bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modelling the Oxidative Dissolution of UO2
Authors: King Fraser , Kolar Miroslav , Shoesmith David W. 
Published: April 2011

AbstractAn electrochemically based model for predicting the effects of α-radiolysis, the precipitation of U(VI) corrosion products and redox processes with Fe and Fe(II) on the dissolution of UO2 is described. Various aspects of the model are presented, including: the underlying mechanism, the reaction-diffusion equations used to describe the mass transport and homogeneous reactions of the various species considered in the model, the geometrical grid used to simulate both experimental and used fuel/container geometries and the electrochemical boundary conditions used for the numerical solution of the reaction-diffusion equations. The results of preliminary simulations are also discussed.
Zirconia – A Ceramic for Excess weapons Plutonium Wastes
Authors: Gong W. L. , Lutze W. , Ewing R. C. 
Published: April 2011

AbstractWe synthesized a ceramic containing simulated excess weapons plutonium waste in solidsolution with zirconia (ZrO2)ss. ZrO2 has a large solubility for other metal oxides. More thantwenty binary systems AxOy- ZrO2 have been reported in the literature, including PuG2, rare earth elements, and oxides of metals contained in weapons plutonium wastes. We show that significant amounts of gadolinium (neutron absorber) and yttrium (stabilizer of the cubic modification) can be dissolved in ZrO2, together with plutonium (simulated by Th4+, Ce4+, or U4+) and impurities (e.g., Ca Mg, Fe, Si). Sol-gel and powder methods were applied to make homogeneous, single phase zirconia solid solutions. Pu waste impurities were completely dissolved in the solid solutions. In contrast to other phases, e.g., zirconolite and pyrochlore, yttrium stabilized cubic zirconia does not undergo amorphization upon irradiation.
A Consistance Model for Anion Exclusion and Surface Diffusion
Authors: Lehikoinent J. , Muurinen A. , Valkiainen M. 
Published: April 2011

AbstractA decomposition of the diffusion flux equation for an electrostatically bound and mobile hydrated ion into two components is proposed. The first component includes the effects arising solely from the irregular pore shape and increase in solvent viscosity in the proximity of negatively charged pore walls. Apart from these effects, the second flux component includes an additional contribution from an increased (decreased) concentration for cations (anions) close to the pore walls. Defining the distribution coefficient, Kd, in a fashion that allows negative values for co-ions readily accounts for their exclusion without the need to introduce somewhat artificial quantities like the “effective co-ion porosity”. In this study, it is thus possible to retain the purely volumetric meaning of the porosity and to maintain consistency throughout the conceptualization for anions, cations and electrically neutral species. Furthermore, the decomposition of the flux equation provides support for surface diffusion, a subject of great controversy and lively debate in the literature. In this connection, the role of concentration to regulate the diffusive flux for ions in relation to neutral species is emphasized. Implications for the theoretical apparent and effective diffusivities in compacted montmorillonite clay are also discussed and a modified form of the macroscopic theory is proposed.
New Crystalline Silicotitanate (CST) Waste Forms: Hydrothermal Synthesis and Characterization of CS-SI-TI-O Phases
Authors: Nyman M. , Nenoff T. M. , Su Y. , Balmer M. L. , Navrotsky A. , Xu H. 
Published: April 2011

AbstractThe radioactivity of the Hanford site waste tanks is primarily from 137Cs and 90Sr, of which can both be selectively removed from solution using a crystalline silicotitanate (CST) ion exchanger. We are currently seeking waste forms alternative to borosilicate glass for Cs-CSTs. In order to obtain a fundamental basis for the development of an alternative waste form, we are investigating synthesis and characterization of CST component phases, namely Cs-Si-Ti-O phases. Two novel Cs-Ti-Si-O phases (one porous, one condensed) have been hydrothermally synthesized, characterized and evaluated as waste form candidates based on chemical and thermal stability, leachability, and ion exchange capabilities.
Deposition Behavior of Latex Particles In Filtration Process Through Glass Packed Column
Authors: Chinju H. , Nagasaki S. , Tanaka S. , Tanaka T. , Ogawa H. 
Published: April 2011

AbstractThe deposition behavior of colloids from a flowing suspension onto a solid collector surface infiltration process was observed by conducting column experiments with polystyrene latex particlesand glass beads packed columns. From the observed particle breakthrough curves, single collectorefficiencies were calculated. By comparing the single collector efficiency obtained in the columnexperiment with the existing approximate expression, the effects of flow velocity on the depositionbehavior was discussed.
Evaluation of thermally converted silicotitanate waste forms
Authors: SU Y. , Balmer M. L. , Wang L. , Bunker B. C. , Nyman M. , Nenoff T. , Navrotsky A. 
Published: April 2011

AbstractCrystalline silicotitanate ion exchangers are highly selective for separating Cs from Narich waste streams. However, use of these ion exchangers for removal of Cs from radioactive tank waste will result in large volumes of secondary wastes. Thermal conversion of silicotitanates produces a durable waste form with reduced volumes up to 40%. Leach tests (MCC-l and PCT) have shown that Cs leach rates of IE-91 1-Na (heat treated at 900°C for an hour) are extremely low, ranging from 0.1 to lwt% Cs loss in Cs fraction release, or 10-1 to 10-8g/m2day in normalized Cs mass loss. These are several orders of magnitude lower than that of borosilicate glass. In order to understand the interplay between the structure and high Cs durability, X-ray diffraction, 133Cs NMR, and thermogravimetric analysis have been used to identify phase(s) responsible for trapping Cs in these silicotitanates. Results indicate that Cs is likely to be contained in a crystalline silicate phase.
Study of Natural Zircon-Xenotime Assemblages for Estimation of the Actinide Waste Forms Stability
Authors: Lapina M. I. , Yudintsev S. V. 
Published: April 2011

AbstractIn order to estimate compositions and structural stability of YPO4-ZrSiO4-USiO4- ThSiO4 solid solution, natural specimens of (U,Th)-enriched zircon-xenotime assemblages were studied. The samples were picked from granitic pegmatites of North Karelia of about 1,850 Ma age. TEM data show that amorphization of the zircon structure is not complete. Xenotime has typical crystal structure. ASEM and EPMA results indicate that zircon and xenotime are micro-heterogeneous in there compositions. Numerous micro-inclusions (<5 μm in diameter) of xenotime and uranium oxide were found within the zircon grains. Xenotime is free from inclusions. The results give rise to the conclusions that xenotime-zircon solid solution is restricted due to high difference in ionic radii (y3+ vs. Zr4+ and p5+ vs. Si4+). Data on high amounts of rare earth elements and phosphorus in natural zircons are result from REE-phosphate inclusions in analyzed zircons. Content of U in the zircon allowed the estimation of the solubility of coffinite end member to be no less than 5 mol.% "USiO4".
Vegetation-Derived Insights on the Mobilization and Potential:Transport of Radionuclides from the Nopal I Natural Analog Site, Mexico
Authors: Leslie Bret W. , Pickett David A. , Pearcy English C. 
Published: April 2011

AbstractThe Nopal I uranium (U) deposit, Peñia Blanca, Mexico is a source term and contaminant transport natural analog to the proposed high-level nuclear waste repository at Yucca Mountain, Nevada. In an attempt to characterize the mobilization and potential transport of adionuclides in the unsaturated zone at the Nopal I deposit, vegetation growing on ore piles was analyzed for238U,235U, and232Th decay-series isotopes. Specimens ofPhacelia robustagrowing on highgrade piles of U ore were collected and analyzed by alpha autoradiography, and by alpha and gamma spectrometry. Activities for U, thorium (Th), and radium (Ra) isotopes (Bq/kg dried plant) were 300, 1000, and 7000 for238U,230Th, and226Ra, respectively. The226Ra activities in these specimens are among the highest ever measured for plants; furthermore, the plant-to-soil226Ra concentration ratio is higher than expected. These results demonstrate the large mobility and bio-availability of Ra in the Nopal I environment, and support previous indications of recent loss of226Ra from the ore body. Comparison between the activities of238U and232Th decay-chain Th isotopes in the plants and in the ore substrate indicate that relative mobilization into pore solutions of228Th >230Th >232Th, in a ratio of about 50 – 25:4:1, respectively. The similarity of the plant's234U/238U activity ratio (˜1.2) to that of a caliche deposit that formed adjacent to the Nopal ore body around 54 ka suggests the234U/238U activity ratio of U released from the ore is approximately 1.2. The U and236Ra isotope activities of the plants and ore substrate, and solubility considerations, are used to assess a source term model of the potential Yucca Mountain repository. These results suggest the use of a natural analog source term model in performance assessments may be non-conservative.
Development of Integrated Mechanistically-Based Degradation-Mode Models for Performance Assessment of High-Level Waste Containers
Authors: Farmer J. C. , Mccright R. D. , Estill J. C. , Gordon S. R. 
Published: April 2011

AbstractAlloy 22 [UNS N06022] is now being considered for construction of high level waste containers to be emplaced at Yucca Mountain and elsewhere. In essence, this alloy is 20.0–22.5% Cr, 12.5–14.5% Mo, 2.0–6.0% Fe, 2.5–3.5% W, with the balance being Ni. Other impurity elements include P, Si, S, Mn, Co and V. Cobalt may be present at a maximum concentration of 2.5%. Detailed mechanistic models have been developed to account for the corrosion of Alloy 22 surfaces in crevices that will inevitably form. Such occluded areas experience substantial decreases in pH, with corresponding elevations in chloride concentration. Experimental work has been undertaken to validate the crevice corrosion model, including parallel studies with 304 stainless steel.
The Stress Corrosion Cracking of Copper:Nuclear Waste Containers
Authors: King F. , Litke C. D. , Ikeda B. M. 
Published: April 2011

AbstractThe extent of stress corrosion cracking (SCC) of copper nuclear waste containers is being predicted on the basis of a “limited propagation” argument. In this argument, it is accepted that crack initiation may occur, but it is argued that the environmental conditions and material properties required for a through-wall crack to propagate will not be present.In this paper, the effect of one environmental parameter, the supply of oxidant (Jox), on the crack growth rate is examined. Experiments have been conducted on two grades of Cu in NaNO2 environments using two loading techniques. The supply of oxidant has been varied either electrochemically in bulk solution using different applied current densities or by embedding the loaded test specimens in compacted buffer material containing O2 as the oxidant. Measured and theoretical crack growth rates as a function of Jox are compared with the predicted oxidant flux to the containers in a disposal vault and an estimate of the maximum crack depth on a container obtained.
Phase Stability and Mechanical Properties of C-22 Alloy Aged in the Temperature Range 590 to 760°C for 16,000 Hours
Authors: Summers Tammy S. Edgecumbe , Wall Mark A. , Kumar Mukul , Matthews Steven J. , Rebak Raú B. 
Published: April 2011

AbstractThe phase stability of C-22 alloy (UNS #N06022) was studied by aging samples at 593, 649, 704 and 760°C for 2000 h (2.7 mo) and 16,000 h (1.8 yr). The tensile properties and the Charpy impact toughness of these samples were measured in the mill annealed condition as well as after aging. The microstructures of samples aged 16,000 hours were examined using scanning and transmission electron microscopy (SEM and TEM). Preliminary TEM results suggest that μse forms at all temperatures investigated. Discrete carbide particles in addition to a film with very uniform thickness which appears to be μ phase formed on grain boundaries in the sample aged at 593°C. The ordered Ni2(Cr, Mo) phase was also seen in this sample. At the higher aging temperatures, mainly μ phase forms covering all the grain boundaries and also distributed throughout the bulk. Although strength increased somewhat with aging. the ductility decreased due to the formation of these grain boundary precipitates and brittle intermetallics.
The Role of H in the Growth Mechanism of PECVD a-Si:H
Authors: Sanden M.C.M. Van de , Kessels W.M.M. , Smets A.H.M. , Korevaar B.A. , Severens R.J. , Schram D.C. 
Published: April 2011

AbstractThis paper describes an extension of the silyl radical based kinetic growth model by atomic hydrogen induced surface hydrogen abstraction processes. It is shown that by including this direct abstraction process several problems of the SiH 3 based model are resolved. The defect density can be predicted with the proper temperature dependence and order of magnitude. The implications for high rate deposition of a-Si:H are discussed
High Quality a-Si:H Films Grown at High Deposition Rates
Authors: Lubianiker Yoram , Tan Yanyang , Cohen J. David , Ganguly Gautam 
Published: April 2011

AbstractIntrinsic a-Si:H samples were grown with and without hydrogen (H2) dilution of silane at different growth rates. We find that the dilution leads to a considerable reduction in the defect density, in particular at high growth rates. The defect density is particularly low for samples grown using H2 dilution conditions at growth rates as high as 10 Å/sec. Using transient photocapacitance measurements we find evidence for a small concentration of microcrystallites embedded in the amorphous films. An increase in the microcrystalline fraction correlates with a decrease in the defect density.
Very Wide-Gap and Device-Quality a-Si:H from Highly H2 Diluted SiH4 Plasma Decomposed by High RF Power
Authors: Terada N. , Yata S. , Terakawa A. , Okamoto S. , Wakisaka K. , Kiyama S. 
Published: April 2011

AbstractThe H2 dilution technique at a high deposition rate (RD) was investigated by depositing hydrogenated amorphous silicon (a-Si:H) under a high if power density of 750 mW/cm2, which is 20 times as large as that of conventional conditions. It was found that the H2 dilution ratio γ ( = [H2 gas flow rate] / [SiH4 gas flow rate]) tendency of the film properties, such as the H content (CH), optical gap (Eopt), SiH2/SiH and photoconductivity (σph) of a-Si:H is different for the high rf power (750 mW/cm2) and the medium rf power (75 mW/cm2) conditions. Under medium rf power, the CH, Eopt and SiH2/SiH decrease as γ increases. Under the high if power, on the contrary, the CH and Eopt, monotonously increase while maintaining a low SiH2/SiH and a high σph of 10-6 S/cm as γ increases. These results suggest that increasing the rf power enhances the H incorporation reactions due to H2 dilution. It is thought that a high rf power causes the depletion of SiH4 and hence the extinction of H radicals, expressed by SiH4 + H* → SiH3* + H2, is suppressed. A high H radical density enhances the incorporation of H into a-Si:H, resulting in very wide-gap a-Si:H with a high CH, Consequently, very wide-gap a-Si:H with device-quality (Eopt of 1.82 eV with an (αhv)1/3 plot, corresponding to > 2.1 eV with Tauc's plot, and σph of 10-6 S/cm) can be obtained at a high RD of 12 Å/s without carbon alloying.
Electrochemical Tailoring and Optical Investigation of Advanced Refractive Index Profiles in Porous Silicon Layers
Authors: Zangooie S. , Jansson R. , Arwin H. 
Published: April 2011

AbstractPorosity depth profiles with exponential or sinusoidal shape were fabricated electrochemically in crystalline silicon using time-variable current densities and studied employing variable angle spectroscopic ellipsometry. Since volume porosity in porous silicon depends on the current density, it was possible to electrochemically tailor porosity depth profiles, which in a first approximation resembled the time modulation of the applied current. Optical characterization of the samples were realized using multilayer optical models and the Bruggeman effective medium approximation allowing variations of the index of refraction according to the applied current density profiles. The analysis also revealed deviations from desired profiles in terms of in-depth inhomogeneities.
Theory of Hydrogen Interactions with Amorphous Silicon
Authors: Walle Chris G. Van De , Tuttle Blair 
Published: April 2011

AbstractWe present an overview of recent results for hydrogen interactions with amorphous silicon (a-Si), based on first- principles calculations. We review the current understanding regarding molecular hydrogen, and show that H2 molecules are far less inert than previously assumed. We then discuss results for motion of hydrogen through the material, as relating to diffusion and defect formation. We present a microscopic mechanism for hydrogen-hydrogen exchange, and examine the metastable ≠ SiH2 complex formed during the exchange process. We also discuss the enhanced stability of Si-D compared to Si-H bonds, which may provide a means of suppressing light-induced defect generation.
Intrinsic, n- and p-Doped a-Si:H Thin Films Grown by DC Magnetron Sputtering with Doped Targets
Authors: Johansson Å.A. , Jårrendahl K. , Birch J. , Hjörvarsson B. , Arwin H. 
Published: April 2011

AbstractIntrinsic, n- and p-type a-Si:H films were deposited by dc magnetron sputtering and analyzed with several techniques. The films were synthesized in a reactive Ar-H2 atmosphere giving H contents in the range of 3-20 at %. The films were sputtered from pure silicon targets and doped silicon targets with 1 at % B or P. Doping by co-sputtering from composite Si/B4C targets was also explored. The doping concentrations were 3 × 1020 − 2 × 1021 cm-3 for the p-type films and 2.6-2.9 × 1019 cm-3 for the n-type films. The conductivity was in the range 10-2−10-4 Ω-1 cm-1 for p-doped films and 10-5 Ω-1 cm-1 for the best n-doped films. Band gap estimations were obtained from dielectric function data and showed an increase with hydrogen content. A comparison to device quality PECVD-samples was also made.
Photoinduced Expansion in Hydrogenated Amorphous Silicon
Authors: Nonomura S. , Gotoh T. , Nishio M. , Sakamoto T. , Kondo M , Matsuda A. , Nitta S. 
Published: April 2011

AbstractThe structural aspect of photodegradation effect in hydrogenated amorphous silicon has been investigated by the use of the simple and sensitive detection technique, the laser optical-lever bending method, for a small expansion or extraction in thin films. The volume change induced by the thermal expansion due to the photothermal effect and the residual expansion was observed in hydrogenated amorphous silicon prepared by PECVD. The latter residual expansion was persistent after light soaking and was recovered by thermal annealing at 200°C.The time dependence of the volume expansion with light soaking shows the same time dependence of photoinduced defect density. The photoinduced volume changes normalized by the initial volume are the order of 10-5~10-5, which values are two orders smaller than chalcogenide glasses such as a-As2S3. The normalized volume change of a-Si:H with the different sample preparation conditions of PECVD such as the hydrogen dilution ratio r (r = SiH4/H2) and substrate temperature is shown. Also it is demonstrated that the photoinduced expansion is observed in hydrogenated amorphous silicon prepared by photo CVD and hot-wire CVD methods. The spatial extent related to a photoinduced defect creation in a-Si:H is estimated.
Slow Degradation of Hydrogenated Amorphous Silicon Photoconductivity Under Pulsed Illumination
Authors: Heck Stephan , Branz Howard M. 
Published: April 2011

AbstractWe degraded hydrogenated amorphous silicon (a-Si:H) using red light pulses of 32 microseconds to 2 milliseconds. These metastable photoconductivity degradations were compared to degradation with continuous light of the same intensity and same exposure time. For a given integrated exposure time we observe higher degraded photoconductivities (by up to 40 %) as we shorten the pulses. For example, to obtain the same amount of degradation with 120 microsecond pulses as with continuous illumination, the integrated sample exposure time must be doubled. Experiments were conducted to exclude thermal effects. Our result cannot be explained with a simple recombination-driven degradation mechanism, because electron and hole populations rise and fall to steady-state values in only a few microseconds. We conclude that carrier recombination creates a more proximate precursor to metastable degradation of a-Si:H. When illumination begins, this precursor's density rises more slowly than the density of photocarriers and therefore degradation is delayed.
Light-Induced Increase in Two-Level Tunneling States in Hydrogenated Amorphous Silicon
Authors: Liu Xiao , Pohl R.O. , Crandall R.S. 
Published: April 2011

AbstractWe observe an increase of the low-temperature internal friction of hydrogenated amorphous silicon prepared by both hot-wire and plasma-enhanced chemical-vapor deposition after extended light-soaking at room temperature. This increase, and the associated change in sound velocity, can be explained by an increase of the density of two-level tunneling states, which serves as a measure of the lattice disorder. The amount of increase in internal friction is remarkably similar in both types of films although the amount and the microstructure of hydrogen are very different. Experiments conducted on a sample prepared by hot-wire chemical-vapor deposition show that this change anneals out gradually at room temperature in about 70 days. Possible relation of the light-induced changes in the low-temperature elastic properties to the Staebler-Wronski effect is discussed.
A Comparison of the Degradation and Annealing Kinetics in Amorphous Silicon and Amorphous Silicon-Germaniumsolar Cells
Authors: Carlson D. E. , Chen L. F. , Ganguly G. , Lin G. , Middya A. R. , Crandall R. S. , Reedy R. 
Published: April 2011

AbstractThe degradation and annealing kinetics of both a-Si:H and a-SiGe:H single-junction solar cells were investigated under varying conditions. In every case, the kinetics associated with degradation and annealing were slower for a-SiGe:H cells than for a-Si:H cells. Since deuterium diffusion studies indicate that the hydrogen in our a-SiGe:H films diffuses more slowly than that in the a-Si:H films, hydrogen motion may play a role in determining both the degradation and annealing kinetics of the devices.
A Molecular Dynamics Study of Band Tails in a-Si:H
Authors: Fedders Peter A. , Drabold D. A. 
Published: April 2011

AbstractBand tail states are routinely invoked in models of a-Si:H including defect pool models and models of light induced defects. These models describe the band tail states as being localized on a single stretched bond. However to our knowledge, there is no theoretical or experimental work to justify these assumptions. In this work we use ab initio calculations to support earlier tight-binding calculations that show that the band tail states are very delocalized ---- involving large numbers of atoms as the energy is varied from miidgap into the tails. Our work also shows that the valence band tail states are statistically associated with short bonds (not long bonds) and the conduction-band states with long bonds. We have slightly modified a 512 atom model of a-Si due to Djordjevic et al. to produce a large, defect free model of a-Si:H with realistic band tails.