bluuebunny/crossref_metadata_2025_split huggingface dataset:
Geogas in Crystalline Bedrock and its Potential Significance for Disposal of Nuclear Waste
Authors: SjÖblom Rolf , Hermansson Hans-Peter , Åkerblom Gustav 
Published: April 2011

AbstractIn 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

AbstractThe 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

AbstractA 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

AbstractDuring 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

AbstractCombined 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

AbstractLocalized 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

AbstractIn 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

AbstractNuclides 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

AbstractIt 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

AbstractThe 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

AbstractA 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

AbstractEffects 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

AbstractHomogenous, 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

AbstractWe 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

AbstractSingle 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

AbstractRadiation-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

AbstractWe 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.
Defect Trapping and Precipitation Processes During Annealing of Cu and Au Implanted Si
Authors: Wong-Leung J. , Nygren E. , Williams J. S. , Eaglesham D. J. 
Published: April 2011

AbstractThe behaviour of metallic impurities in Si is an issue of primary concern in advanced device processing. This paper addresses the annealing behaviour of Cu and Au in Si when nanocavities are present within the substrate as potential gettering and precipitation sites. In-situ, ‘hot’ Rutherford backscattering has been used to study metal accumulation to cavities at the annealing temperature. Transmission electron microscopy has been used to study the final microstructure of the Au-Si and Cu-Si system. A range of interesting phenomena have been observed, including very efficient transient gettering of both Cu and Au to cavities at the annealing temperature and precipitation of second phases at cavities. Dissolution of accumulated metal from defect bands into solution occurs for long-time annealing. This behaviour is discussed in terms of supersaturated solid solubility processes, diffusion, defect-trapping and precipitation of Cu and Au in Si.
Coherent V2O3 Precipitates in ɑ-Al2O3 Co-Implanted With Vanadium and Oxygen
Authors: Gea Laurence A. , Boatner L. A. , Rankin Janet , Budai J. D. 
Published: April 2011

AbstractThe oxides of vanadium VO2 and V2O3 are of fundamental and practical interest since they undergo structural phase transitions during which large variations in their optical and electronic properties are observed. In the present work, we report the formation of buried precipitates of V2O3 in sapphire by ion implantation and thermal annealing. It was found that the co-implantation of oxygen and vanadium was required in order to form nanophase V2O3 precipitates. Additionally, these precipitates, which formed only following an anneal of the co-implanted sample under reducing conditions, are coherent with the sapphire lattice. Two epitaxial relationships were observed: (0001)V2O3//(0001) ɑ-Al2O3 and (11-20)V2O3//(0001) ɑ-Al2O3. This finding is in agreement with results obtained elsewhere for thin films of V2O3 deposited on c-axis-oriented sapphire.