bluuebunny/crossref_metadata_2025_split huggingface dataset:
Am 1 Semiempirical Mo Calculations of Torsional Barriers and Structural Reorganization in Radical Ions of Pbo and Pbzt Model Structures
Authors: Connolly John W. , Dudis Douglas S. 
Published: March 2011

ABSTRACTAM 1 semiempirical calculations on model structures of neutral, anionic and cationic poly(p-phenylenebenzobisthiazole), PBZT, offer an explanation why the PBZT anion has been shown to be an electrical conductor while the PBZT cation is not. To wit, the highest occupied molecular orbital (HOMO) in the PBZT model structure is centered primarily on the sulfur atoms, resulting in concentration of the positive charge on the sulfur atom when the cation is formed. The lowest occupied molecular orbital (LUMO) is far more delocalized and consequently anion formation produces a species which has electronic charge delocalized over the entire model structure. Similar calculations on model structures of neutral, anionic and cationic poly(p-phenyienebenzobisoxazole), PBO, indicate that both frontier orbitals, i. e., the HOMO and the LUMO are delocalized.
Nonlinear Molecular Dynamics and Monte Carlo Simulations of Crystals at Constant Temperature and Tensorial Pressure
Authors: Lill J.V. 
Published: March 2011

ABSTRACTComplimentary molecular dynamics and Metropolis Monte Carlo algorithms for the atomistic simulation of crystals at constant temperature and homogeneous tensorial pressure are summarized. The novel aspect of computational physics which unites these methods is the extension of the virial theorem to nonlinear elastic media. This guarantees the dynamical balance between the internal pressure, as determined by the interatomic potential, and effective external pressure, as determined by the applied laboratory pressure, and includes the elastic response of the material. Numerical examples are presented.
Computational Diagnostics for Detecting Phase Transitions During Nanoindentation
Authors: Lee Susanne M. , Hoover Carol G. , Kallman Jeffrey S. , Hoover William G. , De Groot Anthony J. , Wooten Frederick 
Published: March 2011

ABSTRACTWe study nanoindentation of silicon using nonequilibrium molecular dynamics simulations with up to a million particles. Both crystalline and amorphous silicon samples are considered. We use computational diffraction patterns as a diagnostic tool for detecting phase transitions resulting from structural changes. Simulations of crystalline samples show a transition to the amorphous phase in a region a few atomic layers thick surrounding the lateral faces of the indentor, as has been suggested by experimental results. Our simulation results provide estimates for the yield strength (nanohardness) of silicon for a range of temperatures.
Generalized Melting Criterion for Amorphization
Authors: Devanathan R. , Lam N. Q. , Okamoto P. R. , Meshii M. 
Published: March 2011

ABSTRACTWe present a thermodynamic model of solid-state amorphization based on a generalization of the well-known Lindemann criterion. The original Lindemann criterion proposes that melting occurs when the root-mean-square amplitude of thermal displacement exceeds a critical value. This criterion can be generalized to include solid-state amorphization by taking into account the static displacements. In an effort to verify the generalized melting criterion, we have performed molecular dynamics simulations of radiation-induced amorphization in NiZr, NiZr2, NiTi and FeTi using embedded-atom potentials. The average shear elastic constant G was calculated as a function of the total mean-square atomic displacement following random atom-exchanges and introduction of Frenkel pairs. Our results provide strong support for the generalized melting criterion.
The Si Vacancy: An Example of a Pressure-Sensitive Jahn-Teller System
Authors: Tarnow Eugen 
Published: March 2011

ABSTRACTWe calculate the trends in the Jahn-Teller (JT) parameters of the Si vacancy as a function of the breathing mode coordinate. Using ab-initio local-density-functional theory and Hellman--Feynman forces we find the couplings between the JT modes and the breathing mode to be significant. Since the breathing mode coordinate is very sensitive to pressure we predict a pressure-sensitive JT contribution to the effective electronic correlation energy U. Furthermore we find that the tetragonal JT energy increasingly dominates over the trigonal energy as the breathing coordinate decreases due to a softening of the tetragonal spring constant.
Incorporation of Radionuclides into Mineral Phases via a Thermally Unstable Complexant Ligand
Authors: Apblett Allen W. , Georgieva Galina D. , Mague Joel T. 
Published: March 2011

ABSTRACTThe complexation and thermal behavior of pyruvic acid oxime (PAO) with representative cations associated with nuclear reprocessing waste has been investigated. This ligand has the dual advantage that it precipitates most of the cations from solution and these complexes decompose at ca. 150°C to CO2, acetonitrile, and, initially, the metal hydroxide. Introduction of the appropriate elements required for Synroc phases to a simulated waste stream followed by NaPAO and disodium fumarate leads to precipitation of a metalorganic Synroc precursor in which the components have been intimately mixed at the molecular level. This approach circumvents incorporation of alkali metals in the final ceramic.
The Leaching of Pu, Am, Np and Tc from High-Level Waste Glasses in Clay Media
Authors: Lemmens K. , Van iseghem P. , Wang L. 
Published: March 2011

ABSTRACTThe leaching behaviour of Pu, Am, Np and Tc from the high-level waste glasses of interest to Belgium has been studied. Corrosion tests were carried out on inactive glasses (the Cogéma R7T7 glass SON68 and the DWK Pamela glasses SM513 and SM527), doped with the radionuclides. Nuclide leaching was studied in contact with a slurry of Boom clay, diluted with claywater, at 90°C and 40°C, for durations of up to 4 years. The retention of Pu and Np in the glass surface layers was considerable for SON68 and SM513, but small for SM527. The retention of these elements decreased at lower temperature. Tc was not retained. The largest part of the leached activity is adsorbed onto the clay in an immobile form. Nuclide concentration in mobile form (smaller than 105 MWU) was found to be independent on duration for Pu and Am. Increasing concentrations were found for Np and Tc. The mobile concentrations were smaller in the presence of corrosion products.
Analytical Electron Microscopystudy of Colloids from Nuclear Waste Glass Reaction
Authors: Buck E.C. , Bates J.K. , Cunnane J.C. , Ebert W.L. , Feng X. , Wronkiewicz D.J. 
Published: March 2011

ABSTRACTAn Analytical Electron Microscopy study of colloidal particles formed during reaction of waste glass has been performed. The effect of waste glass test parameters on colloid formation is examined. Characterization of phases present in the leachate of these tests has shown that layers spalled from the glass and precipitated phases are both sources of colloids in the leachate. Elements, such as uranium, have been found to concentrate within colloidal particles in the leachate.
Effects of Relative Surface Area and Leachant Composition on the 137Cs Leach Rate from Cement Waste Forms
Authors: Peric Aleksandar D. , Plecas Ilija B. , Glodic Snezana D. 
Published: March 2011

ABSTRACTCement matrices, doped with 137Cs, were investigated to establish their physicochemical properties, mainly 137Cs leach-rate. Experiments were carried out in leachant media with different degrees of aggressiveness on the cement matrix, e.g., distilled water, Na2SO4 and NaCl solutions. A series of six cement samples, which were orthocylindrically-shaped, were treated in each of three leachant media. For each sample, the surface area exposed to leaching relative to the total surface area was increased from 1:6 to 6:6, at increments of 1/6 of the total surface area. Expected higher 137Cs leach-rates of the samples immersed in the more aggressive environment show very specific behavior and functional relations with respect to the exposed surface area. Radionuclide leach-rate coefficients did not exhibit linear behavior. Instead, leach-rate values were best fit by exponential equations to describe cement matrix behavior in the leachants. Different empirical equations were established to describe 137Cs leach-rates in the various leachants as a function of the relative surface area of the cement matrix exposed to the leachant.
The Role of Oxygen Diffusion in the Release of Technetium from Reducing Cementitious Waste Forms
Authors: Smith Robert W. , Walton John C. 
Published: March 2011

ABSTRACTCementitious materials provide an ideal geochemical environment (e.g., high pH pore fluids and large surface areas for sorption) for immobilizing nuclear waste. The inclusion of reducing agents, such as blast furnace slag (BFS) can immobilize radionuclides by forming of solid sulfide phases. Thermodynamic calculations using the MINTEQ geochemical computer code indicate that elemental sulfur present in BFS reacts with the highly mobile pertechnetate anion (TcO4−) anion to form an insoluble technetium sulfide phase (Tc2S7(8)). Initially, the waste form very effectively immobilizes technetium. However, as oxygen diffuses into the waste form, an outer zone of oxidized concrete and a shrinking core of reduced intact concrete develops. Oxidation of sulfur in the outer zone results in increased technetium concentrations in the pore fluid because Tc2S7(8) oxidizes to the mobile TcO4− anion. The TcO4− anion can then diffuse from the waste form into the environment.A mathematical model that accounts for diffusion of oxygen into concrete coupled with oxidation of sulfur and sulfide to sulfate has been developed. This model assumes the existence of an oxidized outer layer of concrete surrounding a shrinking core of reducing intact concrete. A sharp boundary between the two zones moves slowly inward resulting in oxidation of Tc2S7(8) and subsequent release of TcO4− via aqueous diffusion in the concrete pore fluids. The model indicates that this mechanism results in a linear dependance of release with the square root of time similar to pure diffusion. In addition, the release of technetium is related to the inverse of the square root of the concentration of BFS, indicating that performance will significantly increases with the addition of approximately 20 percent BFS to the cement mix.
Alkali-Activated Fly Ash-Slag Cement Based Nuclear Waste Forms
Authors: Jiang W. , Wu X. , Roy D.M. 
Published: March 2011

ABSTRACTThis paper is based on the results of an in-progress research project on Alkali-Activated Cement System at MRL. The objective of this research is to establish the potential for large volume use of fly ash and slag as main components of the cement system. Alkali-activated Fly ash-slag Cement (AFC) was studied as a matrix for immobilization of nuclear waste. AFC is characterized by high early strength, high ultimate strength, low porosity, lower solubilities of the hydrates, and high resistance to chemical corrosion as well as to freezing and thawing. All these advanced properties are particularly favorable to the immobilization the nuclear wastes.
Critical Conditions for Initiation of Localized Corrosion of Mild Steels in Contact with Bentonite Used in Geological Disposal Packages of Nuclear Waste
Authors: Nakayama Guen , Akashi Masatsune 
Published: March 2011

ABSTRACTIn the current design of geological disposal of high-level nuclear waste, the use of bentonite to stand as an artificial barrier-cum-buffer between the host rock and the packages made of mild steel is being investigated. Although mild steels commomly have been considered to be passivity in alkaline environments, under certain circumstances, they become liable to localized corrosion, e.g., pitting corrosion and crevice corrosion. Since bentonite can turn the environment alkaline to a pH of approximately 10 when it is mixed with groundwater, critical conditions for the initiation of localized corrosion of mild steel must be known to evaluate the extremely long time integrity of disposal packages serving in such an environment. This paper presents and discusses the observations and results acquired in a series of critical conditions for the initiation of localized corrosion of mild steels in various groundwater-bentonite environments at 20C, with a deaerated aqueous solution of 1 mMol/L [HCO3−] +10 ppm [CI−], simulating the natural groundwater and varying the bentonite content.
Aqueous Diffusion in Repository and Backfill Environments
Authors: Conca James L. , Apted Mick , Arthur Randy 
Published: March 2011

ABSTRACTAqueous diffusion coefficients have been experimentally determined in a variety of porous/fractured geologic and engineered media. For performance assessment applications, the purely diffusive flux must be separated from retardation effects. The simple diffusion coefficient, D, does not include any transient chemical effects, e.g., sorption, which lower the diffusion coefficient for some finite time period until equilibrium is reached. D is primarily a function of volumetric water content, θ, and not material characteristics. At high water contents, D gradually declines as water content decreases, from 10−5 cm2/sec at θ ∼ 50% to 10−7 cm2/sec at θ ∼ 5%, followed by a sharp decline to 10−10 cm2/sec at θ ∼ 0.5%. Although surface diffusion has a strong experimental basis in the transport of gases along metal surfaces, experimental evidence for aqueous geologic/backfill/engineered systems strongly indicates that surface diffusion is not important, even in bentonite, because of the extremely poor connectivity among electric double-layers and the extremely low diffusivities and high ∂C/∂x at small area/point contacts which more than negate the increased flux along intragrain surfaces.
Study on Retardation Mechanism of 3H, 99Tc, 137Cs, 237Np and 241AM in Compacted Sodium Bentonite
Authors: Sato H. , Ashida T. , Kohara Y. , Yui M. 
Published: March 2011

ABSTRACTThe apparent diffusion coefficients were measured at room temperature (about 23°C) under atmospheric condition by the one-dimensional non-steady state diffusion method for 3H, 99Tc, 137Cs, 237Np and 241Am in compacted sodium-bentonite saturated with water. Sodium-bentonite, which is commercially available as KunigelVl®, was used in this study. Experiments were carried out in the density range of 0.4–2.0 (×103kg/m3). Bentonite in the cell was prepared to be saturated with distilled water. The measured apparent diffusion coefficient decreases with increasing dry density of bentonite. That the apparent diffusion coefficient of 3H decreased as a function of dry density of bentonite appears to be the effect of the change of porous structure with dry density of bentonite. 99Tc may be retarded by anion-exclusion because dominant diffusion specie of 99Tc is pertechnetate ion under atmospheric condition. Retardation for 137Cs may be caused by ion-exchange on bentonite. The sorption, anion-exclusion and molecular filtration are considered as a retardation mechanism for 237Np and 241Am because those dominant species are negatively charged and of large ionic size.
Measurements of Grain-Boundary Inventories of 137Cs, 90Sr and 99Tc in Used Candu Fuel
Authors: Stroes-Gascoyne S. , Tait J.C. , Porth R.J. , Mcconnell J.L. , Barnsdale T.R. , Watson S. 
Published: March 2011

ABSTRACTTwo methods were used to measure grain-boundary inventories of 137Cs, 90Sr and 99Tc in used CANDU fuel, to corroborate source term estimates based on a fission gas release code. Used fuels were partially oxidized at 200°C in air to overall compositions of UO2+x (0.15≤ × ≤0.25) to expose UO2 grain boundaries, followed by leaching in aqueous solution. Only a fraction (2 to 18%) of the calculated gap + grain-boundary inventories for 37Cs was released. This suggests that the calculations overestimate Cs release or that oxidation does not expose all grain boundaries, or that Cs release from grain boundaries is slow. Release of 90Sr (0.01 to 0.7%) agreed reasonably well with the source term estimates (0.001 to 0.3%). Release of 99Tc (0.3 to 1.5%) suggests that the source term estimate for the upper boundary of 99Tc release (25%) may be too high. A second technique involved leaching of crushed and size-fractionated used fuel in either a static or dynamic system. A direct one-to-one correlation between calculated and measured gap + grain-boundary inventories for 137Cs was found for low- and medium-power fuels.
Temperature and Moisture Distributions in a Clay Buffer Material Due to Thermal Gradients
Authors: Mohamed A.M.O. , Yong R.N. , Kjartanson B. 
Published: March 2011

ABSTRACTSeveral series of one-dimensional tests were used to investigate the nature of transient heat and moisture movements in a clay buffer under different imposed temperature gradients. The measured temperature and moisture profiles were used to calculate the diffusion parameters governing heat and moisture movement in the buffer material. The diffusion parameters are shown to depend on the moisture content, temperature and moisture equilibrium time.
Radiation Damage in NaCI: Calculations with an Extended Jain-Lidiard Model
Authors: Soppe Wim J. , Prij J. 
Published: March 2011

ABSTRACTThe colloid growth due to irradiation in a rock salt formation is calculated with an extended version of the Jain-Lidiard model. The extensions of the model comprise a description of the nucleation stage of the colloids and the role of impurities on the formation of defect centers. Results of model calculations are shown for a representative design for a high-level radioactive waste repository in a rock salt formation. It is concluded that it is unlikely that, near the waste containers, the fraction of NaCl that will be converted to metallic Na and molecular Cl centers will exceed a few mole percent.
Results from Simulated Remote-Handled Transuranic Waste Experiments at the Waste Isolation Pilot Plant (WIPP)
Authors: Molecke Martin A. 
Published: March 2011

ABSTRACTMulti-year, simulated remote-handled transuranic waste (RH TRU, nonradioactive) experiments are being conducted underground in the Waste Isolation Pilot Plant (WIPP) facility. These experiments involve the near-reference (thermal and geometrical) testing of eight full-size RH TRU test containers emplaced into horizontal, unlined rock salt boreholes. Half of the test emplacements are partially filled with bentonite/silica-sand backfill material. All test containers were electrically heated at about 115 W/each for three years, then raised to about 300 W/each for the remaining time. Each test borehole was instrumented with a selection of remote-reading thermocouples, pressure gages, borehole vertical-closure gages, and vertical and horizontal borehole-diameter closure gages. Each test emplacements was also periodically opened for visual inspections of brine intrusions and any interactions with waste package materials, materials sampling, manual closure measurements, and observations of borehole changes. Effects of heat on borehole closure rates and near-field materials (metals, backfill, rock salt, and intruding brine) interactions were closely monitored as a function of time. This paper summarizes results for the first five years of in situ test operation with supporting instrumentation and laboratory data and interpretations. Some details of RH TRU waste package materials, designs, and assorted underground test observations are also discussed. Based on the results, the tested RH TRU waste packages, materials, and emplacement geometry in unlined salt boreholes appear to be quite adequate for initial WIPP repository-phase operations.
Modelling in-situ Matrix Diffusion at Palmottu Natural Analogue Study Site in SW Finland.
Authors: Rasilainen Karl , Suksi Juhani 
Published: March 2011

ABSTRACTRadioactive disequilibria, between U-238, U-234, and Th-230, in crystalline rock adjacent to a fracture, indicates mass transfer of U and Th between water in the fracture and the rock. The matrix diffusion theory was used to interpret the observed profiles of mobilized nuclides around the natural fracture. The interpretation of the profiles was based on the use of uranium series disequilibrium code, URSE, and migration code FTRANS. The model system was characterized using all available site-specific data, and the system evolution was outlined using the geology of the Palmottu site. The simulated concentration profiles, as a function of depth from the fracture surface, indicate that measured profiles can be modelled by matrix diffusion, assuming realistic initial and boundary conditions and diffusion times of 300,000 years.
Surface-Discharging Hydrothermal Systems at Yucca Mountain – Examining the Evidence
Authors: Levy SchÖn S. 
Published: March 2011

ABSTRACTExposures of altered rock that have been thought to form by recent discharge of water from depth were examined to address a concern that hydrothermal processes could compromise the isolation capability of a potential high-level nuclear waste repository at Yucca Mountain. The suspected hot-spring and hydrothermal-vent deposits are more likely the products of infiltration of meteoric water into newly deposited and still-hot pyroclastic flows >12 Myr ago.
Corrosion Model Validation in High Level Nuclear Waste Package Research
Authors: Mcneil M. B. , Moody J.B. 
Published: March 2011

ABSTRACTThe strategies for waste package (WP) performance validation will be based on site specific geologic and hydrogeochemical information plus models which can be used to predict potential WP lifetimes. The development and application of such models will include the evaluation of natural analogues (NA). These analogues are needed to resolve issues related to the validation of models. Natural analogues have not had extensive use or widespread acceptance in the area of waste package failure prediction. This lack of acceptance is due to the anticipated choice of alloys for waste package containers. Few of these alloys are similar to naturally occurring metals, and the proposed HLW repositories are in general in geologic settings not very similar to those in which naturally occurring metals are generally found.Natural and archaeological analogues can be used, however, in analysis of possible waste package failures as a means of testing proposed models for failure. In fact, the analogues are the only available mechanisms for testing models of long-term waste package behavior. A strategy is outlined for incorporating natural and archaeological analogue studies into waste package research, and examples are discussed. The natural/archaeological analogues approach which appears most promising is to use archaeological and mineral samples to develop an understanding of the identities and rates of the mineral alteration reactions at or near the surface of the package, improving present capability for estmating the lifetimes of metallic waste package containers.
Increases in Leach Rate due to Possible Cracking in Silicate Glasses
Authors: Sang J.C. , Barkatt A. , Talmy I.G. , Norr M.K. 
Published: March 2011

ABSTRACTComparative studies of two multi-component silicate glasses have confirmed the observation that glasses with a relatively low SiO2 + AlO3/2 content may exhibit temporary increases in leach rate during the initial stages of their exposure to water. SEM studies of the leached glass surfaces strongly support the assumption that this phenomenon is due to cracking of the leached glass and a consequent increase of the exposed surface area.