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The origins and near-surface distributions of the ~250 known uranium and/or thorium minerals elucidate principles of mineral evolution. This history can be divided into four phases. The first, from ~4.5 to 3.5 Ga, involved successive concentrations of uranium and thorium from their initial uniform trace distribution into magmatic-related fluids from which the first U4+ and Th4+ minerals, uraninite (ideally UO2), thorianite (ThO2), and coffinite (USiO4), precipitated in the crust. The second period, from ~3.5 to 2.2 Ga, saw the formation of large low-grade concentrations of detrital uraninite (containing several wt% Th) in the Witwatersrand-type quartz-pebble conglomerates deposited in a highly anoxic fluvial environment. Abiotic alteration of uraninite and coffinite, including radiolysis and auto-oxidation caused by radioactive decay and the formation of helium from alpha particles, may have resulted in the formation of a limited suite of uranyl oxide-hydroxides. Earth’s third phase of uranium mineral evolution, during which most known U minerals first precipitated from reactions of soluble uranyl (U6+O2)2+ complexes, followed the Great Oxidation Event (GOE) at ~2.2 Ga and thus was mediated indirectly by biologic activity. Most uraninite deposited during this phase was low in Th and precipitated from saline and oxidizing hydrothermal solutions (100 to 300 °C) transporting (UO2)2+-chloride complexes. Examples include the unconformity- and vein-type U deposits (Australia and Canada) and the unique Oklo natural nuclear reactors in Gabon. The onset of hydrothermal transport of (UO2)2+ complexes in the upper crust may reflect the availability of CaSO4- bearing evaporites after the GOE. During this phase, most uranyl minerals would have been able to form in the O2-bearing near-surface environment for the first time through weathering processes. The fourth phase of uranium mineralization began ~400 million years ago, as the rise of land plants led to non-marine organic-rich sediments that promoted new sandstone-type ore deposits. The modes of accumulation and even the compositions of uraninite, as well as the multiple oxidation states of U (4+, 5+, and 6+), are a sensitive indicator of global redox conditions. In contrast, the behavior of thorium, which has only a single oxidation state (4+) that has a very low solubility in the absence of aqueous F-complexes, cannot reflect changing redox conditions. Geochemical concentration of Th relative to U at high temperatures is therefore limited to special magmatic-related environments, where U4+ is preferentially removed by chloride or carbonate complexes, and at low temperatures by mineral surface reactions. The near-surface mineralogy of uranium and thorium provide a measure of a planet’s geotectonic and geobiological history. In the absence of extensive magmatic-related fluid reworking of the crust and upper mantle, uranium and thorium will not become sufficiently concentrated to form their own minerals or ore deposits. Furthermore, in the absence of surface oxidation, all but a handful of the known uranium minerals are unlikely to have formed.

The mobility and availability of arsenite, As(III), in anoxic environments is largely controlled by adsorption onto iron sulfides and/or precipitation of arsenic in solid phases. The interaction of As(III) with synthetic mackinawite (FeS m ) in pH 5 and 9 suspensions was investigated using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), STEM elemental mapping, high resolution TEM, and X-ray photoelectron spectroscopy (XPS). At pH 5, arsenic sulfide phases precipitate among the FeS m particles as discrete particles that are an amorphous hydrous phase of arsenic sulfide. The oxidation state of As in the surface layers of the arsenic sulfide precipitates is ‘realgar-like’ based on XPS results showing that > 75% of the As 3d peak area is due to As with oxidation states between 0 and 2+. Discrete, arsenic sulfide precipitates are absent at pH 9, but elemental mapping in STEM-EDX mode shows that arsenic is uniformly distributed on the FeS m , suggesting that uptake is caused by the sorption of As(III) oxyanions and/or the precipitation of highly dispersed arsenic sulfides on FeS m . XPS also revealed that the FeS m that equilibrated without As(III) has a more oxidized surface composition than the sample at pH 9, as indicated by the higher concentration of O (∼ three times greater than that at pH 9) and the larger fraction of Fe(III) species making up the total Fe (2p 3/2 ) peak. These findings provide a better understanding of redox processes and phase transitions upon As(III) adsorption on iron sulfide substrates.

Swift heavy-ion irradiations of a wide variety of materials have been used to modify and manipulate the properties of solids at the nanoscale. Recently, these high-energy irradiations have been successfully combined with high-pressure experiments. Based on results obtained for zircon (ZrSiO(4)), this paper introduces this new experimental approach involving diamond anvil cells and large ion-accelerator facilities. This technique provides a wide spectrum of geoscience applications from nanoscale simulations of fission-track formation under crustal conditions to phase transitions of radiation-damaged minerals resulting from meteorite impact.

Coffinite, USiO4, is one of the two most abundant and important naturally occurring U4+ phases (the other is UO2), and it is an alteration product of the UO2 in spent nuclear fuel when in contact with silica-rich groundwater under reducing conditions. Despite its ubiquity, there are very limited data on the response of coffinite to radiation. Here, we present the results of the first systematic investigation of energetic ion beam irradiation (1 MeV Kr2+) of ultra-fine, synthetic coffinite (20–50 nm). In situ transmission electron microscopy (TEM) showed that the crystalline-to-amorphous transformation occurs at a relatively low dose, ∼0.27 displacements per atom (dpa) at room temperature. The critical temperature, Tc, above which coffinite cannot be amorphized, is low (∼608 K). Synthetic coffinite is more stable as compared with isostructural zircon (ZrSiO4; Tc = 1000 K) and thorite (ThSiO4; Tc above 1100 K) upon ion beam irradiation at elevated temperature, suggesting enhanced defect annealing behavior in nano-sized synthetic coffinite. Irradiation was found to decrease the temperature required to induce phase decomposition process in coffinite upon thermal annealing. A good correlation among the critical amorphization temperature, Tc, phase decomposition temperature, Tf, and the temperature range of the two-phase (ZrO2 and SiO2) co-existed region was identified.

Coffinite, USiO4, has been produced by hydrothermal synthesis. The synthesis products, coffinite nanoparticles (50 nm in size) with UO2 nanoparticles (a few nanometers), are always associated even if they are not always detected by XRD measurements. The formation of coffinite was shown to be very sensitive to several experimental parameters. The most important of these parameters are the pH, which must be in the range 8-9.5, the pressure, which must be below 50 bars, and the reaction conditions, which must be oxygen-free to maintain uranium in its tetravalent oxidation state. XRD and TEM reveal that tetragonal coffinite accounts for more than 90% of the final products while the by-products UO2 and a Si-rich amorphous phase are also present. The structural formula of the obtained coffinite is close to USiO4 as determined from EMPA (U0.99±0.06Si0.97±0.07O4) . XPS measurements show a peak chemical shift of the U-4f core levels by 1 eV toward higher binding energies in coffinite compared with stoichiometric UO2. The U-4f7/2 and U-4f5/2 positions in coffinite are found to be near 380.8 ± 0.3 eV and 391.7 ± 0.3 eV, respectively.

Cu-rich and As-rich growth zones in pyrite provide new insights into the composition of late-stage magmatic fluids and their host hydrothermal ore deposits. The pyrite is from the Pueblo Viejo (Dominican Republic) and Yanacocha (Peru) high-sulfidation gold-silver deposits, which are thought to form from hydrothermal systems that interacted with magmatic vapor plumes. Electron microprobe analysis, secondary ion mass spectrometry, and elemental maps show that pyrite, the most common sulfide mineral in both deposits, contains three different types of growth zones: (1) As-rich zones that are enriched in Au, Ag, Sb, Te, and Pb, (2) Cu-rich zones with significantly lower concentrations of these elements, and (3) barren pyrite zones with no other elements. These zones are interpreted to result from mixing between the pyrite-forming fluid and vapors that invaded the main hydrothermal system episodically. Comparison to experimental studies of elemental partitioning and analyses of fumaroles and fluid inclusions from magmatic-hydrothermal systems suggests that the As-rich vapor formed at high and possibly magmatic temperatures, whereas the Cu-rich vapor formed at lower temperatures, possibly during migration of the original magmatic vapor. The presence of finely spaced multiple growth zones in pyrite suggests that the composition of at least high-sulfidation hydrothermal systems can be affected intermittently and repetitively by vapors, probably from underlying magmas.

Nanosized (12−17 nm) zirconia particles with a thin alumina shell (

The compressibility, phase stability, and vibrational properties of coffinite (USiO4) were studied by in situ X-ray diffraction and infrared (IR) measurements at high pressures. An irreversible phase transition from the zircon-type to scheelite-type structure was found to occur at 14-17 GPa. Accompanying the structural transition, partial amorphization was also evident in the XRD analysis. The predicted transition pressure calculated by density functional theory is in good agreement with the experimental results. IR spectra also suggest that water is incorporated into the coffinite structure, and a pressure-induced electron transfer (U4+ -> U5+) may also occur.

Inert matrix fuels are an important component of advanced nuclear fuel cycles, as they provide a means of utilizing plutonium and reducing the inventory of ‘minor’ actinides. We examine the neutronic and thermal characteristics of MgO–pyrochlore (A2B2O7: La2Zr2O7, Nd2Zr2O7 and Y2Sn2O7) composites as inert matrix fuels in boiling water reactors. By incorporating plutonium with resonance nuclides, such as Am, Np and Er, in the A-site of pyrochlore, the kinf vs. burn-up curves are shown to be similar to those of UO2, although the Doppler coefficients are less negative than UO2. The Pu depletion rates are 88–90% (239Pu) and 54–58% (total Pu) when the inert matrix fuels experience a burn-up equivalent of 45 GWd/tHM UO2. Because of the high thermal conductivity of MgO, the center-line temperatures of the MgO–pyrochlore composites at 44.0 kW/m are lower than those of UO2 pellets. After burn-up, the A-site cation composition is 15–35 at.% lower than that of the B-site cations in pyrochlore (e.g., A1.84B2.17O7.00) due to the fission of Pu in the A-site and the presence of fission product elements in the A- and B-sites of the pyrochlore structure.

The response of strontium titanate (SrTiO 3 ) to ion and electron irradiation is studied at room temperature. For an accurate energy to depth conversion and a better determination of ion-induced disorder profile from Rutherford backscattering spectrometry measurement, a detailed iterative procedure is described and applied to ion channeling spectra to determine the dechanneling yield and the disorder profiles for the Sr and Ti sublattices. The result shows a large underestimation in disorder depth, ∼40% at the damage peak, which indicates a large overestimation of the electronic stopping power for 1.0 MeV Au ions in SrTiO 3 predicted by the SRIM (Stopping and Range of Ions in Matter) code. Overestimation of heavy ion stopping power may lead to an overestimation of the critical dose for amorphization. The current study also demonstrates possible ionization effects in SrTiO 3 under ion and electron irradiation. Pre-amorphized SrTiO 3 exhibits strong ionization-induced epitaxial recovery at the amorphous/crystalline interface under electron irradiation.

The isometric, pyrochlore structure type, A2B2O7, exhibits a wide variety of properties that find application in a large number of different technologies, from electrolytes in solid oxide fuel cells to actinide-bearing compositions that can be used as nuclear waste forms or inert matrix nuclear fuels. Swift xenon ions (1.43 GeV) have been used to systematically modify different compositions in the Gd2Zr2-xTixO7 binary at the nanoscale by radiation-induced phase transitions that include the crystalline-to-amorphous and order-disorder structural transformations. Synchrotron x-ray diffraction, Raman spectroscopy, and transmission electron microscopy provide a complete and consistent description of structural changes induced by the swift heavy ions and demonstrate that the response of pyrochlore depends strongly on chemical composition. The high and dense electronic energy deposition primarily results in amorphization of Ti-rich pyrochlore; whereas the formation of the fully disordered, defect-fluorite structure is the dominant process for Zr-rich pyrochlore.

Recent studies have attributed toxic effects of ambient fine particulate matter (aerodynamic diameter ≤ 2.5 μm; PM 2.5 ) to physical and/or chemical properties rather than total mass. However, identifying specific components or sources of a complex mixture of ambient PM 2.5 that are responsible for adverse health effects is still challenging. In order to improve our understanding of source-to-receptor pathways for ambient PM 2.5 (links between sources of ambient PM 2.5 and measures of biologically relevant dose), integrated inhalation toxicology studies using animal models and concentrated air particles (CAPs) were completed in southwest Detroit, a community where the pediatric asthma rate is more than twice the national average. Ambient PM 2.5 was concentrated with a Harvard fine particle concentrator housed in AirCARE1, a mobile air research laboratory which facilitates inhalation exposure studies in real-world settings. Detailed characterizations of ambient PM 2.5 and CAPs, identification of major emission sources of PM 2.5 , and quantification of trace elements in the lung tissues of laboratory rats that were exposed to CAPs for two distinct 3-day exposure periods were completed. This paper describes the physical/chemical properties and sources of PM 2.5 , pulmonary metal concentrations and meteorology from two different 3-day exposure periods—both conducted at the southwest Detroit location in July 2003—which resulted in disparate biological effects. More specifically, during one of the exposure periods, ambient PM 2.5 -derived trace metals were recovered from lung tissues of CAPs-exposed animals, and these metals were linked to local combustion point sources in southwest Detroit via receptor modeling and meteorology; whereas in the other exposure period, no such trace metals were observed. By comparing these two disparate results, this investigation was able to define possible links between PM 2.5 emitted from refineries and incinerators and biologically relevant dose, which in turn may be associated with observed health effects.

Colloid-like nanoparticles in groundwater have been shown to facilitate migration of several radionuclides: (239,240)Pu, 137Cs, (152,154, 155)Eu, and 60Co. However, the exact type of nanoparticle and the speciation of the associated radionuclides has remained unknown. We have investigated nanoparticles sampled from the far-field at the Nevada Test Site, Nevada, utilizing advanced electron microscopytechniques, including high-angle annular dark-field scanning TEM (HAADF-STEM). Fissiogenic elements: Cs, rare earth elements (REE), activation elements: Co; and actinides: U and Th, were detected. Cesium is associated with U-forming cesium uranate with a Cs/U atomic ratio of approximately 0.12. Light REEs and Th are associated with phosphates, silicates, or apatite. Cobalt occurs as a metallic aggregate, associated with Cr, Fe, Ni, and +/-Mo. Uranyl minerals; Na-boltwoodite and oxide hydrates are also present as colloids. Because of these chemical associations with nanoscale particles, in the size range

The uranyl peroxide, studtite (UO 4 ·4H 2 O, C 2/ c , Z = 4), is expected to form as a consequence of alpha radiolysis of water in contact with spent nuclear fuel (SNF) in a geologic repository. Investigation of its stability is, therefore, of critical importance because secondary U(VI) phases may incorporate trace amounts of radionuclides and thus retard their mobility away from a repository site. To examine the effect of ionizing radiation on uranyl peroxides, electron-beam irradiation experiments have been conducted on two synthetic uranyl peroxides: studtite and metastudtite (UO 4 ·2H 2 O, Immm , Z = 2). All experiments were done using a transmission electron microscope (TEM) with an acceleration voltage of 200 kV at room temperature. The fluence required to completely amorphize studtite was 0.51–1.54 × 10 17 e/cm 2 , which is equivalent to an absorbed dose of 0.73–1.43 × 10 7 Gy. Metastudtite becomes amorphous at a higher absorbed dose (1.31 × 10 7 Gy) than studtite, most likely because it contains fewer water molecules in its structure. These uranyl peroxides partially amorphize at doses that are one-tenth of the dose required for complete amorphization. With continued irradiation, uraninite nanocrystals form that are a few nanometers in diameter, at 4–20 × 10 10 Gy. In a geologic repository, for spent nuclear fuel, the estimated absorbed doses due to ionizing radiation may be as high as 10 8 –10 11 Gy after 10 6 years. This is well in excess of doses in the laboratory experiments that caused the uranyl peroxides to become amorphous and decompose.

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.

A significant blue shift of the luminescence of CdSe/ZnS quantum dots (QD) conjugated with functionalized multi-wall carbon nanotubes (CNT) is investigated. The observed Stark shift is due to the local electrostatic field induced by the carboxylic anions on the CNT surface. A theoretical model is developed to evaluate the contribution of the surface charges to the observed spectral shift (up to 0. 59 eV). Based on the present model, the Stark shift provides an effective method for evaluating the density of carboxyl groups on the surface of functionalized CNTs and can be used as a charge detector for QD–nanostructures.

The tetragonal orthophosphates, YbPO4 and LuPO4, were studied by in situ X-ray diffraction (XRD) at pressures up to 52 and 43 GPa, respectively. A reversible phase transition from the zircon structure-type to the scheelite structure-type was found at not, vert, similar22 GPa for YbPO4 and 19 GPa for LuPO4. Coinciding with the transition from the zircon structure-type to the scheelite structure-type, there is a not, vert, similar 10% reduction in volume and a significant increase in the bulk modulus for both compounds.

Using a new experimental approach, fission-track formation has been simulated, for the first time, under crustal conditions by exposing natural zircon, at a pressure of 7.5 kbar and a temperature of 250 °C, to a beam of relativistic heavy ions. The latent tracks were investigated using high-resolution transmission electron microscopy, and the diameters of several hundred tracks were measured. The mean values (± σ) of the track diameters were 5.2 ± 0.5 nm and 5.4 ± 0.4 nm for zircon at ambient and elevated pressure-temperature, respectively. Based on the number of measurements, this represents a statistically significant difference between the tracks at ambient vs. high-pressure/temperature conditions. The slightly larger size of the tracks at elevated pressure can be understood in terms of the increased efficiency of the damage process in a strained crystal lattice. This slight variation in track diameter (~ 0.2 nm) at high pressure probably will not affect the dimensions of etched tracks.

A specially designed carbon nanotube (CNT) is developed for use in the early detection and treatment of cancer. The key functionalities for biomedical diagnosis and drug delivery are incorporated into the CNTs. In vivo imaging of live mice is achieved by intravenously injecting quantum dot (QD)-conjugated CNT for the first time. With near infrared emission around 752 nm, the CNT with surface-conjugated QD (CNT-QD) exhibit a strong luminescence for non-invasive optical in vivo imaging. CNT surface modification is achieved by a plasma polymerization approach that deposited ultra-thin acrylic acid or poly(lacticco-glycolic acid) (PLGA) films (� 3 nm) onto the nanotubes. The anticancer agent paclitaxel is loaded at 112.5 � 5.8 m gm g � 1 to PLGA-coated CNT. Cytotoxicity of this novel drug delivery system is evaluated in vitro using PC-3MM2 human prostate carcinoma cells and quantified by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The in vivo distribution determined by inductively coupled plasma mass spectrometry (ICP-MS) indicates CNT-QD uptake in various organs of live animals.

The structural variations of synthetic zircon (ZrSiO4) single crystals irradiated at room temperature by 280 keV Pb+ ions (with fluences up to 1 × 1015 ions/cm2) were investigated using infrared (IR) spectroscopy. Like metamict zircon whose crystal structure is damaged and amorphized by naturally occurring α-decay events, the Pb+-irradiated zircon crystals show a dramatic decrease in reflectivity. However, no significant decrease in wavenumbers of the stretching vibrations of SiO4 tetrahedra in zircon was detected. The Pb+-implanted zircon exhibits new IR bands, indicating irradiation-induced new vibrations or domains, clusters or phases in addition to SiO2 and ZrO2. IR features consistent with those of Pb silicates (with a divalent state, i.e., Pb2+) are also found in the irradiated sample. This finding implies that some of the radiogenic Pb in natural zircon might not actually reside in the zircon lattice or in ZrSiO4 phases, but form new local domains or clusters. Infrared bands of OH-stretching vibrations were also detected in the irradiated synthetic zircon, which was originally free from OH features prior to the irradiation. These results indicate that H can easily diffuse into the irradiated layer or into irradiated-induced phases to form OH or and hydrous species after the irradiated material is damaged. The type and content of hydrous species vary with irradiation fluences.

This report describes a new form of arsenian pyrite, called As3+-pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As1--pyrite, in which As1- substitutes for S [Fe(As,S)2]. As3+-pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As3+-pyrite confirm that As is present largely as As3+. EMPA analyses show that As3+-pyrite incorporates up to 3.05 at % of As and 0.53 at. %, 0.1 at. %, 0.27 at. %, 0.22 at. %, 0.08 at. % and 0.04 at. % of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As3+ in the pyrite could be written like: As3 + + y Au+ + 1 - y (□) ⇔ 2 Fe2 +; where Au+ and vacancy (□) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As3+-pyrite (20-40 nm thick) and co-linear lattice fringes indicating epitaxial growth of As3+-pyrite on As-free pyrite. Overgrowths of As3+-pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8-20 nm, 100-300 nm and 400-900 nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As3+-pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As3+; in contrast, reducing conditions are typical of most environments that host As1--pyrite. As3+-pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments, including high-sulfidation epithermal deposits and shallow groundwater systems, where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of As3+ and toxic heavy metals in oxidizing natural systems.

Both focused electron and ion beams (FEB and FIB) were used for fabricating patterned nano-/micro-structures on the surfaces of an iron phosphate glass (FePO). Periodically arranged, nano-sized holes/tubes/rings with their walls being made of Fe nanoparticles were fabricated by FEB direct writing. The diameters of the holes/tubes can be controlled by the beam size and dwell time, as well as by subsequent electron irradiation. Micro- and nano-sized patterns, also with Fe-nanoparticles sitting on the surfaces, were fabricated by FIB sputtering. These result in the possible exploitation of new applications of the glass, particularly in biomaterials, optical circuits, high-density data storage, and controlling the growth of some nanowires/tubes.

First-principle calculations have been completed on a series of Ln 2 Sn 2 O 7 (Ln = Sm, Gd, Tb, Ho, Er, Lu, Y, La, Pr and Nd) pyrochlores to study the effect of structural geometry and bond-type on the stability of the pyrochlore structure-type. Overlap population analysis showed that the 〈Sn–O 48f 〉 bonds in stannate pyrochlores are much more covalent than the 〈Ln–O 48f 〉 bonds, and a nonlinear relationship is observed between the 〈Sn–O 48f 〉 or 〈Ln–O 48f 〉 bond lengths and the Ln cation radii. The 〈Ln–O 8b 〉 bonds are the most ionic among the metal–oxygen bonds. These results are consistent with experimental results. We note other factors, in addition to bond-type, that affect the stability of the pyrochlore structure.

Coarse-grained coffinite [USiO4•nH2O; n = 0–2] from the Upper Jurassic Morrison Formation in the Grants uranium region, New Mexico, USA, has been investigated in order to understand the processes of coffinite formation and alteration under reducing and oxidizing conditions. Elongated (up to 35 μm) prismatic crystals of coffinite precipitated contemporaneously with layers of organic matter (OM) and a vanadium-rich mica in this sandstone. Three different populations of coffinite were determined based on paragenesis and chemical composition; i) primary coffinite, (U0.924Ca0.218Y,REE0.021)1.163(Si0.835P0.033As0.010)0.878O4•nH2O, that was partially dissolved by migrating organic acids under reducing conditions; ii) recrystallized coffinite, (U0.805Ca0.156Y,REE0.022Zr0.001)0.984(Si0.954P0.036As0.005)0.995O4•nH2O, for which there was a 13% loss of U [apfu] and 28% loss of Ca [apfu], while Y + La + Ce + Nd [apfu] is enriched 5%. Uranium and REEs were incorporated into nano-scale crystals of secondary coffinite within the layers of organic matter; iii) coffinite formed during extensive alteration under oxidizing conditions. This coffinite, (U0.619Ca0.117Y,REE0.018)0.754(Si1.127P0.040As0.004)1.171O4•nH2O, is depleted ~ 23% of U [apfu], 25% of Ca [apfu] and 18% of Y +REE [apfu], as compared with coffinite-(ii). (Na,K)-boltwoodite, [(Na,K)(UO2SiO3OH)(H2O)], and a mixture of various uranyl sulfates, precipitated at the expense of coarse-grained coffinite-(i) as the final products of the alteration under oxidizing conditions. Based on charge balance calculations of coffinite groups (i) and (ii), the amount of U6+ was estimated to be in the range of 0.1–0.19 [apfu]. An upper limit of 0.2 [apfu] U6+ is postulated for the coffinite structure. Thus, the ideal chemical formula of coffinite is: (U4+1 − xU6+x)Si1 − xO4•nH2O, where 0 < x < 0.2; 0 < n < 2. Compositional variations of coffinite are governed by the substitutions: U4+ + Si4+ ⇔ U6+ + [2(OH)−, 0.5 □]; REE3+ + As5+ ⇔ U4+ + Si4+; Ca2+ + (As,P)5+ ⇔ REE3+ + Si4+. The variable, but high, totals of the chemical analyses (92.8–99.7 wt.%) suggest that coffinite contains molecular H2O, which is not an essential component of coffinite structure. The amount of H2O varies between 0 and 2 molecules per formula unit, and a minor amount of (OH)− may be accommodated into the structure. The U–Pb chemical ages limit the age of coffinite precipitation to between 36.6 and 0.7 Ma, which indicates continuous precipitation of coffinite since the mid-Tertiary. These results suggest that organic matter, which has preserved reducing conditions even in the presence of oxidizing fluids, over this long period of approximately 30 million years, plays an important role in the sustained presence of coffinite in these deposits.

Zircon (ZrSiO4), the most important accessory mineral in the Earth's crust, transforms under high pressure to reidite, a scheelite-structured polymorph. Recently, reidite was found in association with meteorite impact structures. Here, we show that the zircon-to-reidite transition, and thus the amount of reidite produced during high-pressure events, strongly depends on the microstructure of the initial zircon. Our results clearly demonstrate that radiation damage, present in natural zircon due to radioactive decay, dramatically modifies the phase stability of crystalline zircon at high pressure. By simulating this radiation damage with ion beams, we show that zircon, pre-irradiated with 1.47-GeV Xe ions, formed only minor amounts of reidite up to 36GPa; whereas, an unirradiated zircon was almost completely transformed to reidite under the same conditions. By means of Raman scattering, X-ray diffraction, and transmission electron microscopy, we confirmed that the stability field of the irradiated zircon is expanded to higher pressures as a result of the interplay between pressure, ion beam-induced amorphization, and the formation of nanoscale damage domains. These results provide insight into the formation-conditions of reidite in nature and illustrate how pressure-induced phase transitions may be affected by defects, in this case those caused by radioactive decay.

The addition of Th to U-based fuels increases resistance to corrosion due to differences in redox-chemistry and electronic properties between UO2 and ThO2. Quantum-mechanical techniques were used to calculate surface energy trends for ThO2, resulting in (1 1 1)

ImagingBy Donglu Shi,* Yan Guo, Zhongyun Dong, Jie Lian, Wei Wang, Guokui Liu, Lumin Wang, and Rodney C. EwingAdv. Mater. 2007, 19, 4033–4037DOI: 10.1002/adma.2007000035The scale bars in Fig. 6 of this article are in error. The correct dimensions of the mice used in this study are given below:Total length (including tail): 18-20 cmLength of the tail: 9 cmHeight of the mice: 3-4 cm

One of the major issues related to the expanded use of nuclear power and the development of advanced nuclear fuel cycles is the fate of plutonium and “minor” actinides. In addition, substantial quantities of plutonium and highly enriched uranium from dismantled nuclear weapons now require disposition. There are two basic strategies for the disposition of the actinides: (1) to “burn” or transmute the actinides using nuclear reactors or accelerators; (2) to “sequester” the actinides in chemically durable, radiation-resistant materials that are suitable for geologic disposal. This paper deals with actinide-bearing materials that support the latter approach. During the past two decades, a considerable amount of research and development has been done in an effort to develop matrices for the immobilization of plutonium and the “minor actinides”, Np, Am and Cm. A variety of waste form materials – oxides, silicates and phosphates – have been developed that have a high capacity for the incorporation of actinides, are chemically durable and, in some cases, resistant to the radiation-induced transformation to the aperiodic state. These waste forms can be selected depending on the composition of the waste stream that contains the actinides, the desired materials' properties of the waste form, and the geochemical and hydrologic conditions of the specific repository. The present state-of-knowledge for these materials is such that now one can design materials for very specific conditions, such as the thermal history and accumulated radiation dose, in a repository.

A pressure-induced splitting of the Cu-O chains was found in SrCuO2 at pressures >7 GPa and room temperature. The high-pressure phase is a superstructure of the infinite-layered tetragonal superconducting phase of SrCuO2 and is not stable under ambient conditions. The Cu-O chains buckle with further increasing pressure, and a new high-density polymorph of SrCuO2 is formed at 34.2 GPa.

U-, Th- and Ce-doped pyrochlores were synthesized by solid state reaction: (Ca0.62Gd0.97U0.23)(Zr0.84Ti1.34)O6.9, (Ca0.47Gd0.95Th0.40)(Zr1.29Ti0.89)O7.05, (Ca0.44GdTh0.42)Zr2.13O7.05, Ca0.91Th0.84Zr2.25O7.09 and Ca1.04Ce0.97Ti1.99O6.96. Their response to the radiation damage from the recoils of the alpha-decay events has been simulated by 1 MeV Kr2+ ion irradiation and studied by in situ transmission electron microscopy (TEM). An ion beam-induced pyrochlore-to-fluorite structural transition occurred for all compositions, and independent kinetics were observed for cation and anion disordering processes. Similar to the stoichiometric titanate and zirconate pyrochlores, the Ti-rich compositions are susceptible to ion beam-induced amorphization; while, the Zr-rich compositions are more radiation “resistant”. A close relation between the critical amorphization temperature of the U-, Th- and Ce-doped pyrochlores and the average cation ionic radius ratio of the A- and B-sites was observed.

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.

Alteration of uraninite from a hydrothermal vein-type U-deposit in Marshall Pass, Colorado, has been examined by electron microprobe analysis in order to investigate the release and migration of trace elements W, As, Mo, Zr, Pb, Ba, Ce, Y, Ca, Ti, P, Th, Fe, Si, Al, during alteration, under both reducing and oxidizing conditions. The release of trace elements from uraninite is used to establish constraints on the release of fission product elements from the UO2 in spent nuclear fuels. Uraninite occurs with two different textures: (1) colloform uraninite and (2) fine-grained uraninite. The colloform uraninite contains 1.04-1.75 wt% of WO3, 0.16-1.70 wt% of As2O3, 0.06-0.88 wt% of MoO3; whereas, the fine-grained uraninite retains 2.25-4.93 wt% of WO3, up to 5.76 wt% of MoO3, and 0.26-0.60 wt% of As2O3. The near constant concentration of incompatible W in the colloform uraninite suggests W-incorporation into the uraninite structure or homogeneous distribution of W-rich nano-domains. Incorporation of W and Mo into the uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of Cs during the corrosion of spent nuclear fuel, as there is a strong affinity of Cs with W and Mo. Zoning in the colloform texture is attributed to variation in the amount of impurities in uraninite. For unaltered zones, the calculated amount of oxygen ranges from 2.08 to 2.32 [apfu, (atom per formula unit)] and defines the stoichiometry as UO2+x and U4O9; whereas, for the altered zones of the colloform texture, the oxygen content is 2.37-2.48 [apfu], which is probably due to the inclusion of secondary uranyl phases, mainly schoepite. The supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of uraninite. Four stages of colloform uraninite alteration are proposed: (i) formation of an oxidized layer at the rim, (ii) corrosion of the oxidized layer, (iii) precipitation of U6+-phases with well-defined cleavage, and (iv) fracture of the uraninite surface along the cleavage planes of the U6+-phases.

The structural characteristics of Sm2Zr2O7, an ordered pyrochlore, were studied by angle-dispersive X-ray diffraction (XRD) and Raman scattering methods with increasing pressure. A structural distortion was observed at low pressures; but the distortion disappeared at pressures above 13.5 GPa. Above 18 GPa, the pyrochlore structure is unstable and a pressure-induced phase transition occurred. The high-pressure phase is a distorted fluorite structure in which the cations and anion vacancies are disordered. The disordered structure is accompanied by the formation of partially amorphous domains. With the release of pressure, the high-pressure phase partially transforms to the amorphous state.

Y 2 Ti 2 O 7 pyrochlores doped with La have been sintered at 1373 K for 12 h with the designed compositions of the (La x Y 1− x ) 2 Ti 2 O 7 system ( x = 0, 0.08, 0.5, and 1), and the phase compositions were analyzed by X-ray diffraction. Limited amounts of La were incorporated into yttrium titanate pyrochlore structure for La-doped samples; while, the end member composition of La 2 Ti 2 O 7 formed a layered perovskite structure. Ion beam-induced amorphization occurred for all compositions in the (La x Y 1− x ) 2 Ti 2 O 7 binary under 1 MeV Kr 2+ irradiation at room temperature, and the critical amorphization dose decreased with increasing amounts of La 3+ . The critical amorphization temperatures for Y 2 Ti 2 O 7 , (La 0.162 Y 0.838 ) 2 Ti 2 O 7 and La 2 Ti 2 O 7 were determined to be ∼780, 890 and 920 K, respectively. Th 4+ and Fe 3+ -doped yttrium titanate pyrochlores were synthesized at 1373 K by sintering Y 2 Ti 2 O 7 with (ThO 2 + Fe 2 O 3 ). Pyrochlore structures and the chemical compositions were primarily identified by the X-ray diffraction and energy dispersive X-ray (EDX) measurements. The lattice parameter and the critical amorphization dose (1 MeV Kr 2+ at room temperature) increase for yttrium titanate pyrochlores with the addition of Th. The increasing ‘resistance’ to amorphization with less La and greater Th and Fe contents for (Y 1− x La x ) 2 Ti 2 O 7 and Y 2 Ti 2 O 7 –Fe 2 O 3 –ThO 2 systems, respectively, are consistent with the changes in the average ionic radius ratio at the A-sites and B-sites. These results suggest that the addition of lanthanides and actinides (e.g., Th, U, or Pu) will affect the structural stability, as well as the radiation response behavior of the pyrochlore structure-type.

There are two compelling reasons for understanding source term and near-field processes in a radioactive waste geologic repository. First, almost all of the radioactivity is initially in the waste form, mainly in the spent nuclear fuel (SNF) or nuclear waste glass. Second, over long periods, after the engineered barriers are degraded, the waste form is a primary control on the release of radioactivity. Thus, it is essential to know the physical and chemical state of the waste form after hundreds of thousands of years. The United States Department of Energy's Yucca Mountain Repository Program has initiated a long-term program to develop a basic understanding of the fundamental mechanisms of radionuclide release and a quantification of the release as repository conditions evolve over time. Specifically, the research program addresses four critical areas: a) SNF dissolution mechanisms and rates; b) formation and properties of U6+-secondary phases; c) waste form–waste package interactions in the near-field; and d) integration of in-package chemical and physical processes. The ultimate goal is to integrate the scientific results into a larger scale model of source term and near-field processes. This integrated model will be used to provide a basis for understanding the behavior of the source term over long time periods (greater than 105 years). Such a fundamental and integrated experimental and modeling approach to source term processes can also be readily applied to development of advanced waste forms as part of a closed nuclear fuel cycle. Specifically, a fundamental understanding of candidate waste form materials stability in high temperature/high radiation environments and near-field geochemical/hydrologic processes could enable development of advanced waste forms “tailored” to specific geologic settings.

Molybdenum oxide thin films were produced by magnetron sputtering using a molybdenum (Mo) target. The sputtering was performed in a reactive atmosphere of an argon–oxygen gas mixture under varying conditions of substrate temperature ( T s ) and oxygen partial pressure ( p O 2 ). The effect of T s and p O 2 on the growth and microstructure of molybdenum oxide films was examined in detail using reflection high-energy electron diffraction (RHEED), Rutherford backscattering spectrometry (RBS), energy-dispersive X-ray spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) measurements. The analyses indicate that the effect of T s and p O 2 on the microstructure and phase of the grown molybdenum oxide thin films is remarkable. RHEED and RBS results indicate that the films grown at 445 °C under 62.3% O 2 pressure were stoichiometric and polycrystalline MoO 3 . Films grown at lower p O 2 were non-stoichiometric MoO x films with the presence of secondary phase. The microstructure of the grown Mo oxide films is discussed and conditions were optimized to produce phase pure, stoichiometric, and highly textured polycrystalline MoO 3 films.

The perovskite-related layered structure of La 2 Ti 2 O 7 has been studied at pressures up to 30 GPa using synchrotron radiation powder X-ray diffraction (XRD) and Raman scattering. The XRD results indicate a pronounced anisotropy for the compressibility of the monoclinic unit cell. The ratio of the relative compressibilities along the [100], [010] and [001] directions is ∼1:3:5. The greatest compressibility is along the [001] direction, perpendicular to the interlayer. A pressure-induced phase transition occurs at 16.7 GPa. Both Raman and XRD measurements reveal that the pressure-induced phase transition is reversible. The high-pressure phase has a close structural relation to the low-pressure monoclinic phase and the phase transition may be due to the tilting of TiO 6 octahedra at high pressures.

This paper reports on the results of a structural investigation of an important group of synthetic compounds with derivative structures of the mineral murataite, a potential matrix for immobilization of radioactive wastes. A model describing the structure of the entire group of compounds is proposed on the basis of analyzing the results obtained using chemical (electron microprobe) analysis methods, high-resolution transmission electron microscopy (high-resolution electron images and microdiffraction patterns), and X-ray diffractometry of a large array of artificial murataite ceramics. In essence, the proposed structural model is as follows: in the structure, layers of two (pyrochlore and murataite) packing types alternate along the [111] direction of the cubic supercell, so that the resulting structures can undergo substitutional and displacive modulations. Experiments on irradiation of different murataite modifications with heavy ions demonstrate that these modifications have close values of the radiation resistance, which is higher than that of the pyrochlore coexisting with them.

Single crystal samples of cubic zirconia stabilized by 9.5 mol% Y2O3 (YSZ) were implanted with 200 KeV Xe ions and 400 KeV Cs ions up to a dose of 5 x 10(16) ions/cm(2), then annealed at 200, 300, and 400 degrees C for 1 h isochronously, respectively. Electron paramagnetic resonance (EPR) and cross-sectional transmission electron microscopy were used to study defect structure and radiated damage mechanism. EPR spectra showed the trigonal signal with g(parallel to) = 1.989 and g(perpendicular to) = 1.869, which exhibited axial symmetry with (111) direction as symmetry axis composed of six-fold-coordinated Zr3+ sites. Peak-to-peak intensity (per unit volume) of Cs-ion irradiated YSZ is as about 150 time as that of Xe-ion irradiated YSZ. This indicated that the concentration of six-fold-coordinated Zr3+ defects produced by Cs-ion irradiation was far more than that of Xe-ion irradiated. The cross-sectional image of the irradiated samples showed that the Cs-ion irradiation in YSZ produced more defect clusters than Xe-ion irradiation. Annealing did not produce any change for EPR signals and indicated six-fold-coordinated Zr3+ defects was stable defect structure below 400 degrees C. (c) 2006 Elsevier B.V. All rights reserved.

The effect of 1.5 keV Ar+ ion irradiation on the (001) surfaces of potassium titanyl arsenate, KTiOAsO4 (KTA), has been investigated using reflection high-energy electron diffraction (RHEED) and X-...

Complexly zoned zircons (19 grains, ∼ 3.3 Ga) from a porphyritic granite in the Jack Hills, Western Australia, have been investigated using electron microprobe analysis (EMPA) and transmission electron microscopy (TEM) in order to examine the effects of radiation as a function of dose, as well as the nano-scale microstructure and composition of impurities and secondary alteration phases. In back-scattered electron (BSE) images, zones with bright contrast consist of an almost unaltered zircon with limited amounts of impurity elements. In contrast, the dark zones contain higher concentrations of trace elements: U, Th, Pb, Fe, Y, Ce, Ca and Al. The cumulative doses due to alpha-decay in the dark zones are calculated to be 0.21–1.0 × 10 17 (alpha-decay events/mg), equivalent to 1.0–4.7 dpa (displacements per atom). These doses are much higher than the dose required for radiation-induced amorphization, as determined by ion-beam irradiation of synthetic zircon, 0.3–1.0 dpa (0–600 °C). However, based on high-resolution TEM observations, none of the zircons are fully amorphous, to the result of annealing under ambient conditions. The concentrations of Ca and Al, which was considered to indicate alteration, increase dramatically at a cumulative dose of 1.6 × 10 16 (alpha-decay events/mg). This is the dose at which the first percolation point occurs, as amorphous domains overlap and form an interconnected network. In the altered zones, nanocrystallites of zircon are present with a random orientation, and the amorphous matrix contains the impurity elements. Although the Ce-concentration is extremely high, 1–2 wt.%, the Ce anomaly, Ce/Ce⁎, does not vary significantly as a function of dose or degree of alteration, indicating that the LREE patterns were overprinted by the fluids with a similar Ce-anomaly. The valence of Ce analyzed by EELS (electron energy loss spectroscopy) is tetravalent in the altered zone, suggesting that the altering fluids were oxidizing.

The oxidative alteration of uraninite and the fate of trace elements (Y, LREE, Zr, and Th) in a granite-hosted uranium ore deposit in north Guangdong province, China, were investigated to understand the geochemical behavior of spent UO2fuel and associated fission products and transuranium elements under oxidizing conditions. In light of the paragenetic relationship of the alteration products, two alteration series of uraninite were identified: one is the silicate series with a mineral paragenesis of uraninite → uranyl oxide hydrates → Si-rich uranyl phase → uranophane (Ca[(UO2)(SiO3OH)]2(H2O)5), and the other is the phosphate series with a mineral paragenesis of uraninite → uranyl oxide hydrates → autunite (Ca[(UO2)(PO4)]2(H2O)6) → yingjiangite ((K2,Ca)(UO2)7(PO4)4(OH)6(H2O)6). In contrast to the wide distribution and abundance of uranophane and the uranyl phosphate minerals, uranyl oxides were only occasionally found in the ore samples, suggesting that the uranyl silicates and phosphates should be the predominant alteration products of UO2under oxidizing conditions, although uranyl oxide hydrates would be the solubility-limiting phase of uranium in the very early stage of alteration. Furthermore, the interlayer cation of the uranyl phases in the Xiazhuang uranium ore field is dominated by Ca2+, indicating that the release of uranium and other radionuclides will be limited mainly by uranophane and autunite during the oxidative alteration of spent UO2fuel in underground repositories where enhanced calcium concentration is expected due to cement/water reactions.Compositionally, the cation atomic ratios in uranyl phases often deviate considerably from their respective stoichiometric values as indicated by the nominal formulae, but the compositional variation does not result in significant structural change as indicated by X-ray diffraction patterns. This observation indicates that the structure of U6+minerals may easily be adjusted to accommodate impurity elements including crystallographically compatible radionuclides. Compared with the primary uraninites, the secondary minerals are slightly enriched in light REE, but significantly depleted in Y3+probably due to its cation-radius mismatch with interlayer Ca2+. The apparent enrichment of Zr4+in uranophane and uranyl phosphates relative to uraninite may result from the coupled substitutions: Zr4+↔ U6+and REE3+↔ Ca2+(K+). Because an adequate charge-balance mechanism and significant distortion of the coordination polyhedra are required for the substitution An4+↔ U6+, this type of substitution may not be common.

The migration and dispersion of radioactive Cs (mainly (134)Cs and (137)Cs) are of critical concern in the area surrounding the Fukushima Daiichi Nuclear Power Plant (FDNPP). Considerable uncertainty remains in understanding the properties and dynamics of radioactive Cs transport by surface water, particularly during rainfall-induced flood events to the ocean. Physical and chemical properties of unique estuary sediments, collected from the Kuma River, 4.0km south of the FDNPP, were quantified in this study. These were deposited after storm events and now occur as dried platy sediments on beach sand. The platy sediments exhibit median particle sizes ranging from 28 to 32μm. There is increasing radioactivity towards the bottom of the layers deposited; approximately 28 and 38Bqg(-1) in the upper and lower layers, respectively. The difference in the radioactivity is attributed to a larger number of particles associated with radioactive Cs in the lower part of the section, suggesting that radioactive Cs in the suspended soils transported by surface water has decreased over time. Sequential chemical extractions showed that ~90% of (137)Cs was strongly bound to the residual fraction in the estuary samples, whereas 60~80% of (137)Cs was bound to clays in the six paddy soils. This high concentration in the residual fraction facilitates ease of transport of clay and silt size particles through the river system. Estuary sediments consist of particles

Radioactive Cs isotopes (137Cs, t1/2 = 30.07 years and 134Cs, t1/2 = 2.062 years) occur in severely contaminated soils within a few kilometer of the Fukushima Daiichi nuclear power plant at concentrations that range from 4 × 105 to 5 × 107 Bq/kg. In order to understand the mobility of Cs in these soils, both bulk and submicron-sized particles elutriated from four surface soils have been investigated using a variety of analytical techniques, including powder X-ray diffraction analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and analysis of the amount of radioactivity in sequential chemical extractions. Major minerals in bulk soil samples were quartz, feldspar, and minor clays. The submicron-sized particles elutriated from the same soil consist mainly of mica, vermiculite, and smectite and occasional gibbsite. Autoradiography in conjunction with SEM analysis confirmed the association of radioactive Cs mainly with the submicron-sized particles. Up to ~3 MBq/kg of 137Cs are associated with the colloidal size fraction (98% of Cs within top ~5 cm of the soil. These results suggest that the mobility of the aggregates of submicron-sized sheet aluminosilicate in the surface environment is a key factor controlling the current Cs migration in Fukushima.

Coffinite, USiO4, is the second most abundant U4+ mineral on Earth, and its formation by the alteration of the UO2 in spent nuclear fuel in a geologic repository may control the release of radionuclides to the environment. Despite its abundance in nature, the synthesis and characterization of coffinite have eluded researchers for decades. On the basis of the recent synthesis of USiO4, we can now define the experimental conditions under which coffinite is most efficiently formed. Optimal formation conditions are defined for four parameters: pH, T, heating time, and U/Si molar ratio. The adjustment of pH between 10 and 12 leads probably to the formation of a uranium(IV) hydroxo-silicate complex that acts as a precursor of uranium(IV) silicate colloids and then of coffinite. Moreover, in this pH range, the largest yield of coffinite formation (as compared with those of the two competing byproduct phases, nanometer-scale UO2 and amorphous SiO2) is obtained for 250 °C, 7 days, and 100% excess silica.

The focus of the research was on the application of high-pressure/high-temperature techniques, together with intense energetic ion beams, to the study of the behavior of simple oxide systems (e.g., SiO2, GeO2, CeO2, TiO2, HfO2, SnO2, ZnO and ZrO2) under extreme conditions. These simple stoichiometries provide unique model systems for the analysis of structural responses to pressure up to and above 1 Mbar, temperatures of up to several thousands of kelvin, and the extreme energy density generated by energetic heavy ions (tens of keV/atom). The investigations included systematic studies of radiation- and pressure-induced amorphization of high P-T polymorphs. By studying the response of simple stoichiometries that have multiple structural “outcomes”, we have established the basic knowledge required for the prediction of the response of more complex structures to extreme conditions. We especially focused on the amorphous state and characterized the different non-crystalline structure-types that result from the interplay of radiation and pressure. For such experiments, we made use of recent technological developments, such as the perforated diamond-anvil cell and in situ investigation using synchrotron x-ray sources. We have been particularly interested in using extreme pressures to alter the electronic structure of a solid prior to irradiation. We expected that the effectsmore » of modified band structure would be evident in the track structure and morphology, information which is much needed to describe theoretically the fundamental physics of track-formation. Finally, we investigated the behavior of different simple-oxide, composite nanomaterials (e.g., uncoated nanoparticles vs. core/shell systems) under coupled, extreme conditions. This provided insight into surface and boundary effects on phase stability under extreme conditions.« less

Coffinite, USiO4, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.

To design reliable and safe geological repositories it is critical to understand how the characteristics of spent nuclear fuel evolve with time, and how this affects the storage environment.

Coffinite, USiO4, is the second most abundant U(4+) mineral on Earth, and its formation by the alteration of the UO2 in spent nuclear fuel in a geologic repository may control the release of radionuclides to the environment. Despite its abundance in nature, the synthesis and characterization of coffinite have eluded researchers for decades. On the basis of the recent synthesis of USiO4, we can now define the experimental conditions under which coffinite is most efficiently formed. Optimal formation conditions are defined for four parameters: pH, T, heating time, and U/Si molar ratio. The adjustment of pH between 10 and 12 leads probably to the formation of a uranium(IV) hydroxo-silicate complex that acts as a precursor of uranium(IV) silicate colloids and then of coffinite. Moreover, in this pH range, the largest yield of coffinite formation (as compared with those of the two competing byproduct phases, nanometer-scale UO2 and amorphous SiO2) is obtained for 250 °C, 7 days, and 100% excess silica.

International audience; Gd2Ti x Zr2−x O7 (x = 0 to 2) pyrochlore was irradiated by 30 MeV C60 clusters, which provide an extremely high ionizing energy density. High-resolution transmission electron microscopy revealed a complex ion-track structure in Gd2Ti2O7 and Gd2TiZrO7, consisting of an amorphous core and a shell of a disordered, defect-fluorite structure. As compared with the irradiation by 1.5 GeV U ions with the highest energy loss, the track structure is consistent with tracks created by monoatomic swift heavy ions, but the diameters (with the entire diameter of 17 nm for Gd2Ti2O7 and 15 nm for Gd2TiZrO7) are significantly larger due to the much smaller velocity and higher energy density of the C60 ions. Ion tracks created by monoatomic ions are challenging to describe by HRTEM, as the boundary between disordered fluorite and pyrochlore matrix is less distinct. However, the C60 irradiation shows a clearly resolved ion track with completely crystalline, disordered, defect-fluorite structure around an amorphous core. Based on the distinct boundaries of the track morphology, inelastic thermal-spike calculations were used to describe the track size and extract critical energy densities for the interpretation of the complex core–shell morphologies for the different pyrochlore compositions.