the deposit and are overgrown by silica glaze. These glazes consist mainly of opal A, which contains small particles of uraninite, UO2, and weeksite. Close to a fault between brecciated volcanic rocks and welded tuff, a greenish silica glaze coats the altered breccia. Yellow silica glazes from the center of the breccia pipe and from the high-grade pile coat uranyl-silicates, predominantly uranophane and weeksite. All silica glazes are strongly zoned with respect to U and Ca, and the distribution of these elements indicates curved features and spherical particles inside the coatings. The concentrations of U and Ca correlate in the different zones and both elements inversely correlate with the concentration of Si. Zones within the silica glazes contain U and Ca in a 1:1 ratio with maximum concentrations of 0.08 and 0.15 at.% for the greenish and yellow glazes, respectively, suggesting trapping of either Ca1U1-aqueous species or -particles in the colloidal silica. X-ray photoelectron spectroscopy (XPS), Fourier-transform infra-red spectroscopy (FTIR), and oxygen-isotope ratios measured by secondary-ion mass spectrometry (SIMS) indicate higher U6+/U4+ ratios, higher proportions of Si–OH groups and lower δ18O values for the greenish silica glaze than for the yellow silica glaze. These differences in composition reflect increasing brecciation, porosity, and permeability from the center of the breccia pipe (yellow silica glaze) toward the fault (green silica glaze), where the seepage of meteoric water and Eh are higher.

The Um Ara area, in the south Eastern Desert of Egypt contains a number of uranium occurrences related to granitic rocks. U-rich thorite, thorite and zircon are the main primary uranium- and thorium-bearing minerals found in mineralized zones of the Um Ara alkali-feldspar granites; uranophane is the most common secondary uranium mineral. U-rich thorite contains blebs of galena, has rims of uranophane and contains inclusions of Zr-rich thorite. Electron probe microanalysis (EPMA) provides an indication of a range of solid solution between thorite and zircon, in which intermediate phases, such as Th-rich zircon and Zr-rich thorite, were formed. These phases have higher sum of all cations per formula (2.05 to 2.06 apfu, for 4 oxygen atoms) than that of ideal thorite and zircon. This is attributed to the presence of substantial amount of interstitial cations such as Ca, U and Al in these phases. Some zircon grains are stoichiometric in composition, other altered grains display lower SiO 2 and ZrO 2 contents. Enrichment of Th and U in altered zircon preferentially involves coupled substitution (Ca 2+ + (Th,U) 4+ ↔ 2Zr 4+ + 2Si 4+ ), implying that significant U and Th may enter the Zr and Si position in zircon. Negative correlation of Zr vs. Hf and Al may indicate that Hf and Al have been introduced to the zircon during later fluid alteration rather than during the primary magmatic event. A two-stage metallogenetic model is proposed for the alteration processes and origin of U- and Th-bearing minerals in the Um Ara alkali-feldspar granite: 1) the first stage was dominated by hydrothermal alteration and accompanied by albitization, k-feldspathization, desilicification, chloritization, hematitization, silicification, argillization, fluoritization and corrosion of primary U-bearing minerals. Solid-solution between thorite and zircon occurred during this stage. The second stage occurred at the near-surface profile where circulating meteoric water played an important role in mobilizing the early formed primary U-bearing minerals. Uranium was likely transported as a calcium uranyl carbonate complexes. When these complexes lost their stabilities by precipitation of calcite, they decomposed in the presence of silica to form uranophane.

Dissolution experiments on single crystals of uranophane and uranophane-β, Ca(H2O)5[(UO2)(SiO3(OH)]2, from the Shinkolobwe mine of the Democratic Republic of Congo, were done in an aqueous HCl solution of pH 3.5 for 3 h, in HCl solutions of pH 2 for 5, 10 and 30 min, and in Pb2+–, Ba–, Sr–, Ca– and Mg–HCl solutions of pH 2 for 30 min. The basal surfaces of the treated uranophane crystals were examined using atomic-force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Solutions after dissolution experiments on single crystals and synthetic powders were analysed with inductively coupled plasma-optical emission spectroscopy (ICP-OES) and mass spectroscopy (ICP-MS). The morphology of the observed etch pits (measured by AFM) were compared to the morphology, predicted on the basis of the bond-valence deficiency of polyhedron chains along the edges of the basal surface. Etch pits form in HCl solutions of pH 2. Their decrease in depth with the duration of the dissolution experiment is explained with the stepwave dissolution model, which describes the lowering of the surrounding area of an etch pit with continuous waves of steps emanated from the etch pit into the rest of the crystal surface. Hillocks form in an HCl solution of pH 3.5, and the chemical composition of the surface (as indicated by XPS) shows that these hillocks are the result of the precipitation of a uranyl-hydroxy-hydrate phase. Well-orientated hillocks form on the surface of uranophane in a SrCl2–HCl solution of pH 2. They are part of an aged silica coating of composition Si2O2(OH)4(H2O)n. An amorphous layer forms on the surface of uranophane in a MgCl2–HCl solution of pH 2, which has a composition and structure similar to silicic acid. Small crystallites of uranyl-hydroxy-hydrate phases form on the surface of uranophane after treatment in Pb(NO3)2–HCl and BaCl2–HCl solutions of pH 2. Dissolution experiments on synthetic uranophane powders show that in the early stage of the experiments, the dissolution rate of uranophane increase in the sequence Pb(NO3)2–HCl

In order to investigate the interactions between spent nuclear fuel, corroding iron waste packages, and water under conditions likely to be relevant at the proposed repository at Yucca Mountain, six small-scale waste packages were constructed. Each package differed with respect to water input, exposure to the atmosphere and temperature. Two of the packages contained 0.1 g UO2. Simulated Yucca Mountain process water (YMPW) was injected into five of the packages at a rate of 200 μl per day for up to 2 years, at which point the solids were characterized with X-ray powder diffraction, scanning electron microscopy, wet chemistry and electron microprobe analysis. Fe(II) is abundant in the corrosion products that form, and the dominant crystalline product in all cases according to X-ray diffraction is magnetite or the structurally similar maghemite. Minor phases included akaganeite (β-FeOOH) and possibly also hematite (Fe2O3), lepidocrocite (γ-FeOOH) and green rust (Fe(II)1−xFe(III)x(OH)2Yx/n). Under these conditions, UO2 is expected to alter to the uranyl silicate uranophane (Ca[(UO2)SiO3(OH)]2·5H2O). Neither oxidation of the UO2 nor any oxidized (uranyl) solid was observed, suggesting that conditions were sufficiently reducing to kinetically hinder U(IV) oxidation.

The dissolution and growth of uranophane [Ca(UO 2 ) 2 (SiO 3 OH) 2 ·5H 2 O] have been examined in Ca- and Si-rich test solutions at low temperatures (20.5 ± 2.0 °C) and near-neutral pH (∼6.0). Uranium-bearing experimental solutions undersaturated and supersaturated with uranophane were prepared in matrices of ∼10 −2 M CaCl 2 and ∼10 −3 M SiO 2 (aq). The experimental solutions were reacted with synthetic uranophane and analyzed periodically over 10 weeks. Interpretation of the aqueous solution data permitted extraction of a solubility constant for the uranophane dissolution reaction ( log K sp o = 11.01 ± 0.09 ) and standard state Gibbs free energy of formation for uranophane ( Δ G f o = - 6196.1 ± 0.5 kJ mol −1 ).

As a natural analogue of the processes and products of spent nuclear fuel (SNF) alteration, we have examined the sequence of phases that form during the alteration of natural UO2+x in a U-deposit at Marshall Pass, Colorado. We have determined the paragenesis of U(VI)-phases including the fate of trace elements: W, Mo, As, Sb, Cu, Ba, Ce, Y, Pb and Th. Enrichment of trace elements, especially W and Mo, in this system resulted in a unique alteration sequence: uraninite → amorphous U-oxyhydrate gel → schoepite(I)/vandendriesscheite/compreignacite → uranophane → schoepite(II)/"dehydrated" schoepite(I) → Ba-Mo-W-U phase/U-arsenates/U-Sb phase → "dehydrated schoepite" (II) → soddyite/swamboite. In this sequence, the Ba-Mo-W uranyl phase and U-Sb phase are newly characterized phases. These results suggest that the UO2+x alteration, involving higher concentrations of certain radionuclides and metallic compounds, may lead to a different paragenesis of U(VI)-phases, as compared with the expected alteration sequence of UO2 interacting with a typical groundwaters. This was also noted in a previous study of the alteration of Pb-rich uraninite [R.J. Finch, R.C. Ewing, J. Nucl. Mater. 190 (1992) 133-156]. Some trace elements, such as CaO 2.08 wt.%, PbO 1.69 wt.%, WO3 1.39 wt.%, As2O3 0.50 wt.% and MoO3 0.41 wt.%, can locally concentrate, but still form uranyl phases. As a consequence, the mobility of U and radionuclides is governed by the stability of these metal-uranyl phases, such as Pb-oxide hydrates, Ba-uranyl molybdates/ tungstates and U-antimonate.

Uranyl silicates such as uranophane and Na-boltwoodite appear to control the solubility of uranium in certain contaminated sediments at the US Department of Energy Hanford site [Liu, C., Zachara, J.M., Qafoku, O., McKinley, J.P., Heald, S.M., Wang, Z. 2004. Dissolution of uranyl microprecipitates in subsurface sediments at Hanford Site, USA. Geochim. Cosmochim. Acta 68, 4519–4537.]. Consequently, the solubility of synthetic Na-boltwoodite, Na(UO2)(SiO3OH) · 1.5H2O, was determined over a wide range of bicarbonate concentrations, from circumneutral to alkaline pH, that are representative of porewater and groundwater compositions at the Hanford site and calcareous environments generally. Experiments were open to air. Results show that Na-boltwoodite dissolution was nearly congruent and its solubility and dissolution kinetics increased with increasing bicarbonate concentration and pH. A consistent set of solubility constants were determined from circumneutral pH (0 added bicarbonate) to alkaline pH (50 mM added bicarbonate). Average log K sp o = 5.86 ± 0.24 or 5.85 ± 0.0.26; using the Pitzer ion-interaction model or Davies equation, respectively. These values are close to the one determined by [Nguyen, S.N., Silva, R.J., Weed, H.C., Andrews, Jr., J.E., 1992. Standard Gibbs free energies of formation at the temperature 303.15 K of four uranyl silicates: soddyite, uranophane, sodium boltwoodite, and sodium weeksite. J. Chem. Thermodynamics 24, 359–376.] under very different conditions (pH 4.5, Ar atmosphere).

Raman spectra at 298 and 77 K of three uranophane samples from different localities are described and interpreted. The spectra are sample dependent. U–O bond lengths in uranyls are calculated from the spectra and compared with the published data of single crystal structure and EXAFS spectroscopy. Hydrogen-bonding of water molecules and silanols is discussed and the ‘proton mobility’ in uranophane sheet crystal structure is assumed.

Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ∼ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O = U = O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm −1 for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60% of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.

A new lead uranyl divanadate, PbUO2(V2O7), has been synthesized by high temperature solid-state reaction and its crystal structure was solved by direct methods using single-crystal X-ray diffraction data. It crystallizes in the monoclinic system with space group P21/n and following cell parameters: a=6.9212(9) A, b=9.6523(13) A, c=11.7881(16) A, β=91.74(1)°, V=787.01(2) A3, Z=4, ρmes=5.82(3), ρcal=5.83(1) g/cm3. A full-matrix least-squares refinement on the basis of F 2 yielded R1=0.029 and wR2=0.064 for 2136 independent reflections with I > 2 σ ( I ) collected with a Bruker AXS diffractometer (MoKα radiation). The crystal structure of PbUO2(V2O7) consists of a tri-dimensional framework resulting from the association of V2O7 divanadate units formed by two VO4 tetrahedra sharing corner and UO7 uranyl pentagonal bipyramids and creating one-dimensional elliptic channels occupied by the Pb2+ ions. In PbUO2(V2O7), infinite ribbons of four pentagons wide are formed which can be deduced from the sheets with Uranophane type anion-topology that occurs, for example, in the uranyl divanadate (UO2)2(V2O7), by replacement of half-U atoms of the edge-shared UO7 pentagonal bipyramids by Pb atoms. Infrared spectroscopy was investigated at room temperature in the frequency range 400–4000 cm−1, showing some characteristic bands of uranyl ion and of VO4 tetrahedra.

The dissolution of uranium was investigated from contaminated sediments obtained at the US. Department of Energy (U.S. DOE) Hanford site. The uranium existed in the sediments as uranyl silicate microprecipitates in fractures, cleavages, and cavities within sediment grains. Uranium dissolution was studied in Na, Na-Ca, and NH4 electrolytes with pH ranging from 7.0 to 9.5 under ambient CO2 pressure. The rate and extent of uranium dissolution was influenced by uranyl mineral solubility, carbonate concentration, and mass transfer rate from intraparticle regions. Dissolved uranium concentration reached constant values within a month in electrolytes below pH 8.2, whereas concentrations continued to rise for over 200 d at pH 9.0 and above. The steady-state concentrations were consistent with the solubility of Na-boltwoodite and/or uranophane, which exhibit similar solubility under the experimental conditions. The uranium dissolution rate increased with increasing carbonate concentration, and was initially fast. It decreased with time as solubility equilibrium was attained, or dissolution kinetics or mass transfer rate from intraparticle regions became rate-limiting. Microscopic observations indicated that uranium precipitates were distributed in intragrain microfractures with variable sizes and connectivity to particle surfaces. Laser-induced fluorescence spectroscopic change of the uranyl microprecipitates was negligible during the long-term equilibration, indicating that uranyl speciation was not changed by dissolution. A kinetic model that incorporated mineral dissolution kinetics and grain-scale, fracture-matrix diffusion was developed to describe uranium release rate from the sediment. Model calculations indicated that 50–95% of the precipitated uranium was associated with fractures that were in close contact with the aqueous phase. The remainder of the uranium was deeply imbedded in particle interiors and exhibited effective diffusivities that were over three orders of magnitude lower than those in the fractures.

Two hydrated uranyl arsenates and a uranyl phosphate were synthesized by hydrothermal methods in the presence of amine structure-directing agents and their structures determined: (N2C6H14)[(UO2)(AsO4)]2(H2O)3, DabcoUAs, {NH(C2H5)3}[(UO2)2(AsO4)(AsO3OH)], TriethUAs, and (N2C4H12)(UO2)[(UO2)(PO4)]4(H2O)2, PiperUP. Intensity data were collected at room temperature using MoKα X-radiation and a CCD-based area detector. The crystal structures were refined by full-matrix least-squares techniques on the basis of F2 to agreement indices (DabcoUAs, TriethUAs, PiperUP) wR2=5.6%, 8.3%, 7.2% for all data, and R1=2.9%, 3.3%, 4.0%, calculated for 1777, 5822, 9119 unique observed reflections (|Fo|⩾4σF), respectively. DabcoUAs is monoclinic, space group C2/m, Z=2, a=18.581(1), b=7.1897(4), c=7.1909(4) A, β=102.886(1)°, V=936.43(9) A3, Dcalc=3.50 g/cm3. TriethUAs is monoclinic, space group P21/n, Z=4, a=9.6359(4), b=18.4678(7), c=10.0708(4) A, β=92.282(1)°, V=1790.7(1) A3, Dcalc=3.41 g/cm3. PiperUP is monoclinic, space group Pn, Z=2, a=9.3278(4), b=15.5529(7), c=9.6474(5) A, β=93.266(1)°, V=1397.3(1) A3, Dcalc=4.41 g/cm3. The structure of DabcoUAs contains the autunite-type sheet formed by the sharing of vertices between uranyl square bipyramids and arsenate tetrahedra. The triethylenediammonium cations are located in the interlayer along with two H2O groups and are disordered. Both TriethUAs and PiperUP contain sheets formed of uranyl pentagonal bipyramids and tetrahedra (arsenate and phosphate, respectively) with the uranophane sheet-anion topology. In TriethUAs, triethlyammonium cations are located in the interlayer. In PiperUP, the sheets are connected by a uranyl pentagonal bipyramid that shares corners with phosphate tetrahedra of adjacent sheets, resulting in a framework with piperazinium cations and H2O groups in the cavities of the structure.

The hydrated neutral uranyl phosphate, (UO2)3(PO4)2(H2O)4, was synthesized by hydrothermal methods. Intensity data were collected using MoKα radiation and a CCD-based area detector. The crystal structure was solved by direct methods and refined by full-matrix least-squares techniques to agreement indices wR2=0.116 for all data, and R1=0.040, calculated for the 2764 unique observed reflections (∣Fo∣≥4σF). The compound is orthorhombic, space group Pnma, Z=4, a=7.063(1) A, b=17.022(3) A, c=13.172(3) A, V=1583.5(5) A3. The structure consists of sheets of phosphate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO2)(PO4)]− and the uranophane sheet anion topology. The sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with a phosphate tetrahedron on each of two adjacent sheets, resulting in an open framework with isolated H2O groups in the larger cavities of the structure.

Secondary U mineralisation is found in the oxidised zone pervading fractured albitised and alkali-feldspar granites emplaced at the northern boundary of Um Ara Pluton. It occurs as stains along crevices and fracture surfaces and as acicular crystals filling cavities. X-ray diffraction and SEM were used to identify secondary U minerals and the associated alteration products. Uranophane and β-uranophane are the most abundant U minerals, whereas Ca-montmorillonite and illite represent low temperature alteration products of the host granitic rocks. The genesis of secondary U minerals is mainly attributed to the action of oxic groundwater on previously corroded primary U minerals. These secondary U minerals were deposited near the surface from the circulating groundwater by evaporation.

The thermodynamic and kinetic properties of a synthetic uranophane (Ca(H3O)2(UO2)2(SiO4)2 · 3H2O) have been determined from dissolution experiments in test solutions of different bicarbonate concentrations at 25°C. The experiments were performed using batch and continuously stirred tank flow-through reactors. From the experimental data obtained with the batch reactor the solubility constant for the reaction: Ca(H3O)2(UO2)2(SiO4)2 · 3H2O + 6H3O+ ⇔ Ca+2 + 2UO2+2 + 2H4SiO4 + 11H2O was determined to be log Ks00 = 11.7 ± 0.6 and the rate equation for the dissolution process is log rdissol (mol s−1 m−2) = −8.3 (±0.6) + 0.7 (±0.3) log [HCO3−]. By using the continuously stirred tank flow-through reactor we obtained a rate equation in reasonably good agreement with that obtained in the batch reactor: log rdissol (mol s−1 m−2) = −9.2 (±0.4) + 0.7 (±0.2) log [HCO3−].

The structure of natural uranium-bearing, recently formed products arising from oxidative weathering of a U deposit (Peny, Massif Central, France) was studied to determine mechanisms of U-trapping by natural gels. Sampled waters are close to saturation with respect to amorphous silica and crystalline-hydrated U-hydroxides and silicates. All products are U-bearing Si/Al- and Fe-rich gels with minor Ca, Mg and water. Fourier Transform InfraRed spectrometry (FTIR) and X-ray absorption spectroscopy at Al–K, Fe–K and U–LIII edges show that these gels are composed of intimate mixtures of short-range ordered aluminosilicates and hydrous ferric oxides. In Si/Al-rich gels, Al is 6-fold coordinated to oxygens and Si tetrahedra are mostly isolated as in Al-rich allophane (Al:Si=2). Fe-rich gels exhibit a local structure dominated by edge-sharing octahedra, with d(Fe–Fe)=3.03 A, likely resulting from hydrolysis and oxidation from Fe2+ solution, and poisoning by Si/Al during Fe3+ precipitation. The local Al environment is close to that measured in Al-substituted goethite, and the Si tetrahedra are poorly polymerized as in natural Si-bearing ferrihydrite. The local structure around U was solved up to 3–4 A by Extended X-ray Absorption Fine Structure (EXAFS). Uranium is present in the hexavalent oxidized state, as uranyl complexes (UO22+) characterized by two axial oxygen atoms at d(U–Oax)=1.80 A and four equatorial oxygen atoms, at two distinct distances (d(U–Oeq1=2.33 A, d(U–Oeq2)=2.48 A). The collinear Oax–U–Oax geometry explains the presence of multiple scattering (MS) events in the EXAFS spectra of the U-bearing gels. In Si/Al-rich gels, U–U pairings at 3.82 A are consistent with polymers based on edge-sharing uranyls, in accordance to the speciation calculated in associated solutions. Alternatively to U–Oax MS, the contribution of Si neighbors at 3.3 and 3.7 A accounts for structural data beyond uranyl polyhedra. It is consistent with the local structure of uranophane, suggesting a coprecipitation process of U and Si. Elements such as Al may have poisoned crystal growth. In contrast, no U–Fe, U–Si/Al nor U–U contributions are evidenced in the Fe-rich gels. Only a weak contribution due to U–Oax MS is present. The geometry of Oeq coordination shell suggests that uranyl is complexed or sorbed onto mineral surfaces. It is proposed that U has been complexed by low Z elements like Si, Al in a first step, then trapped within hydrous ferric oxides during iron precipitation in a second step, i.e., during the final oxidation of the solutions inside the mine galleries.

Prominent shear zones in the low Ca granitic plutons of El-Missikat and El-Erediya, central Eastern Desert of Egypt trend northwest-southeast, north-south and northeast-southwest. The shear zones are filled with jasper containing uraniferous mineralisation with accessory uranophane, pitchblende and uraninite, accompanied by pyrite, galena, magnetite-titanomagnetite, ilmenite, hematite, rutile, titanite, fluorite, zircon, monazite, apatite and tourmaline. It is possible to discriminate between the mineralisation of the two prospects on the basis of the Pb and Cu content. The El-Missikat veins contain more Pb and less Cu, compared to those of El-Erediya. The mineralising hydrothermal fluid was ferruginous and highly enriched in many trace elements but was markedly deficient in europium. The content of the high field strength elements is mostly determined by the amount of zircon, monazite and rutile. Colouration in fluorite is associated with considerable enrichment in the REEs.

The interpretation of airborne gamma ray spectrometric data from the Magal Gebriel area, Eastern Desert, Egypt, has shown that a statistical treatment of these data in terms of individual geologic units is useful in detecting surface uranium mineralization. This is achieved through outlining the probable boundaries of potential uraniferous provinces. The locations and magnitudes of deviations in units equal to the standard deviation from the mean for e U , e U e Th and e U K are shown on a uranium anomaly map. This map shows two significant groupings of elevated eU values that coincide with elevated e U e Th and e U K values, associated with granitic rocks. This is in agreement with a recent geological study carried out on the Magal Gebriel area, which has reported the presence of secondary uranium minerals, namely B-uranophane, in these two parts. From the present study, the granitic bodies are considered to be the most promising for uranium prospecting in the area. The potential for significant uranium mineralization is still a matter of speculation. Uranium, thorium and potassium associations in these granitic bodies were studied applying correlation diagrams, indicating that uranium anomalies have the same pattern and are spatially related to both thorium and potassium anomalies, which suggests that there has been no uranium mobilization in these anomalous granites.

At the Nopal I uranium deposit, primary uraninite (nominally UO2+x) has altered almost completely to a suite of secondary uranyl minerals. The deposit is located in a Basin and Range horst composed of welded silicic tuff; uranium mineralization presently occurs in a chemically oxidizing and hydrologically unsaturated zone of the structural block. These characteristics are similar to those of the proposed U.S. high-level nuclear waste (HLW) repository at Yucca Mountain, Nevada. Petrographic analyses indicate that residual Nopal I uraninite is fine grained (5–10 μm) and has a low trace element content (average about 3 wt%). These characteristics compare well with spent nuclear fuel. The oxidation and formation of secondary minerals from the uraninite have occurred in an environment dominated by components common in host rocks of the Nopal I system (e.g. Si, Ca, K, Na and H2O) and also common to Yucca Mountain. In contrast, secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the Yucca Mountain environment (e.g. Pb, P and V). The oxidation of Nopal I uraninite and the sequence of alteration products, their intergrowths and morphologies are remarkably similar to those observed in reported corrosion experiments using spent fuel and unirradiated UO2 under conditions intended to approximate those anticipated for the proposed Yucca Mountain repository. The end products of these reported laboratory experiments and the natural alteration of Nopal I uraninite are dominated by uranophane [nominally Ca(UO2)2Si2O7·6H2O] with lesser amounts of soddyite [nominally (UO2)2SiO4·2H2O] and other uranyl minerals. These similarities in reaction product occurrence developed despite the differences in time and physical—chemical environment between Yucca Mountain-approximate laboratory experiments and Yucca Mountain-approximate uraninite alteration at Nopal I, suggesting that the results may reasonably represent phases likely to form during long-term alteration of spent fuel in a Yucca Mountain repository. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system conditions.

Experimental results indicate that UO2 will readily react after being exposed to dripping oxygenated ground water at 90°C. A pulse of rapid U release, combined with the formation of dehydrated schoepite characterizes reactions between one to two years. Rapid dissolution of intergrain boundaries and spallation of UO2 granules appears to be responsible for the rapid U release. Less than 5% of the U is released in a soluble or suspended form. After two years, U release rates decline and a more stable assemblage of uranyl silicate phases form by incorporating cations from the leachant. Uranophane, boltwoodite, and sklodowskite are the final solubility-limiting phases for U in these tests. This observed paragenetic sequence (from uraninite to schoepite to uranyl silicates) is identical to those observed in weathered uraninite deposits. Dispersion of particulate matter may be an important release mechanism for U and other radionuclides in spent nuclear fuel.

The solubility products and standard molar Gibbs free energies of formation were obtained from solubility measurements at 303.15 K for the following synthetic uranium-silicate phases: soddyite, (UO2)2SiO4·2H2O; uranophane, Ca(H3O)2(UO2)2(SiO4)2·3H2O; sodium boltwoodite, Na(H3O)(UO2)SiO4·H2O; and sodium weeksite, Na2(UO2)2(Si2O5)3·7H2O. These phases were identified as uranium precipitates likely to form near a high-level nuclear waste repository. The experimental standard molar Gibbs free energies of formation for soddyite, uranophane, sodium boltwoodite, and sodium weeksite are: −(3658.0±4.8) kJ·mol−1, −(6210.6±7.6)kJ·mol−1, −(2966.0±3.6)kJ·mol−1, and −(9088.5±18.4) kJ·mol−1, respectively. The value for sodium boltwoodite is a lower bound and therefore should be used with caution.

Ion exchange equilibria between silicate gels p SiO 2 −( A,B )O− n H 2 O and aqueous solutions ( A 2+ , B 2+ ), where A 2+ is UO 2 2+ or NpO 2 2+ and B 2+ is Mg 2+ , Ni 2+ or Ca 2+ , were studied. For the uranyl cation, all the pairs ( A 2+ , B 2+ ) investigated exhibit normal simple ion-exchange behaviour, except the pair (UO 2 2+ −Ca 2+ ). Although normal, this behaviour is not totally in conformity with the thermodynamic cation-hydroxide model, previously proposed for Mg 2+ and the divalent cations of the transition elements of the first series. Several explanations are put forward: structural, chemical and, possibly, thermodynamic. For the pair UO 2 2+ −Ca 2+ , the anomalous behaviour recorded can be explained by the formation of a stoichiometric analogue of uranophane, Ca(UO 2 ) 2 (SiO 3 OH) 2 , which precludes any quantitative comparison with the model. Supplementary distribution data obtained with the pairs Mg 2+ −Ca 2+ and Ni 2+ −Ca 2+ give indirect evidence of an order of affinity of UO 2 2+ for silicate gels: Ni 2+ 2+ 2+ , also in concordance with the one for hydroxides as in the model. The behaviour of the neptunyl cation, which was only investigated in the high dilution range, appears to be similar to that of the uranyl cation. The overall results suggest a selective fixation of both actinide cations by silicate gels, which appear to be materials of interest in the problems of radioactive waste repository contamination.

Gibbs free energies, enthalpies and entropies of 42 dissolved uranium species and 30 uranium-bearing solid phases have been critically evaluated from the literature and estimated when necessary for 25°C. Application of the data shows that the uranium in natural waters is usually complexed. At typical concentrations of chloride, fluoride, phosphate and sulfate, uranous (U4+) fluoride complexes are important in anoxic waters below pH 3–4. An intermediate Ehs (between about +0.2 and −0.1 V) and pH values 1–7, UO2+ ion may predominate. In oxidized waters, uranyl (U22+) fluoride complexes and uranyl ion predominate below pH 5; from about pH 4 to 7.5, UO2(HPO4)22− is the principal species; while at higher pHs, UO2CO30 and the di- and tri-carbonate complexes predominate. Uraninite [UO2-UO2.25], α-U3O8 and schoepite are the stable uranium oxides and hydroxides in water at 25°C. Coffinite, USiO4 (c), is probably stable relative to UO2(c) when dissolved silica exceeds about 60 ppm (as SiO2). At low Ehs and pH 4–6, the solubilities of stoichiometric crystalline uraninite and coffinite are below roughly 10−4 ppb. But at intermediate Ehs and neutral to alkaline pHs in the presence of phosphate or carbonate, the formation of uranyl phosphate or carbonate complexes can increase the solubilities of these minerals by several orders of magnitude. The uranyl minerals carnotite, tyuyamunite, autunite, potassium autunite and uranophane are least soluble at pHs in the range 5–8.5 and, in the case of carnotite and tyuyamunite, have solubilities as low as 0.2 and 1 ppb uranium, respectively. The autunites and uranophane are usually several orders of magnitude more soluble than this, consistent with their natural occurrences. Sorption of uranyl on to natural materials is maximal in the same pH range of 5–8.5.

Thermal decomposition /combined TG and OTA/ of some secondary uranium minerals of the uranyl silicate group /uranophane, β-uranophane, sklodowskite, cuprosklodowskite, kasolite and soddyite/ was studied with special regard to the water content in the minerals. The obtained results were correlated with the infrared absorption spectra and published crystal structure data of the minerals studied.

The mechanisms of immobilization of nuclear waste elements such as Cs, Sr, Ba, U, La and Nd (the latter two simulating Am and Cm) by three cement minerals, one cement and one mortar were investigated. Cement minerals did not immobilize Cs or Sr or Ba in the absence of CO2 but immobilized 62 to 91% of the added Cs and all of the added Sr by forming carbonates when CO2 was bubbled through the cement mineral suspensions. The elements La, Nd and U reacted significantly with various cement minerals, cement and mortar and precipitated as hydroxides. For example, C3S★ mineral immobilized 92, 73 and 99.2 of the added La, Nd and U respectively. Reaction of cement with U resulted in the formation of basic calcium uranyl silicate hydrate or uranophane under simulated repository conditions. These results suggest that cements serve as not only physical barriers but also act as chemical barriers for the migration of especially U and transuranic elements of nuclear waste.

The Todilto formation of the Grants, New Mexico, uranium belt has been a significant source of uranium from limestone. It is the only carbonate in a thick sequence of Mesozoic shales, mudstones and sandstones in which four sandstone units together contained the largest uranium resources yet identified in sandstone. The Todilto expresses nearly orthogonal sets of tectonic folds ranging downward to 6th order (several meters long) in size (the Zuni uplift axis being 1st order) in alternating N-E or NE-NW orientations. Those shorter than 5th order (about 50 m long) are intraformational. Smaller irregular diagenetic slump folds are abundant. Tectonic folds are attributed to mild orogeny during the lower and middle Cretaceous hiatus, probably reactivated during the Laramide orogeny with similar stress orientations. Primary uranium deposits are controlled by orthogonal fold sets and recrystallization-induced permeability zones. Secondary uranium deposits are controlled by joint stockworks. Compressional shear was relieved partly by flow recrystallization in envelopes enclosing the folds. Within these recrystallization to coarse granularity incompletely destroyed bedding and dessication joints, bleached the rock of hydrocarbon and other coloring agents and greatly enhanced textural permeability. The Barbara “J” No. 1 orebody was controlled by two echelon 4th order anticlines. The deposit was 122 m deep (±20 m below the water table), 100 m long and 35–40 m wide. In detail it was a mosaic of smaller richer bodies controlled by sets of 5th and 6th order folds. It produced 8691 short tons of ore grading 0.20% U3O8. Like all primary unoxidized ores in the Todilto, the Barbara “J” No. 1 orebody was restricted to the pressure-recrystallization envelope enclosing the folds. Pre-uranium alterations were metasomatic leaching/bleaching (distinct from recrystallization bleaching), illitization of siltstone hematization and silicification. Coffinite and uraninite/pitchblende occurred as isolated replacement grains or aggregates up to 2 cm in diameter, randomly distributed through a matrix of hematitic recrystallized limestone. Associated ore-stage minerals included purple fluorite, barite, apatite, cassiterite, silver and rare earth (Nd, La, Ce) minerals. Post-ore coarsely crystalline calcite fills axial tension fractures. Supergene modification was minimal, limited to a few uranophane aggregates in the calcite veins. Because unmineralized Todilto has unusually low radioactivity and is dense and impermeable, uranium is considered extrinsic. Migration through the rock texture has been possible only in the permeable recrystallization envelopes to which access probably was through the fracture system.

Petrographic, Gandolfi-XRD and scanning electron microprobe analyses of samples from the Otish and Mistassini Basins, Quebec, Canada, have demonstrated that: 1. (1) hydrated uraniferous mineral assemblages characterize each of the major lithologies studied. In particular, prismatic and radial polycrystalline aggregates of kasolite and dewindtite have been found within samples of conglomeratic sandstone of the Papaskwasati Formation, sector 1, Takwa and within samples of conglomeratic sandstones of the Indicator Formation, sector 3, Lac Indicateur; 2. (2) microcrystalline, flaky francevillite and crust-like fourmarierite, minor uranophane and coffinite have been found within samples from erratic boulders of dolomite from sector 4, Lac Indicateur. Only one rare occurrence of uraninite has been identified in a sample of conglomeratic sandstone from the Papaskwasati Formation, sector 1, Takwa; 3. (3) no uraniferous mineralization has been found in samples of north-trending gabbroic rocks cropping out within sector 2, Holton; 4. (4) the presence of the uraniferous hydrated phases suggests that the mineralization has been derived from a primary uraninite source undergoing strong weathering by acid meteoric waters; 5. (5) the gabbros which cross-cut the lithologies of sector 2, Holton, do not appear to have been able to cause a thermal rise important enough to generate a major remobilization of the system studied; and 6. (6) it is suggested that special weathering conditions, including the presence of acid rains and extensive hydrolysis of the silicates, within the source region, before and/or during the transport and deposition of the granitic detrital fraction, must have prevailed during the Archean.

More or less intensive supergene redeposition is displayed in the upper parts of all of the U occurrences in the basement rocks of northeastern Bavaria. The secondary U minerals formed during this redeposition — uranophane, torbernite, autunite, uranocircite and saleeite — were dated using the U/Pb and ionium-uranium methods. All of the samples yielded discordant U Pb ages. The upper intersection of the discordia with the concordia in the graph of 207 Pb 206 Pb vs. 238U/206Pb can be regarded only in some cases as the age of the primary pitchblende from which the U mineral was formed, whereas in all cases the lower intersection gives the age of the secondary U mineralization. Ages between 0.110 ± 0.006 Ma (by the ionium—uranium method) and 8.6 Ma were obtained; these can be divided into three groups: Group I comprises all of the minerals whose 230 Th 234 U activity ratio was lower than 1. Assuming that there was no 230Th present at the time of mineral formation, ages between 0.110 and 0.164 Ma were calculated for three torbernite occurrences. Group II consists of those U minerals which are in a state of radioactive equilibrium (i.e. older than 0.4 Ma), but which yielded U Pb ages between 0 and 3 Ma. Owing to the limited precision of the lower intersections, more accurate dates cannot be given. Group III contains the oldest of the secondary U minerals and is characterized by well-defined ages between 5.6 and 8.4 Ma.