bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature Dependence of Magneto-Mechanical Response in Ni-Mn-Ga Magnetic Shape Memory Alloys
Authors: Cheng Leon M. , Landry Garrett , Farrell hannon P. , Ham-Su Rosaura , Hyatt Calvin V.
Published: April 2011

In this work, a systematic investigation is being carried out on single crystals of Ni47.8Mn27.5Ga24.7 alloy to determine the effect of temperature on the magneto-mechanical behaviour of the Ni-Mn-Ga alloys. Repeated mechanical and magnetic forces have been applied at various temperatures below the martensite finish (MF) temperature. It has been observed that twinning start and finish stresses, critical magnetic field and maximum magnetic-field-induced strain all remain almost constant within about 20K below MF and then change substantially at lower temperatures. Eventually no magnetic-field-induced strain can be observed at temperatures below 262K. It is proposed that although magnetic anisotropy constant increases with decreasing temperature, it is not sufficient to overcome the increasing twinning stresses required for twin boundary motion at lower temperatures.
Development of Field-responsive Polymeric Magnetic Composite Fibers via Electrospinning
Authors: Wang M. , Singh H. , Hatton T.A. , Rutledge G.C.
Published: April 2011

A mathematical model is developed to predict the changing stiffness of magnetic fibers within a uniform external magnetic field. Magnetite nanoparticles 16nm in diameter were synthesized by an organic route. An ultrafine PMMA fiber containing 16nm magnetite nanoparticles was produced by electrospinning. The magnetic properties of the fibers were also characterized.
Electroactive Polymer Based Micro-ElectroMechanical System as Biosensor Platform
Authors: Li Zhimin , Li Suiqiong , Cheng Z.-Y.
Published: April 2011

A micro-electromechanical diaphragm (MEMD) as micro-sensor platform is introduced. The performance of a MEMD is compared with that of a microcantilever (MC). It is theoretically found that the sensitivity of a MEMD is about 50 times higher than that of a MC. The measured resonance frequencies in air proved the validity of the MEMD design. More importantly, the quality merit factor (Q value) of MEMD is higher than that of MC. The damping effect of liquid medium on a MEMD is much smaller than on a MC. It is experimentally demonstrated that the MEMD works well in both air and liquid.
Enhanced Strain in Electrochemical Deformation of Polypyrrole Film doped with Sulfonated Polyaniline
Authors: Tanaka Akihisa , Takashima Wataru , Kaneto Keiichi
Published: April 2011

Composite films of polypyrrole (PPy) and poly(2-methoxyaniline-5-sulfonic acid) (PMAS) have been electrodeposited from aqueous electrolytes solution consisting of pyrrole and various ratios of dodecylbenzene sulfonic acid (DBS) and PMAS. The obtained PPy(DBS/PMAS) films exhibited different characteristics in water contents, surface morphology and electrochemomechanical deformations (ECMD) depending on the content of PMAS. A film having the content of PPy(95%DBS/5%PMAS) simply showed the large cathodic ECMD with the strain of 6.1% in non-aqueous electrolyte solution of 0.1M tetrabutylammonium chloride/acetonitrile electrolyte. The improvement of cathodic ECMD is conjectured to the large porosity of the composite film. The ECMD behaviors of the films operated in various electrolyte solutions including aqueous solution are also reported.
Growth of Single-Walled Carbon Nanotubes by Microwave Plasma Enhanced Chemical Vapor Deposition
Authors: Maschmann Matthew R. , Goyal Amit , Iqbal Zafar , Fisher Timothy S. , Gat Roy
Published: April 2011

Single-walled carbon nanotubes (SWCNTs) have been grown for the first time by microwave plasma-enhanced chemical vapor deposition (PECVD) at 800°C using methane as the precursor and bimetallic Mo/Co catalyst supported on MgO dispersed on a silicon wafer. The nanotubes grown consist of bundles, each composed of individual tubes of a single diameter associated with either metallic or semiconducting SWCNTs, based on characterization by Raman spectroscopy. Field-emission scanning electron microscopy and atomic force microscopy show that the bundles are relatively thin – 5 to 10 nm in diameter – and up to a few micrometers in length. The results are compared with those obtained on recently reported SWCNTs grown by radio frequency PECVD.
Ultrafast Transient Absorption Spectroscopy Investigations of Excited State Dynamics in SWNT/Polymer Composites
Authors: Styers-Barnett David J. , Ellison Steven P. , Park Cheol , Wise Kristopher E. , Papanikolas John M.
Published: April 2011

Wavelength-resolved femtosecond transient absorption spectroscopy is used to follow the electronic dynamics of single-walled carbon nanotubes in polymers following visible and near IR photoexcitation. Electron-hole (e-h) pairs give rise to sharp features in the transient spectra that decay in amplitude and exhibit rapid spectral shifts. The decay reflects (e-h) recombination on both short (1.3 ps) and long (35 ps) time scales. Transient spectra also exhibit a broad photobleach at early times that arises from the cooling of a hot electron gas created via excitation at the red edge of a π-plasmon band.
Light Emission from Carbon Nanotubes Induced by Field Electron Emission from Oriented MWCNT Arrays Accompanied by Re-Deposition
Authors: Nanjundaswamy R. , Zakhidov Al. A , Zhang M. , Lee S. B. , Sampson W. M. , Obraztsov A. N. , Cunningham A. , Zakhidov A. A.
Published: April 2011

Well-ordered aligned arrays of multiwalled carbon nanotubes were synthesized by the catalytic thermal chemical vapor deposition (CVD) in acetylene gas at the atmospheric pressure. Abrupt spark–type light emission spots and release of the carbon nanotubes from the cathode and succeeding deposition onto the anode without decomposition has been detected above the threshold electron emission current. Spectral analysis of the light showed that the spectra consist of a background similar to blackbody radiation and a set of sharp lines identified with the radiative transitions of excited carbon and iron atoms. Light emission spectra were found to be essentially different depending on the type (AC or DC) of the applied bias voltage and its value. As-grown and re-deposited materials were characterized by Raman spectroscopy and scanning electron microscopy. It was found that the structural properties of the re-deposited carbon nanotubes remained intact, with only negligible amorphization. A mechanism of the spark emission and re-deposition processe is proposed and discussed in terms of fast overheating of CNT, after explosion type melting of Fe-catalyst nanoclusters, followed by CNT transfer from cathode to anode.
Carbon Nanotubes' and Silicon Carbide Whiskers' Growth on Metal Catalysts: Common Features of Formation Mechanisms
Authors: Kuznetsov Vladimir , Usoltseva Anna , Mazov Ilya
Published: April 2011

The formation mechanisms of carbon deposits and silicon carbide whiskers on metal surface catalysts have some common steps. The most important are: (1) the formation of metal particle alloys oversaturated with carbon or silicon and carbon atoms and (2) the nucleation of corresponding deposits on the metal catalyst surface. A thermodynamic analysis of the carbon and/or silicon carbide nucleation on the metal surface was performed. The master equations for the dependence of critical radius of carbon or SiC nucleus on reaction parameters, such as reaction temperature, supersaturation degree of catalyst particles with C (or Si and C), work of adhesion of metal to carbon (or metal to SiC), were obtained. These equations combined with the phase diagram approach can be used for the description of different scenarios of carbon and/or SiC deposits formation and for the development of the main principles of catalyst and promoters design.
Growth kinetics changes of vertically aligned carbon nanostructures synthesised at low substrate temperatures
Authors: Chen G. Y. , Poa C. H. P. , Stolojan V. , Henley S. , Silva S. R. P.
Published: April 2011

Carbon nanotubes and nanofibres are typically synthesised under substrate temperatures above 600°C. Here we investigate the influence of the substrate temperature and the plasma conditions on the growth of vertically aligned carbon nanostructures using Direct Current plasma Chemical Vapour Deposition, at temperatures below 550°C. These nanostructures are produced using a C2H2 based plasma and nickel thin film as the catalyst. We found that preferential deposition of amorphous carbon takes place as the synthesis temperature is lowered below 500°C. However, lowering the carbon concentration in the gas feedstock (<2% conc.) allows for the nucleation of nanofibre-like structures, whilst balancing the buildup of amorphous carbon. This method allows for the synthesis of vertically aligned structures at low temperatures (around 230°C) without intentional heating, while still achieving reasonable average growth rates up to 27 nm/min. The only heating was provided by the plasma, which typically consumes ∼ 4 W/cm2. It was found that by varying the applied plasma bias during high temperature synthesis, we increased the growth rate up to 165 nm/min. Based on the observations of experimental process variations and the morphology of the synthesised structures, we propose a growth mechanism for such low temperature growth and examine the resulting morphology changes.
Surface characteristics and wetting behavior of carbon nanotubes
Authors: Barber Asa H. , Liu Luqi , Cohen Sidney R. , Daniel H.
Published: April 2011

The wetting properties and surface characteristics of individual carbon nanotubes are elucidated by immersing the nanotube into various organic liquid. The resultant force acting on the nanotube can be used to evaluate a liquid contact angle at the nanotube surface from classical methods. This technique was shown to be accurate enough to discern differences in wetting behavior due to both structural and chemical changes in the nanotube structure.
Phonon Dispersions of a Single-Wall (8,0) Carbon Nanotube: Effects of the Rotational Acoustic Sum Rule and of Surface Attachment
Authors: Mounet Nicolas , Marzari Nicola
Published: April 2011

The lattice dynamics of single-walled carbon nanotubes (SWCNT) is studied from first-principles using density-functional perturbation theory (DFPT) at the GGA-PBE level. The phonon dispersions of a pristine, infinite zigzag (8,0) SWCNT are obtained and the effect of applying the rotational acoustic sum rule on vibrational properties is discussed. Finally we study the effects of covalent functionalizations on the SWCNT phonon frequencies by selectively increasing the effective mass of the carbon atoms that would link to the functional groups.
The Characteristics of Carbon Nanotubes with Electroless Plating Deposited Ni Catalysts
Authors: Huang C. S. , Huang B. R. , Lin C. Y. , Wu C. C. , Yu T. E.
Published: April 2011

We have synthesized multi-walled carbon nanotubes (MWNTs) on nickel-deposited n-type silicon substrates by thermal chemical vapor deposition (Thermal CVD). The electroless plating method was adopted in the deposition of the Ni catalytic layer. Sulfuric acid solution was used as a buffer to adjust and maintain a pH value of 4.6. Both the deposition time of the nickel catalyst layer and the growth time of MWNTs were adopted to control the length and density of MWNTs. Scanning electron microscopy (SEM) images showed that the density of carbon nanotubes increased as the deposition time of nickel catalyst layer increased. The formation of nickel nucleation becomes rich as the immersion time of the substrate in electroless plating solution was longer, and this benefited the growth of carbon nanotubes. In addition, the Raman spectrum demonstrated that the ID/IG ratio of MWNTs decreases as the deposition and growth time increase, indicating that more graphene MWNT structures were formed.
Atomic-Scale Physics and Modeling of Schottky Barrier Effect in Carbon Nanotube Nanoelectronics
Authors: Xue Yongqiang
Published: April 2011

We present an atomistic self-consistent study of the electronic and transport properties of semiconducting carbon nanotubes in contact with metal electrodes at different contact geometries. We analyze the Schottky barrier effect at the metal-nanotube interface by examining the electrostatics, the band line up and the conductance of the metal-nanotube wire-metal junction as a function of the nanotube channel length, which leads to an effective decoupling of interface and bulk effects in electron transport through nanotube junction devices.
Structure and dynamics of carbon buckyballs encapsulated into Single-Walled carbon Nanotubes
Authors: Cambedouzou Julien , Rols Stéphane , Almairac Robert , Sauvajol Jean-Louis , Kataura H. , Schober H.
Published: April 2011

The structural and dynamical properties of C60 chains inserted inside Single Walled carbon Nanotubes (SWNT) have been investigated by x-ray and neutron diffraction, and inelastic neutron scattering (INS). The measurements have been performed on a large mass of a very high quality carbon peapods sample. We showed that powder diffraction could hardly give definitive response on the way the C60s are bonded inside SWNT. However, the comparison of the neutron-derived generalized density of states (GDOS) of the inserted C60 peas with the GDOS of the same objects derived from lattice dynamics calculations led to unambiguous results. The observation of excitations in the 8–15 meV range is a clear evidence for the presence strong bonds between C60s in the sample. In the same time, the observation of a very weak quasi-elastic signal may be related to rotational motions of C60 monomers inside the SWNT. These results suggest that peapods are made of a mixed phase of C60 monomers and C60 n-mers.
Neptunium Solubility in the Near-Field Environment of a Proposed Yucca Mountain Repository
Authors: Sassani David C. , Van Luik Abraham , Summerson Jane
Published: April 2011

For representing the source-term of a proposed repository at Yucca Mountain, NV, the performance assessment (PA) approach evaluates the disequilibrium degradation of the waste forms to capture a bounding rate for radionuclide source-term availability and use solubility constraints that are more representative of longer-term, equilibrium processes to limit radionuclide mass transport from the source-term. These solubility limits capture precipitation processes occurring either as the waste forms alter, or in the near-field environment as chemical conditions evolve. A number of alternative models for solubility controls on dissolved neptunium concentrations have been evaluated. These alternatives include idealized models based on precipitation of simple, discrete neptunium phases and more complex considerations of trace amounts of neptunium being incorporated into secondary uranyl phases that form during waste form alteration. Thermodynamic constraints for neptunium under oxidizing conditions indicate that tetravalent neptunium solids (e.g., NpO2) are more stable relative to pentavalent phases (e.g., Np2O5), and therefore have solubilities that set lower dissolved concentrations of neptunyl species. Within solution, the pentavalent neptunyl ion (NpO2+) and its complexes dominate. Data on solids and solutions from slow flow-through (dripping) tests on spent nuclear fuel (SNF) grains indicate that neptunium is tetravalent in the SNF and that, over the ∼9 years duration of the tests, the dissolved neptunium concentrations are near to, or below, calculated NpO2 solubility. Currently, the NpO2 solubility model is applied at the waste forms within waste packages because the degradation of SNF and corrosion of alloys ensures active reduction reactions. The more conservative Np2O5 solubility model is applied in environments distal to the waste forms (i.e., the invert below waste packages) where reduction of dissolved pentavalent neptunium is less certain. Consideration of Np incorporation into secondary uranyl phases suggests that both of these idealized models will provide conservative estimates of neptunium release to the geosphere.
Modeling of Groundwater Flow and Radionuclide Transport in a High-Level Radioactive Waste Repository with Multiple Canisters
Authors: Lim Doo-Hyun
Published: April 2011

A two-dimensional numerical model incorporating a multiple-canister configuration and a non-uniform flow field of the host rock is developed for more reliable performance assessment of a water-saturated high-level radioactive waste repository. A space-dependent flow velocity distribution is established numerically using the finite-element method. Transport of Cs-135 is simulated using the random-walk method. Numerical results for a source condition of instantaneous-pulse-input show that (i) migration of Cs-135 is influenced significantly not only by the canister configurations but also by flow boundary conditions, ii) effect of applied hydraulic gradient direction on Cs-135 migration becomes more significant as the number of canisters increases, while the effect is negligible for a single-canister configuration, (iii) the direction of the hydraulic gradient orthogonal to the orientation of the disposal tunnel is most advantageous with respect to the isolation of the radionuclide, and (iv) the single-canister configuration yields conservative results compared with multiple-canister configurations.
Studies on Sealing Performance of Clay Plug by the Tunnel Sealing Experiment
Authors: Kawakami S. , Fujita T. , Masumoto K. , Yui M.
Published: April 2011

To establish sealing performance suitable for geological environmental conditions of Japan, it is necessary to obtain basic data related for the sealing function through laboratory and in-situ experiments. These data are to be used for practical design and to establish the method for analysis of the sealing performance.

Japan Nuclear Cycle Development Institute (JNC) has joined the international project, the Tunnel Sealing Experiment (TSX), to demonstrate the sealing performance of full scale plugs in-situ, and to develop analytical method of the sealing performance of the plugs at Underground Research Laboratory (URL) of Atomic Energy of Canada Limited (AECL) in Canada [1]. This experiment is in the final stage and fundamental data for the sealing function of clay and concrete plugs have been obtained.

This paper presents the total data handling and interpretation of the sealing experiment, as well as the numerical analysis and interpretation for clay plug performance based on tracer test results. As a result of the numerical analysis, the sealing performance of the clay plug was confirmed and it was shown that the excavation disturbed zones (EDZ) around the clay plug and the tunnel could be a dominant transport pathway for radionuclides.
IAEA's Coordinated Research Programmes on Performance of High Level Wastes in Deep Geological Repositories
Authors: Gonzalez J.L. , Iseghem P. Van
Published: April 2011

The main conclusions of a Coordinated Research Programmes organized by the International Atomic Energy Agency on the performance of high-level wastes in deep geological repositories are summarized in this paper. The programme ran from 1997 till 2004. Glass, spent fuel and ceramics were the waste forms considered. The 13 participating countries reported their R&D including waste form development, basic understanding of the waste form properties and the performance in simulated disposal conditions. Significant progress has been achieved on the various issues. Recommendations are formulated as to integrate the R&D with the geological disposal conditions and performance assessment considerations, or to conceive generic studies in preparation of the above approach.
Effects of the Orientation of Clay Particles and Ionic Strength on Diffusion and Activation Enthalpies of I−and Cs+Ions in Compacted Bentonite (II)
Authors: Sato Haruo
Published: April 2011

The apparent diffusivities (Da) and activation enthalpies (ΔEa) for I−and Cs+ions in compacted Na-smectite were obtained in the parallel and perpendicular directions to the orientation of smectite particles as a function of smectite's dry density (0.9-1.4 Mg/m3), salinity ([NaCl]=0.01, 0.51 M) and temperature (295-333 K). TheDa-values for both ions tended to be higher in the parallel direction than in the perpendicular direction to the orientation of smectite particles. TheDa-values of I−ions in the parallel direction decreased with increasing salinity only at low dry density, but those of Cs+ions increased with increasing salinity for all conditions. Based on this, it is interpreted that I−ions predominantly diffuse in external pores and Cs+ions diffuse in both interlayer and external pores. The ΔEa-values forI−ions are at similar levels as those for the ionic diffusivity in free water (D°) of I−ions at low dry density and increased with increasing dry density. The ΔEa-values for Cs+ions are higher than those for theD° of Cs+ions even at low dry density and increased with increasing dry density. Such high ΔEa-values for Cs+ions are considered to be due to the combined effects of the ion exchange enthalpy between Cs+and Na+ions in smectite and the lowering in the activity of porewater.
Prediction of Long Term Corrosion Behaviour in Nuclear Waste Systems
Authors: Féron Damien , Macdonald Digby D.
Published: April 2011

The corrosion resistance of container materials in underground repositories is an important issue for the safe disposal of High Level Nuclear Waste (HLNW). The reliable prediction of container degradation rate and engineering barrier integrity over extended periods, up to several thousands years or even several hundreds of thousands of years, represents one of the greatest scientific and technical challenges. The first and the second International Workshops on Prediction of Long Term Corrosion Behaviour in Nuclear Waste Systems, which were held in 2001 (Cadarache) and 2004 (Nice), sought to compare the scientific and experimental approaches that are being developed in various organisations worldwide for predicting long term corrosion phenomena, including corrosion strategies for interim storage and geological disposal. The lessons learned during these Workshops, include the necessity of developing two approaches based on semi-empiricism and determinism in a complementary manner for effective prediction. The use of archaeological artefacts to demonstrate the feasibility of long term storage and to provide a database for testing and validating modelling work was also emphasized.
Probing the Water Phases and Microstructure in a Model Cement Blend Matrix used for the Encapsulation of Intermediate Level Nuclear Wastes
Authors: Gorce Jean-Philippe , Milestone Neil B. , Mcdonald Peter J.
Published: April 2011

The changes in microstructure and content of water phases during hydration of a 3:1 BFS:OPC blend are investigated by Mercury Intrusion Porosimetry (MIP), freeze-drying, Thermal Gravimetric Analysis (TGA) and 1H Nuclear Magnetic Resonance (NMR) relaxometry. MIP indicates that during the blend hydration, a reduction in the population of capillary pores (larger than about 100 nm) occurs while the population of gel pores (smaller than few tens of nanometres) increases. Between 3 and 90 days, the porosity estimated by MIP decreases from about 36% down to 18% while the median pore size decreases from about 140 nm down to 6 nm.

1H NMR relaxometry shows that after 1 day of hydration, nearly 70% of the evaporable water is held in capillary pores while about 30% is present in gel pores. After two weeks, most of the evaporable water (90%) is found in pores smaller than few tens of nanometres.

The amount of evaporable water detected by freeze drying decreases from less than 20 wt.% after one week of hydration down to about 16.3 wt.% after 90 days while the amount of chemically bound water related to the degree of advancement of the cement hydration and detected by TGA increases from 8 wt.% to 10.3 wt.%.

During hydration the BFS:OPC blend matrix evolves from an open microporous network to one of a poorly connected network of water rich nanopores with increasing amounts of chemically bound water.
Sodium Sulphate Activated GGBS/PFA and Its Potential as a Nuclear Waste Immobilisation Matrix
Authors: Bai Yun , Milestone Neil B. , Yang Changhui
Published: April 2011

The UK currently uses Portland cement composite blends to immobilise/encapsulate intermediate and low level radioactive wastes (ILW and LLW). However, among other things, the high pH of these systems causes the corrosion of some metals, which can lead to expansion and excess generation of hydrogen. Therefore, in order to immobilise nuclear waste where corrosion is an issue, a near neutral cementing system is desirable.

Among the activators which can be used in the alkali-activated slag (AAS) systems, a solution of Na2SO4 is near neutral but the ground granulated blast-furnace slag (GGBS) itself has a pH about 11, increasing the pH within the Na2SO4 activated AAS. As low calcium Pulverized Fuel Ash (PFA) only has a pH about 9, using a GGBS/PFA blend activated by Na2SO4 offers the potential to develop a near neutral cementing system for nuclear waste immobilisation purposes.

In this paper, the replacement of GGBS in the Na2SO4 activated AAS system with PFA at 0%, 10%, 20% and 30% by mass was examined. The pH and corrosion of Al were determined and used as primary criteria for judging the feasibility for further development of a Na2SO4 activated GGBS/PFA matrix for immobilising nuclear wastes. The microstructure of the matrices was studied by SEM. Leaching studies were carried out to examine the possibility of immobilising Cs+ within these Na2SO4 activated GGBS/PFA matrices. The potential of using Na2SO4 activated GGBS/PFA for immobilising nuclear wastes is discussed.
Kinetics of Mixed Low-Level Waste Incapsulation Using Iron Phosphate Chemically Bonded Cement
Authors: Aloy A.S. , Koltsova T.I. , Kovarskaya E.N. , Silin M.Yu.
Published: April 2011

The iron phosphate cement (IPC) structure formation process has been studied by Mossbauer spectroscopy. The IPC is formed as a result of interaction of iron oxides with orthophosphoric acid and could be used as a matrix for immobilization of low-level radioactive waste.

The structural formation process has been shown may be considered to consist of two stages. The first stage lasts to the moment of setting, when the diffusion process goes on in the liquid phase. At this stage the main fragments of the structural polymeric frame of the IPC are developed consisting of iron (+2) and (+3) phosphates. The rate of the oxides-with-phosphoric-acid reaction as well as the time of hardening depends on the bivalent iron content.

The second stage begins from the moment of setting when the diffusion becomes slower. At this stage the process is characterized by the negligible increase in the iron phosphates (+2), (+3) content and transformation of the previously formed phosphates.

The nuclear gamma-resonance (NGR) parameters have been determined of FeH3(PO4)2·2.5H2O, forming in the hematite (Fe2O3) based IPC: isomeric shift (IS) = 0.46 mm/s, quadrupole splitting (QS) = 0.197 mm/s, FWHM =0.282 mm/s.
Reactions in Cemented Nuclear Waste forms – the Need for a Toolbox of Different Cement Types
Authors: Milestone Neil B , Bai Yun , Borges Paulo R , Gorce Nick C Collier Jean-Phillipe , Gordon Laura E , Setiadi Anthony , Utton Claire A , Zhou Qizhi
Published: April 2011

Encapsulation of low and intermediate level waste (LLW and ILW) into a monolithic waste form is a key method for preparing such wastes for storage and ultimate disposal. One of the most efficient ways of doing this is by cementation, successfully used for much of today's operational waste in the UK. Formulations are based on composite cements where high levels ofpulverised fuel ash or blast furnace slag replace ordinary Portland cement (OPC). These provide cementing systems with good fluidity, lowered heat of hydration and a matrix with lower porosity and permeability. Further the alkaline hydrated calcium silicate matrix ensures insolubility of many metal hydroxides and hydrated oxides. Dealing with historic or legacy wastes presents a challenge as for many, a suitable cementing formulation has yet to be devised. To achieve ultimate durability, likely interactions between the waste and the cementing matrix need to be well understood when choosing the cement system to be used. The alkaline OPC based matrix causes corrosion with metals such as Al and Mg and porous silicates and layered metal hydroxides used to selectively remove radionucleides react in the alkaline environment, potentially releasing the adsorbed species. Slurries of materials normally considered inert show interactions within the OPC composite systems, although these are not considered detrimental for durability. This paper describes the chemistry of some interactions that occur within OPC composite cements. It also details some results for alternative cementing systems that may provide a toolbox of cement types that may address many of the interaction issues. These include calcium sulphoaluminate cements, inorganic polymers or geopolymers, and activated slag systems.
Crystal Chemistry and Cation Ordering in Zirconolite 2M
Authors: Lumpkin G.R. , Whittle K.R. , Howard C.J. , Zhang Z. , Berry F.J. , Oates G. , Williams C.T. , Zaitsev A.N.
Published: April 2011

Structural studies of single phase or nearly single phase zirconolite ceramic samples have been conducted using electron microscopy and microanalysis, X-ray diffraction, neutron diffraction, and spectroscopic methods. We show that it is possible to produce a complete series of zirconolite 2M samples with substitution of 2Ti by Nb+Fe in the HTB layer. The samples are single phase up to about 80% Nb +Fe substitution, with the appearance of a minor perovskite phase at higher Nb+Fe levels. Electron probe microanalysis reveals that the samples are homogeneous and close to their nominal compositions, except for those containing perovskite, which have a slight excess of Zr and a deficiency in the Fe content. The lattice parameters and the positions of certain Raman bands are non-linear as a function of composition, suggesting the possibility of cation ordering over the three available Ti sites within the HTB layer. Rietveld refinement of Synchrotron X-ray powder data for the Nb+Fe end-member have been conducted for the disordered case and for six trial models each with a different ordering scheme. Results of this exercise indicate that Fe preferentially occupies the Ti2 (split) site with partial ordering of Nb and the remaining Fe over the Ti1 and Ti2 octahedra. The preference of Fe for the five coordinated Ti2 site has been confirmed by 57Fe Mossbauer spectroscopy.
Composition driven structural phase transitions in the (Y1−xLax)2Ti2O7 system
Authors: Harvey Elizabeth J. , Ashbrook Sharon E. , Lumpkin Gregory R. , Redfern Simon A.T.
Published: April 2011

The structural characteristics of samples in the (Y1−xLax)2Ti2O7 system have been studied as a function of bulk composition using time-of-flight (TOF) powder neutron diffraction, powder X-ray diffraction (XRD), Electron Microscopy and 89Y (I = ½) Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Analysis of the neutron diffraction data suggests the following: For compositions where 0 ≤ x ≤ 0.132, a single cubic phase is present (Fd3, m, Z = 8). Between 0.832 ≤ − ≤ 1, a solid solution with the La2Ti2O7 structure (P21, Z = 4) forms. The two phases coexist between x values of 0.132 and 0.832. XRD suggests that the limits of the two regions of solid solution fall within the ranges of x = 0.1-0.125 and x = 0.875-0.9. The variation in the limits found by the two diffraction methods may be due to stoichiometric errors in the samples. Line-shape analysis of NMR spectra for Y rich compositions suggests that increasing proportions of La are incorporated onto the pyrochlore A site in a statistically random manner, up to the limit of solid solution. The NMR spectra of the monoclinic phase suggest that occupation of the four crystallographically distinct ‘A’ type sites within the structure is not random. At low Y concentrations, two of these sites are preferentially occupied by Y. These sites are suggested to be those found at the edge of the perovskite slabs, which exhibit smaller coordination numbers (based on the number of oxygens within bonding distance of the ‘A’ type cation) than the true perovskite A sites found at the slab centres.
Phase Relations In The Systems Based On Yttrium, Gadolinium, Manganese, Titanium, and Uranium Oxides Doped with Calcium, Aluminum and Iron Oxides
Authors: Stefanovsky Sergey V. , Stefanovsky Olga I. , Yudintsev Sergey V. , Nikonov Boris S.
Published: April 2011

Titanate-based ceramics with yttrium, gadolinium, manganese, uranium, calcium, aluminum, and iron oxides being of interest as matrices for radioactive wastes containing lanthanides, actinides and corrosion products were synthesized using cold pressing and sintering and melting routes. Major constituents of the ceramics are fluorite structure phases (pyrochlore, “murataite”, cubic oxide with fluorite-related lattice) as well as brannerite. Incorporation of aluminum and iron oxides (corrosion products) forms second phases with perovskite or/and crichtonite structure.
Phase composition of the ceramics based on Rare Earth, Manganese and Titanium Oxides
Authors: Stefanovsky S.V. , Yudintsev S.V. , Nikonov B.S. , Stefanovsky O.I.
Published: April 2011

Ceramics in systems with rare earth (REE = Y, Sm, Gd), manganese, and titanium oxides are considered as potential matrices for immobilization of REE- and actinide-containing radioactive wastes. Ceramics of this type were synthesized from oxide mixtures by a cold pressing and sintering method at temperatures of 1300-1400 °C as well as plasma heating. Major phases in the ceramics were found to be the phases of the pyrochlore-murataite polysomatic series with three- (3C), five- (5C), and seven-fold (7C) elementary fluorite unit cell. Perovskite/pyrophanite, hibonite/loveringite as well as garnet in the samples contained extra Fe and Al oxides were present as minor phases. Formation of the murataite, to be exact – structures built from murataite and pyrochlore modules, is more characteristic of the Y-bearing systems rather than the systems with heavier lanthanides. Increase of ionic radius of the REE trends towards formation of the pyrochlore structure phases whereas occurrence of large-size cations (La) stimulates formation of the perovskite structure phase. In the Y-bearing system murataite formation takes place at relative low Y2O3 concentrations (<15 mole %) and TiO2 content as high as ∼60 mole % and MnOx – 15-20 mole %. Higher Y2O3 concentrations and Gd2O3 substitution for Y2O3 yields preferably the pyrochlore phase or may favor formation of the phase with the 7C polytype.
Modelling experimental results on radiolytic processes at the spent fuel water interface. II. Radionuclides release
Authors: Cera E. , Grivé M. , Bruno J. , Eriksen T.E.
Published: April 2011

Experimental and modelling efforts in the last decade in the frame of nuclear waste management field have been focused on studying the role of the UO2surfaces in poising the redox state of solid/water systems as well as the radionuclides release behaviour. For this purpose, an experimental programme was developed consisting on dissolution experiments with PWR spent fuel fragments in an anoxic environment and by using different solution compositions.

Some of the collected data has been previously published [1], specifically those data concerning radiolysis products and dissolution of the matrix. The results and the modelling tasks indicated an overall balance of the generated radiolytic species and that uranium dissolution was controlled by the oxidation of the spent fuel matrix in 10mM bicarbonate solutions while in the tests carried out at lower or without carbonate concentrations uranium in the aqueous phase was governed by the precipitation of schoepite.

This paper is the continuation of a series accounting for the data and modelling work related to investigating the release behaviour of minor radionuclides from the spent fuel.

Uranium concentrations as a function of time showed an initial increase until reaching a steady state, indicating a matrix dissolution control. The same behaviour is observed for neptunium, caesium, strontium, technetium and molybdenum indicating a congruent release of these elements with the major component of the fuel matrix. On the other hand, no cler tendency is observed for plutonium data where additional solubility limiting mechanisms may apply.

Kinetic modelling of the trace elements: caesium, strontium, technetium and molybdenum is based on the congruent release of these elements with the major component of the fuel matrix. Rate constants have been determined. Kinetic modelling of neptunium data took also into account the subsequent precipitation as Np(IV) hydroxide. Finally, measured Pu concentrations may be explained by the precipitation of Pu(IV) and/or Pu(III) solid phases.
Static dissolution tests of α-doped UO2 under repository relevant conditions: Influence of Boom Clay and α-activity on fuel dissolution rates
Authors: Salah Sonia , Cachoir Christelle , Lemmens Karel , Maes Norbert
Published: April 2011

Since reprocessing is no longer the reference policy in Belgium, studies on the direct disposal of spent fuel in a clay formation have gained increased interest in the last years. In order to determine to what extent the clay properties and the α-activity influence the dissolution kinetics of spent fuel for the long term disposal, static dissolution tests have been performed on 5 different types of α-doped UO2, representing a PWR fuel with a burn-up of 45 or 55 GWd · tHM−1 and fuel ages ranging between 150 and 90,000 years, in different Boom Clay (BC) media at room temperature and in an anoxic atmosphere for 90 to 720 days. The uranium activity in the clay fraction over time was found to be much higher than the U-activity in the leachates, which has been mainly ascribed to the high retention capacity of the BC. The average dissolution rate between 0 and 90 days obtained for the 5 types of α-doped UO2 were all found to be high and quite similar at ∼263 µg · m−2· d−1and a “longer-term” rate (181 to 720 days) ranging between zero and 15 µg · m−2· d−1. These results suggest that the activity of the fuels does not seem to have an effect on the UO2 dissolution rates under the considered test conditions. In order to study the partition/redistribution of U during UO2dissolution, sequential extraction experiments were performed. Results of the latter have provided a better mechanistic understanding of BC/spent fuel interaction processes as well as information on the role of the different minerals controlling the U-retention/immobilization.
Modelling of the spent fuel dissolution rate evolution for repository conditions. Matrix Alteration Model results and sensitivity analysis
Authors: Quiñones J. , Iglesias E. , Martínez-Esparza A. , Merino J. , Cera E. , Bruno J. , de Pablo J. , Casas I. , Giménez J. , Clarens F. , Rovira M.
Published: April 2011

This paper focuses on how to extrapolate current knowledge of spent fuel matrix alteration processes from laboratory to repository conditions, i.e., the influence of changes in both the environmental conditions and the range of time scale considered. Therefore, a spent fuel matrix alteration model allowing the alteration rate evolution to be predicted as a function of both the host rock considered and evaluation time scale of interest is described.

At present, the model assumes that alteration of the spent fuel will start when the groundwater reaches the solid surface and that only the radiolytic species of the groundwater (oxidants generated by α-radiation of spent fuel) will produce the surface oxidation process and subsequent matrix dissolution; O2, H2O2 and OH· are the species that react with UO2(s) for oxidation of the pellet surface. The dissolution process of the surface sites that are oxidized is modelled in two steps: first, a surface co-ordination of the oxidized layer with aqueous ligands and, second, detachment (dissolution) of the product species. Taking this mechanism into account, the model gives the evolution of the spent fuel matrix alteration rate over periods as long as 1,000,000 years.

In this work the matrix alteration rate results obtained for two repository environments, granitic and argillaceous, will be presented. Furthermore, a sensitivity analysis study has been performed on the influence of the following variables: type of spent fuel considered, α-dose rate evolution, α-range in groundwater, carbonate and iron concentration in groundwater, H2 partial pressure, container time failure and specific surface area of the pellet.
Obtaining Solubility Constants to Describe the Incongruent Dissolution of SON68 Waste Glass by an Equilibrium Ideal Solid Solution Model Approach
Authors: Jacques Diederik , Lemmens Karel
Published: April 2011

One of the waste forms resulting from the Belgian programme for nuclear energy is the R7T7 high level waste glass, simulated by the SON68 glass. When exposed to distilled water or synthetic interstitial clay water, SON68 glass dissolves incongruently, releasing some elements (B, Li, Na) faster to the solution than others (Si, Al, Ca). The objective of this study is to describe the composition of the leachate in contact with inactive SON68 glass by assuming congruent glass dissolution followed by the precipitation of a secondary solid phase represented as an ideal solid solution in equilibrium with the leachate. Experimental SON68 dissolution data in distilled water was available at three temperatures. The solubility of the different end members in the solid solution is optimized using the available data for each temperature. The temperature dependence of the different end members was then obtained by Van 't Hoff equation. The calibrated model can describe the composition of the leachate and the altered glass phase during glass dissolution at different temperatures. The model, calibrated for the distilled water system, could successfully describe the composition of the leachate during glass dissolution experiments in a synthetic clay water system.
Temperature Dependence of Long-Term Alteration Rate for Aqueous Alteration of P0798 Simulated Waste Glass under Smectite Forming Conditions
Authors: Inagaki Y. , Saruwatari T. , Idemitsu K. , Arima T. , Shinkai A. , Yoshikawa H. , Yui M.
Published: April 2011

Several kinetic models have been proposed to evaluate the aqueous dissolution/alteration rate of nuclear waste glass for long-term. However, reaction processes controlling the long-term rate are much more subjected to controversy. Temperature dependence of the long-term alteration rate is an essential issue to understand the rate controlling processes. In the present study, the static aqueous alteration tests were performed with a Japanese simulated waste glass P0798 as a function of temperature from 60°C to120°C, and the temperature dependence of the long-term alteration rate was evaluated to understand the rate controlling processes. The tests were performed in 0.001M NaOH solution to maintain a constant solution pH of around 10 during the test period and to provide smectite forming conditions where smectite forms as the major secondary phase without zeolite formation. From the test results on dissolution of boron, the alteration rate at each temperature was analyzed by use of a water-diffusion model. The water-diffusion model used is based on a simple assumption; the glass alteration is controlled by water diffusion with ion-exchange between water (hydronium ion: H3O+) and soluble elements (B, Na, Li, etc) at the glass surface layer with the apparent diffusion coefficient Di. A good agreement was observed between the model analysis and the test results, and the value of Di was evaluated to be 1.2 × 10−22 m2/s at 60°C to 1.8 × 10−21 m2/s at 120°C. The Arrhenius plot of Di showed a good linearity to give the activation energy of 49 kJ/mol, which value is close to that for the residual dissolution rate of French waste glass (53 kJ/mol) by Gin [1], and is very close to that for ion-exchange in sodium aluminosilicate glass (49 kJ/mol) by McGrail [2]. These results suggest that water diffusion with ion-exchange can be the dominant process controlling the alteration rate under smectite forming conditions. At elevated temperatures (100°C and 120°C), however, the model-predicted boron releases deviated from the experimental data at the later stage beyond 50-80 days, which suggests that the alteration layer developing at the glass surface may evolve to be protective against the water diffusion to depress the alteration rate as the alteration proceeds.
Modelling of transport in fractures with complex matrix properties
Authors: Moreno Luis , Crawford James , Neretnieks Ivars
Published: April 2011

In the ongoing Swedish site investigations it has been found that the rock matrix adjacent to many open fractures has been altered. The alteration can extend from mm to several cm. The altered rock can have different sorption and diffusion properties compared to the undisturbed rock and this may influence the retardation of the nuclides. The paper presents how the Channel Network model has been adapted to handle diffusion into a matrix composed of several layers with different properties in addition to the infinite undisturbed matrix. For one channel, the solution for the Residence Time Distribution, RTD, may be found in the Laplace-plane. For the transport in the Channel Network, a particle tracking technique is used to determine the paths followed by solute particles. The RTD for this path is obtained using convolution, which in the Laplace-plane means multiplication of the transfer functions for each channel. The inversion to the time-plane is made by numerical inversion of the Laplace transforms for each path. The method has been tested with data from the TRUE (Tracer Retention Understanding Experiments) project, Task 6F, fluid flow and solute transport in two features in 100 m scale where a complex matrix was modelled. The model was used to predict the transport of the tracers (I-129), Cs-137, and Am-241) over some 20 m. The paper also addresses how the RTD is influenced by the different retardation mechanisms under Site Characterisation (SC) as well as Performance Assessment (PA) conditions.
Impact of Device Variability and Circuit Phase Shift in Synchronized Spin Torque Oscillators
Authors: Persson Johan , Zhou Yan , Akerman Johan
Published: April 2011

Current-induced magnetization dynamics in a system composed of two electrically coupled spin torque oscillators (STOs) is examined. The dynamics of the STOs is modeled by the Landau–Lifshitz–Gilbert equations modified with a Slonczewski spin-transfer torque term. To study the impact of realistic process variations on STO synchronization we let the two STOs have different in-plane anisotropy fields (Hk). The simulation also provides for a time delay τ. We construct a phase diagram of the STO synchronization as a function of Hk and direct current (Idc) at different τ. The phase diagram turns out to be quite rich with different types of synchronized precession modes. While the synchronized state is originally very sensitive to STO process variations and can only sustain up to 4% Hk variation, the addition of a small time delay dramatically improves its robustness and allows as much as 145% Hk variation in the entire out-of-plane precession regime. It is also shown that the two STOs can not only be locked in frequency, but also in phase at a given τ and the phase difference between the two STOs can be tuned by varying the dc current.