bluuebunny/crossref_metadata_2025 huggingface dataset:
Molecular Statics of a Ni/Zr Heterophase Interface
Authors: Fernandes M. G. , Pontikis V. , Bristowe P.D.
Published: March 2011

An atomistic model for a Ni/Zr hetero-interface formed between closed packed atomic planes is constructed and simulated using energy minimization techniques. The results show how misfit dislocations are introduced in the system and form an hexagonal array, and that the strain field in Ni can reach values two times larger than those in Zr. Calculated values for antisite defect energies are positive and are a strong function of position at the interface, reflecting the role of the dislocation network.
Boron and Hydrogen in NI3AL: Part II. Mechanical Testing of Bicrystals
Authors: Johnson Patricia E. , Gourdin W. , Gonis A. , Kioussis N. , Vaudin M.
Published: March 2011

To provide a sensitive measurement of the effect of boron segregation on the strength and ductility of Ni3A1 grain boundaries, bicrystal tensile tests were performed on small specimens of boron doped Ni76A124 cut from extremely large-grained boules. Five specimens with the same “random” or low-symmetry grain boundary (disorientations measured by means of backscattered Kikuchi patterns) and two specimens with a second random grain boundary were tested in quenched and slow-cooled conditions. Duplicate tests performed in a low (7 ppm) water-vapor environment showed that the fracture mode and the stress and strain at fracture are altered by environmental embrittlement at individual, partially strengthened grain boundaries.
Defect-Solute Interactions Near Irradiated Grain Boundaries
Authors: Simonen E. P. , Vetrano J. S. , Heinisch H. L. , Bruemmer S. M.
Published: March 2011

Defect-solute interactions control radiation-induced segregation (RIS) to interfacial sinks, such as grain boundaries, in metallic materials. The best studied system in this regard has been austenitic stainless steels. Measurements of grain boundary composition indicate that RIS of major alloying elements is in reasonable agreement with inverse-Kirkendall predictions. The steep and narrow composition profiles are shown to result from limited back diffusion near the boundary. Subsequently, defect-solute interactions that affect the near-boundary defect concentrations strongly affect RIS. The variability in measured RIS may in part be caused by grain boundary characteristics.
Bias Annealing Effect on Hydrogen-Containing Au/N-Silicon Schottky Barrier
Authors: Yuan M.H. , Jia Y.Q. , Qin G.G.
Published: March 2011

Au/n-Si Schottky barrier (SB) incorporated by hydrogen has a 0.13 eV lower SB height (SBH) than that without hydrogen incorporation. For the hydrogen-containing SB, zero bias annealing (ZBA) decreases the SBH while reverse bias annealing (RBA) increases it. Besides, the ZBA and RBA cycling experiments reveal a reversible change of the SBH with in at least three cycles. The higher annealing temperature of RBA results in higher SBH. We interpret the above experimental facts as that hydrogen has an effect on metal-semiconductor interface states and then on the SBH, and both the bias on SB and temperature of annealing can influence the hydrogen effects on metal-semiconductor interface states.
The Nature of Oxygen-Related Polytypoids in the Aluminum Nitride-Aluminum Oxide System
Authors: Youngman R.A.
Published: March 2011

Previous investigations into the nature of polytypoid structures in the A1N-A12O3 and A1NSiO2 systems have concluded that these structures are comprised of ordered stacking faults which accommodate oxygen (and silicon) in the basic wurtzite (2H) AIN structure. The polytypoids are distinct chemical phases intimately related to the pure 2H AIN. More recent work in low oxygen content (<6 at.%) A1N has elucidated the evolution of oxygen-related point defects, and transformation of these defects into extended structures. These studies have shown that all oxygenrelated extended defects in the A1N-A12O3 system are inversion domain boundaries (IDBs).

We present here extensions of the concepts developed from low oxygen content studies which lead to direct application in understanding the polytypoid structures. High resolution electron microscopy (HREM), specific electron diffraction experiments, and structural models are utilized to prove that the polytypoid structures are not based on stacking faults, but, in fact are based on IDBs.
An Investigation on Interconnect Via Failure
Authors: Zhao Joe W. , Doherty John , Whitefield Bruce
Published: March 2011

The formations of insulating material and voids at the interface between metal 1 and metal 2 in interconnect vias in Al-1%Si-0.25Cu% alloy has been investigated. A thin layer between metal 1 and 2 interface was identified using Transmission Electron Microscopy(TEM). The material was analyzed using TEM with Energy Dispersive X-ray spectroscopy(EDX) confirming that the insulating layer was Silicon. It is believed that the via failure was due to the formation of silicon precipitate at the interface and the coalescence of voids after High Temperature Storage(HTS) test for a long time period. The formation of Si precipitate and the growth of voids at the interface of aluminum alloy in interconnect via is dependent on the grain structure. The formation of voids at via interface is related to thermal treatment position in backend process. A change of thermal treatment position has been made, and no via failure has been found after HTS test for 2000 hours. A model is proposed for the mechanism of the via open failure.
Interfaces in Silicon Carbide Whisker and Carbon Fiber Reinforced Calcium Phosphate Composites
Authors: Park K. , Sung C. , Sundaresan S. , Vasilos T.
Published: March 2011

The microstructural and chemical characteristics of silicon carbide whisker/calcium phosphate and carbon fiber/calcium phosphate composites were studied using analytical transmission electron microscopy. The interface at these calcium phosphate-based composites was studied, with particular attention to finding evidence of a chemical reaction at the interface. Structural observations made on these composites explain the fact that their mechanical properties may be strongly affected by the structural properties.
The use of thin Film Substrates to Study Enhanced Solid-State Phase Transformations
Authors: Kotula PAUL G. , Erickson Dwight D. , Carter C. Barry
Published: March 2011

A thin-film substrate geometry is described for the study of enhanced or seeded solid-state phase transformations. As an example of this approach, thin films of hematite have been used as substrates for the study of the seeded phase transformation of a boehmite-derived transition-alumina to α-A12O3. The hematite films were grown on bulk (0001) α-A12O3 single crystal substrates by pulsed-laser ablation. A layer of a boehmite sol was then spin-coated onto the thin film. The assemblages were then heated to 950°C, or 1000°C in order to induce the phase transformation. Specimens were imaged in cross section by transmission electron microscopy. No transformation was observed for specimens heated to 950°C. In specimens heated to 1000°C, the transition alumina was found to transform to alpha-alumina, starting at the surface of the hematite film, via solid-state heteroepitaxy. In this case, islands, growing out from the hematite film into the transition alumina layer, were observed.
Observation of the Phase-Boundary Controlled Formation of NiA12O4 From a Single-Crystal NiO Thin Film and a Single-Crystal α-A12O3 Substrate
Authors: Kotula Paul G. , Carter C. Barry
Published: March 2011

High-quality NiO thin films have been grown on single-crystal α-A12O3 substrates ((0001) orientation) by pulsed-laser ablation, forming essentially the idealized solid-state reaction geometry-a single-crystal film in intimate contact with a single-crystal substrate. These reaction couples have been characterized by cross-section transmission electron microscopy and scanning electron microscopy both before and after being heated in air to induce the solid-state reaction (i.e., Ni-spinel formation). The NiO films consisted of two twin variants which were found to conform to the underlying substrate surface steps. The substrate surface steps were produced by heat-treating the substrates prior to thin film deposition. Using this reaction geometry, it has been found that the initial reaction of the spinel takes place where twin boundaries in the NiO films meet the substrate. The initial reaction corresponds to the nucleation of the spinel. This interpretation is supported by the fact that the reaction proceeded faster up the NiO twin boundaries than elsewhere along the reaction layer (i.e., nucleation of the spinel is easier at twin boundaries in the NiO film). Scanning electron microscopy has been used with the present thin-film reaction geometry to measure reaction layer width along interfaces up to 2 mm long.
Tem Analysis of Interfacial Reactions Between TiWn, Wn Gate Metallizations and GaAs in Mesfet Devices
Authors: Jones K.S. , Robinson H.G. , Jasper C. , Cronin W. , Durlam M.
Published: March 2011

In an effort to improve the uniformity of the threshold voltage, the Schottky barrier height, and the ideality factor, Ti0.29W0.52N0.19 and W0.81N0.19 gate metal depositions have been investigated as a function of annealing conditions for GaAs based MESFET devices. Crosssectional TEM samples were made of each device. The results of these studies indicate there is a systematic and significant reaction occurring between the gate metal and the GaAs upon 850°C and 900°C rapid thermal annealing. These reactions take different forms depending on whether Ti is present or not. If Ti is absent (i.e. WN gates) then the interfacial roughness between the gate and the GaAs is less than 30Å indicating the metallization is very unreactive. The WN contacts for some gates show void formation indicative of GaAs decomposition also for some devices an amorphous layer is observed at the interface. Selected area diffraction patterns indicate only the alpha-W and beta-W2N phases are present. For the TiWN gates the interface roughness is as large as 200Å upon 900°C 10 second RTA. However no voids or interfacial amorphous layers were observed. Again only alpha-W and beta-W2N were observed in the bulk of the gates. For both systems, beta-W2N appears to form at the interface however the morphology of the beta-W2N grains are much larger for the TiWN gates. Electrical results indicate the TiWN gates have a lower ideality factor (near 1.1) and greater uniformity across the wafer compared to the WN gates. It is proposed that the presence of Ti in the gate metal aids in reducing any surface oxides thus improving the ideality and uniformity of the gate metal/GaAs contact.
Waste Glass Corrosion Modeling: Comparison with Experimental Results
Authors: Bourcier William. L.
Published: March 2011

Models for borosilicate glass dissolution must account for the processes of (1) kinetically-controlled network dissolution, (2) precipitation of secondary phases, (3) ion exchange, (4) rate-limiting diffusive transport of silica through a hydrous surface reaction layer, and (5) specific glass surface interactions with dissolved cations and anions. Current long-term corrosion models for borosilicate glass employ a rate equation consistent with transition state theory embodied in a geochemical reaction-path modeling program that calculates aqueous phase speciation and mineral precipitation/dissolution. These models are currently under development. Future experimental and modeling work to better quantify the rate-controlling processes and validate these models are necessary before the models can be used in repository performance assessment calculations.
Science and Licensing: Let's Get off the Collision Course
Authors: Van Konynenburg R. A.
Published: March 2011

The best possibility for gaining an understanding of the likely future behavior of a high level nuclear waste disposal system is use of the scientific method. However, the scientific approach has inherent limitations when it comes to making long-term predictions with confidence. This paper examines these limiting factors as well as the criteria for admissibility of scientific evidence in the legal arena, and concludes that the prospects are doubtful for successful licensing of a potential repository under the regulations that were binding until recently. Suggestions are made for remedying this situation.
Demonstration of Safety for Geologic Disposal
Authors: Taylor Edward C. , Ramspott Lawrence D. , Sprecher William M.
Published: March 2011

The U. S. Department of Energy (DOE) is developing a nuclear waste management system that will accept high-level radioactive waste, transport it, store it, and ultimately emplace it in a deep geologic repository. The key activity now is determining whether Yucca Mountain, Nevada is suitable as a site for the repository. If so, the crucial technological advance will be the demonstration that disposal of nuclear waste will be safe for thousands of years after closure. This paper assesses the impact of regulatory developments, legal developments, and scientific developments on such a demonstration.
Room Closure Response to Gas Generation and Mechanical Strength of Different Waste Forms in a Bedded Salt Repository
Authors: Mendenhall F. T. , Stone C.M.
Published: March 2011

Finite element calculations of the porosity history of a nuclear waste disposal room for transuranic (TRU) waste in a bedded salt formation have been completed. The analyses include a reduced elastic/secondary creep model for the host halite and a nonlinear consolidation model for the crushed salt backfill. Separate gas generation and constitutive models were used for three distinct hypothetical waste forms: 1) unaltered defense-related, contact-handled transuranic (CH-TRU) waste, 2) shredded and cemented CH-TRU waste, and 3) incinerated and vitrified CH-TRU waste. Histories were determined for a 2000 year time period starting from the emplacement of room seals. The resulting final room porosities varied from roughly 55% to less than 10%, according to waste forms.
Zirconolite-Rich Titanate Ceramics for High-Level Actinide Wastes
Authors: Vance Eric R. , Angel P.J. , Begg B.D. , Day R.A.
Published: March 2011

New diffraction and microstructural data on the incorporation of rare earths, Np and Pu in nominally single-phase zirconolite (CaZrTi2O7) are presented. Such data are vital for the crystal-chemical design of zirconolite-rich titanate ceramics of the Synroc family, for the immobilization of up to 30 wt% of actinide wastes from advanced reprocessing/partitioning schemes. Some trial zirconolite-rich ceramics were made with U and Nd simulating tetravalent and trivalent actinides respectively. Some preliminary results to examine chemical flexibility with regards to variations in waste/precursor are given, for different actinide waste compositions.
Production of Synroc Ceramics from Titanate Gel Microspheres
Authors: Sizgek Erden , Bartlett J.R. , Woolfrey J.L. , Vance E.R.
Published: March 2011

Synroc is a multi-component titanate ceramic, designed to immobilise High Level Waste (HLW) from nuclear fuel reprocessing plants [1]. Synroc precursor powders have been previously produced by various methods, such as oxide and alkoxide-hydrolysis routes [2]. However, various technological aspects of HLW processing make the use of free-flowing, dustfree, highly sinterable precursor powders desirable. Such powders have been produced by spray-drying colloidal precursors, yielding microspherical particles with controlled porosity.
Electrochemical Characterization of Grouted Radioactive Waste.
Authors: Kahanda G. L. M. K. S. , Kruger A. A. , Gu J. , Shen W. M. , Tomkiewicz M.
Published: March 2011

We are investigating the possibility of monitoring the degree of immobilization of the low level radioactive waste by embedding an array of long lasting electrodes in grout. This work describes our ongoing attempts to understand the physics and chemistry of charge carrier in the grout under various load conditions.
Mortar-Radwaste Mixture Form Characterization Due to 137Cs Leach-Rate Using Accelerated Ageing Processes
Authors: D.Perić Aleksandar , B.Plećaš Ilija , S.Pavlović Radojko , D.Pavlović Snežana
Published: March 2011

Series of mortar-radwaste mixture samples, orthocylinder shaped, with the external surface completely open to contact with leachant medium, were investigated in experiments that tried to characterize long-term physico-chemical and mechanical properties of the matrix material. After curing period of 28 days in the atmosphere of defined parameters, one group of the samples was treated on leaching in the distilled water for a one year period, to establish the 137Cs cumulative leached fraction. The second group of the samples of the same batch as the first one, after one year curing period , was investigated under temperature stress conditions. The values of the temperature climaxes were: Tmin=-20°C and Tmax=+70°C. Samples were periodically immersed in distilled water at the room temperature, after each freezing and heating treatment. The experimental goal was to bring mortar-radwaste mixture forms, during a short period of experimental time, into the state that has nearly the same physico-chemical properties, mainly radionuclide leach-rate, as the first sample group has, which was treated in the normal one-year leaching process. Data obtained from these investigations, using accelerated ageing processes, do not correspond completely to the appropriate values that could define the state of the samples treated excusively in the leachant medium. Nevertheless, accelerated ageing processes are suitable for the relatively fast estimation of immobilizing properties of mortar matrices, and their behaviour in the extended experimental time.
A Study of Fission Product Migration and Selective Leaching by Means of a Power-Bump Test.
Authors: Forsyth Roy.S. , Eklund Ulla-Britt , Werme Lars O
Published: March 2011

As part of a programme to identify and quantify the mechanisms controlling the aqueous corrosion of spent fuel,a short fuel rod was refabricated from a full-length rod from the Swedish Ringhals 1 BWR (burnup 44-48 MWd/kgU). After a conditioning period at 25 kW/m,the rod was power-bumped in Studsvik Nuclear´s R2 test reactor up to a maximum linear heat rating of 43 kW/m,with a hold time at power of 3 hours,imposing steeper radial temperature gradients on the fuel pellets. A few hours after termination of irradiation,the rod was shipped to Studsvik Nuclear´s Hot Cell Laboratory,where the pellet inventories of gamma-emitting fission products were determined non-destructively.Two 20mm long fuel/clad sections were cut from the rod at positions corresponding to power-bump LHR values of 43 and 33 kW/m.Two similar sections were cut from the original rod as references,together with a shorter section for destructive inventory analysis.

The fuel/clad sections were leached in 200ml of synthetic groundwater (l23ppm HCO3) under oxic conditions for consecutive periods of 7,7,14 and 14 days.The samples arising from the tests were analysed by conventional radiometric and radiochemical methods,and also using an ICP-MS (Inductively Coupled Plasma Mass Spectrometer) instrument commissioned for active use.

Fractional release data for both radioactive and stable fission products has been obtained. Since the main aim of the test was the study of the early stages of corrosion,expected to be dominated by migrational effects in the fuel during irradiation,particular attention has been paid to Cs,I,Rb,Sr, Ba,Mo and Tc. Although the migration and selective leaching of Cs and I are well-established,it is shown that there is also appreciable leaching of Rb nuclides,whereas the results show no significant Sr migration.This represents support for the hypothesis that the apparent enhanced leach rate of Sr-90 observed experimentally can be due to migration of Rb-90 precursors.
Initial Results from Dissolution Testing of Various Air-Oxidized Spent Fuels
Authors: Gray W. J. , Thomas L. E.
Published: March 2011

Flowthrough dissolution tests have been conducted on two different light-water-reactor spent fuels oxidized to U4O9+x or U3O8. Oxidation had a bigger impact on the dissolution of U and a smaller impact on the dissolution of Tc from the fuel with higher burnup and higher fission gas release. Possible reasons for the observed differences in test results are discussed, but clarification awaits results from tests on other fuels, which are in progress.
Waste Glass Weathering
Authors: Bates John K. , Buck Edgar C.
Published: March 2011

The weathering of glass is reviewed by examining processes that affect the reaction of commercial, historical, natural, and nuclear waste glass under conditions of contact with humid air and slowly dripping water, which may lead to immersion in nearly static solution. Radionuclide release data from weathered glass under conditions that may exist in an unsaturated environment are presented and compared to release under standard leaching conditions. While the comparison between the release under weathering and leaching conditions is not exact, due to variability of reaction in humid air, evidence is presented of radionuclide release under a variety of conditions. These results suggest that both the amount and form of radionuclide release can be affected by the weathering of glass.
Chemical Durability of High Burnup Lwr-Spent Fuel in Concentrated Salt Solutions
Authors: Loida A. , Grambow B. , Dressier P. , Friese K. , Geckeis H. , Karlsruhe Kernforschungszentrum
Published: March 2011

High-burnup (<50 MWd/kgU) spent fuel samples of various sizes were exposed to NaCl solutions under static, anaerobic and reducing conditions. The accumulated corrosion time was about 200 days. Gas phase and leach solutions were analyzed. By dissolving mm sized fragments in large volumes of solution, saturation effects were avoided and upper limits for intrinsic dissolution rates of about 5-20 mg/(m2d) were measured. Surface area normalized reaction rates were significantly lower when using fine grained fuel powder (estimated sample surface area to solution volume ratio S/V ca. 3000 m-1), indicating saturation effects. The maximum concentrations of Pu and Am in the tests are close to reported solubility limited concentrations in pure 5m NaCl solutions in the absence of radiolysis effects. The presence of iron effectively reduces the solution concentration of all measured radionuclides (except Cs).
Photothermal Deflection Spectroscopy Investigations of Uranium Electrochemistry - II
Authors: Rudnicki James D. , Russo Richard E.
Published: March 2011

Photothermal Deflection Spectroscopy (PDS) has been applied to the study of uranium oxide electrochemistry. PDS measures the optical absorption of the sample surface and concentration gradients formed in the electrolyte. Both of these measurements are performed in situ under dynamic conditions. The combination of these two measurements provides information that can be used to infer the mechanism of the UO2 surface chemistry. These studies of the uranium dissolution mechanism are performed in pH 10.5 sodium sulfate electrolytes at 22°C. The electrolytes are free from oxygen, and complexing species. Our results suggest that dissolution of UO2 can occur at oxidizing potentials as low as -300 mV vs. saturated calomel electrode (SCE). The optical absorption and concentration gradient results provide evidence for a substantial surface change that occurs at an oxidation potential of +300 mV. The results show that the surface layer formed by this change dissolves slowly by a non-electrochemical reaction.
Distribution of Uranium-Bearing Phases in Soils From Fernald
Authors: Buck E. C. , Brown N. R. , Dietz N. L.
Published: March 2011

Electron beam techniques have been used to characterize uranium-contaminated soils at the Fernald Site, Ohio. Uranium particulates have been deposited on the soil through chemical spills and from the operation of an incinerator plant on the site. The major uranium phases have been identified by electron microscopy as uraninite, autunite, and uranium phosphite [U(PO3)4]. Some of the uranium has undergone weathering resulting in the redistribution of uranium within the soil.
Characterization of High-Level Nuclear Waste Glass Using Magnetic Measurements
Authors: Senftle Frank E. , Thorpe Arthur N. , Grant Julius R. , Barkatt Aaron
Published: March 2011

Magnetic measurements constitute a promising method for the characterization of nuclear waste glasses in view of their simplicity and small sample weight requirements.

Initial studies of simulated high-level waste glasses show that the Curie constant is generally a useful indicator of the Fe2+:Fe3+ ratio. Glasses produced by air-cooling in large vessels show systematic deviations between experimental and calculated values, which are indicative of the presence of small amounts of crystalline iron-containing phases. Most of the iron in these phases becomes dissolved in the glass upon re-heating and more rapid quenching. The studies further show that upon leaching the glass in water some of the iron in the surface regions of the glass is converted to a form which has high temperature-independent magnetic susceptibility.
Reaction Sequences in Simulated Neutralized Current Acid Waste Slurry During Processing with Formic Acid
Authors: Smith H.D. , Wiemers K.D. , Langowski M.H. , Powell M.R. , Larson D.E.
Published: March 2011

The Hanford Waste Vitrification Plant (HWVP) is being designed for the Department of Energy to immobilize high-level and transuranic wastes as glass for permanent disposal. Pacific Northwest Laboratory is supporting the HWVP design activities by conducting laboratory-scale studies using a HWVP simulated waste slurry. Conditions which affect the slurry processing chemistry were evaluated in terms of offgas composition and peak generation rate and changes in slurry composition. A standard offgas profile defined in terms of three reaction phases, decomposition of H2CO3, destruction of NO2-1, and production of H2 and NH3 was used as a baseline against v/hich changes were evaluated. The test variables include nitrite concentration, acid neutralization capacity, temperature, and formic acid addition rate. Results to date indicate that pH is an important parameter influencing the N2O/NOx generation ratio; nitrite can both inhibit and activate rhodium as a catalyst for formic acid decomposition to CO2 and H2; and a separate reduced metal phase forms in the reducing environment. These data are being compiled to provide a basis for predicting the HWVP feed processing chemistry as a function of feed composition and operation variables, recommending criteria for chemical adjustments, and providing guidelines with respect to important control parameters to consider during routine and upset plant operation.
Surface Chemistry and Durability of Borosilicate Glass
Authors: Carroll S. A. , Bourcier W. L. , Phillips B. L.
Published: March 2011

Preliminary results show that glass durability is dependent on reactions occurring at the glass-solution interface. CSG glass (18.2 wt. % Na2O,5.97 wt. % CaO, 11.68 wt. % Al2O3, 8.43 wt. % B2O3, and 55.73 wt. % SiO2) dissolution and net surface H+ and OH- adsorption are minimal at near neutral pH. In the acid and alkaline pH regions, CSG glass dissolution rates are proportional to 2 to , respectively. In contrast, silica gel dissolution and net H+ and OH" adsorption are minimal and independent of pH in acid to neutral solutions. In the alkaline pH region, silica gel dissolution is proportional 0.9 to . Although Na adsorption is significant for CSG glass and silica gel in the alkaline pH regions, it is not clear if it enhances dissolution, or is an artifact of depolymerization of the framework bonds.
Surface Layer Effects on Waste Glass Corrosion
Authors: Feng Xiangdong
Published: March 2011

Water contact subjects waste glass to chemical attack that results in the formation of surface alteration layers. Two principal hypotheses have been advanced concerning the effect of surface alteration layers on continued glass corrosion: (1) they act as a mass transport barrier and (2) they influence the chemical affinity of the glass reaction. In general, transport barrier effects have been found to be less important than affinity effects in the corrosion of most high-level nuclear waste glasses. However, they can be important under some circumstances, for example, in a very alkaline solution, in leachants containing Mg ions, or under conditions where the matrix dissolution rate is very low. The latter suggests that physical barrier effect may affect the long-term glass dissolution rate. Surface layers influence glass reaction affinity through the effects of the altered glass and secondary phases on the solution chemistry. The reaction affinity may be controlled by various precipitates and crystalline phases, amorphous silica phases, gel layer, or all the components of the glass. The surface alteration layers influence radionuclide release mainly through colloid formation, crystalline phase incorporation, and gel layer retention. This paper reviews current understanding and uncertainties.
Effects of Container Material on PCT Leach Test Results for High-Level Nuclear Waste Glasses
Authors: Xing Shi-Ben , Pegg Ian L.
Published: March 2011

A glass-based waste form used for the immobilization of high-level nuclear wastes should exhibit good resistance to aqueous corrosion since typically this is the primary process by which radionucleides could be released into the environment upon failure of other barriers. In the USA, the Waste Acceptance Product Specifications (WAPS) provides a set of requirements to ensure the consistency of the waste forms produced and specifies the Product Consistency Test (PCT) as a measure of relative chemical durability. While the PCT procedure permits usage of both Teflon and stainless steel vessels for testing of simulated development glasses, Teflon is not permitted for testing of production glasses due to radiative degradation. The results presented in this paper indicate that there are very significant differences between tests conducted in the two types of vessels due to the well-known permeability of Teflon to atmospheric carbon dioxide which results in lowering of the solution pH and a consequent reduction in the leach rate of silicate glasses. A wide range of nuclear waste glass compositions was subjected to the PCT procedure using both Teflon and stainless steel vessels. The magnitude of the effect (up to a factor of four for B, Na, Li concentrations) depends strongly on glass composition, therefore the isolated checks performed previously were inconclusive. The permeability to CO2 of two types of Teflon vessels specified in the PCT procedure was directly measured using buffer solutions: ingress of CO2 is linear in time, strongly pH-dependent, and was as high as 100 ppm after 7 days. In actual PCT tests in Teflon vessels, the total CO2 content was 560 ppm after 87 days and 1930 ppm after one year.
Dissolution Behavior of Waste Glass Under Reducing Condition
Authors: Imakita Tsuyoshi , Sasakawa Kaoru , Matsuda Fumio , Wada Ryutaro
Published: March 2011

A corrosion test of a model waste glass was carried out with and without carbon steel, and the leaching behavior of the glass was studied under reducing condition in a glove box purged with N2. The redox potential and pH were monitored, and the concentration of Fe, Na, Si, and Mo in the leachates were measured to understand the leaching behavior of the glass. The redox potential of the leachates obtained in the corrosion test coexisting with carbon steel under reducing conditions showed initially that it was based on an Fe0/Fe2+ redox potential, and gradually it increased to that based on Fe2+/Fe3+ one. The corrosion rate of the glass under oxidizing conditions was ten times greater than that obtained with carbon steel, and that under reducing condition with carbon steel was negligible small.
Thermodynamic and Structural Models Compared with the Initial Dissolution Rates of “SON” Glass Samples
Authors: Tovena I. , Advocat T. , Ghaleb D. , Vernaz E. , Larche F.
Published: March 2011

The experimentally determined initial dissolution rate R0 of nuclear glass was correlated with thermodynamic parameters and structural parameters. The initial corrosion rates of six “R7T7” glass samples measured at 100°C in a Soxhlet device were correlated with the glass free hydration energy and the glass formation enthalpy. These correlations were then tested with a group of 26 SON glasses selected for their wide diversity of compositions. The thermodynamic models provided a satisfactory approximation of the initial dissolution rate determined under Soxhlet conditions for SON glass samples that include up to 15 wt% of boron and some alumina. Conversely, these models are inaccurate if the boron concentration exceeds 15 wt% and the glass contains no alumina. Possible correlations between R0 and structural parameters, such as the boron coordination number and the number of nonbridging oxygen atoms, were also investigated. The authors show that R0 varies inversely with the number of 4-coordinate boron atoms; conversely, the results do not substantiate published reports of a correlation between R0 and the number of nonbridging oxygen atoms.
Volcanic Glass as a Natural Analog for Borosilicate Waste Glass
Authors: Morgenstein Maury E. , Shettel Don L.
Published: March 2011

Obsidian and basaltic glass are opposite end-members of natural volcanic glass compositions. Syngenetic and diagenetic tensile failure in basaltic glass (low silica glass) is pervasive and provides abundant alteration fronts deep into the glass structure. Perlitic fracturing in obsidian (high silica glass) limits the alteration zones to an “onion skin” geometry. Borosilicate waste glass behaves similarly to the natural analog of basaltic glass (sideromelane).

During geologic time, established and tensile fracture networks form glass cells (a three-dimensional reticulated pattern) where the production of new fracture surfaces increases through time by geometric progression. This suggests that borosilicate glass monoliths will eventually become rubble. Rates of reaction appear to double for every 12C° of temperature increase. Published leach rates suggest that the entire inventory of certain radionuclides may be released during the 10,000 year regulatory time period. Steam alteration prior to liquid attack combined with pervasive deep tensile failure behavior may suggest that the glass waste form is not license defensible without a metallic- and/or ceramic-type composite barrier as an overpack.
Use of the QUasI-Stationary State Approximation to Determine the Migration of Mineral Alteration Zones at a Natural Analogue for the Disturbed Zone of a Cementitious Radioactive Waste Repository
Authors: Chambers A.V.
Published: March 2011

Calculation of the movement of chemical fronts over long timescales could be important in underpinning performance assessments for radioactive waste disposal. A quasi-stationary state model, MARQUISS (Mineral Alteration Reactions using the QUasI-Statίonary State approximation), has been developed to achieve this objective by avoiding many of the problems encountered using more conventional approaches to coupled chemistry and transport calculations. MARQUISS simulates advective, dispersive and diffusive transport through a one-dimensional porous medium coupled with the chemical kinetics of mineral precipitation and dissolution. A description of its development and verification for simple systems is provided, together with its application in a study of the migration of mineral alteration zones at a natural analogue for a cementitious radioactive waste repository located at Maqarin in northern Jordan.
Case Studies of Corrosion of Mixed Waste and Transuranic Waste Drums
Authors: Kosiewicz Stanley T.
Published: March 2011

This paper presents three case studies of corrosion of waste drums at the Los Alamos National Laboratory (LANL). Corrosion was not anticipated by the waste generators, but occurred because of subtle chemical or physical mechanisms. In one case, drums of a cemented transuranic (TRU) sludge experienced general and pitting corrosion. In the second instance, a chemical from a commercial paint stripper migrated from its primary containment drums to chemically attack overpack drums made of mild carbon steel. In the third case, drums of mixed low level waste (MLLW) soil corroded drum packaging even though the waste appeared to be dry when it was placed in the drums.

These case studies are jointly discussed as “lessons learned” to enhance awareness of subtle mechanisms that can contribute to the corrosion of radioactive waste drums during interim storage.
The Degradation of Cellulose in the Near Field of a Radioactive Waste Repository
Authors: Greenfield B.F. , Hurdus M.H. , Pilkington N.J. , Spindler M.W. , Williams S.J.
Published: March 2011

UK Nirex Ltd is seeking to develop a deep underground repository for the disposal of solid low- and intermediate-level radioactive wastes in the UK. The Nirex Safety Assessment Research Programme (NSARP) comprises scientific research to support the post-closure performance assessment of the repository. One of the investigations carried out by AEA Technology under the NSARP is the investigation of the formation of water-soluble complexants from the degradation of the solid organic polymers and cellulosic materials present in such wastes. The influence of these complexants on the solubility and sorption of a number of radionuclides is also being investigated by a combination of experimental measurements and thermodynamic modelling. Under the alkaline, anaerobic conditions representative of the near field of the repository, cellulose has previously been shown to degrade to yield soluble products which enhance the solubilities and reduce the sorption of several radionuclides. As part of this investigation, cellulose was chemically degraded under conditions similar to those expected in the near field and the leachate analysed. Separation of the products by HPLC gave direct identification of 2-C-(hydroxymethyl)-3-deoxy-D-erythro-pentonic (isosaccharinic) acid as one of the degradation products. More tentative assignments were made for glycolic, formic, lactic and acetic acids. A comparison was made with the degradation products suggested in the literature. The possible mechanisms and extent of cellulose degradation are discussed, as well as the factors affecting its degradation in the near field. The sorption of plutonium on to cement from a leachate produced under alkaline, anaerobic conditions is reported.
Effects of Recrystallization on Time Variant Sorption of Radionuclides onto Steel Corrosion Products
Authors: Smith Robert W. , Walton John C. , Rahman Masudur
Published: March 2011

Corrosion products resulting from the oxidation of steel containers and iron-containing waste can alter the local geochemical environment in radioactive waste disposal vaults and trenches. In addition, corrosion products can enhance the retention of radionuclides by providing a highly sorptive media. Initial hydrous corrosion products are poorly crystalline with large surface area, high sorption capacity, and large Kds. Over time, corrosion products recrystallize by step-wise dehydration to phases with significantly smaller surface areas. The decreased surface area results in decreased sorption and the potential for remobilization of previously sorbed radionuclides. The impact of time-dependent variation in sorption on radionuclide transport is a complex function of the rate of recrystallization and the radionuclide half-life.

A mathematical model has been developed, which coupling mass transport with corrosion product recrystallization and equilibrium sorption of radionuclides. The model is applied release of radionuclides with a range of half-lives (year to 100,000s of years) from radioactive waste disposal facilities to illustrate the significance of corrosion product recrystallization on release.
Thermodynamics of Ion-Exchange Between Na+/Sr2+ Solutions and the Zeolite Mineral Cinoptilolite
Authors: Pabalan Roberto T. , Paul Bertetti F.
Published: March 2011

Ion-exchange experiments were conducted at 25 °C between the zeolite mineral clinoptilolite and aqueous solutions of varying equivalent ratios of Na+ and Sr2+ and total concentrations of 0.005, 0.05, and 0.5 N. The experiments were designed to investigate the effects of changes in total solution concentration and in the relative concentrations of exchangeable cations on the following ion-exchange equilibrium:

Sr2+ + 2NaZ ⇄ SrZ2 + 2Na+

Using the isotherm data at 0.05 N solution concentration, a thermodynamic model for the ion-exchange reaction was derived using a Margules formulation for the activity coefficients of zeolite components and the Pitzer ion-interaction approach for activity coefficients of aqueous ions. The results of the forward experiments showed that the ion-exchange isotherm strongly depends on the total solution concentration. Additional experiments demonstrated that the above ion-exchange reaction is reversible. The derived equilibrium constant, K, and Gibbs energy of ion-exchange, ΔG°, are equal to 0.321±0.021 and 2,820±170 J/mol, respectively.

Using thermodynamic parameters derived from the 0.05 N isotherm experiment, the model was used to predict isotherm values at 0.005 and 0.5 N, which showed excellent agreement with measured data. Because the thermodynamic model used in this study can be easily extended to ternary and more complicated mixtures, it may be useful for modeling ion-exchange equilibria in multicomponent geochemical systems.
Technical Workshop on Near-Field Performance Assessment of High-Level Waste Disposal
Authors: Apted Michael , Andersson Kjell
Published: March 2011

An international Workshop was conducted to discuss the present state of modelling of near-field performance of high-level waste repositories, with emphasis on the conceptual and mathematical models for source-term calculations. The Workshop was based on an international survey and review of this topic by the authors. Technical position papers developed by the participants are summarized here. A Proceedings of the Workshop, including the full position papers and the survey and review document, are to be published by Nuclear Energy Agency by the end of 1993.
Testing Long-Term Predictions from Hydro-Geochemical Models
Authors: Glassley William E. , Bruton Carol J. , Bourcier William L.
Published: March 2011

Thermally induced flow of liquid water and water vapor at the potential repository site at Yucca Mountain, Nevada, will extend hundreds of meters away from the repository edge. The resultant transfer of heat and mass will sufficiently perturb the ambient conditions such that a variety of mineralogical and chemical reactions will occur that may modify hydrological properties. The consequences of this “coupling” of geochemical and hydrological processes will vary through time, and will occur to different degrees in four regimes (T < Tboiling; T = Tboiling; T > T boiling; cooling) that will develop within the repository block. The dominant processes in the regimes differ, and reflect the local balance between: 1) kinetics and equilibrium; 2) dissolution and precipitation; 3) evaporation and boiling; and 4) fluid flow in matrix and fractures. Simulations were conducted of the evolution of these regimes, using laboratory derived kinetics and thermodynamic data, and site specific mineralogical and hydrological properties. These simulations identify regions where chemical and mineralogical equilibrium is likely to be achieved, and where net changes in hydrological properties will be concentrated. Tests of the results of these simulations have been initiated using field data from the Taupo Volcanic Zone, New Zealand. A preliminary series of calculations suggest that relative changes in porosity of as much as ± 20% to 30% may be possible for rocks with an initial porosity of 10%.
The Modification and Application of the Geochemical Modelling Code CHEQMATE to Simulate Transport and Chemical Processes within Fractured Media
Authors: Chambers A. V. , Smith A.C.
Published: March 2011

The UK current plans for a repository for the disposal of intermediate-level radioactive waste involve the use of a cementitious backfill within the repository vaults. Radionuclide transport could be concentrated within cracks that may form in the cement, or in fractures within the rock surrounding the repository. A version of CHEQMATE (CHemical EQuilibrium with Migration And Transport Equations) that includes a representation of transport through fractured media has been developed. This paper describes modifications of the code, to allow simulation of transport through either a single fracture, or, by adopting a continuum approach to represent fracture distribution, through multiple fractures. Successful verification of the modified program against analytical solutions for transport through fractured rock and cracked cementitious media is presented. An application of the modified program to study the evolution of the aqueous chemistry within a repository vault containing a single fracture through the cementitious backfill is also discussed.
Performance Assessment Modeling of a Multi-Purpose Container
Authors: Bullen Daniel B.
Published: March 2011

A mathematical model to predict the cumulative failure distribution for the engineered barrier system employed in a deep geologic disposal facility as a function of container design and near-field environmental conditions has been developed. The model employs Weibull and exponential distributions to describe cumulative container failures as a function of time. Parameter values employed in the model are based upon simple, time-dependent, mechanistic models and relevant corrosion data, which describe failure of individual components of the container as a function of environmental conditions.

Recent developments in container design for the Yucca Mountain site center on the possible deployment of Multi-Purpose Containers (MPC). These containers will be designed and constructed to serve as transport casks, interim storage containers, and disposal containers. The current container performance assessment model is applied to evaluate the long-term performance of various MPC designs under the areal power density and heat transfer regimes expected in the Yucca Mountain environment. This model has previously been employed to describe the performance of the container as one part of a risk-based performance assessment of the Yucca Mountain site.

The relative importance of container design, areal power density, and dominant heat transfer mode on predicted MPC performance is demonstrated through comparison of the cumulative container failure distributions for each MPC design when exposed to expected Yucca Mountain environmental conditions.
Development of Modelling Criteria for Predicting Lifetimes of Titanium Nuclear Vaste Containers
Authors: Shoesmith D. W. , Ikeda B. M.
Published: March 2011

The parameters that must be known if the extent of damage due to the crevice corrosion of Grade-2 titanium waste containers is to be predicted are discussed. These parameters include the kinetics of crevice propagation as a function of temperature and oxygen concentration, the critical values of temperature and oxygen concentration for which repassivation occurs, the rate of hydrogen absorption by the metal under crevice corrosion conditions, and the threshold value of absorbed hydrogen concentration above which the material may be susceptible to hydrogen-induced cracking. The results of experiments to measure these parameters are briefly summarized, and their significance as modelling parameters or criteria are discussed.