bluuebunny/crossref_metadata_2025 huggingface dataset:
Interfacial Structure of Electrically Conducting Adsorbed Multilayers
Authors: Kellogg G. J. , Mayes A.M. , Stockton W. B. , Ferreira M. , Rubner M.F. , Satija S.K.
Published: April 2011

The multilayer organization of sulfonated polyaniline/polyallylamine (SPAN/PAH) films was investigated by neutron reflectivity. Films were prepared by sequential adsorption of polycations and polyanions from dilute solution. Scattering contrast was achieved by selective deuteration of blocks of bilayers at varying intervals along the films. The internal organization of the multilayer structure was found to decay significantly with increasing number of bilayer depositions. In addition, a high degree of roughness was determined at the free surfaces of the films. Together, these results suggest that both conformal and nonconformal roughness are present in the films, i.e., defects introduced during individual layer depositions are to some degree transferred to the surface of subsequently deposited layers.
Determination of the Bond Strength Between Nanosized Particles
Authors: Weber Alfred P. , Friedlander Sheldon K.
Published: April 2011

A method has been developed for determining the bond energies between nanosized particles from the kinetics of the rearrangement of aerosol agglomerates. The method of calculation is based on the change in Gibbs' free energy during restructuring. For Ag and Cu agglomerates, bond energies between the nanosized particles are of the order of magnitude calculated from bulk Hamaker constants.
Facet Formation of Lineshaped Silicon Mesas Grown with Micro Shadow Masks
Authors: Gossner H. , Fehlauer G. , Kiunke W. , Eisele I. , Stolz M. , Hintermaier M. , Knapek E.
Published: April 2011

As reported previously, perfect facets can be achieved at the side walls of submicron silicon mesa structures grown by molecular beam epitaxy (MBE) with micro shadow masks [1]. An essentially self organizing, three-dimensional growth was observed. In this paper we present the results of the epitaxial growth on (001) substrates using long (≥ 1μm), lineshaped mask apertures, which put constraints on the formation of facets. At a growth temperature of 500°C {111} facet formation is observed for lineshaped mesas oriented along the <110> direction of the substrate. Side walls with a length of I μm are perfectly plane, while mesas with a length of 10 μm and more show rough sidewalls. This is explained by a limited silicon adatom diffusion on the facet. For higher flux rates the facet formation is suppressed. This can be understood in terms of a reduced adatom diffusion.

A crossover from {111} to {113} facet formation is observed at growth temperatures above 500°C. A model for the temperature dependent formation of {111} and {113} facets is given.
Blue Light Emission from Silicon Ultrafine Particles
Authors: Nozaki Shinji , Sato S. , Ono H. , Morisaki H.
Published: April 2011

The oxygen-containing silicon (Si) ultrafine particles have been deposited onto Si and SiO2 substrates by evaporation of Si powder in an oxygen-containing argon atmosphere. The asdeposited Si ultrafine particles exposed to the ultraviolet light emit blue light, which is strong enough to be seen with the naked eye. The blue light emission is associated with a broad photoluminescence (PL) peak at 2.7 eV, which is attributed to radiative recombination via a radiative recombination center. The proposed model with one radiative and two nonradiative recombination centers well explains the temperature-dependent PL peak intensity.
IR-Visible Photoluminescence Study of Nanometer-Size Amorphous Silicon Powder Produced by Square-Wave-Modulated RF Glow Discharge
Authors: Costa J. , Roura P. , Sardin G. , Morante J.R. , Bertran E.
Published: April 2011

Amorphous silicon powder exhibiting microstructural properties such as nanometer-size, large presence of silicon-hydride groups and high surface/volume ratio has been produced in a plasma enhanced chemical vapor deposition (PECVD) reactor using pure silane gas and low frequency square-wave-modulated (SQWM) rf power (13.56 MHz). The possible crystallinity of nanometer-size domains embedded in the amorphous network was studied with Raman spectroscopy and electron diffraction analysis.

Photoluminescence (PL) in the near IR-VIS region was excited by an Ar laser. The PL intensity exhibits very unusual properties: a supralinear dependence on excitation power and an exponential decrease with pressure (below 50 Pa). The analysis of PL dynamics led us to propose a mathematical model of excitation of the PL. This model involves the existence of an intermediate step, with an extremely long lifetime, that controls the PL dynamics.
A Molecular Approach to Ultrathin Multilayered Films of Titanium Dioxide
Authors: Kleinfeld Elaine R. , Ferguson Gregory S.
Published: April 2011

We have used solution-phase inorganic chemistry to form ultrathin, covalently bonded layers of TiO2 on silicon wafers. Ellipsometry and x-ray photoelectron spectroscopy (XPS) were used to monitor the growth of the films. The ellipsometric thickness of the TiO2 films increased linearly with the number of reaction cycles, the first absorption cycle resulting in about 3–4 Å of growth, and subsequent cycles resulting in about 1 Å of growth. We attribute this limited growth to the limited number of reactive sites on the surface.
The Control of Dna Structure and Topology: An Overview
Authors: Seeman Nadrian C. , Zhang Yuwen , Du Shou Ming , Wang Hui , Mueller John E. , Wang Yinli , Liu Bing , Qi Jing , Chen Junghuei
Published: April 2011

The control of structure on the nanoscale relies on intermolecular interactions whose specificity and geometry can be treated on a predictive basis. DNA fulfills this criterion, and provides an extremely convenient construction medium: The sticky-ended association of DNA molecules occurs with high specificity, and it results in the formation of double helical DNA, whose structure is well known. The use of stable branched DNA molecules permits one to make stick-figures. We have used this strategy to construct in solution a covalently closed DNA molecule whose helix axes have the connectivity of a cube: The molecule has twelve double helical edges; every edge is two helical turns in length, resulting in a hexacatenane, each of whose strands corresponds to a face of the object. We have developed a solid-support-based synthetic methodology that is more effective than solution synthesis. The key features of the technique are control over the formation of each edge of the object, and the topological closure of each intermediate. The isolation of individual objects on the surface of the support eliminates cross-reactions between growing products. The solid-support-based methodology has been used to construct a molecule whose helix axes have the connectivity of a truncated octahedron. This figure has 14 faces, of which six are square and eight are hexagonal; this Archimedean polyhedron contains 24 vertices and 36 edges, and is built from a 14-catenane of DNA. Knotted molecules appear to be the route for cloning DNA objects. It is possible to construct three knotted topologies, as well as a simple cyclic molecule from a single precursor, by control of solution conditions. Control of both branching and braiding topology is strong in this system, but control of 3-D structure remains elusive. Our key aim is the formation of prespecified 2-D and 3-D periodic structures for use in diffraction experiments. Another application envisioned is scaffolding for the assembly of molecular electronic devices.
Langmuir-Blodgett Films of Calcium Stearate
Authors: Costa Necesio , Aindow Mark , Marquis Peter M.
Published: April 2011

The Langmuir Blodgett (LB) Process has been shown to be an appropriate method for use in mimicking of biological processes for producing engineering materials such as bioceramics. The main advantages of this approach are that the layers form at low temperatures, that they are fully dense and that the process of densification is by infiltration rather than by sintering. Moreover, biological hard tissues are self-assembled to perform certain functions; the architecture being controlled by an epitaxial organic matrix. Clearly, if this process can be understood in detail then it is possible that LB films may be used to replicate this architecture for engineering purposes.

Atomic Force Microscopy (AFM) and X-ray diffraction (XRD) have been used to study and characterise LB films of calcium stearate obtained by the repeated dipping into and withdrawal of a (001) Si wafer from a subphase containing calcium ions and using stearic acid as the surfactant. Contact-mode AFM images of the film surface have been used to measure the thickness of the LB layers and to reveal the nature and distribution of defects in the film. The measured thickness of the calcium stearate layers is about 2.5 nm; a value consistent with that obtained by XRD, but smaller than the length of an individual calcium stearate molecules.
Adsorption of Carbon-14 on Mortar
Authors: Matsumoto Junko , Banba Tsunetaka , Muraoka Susumu
Published: April 2011

The sorption experiments of carbon-14 on the mortar grain (grain size : 0.50 – 1.0 mm) focused on the chemical form of the carbon-14 were carried out by the batch method. Three kinds of carbon-14 chemical form were used for the experiments : sodium carbonate (Na214CO3) as the inorganic radiocarbon, and sodium acetate (CH314COONa) and acetaldehyde (14CH314CHO) as the organic radiocarbons. 0.30 gram samples of mortar were soaked in the solution with carbon-14 at 15°C for periods of up to 160 days. At the end of each run, carbon-14 concentrations in the supernatants were determined before and after centrifugation (3,500 rpm., 1 hr.).

In the mortar-sodium carbonate system, the retention process of carbon-14 related to reaction on the surface of the mortar was speculated as follows. First, 3CaO-SiO2. and 2CaO-SiO2, of the mortar components contact with water and produce Ca(OH)2. Ca(OH)2 produces Ca2+ and OH− in the solution. Then, calcite forms from Ca2+ and CO32− in the solution. Thus, the sorption ratio of carbon-14 onto mortar will be high until mortar nas been completely carbonated because Ca2+ is rich in the mortar and the solubility of calcite is low.

In the mortar-organic carbon system, the soluble organic carbon-14 is hardly sorbed on the surface of the mortar. Therefore, the cementitious materials may not inhibit the release of organic radiocarbons from the low-level radioactive wastes, contrary to the case of inorganic radiocarbon.
Migration Behavior of Carbonate-14C in Tuffaceous Soil
Authors: Nagao S. , Senoo M.
Published: April 2011

A simple closed column system was developed to study migration of bicarbonate and carbonate 14C in soil layer. Experimental data for glass beads having less the 14C adsorption indicate that the system can be performed on column experiment keeping mass balance of 14C. Migration behavior of bicarbonate and carbonate 14C in a tuffaceous sandy soil was investigated by this column system in the pH range from 8 to 12. The 14C was delayed to tritiated water, and the retardation became stronger in order, pH 12 < pH 10 < pH 8. This is considered to be due to pH-dependent adsorption of bicarbonate and carbonate ions onto the soil sample.
The Solubility of Th(IV) and U(IV) Hydrous Oxides in Concentrated Nahco3 and Na2Co3 Solutions
Authors: Rai Dhanpat , Felmy Andrew R. , Moore Dean A. , Mason Marvin J
Published: April 2011

The solubility of Th(IV) and U(IV) hydrous oxide was studied in the aqueous HCO3−-CO32−-OH−-H2O system extending to high concentration. The solubility of the Th(IV) and U(IV) hydrous oxides increases dramatically in both high bicarbonate and carbonate solutions and decreases with the increase in hydroxide at a fixed carbonate concentration. In general, the observed solubility of Th(IV) hydrous oxide at a given total carbonate concentration was three to four orders of magnitude higher than the solubility of U(IV) hydrous oxide. In the studies of the U(IV) system, extreme caution was used to ensure that the dissolved U was present as U(IV). Oxidation state analyses and systematic trends in the U(IV) solubility data similar to those for Th(IV), which is not redox sensitive, indicated that the dissolved U was present as U(IV).
Enhanced Recovery of Technetium(VII) by Primary Amine Extraction from Aqueous Fluoride Solutions and Aqueous Alkaline Solutions
Authors: Ito K.
Published: April 2011

The extraction behavior of technetium from aqueous hydrofluoric acid solution has been investigated. The recovery of Tc using two techniques, primary amine extraction in mixed heptane and 1–octanol solution and stripping with a mixed aqueous ammonium hydroxide – ammonium carbonate solution has been studied. The determination of distribution ratios and percentages of technetium in both extraction and stripping cycles was established as were the volume ratios between organic and aqueous phases. The stripping mechanism in the aqueous solution has also been investigated by the slope analysis method.
Recycle Processes for Metal Fuels by Dissolution into Organic Solution
Authors: Sano Y. , Hayami H. , Miyake C. , Horie M.
Published: April 2011

The dissolution behavior of actinide and fission product metals into THF with 10 vol% Br2 was examined in this study. The solubilities of niobium, ruthenium, rhodium, zirconium and molybdenum in both aerated and nitrogenated THF solutions with 10 vol% Br2 are negligible. On the other hand, the solubilities of lanthanum, neodymium, samarium and strontium in aerated and nitrogenated THF solutions with 10 vol% Br2 are large. Among metals examined in this study, uranium had the largest solubility in nitrogenated THF with 10 vol% Br2 and under this condition uranium will be separated from other metals. In the nitrogenated organic solution U(IV), U(V) and UO22+ (VI) coexist, while only UO22+ (VI) exists in the aerated organic solution.
Development of the Hydrogeological Survey Method Using the Natural Dissolved Noble Gases for the Site-Characterization for Radwastes Disposal
Authors: Mahara Yasunori
Published: April 2011

This study focused on developing the tracer method using the dissolved natural noble gases. The method measures precisely helium and neon dissolved in groundwater for dating. Since the dissolved helium consists of three different components (Atmospheric He, Radiogenic He and Mantle He), if it is possible to separate total dissolved helium into each component, the mixing rate between young groundwater and ancient will be predicted in in-situ groundwater. Then, dating of young groundwater is determined by measuring tritium and tritiogenic 3He in a groundwater sample. On the other hand, the dating method for old groundwater is not still verified. In this study, the old groundwater age is predicted by the accumulating rate of radiogenic helium through the alpha decay of uranium and thorium contained in rock.

This tracing method was applied to a confined aquifer in the Kumamoto plain (10km × 20km ), where the groundwater flow and the geohydrological formation of aquifer was investigated in detail in the past. The groundwater in the area consists of mixture of young groundwater (less than 20 yrs old) and ancient isolated groundwater (more than 1000 yrs old). Furthermore, the drastic change in the helium isotopic ratio revealed unexpectedly the existence of active faults covered in groundwater basins. Consequently, the method using the dissolved noble gases is practicably available for investigating the geohydrologic formation of aquifer at the early stage of the site characterization.
Effects of Alpha Decay on the Properties of Actual Nuclear Waste Glass
Authors: Banba T. , Matsumoto S. , Muraoka S. , Yamada K. , Saito M. , Ishikawa H. , Sasaki N.
Published: April 2011

An accelerated experiment using the actinide doping technique was performed to investigate the effects of alpha decay on the properties of actual nuclear waste glass at high radiation doses. A fully radioactive borosilicate waste glass, containing the actual high-level radioactive liquid waste generated from the Tokai Reprocessing Plant of PNC, was prepared by JAERI, and a powder mixture of the ground fully radioactive glass and 244CmO2 was melted at 1200°C for 2 hrs. The radioactivity concentration of 244Cm was 1.0 } 1010 Bq/g-glass at the date of preparation. The homogeneity of curium-doped glass samples was confirmed by the density measurement, heat load measurement and alpha autoradiography. The properties of the irradiated samples were investigated by the mass spectrometer for helium determination, the optical microscope, the electron probe microanalyzer, the densitometer, the Soxhlet and MCC-1 leach testing apparatus. By measuring the amount of helium released from the curium-doped glass samples, more than 99% of helium remained in the matrix at room temperature. The density of the sample slightly decreased with the increase of cumulative alpha decays and the decrease of 0.77% was observed at a dose of 1.55 } 1018 alpha decays/g, corresponding to an equivalent age of 150000 yrs. Optical and scanning electron micrographs showed that no cracks were observed on all samples having up to a dose of 1.5 } 1018 alpha decays/g. The leach rates, based on weight loss, in both the Soxhlet (100°C, 7 days) and MCC-1 (90°C, 28 days) tests did not significantly change with alpha decay dose.
Experimental Investigation on the Active Range of Sulfate-Reducing Bacteria for Geological Disposal
Authors: Fukunaga S. , Yoshikawa H. , Fujiki K. , Asano H.
Published: April 2011

The active range ofDesulfovibrio desulfuricans. a species of sulfate-reducing bacteria, was examined in terms of pH and Eh using a fermenter at controlled pH and Eh. Such research is important because sulfate-reducing bacteria (SRB) are thought to exist underground at depths equal to those of supposed repositories for high-level radioactive wastes and to be capable of inducing corrosion of the metals used in containment vessels.

SRB activity was estimated at 35°C, with lactate as an electron donor, at a pH range from 7 to 11 and Eh range from 0 to -380 mV. Activity increased as pH approached neutral and Eh declined. The upper pH limit for activity was between 9.9 and 10.3, at Eh of -360 to -384 mV. The upper Eh limit for activity was between -68 and -3 mV, at pH 7.1. These results show that SRB can be made active at higher pH by decreasing Eh, and that the higher pH levels of 8 to 10 produced by use of the buffer material bentonite does not suppress SRB completely.

A chart was obtained showing the active range ofDesulfovibrio desulfuricansin terms of pH and Eh. Such charts can be used to estimate the viability of SRB and other microorganisms when the environmental conditions of a repository are specified.
Experimental Validation of a Speciation Model Applied to Organic-Rich Groundwaters
Authors: Crawford M. B. , Higgo J. J. W. , Davis J.
Published: April 2011

Natural organic compounds can play an important role in the transport of radionuclides through the geosphere, but the inclusion of polyelectrolytic fulvic and humic acids into geochemical speciation codes is not straightforward. Schubert ion-exchange experiments have been used to parameterise an equilibrium-based, discrete site-electrostatic model, Model V, which is concerned with predicting trace metal-humic interactions in solution. However, before such a model can be applied to natural groundwaters, its ability to deal with competition between trace cations and anionic ligands has to be tested.

PHREEQEV, which incorporates Model V with the inorganic speciation code PHREEQE, has been used to guide experiments based on a modified Hummel-Dreyer method. To improve speed and precision, these experiments used a HPLC method rather than Sephadex gel, and measured the effect of Ca on Co and Ni binding to fulvic acid. The results were compared with the predictions made by PHREEQEV. Although the system was further complicated by the need to add citric acid to prevent binding of metals to the HPLC column, predictions at pH 5 were good for both metals. At pH 7, the Co binding results were not well predicted and the reasons for this are still unclear. Comparisons between the Schubert and Hummel-Dreyer-type experiments show that the two methods are compatible.
Clay-Based Materials for Engineered Barriers : A Review
Authors: Lajudie Alain , Raynal Joël , Petit Jean-Claude , Toulhoat Pierre
Published: April 2011

The potential importance of backfilling and plugging in underground radioactive waste repositories has led differents research institutions around the world (SKB in Sweden, CEA in France, AECL in Canada, etc.) to carry out extensive studies of swelling clay materials for the development of engineered barriers. These materials, which have to be emplaced in underground conditions, should combine a variety of complementary properties from both the hydro-thermo-mechanical and geochemical viewpoints: impermeability, swelling ability in order to fill all void space, heat transfer and retention capacity for the most noxious radionuclides. For years, the scientific community has acknowledged the fact that smectite clays best exhibit these properties and, thus, most of the research effort has been devoted to this type of materials. The aim of such studies is to try and link the microscopic characteristics of the material (mineralogy, geochemical properties, microstructure, etc.) to its macroscopic behaviour (swelling properties, etc…).
Permeability Variation of Bentonite-Sand Mixture Due to Shear Strain
Authors: Zhang Ming , Esaki T. , Mitani Y.
Published: April 2011

The objective of this study is to investigate the permeability variation of the compacted bento-nite-sand mixture, a candidate material to be used in low level radioactive nuclear waste geotechnical disposal facility, due to shear deformation which might occur in the mixture caused by the earthquake and/or the nonuniform deformation in its long term performance. To execute the study systematically, a coupled shear and permeability testing apparatus was firstly developed in which a hollow cylindric specimen and the constant flow pump was adopted. It enabled us to measure the permeability along the shear plane in the specimen successfully and rapidly. The result of this study revealed that the permeability of the compacted bentonite-sand mixture with 15% of bentonite was not significantly influenced by the shear deformation. This has tentatively shown the effectiveness of both the new coupled shear and permeability testing apparatus and the bentonite-sand mixture to be used for retarding the radionuclide migration around the radioactive waste repository accompanying water movement.
Simultaneous Transport of Synthetic Colloids and a Nonsorbing Solute Through Single Saturated Natural Fractures
Authors: Reimus Paul W. , Robinson B. A. , Nuttall H. E. , Kale R.
Published: April 2011

Tracer transport experiments involving colloids that showed little tendency to attach to rock surfaces and a nonsorbing solute (iodide) were conducted in three different well-characterized natural fractures in tuff. The objective was to investigate the potential for using nonattaching colloids in tracer experiments to (1) obtain a measure of the dispersion in a fractured system in the absence of diffusion into the matrix or into stagnant zones along the fracture walls, and (2) provide insights into predicting colloid transport in fractures under conditions of minimal deposition.

In the experiments, the colloids always arrived earlier in the effluent than the iodide, which we believe is evidence of (1) hydrodynamic chromatography and/or (2) the fact that the colloids experience a smaller effective volume in the fracture because they diffuse too slowly to enter low-velocity regions (dead zones) along the rough fracture walls. The iodide also approached the inlet concentration in the effluent more slowly than the colloids, with the concentration at a given elution volume being greater at higher flow rates. By contrast, the rate of approach of the colloid concentration to the inlet concentration did not vary with flow rate. We attribute this behavior to matrix diffusion of the iodide, with the colloids being too large/nondiffusive to experience this phenomenon. Dispersion of all tracers was greatest in the fracture of widest average aperture and least in the fracture of narrowest aperture, which is consistent with Taylor dispersion theory.

The tracer experiments were modeled/interpreted using a three-step approach that involved (1) estimating the aperture distribution in each fracture using surface profiling techniques, (2) predicting the flow field in the fractures using a localized parallel-plate approximation, and (3) predicting tracer transport in the fractures using particle-tracking techniques. Although considered preliminary at this time, the model results were in qualitative agreement with the experiments.
Hydrologic Imaging of Fractured Rock
Authors: Karasaki Kenzi , Cohen Andrew , Cook Paul , Freifeld Barry , Grossenbacher Ken , Peterson John , Vasco Don
Published: April 2011

Various geophysical and hydrologic tests were conducted in a cluster of nine wells to image the hydrologic connections of a fractured rock mass. Results of intra-borehole flow surveys and cross-hole radar and seismic tomography surveys correlated very well, and indicated that there is a major feature at a depth of 30m. Systematic injection tests were conducted in all nine wells. Three to four intervals in each well were isolated using pneumatic packers. Each interval was equipped with a high resolution pressure transducer. Some 130 injection tests were conducted, and more than 4,100 cross-hole transient pressure measurements were obtained. A computer algorithm was developed to analyze such massive interference data systematically. As a result of the analysis, an image of the fracture connections emerged which is consistent with the geophysical data.
A Channel-Network-Model for Radionuclide Transport in Fractured Rock-Testing Against Field Data
Authors: Gylling Björn , Moreno Luis , Neretnieks Ivars
Published: April 2011

A new model concept for describing flow and transport in fractured rock has been tested on some field tracer experiments. The flowing water in the rock is envisaged to take place in a three-dimensional network of channels with stochastic properties. For times of interest for deep geological repositories for nuclear waste it has been found that the volume of the channels, i.e. the flow porosity of the rock, has no practical influence on the Residence Time Distribution of the nuclides. It is totally dominated by the matrix diffusion effects. The specific Flow Wetted Surface, FWS, of the channels therefore needs to be known. A method to evaluate the FWS from hydraulic measurements in boreholes is presented.

Tracer tests at the Swedish ÄSPÖ rock laboratory have been predicted using this model and data obtained in the way outlined in the paper. The main tracer test was predicted reasonably well without using any adjustable parameters. Some other tracers also used were not detected in the tests as they were expected to be. We discuss some of the possible reasons for this in relation to geological observations made in the expected pathways for these tracers. The sensitivity of results to uncertainties in parameter values is also discussed.
Geogas in Crystalline Bedrock and its Potential Significance for Disposal of Nuclear Waste
Authors: SjÖblom Rolf , Hermansson Hans-Peter , Åkerblom Gustav
Published: April 2011

In assessments of the safety of final repositories for nuclear waste situated in crystalline basement rock it is usually postulated that the transfer of radionuclides to the biosphere can take place only through transport by water. However, in order for such an assumption to be valid, it must be verified that any geogas that is present will not affect the transport - at least not to any significant degree. (The word geogas refers to the occurrence in the crystalline basement rock of substances which become gaseous at normal pressures and temperatures.)

Geogas in crystalline rock consists of species such as nitrogen, argon, helium, hydrogen, methane, carbon monoxide, carbon dioxide, hydrogen sulfide, and oxygen. The gas originates from the atmosphere, chemical reactions in the rock, the decay of radioactive elements in the rock, and degassing from the mantle of the earth. In most observed cases, geogas is dissolved in the groundwater. However, at elevated pressures and at low temperatures, methane may combine with water to form a solid phase commonly called methane-ice.

The transfer of geogas through the rock and to the surface takes place through flow in fractures. Firstly, dissolved geogas migrates due to the flow of the groundwater, and secondly, pockets of gas may form and eventually be released in the form of bursts. In the second case, the gas might act as a carrier for heavy elements through four different mechanisms: 1) formation of volatile compounds, 2) formation of surface active complexes, 3) flotation, and 4) formation of aerosols.

When a potential site for waste disposal is being evaluated, studies of geogas should form part of such a characterization programme. Favourable conditions for the formation of free gas may develop as a result of the heating of the rock by radioactive decay in the waste. It is also conceivable that methane-ice might form in the backfill of a repository in connection with a glaciation. The decomposition behaviour of such methane-ice appears to be largely unknown. Positive aspects may include the possibility of utilizing geogas flow for the non-destructive monitoring of a site after closure of the repository.
Common Aspects of the PNC and Nagra Assessments of Deep Repositories for Vitrified HLW
Authors: Smith P.A. , Umeki H. , Neall F. , McKinley I.G.
Published: April 2011

The repository concepts developed by PNC and Nagra for the disposal of vitrified high-level waste show many common features; both concepts involve deep geological disposal, with massive engineered barriers of similar design. PNC and Nagra have recently published comprehensive performance assessments based on their repository concepts and these are compared in order to gain assurance that the conceptual and mathematical models employed are state-of-the-art and to build confidence in the datasets in the assessment results. The predicted performance of the engineered barriers is more favourable to safety in the case of the Nagra Assessment, whereas the opposite is true of the geological barriers. These differences in predicted performance can be traced to a small number of differences in model assumptions and data. It is concluded that the differences are consistent with the somewhat different requirements of the two assessments and that the comparison enhances confidence in both assessments.
Arest-Ct: A New Chemical Transport code for Waste Disposal System Performance Analysis
Authors: McGrail B. P. , Steefel C. I. , Fort J. A. , Engel D. W. , Yabusaki S. B.
Published: April 2011

A new computer code, Analysis of RadionuclidESource-Term with Chemical Transport (AREST-CT), is described in this paper. The code is being designed to support performance assessment analyses of engineered systems for subsurface isolation of hazardous and radioactive wastes. Radionuclide releases from an engineered system are modeled by solving governing equations describing conservation of water mass, air mass, thermal energy, and chemical species mass. As such, the AREST-CT code will be capable of simulating radionuclide release and transport in a non-isothermal, unsaturated-saturated setting. Constituitive equations are implemented that describe corrosion of iron-based container materials, glass, and spent fuel waste forms. The governing equations are solved in a two-dimensional domain using an integrated finite-volume method. A third-order total variation diminishing (TVD) numerical scheme is evaluated to minimize numerical oscillations and dissipation of steep concentration gradients in advection-dominated transport problems.
Gas Generation and Migration from Radioactive Waste Repositories
Authors: Agg P. J. , Nash P. J. , Rodwell W. R. , Lineham T. R.
Published: April 2011

During the post-closure phase of a repository for low- and intermediate-level radioactive wastes significant quantities of gas will be generated, primarily by the anaerobic corrosion of metals and the degradation of organic wastes. A description is given of recent work carried out within the Nirex Safety Assessment Research Programme to address the generation of gas within a repository and the migration of gas from the repository to the biosphere. Theoretical modelling capabilities have been developed to address both of these issues. In order to gain confidence in such theoretical models, it is important to validate model predictions against observed experimental data. Preliminary experiments carried out to help validate the modelling approaches are described.
Measurement of Gap and Grain-Boundary Inventories of 129I Used in Candu Fuels
Authors: Stroes-Gascoyne S. , Moir D. L. , Kolar M. , Porth R. J. , McConnell J. L. , Kerr A. H.
Published: April 2011

Combined gap and grain-boundary inventories of 129I in 14 used CANDU fuel elements were measured by crushing and simultaneously leaching fuel segments for 4 h in a solution containing KI carrier. From analogy with previous work a near one-to-one correlation vas anticipated between the amount of stable Xe and the amount of 129I in the combined gap and grain-boundary regions of the fuel. However, the results showed that such a correlation was only apparent for low linear power rating (LLPR) fuels with an average linear power rating of 642 kV/m. For high linear power rating (HLPR) fuels (>44 kw/m), the 129I values were considerably smaller than expected. The combined gap and grain-boundary inventories of 129I in the 14 fuels tested varied from 1.8 to 11.01, with an average value of 3.6 ± 2.4% which suggests that the average value of 8.1 ± 1% used in safety assessment calculations overestimates the instant release fraction for 129I. Segments of used CANDU fuels were leached for 92 d (samples taken at 5, 28 and 92 d) to determine the kinetics of 129I release. Results could be fitted tentatively to half-order reaction kinetics, implying that 129I release is a diffusion-controlled process for LLPR fuels, and also for HLPR fuels, once the gap inventory has been leached. However, more data are needed over longer leaching periods to gain more understanding of the processes that control grain-boundary release of 129I from used CANDU fuel.
The Use of Repassivation Potential in Predicting The Performance of High-Level Nuclear Waste Container Materials
Authors: Sridhar Narasi , Dunn Darrell , Cragnolino Gustavo
Published: April 2011

Localized corrosion in aqueous environments forms an important bounding condition for the performance assessment of high-level waste (HLW) container materials. A predictive methodology using repassivation potential is examined in this paper. It is shown, based on long-term (continuing for over 11 months) testing of alloy 825, that repassivation potential of deep pits or crevices is a conservative and robust parameter for the prediction of localized corrosion. In contrast, initiation potentials measured by short-term tests are non-conservative and highly sensitive to several surface and environmental factors. Corrosion data from various field tests and plant equipment performance are analyzed in terms of the applicability of repassivation potential. The applicability of repassivation potential for predicting the occurrence of stress corrosion cracking (SCC) and intergranular corrosion in chloride containing environments is also examined.
Chemical Durability of Copper Canisters Under Crystalline Bedrock Repository Conditions
Authors: Sjöblom Rolf , Hermansson Hans-Peter , Amcoff Örjan
Published: April 2011

In the Swedish waste management programme, the copper canister is expected to provide containment of the radionuclides for a very long time, perhaps millions of years. The purpose of the present paper, is to analyse prerequisites for assessments of corrosion lifetimes for copper canisters.

The analysis is based on compilations of literature from the following areas: chemical literature on copper and copper corrosion, mineralogical literature with emphasis on the stability of copper in near surface environments, and chemical and mineralogical literature with emphasis on the stabilities and thermodynamics of species and phases that may exist in a repository environment.

Three main types of situations are identified: (1) under oxidizing and low chloride conditions, passivating oxide type of layers may form on the copper surface; (2) under oxidizing and high chloride conditions, the species formed may all be dissolved; and (3) under reducing conditions, non-passivating sulfide type layers may form on the copper surface.

Considerable variability and uncertainty exists regarding the chemical environment for the canister, especially in certain scenarios. Thus, the mechanisms for corrosion can be expected to differ greatly for different situations. The lifetime of a thick-walled copper canister subjected to general corrosion appears to be long for most reasonable chemistries. (It is assumed that the canister has no defects from manufacturing and that the bentonite buffer is intact). Localized corrosion may appear for types (1) and (3) above but the mechanisms are widely different in character. The penetration caused by localized corrosion can be expected to be very sensitive to details in the chemistry.
Nuclear Waste Repositories in Crystalline Rock- an Overview of Flow and Nuclide Transport Mechanisms
Authors: Neretnieks Ivars
Published: April 2011

Nuclides eventually escaping from nuclear waste repositories in crystalline rock will move with the seeping water in the fracture network. Most important nuclides interact physically or chemically with the rock and are expected to be considerably retarded allowing them to decay to insignificant concentrations. Velocity variations may allow some portions of the nuclides to move faster. Matrix diffusion and sorption on the surfaces of the rock are by far the most powerful retardation mechanisms and depend, in addition to the sorption and diffusion properties, directly on the magnitude of the “flow wetted surface”which is the contact surface between the mobile water carrying the nuclides and the fracture surfaces over which the nuclides diffuse into the matrix.

A number of field experiments have been performed over the last 15 years to help validate the concepts and models and to obtain data. A number of such experiments are described and discussed in relation to the above issues.
Modelling Radionuclide Transport in Cracks Through Cemented Radioactive Waste
Authors: Chambers A. V.
Published: April 2011

It is currently proposed to site the UK repository for intermediate-level radioactive waste (ILW) underground at a depth of several hundred metres, and to use a cementitious backfill to fill spaces between disposal packages within the repository vaults. Flow of water through the repository could be concentrated within cracks that may form in the backfill. This paper describes the CRACK and CRACK2 computer programs. These are used to predict radionuclide release via cracks, and radionuclide distributions within the vault. The programs are applied to an assessment of the release of inventory- and solubility-limited radionuclides from hypothetical ILW vaults. Calculations are reported for different densities of cracking, and for different repository designs, and the dominant modes of transport within cracks in repository vaults are discussed.
Modeling of the Association of Metal Ions with Heterogeneous Environmental Sorbents
Authors: Westall John C.
Published: April 2011

The use of mechanistic (surface-complexation, electric-double layer) and semi-empirical (affinity spectrum) models for representation of the association of metal ions with heterogeneous environmental materials, such as humic acids and soil particle surfaces, is compared. It is seen that mechanistic models are not nearly as mechanistic as one generally assumes, and that semi-empirical models are much more valuable than one might assume by comparison to simple Kd, models. A semi-empirical discrete-log-K-spectrum model was used to describe the binding of Co(II), as a function of pH and NaClO4 concentration, to two environmental substrates: leonardite humic acid and a kaolinitic subsoil. Excellent agreement of the model and the data was obtained over a wide range of solution composition. These models appear to be the most promising among several alternatives for modeling interactions of metal ions with complex heterogeneous environmental materials over a wide range of solution composition.
Etching and Charging Effects on Dose in Plasma Immersion Ion Implantation
Authors: Shao Jiqun , Corporation Eaton , Qin Shu , Zhao Zhuofan , Chan Chung
Published: April 2011

A general relation between the implanted dose and the processing time for plasma immersion ion implantation (PHI) can be established through the dynamic sheath model. In practice, etching and charging effects have to be taken into account in PIII dose estimation.

Plasma immersion ion implantation (PII) has been tested in fabrication of semiconductor devices with shallow junctions and in hydrogénation of poly-Si thin film transistors (TFT). PIII doping is more suitable than conventional implantation for such applications because of its high dose rate at lower energy. Since the dose rate in PIII does not depend on the area being treated, the effective current will be higher if a larger implanted area is involved. However, the relation between dose and time is not always straightforward. During PIII processing possible etching and charging will affect the total accumulated doses. This paper presents a model for each which allows a proper compensation to be performed.
Low Energy Ion Irradiation Effect on Electron Transport in Gaas/Algaas Heterostructures
Authors: Yanagisawa J. , Nozawa A. , Yuba Y. , Takaoka S. , Murase K. , Gamo K.
Published: April 2011

Effects of low energy ion beam induced damages on transport properties of a two-dimensional electron gas (2DEG) system in GaAs/AlGaAs heterostructures have been investigated. 1 keV Ar ions were irradiated on the sample surface at several ion doses (1011 - 1013 cm-2). Carrier density and electron mobility of the 2DEG formed at about 90 nm below the GaAs/AlGaAs heterostructure surface were estimated at 1.5 K by Hall resistance and longitudinal resistance measurements before and after annealing at 400°C for 10 min in an Ar gas ambient. The temperature dependence of those values was also measured for as-grown and for 1013 cm-2 ion irradiated and subsequently annealed samples. Typical results show that carrier density and mobility are not degraded severely by Ar ion irradiation at doses of 1013 cm-2 and suggest the possibility to fabricate buried structures in GaAs/AlGaAs heterostructures using low energy Si focused ion beam (FIB) irradiation and subsequent in situ overlayer growth by MBE.
keV- and MeV- Ion Beam Synthesis of Buried SiC Layers in Silicon
Authors: Lindner J.K.N. , Frohnwieser A. , Rauschenbach B. , Stritzker B.
Published: April 2011

Homogenous, epitaxial buried layers of 3C-SÍC have been formed in Si(100) and Si(lll) by ion beam synthesis (IBS) using 180 keV high dose C ion implantation. It is shown that an annealing temperature of 1250 °C and annealing times of 5 to 10 h are sufficient to achieve well-defined Si/SiC/Si layer systems with abrupt interfaces. The influence of dose, annealing time and temperature on the layer formation is studied. The favourable dose is observed to be dependent on the substrate orientation. IBS using 0.8 MeV C ions resulted in a buried SiC precipitate layer of variable composition.
Ion Beam Synthesis by Tungsten-Implantation Into 6H-SIC
Authors: Weishart Hannes , Schöneich J. , Steffen H. J. , Matz W. , Skorupa W.
Published: April 2011

We studied high-dose implantation of tungsten into 6H-silicon carbide in order to synthesize a highly conductive surface layer. Implantation was performed at 200 keV at room temperature. Subsequently, the samples were annealed in two steps at 500°C and 700°C or 950°C, respectively. The influence of dose and annealing temperature on the reaction of W with SiC was investigated. Rutherford Backscattering Spectrometry (RBS), X-Ray Diffraction (XRD) and Auger Electron Spectroscopy (AES) contributed to study structure and composition of the layer as well as chemical states of the elements. During implantation sputtering became significant at a dose exceeding 1.0×1017 W+cm−2. Formation of tungsten carbide and suicide was observed already in the as-implanted state. An annealing temperature of 950°C was necessary to crystallize tungsten carbide. However, tungsten suicide remained amorphous at this temperature. Therefore, a mixture of polycrystalline tungsten carbide and amorphous tungsten suicide evolved under these conditions. The resistivity of such a layer implanted with 1.0×1017 W+ cm−2 and annealed at 950°C is 565 μΩcm.
Platinum Ion Implantation Into Single Crystal Zirconia With A Carbon Sacrificial Layer on The Surface
Authors: Cao D.X. , Chu J.W. , Pogany A.P. , Sood D.K. , Brown I.G.
Published: April 2011

Single crystal samples of (100) oriented yttria stabilized cubic zirconia are implanted with Pt ions to doses up to 6x1017 /cm2. Our previous studies showed that the retained Pt dose was too small due to the sputter limit. To overcome this problem, a C film was deposited on the surface of some samples before implantation. The implanted samples were annealed isothermally in air at 400°-1200°C, and were analysed with RBS-C and TEM. The C-layer has been found to be very effective for protecting the sample surface from sputtering and thus increasing the Pt concentration by a factor of two, up to 33 mol.% Pt. The as-implanted Pt was non-substitutional. After annealing at 1200°C for 1 h, Pt diffused to large depths, and showed a substitutional fraction of about 23%. The detailed TEM measurements on the microstructure and damage of the implanted layer show that after annealing at these temperatures the recovery of ion damage is incomplete. Pt atoms undergo substantial migration concurrently. The Pt at first dissolves in the matrix (though not substitutionally), diffuses and precipitates again at high temperature.
Damage and Lattice Strain in Ion-Irradiated AlxGai-xAs
Authors: Partyka P. , Averback R.S. , Forbes D.V. , Coleman J.J. , Ehrhart P. , Jager W.
Published: April 2011

Radiation-induced damage and strain in AlxGai-xAs (x=5 to 1) were investigated by measurements of the lattice parameter using x-ray diffraction. Irradiations employed MeV C, Ar and Au ion beams with a substrate temperature of 80 K. For samples with high Al content, the out-of-plane lattice parameter increased with fluence at low doses, saturated, and then decreased to nearly its original value. The in-plane lattice parameter did not change, throughout. These results were independent of the irradiation particle when scaled by damage energy. For the Al.5Ga.5As samples, however, the out-of-plane lattice parameter increased monotonically with dose to large strains until the layer amorpnized. Selected samples were examined by high resolution and conventional transmission electron microscopy (TEM). Channeling Rutherford backscattering spectrometry (CRBS) was also employed to monitor the buildup of damage in many samples. Recovery of the lattice parameter during subsequent thermal annealing was also investigated.
Ion Beam Synthesis of Silicon Carbide : Infra-Red and RBS Studies
Authors: Simon L. , Mesu A. , Grob J. J. , Heiser T. , Balladore J. L.
Published: April 2011

We report on p-SiC thin layer synthesis by carbon ion implantation at high temperature. Infra-red and RBS analysis were performed on samples implanted at temperatures ranging from 200 to 900°C and for carbon doses varying in the range 1017to2.1018 cm . RBS analysis does not reveal any diffusion or segregation of carbon up to 900°C. At this temperature we obtained the optimum Infra-red signature. The (3-SiC formation is shown to be a thermally activated process with an energy of 0.1 eV leading us to speculate that the diffusion of point defects could be the limiting factor of the process.