bluuebunny/crossref_metadata_2025 huggingface dataset:
Processing of InP and GaAs Surfaces by Hydrogen and Oxygen Plasmas: In Situ Real Time Ellipsometric Monitoring
Authors: Losurdo M. , Capezzuto P. , Bruno G.
Published: September 2012

Remote radiofrequency H2 and O2 plasma processing of InP and GaAs surfaces was investigated by in situ real time spectroscopic ellipsometry. Hydrogen plasmas were used for the native oxide removal and the defect passivation of III-V surfaces. The effect of hydrogen exposure time and of crystallographic orientation (GaAs (100), (110), (111)) on the chemistry and kinetics of oxygen removal and of phosphorus/arsenic depletion was investigated. Oxygen plasma anodization was used to grow oxide films on GaAs (100), (110) and (111) substrates. The effect of bias voltage and UV-light irradiation on the chemistry and kinetics of oxidation process and on the oxide properties was studied. The composition and morphology of the InP and GaAs surfaces resulting from these plasma treatments was described.
Solid Phase Crystallization of LPCVD Amorphous Si Films by Nucleation Interface Control
Authors: Hwang Eui-Hoon , Ro Jae-Sang
Published: September 2012

A novel method for the fabrication of poly-Si films with a large grain size is reported using solid phase crystallization (SPC) of LPCVD amorphous Si films by nucleation interface control. The reference films used in this study were 1000 Ǻ -thick a-Si films deposited at 500°C at a total pressure of 0.35 Torr using Si2H6/He. Since the deposition condition changes the incubation time, i.e. nucleation rate, and since nucleation occurs dominantly at a-Si/SiO2 interface, we devised the following deposition techniques for the first time in order to obtain the larger gain size. A very thin a-Si layer (~ 50 Ǻ) with the deposition conditions having long incubation time is grown first and then the reference films (~ 950 Ǻ) are grown successively. Various composite films with different combinations were tested. The crystallization kinetics of composite films was observed to be determined by the deposition conditions of a thin a-Si layer at the a-Si/SiO2 interface. Nucleation interface was also observed to be modified by interrupted gas supply resulting in the enhancement of the grain size.
Microstructure and Interfacial Properties of Laterally Oxidized AlxGa1-xAs
Authors: Twesten R.D. , Follstaedt D. M. , Choquette K. D.
Published: September 2012

The oxidation of high Al content AlxGa1-xAs has received much attention due to its use in oxide-aperture, vertical-cavity surface emitting lasers (VCSELs) and for passivating AlAs against environmental degradation. We have recently identified the spinel, gamma phase of Al2O3 in layers laterally oxidized in steam at 450°C for =0.98 & 0.92 and have seen evidence for an amorphous precursor to the gamma phase. At the interface with the unoxidized AlxGa1-xAs , an ~17nm amorphous phase remains which could account for the excellent electrical properties of oxide-confined VCSELs and help reduce stress concentrations at the oxide terminus.
The effect of processing conditions on the structure of buried interfaces between silicon and silicon dioxide
Authors: Chen Xidong , Murray Gibson J.
Published: September 2012

A transmission electron microscope technique is used to image atomic steps at buried interfaces between silicon and silicon dioxide. We have studied the effect of processing conditions on the interfacial structure of Si/SiO2 with this technique. We observed a dramatic effect of post-oxidation annealing on silicon (100). Roughening of Si(111) interfaces due to chemical preparation is also reported.
Improvement of Ultrathin Oxides by Post-Oxidation Annealing
Authors: Sakoda Tomoyuki , Matsumura Mieko , Nishioka Yasushiro
Published: September 2012

The low temperature oxidation is effective for the atomically-controlled gate oxide growth. We focused the effects of post-oxidation annealing (POA) and attempted to improve the properties of the low-temperature-grown ultrathin oxides with a thickness of 3nm by POA. POA abruptly reduced the leakage current at a low gate voltage below 1.5V and the interface trap density. The correlation between the interface trap and the leakage current at a low applied voltage region were confirmed. We found that the stressing immunity of the ultrathin oxides grown at a low temperature, 650°C, is drastically improved by POA at 850°C.
Charge Trapping and Degradation of High Permittivity TiO2 Dielectric Metal-Oxide-Semiconductor Field Effect Transistors
Authors: Kim Hyeon-Saeg , Campbell S.A. , Gilmer D.C. , Polla D.L.
Published: September 2012

Suitable replacement materials for ultrathin SiO2 in deeply scaled MOSFETs such as lattice polarizable films, which have much higher permittivities than SiO2, have bandgaps of only 3.0 to 4.0 eV. Due to these small bandgaps, the reliability of these films as a gate insulator is a serious concern. Ramped voltage, time dependent dielectric breakdown, and capacitance-voltage measurements were done on 190Å layers of TiO2 which were deposited through the metal-organic chemical vapor deposition of titanium tetrakis-isopropoxide. Measurements of the high and low frequency capacitance indicate that virtually no interface states are created during constant current injection stress. The increase in leakage upon electrical stress suggests that uncharged, near-interface states may be created in the TiO2 film near the SiO2 interfacial layer that allow a tunneling current component at low bias.
Field emission through diamond/Mo interfaces
Authors: Choi W.B. , Myers A.F. , Cuomo J.J. , Hren J.J.
Published: September 2012

The influence of interfacial nanostructure on electron emission from diamond coated Mo emitters is presented. Diamond coating is known to increase electron emissivity, but interfacial oxides and an amorphous layer change the magnitude. After annealing, emissivity was enhanced further and Mo2C was formed at the interface, as well as a decrease in the oxides and amorphous layers. The shape of the FEM image changed but no individual emitting sites were observed. Possible mechanisms causing these changes are discussed.
Segregation of Copper to (100) and (111) Silicon Surfaces in Equilibrium with Internal Cu3Si Precipitates
Authors: Wampler W. R.
Published: September 2012

The energetics of copper segregation to silicon surfaces were examined by measuring the Cu coverage after equilibration between Cu on the surface and internal Cu3Si, for which the Cu chemical potential is known. For oxide-free surfaces the Cu coverage was close to one monolayer on (111) surfaces but was much smaller on (100) surfaces. The Cu coverage was greatly reduced by oxide passivation of the surface. LEED showed the 7×7 structure of the clean (111) silicon surface converted to a quasiperiodic 5×5 structure after equilibrating with Cu3Si. The 2×1 LEED patterns for (100) surfaces indicated no change in surface structure due to the Cu3Si. These results show that the free energy of copper in Cu3Si is higher than that of copper on (111) surfaces but lower than that of copper on (100) surfaces.
Surface and Interface Analysis of Thin-Film/Si(Substrate) Contacts by Sxes
Authors: Heck C. , Kusaka M. , Hirai M. , Nakamura H. , Iwami M. , Watabe H.
Published: September 2012

A soft X-ray emission spectroscopy(SXES) study under an energetic electron irradiation has been applied to a nondestructive buried interface analysis of a thin-film(e.g., Cr)/Si(substrate) contact system, where the energy of primary electrons, Ep , is less than 20keV. An interesting point of this method is that we can have a specific signal for an element to be used as a finger print, otherwise it is difficult. By using this e-beam excited SXES, we can study an interface buried deep in a rather thick overlayer, e.g., more than a hundred of nm, which is due to the fact that a mean free path of a soft X-ray or an X-ray production depth is much larger than the mean free path of an energetic electron in solids. Electronic structural study of silicides by SXES is also shown.
Alloying Behavior and Reliabilty of Pt Embedded Metal/n+-GaAs Thin Ohmic Contact System
Authors: Kim C.Y. , Lee W.S. , Kwon H.J. , Jeong Y.W. , Lee J.S. , Whang C.N.
Published: September 2012

Pt embedded ohmic contacts to n+-GaAs (AuGe-800 Å/ Ni-150 Å/Pt-200 Å/Au-500 Å and AuGe-800 Å/Pt-200 Å/Ni-150 Å/Au-500 Å/n+-GaAs) have been developed for the advanced discrete devices and MMIC (monolithic microwave integrated circuit) applications. The specific contact resistance investigated by Transmission Line Method is 1x10-6 Ω cm2. Ohmic contact reliability investigated by thermal storage test at 300 °C under N2 ambient demonstrated nearly the same contact characteristics after 3000 hours. In both systems, X-ray diffraction results and Auger depth profiles show that the good ohmic contact is related to the formation of Au7Ga2, PtAs2, and Ni19Gen12 phases. AuGa compound enhances the creation of Ga vacancies, allowing incorporation of Ge into Ga sites, and PtAs compound is piled up in the middle of AuGa layer to suppress As outdifrusion from GaAs substrate. TEM cross-sectional view indicates that metal/n+-GaAs reaction layer is ∼ 1200 Å beneath GaAs. Surface and interface are very smooth and abrupt in comparison to conventional AuGe/Ni/Au contact.
Cl2 Plasma Etching of Si(100): Surface Chemistry and Damage
Authors: Layadi N. , Donnelly V. M. , Lee J. T. C.
Published: September 2012

The interaction of a Cl2 plasma with a Si(100) surface has been investigated by angle resolved x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. From XPS, it was found that the amount of chlorine incorporated at the Si surface increases with ion energy. Chlorine is present as SiClx (x = 1-3) with average relative coverages (integrated over depth) of [SiCl]:[SiCl2]:[SiCl3] ≅ 1:0.33:0.1. These relative coverages don’t depend strongly on ion energy between 40 and 280 eV. Real-time spectroscopic ellipsometry measurements showed that the layer present during etching is stable when the plasma is extinguished and the gas pumped away. In addition, the equivalent thickness of damaged silicon and silicon-chloride within the surface layer increases with ion energy.
Improvement of the Refractory Metal/n-GaAs Interface by Low Temperature Anneal
Authors: Singh A. , Velásquez L.
Published: September 2012

The W/n-GaAs Schottky junctions A and B of area 1.75×l0-2 cm2 were fabricated by deposition of W on the chemically etched polished surfaces of n-GaAs samples by rf sputtering using a rf powers of 300 Watt for 30 min. The W contact B was subjected to a 90 min. thermal anneal at 390 °C. The room temperature I-V and C-V/f (with 200 Hz < f < 1 MHz) measurements were carried out for both the as-deposited and thermally annealed W/n-GaAs Schottky junctions A and B, respectively. From the direct I-V data, the values of 1.09 and 8.1×10-8 A for the ideality factor (n) and the reverse saturation current (Io), respectively, were estimated for the diode B, compared to the values of n=1.70 and Io=6.3×10-6 A for the diode A. The observed frequency dispersion in the zero bias capacitance in the diode B was attributed to fast interface states with a time constant, τ2=6 μs and density, Nss2=5.8×1010 eV-1cm-2, whereas, both the slow interface states (with τ1=4 ms and density, Nss1=7.8×1012 eV-1cm-2) and fast states (with τ2=1 μs and density Nss2=8.6×1010 eV-1cm-2) were responsible for the observed frequency variation of the zero bias capacitance in the diode A. For the forward bias values in the range 20-100 mV, the frequency dispersion in the measured capacitance suggested the presence of both the fast and slow interface states (with time constants differing by three orders of magnitude) in the as-deposited and the heat treated W/n-GaAs interfaces. Thermal anneal at 390 °C for 90 min. lowered the density of states at the W/n-GaAs interface by two orders of magnitude and resulted in the formation of a high quality rectifying W contact to n-GaAs with a rectification ratio of 1.4×104, a low Io and an ideality factor close to unity.
Relationship between Structural and Electrical Properties of Zn-Based Contacts to p-GaAs : Towards the Mechanism of the Ohmic Contact Formation
Authors: Kamińska E. , Piotrowska A. , Kasjaniuk S. , Gierlotka S.
Published: September 2012

The relationship between electrical properties and microstructure of pure Zn and Au(Zn) contacts to p-GaAs has been studied. Thermally activated changes in ФB correlate with structural processes at MS interfaces. For Zn/GaAs contacts, lowering of ФB from 0.63 eV to 0.35 eV corresponds to the penetration of Zn into the native oxide layer. In AuZn/p-GaAs contacts, β-AuZn phase is responsible for the formation of ФB=0.4 eV in as-deposited contacts. The onset of the ohmic behaviour of Au(Zn)/p-GaAs contacts (ФB=0.3 eV) coincides with the appearance of α3-AuZn phase for Zn content less than 20 at.% or α1-AuZn, for higher Zn concentrations.

The obtained results prove that the mechanism responsible for the formation of low-resistance Zn -based contacts to p-type GaAs is associated with the lowering of the Schottky barrier at the metal/semiconductor interface. We suggest that the ultimate properties of these contacts are determined by the presence of a single, specific phase in a direct contact with the semiconductor.
Novel Metal-Semiconductor-Metal Photodetectors on Semi-Insulating Indium Phosphide
Authors: Palmer J.W. , Anderson W.A.
Published: September 2012

Depositing Pd or Au on InP at substrate temperatures near 77K has previously been found to significantly reduce the interaction between the metal and semiconductor upon formation of the interface. In this work, this technique was used to fabricate metal-semiconductor-metal photodetectors (MSMPD’s) on semi-insulating (SI) InP substrates with superior characteristics compared to detectors formed using standard room temperature (RT) metal deposition. The low-temperature (LT) metallizations were patterned using a polyimide/SiO2 lift-off mask, and a SiO antireflection coating was used to attain near-zero reflection at λ=840nm. The detectors had an active area of 200μm × 200μm, and line widths and line spacings of 3μm. Detectors having a LT-Pd/SI-InP structure had a dark current of 80nA at 5V, which was a factor of 4 lower than the dark current of conventional MSMPD’s. Additionally, LT-Pd/SI-InP MSMPD’s exhibited excellent saturation characteristics and a responsivity of 0.75 A/W. Detectors with an indium-tin-oxide (ITO)/LT-Au(200Å)/SI-InP structure had a higher responsivity of 1.0 A/W, due to the relative transparency of this metallization. In contrast, MSMPD’s with RT metallizations had poor saturation characteristics, consistent with the results of others. The difference in the illuminated characteristics of MSMPD’s with RT and LT metallizations was due to a change in the internal photoconductive gain mechanism. In RT detectors, hole trapping at interface states near the cathode dominated the gain mechanism. In LT detectors, the difference in carrier transit-times dominated.
Enhanced Photoyield with Decreasing Film Thickness on Metal-Semiconductor Structures
Authors: Hoffmann V. , Brauer M. , Schmidt M.
Published: September 2012

Experimental results of the internal quantum yield Yi associated with the internal photoemission on Au/n-Si structures are presented. The samples were prepared on Si(100) and Si(111) substrates with photoemitter layer thicknesses ranging from 5 nm to 50 nm. The Yi was measured at temperatures between 165 K and 300 K with the photoexciting energy varying from 0.72 eV to 1.07 eV. It was found that the Yi increases with decreasing Au layer thickness with a strong enhancement (40 times) in regard to the conventional Fowler theory. This experimental result is in good agreement with model calculations taking account of hot carrier scattering in the photoemitter layer. Barrier energies are larger than deduced from the Fowler plot.
Strain-Related Excitonic In-Plane Optical Anisotropy in (100) InGaAs/InAlAs/InP MQW
Authors: Dimoulas A. , Tober R. , Leavitt R. , Feng T. , Christou A.
Published: September 2012

Strained (x=0.48) and lattice-matched (x=0.53) InxGal-xAs/InAlAs/InP (100) MQWs have been investigated by photoreflectance. In the strained sample the relative intensities of the light-hole and heavy-hole excitonic transitions is different for the two different polarizations of the incident light parallel and perpendicular to the ]0-1-1 ] direction. This polarization anisotropy is explained in terms of the spontaneous formation of “quantum wires” and the presence of anisotropic strain due to spinodal-like phase decomposition of the InGaAs alloy in In-rich and Ga-rich regions.
Band Lineup of Van Der Waals-Epitaxy Interfaces
Authors: Schlaf R. , Löher T. , Lang O. , Klein A. , Pettenkofer C. , Jaegermann W.
Published: September 2012

Epitaxial lattice mismatched heterointerfaces between layered semiconductors and themselves and II-VI semiconductors (CdS, CdTe), respectively, have been prepared and their band lineup determined by photoemission. Different physical mechanisms, which govern the heterointerface formation, can be discriminated due to the specific properties of the van der Waals (vdW) surface. The interfaces between layered semiconductors mostly follow the electron affinity rule with a small but systematic deviation, which is assigned to the influence of interfacial quantum dipoles. However, the band lineup to the II-VI semiconductors shows a large interface dipole, which is related to a structural dipole from the polar, Cd terminated, face of the (111)- in case of Zinkblende CdTe- and the (0001)- in case of Wurtzite CdS- oriented overlayer film.
Contactless Electroreflectance Study of InxGa1-xAs/InP Multiple Quantum Well Structures Including the Observation of Surface/Interface Electric Fields
Authors: Malikova L.V. , Wan J.Z. , Pollak Fred H. , Simmons J.G. , Thompson D.A.
Published: September 2012

Contactless electroreflectance measurements at 300 K were performed on two InxGal-xAs/InP ]x = 0.53 (lattice-matched) and 0.75] samples containing three quantum wells (QWs) grown by gas-source molecular beam epitaxy. The spectra consisted of two excitonic transitions (le-l hh and le-l lh), corresponding to the fundamental conduction to heavy (h)- and light(l)- hole transitions, respectively, in the QW portion and a complicated Franz-Keldysh oscillation (FKO) pattern originating in the InP regions. Comparison between the experimental energies of le-l hh/le-llh and a theoretical envelope function calculation (including the effect of strain) made it possible to evaluate the conduction band offset parameters Qc =0.34+0.03 and 0.57+0.03 for x = 0.53 and 0.75, respectively. The InP related FKO beat patterns were analyzed by a Fourier transform method. It was found that the FKO spectra were due to the simultaneous contribution of at least three different fields (106 kV/cm, 36 kV/cm, and 23 kV/cm), which originate in the various interfaces, i.e., substrate/buffer, cap layer/surface, and buffer/QW structure. Identification of the different fields has been accomplished by comparison of the Fourier-transformed spectra before and after sulfur passivation of the structure surface.
In Situ Infrared Observation of Hydrogenation, Oxidation, And Adsorption On Silicon Surfaces in Solutions
Authors: Sugita Yoshihiro , Watanabe Satoru
Published: September 2012

Fourier transform infrared attenuated total reflection method was employed to observe Si surfaces during wet chemical treatments. We observed the time evolution of the surface chemical structure during the oxidizing of hydrogenated Si surfaces in such oxidants as ozonized water and hydrogen peroxide using (100) and (111) surfaces. We also examined the adsorption of surfactants which were introduced in an HF solution. The interaction of adsorbates at the interface and with molecules in a liquid phase was discussed based on our in situ observations.
Analysis of InP Passivated with Thiourea/Ammonia Solutions and Thin CdS Films
Authors: Dauplaise H. M. , Davis A. , Vaccaro K. , Waters W. D. , Spaziani S. M. , Martin E. A. , Lorenzo J. P.
Published: September 2012

The use of thiourea/ammonia pre-treatments on (100) InP, followed by chemical bath deposition (CBD) of CdS thin films (∼ 30 Å), with low-temperature, low-pressure chemical vapor deposited SiO2 has been shown to produce metal-insulator-semiconductor (MIS) samples with near-ideal capacitance-voltage (C-V) response. Here, we report on x-ray photoelectron spectroscopy (XPS) analysis of the near-surface of InP following pre-treatment and CdS deposition. The pre-treatment was shown by XPS to form an indium sulfide layer and effectively remove native oxides from the InP surface. The subsequent deposition of CdS on a sulfur-passivated surface forms a stable layer which protects the substrate from oxidation during SiO2 chemical vapor deposition. MIS samples prepared using the pre-treatment without CdS deposition showed improved C- V response, while samples prepared with both the pre-treatment and CdS deposition showed a dramatic reduction in the density of interface states.
Surface Oxidation Study of Silicon-Doped GaAs Wafers by Ftir Spectroscopy
Authors: Chang R.-H. , Al-Sheikhly M. , Christou A. , Varmazis C.
Published: September 2012

A surface study of Si-doped GaAs (100) oriented wafers treated with NH4OH and HC1 following exposure to fluorine containing plasma was conducted using fourier transform infrared spectroscopy (FTIR). These treatments were observed to produce various oxidation products, such as AS2O5 and GaO. Though inorganic salts, such as (NH4)3GaF6, can be formed on the Si-doped GaAs wafers during cleaning with hydrofluoric acid buffered with ammonium fluoride, the applied cleaning method which consisted of NH4OH and HCl treatments subsequent to exposure to fluorine containing plasma did not induce formation of any inorganic salts. A small amount of hydroxide group was also presented in the samples. Water molecules and ammonium hydroxide can be sources of OH which can then be incorporated interstitially into the wafer surfaces.
Ultra High Vacuum Scanning Tunneling Microscopy Observation of Multilayer Step Structure on GaAs and AlAs Vicinal Surface Grown by Metalorganic Vapor Phase Epitaxy
Authors: Ishizaki Jun-Ya , Ishizaki Yasuhiko , Fukui Takashi
Published: September 2012

We observe the atomic structures at the multilayer step region on MOVPE-grown GaAs (001) vicinal surface using ultra high vacuum scanning tunneling microscopy (UHV-STM), and clarify that (4×2) or (4×3) like reconstruction units are dominant. Oxide free AlAs surfaces grown on GaAs vicinal surface are also successfully observed by UHV-STM. The reconstruction units at the multilayer step region on AlAs surface have the same units on GaAs vicinal surface. GaAs surface has the lack of dimmer rows on the terrace region just below the multilayer step region, while AlAs surface has dimmer rows even on the terrace just below the multilayer step region. GaAs layer growth leads tothe step bunching phenomenon and AlAs surface leads to the step debunching phenomenon.
Transport from the Canister to the Biosphere: Using an Integrated Near-and Far-Field Model
Authors: Gylling B. , Romero L. , Moreno L. , Neretnieks I.
Published: September 2012

A coupled model concept which may be used for performance assessment of a nuclear repository is presented. The tool is developed by integration of two models, one near field and one far field model. A compartment model, NUCTRAN, is used to calculate the near field release from a damaged canister. The far field transport through fractured rock is simulated by using CHAN3D, based on a three-dimensional stochastic channel network concept. The near field release depends on the local hydraulic properties of the far field. The transport in the far field in turn depends on where the damaged canister(s) is located. The very large heterogeneities in the rock mass makes it necessary to study both the near field release properties and the location of release at the same time. In order to demonstrate the capabilities of the coupled model concept it is applied on a hypothetical repository located at the Hard Rock Laboratory in Äspö, Sweden. Two main items were studied; the location of a damaged canister in relation to fracture zones and the barrier function of the host rock. In the study of the near field rock as a transport barrier the effect of different tunnel excavation methods which may influence the damage level of the rock around the tunnel was addressed.
Development of a New Mechanistic Low-Level Waste Source Term Model
Authors: Yim Man-Sung
Published: September 2012

A new mechanistic low-level waste source term model was developed. Key features of this effort are: use of cumulative probability functions to describe the failures of waste containers; capability to describe diffusion controlled release which is dependent on the conditions of the waste package surroundings; consideration of the effects of gas generation on the source term, and; use of a source inventory characterization routine to provide a built-in capability for defining the distributions of radionuclides in various waste forms and streams. The model is capable of describing the diffusion of radionuclides in waste forms with the use of concentration and flux continuity boundary conditions. Release of radionuclides from various waste streams is modeled by the combination of diffusion, dissolution, and surface release. Failures of waste containers are portrayed by the use of probabilistic failure functions based on Weibull and lognormal distributions. The model for radionuclide release from waste packages is coupled with the near-field transport model to describe the effects of migration and dispersion within the disposal unit. Characteristics of the new model were evaluated through sensitivity analysis and compared with existing source term codes.
Incorporation of “Corrosion-Time” and Effects of
Corrosion-Product Spalling in Waste Package Degradation Simulation in the
Potential Repository at Yucca Mountain
Authors: Lee Joon H. , Atkins Joel E. , Dunlap Bryan
Published: September 2012

A two-layer waste package (carbon steel outer barrier and Alloy 825 inner
barrier) is specified to dispose of high-level nuclear waste at the
potential repository at Yucca Mountain. A set of improvements and more
realism have been added to a stochastic waste-package degradation model
which was developed for a recent total system performance assessment of the
potential repository [1]. The waste-package surface is divided into
“patches” to better represent the general corrosion of the carbon-steel
outer barrier. The “corrosion-time” concept is developed to represent the
corrosion of the carbon-steel outer barrier in changing exposure conditions
with time such as those expected in the potential repository. With the
patches approach and the corrosion-time concept implemented into the
waste-package degradation model, sensitivity of the waste package
degradation (failure and pitting degradation) to different threshold
spalling thicknesses of the corrosion products from the carbon-steel outer
barrier is analyzed. The results show that the waste-package pitting
degradation is sensitive to the corrosion-products spalling thickness of the
carbon-steel outer barrier. A greater pitting degradation of the waste
packages is predicted with a smaller spalling thickness. Further
understanding of the corrosion-products spalling in different exposure
conditions (i.e., water chemistry, water contact mode, etc.) and its effects
on carbon steel corrosion is needed to enhance the confidence in the
waste-package performance modeling in the potential repository.
Nuclear Repository Performance Assessment: Insights into
Critical Models and Parameters Affecting Projected Future
Doses
Authors: Mcguire R. K. , Kessler J. H. , Vlasity J. A.
Published: September 2012

The Phase 3 Total System Performance Assessment (TSPA) sponsored by the
Electric Power Research Institute (EPRI) has led to new insights into
critical models and parameters affecting estimated doses to humans from a
potential repository of high-level radioactive wastes at Yucca Mountain,
Nevada. The Phase 3 model has been extended to encompass time-varying
climate and infiltration, detailed modeling of the source term and
hydrology, and detailed specification of possible interaction between
percolating ground water and waste containers. The model estimates doses to
a time of one million years.

The three key radionuclides contributing to estimated total doses are
Tc-99,1–129, and Np-237. Five other nuclides contributing to dose in lesser
(but significant) amounts are U-233, Th-229, Pa-231, Ac-227, and Se-79.
These results are consistent with other TSPAs.

From sensitivity studies, the most critical models and parameters are as
follows. Infiltration and percolation assumptions, including the amount of
lateral diversion of infiltration water, are important and need verification
with site data and/or more detailed modeling. Parameters of the unsaturated
zone (UZ) and saturated zone (SZ) determine dilution and delay of
concentrations and peak doses downstream. The fraction of containers that
become wet are not critical in our model, but this lack of sensitivity
reflects our coupling of the fraction with a model of focused flow past the
containers; an different model might indicate higher sensitivity. Also, the
degree of coupling between fracture and matrix flow is important in
affecting the times of peak doses but not their magnitudes.

Other critical design assumptions that could lead to reduced and/or delayed
doses are a more robust container design, a capillary barrier around each
container, the dilution during hydrologie transport from the repository to a
potential agricultural community downstream, and the characteristics of an
“average” individual in that community who might receive a dose.
Maximum Individual Dose and Vicinity-Average Dose for a Geologic
Repository
Authors: Pigford T. H. , Zwahlen E. D.
Published: September 2012

Recent proposals for a new U.S. standard for high-level waste disposal would
limit the average dose to individuals in the vicinity surrounding a geologic
repository. This would be a new approach to protecting the public from
environmental releases of radioactivity. Heretofore, criteria adopted for
geologic disposal have limited the reasonable maximum exposure to a future
hypothetical individual. Here we present quantitative analyses of the
relation between maximum exposure and vicinity-average exposure, resulting
from future human use of ground water contaminated by radioactive releases
from a repository.

Estimating the vicinity-average exposure would require postulates and
guesses of location and habits of future people. Exposure probabilities
postulated by others show that proposed dose limit to the vicinity-average
individual would be a far more lenient standard than the traditional dose
limit to reasonably maximally exposed individuals. The proposed
vicinity-average dose limit would allow far greater concentrations of
contaminants in ground water than would be allowed by normal standards of
ground water protection. A safety standard that limits vicinity-average
exposure should also include limits on maximum exposure.
Thermal/Hydrological Modeling of the Radioactive Scrap and Waste
Facility (RSWF) with the Tough2 (Transport of Unsaturated Groundwater and
Heat) Code
Authors: Esh D. W. , Benedict R. W.
Published: September 2012

Thermal/hydrological modeling of the Radioactive Scrap and Waste Facility
(RSWF) has been completed with the TOUGH2 (Transport of Unsaturated
Groundwater and Heat) Code.[1] The RSWF will be utilized as an interim
storage facility for ceramic and metallic waste forms developed from the
electrometallurgical treatment of spent nuclear fuel. The RSWF is an array
of 1,350 carbon steel liners located at grade near Argonne National
Laboratory-West on the Idaho National Engineering Laboratory (INEL).

The primary driving force for this modeling research was to assess thermal
capacity limits for RSWF liners so that heat generating materials can be
safely stored. Maximum wasteform temperatures will be governed by both the
amount of heat the system can dissipate and the orientation and
characteristics of the wasteform. The focus of this report is on the amount
of heat the interim storage system can safely dissipate. The effect of the
temporal variation of soil moisture on the performance of the RSWF is
assessed. The facility was analyzed to determine the maximum allowable
thermal loading of the RSWF liners.
An Analysis of Bias in Groundwater Modelling Due to the
Interpretation of Site Characterization Data
Authors: Clark K. J. , Ikeda T. , Impey M. D. , McEwen T. , White M.
Published: September 2012

Bias is a difference between model and reality. Bias can be introduced at
any stage of the modelling process during a site characterisation or
performance assessment programme. It is desirable to understand such bias so
as to be able to optimally design and interpret a site characterisation
programme. The objective of this study was to examine the source and effect
of bias due to the assumptions modellers have to make because reality cannot
be fully characterised in the prediction of ground-water fluxes. A
well-defined synthetic “reality” was therefore constructed for this study. A
limited subset of these data were independently interpreted and used to
compute groundwater fluxes across specified boundaries in a cross section.
The modelling results were compared to the “true” solutions derived using
the full dataset. This study clarified and identified the large number of
assumptions and judgements which have to be made when modelling a limited
site characterisation dataset. It is concluded that bias is introduced at
each modelling stage, and that it is not necessarily detectable by the
modellers even if multiple runs with varied parameter values are
undertaken.
Thermal Analysis of Options for Spent Fuel Disposal in
Switzerland
Authors: Sasaki Tetsuo , Ando Kenichi , Amura Hideki Kaw , Schneider Jürg W. , McKinley Ian G.
Published: September 2012

In parallel to studies of disposal of vitrified high-level waste from
reprocessing, projects have been initiated to examine options for direct
disposal of spent fuel in Switzerland. The basic concept involves in-tunnel
emplacement of encapsulated spent fuel in a deep repository which is
backfilled with compacted bentonite. Two possible host rocks are considered
- crystalline basement and Opalinus Clay. This paper reports the results of
a thermal analysis which was carried out to evaluate constraints on
repository layout set by the desire to limit temperatures experienced by the
bentonite backfill.
Summary: Panel Discussion on the Role of Performance Assessment
for Radioactive Waste Management
Authors: Triay I. R. , Apted M. J.
Published: September 2012

The role of performance assessment was discussed by a group of panelists and
the participants of the 20th MRS symposium on the “Scientific Basis for
Nuclear Waste Management.” Panel members were Professor Thomas Pigford, Dr.
Alan Cooper, and Dr. Patrik Sellin; Dr. Michael J. Apted served as
moderator. For discussion purposes, “performance assessment” (PA) was
defined as the analysis of the release of radionuclides from a
repository system of barriers to the accessible environment.
Reconciliation of Experimental and Modelling Concepts in a
Natural Analogue of Radionuclide Migration
Authors: Suksi J. , Rasilainen K.
Published: September 2012

One of the major problems in testing radionuclide transport models with a
natural analogue is the potential discrepancy between experimental and
modelling concepts. In the matrix diffusion studies at Palmottu the measured
U-series concentration profiles and mathematical simulations showed
disagreement, suggesting discrepancy in respective concepts of radionuclide
attachment on the rock pores. Here, the discrepancy was approached by
studying uranium fixation in more detail within the most loosely-bound
fraction.
On the Accuracy of Uranium-Series Dating: a Comparison with a
Known Matrix Diffusion Case
Authors: Rasilainen K. , Suksi J.
Published: September 2012

The accuracy of uranium-series dating was studied quantitatively using the
natural long-lived decay chains 4n+2 and 4n+3. An unusually well-bounded
matrix diffusion case was used as the reference for USD simulations. The
simulations applied the traditional closed and open system models, as well
as detailed multistage models that incorporated the known accumulation
history of the decay chain members. The results show clearly the improved
accuracy and consistency in dating with detailed mass flow information.
The Crystal Structure of Ianthinite, a Mixed-Valence Uranium Oxide Hydrate
Authors: Burns Peter C. , Finck Robert J. , Hawthorne Frank C. , Miller Mark L. , Ewing Rodney C.
Published: September 2012

Ianthinite, [U4+2(UO2)4O6(OH)4(H2O)4](H2O)5, is the only known uranyl oxide hydrate mineral that contains U4+, and it has been proposed that ianthinite may be an important Pu4+-bearing phase during the oxidative dissolution of spent nuclear fuel. The crystal structure of ianthinite, orthorhombic,a7.178(2),b11.473(2),c.30.39(1) Å,V2502.7 Å3,Z= 4, space groupP21cn, has been solved by direct methods and refined by least-squares methods to anRindex of 9.7 % and awRindex of 12.6 % using 888 unique observed [ | F | ≥ 5σ | F | ] reflections. The structure contains both U6+and U4+. The U6+cations are present as roughly linear (U6+O2)2+uranyl ions (Ur) that are in turn coordinated by five O2-and OH located at the equatorial positions of pentagonal bipyramids. The U4+cations are coordinated by O2-, OH and H2O in a distorted octahedral arrangement. TheUrφ5and U4+φ6(φ: O2-, OH, H2O) polyhedra link by sharing edges to form two symmetrically distinct sheets atzz ≈ 0.0 and z ≈ 0.25 that are parallel to (001). The sheets have the β-U3O8sheet anion-topology. There are five symmetrically distinct H2O groups located at z ≈ 0.125 between the sheets of Uφnpolyhedra, and the sheets of Uφnpolyhedra are linked together only by hydrogen bonding to the intersheet H2O groups. The crystal-chemical requirements of U4+and Pu4+are very similar, indicating that extensive Pu4+↔ U4+substitution can occur within the sheets of Uφnpolyhedra in the structure of ianthinite.
Proliferation Resistance of Borosilicate Glass as a Host Form
for Weapons-Grade Plutonium
Authors: Cerefice G. S. , Wenzel K. W.
Published: September 2012

To examine the proliferation resistance of borosilicate glass, a process to
extract and recover a plutonium analog (thorium) from borosilicate glass was
developed and examined. The glass matrix examined was a modified standard
frit consisting of the ARM-1 frit (with simulated fission products) loaded
with 2 wt. % thorium (as an analog for plutonium) and 2 wt. % each of three
rare earth elements (Gd, Sm, Eu), which were added for criticality control
and to possibly increase the proliferation resistance of the glass matrix.
The plutonium analog was extracted from the crushed glass with a nitric acid
dissolution process, and subsequently decontaminated using a solvent
extraction process. The acid dissolution process was able to extract 88.4 ±
6.8 % of the plutonium surrogate from the glass host form. The bench top
solvent extraction process was 30.2 ± 10.9 % efficient in recovering the
plutonium analog as a purified product. Overall, this process was able to
extract 26.7 ± 9.9 % of the plutonium analog from the glass as a purified
product. To quantify the proliferation resistance of borosilicate glass as a
host form for weapons-grade plutonium, MCNP was used to determine the
compressed critical mass of a plutonium alloy with the same composition as
the product of the extraction process. For the average product composition,
the compressed critical mass was 4.7 kg of material. On average, one
compressed critical mass could be recovered from 613 kg of borosilicate
glass (2 wt. % Pu loading).
Characterization of a Plutonium-Bearing Zirconolite-Rich Synroc
Authors: Buck E. C. , Ebbinghaus B. , Bakel A. J. , Bates J. K.
Published: September 2012

A titanate-based ceramic waste form, rich in phases structurally related to zirconolite (CaZrTi2O7), is being developed as a possible method for immobilizing excess plutonium from dismantled nuclear weapons. As part of this program, Lawrence Livermore National Laboratory (LLNL) produced several ceramics that were then characterized at Argonne National Laboratory (ANL). The plutonium-loaded ceramic was found to contain a Pu-Gd zirconolite phase but also contained plutonium titanates, Gd-polymignyte, and a series of other phases. In addition, much of the Pu was remained as PuO2-x. The Pu oxidation state in the zirconolite was determined to be mainly Pu4+, although some Pu3+was believed to be present.
Characterization and Leaching Behavior of Plutonium-Bearing
Synroc-C
Authors: Smith Katherine L. , Lumpkin Gregory R. , Blackford Mark G. , Hambley Michelle , Arthur Day R. , Hart Kaye P. , Jostsons Adam
Published: September 2012

Synroc-C containing 10wt% simulated PW-4b-D HLW including 0.62 wt% 239Pu was
subjected to MCC-1 type leach tests at 70°C in deionised water, silicate and
carbonate leachates for 53 d and deionised water for 2472 d. The normalised
total (i.e. unfiltered leachate + vessel wall) Pu leach rates in deionised
water, silicate and carbonate leachates for periods up to 53 d were found to
be of the order of 10-5, 10-4 and 10-4 g
m-2 d-1 respectively. After 2472 d, the
differential, normalised, Pu leach rate in deionised water dropped to ∼5 ×
10-6 (total) and ∼5 × 10-8 (solution - after
filtration through a 1000NMW filter) g m-2 d-1. SEM
and AEM were used to characterise our starting material and investigate the
secondary phases on the surfaces of leached Synroc-C discs. Calculated and
measured normalised Pu leach rates are compared and the partitioning of Pu
between zirconolite and perovskite is discussed.
Underground Criticality in Geologic Disposal
Authors: Choi J-S , Pigford T. H.
Published: September 2012

Disposition of weapons fissile material as once-through MOx fuel in reactors
or immobilization in waste glass would result in end products requiring
geologic disposal. Similar considerations apply to some spent fuels in a
geologic repository. The criticality potential of these end products in a
geologic setting is an important consideration for the final disposal of
these materials. In this document we present calculations that help define
parameters and configurations for criticality, including those that, if the
criticality is achieved, could lead to autocatalytic power transients. It is
not the purpose of this paper to define scenarios that could reasonably lead
to criticality or to establish the probabilities that critical
configurations could occur. We do suggest some parameters that should be
considered by waste package designers and repository analysts to ensure that
criticality cannot occur.
Interaction of UO2 and Zircaloy During the Chernobyl
Accident
Authors: Ushakov S. V. , Burakov B. E. , Shabalev S. I. , Anderson E. B.
Published: September 2012

A summary of the results collected during the studies of the products of a
chemical interaction between uranium oxide fuel and Zircaloy cladding in the
Chernobyl accident is presented in this paper. The reaction products are
mainly Zr-U-containing phases with different U/Zr ratio and are described on
the basis of electron microprobe and X-ray diffraction (XRD) analyses. The
Zr-U-bearing phases were discovered among the inclusions in different types
of Chernobyl fuel-containing masses (”lava”) inside the destroyed 4th Unit
and in hot particles collected up to 12 km from the 4th Unit along the West
Plume. A correlation of data on the chemical composition and phase
interrelations obtained in investigated samples with a phase diagram of
Zr(O) - UO2 shows, that a temperature >1900 °C was reached in
a part of the core before the explosion. The detection of hot particle with
segregated morphology points out that liquid immiscibility existed between
U-rich and Zr-rich melts. This and other observations indicate that the core
temperature locally was above 2400–2600°C.
Stability of Mineral Matter in Aqueous Media of the Chernobyl
Unit-4 Shelter: Thermodynamic Evaluation
Authors: Sinitsyn V. A. , Kulik D. A. , Skhodorivski M. , Kurepin V. A. , Abramis A. Y. , Kolyabina I. L. , Shurpach N. A.
Published: September 2012

A special geochemical environment exists within the Shelter (”Sarcophagus”)
erected in 1986 over the destroyed Unit-4 of Chernobyl nuclear power plant
(NPP). Based upon the available in situ and compositional
data, thermodynamic models of solid-aqueous interactions were developed to
clarify the leaching behaviour of various materials within the Shelter. The
“Selektor-A” code, based on a convex programming approach to Gibbs free
energy minimization, was used for the calculations. A built-in flexible
hybrid thermodynamic database for the system
Na-K-Ca-Mg-Cl-S-N-H-O-Si-P-Fe-Al-Sr-Cs was extended with the critically
selected and matched parameters for aqueous species and solid phases in the
U-Zr-Si-O-H subsystem, secondary U-minerals, mineral phases of fully
hydrated Portland cements and U-bearing zircons. Modeling results show that
the “Shelter waters” can selectively leach a significant quantity of U and
Si from the fuel-containing masses, while Zr, Fe, Ca, Mg and some other
components are rather insoluble. Serpentinite, assemblages of fully-hydrated
phases of Portland cements, and oxidation products of steel structural
elements are estimated to be sufficiently stable in the aqueous environment
of the Shelter. Our calculations also define some feasible pathways for
secondary mineral formation from evaporation of Shelter water solutions and
interactions between these waters with the mineral matter inside the
Shelter.
Modeling the Migration of 99Sr and 137Cs
in the Subsurface from the Chernobyl NPP Unit-4 Shelter
Authors: Kivva S. L. , Stelya O. B. , Proskura N. I.
Published: September 2012

The Shelter constructed above the destroyed Unit-4 of the Chernobyl NPP
contains 20 MCi of nuclear fuel. More than 1000 m3 of
intermediate-level radioactive water is disposed in its basement. The
purpose of this work was to simulate migration of the radionuclides
90Sr and 137Cs from the Shelter into the subsurface
environment to evaluate their migration rate and migration paths. A
mathematical model accounting for the coupled transport of water and
radionuclides in variably saturated media was used. The lack of suitable
experimental and field studies excluded the possibility of complete
validating the model. Results of the simulations will be useful for future
field studies.
Optimization of Savannah River M-Area Mixed Waste for
Vitrification
Authors: Fu Sabrina S. , Gan Hao , Muller Isabelle S. , Pegg Ian L. , Macedo Pedro B.
Published: September 2012

Vitrification studies of actual Savannah River M-Area mixed wastes have
shown that the limiting factor for high waste loading of this waste stream
is its chemical durability as defined by the toxicity characteristics
leaching procedure (TCLP). As part of the optimization study of Savannah
River M-Area wastes, a number of additives were examined including
Na2O, Li2O, B2O3,
ZrO2, and TiO2. This paper reports on the effect of
varying the boron to total alkali ratio and on the effect of substitutions
such as ZrO2 for waste and TiO2 for SiO2 on
the chemical durability and processability of M-Area waste glasses.