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Stability of uranium incorporated into Fe(hydr)oxides under fluctuating redox conditions

Description: Reaction pathways resulting in uranium bearing solids that are stable (i.e., having limited solubility) under both aerobic and anaerobic conditions will limit dissolved concentrations and migration of this toxin. Here we examine the sorption mechanism and propensity for release of uranium reacted with Fe (hydr)oxides under cyclic oxidizing and reducing conditions. Upon reaction of ferrihydrite with Fe(II) under conditions where aqueous Ca-UO{sub 2}-CO{sub 3} species predominate (3 mM Ca and 3.8 mM CO{sub 3}-total), dissolved uranium concentrations decrease from 0.16 mM to below detection limit (BDL) after 5 to 15 d, depending on the Fe(II) concentration. In systems undergoing 3 successive redox cycles (15 d of reduction followed by 5 d of oxidation) and a pulsed decrease to 0.15 mM CO{sub 3}-total, dissolved uranium concentrations varied depending on the Fe(II) concentration during the initial and subsequent reduction phases - U concentrations resulting during the oxic 'rebound' varied inversely with the Fe(II) concentration during the reduction cycle. Uranium removed from solution remains in the oxidized form and is found both adsorbed on and incorporated into the structure of newly formed goethite and magnetite. Our 15 results reveal that the fate of uranium is dependent on anaerobic/aerobic conditions, aqueous uranium speciation, and the fate of iron.
Date: April 1, 2009
Creator: Stewart, B.D.; Nico, P.S. & Fendorf, S.
Partner: UNT Libraries Government Documents Department

Actinide (III) solubility in WIPP Brine: data summary and recommendations

Description: The solubility of actinides in the +3 oxidation state is an important input into the Waste Isolation Pilot Plant (WIPP) performance assessment (PA) models that calculate potential actinide release from the WIPP repository. In this context, the solubility of neodymium(III) was determined as a function of pH, carbonate concentration, and WIPP brine composition. Additionally, we conducted a literature review on the solubility of +3 actinides under WIPP-related conditions. Neodymium(III) was used as a redox-invariant analog for the +3 oxidation state of americium and plutonium, which is the oxidation state that accounts for over 90% of the potential release from the WIPP through the dissolved brine release (DBR) mechanism, based on current WIPP performance assessment assumptions. These solubility data extend past studies to brine compositions that are more WIPP-relevant and cover a broader range of experimental conditions than past studies.
Date: September 1, 2009
Creator: Borkowski, Marian; Lucchini, Jean-Francois; Richmann, Michael K. & Reed, Donald T.
Partner: UNT Libraries Government Documents Department

Fate of Contaminants in Contact with West Valley Grouts

Description: The objective of the work described here is to determine to what extent a variety of contaminants, including fission products, actinides, and RCRA elements are sequestered by the two grout formulations. The conceptual model for this study is as follows: a large mass of grout having been poured into a high-level waste tank is in the process of aging and weathering for thousands of years. The waste remaining in the tank will contain radionuclides and other contaminants, much of which will adhere to tank walls and internal structures. The grout will encapsulate the contaminants. Initially the grout will be well sequestered, but over time rainwater and groundwater will gain access to it. Ultimately, the grout/waste environment will be an open system. In this condition water will move through the grout, exposing it to O{sub 2} and CO{sub 2} from the air and HCO{sub 3}{sup -} from the groundwater. Thus we are considering an oxic environment containing HCO{sub 3}{sup -}. Initially the solubility of many contaminants, but not all, will be constrained by chemistry dominated by the grout, primarily by the high pH, around 11.8. This is controlled and buffered by the portland cement and blast furnace slag components of the grout, which by themselves maintain a solution pH of about 12.5. Slowly the pH will diminish as Ca(OH){sub 2} and KOH dissolve, are carried away by water, and CaCO{sub 3} forms. As these conditions develop, the behavior of these elements comes into question. In our conceptual model, although the grout is formulated to provide some reducing capacity, in order to be conservative this mechanism is not considered. In addition to solubility constraints imposed by pH, the various contaminants may be incorporated into a variety of solid phases. Some may be incorporated into newly forming compounds as the grout sets and ...
Date: July 1, 2009
Creator: Fuhrmann,M. & Gillow, J.
Partner: UNT Libraries Government Documents Department

Implementation of equilibrium aqueous speciation and solubility (EQ3 type) calculations into Cantera for electrolyte solutions.

Description: In this report, we summarize our work on developing a production level capability for modeling brine thermodynamic properties using the open-source code Cantera. This implementation into Cantera allows for the application of chemical thermodynamics to describe the interactions between a solid and an electrolyte solution at chemical equilibrium. The formulations to evaluate the thermodynamic properties of electrolytes are based on Pitzer's model to calculate molality-based activity coefficients using a real equation-of-state (EoS) for water. In addition, the thermodynamic properties of solutes at elevated temperature and pressures are computed using the revised Helgeson-Kirkham-Flowers (HKF) EoS for ionic and neutral aqueous species. The thermodynamic data parameters for the Pitzer formulation and HKF EoS are from the thermodynamic database compilation developed for the Yucca Mountain Project (YMP) used with the computer code EQ3/6. We describe the adopted equations and their implementation within Cantera and also provide several validated examples relevant to the calculations of extensive properties of electrolyte solutions.
Date: June 1, 2009
Creator: Moffat, Harry K. & Jove-Colon, Carlos F.
Partner: UNT Libraries Government Documents Department

Interdiffusion in Diffusion Couples: U-Mo v. Al and Al-Si

Description: Interdiffusion and microstructural development in the U-Mo-Al system was examined using solid-tosolid diffusion couples consisting of U-7wt.%Mo, U-10wt.%Mo and U-12wt.%Mo vs. pure Al, annealed at 600°C for 24 hours. The influence of Si alloying addition (up to 5 wt.%) in Al on the interdiffusion microstructural development was also examined using solid-to-solid diffusion couples consisting of U-7wt.%Mo, U-10wt.%Mo and U-12wt.%Mo vs. pure Al, Al-2wt.%Si, and Al-5wt.%Si annealed at 550°C up to 20 hours. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA) were employed to examine the development of a very fine multiphase intermetallic layer. In ternary U-Mo-Al diffusion couples annealed at 600°C for 24 hours, interdiffusion microstructure varied of finely dispersed UAl3, UAl4, U6Mo4Al43, and UMo2Al20 phases while the average composition throughout the interdiffusion zone remained constant at approximately 80 at.% Al. Interdiffusion microstructure observed by SEM/TEM analyses and diffusion paths drawn from concentration profiles determined by EPMA appear to deviate from the assumption of “local thermodynamic equilibrium,” and suggest that interdiffusion occurs via supersaturated UAl4 followed by equilibrium transformation into UAl3, U6Mo4Al43, UAl4 and UMo2Al20 phases. Similar observation was made for U-Mo vs. Al diffusion couples annealed at 550°C. The addition of Si (up to 5 wt.%) in Al significantly reduced the thickness of the intermetallic layer by changing the constituent phases of the interdiffusion zone developed in U-Mo vs. Al-Si diffusion couples. Specifically, the formation of (U,Mo)(Al,Si)3 with relatively large solubility for Mo and Si, along with UMo2Al20 phases was observed along with disappearance of U6Mo4Al43 and UAl4 phases. Simplified understanding based on U-Al, U-Si, and Mo-Si binary phase diagrams is discussed in the light of the beneficial effect of Si alloying addition.
Date: November 1, 2009
Creator: D. D. Keiser, Jr.; Perez, E.; Yao, B. & Sohn, Y. H.
Partner: UNT Libraries Government Documents Department

CSSX Radiolytic H2 Generation ("Thermolysis") -- Final Report

Description: The purpose of this work was to determine the radiolytic hydrogen gas yield of irradiated CSSX solvent at several temperatures. The active ingredient of this solvent is calix[4]arene-bis-(t-octylbenzo) crown-6, a calixarene crown ether used for cesium complexation. The solvent also contains 1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol, a fluorinated alcoholic solvent modifer used to improve the solubility of the calixarene and its cesium complex in the Isopar L diluent. Isopar L is a branch-chain alkane and comprises most of the mixture. Samples of this solution were irradiated to various absorbed ?-ray doses in gas-tight sample containers, which were then sampled for hydrogen gas content. The methods are described below.
Date: January 1, 2009
Creator: Mincher, Bruce J.
Partner: UNT Libraries Government Documents Department