bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamical Structure Factor and Vibrational Normal Modes of SiO2 Glass
Authors: Jin Wei , Kalia Rajiv K. , Vashishta Priya
Published: March 2011

We study the atomic vibrational dynamics in silica glass (a-SiO2) using molecular-dynamics (MD) simulations and classical lattice dynamics method. The SiO2 glasses were generated by molecular-dynamics and steepest-descent quench (SDQ) using an effective interatomic potential consisting of two-body and three-body interactions. The frequency and eigenvectors of vibrational normal modes are obtained by diagonalization of the dynamical matrix. The partial and total vibrational density of states (DOS), bond-projected DOS, participation ratio (PR), and neutron-weighted dynamic structure factor are calculated. The results are compared with inelastic neutron scattering experiments on SiO2 glass.
Calculations of the Electronic Structure of Vacancies in a-Si:H
Authors: Valladares Ariel A. , Sansores L. Enrique
Published: March 2011

The electronic structure of random clusters has been used in the literature as representative of the electronic structure of random solids. In this work a calculation of the local density of states (LDOS) and charge density contours for clusters of the type XSi20H28 with X an Si atom, a vacancy or 4 hydrogen atoms, has been carried out. The method used was a pseudopotential SCF Hartree-Fock and the HONDO program. It is found that the generation of a vacancy in the center of the cluster (removal of the central Si atom), introduces p-like states in the energy gap of the LDOS for the region near the center of the cluster. The saturation of the dangling bonds of the vacancy with 4 hydrogen atoms removes the states within the gap. These results are also borne out by the charge density contours, thereby reinforcing the importance of amorphous cluster calculations in the understanding of the electronic structure of amorphous solids.
Monte Carlo Simulations of Surface Segregation in Cu Ni Alloys
Authors: Pasturel A. , Drchal V. , Kudrnovsky J. , Weinberger P.
Published: March 2011

A new method coupling electronic structure calculations with Monte Carlo simulations has been developed to determine surface compositions in Cu-Ni alloys. The calculations are based on an effective Ising model with parameters as defined within the framework of the Generalized Perturbation Method (GPM) and as calculated by means of the tight-binding version of the linear muffin-tin orbital method. The composition profiles are obtained for the fcc(OOl) surface for three bulk compositions, namely Cu75Ni25, Cu50Ni50, and Cu25Ni75 and compared with available experimental data.
Phase Transformations in Hexagonal-Close-Packed Alloys: Analysis with the Cluster Variation Method
Authors: Mccormack Ryan , Asta Mark , Ceder Gerbrand , Fontaine Didier de
Published: March 2011

We present a study of the hexagonal close-packed (hcp) Ising model for binary alloys within the cluster variation approximation. Groundstates of order stabilized by nearest-neighbor (NN) pair, triplet, and tetrahedron interactions were determined using the cluster configuration polyhedron method; no previous hcp ground-state study has considered all of these interactions. We predict physically realizable groundstates with stoichiometries A, AB (3 distinct structures), A2B, A3B, and A4B3. The previously unpredicted A4B3 structure is stabilized by multiatom (i.e. triplet, tetrahedron) interactions, while the others are stabilized by the two NN pair interactions. The Cluster Variation Method (CVM) was used to calculate the finite-temperature phase-equilibria for prototypical binary alloys. We present the first ordering phase diagrams computed with the CVM which contain all relevant groundstates for both isotropic and anisotropic NN pair interactions.
On the Choice of a Maximal Cluster in the Cluster Variational Method
Authors: Vul David A. , Fontaine Didier de
Published: March 2011

An explanation of the occurrence of unphysical solutions in the cluster variational method is given. A simple algorithm for the construction of an optimal maximal cluster, i.e., a cluster that guarantees correct results for any set of interatomic interactions, is suggested. Examples of optimal maximal clusters for various two- and three-dimensional lattices are presented.
Monte Carlo Simulations of Phase Transformations in Fe-Cr Alloys
Authors: Reinhard L. , Turchi P. E. A.
Published: March 2011

Phase stability properties of bcc based Fe-Cr alloys are examined in the framework of the first-principles KKR-CPA-GPM formalism and Monte Carlo simulations. For Fe-rich alloys, ordered configurations are found stable with respect to the random state of the alloy, but unstable with respect to the pure Fe and Cr metals. The results are compared with the ones obtained by using energy parameters extracted from experimental diffuse scattering data.
Pump-Probe Study of Bi1-xSbx Alloys
Authors: Vidal J. , Cheng T.K. , Fung A.W. , Zeiger H.J. , Dresselhaus G. , Dresselhaust M.S. , Ippen E.P.
Published: March 2011

Bi1-xSbx alloys undergo a semimetal-semiconductor transition at x = 0.065 and 0.22 and are semiconducting between these limits. We investigate this system by means of femtosecond time-resolved experiments, and observe an optical response similar to those in Bi, Sb, Te and Ti2O3. A previously-reported model describes the mechanism of generation of coherent lattice vibrations via the optically excited carriers in this class of materials. As predicted by the model, the modulated reflectivity signal ΔR/R in Bi.88Sb.12 shows a beating oscillatory behavior with the Bi-Bi and Bi-Sb A1g vibrational frequencies. Model-based calculations of the ΔR background relaxation time at different Sb compositions provide understanding of the carrier and lattice dynamics throughout the transition.
Direct Activity Measurements in Liquid Ag-Au=Ge System and its Solution Model Development by Computational Techniques
Authors: Yu Qiling , Howard Stanley M.
Published: March 2011

Ag activities in the Ag-Au-Ge solution were determined by the mass spectrometric analysis of effusates from a unique valved Knudsen cell at 1416 K. A mathematical model was established for Ag activity coefficient as a function of alloy composition. Activities of Au and Ge were calculated by numerical ternary Gibbs-Duhem integration on a spreadsheet. The data obtained from this investigation show that the activities on all three binary systems occurring in the ternary system are in good agreement with accepted literature values. The behavior of all pseudo binaries was found to be consistent with Darken's quadratic formalism. Darken surface of Ag is shown.
Formation of Intermetallic Phases in Metal Matrix Composites
Authors: Dorfman S. , Fuks D. , Pelleg Joshua
Published: March 2011

A theoretical study of the interaction between fibers and a matrix is performed, which is based on several approaches that are connected with the calculation of mixing energy in multicomponent alloys or with the calculation of the diffusion coefficients in such systems. As an example, interatomic potentials for the alloys (Al,Si)-B, (Al,Mg)-B, (Al,Cr)-B, (Al,Mn)-B and (Al,Ga.)-B are calculated. The values of the potential barrier Δu for binary alloys are compared with those in ternary systems. It is shown that alloying of a binary alloy by Si reduces the height of potential in comparison with the binary alloy. Other elements make this barrier higher.
A Dynamical System Theory of Large Deformations and Patterns in Non-Cohesive Solids
Authors: Evesque Pierre , Sornette Didier
Published: March 2011

We propose a dynamical system theory of triaxial-test deformationS and localization bifurcation in brittle media. We apply it to predict that localization may occur in a packing looser than “critical” and that the general localization shape is a spiral staircase in axisymmetric 3-D cells. These two facts have recently been confirmed experimentally. This theory provides a framework for understanding the development of complex deformation patterns from the mechanics of localization and rupture.
Computer Simulation of Fracture in Brittle Polycrystalline Solids: Effect of Modulus Amisotropy
Authors: Ling Shiun , Anderson Michael P.
Published: March 2011

A simulation procedure based on the spring network model has been developed for studying the fracture behavior in brittle, polycrystalline solids. In this 2-D model, the effect of crystalline symmetry is accounted for by imparting to individual bonds a constitutive relationship using the material compliance matrix. Using this model, it was found that the fracture morphology becomes more intragranular in nature with increasing modulus anisotropy. Careful analysis suggests that this is due to the wider stress distributions, and the resulting larger number of cracks generated in the interior of anisotropic grains.
Relationship Between Dilatancy, Stresses and Plastic Dissipation in a Granular Material with Rigid Grains
Authors: Evesque Pierre , Stefani Christian
Published: March 2011

By considering a drained cohesionless granular sample made up of rigid grains and submitted to a triaxial test, we derive an equation relating the dilatancy K, the deviatoric stress q and the confining pressure p to the energy losses D plasticdue to plastic yielding. We demonstrate that the system is contracting (K ≤ 0) at q=0, when q is increasing and that spontaneous uncontrolled yielding begins occurring when dilatancy K is maximum. We also demonstrate the existence of the characteristic state introduced by Luong and Habib and the existence of the critical state of Schofield and Wroth. We give at last a method to determine the plastic losses during a triaxial cell test using the experimental data.
Structure and Elastic Properties of Ni/Cu and Ni/Au Multilayers
Authors: Taiwo Ademola , Yan Hong , Kalonji Gretchen
Published: March 2011

The structure and elastic properties of Ni/Cu and Ni/Au multilayer systems are investigated as a function of the number of Ni monolayers built into the systems. We employed lattice statics simulations with the interatomic potentials described by the embedded-atom method. For the Ni/Cu systems, coherent interfaces and FCC structure are maintained, and no elastic anomaly is found. For the Ni/Au systems, when the Ni layers are thick enough, they undergo a strain-induced phase transformation from FCC to HCP structure. An enhancement of Young’s modulus of these systems is found to be associated with this structural change.
Phonons and Local Elastic Moduli in Symmetrical Tilt Boundaries
Authors: Wang Gui Jin , Vitek V. , Alber I. , Bassani J.
Published: March 2011

Lattice vibrations and atomic level elastic moduli have been studied for a bicrystal containing a fully relaxed symmetrical tilt boundary in Au. Central force many body potentials have been employed to describe atomic interactions. In the long-wavelength limit the phonons localized at grain boundaries can be identified with Stoneley waves known from continuum analyses. These waves are localized at the grain boundary and their velocity agrees well with that evaluated using the local effective elastic moduli of the interfacial region. However, the usually used continuum model assuming an ideal match across the interface is not sufficient to analyze these waves fully and an explicit description of interfacial properties need to be included into the continuum models of interfaces.
Normal Coordinate Analysis of Benitoite (BaTiSi3O9) and its Constituent Ring
Authors: Kim Charles C. , Bell M. I. , McKeown D. A.
Published: March 2011

The normal modes of vibration and their frequencies are calculated for benitoite, a mineral whose crystal structure (space group D23h) consists of three-membered silicate rings (Si3O9) linked by Ba+2 and Ti+4 ions. The calculation unambiguously assigns the normal modes to the lines in the Raman and infrared spectra and determines the atomic interactions. On the assumption that mode mixings and splittings due to inter-ring interactions are small, the normal frequencies of the isolated ring of C3h, symmetry are determined. The identification of normal modes characteristic of three-membered silicate rings may prove to be a valuable guide in the interpretation of the infrared and Raman spectra of amorphous silicates, potentially leading to new information on the ring statistics of these materials.
Effects of Surface Defects on the Local Electric Field in Inhomogeneous Media
Authors: Ferris Kim F. , Risser Steven M.
Published: March 2011

Surface defects both focus the local electric field and enhance its intensity in the vicinity of the defect. Using a finite element model of an inhomogeneous dielectric film, we have examined the relationship between the defect shape and local electric field in the film and defect. We find that raised defect regions that are peaked have the largest local fields while the slope of removed regions has little effect on the field in raised defects. Both peaked and flat surface defects can lead to large enhancements of the local electric field above that predicted by effective medium approximation (EMA) methods.
Order-Disorder Transformation in Cu3AAu: A Molecular Dynamics Study
Authors: Cleri Fabrizio , Mazzone Giorgio , Enea Vittorio Rosato
Published: March 2011

The order-disorder transformation in CU3Au has been investigated using a combined Molecular Dynamics and Cluster Variation Method approach. Free energy minimization has been performed using the Natural Iteration technique. The calculated temperature dependence of enthalpy, lattice dynamics, short-range and long-range order parameters have been successfully compared with experimental data.
H Motion in Pd and Nb: A Molecular-Dynamics Study
Authors: Li Yinggang , Wahnström Göran
Published: March 2011

Based on realistic many-body potentials molecular-dynamics simulations are carried out for PdH0.03 and NbH0.02. The H motion is investigated at two different temperatures, T = 300 and 600K, paying attention to the vibrational and diffusive motion. We find that the motion of H in Nb, a bcc metal, is more complicated than in Pd, a fee metal, and the differences are discussed. When detailed comparison is made with quasielastic neutron scattering data for H in Pd at 600K, we argue that in order to characterize the diffusion correctly, one has to include nonadiabatic effects.
Simulation of Tensile Deformation of Small Copper Single Crystals
Authors: Doyama Masao
Published: March 2011

The molecular dynamics method was used to simulate the plastic deformation of small copper single crystals. Dislocations were created near the tips of cracks. A very sharp yield stress was found.
Computer Modelling of Dynamically-Induced Dislocation Patterning
Authors: Devincre Benoit , Pontikis Vassilis
Published: March 2011

The evolution of a random and initially homogeneous distribution of parallel and infinitely extended edge dislocations is studied by using elastic energy minimization without and in presence of a periodic external stress, τa. During the energy minimization without external stress (relaxation), randomly distributed dislocation dipoles are formed whereas, when the external stress is acting, the dislocations condense in walls. We investigated the spatial periodicity of this microstructure, λ, as a function of, τa, and of the total dislocation density. The elastic energy of the stress-induced microstructure is found to be comparable to the value obtained by relaxation. Thereby, emphasis is given to the dynamical character of patterning. A phenomenological model has been developed, explaining the correlation between λ and τa found in the simulations and comparing favorably with existing experimental data.
Embedded - Atom - Method Interatomic Potentials for BCC - Iron
Authors: Simonelli G. , Pasianot R. , Savino E.J.
Published: March 2011

An embedded-atom-method (EAM) interatomic potential [1] for bcc-iron is derived. It is fitted exactly to the lattice parameter, elastic constants, an approximation to the unrelaxed vacancy formation energy, and Rose's expression for the cohesive energy [2]. Formation energies and relaxation volumes of point defects are calculated. We find that the relative energies of the defect configurations depend on the functional fitting details of the potential considered, mainly its range: the experimental interstitial configuration of lowest energy can be reproduced by changing this parameter. This result is confirmed by calculating the same defect energies using other EAM potentials, based on the ones developed by Harrison et al. [3].
Can the Strength of Brittle Materials be Enhanced?
Authors: Monette L. , Anderson M. P. , Grest G. S.
Published: March 2011

We have employed a two-dimensional computer model to study the effect of volume fraction of second phase constituents on load transfer (stiffness) and strength in brittle short-fiber composites, i.e. composites containing a random distribution of aligned fibers, and brittle particulate composites. We find that the efficiency of load transfer to the second phase consituent increases with volume fraction in particulate composites, while it decreases for short-fiber composites. The strength of brittle particulate composites is found to decrease, while the strength of brittle short-fiber composites marginally increases only at fiber volume fractions equal or greater than 0.25.
Massively Parallel Molecular Dynamics Simulations of Two-dimensional Materials at High Strain Rates
Authors: Wagner Norman J. , Holian Brad Lee
Published: March 2011

Large scale molecular dynamics simulations on a massively parallel computer are performed to investigate the mechanical behavior of 2-dimensional materials. A model embedded atom many- body potential is examined, corresponding to “ductile” materials. A parallel MD algorithm is developed to exploit the architecture of the Connection Machine, enabling simulations of > 106atoms. A model spallation experiment is performed on a 2-D triagonal crystal with a well-defined nanocrystalline defect on the spall plane. The process of spallation is modelled as a uniform adiabatic expansion. The spall strength is shown to be proportional to the logarithm of the applied strain rate and a dislocation dynamics model is used to explain the results. Good predictions for the onset of spallation in the computer experiments is found from the simple model. The nanocrystal defect affects the propagation of the shock front and failure is enhanced along the grain boundary.
Quantitative X-Ray Diffraction Analysis of Titanate Waste Forms and its Application to Damage Ingrowth
Authors: White T.J. , Mitamura H.
Published: March 2011

Quantitative powder X-ray diffraction was used to analyse the phase assemblages of titanate nuclear waste ceramics doped with 244Cm and accumulating a-decay doses up to 1.24 × 1018 alpha-decay-events g−1. Among the Cm-bearing phases, perovskite is more susceptible to damage than zirconolite. Phases devoid of curium (hollandite and freudenbergite) experience a flux of ionizing radiation, and may also undergo radiation-induced structural degradation. The analysis demonstrated that at a dose of 1.24 × 1018 alpha-decay-events g−1 approximately 20 wt% of the waste phases remain aperiodic after annealing for 2 hours at 800°C.
Uranium and Rare Earth Partitioning in Synroc
Authors: Smith K. L. , Lumpkin G. R. , Blackford M. G.
Published: March 2011

Improved AEM techniques were used to investigate three Synrocs containing 10 wt/ simulated HLW and a fourth sample with ∼18 wt/ simulated HLW. One of the 10 wt./ loaded Synrocs also contained an addition of 1.0 wt/ Na2O and another contained an addition of 2.0 wt/ Fe2O3. This work is part of a larger study initiated with the objective of determining if the bulk composition of Synroc affects the partitioning of elements between individual phases. Results from the four samples in this study show that, as expected, elemental partitioning is mainly controlled by the ionic radius criterion, with smaller Y, Gd, and U ions having a preference for zirconolite and the larger Ce and Nd ions favouring perovskite. Additions of Na and Fe lead to the formation of CAT and loveringite at the expense of rutile or Magneli phases, but only have minor effects on partitioning coefficients. Partitioning coefficients, DZ/P, for REE, Y, and U in the four Synrocs are the same (within experimental error).
The Reaction of SRL 202 Glass in J-13 and DIW
Authors: Ebert W. L. , Bates J. K. , Buck E. C.
Published: March 2011

Static leach tests were performed in both 304L stainless steel and Teflon vessels using a synthetic high-level waste glass with either deionized water (DIW) or a tuff groundwater solution as the leachant to assess the effects of the vessel and the initial leachant composition on the extent and nature of the glass reaction. The tests were performed using monolith samples at 340 m−1 and crushed samples at 2000 m−1 for times up to 1 year. The results show less silicon is released from the glass into the groundwater solution than into DIW at both high and low glass surface area/leachant volume ratios (SA/V), but the alkali metal and boron releases are not affected by the leachant used. Tests performed in a stainless steel vessel resulted in slightly lower leachate pH values, but similar reaction rates to those performed in a Teflon vessel, as measured by the boron release. Blank tests with DIW or EJ-13 in the vessels showed the Teflon vessels to release small amounts of fluoride (1 to 2 ppm) and to acidify the DIW slightly (4.0 < pH < 5.6). The pH values of blank tests with EJ-13 increased from 8.2 to about 8.6 in steel and to about 9.2 in Teflon vessels. The slightly higher pH values attained in Teflon vessels are attributed to outgassing of CO2 during the test.
Enhancement of the Glass Corrosion in the Presence of Clay Minerals: Testing Experimental Results with an Integrated Glass Dissolution Model
Authors: Curti Enzo , Godon N. , Vernaz E.Y.
Published: March 2011

Recent glass dissolution experiments, conducted at 90°C in the presence of potential backfill materials, indicate remarkably faster glass corrosion in the presence of clay, compared to tests where the glass is leached either alone or with alternative backfill materials. This effect correlates with the clay content in the backfill, and may be attributed to the removal of silica from solution. Sorption, or dissolution with reprecipitation of a silica-rich clay, have been proposed as possible mechanisms for the silica consumption.

The results of some experiments have been tested against a glass dissolution model, in which a widely used kinetic equation for glass corrosion is coupled with diffusive silica transport through a single porosity, linearly sorbing medium, which represents the backfilling. Because the glass corrosion rates imposed by the kinetic equation are inversely proportional to the silicic acid concentration of the leachant contacting the glass, the model predicts enhanced glass dissolution if silica is sorbed by the porous medium.

The experimental data proved to be consistent with the predicted enhancement of the glass dissolution. Moreover, the model-estimated distribution coefficients for silica sorption (Kd) fall within the range of values extracted from available literature data, thus supporting the hypothesis that the observed high corrosion rates are due to sorption of silica on the clay mineral surfaces.
Does Fully Radioactive Glass Behave Differently than Simulated Waste Glass?
Authors: Feng X. , Bates J. K. , Bradley C. R. , Buck E. C.
Published: March 2011

Static tests at SA/V (ratio of surface area of glass to solution volume) 20,000 m−1 on SRL 200 glass compositions show that, at long test periods, the simulated nuclear waste glass (nonradioactive) leaches faster than the corresponding radioactive glass by a factor of about 40, although comparative tests, done through 560 days, at lower SA/V, 2000 m−1, indicate little difference in the leach behavior of the two types of glasses. The similarity in leach behavior between radioactive and simulated glasses at SAN of 2000 m−1 or lower is also observed for SRL 165/42 and 131/11 compositions. The accelerated glass reaction with the simulated glass 200S is associated with the formation of crystalline phases such as clinoptilolite (or potassium feldspar), and a pH excursion. The radiation field generated by the fully radioactive glass reduces the solution pH. This lower pH, in turn, may retard the onset of increased reaction rate. The radiation field generated by the radioactive glasses does not directly affect the stability of the glass surface alteration layer under those conditions where the radioactive and simulated glasses react at the same rate. These results suggest that the fully radioactive nuclear waste glass 200R may maintain a much lower leach rate than the simulated 200S, if the lower pH in the 200R leachate can be sustained. Meaningful comparison tests between radioactive and simulated nuclear waste glasses should include long-term and high SA/V tests.
Evaluation of Some Low-Activation Structural Materials for Minimizing Radioactive Waste Production by Future Nuclear Fusion Power Plants
Authors: Donato Aldo
Published: March 2011

Taking into account the neutron activation (Bq/kWht) and the environmental impact in terms of Ingestion Toxic Potential IT (m3 H2O/kWht) of radioactive wastes coming from several low-activation structural material candidates for future nuclear fusion power reactors, the contribution of the material chemical composition elements and impurities to the overall radioactivity and IT has been assessed. The chemical impurities, also at very low concentrations, have been ascertained to give, in many cases, a considerable contribution to the radwastes environmental impact, as in the case of SiC/SiC fiber composites materials or low-activation martensitic steels.

The orign of impurities origin has been identified and efforts to use this information in the material production processes for nuclear fusion application are suggested.
in-situ Electrochemical Characterization of Grouted Radioactive Waste
Authors: Kruger Albert A. , Gu Jingyan , Tomkiewicz Micha
Published: March 2011

This work is based upon the notion that a grid of electrodes will be incorporated in the slurry of radioactive waste and grout-forming solids. These electrodes will remain in the hardened grout, and will serve to monitor the ionic concentration of the liquids contained within the matrix, the structural integrity of the matrix, and the rates of out-diffusion of the electrolytes. Ultimately, we will investigate four techniques: (1) ionic conductivity, (2) electrochemical potential, (3) cyclic voltammetry, and (4) impedance measurements. Eventually, the results will be correlated with present quality verification methods, which include recovering samples and following the Toxicity Characteristic Leach Procedure (or other such accepted standard tests). This paper presents the preliminary results of a recently initiated effort.
Use of Fly-Ash for Sealing a Radioactive Waste Repository
Authors: Ollagnier Marc , Tauziede Christian , Olivier Jodl
Published: March 2011

The mining industry currently uses fly-ash from coal-fired power-plants to close access shafts in abandoned mines, in sedimentary formations. The technique consists of installing a plug of fly-ash, about fifty meters high, at the base of the shaft-lining. In this study, fly-ash is considered as a possible sealant for radioactive repositories in sedimentary formations, as well as in hard rock.

Specific studies have been conducted in order to assess the feasibility of this technique, and to improve the long-term performance of the fly-ash seals. The hydraulic and mechanical characteristics of fly-ash, taken from the plug of an abandoned shaft, as well as from dumps of various ages, were measured. Laboratory tests showed that it is possible to reduce the hydraulic conductivity of fly-ash by a factor of thousand, with the addition of ten percent bentonite. Moreover, it seems possible to block fine fissures in the surrounding damaged rock by injecting grouts made of cement and fly-ash having maximum diameters of twenty micrometers.
Crevice Corrosion - NaCI Concentration Map for Grade-2 Titanium at Elevated Temperature
Authors: Tsujikawa Shigeo , Kojima Yoichj
Published: March 2011

The repassivation potential, ER, for metal/metal-crevice of Commercially Pure Titanium, C.P.Ti, was determined in NaC1 solutions at temperatures up to 250C. The ER has its least noble value near 100C and becomes more noble as the temperature increases. As shown in previous research[1], the shrinkage of the repassivation region should continue with increasing temperatures. However, in conducting this same experiment at temperatures higher than 100C, an examination of the NaCI concentration - temperature - crevice corrosion map verifies that the repassivation region began to expand again when the temperature exceeded 140C. This expansion continued as the temperature continued to increase.
Corrosion of Stainless Steel for HLW Containers Under Gamma Irradiation
Authors: Osada K. , Muraoka S.
Published: March 2011

The corrosion behavior of type 304 stainless steel was studied under gamma irradiation as part of the evaluation for the long-term durability of high-level radioactive waste (HLW) disposal containers. Gamma rays, generated from fission products in high-level radioactive waste, are considered to change the environment around the canisters and overpacks. The redox potentials for NaCl solutions and corrosion potentials of stainless steel were measured to consider the effects of gamma irradiation, by using an electrochemical method. The pitting potentials of stainless steel for NaCl solutions were also measured to examine the pitting corrosion under gamma irradiation. As a result of this experiment, it is concluded that the oxidizing properties as a result of the formation of H2O2 and H2 produced by gamma irradiation depended on the concentration of Cl−, and that the strength of oxidizing properties of 1M (mol·dm−3) NaCl solution was particularly high. and the pitting corrosion was found for 1M NaCI solution under gamma irradiation at the dose rate of 2.6×102 C/kg·h (1.0×106 R/h) at 60°C, by using an electrochemical method.
Effects of Chloride, Bromide, and Thiosulfate Ions on the Critical Conditions for Crevice Corrosion of Several Stainless Alloys as a Material for Geological Disposal Packages for Nuclear Waste
Authors: Nakayama Guen , Wakamatsu Hisao , Akashi Masatsune
Published: March 2011

In addition to mild steel, several stainless alloys are being proposed as materials for packages for geological disposal of high-level nuclear waste. When buried deep underground, the greatest detriment to the integrity of packages made of these alloys is localized corrosion, for which critical conditions for initiation of crevice corrosion in chloride environments, with or without other ions, need be precisely known.

Crevice corrosion behavior of Type 304 stainless steel, Type 316 stainless steel, Alloy 825, Ti-Gr.1, and Ti-Gr.12 in solutions containing ions of chloride, bromide (these two for their ordinary presence in natural waters), or thiosulphate (this for the likelihood of microbially influenced corrosion) to varying concentrations have been empirically examined. All of these alloys exhibit much the same concentration dependency of crevice corrosion sensitivity for chloride and bromide ions, while Type 304 stainless steel is particularly sensitive to the thiosulphate ion. The region of insensitivity for chloride ion is wider in the increasing order of Type 304 stainless steel, Type 316 stainless steel, Ti-Gr. 1, and Ti-Gr. 12, with that of Alloy 825 lying somewhere in between.
Use of Sputter-Deposited 316L Stainless Steel Ultrathin Films for Microbial Influenced Corrosion Studies
Authors: Suci P.A. , Pedraza A.J. , Godbole M.J. , Geesey G.G.
Published: March 2011

Ultra thin films (12nm) were sputter deposited onto cylindrical germanium internal reflection elements pre-coated with a thin (2nm) layer of Cr2O3. Two crystals were inserted into Circle cell flow-through chambers and mounted on the optical bench of an Fourier Transform Infrared (FT-IR) spectrometer. One chamber was maintained as a sterile control while the other was sequentially inoculated with four bacterial species:Pseudomonas aeruginosa, Bacillus subtillis, Hafnia alvei, andDesulfovibrio gigas, in that order. The water absorption band (1640cm−1) was monitored and used to follow the deterioration of the ultra thin films. In this respect, the sterile control and inoculated films exhibited only slight differences during the 1000h course of the experiment. Assay of the visible biofilm that had accumulated on the surface of the inoculated crystal after 1000h revealed that the film incorporated viable cells from all four strains.
Dealing with Uncertainty in the Chemical Environment in Bentonite Backfill
Authors: Arthur R.C. , Apted M. J. , Conca J. L.
Published: March 2011

Analytical and conceptual deficiencies in understanding compositional variability in the smectite clays are expected to generate uncertainty in models used to simulate the chemical environment in bentonite backfill. Equilibrium models accounting for nonstoichiometry in smectite can nevertheless bound ranges in aqueous solution compositions that are an explicit function of the uncertainty in smectite compositions. In one approach, we quantify uncertainty in terms of ranges in concentrations of octahedral and tetrahedral Al, and exchange-site cations and vacancies. Heterogeneous mass transfer in bentonite-water systems is modeled using conventional mass-action relations and standard Gibbs energies for stoichiometric minerals, and the site-occupancy constraints combined with site-mixing relations for smectite. The resultant bounding conditions in groundwater compositions may be large or small depending on which aqueous species are of interest in a given situation, but they are valid irrespective of whether equilibrium in smectite-water reactions is attained or is inhibited by slow intracrystalline reaction rates.
Diffusion of Uranium in Compacted Sodium Bentonite
Authors: Muurinen Arto , Ollila Kaija , Lehikoinen Jarmo
Published: March 2011

In this study the diffusion of uranium dissolved from uranium oxide fuel was studied experimentally in compacted sodium bentonite (Wyoming bentonite MX-80). The parameters varied in the study were the density of bentonite, the salt content of the solution and the redox conditions. In the studies with non-saline water of total dissolved solids about 300 ppm, uranium was both in aerobic and anaerobic experiments as anionic complexes and followed the anionic diffusion mechanism. Anion exclusion decreased effective diffusion coefficients, especially in more dense samples. In the studies with saline water of total dissolves solids about 35000 ppm, uranium appeared in the aerobic experiments probably as cationic complexes and followed the cationic diffusion mechanism. Uranium in the saline, anaerobic experiment was probably U(OH)4 and followed the diffusion mechanism of neutral species.
Compared Study of Radiolysis-Induced Gas Liberation in Rocksalt from Various Origins
Authors: Akram N. , Blanchard J.C. , Gaudez M.T. , Toulhoat P.
Published: March 2011

The evolution of impure rocksalt samples under irradiation has been investigated and compared to the evolution of pure halite, concerning the radiolytic release of gases. Four different types of rocksalt (pure halite, anhydrite-bearing, sylvinite-bearing, and marneous) have been irradiated with spent fuel at doses ranging from 104Gy to 107Gy, under synthetic air or helium at 50°C, 150°C, and 200°C. Marneous and anhydrite-bearing salt produced higher amounts of gas than the two other types, especially for H2, CO, and CH4. Radiolytic gas production essentially originates from the decomposition of traces of different kind of impurities : fluid inclusions, which can be composed of brine and/or organic fluids and gases; minerals such as carbonates, sulfates, or hydrated minerals; organic matter, in general (kerogen). Pure halite decomposes only at very high doses (more than 107Gy) giving off some Cl2 and other corrosive gases. Gas production increases steadily with dose for H2, CO, and CH4, contrary to what is observed for the pure Asse salt, where H2 and CO, respectively, peak at 106, and 105 Gy. The increase, with dose, in the production of CH4, C2H6, and C3H8 is far higher for marneous salt and higher for anhydrite-bearing salt. The decrease of the O2/N2 (oxygen consumption) ratio in the irradiation atmosphere is in the following order: marneous>anhydrite>halite=sylvinite. The overall effect of γ-irradiation on impure rocksalt can be described as follows : destruction of organic matter that produces chemically reduced gases (H2, CO, hydrocarbons), after an initial stage of oxygen consumption, which is reached more or less quickly, depending on the availability and quantity of organic matter. The use of impure, i.e, organic matter-bearing salt, as host-rock or engineered barrier for nuclear wastes, may be questioned, because of the high amount of gases produced in the immediate vicinity of the container (explosion hazards, pressure build-up). The present study will be followed by a modelling of the in-situ radiolytic gas production, taking into account: 1) the dose distribution with time, 2) the temperature distribution with time, and 3) the geometry of the repository, and will allow us to estimate the potentiel safety impact of radiolytic gas production in the case of a repository emplaced in impure rocksalt.
Effects of Air Oxidation on the Dissolution Rate of LWR Spent Fuel
Authors: Gray W. J. , Thomas L. E. , Einziger R. E.
Published: March 2011

Dissolution rates for air-oxidized spent fuel were measured in flowthrough tests where U concentrations were kept well below the solubility limit. Results from two types of specimens, separated grains and coarse particles, both in oxidized (U4O9+x) and unoxidized (UO2) conditions indicated only minor effects of oxidation on the surface-area-normalized rates. Similar results were obtained for unirradiated specimens in three different oxidation states (UO2, U3O7, and U3O8). These observations have important practical implications for disposal of spent fuel in a geologic repository as well as implications regarding the oxidative dissolution mechanism of UO2fuel.
The Alteration of Uraninite to Clarkeite
Authors: Finch Robert J. , Ewing Rodney C.
Published: March 2011

The oxidative alteration of uraninite by alkali-bearing, hydrothermal (200-400° C) solutions in granitic rocks produces the rare mineral clarkeite. The general formula for Na-clarkeite is

{(Na,K)6-y-z(M2+PM3+qM4+r)y(□1+z-xPbx)}[UO2)7-xOt0]·nH2O. z ≈ p+2q+3r.

The terms in square brackets designate the sheet structure; the other elements and vacancies (0) occur in interlayer sites. Clarkeite can accommodate Ca, Sr, Ba, Y, Th, and lanthanides. Thus, clarkeite is a potential actinide and fission product host. The replacement of uraninite by clarkeite occurs in the solid state and results in the loss of one-half of the uranium from uraninite. As U decays to Pb, the radiogenic Pb enters interlayer vacancies in clarkeite. This destabilizes the structure and clarkeite eventually decomposes to wölsendorfite, (Pb,Ca)2U2O7·2H2O, or curite, Pb3U8O27·3H2O.

Clarkeite from Spruce Pine, North Carolina is zoned compositionally, with a K-rich core (atomic ratio K:Na:Ca = 1.0 : 0.11 : 0.12) surrounded by Na-clarkeite (Na:K:Ca = 1.0 : 0.03 : 0.03), rimmed and veined by Ca-clarkeite (Ca:Na:K = 1.0 : 1.25 : 0.05). Volumetrically, Na-clarkeite is the most important constituent. The chemical zoning reflects the disparate chemistries of K+, Na+, and Ca2+. Na-clarkeite and Ca-clarkeite are structurally similar, but have different sheet structures. The solubility of K in Na-clarkeite is less than five mole percent and is due to the different sizes of Na+ and K+ ions. The K-phase may not be related structurally to clarkeite.

Exposed to ground water at low temperatures, clarkeite alters to the uranyl silicates, uranophane and kasolite. The alkalis are leached by ground water. The fate of Th, Y, and lanthanides from clarkeite is uncertain.
Kinetic Studies of Natural Uranium Minerals for the Long-Term Evolution of Spent Nuclear Fuel Under Oxidizing Conditions
Authors: Casas Ignasi , Cera E. , Bruno J.
Published: March 2011

The time scale of spent fuel dissolution studies is of the order of magnitude of 2 to 10 years, while the performance of a spent fuel repository should be assessed for much longer times (105-106 years). These time scales can be bridged using appropriate natural analogues. Among other important information, the study of natural systems can give insight of which can be the oxidative alteration of spent fuel in granitic environments. However, in studying such systems, thermodynamic and kinetic data of relevant natural solid phases are needed.

In this work we present preliminary results of dissolution experiments carried out under oxidizing conditions with selected and well characterized natural samples of the alteration chain of uraninite (i.e., uraninite, schoepite, uranophane).

The experiments have been performed using a synthetic granitic groundwater as a leachant, in contact with air and at 25 °C.
A Modelling Study of the Effect of Rock Alteration on the Redistribution of Uranium
Authors: Murakami Takashi , Kimura Hideo
Published: March 2011

A modelling study was carried out to understand the effect of rock alteration on uranium radionuclide concentrations in rocks, in the vicinity of the Koongarra ore deposit, Australia. The one-dimensional, advection-dispersion-sorption model considers two important factors resulting from the process, mechanism and rate of chlorite weathering, one type of rock alteration that has occurred over a million-year period; (a) the changes in the distribution coefficients of uranium and thorium over time, and (b) that in rock porosity. The distribution coefficient and rock porosity at a given time, are expressed as average values of those of the coexisting minerals. By assuming a Darcy velocity of 0.9 m/y and an initial uranium concentration in the groundwater of 0.5 ppm, similar to present-day values, we were able to derive, from the finite-element modelling, uranium concentrations in the rock which are in good agreement with observed values. The calculated values of 230Th/234U activity ratios show a similar trend to those observed, although more experimental data are necessary to confirm the similarity. However, in the absence of the rock alteration, the calculated results did not agree with those observed. The present study suggests that rock alteration should be included in models used to predict uranium migration over long, geologic timescales.
The Long-Term Performance of Nuclear Waste Forms: Natural Materials - Three Case Studies
Authors: Ewing Rodney C.
Published: March 2011

Natural materials may be used to advantage in the evaluation of the long-term performance of nuclear waste forms. Three case studies are presented: (I) radiation effects in ceramic waste forms; (II) corrosion products of U02 under oxic conditions; (III) corrosion rate of nuclear waste glasses. For each case, a natural phase which is structurally and chemically analogous to the waste form is identified and used to evaluate the long-term behavior of a nuclear waste form. Short-term experimental results are compared to the observations made of analogous natural phases. The three case studies illustrate that results may range between providing fundamental data needed for the long-term evaluation of a waste form to only providing qualitative data of limited use. Although in the most rigorous view the long-term behaviour of a phase cannot be predicted, the correspondence between short-term experimental results and observations made of natural phases provides confidence in the “predicted” behavior of the waste form. The strength of this approach rests with the degree to which a mechanistic understanding of the phenomenon is attained.
Modeling Surface Area to Volume Effects on Borosilicate Glass Dissolution
Authors: Bourcier William L. , Ebert W. L. , Feng X.
Published: March 2011

We simulated the reaction of SRL-131 glass with equilibrated J-13 water in order to investigate the effects of surface area to volume ratio (SAN/) on glass dissolution. We show that glass-fluid ion exchange causes solution pH to rise to progressively higher values as SA/V increases. Because the ion exchange is rapid relative to the duration of the glass dissolution experiment, the pH effect does not scale with (SA/V)*time. Experiments compared at the same (SA/V)*time value therefore have different pHs, with higher pHs at higher SA/V ratios. Both experimental data and our simulation results show similar trends of increasing reaction rate as a function of SAN ratio when scaled to (SA/V)*time. Glasses which react in systems of differing SA/V ratio therefore follow different reaction paths and high SAN ratios cannot be used to generate data which accurately scales to long time periods unless the ion exchange effect is taken into account. We suggest some simple test designs which enable more reliable high SAN accelerated tests.
Observation by Sem/edxs of Leached Layers on SRL-165 Glass from 5-Year MIIT Test
Authors: Macedo Pedro B. , Buechele Andrew C.
Published: March 2011

Long-term tests on the durability of nuclear waste glasses, both under laboratory conditions and in actual burial environments, have shown that after periods of several months or years the rates of glass corrosion can show unexpected increases. In burial tests in brine environments the growth of the thickness of the surface layer during years 3 to 5 as revealed by SIMS and SEM measurements was found to be faster than expected on the basis of findings during the first 2 years. In this paper we report on our SEM-EDXS observations of a 5 year burial sample of SRL-165 glass from the MIIT test, and compare our findings to those of other studies, especially SIMS measurements. Our average thickness measurement for the leached layer at the glass-metal interface is 3.70 μm, but wide variation is observed, as well as evidence of partial loss of the leached layer in some areas. A thicker, better retained layer is observed on the glass-brine interface near the core heater averaging 8 to 12 μm in thickness with occasional local attack producing leached layers of greater depth, in one case nearly 40 μm. These findings show that it is important to extend durability tests to long periods in order to establish a reliable basis for predictive modeling of long-term glass durability.
The Potential Role of Diffusion-Induced Grain-Boundary Migration in Extended Life Prediction
Authors: Handwerker C.A. , Blendell J.E. , Interrante C.G. , Ahn T.M.
Published: March 2011

The selection of materials that are suitable for various high-level waste-packaging designs must reflect the need to meet requirements for long-term performance in repository environments that change with time. With this in mind, we examine how grain boundaries in materials are induced to migrate as a result of solute diffusion even at low temperatures, how the composition of the matrix material is changed significantly by this diffusion-induced grain boundary migration (DIGM), and how the changing microstructures and compositions during DIGM lead to major changes in materials performance, such as corrosion or embrittlement. Methods are discussed for prediction of the long-term behavior of materials affected by DIGM.
The Engineered Barrier System: Performance Issues
Authors: Pigford T. H.
Published: March 2011

The engineered barrier system (EBS) for geologic disposal of high-level waste may typically consist of the waste solid, a container, and a porous backfill. Whether the surrounding rock is part of the EBS is arguable, but the properties of the surrounding rock strongly affect the EBS performance. The EBS functions and performance issue are addressed.