New Measurements of the Solubility of Metal Oxides at High Temperature

The results of high temperature solubility studies at ORNL are presented in which mainly direct pH measurements were made of aqueous solutions in contact with the crystalline solid phases: Al(OH){sub 3}, AlOOH, Fe{sub 3}O{sub 4}, Mg(OH){sub 2}, Nd(OH){sub 3}, and ZnO. Examples are highlighted of specific phenomena such as: the kinetics of gibbsite and boehmite dissolution and precipitation; the appearance of metastable equilibria in the dissolution of Fe{sub 3}O{sub 4}; the extremely rapid precipitation of crystalline brucite, Mg(OH){sub 2}; and anomalies in the apparent solubility profiles of AlO(OH) and ZnO. General trends associated with the effects of temperature and ionic strength are mentioned. Some of the potentiometric investigations were augmented by conventional batch [AlO(OH) and ZnO], and flow-through column (ZnO) experiments. In the additional case of ZnCr{sub 2}O{sub 4}, the extremely low solubility of this spinel permitted application of only the latter technique and these results are discussed in terms of the measured chromium levels that resulted from incongruent dissolution

Density-functional embedding using a plane-wave basis

The constrained electron density method of embedding a Kohn-Sham system in a substrate system (first described by P. Cortona, Phys. Rev. B {\bf 44}, 8454 (1991) and T.A. Wesolowski and A. Warshel, J. Phys. Chem {\bf 97}, 8050 (1993)) is applied with a plane-wave basis and both local and non-local pseudopotentials. This method divides the electron density of the system into substrate and embedded electron densities, the sum of which is the electron density of the system of interest. Coupling between the substrate and embedded systems is achieved via approximate kinetic energy functionals. Bulk aluminium is examined as a test case for which there is a strong interaction between the substrate and embedded systems. A number of approximations to the kinetic-energy functional, both semi-local and non-local, are investigated. It is found that Kohn-Sham results can be well reproduced using a non-local kinetic energy functional, with the total energy accurate to better than 0.1 eV per atom and good agreement between the electron densities.Comment: 11 pages, 4 figure

LIFETIME PREDICTIONS OF TOXIC AND RADIOACTIVE WASTE DISPOSAL AND REMEDIATION SCHEMES

Nuclear power production epitomizes the need for predictive geoscience (Ewing, 2004). Current global carbon emissions of {approx}7 Gt/y, largely from fossil fuel consumption, are expected to grow and result in a variety of adverse global effects, including acid rain, toxic smog, and hypothetically, sea level rise and increased frequency and severity of adverse weather conditions. One of the most reliable and sufficiently large alternative sources of energy is nuclear power, which currently provides about 17% of the world's electricity, equivalent to a reduction in carbon emissions of {approx}0.5 Gt/y. The U.S. currently consumes {approx}40% of the world's fossil fuel production, but generates only about 20% of it's electricity from nuclear plants. One major factor inhibiting increased power production form this source in the US. is the lack of a licensed repository for spent nuclear fuel, and Yucca Mountain is the only site being considered at this time. The licensing issue hinges on DOE's ability to present a credible case before the Nuclear Regulatory Commission that releases of radionuclides from the repository will not pose a threat to the accessible environment. This case is being built using a performance assessment model that incorporates a thermochemical database (EQ3/6) fed by experiments and theoretical developments in aqueous geochemistry and fluid rock interactions, coupled reaction/transport models which combine both the chemical and physical aspects of fluid and heat transport through porous and fractured media, geohazard and climate change models, and information gleaned from natural analogs. The assessment period is currently 10,000 years, but this has recently been challenged in a court of law, and may be extended to 300,000 years or more. Yucca Mountain has a design capacity that only marginally exceeds the current U.S. inventory of commercial spent fuel, currently stored on site at power plants through the country. Some analysts suggest that in order to have a significant impact on global carbon emissions, worldwide nuclear and other carbon-free energy sources would have to increase tenfold by 2050. If this increase came entirely from electrical power plants using the once-through nuclear fuel cycle, about 3,500 new 1-GW plants would be needed, that would generate enough spent fuel to fill a Yucca Mountain-sized repository every year. Though this extreme scenario is not likely to unfold, it seems inevitable that we need this source of energy, if the public can be assured that the operation of these plants, and the disposal of the wastes generated from their operation, can be made acceptably safe. The Yucca Mountain field trip provided an excellent opportunity for a diverse cross section of engineers and geoscientists to gain a clearer perspective on the nature and problems related to this particular type of repository. The symposium not only brought together a similar broad cross section of scientists and engineers, but provided a forum for comparing and contrasting different repository designs being considered throughout the world, different methods of assessing their performance characteristics, and the surprisingly broad array of geochemical inputs needed in order to succeed in this Grand Challenge

Direct Measurements of Pore Fluid Density by Vibrating Tube Densimetry

The densities of pore confined fluids were measured for the first time by means of vibrating tube densimetry VTD . A custom built high pressure, high temperature vibrating tube densimeter was used to measure the densities of propane at subcritical and supercritical temperatures between 35 and 97 C and carbon dioxide at supercritical temperatures between 32 and 50 C saturating hydrophobic silica aerogel 0.2 g cm3, 90 porosity synthesized inside Hastelloy U tubes. Additionally, supercritical isotherms of excess adsorption for CO2 and the same porous solid were measured gravimetrically using a precise magnetically coupled microbalance. Pore fluid densities and total adsorption isotherms increased monotonically with increasing density of the bulk fluid, in contrast to excess adsorption isotherms, which reached a maximum and then decreased toward zero or negative values above the critical density of the bulk fluid. The isotherms of confined fluid density and excess adsorption obtained by VTD contain additional information. For instance, the maxima of excess adsorption occur below the critical density of the bulk fluid at the beginning of the plateau region in the total adsorption, marking the end of the transition of pore fluid to a denser, liquidlike pore phase. Compression of the confined fluid significantly beyond the density of the bulk fluid at the same temperature was observed even at subcritical temperatures. The effect of pore confinement on the liquid amp; 8722;vapor critical temperature of propane was less than amp; 8764;1.7 K. The results for propane and carbon dioxide showed similarity in the sense of the principle of corresponding states. Good quantitative agreement was obtained between excess adsorption isotherms determined from VTD total adsorption results and those measured gravimetrically at the same temperature, confirming the validity of the vibrating tube measurements. Thus, it is demonstrated that vibrating tube densimetry is a novel experimental approach capable of providing directly the average density of pore confined fluids, and hence complementary to the conventional gravimetric or volumetric piezometric adsorption techniques, which yield the excess adsorption the Gibbsian surface exces

Surface speciation of yttrium and neodymium sorbed on rutile: Interpretations using the charge distribution model

The adsorption of Y3+ and Nd3+ onto rutile has been evaluated over a wide range of pH (3-11) and surface loading conditions, as well as at two ionic strengths (0.03 and 0.3 m), and temperatures (25 and 50 degrees C). The experimental results reveal the same adsorption behavior for the two trivalent ions onto the rutile surface, with Nd3+ first adsorbing at slightly lower pH values. The adsorption of both Y3+ and Nd3+ commences at pH values below the pH(znpc) of rutile. The experimental results were evaluated using a charge distribution (CD) and multisite complexation (MUSIC) model, and Basic Stern layer description of the electric double layer (EDL). The coordination geometry of possible surface complexes were constrained by molecular-level information obtained from X-ray standing wave measurements and molecular dynamic (MD) simulation studies. X-ray standing wave measurements showed an inner-sphere tetradentate complex for Y3+ adsorption onto the (110) rutile surface (Zhang et al., 2004b). The MD simulation studies suggest additional bidentate complexes may form. The CD values for all surface species were calculated based on a bond valence interpretation of the surface complexes identified by X-ray and MD. The calculated CD values were corrected for the effect of dipole orientation of interfacial water. At low pH, the tetradentate complex provided excellent fits to the Y3+ and Nd3+ experimental data. The experimental and surface complexation modeling results show a strong pH dependence, and suggest that the tetradentate surface species hydrolyze with increasing pH. Furthermore, with increased surface loading of Y3+ on rutile the tetradentate binding mode was augmented by a hydrolyzed-bidentate Y3+ surface complex. Collectively, the experimental and surface complexation modeling results demonstrate that solution chemistry and surface loading impacts Y3+ surface speciation. The approach taken of incorporating molecular-scale information into surface complexation models (SCMs) should aid in elucidating a fundamental understating of ion-adsorption reactions