Publications

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In the supercritical CO2-water-mineral systems relevant to subsurface CO2 sequestration, interfacial processes at the supercritical fluid-mineral interface will strongly affect core- and reservoir-scale hydrologic properties. Experimental and theoretical studies have shown that water films will form on mineral surfaces in supercritical CO2, but will be thinner than those that form in vadose zone environments at any given matric potential. The theoretical model presented here allows assessment of water saturation as a function of matric potential, a critical step for evaluating relative permeabilities the CO2 sequestration environment. The experimental water adsorption studies, using Quartz Crystal Microbalance and Fourier Transform Infrared Spectroscopy methods, confirm the major conclusions of the adsorption/condensation model. Additional data provided by the FTIR study is that CO2 intercalation into clays, if it occurs, does not involve carbonate or bicarbonate formation, or significant restriction of CO2 mobility. We have shown that the water film that forms in supercritical CO2 is reactive with common rock-forming minerals, including albite, orthoclase, labradorite, and muscovite. The experimental data indicate that reactivity is a function of water film thickness; at an activity of water of 0.9, the greatest extent of reaction in scCO2 occurred in areas (step edges, surface pits) where capillary condensation thickened the water films. This suggests that dissolution/precipitation reactions may occur preferentially in small pores and pore throats, where it may have a disproportionately large effect on rock hydrologic properties. Finally, a theoretical model is presented here that describes the formation and movement of CO2 ganglia in porous media, allowing assessment of the effect of pore size and structural heterogeneity on capillary trapping efficiency. The model results also suggest possible engineering approaches for optimizing trapping capacity and for monitoring ganglion formation in the subsurface.

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An in-package chemistry model is presented to calculate pH in the pore space of degradation products inside a breached waste package in the unsaturated environment of the Yucca Mountain repository. The pH is calculated as a function of liquid influx rate, partial pressure of carbon dioxide, solid-water volume ratio in the porous degradation products (provided by a coupled water balance model), and the relative rate of steel and waste form degradation. The EQ3/6 code is used to calculate pH at high liquid influx rates and zero liquid influx rates (vapor influx only). For mid-range liquid influx rates, a Damkohler ratio is defined and used to interpolate between the pH values calculated at the two extremes. This approach allows the in-package pH to be calculated over broad ranges of key parameters in a total system performance assessment.

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This report describes a gap analysis performed in the process of developing the Waste Integrated Performance and Safety Codes (IPSC) in support of the U.S. Department of Energy (DOE) Office of Nuclear Energy Advanced Modeling and Simulation (NEAMS) Campaign. The goal of the Waste IPSC is to develop an integrated suite of computational modeling and simulation capabilities to quantitatively assess the long-term performance of waste forms in the engineered and geologic environments of a radioactive waste storage or disposal system. The Waste IPSC will provide this simulation capability (1) for a range of disposal concepts, waste form types, engineered repository designs, and geologic settings, (2) for a range of time scales and distances, (3) with appropriate consideration of the inherent uncertainties, and (4) in accordance with rigorous verification, validation, and software quality requirements. The gap analyses documented in this report were are performed during an initial gap analysis to identify candidate codes and tools to support the development and integration of the Waste IPSC, and during follow-on activities that delved into more detailed assessments of the various codes that were acquired, studied, and tested. The current Waste IPSC strategy is to acquire and integrate the necessary Waste IPSC capabilities wherever feasible, and develop only those capabilities that cannot be acquired or suitably integrated, verified, or validated. The gap analysis indicates that significant capabilities may already exist in the existing THC codes although there is no single code able to fully account for all physical and chemical processes involved in a waste disposal system. Large gaps exist in modeling chemical processes and their couplings with other processes. The coupling of chemical processes with flow transport and mechanical deformation remains challenging. The data for extreme environments (e.g., for elevated temperature and high ionic strength media) that are needed for repository modeling are severely lacking. In addition, most of existing reactive transport codes were developed for non-radioactive contaminants, and they need to be adapted to account for radionuclide decay and in-growth. The accessibility to the source codes is generally limited. Because the problems of interest for the Waste IPSC are likely to result in relatively large computational models, a compact memory-usage footprint and a fast/robust solution procedure will be needed. A robust massively parallel processing (MPP) capability will also be required to provide reasonable turnaround times on the analyses that will be performed with the code. A performance assessment (PA) calculation for a waste disposal system generally requires a large number (hundreds to thousands) of model simulations to quantify the effect of model parameter uncertainties on the predicted repository performance. A set of codes for a PA calculation must be sufficiently robust and fast in terms of code execution. A PA system as a whole must be able to provide multiple alternative models for a specific set of physical/chemical processes, so that the users can choose various levels of modeling complexity based on their modeling needs. This requires PA codes, preferably, to be highly modularized. Most of the existing codes have difficulties meeting these requirements. Based on the gap analysis results, we have made the following recommendations for the code selection and code development for the NEAMS waste IPSC: (1) build fully coupled high-fidelity THCMBR codes using the existing SIERRA codes (e.g., ARIA and ADAGIO) and platform, (2) use DAKOTA to build an enhanced performance assessment system (EPAS), and build a modular code architecture and key code modules for performance assessments. The key chemical calculation modules will be built by expanding the existing CANTERA capabilities as well as by extracting useful components from other existing codes.

This report describes the progress in fiscal year 2010 in developing the Waste Integrated Performance and Safety Codes (IPSC) in support of the U.S. Department of Energy (DOE) Office of Nuclear Energy Advanced Modeling and Simulation (NEAMS) Campaign. The goal of the Waste IPSC is to develop an integrated suite of computational modeling and simulation capabilities to quantitatively assess the long-term performance of waste forms in the engineered and geologic environments of a radioactive waste storage or disposal system. The Waste IPSC will provide this simulation capability (1) for a range of disposal concepts, waste form types, engineered repository designs, and geologic settings, (2) for a range of time scales and distances, (3) with appropriate consideration of the inherent uncertainties, and (4) in accordance with robust verification, validation, and software quality requirements. Waste IPSC activities in fiscal year 2010 focused on specifying a challenge problem to demonstrate proof of concept, developing a verification and validation plan, and performing an initial gap analyses to identify candidate codes and tools to support the development and integration of the Waste IPSC. The current Waste IPSC strategy is to acquire and integrate the necessary Waste IPSC capabilities wherever feasible, and develop only those capabilities that cannot be acquired or suitably integrated, verified, or validated. This year-end progress report documents the FY10 status of acquisition, development, and integration of thermal-hydrologic-chemical-mechanical (THCM) code capabilities, frameworks, and enabling tools and infrastructure.

The objective of the U.S. Department of Energy Office of Nuclear Energy Advanced Modeling and Simulation Waste Integrated Performance and Safety Codes (NEAMS Waste IPSC) is to provide an integrated suite of computational modeling and simulation (M&S) capabilities to quantitatively assess the long-term performance of waste forms in the engineered and geologic environments of a radioactive-waste storage facility or disposal repository. To meet this objective, NEAMS Waste IPSC M&S capabilities will be applied to challenging spatial domains, temporal domains, multiphysics couplings, and multiscale couplings. A strategic verification and validation (V&V) goal is to establish evidence-based metrics for the level of confidence in M&S codes and capabilities. Because it is economically impractical to apply the maximum V&V rigor to each and every M&S capability, M&S capabilities will be ranked for their impact on the performance assessments of various components of the repository systems. Those M&S capabilities with greater impact will require a greater level of confidence and a correspondingly greater investment in V&V. This report includes five major components: (1) a background summary of the NEAMS Waste IPSC to emphasize M&S challenges; (2) the conceptual foundation for verification, validation, and confidence assessment of NEAMS Waste IPSC M&S capabilities; (3) specifications for the planned verification, validation, and confidence-assessment practices; (4) specifications for the planned evidence information management system; and (5) a path forward for the incremental implementation of this V&V plan.

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Waste IPSC Objective is to develop an integrated suite of high performance computing capabilities to simulate radionuclide movement through the engineered components and geosphere of a radioactive waste storage or disposal system: (1) with robust thermal-hydrologic-chemical-mechanical (THCM) coupling; (2) for a range of disposal system alternatives (concepts, waste form types, engineered designs, geologic settings); (3) for long time scales and associated large uncertainties; (4) at multiple model fidelities (sub-continuum, high-fidelity continuum, PA); and (5) in accordance with V&V and software quality requirements. THCM Modeling collaborates with: (1) Other Waste IPSC activities: Sub-Continuum Processes (and FMM), Frameworks and Infrastructure (and VU, ECT, and CT); (2) Waste Form Campaign; (3) Used Fuel Disposition (UFD) Campaign; and (4) ASCEM.

Carbon capture and sequestration (CCS) is an option to mitigate impacts of atmospheric carbon emission. Numerous factors are important in determining the overall effectiveness of long-term geologic storage of carbon, including leakage rates, volume of storage available, and system costs. Recent efforts have been made to apply an existing probabilistic performance assessment (PA) methodology developed for deep nuclear waste geologic repositories to evaluate the effectiveness of subsurface carbon storage (Viswanathan et al., 2008; Stauffer et al., 2009). However, to address the most pressing management, regulatory, and scientific concerns with subsurface carbon storage (CS), the existing PA methodology and tools must be enhanced and upgraded. For example, in the evaluation of a nuclear waste repository, a PA model is essentially a forward model that samples input parameters and runs multiple realizations to estimate future consequences and determine important parameters driving the system performance. In the CS evaluation, however, a PA model must be able to run both forward and inverse calculations to support optimization of CO{sub 2} injection and real-time site monitoring as an integral part of the system design and operation. The monitoring data must be continually fused into the PA model through model inversion and parameter estimation. Model calculations will in turn guide the design of optimal monitoring and carbon-injection strategies (e.g., in terms of monitoring techniques, locations, and time intervals). Under the support of Laboratory-Directed Research & Development (LDRD), a late-start LDRD project was initiated in June of Fiscal Year 2010 to explore the concept of an enhanced performance assessment system (EPAS) for carbon sequestration and storage. In spite of the tight time constraints, significant progress has been made on the project: (1) Following the general PA methodology, a preliminary Feature, Event, and Process (FEP) analysis was performed for a hypothetical CS system. Through this FEP analysis, relevant scenarios for CO{sub 2} release were defined. (2) A prototype of EPAS was developed by wrapping an existing multi-phase, multi-component reservoir simulator (TOUGH2) with an uncertainty quantification and optimization code (DAKOTA). (3) For demonstration, a probabilistic PA analysis was successfully performed for a hypothetical CS system based on an existing project in a brine-bearing sandstone. The work lays the foundation for the development of a new generation of PA tools for effective management of CS activities. At a top-level, the work supports energy security and climate change/adaptation by furthering the capability to effectively manage proposed carbon capture and sequestration activities (both research and development as well as operational), and it greatly enhances the technical capability to address this national problem. The next phase of the work will include (1) full capability demonstration of the EPAS, especially for data fusion, carbon storage system optimization, and process optimization of CO{sub 2} injection, and (2) application of the EPAS to actual carbon storage systems.

This report describes the specification of a challenge problem and associated challenge milestones for the Waste Integrated Performance and Safety Codes (IPSC) supporting the U.S. Department of Energy (DOE) Office of Nuclear Energy Advanced Modeling and Simulation (NEAMS) Campaign. The NEAMS challenge problems are designed to demonstrate proof of concept and progress towards IPSC goals. The goal of the Waste IPSC is to develop an integrated suite of modeling and simulation capabilities to quantitatively assess the long-term performance of waste forms in the engineered and geologic environments of a radioactive waste storage or disposal system. The Waste IPSC will provide this simulation capability (1) for a range of disposal concepts, waste form types, engineered repository designs, and geologic settings, (2) for a range of time scales and distances, (3) with appropriate consideration of the inherent uncertainties, and (4) in accordance with robust verification, validation, and software quality requirements. To demonstrate proof of concept and progress towards these goals and requirements, a Waste IPSC challenge problem is specified that includes coupled thermal-hydrologic-chemical-mechanical (THCM) processes that describe (1) the degradation of a borosilicate glass waste form and the corresponding mobilization of radionuclides (i.e., the processes that produce the radionuclide source term), (2) the associated near-field physical and chemical environment for waste emplacement within a salt formation, and (3) radionuclide transport in the near field (i.e., through the engineered components - waste form, waste package, and backfill - and the immediately adjacent salt). The initial details of a set of challenge milestones that collectively comprise the full challenge problem are also specified.

The United States is now re-assessing its nuclear waste disposal policy and re-evaluating the option of moving away from the current once-through open fuel cycle to a closed fuel cycle. In a closed fuel cycle, used fuels will be reprocessed and useful components such as uranium or transuranics will be recovered for reuse. During this process, a variety of waste streams will be generated. Immobilizing these waste streams into appropriate waste forms for either interim storage or long-term disposal is technically challenging. Highly volatile or soluble radionuclides such as iodine ({sup 129}I) and technetium ({sup 99}Tc) are particularly problematic, because both have long half-lives and can exist as gaseous or anionic species that are highly soluble and poorly sorbed by natural materials. Under the support of Sandia National Laboratories (SNL) Laboratory-Directed Research & Development (LDRD), we have developed a suite of inorganic nanocomposite materials (SNL-NCP) that can effectively entrap various radionuclides, especially for {sup 129}I and {sup 99}Tc. In particular, these materials have high sorption capabilities for iodine gas. After the sorption of radionuclides, these materials can be directly converted into nanostructured waste forms. This new generation of waste forms incorporates radionuclides as nano-scale inclusions in a host matrix and thus effectively relaxes the constraint of crystal structure on waste loadings. Therefore, the new waste forms have an unprecedented flexibility to accommodate a wide range of radionuclides with high waste loadings and low leaching rates. Specifically, we have developed a general route for synthesizing nanoporous metal oxides from inexpensive inorganic precursors. More than 300 materials have been synthesized and characterized with x-ray diffraction (XRD), BET surface area measurements, and transmission electron microscope (TEM). The sorption capabilities of the synthesized materials have been quantified by using stable isotopes I and Re as analogs to {sup 129}I and {sup 99}Tc. The results have confirmed our original finding that nanoporous Al oxide and its derivatives have high I sorption capabilities due to the combined effects of surface chemistry and nanopore confinement. We have developed a suite of techniques for the fixation of radionuclides in metal oxide nanopores. The key to this fixation is to chemically convert a target radionuclide into a less volatile or soluble form. We have developed a technique to convert a radionuclide-loaded nanoporous material into a durable glass-ceramic waste form through calcination. We have shown that mixing a radionuclide-loaded getter material with a Na-silicate solution can effectively seal the nanopores in the material, thus enhancing radionuclide retention during waste form formation. Our leaching tests have demonstrated the existence of an optimal vitrification temperature for the enhancement of waste form durability. Our work also indicates that silver may not be needed for I immobilization and encapsulation.

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Layered double hydroxides (LDHs) have shown great promise as anion getters. In this paper, we demonstrate that the sorption capability of a LDH for a specific oxyanion can be greatly increased by appropriately manipulating material composition and structure. We have synthesized a large set of LDH materials with various combinations of metal cations, interlayer anions, and molar ratios of divalent cation M(II) to trivalent cation M(III). The synthesized materials have then been tested systematically for their sorption capabilities for pertechnetate (TcO-4). It is discovered that for a given interlayer anion (either CO2-3 or NO-3) the Ni-Al LDH with a Ni/Al ratio of 3:1 exhibits the highest sorption capability among all the materials tested. The sorption of TcO-4 on M(II)-M(III)-CO3 LDHs may be dominated by the edge sites of LDH layers and correlated with the basal spacing d003 of the materials, which increases with the decreasing radii of both divalent and trivalent cations. The sorption reaches its maximum when the layer spacing is just large enough for a pertechnetate anion to fit into a cage space among three adjacent octahedra of metal hydroxides at the edge. Furthermore, the sorption is found to increase with the crystallinity of the materials. For a given combination of metal cations and an interlayer anion, the best crystalline LDH material is obtained generally with a M(II)/M(III) ratio of 3:1. Synthesis with readily exchangeable nitrate as an interlayer anion greatly increases the sorption capability of a LDH material for pertechnetate. The work reported here will help to establish a general structure-property relationship for the related layered materials. © 2006 Elsevier Inc. All rights reserved.

Interstratification-periodic or nonperiodic stacking of two different silicate layers along a c*-axis-is common in phyllosilicates. Published evidence indicates that some interstratified minerals precipitate directly from aqueous solutions. In this paper, we have demonstrated, based on chaos theory, that both periodic and nonperiodic interstratification can autonomously arise from simple kinetics of mineral growth from a solution. Growth of a mixed-layer mineral is assumed to proceed layer by layer, and each layer starts with the formation of a base (Si, Al)-O tetrahedral sheet, whose structural configuration in a-b dimensions determines the type of new layer that forms. The sequence of layer stacking can be described by a one-dimensional map (i.e., a difference equation), which accounts for two competing factors: (1) the affinity of each end-member structural component for attaching to the surface of the preceding layer, and (2) the strain energy created by stacking next to each other two silicate layers with different structural configurations. Chaotic (or nonperiodic) interstratification emerges when the contacting solution becomes slightly supersaturated with respect to both structural components. The transition from one interstratification pattern to another reflects a change in chemical environment during mineral crystallization. Our model can successfully predict the occurrence of mixed-layer phyllosilicates and the associated layer stacking sequences observed in both hydrothermal alteration and sediment diagenesis. The model suggests that the diagenetic transition of smectite → nonperiodic illite/smectite → ordered illite/smectite → illite may reflect relative changes in the saturation degree of pore water with respect to two end-member phases as a result of increasing burial temperatures. © 2006 Elsevier Inc. All rights reserved.

Microorganisms are ubiquitous in subsurface environments and play a major role in the biogeochemical recycling of various elements. In this paper, we have developed a general approach for a systematic evaluation of microbial impact on the long-term performance of the repository. We have demonstrated that data on microbial population alone are not sufficient for the evaluation of microbial impact on repository performance and a sensible approach for such evaluation must be based on the consideration of environmental constraints on microbial reaction pathways. We have applied our approach to both the Yucca Mountain (YM) repository and the Waste Isolation Pilot Plant (WIPP). We have demonstrated that the effect of microbial activity on the near-field chemistry in the Yucca Mountain repository is negligible because of limited nutrient supply and harsh environmental conditions created by waste emplacement. Whereas for the WIPP, we have shown that, due to the presence of a large quantity of organic materials and nutrients in the wastes, a significant microbial activity can potentially be stimulated and its impact on repository performance can be evaluated with carefully designed incubation experiments coupled with performance assessment calculations. The impact of microbial gas generation on disposal room chemistry in the WIPP can be mitigated by introducing MgO as a backfill material.

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This report summarizes the results obtained from a Laboratory Directed Research & Development (LDRD) project entitled 'Investigation of Potential Applications of Self-Assembled Nanostructured Materials in Nuclear Waste Management'. The objectives of this project are to (1) provide a mechanistic understanding of the control of nanometer-scale structures on the ion sorption capability of materials and (2) develop appropriate engineering approaches to improving material properties based on such an understanding.

Waste characterization is probably the most costly part of radioactive waste management. An important part of this characterization is the measurements of headspace gas in waste containers in order to demonstrate the compliance with Resource Conservation and Recovery Act (RCRA) or transportation requirements. The traditional chemical analysis methods, which include all steps of gas sampling, sample shipment and laboratory analysis, are expensive and time-consuming as well as increasing worker's exposure to hazardous environments. Therefore, an alternative technique that can provide quick, in-situ, and real-time detections of headspace gas compositions is highly desirable. This report summarizes the results obtained from a Laboratory Directed Research & Development (LDRD) project entitled 'Potential Application of Microsensor Technology in Radioactive Waste Management with Emphasis on Headspace Gas Detection'. The objective of this project is to bridge the technical gap between the current status of microsensor development and the intended applications of these sensors in nuclear waste management. The major results are summarized below: {sm_bullet} A literature review was conducted on the regulatory requirements for headspace gas sampling/analysis in waste characterization and monitoring. The most relevant gaseous species and the related physiochemical environments were identified. It was found that preconcentrators might be needed in order for chemiresistor sensors to meet desired detection {sm_bullet} A long-term stability test was conducted for a polymer-based chemresistor sensor array. Significant drifts were observed over the time duration of one month. Such drifts should be taken into account for long-term in-situ monitoring. {sm_bullet} Several techniques were explored to improve the performance of sensor polymers. It has been demonstrated that freeze deposition of black carbon (CB)-polymer composite can effectively eliminate the so-called 'coffee ring' effect and lead to a desirable uniform distribution of CB particles in sensing polymer films. The optimal ratio of CB/polymer has been determined. UV irradiation has been shown to improve sensor sensitivity. {sm_bullet} From a large set of commercially available polymers, five polymers were selected to form a sensor array that was able to provide optimal responses to six target-volatile organic compounds (VOCs). A series of tests on the response of sensor array to various VOC concentrations have been performed. Linear sensor responses have been observed over the tested concentration ranges, although the responses over a whole concentration range are generally nonlinear. {sm_bullet} Inverse models have been developed for identifying individual VOCs based on sensor array responses. A linear solvation energy model is particularly promising for identifying an unknown VOC in a single-component system. It has been demonstrated that a sensor array as such we developed is able to discriminate waste containers for their total VOC concentrations and therefore can be used as screening tool for reducing the existing headspace gas sampling rate. {sm_bullet} Various VOC preconcentrators have been fabricated using Carboxen 1000 as an absorbent. Extensive tests have been conducted in order to obtain optimal configurations and parameter ranges for preconcentrator performance. It has been shown that use of preconcentrators can reduce the detection limits of chemiresistors by two orders of magnitude. The life span of preconcentrators under various physiochemical conditions has also been evaluated. {sm_bullet} The performance of Pd film-based H2 sensors in the presence of VOCs has been evaluated. The interference of sensor readings by VOC has been observed, which can be attributed to the interference of VOC with the H2-O2 reaction on the Pd alloy surface. This interference can be eliminated by coating a layer of silicon dioxide on sensing film surface. Our work has demonstrated a wide range of applications of gas microsensors in radioactive waste management. Such applications can potentially lead to a significant cost saving and risk reduction for waste characterization.

This report summarizes the results obtained from a Laboratory Directed Research & Development (LDRD) project entitled 'Investigation of Potential Applications of Self-Assembled Nanostructured Materials in Nuclear Waste Management'. The objectives of this project are to (1) provide a mechanistic understanding of the control of nanometer-scale structures on the ion sorption capability of materials and (2) develop appropriate engineering approaches to improving material properties based on such an understanding.

Acid-base titration and metal sorption experiments were performed on both mesoporous alumina and alumina particles under various ionic strengths. It has been demonstrated that surface chemistry and ion sorption within nanopores can be significantly modified by a nano-scale space confinement. As the pore size is reduced to a few nanometers, the difference between surface acidity constants (ΔpK = pK2 - pK1) decreases, giving rise to a higher surface charge density on a nanopore surface than that on an unconfined solid-solution interface. The change in surface acidity constants results in a shift of ion sorption edges and enhances ion sorption on that nanopore surfaces.

Nanopores are ubiquitous in porous geologic media and may account for >90% of total mineral surface areas. Surface chemistry, ion sorption, and the related geochemical reactions within nanopores can be significantly modified by a nanometer-scale space confinement. As the pore size is reduced to a few nanometers, the difference between surface acidity constants (ΔpK = pK2 - pK1) decreases, giving rise to a higher surface charge density on a nanopore surface than that on an unconfined mineral-water interface. The change in surface acidity constants results in a shift of ion sorption edges and enhances ion sorption on nanopore surfaces. Also, the water activity in a nanopore is greatly reduced, thus increasing the tendency for inner sphere complexation and mineral precipitation. All these effects combine to preferentially enrich trace elements in nanopores, as observed in both field and laboratory studies. The work reported here sheds new light on such fundamental geochemical issues as the irreversibility of ion sorption and desorption, the bioavailability of subsurface contaminants, and the enrichment of trace metals in ore deposits, as well as the kinetics of mineral dissolution and/or precipitation.

Correctly identifying the possible alteration products and accurately predicting their occurrence in a repository-relevant environment are the key for source-term calculations in a repository performance assessment. Uraninite in uranium deposits has long been used as a natural analog to spent fuel in a repository because of their chemical and structural similarity. In this paper, a SEM/AEM investigation has been conducted on a partially alterated uraninite sample from a uranium ore deposit of Shinkolobwe of Congo. The mineral formation sequences were identified: uraninite→uranyl hydrates→uranyl silicates→Ca-uranyl silicates or uraninite→uranyl silicates→Ca-uranyl silicates. Reaction-path calculations were conducted for the oxidative dissolution of spent fuel in a representative Yucca Mountain groundwater. The predicted sequence is in general consistent with the SEM observations. The calculations also show that uranium carbonate minerals are unlikely to become major solubility-controlling mineral phases in a Yucca Mountain environment. Some discrepancies between model predictions and field observations are observed. Those discrepancies may result from poorly constrained thermodynamic data for uranyl silicate minerals.

Crystalline phases of pyrochlore (e.g., CaPuTi2O7, CaUTi2O7) have been proposed as a durable ceramic waste form for disposal of high level radioactive wastes including surplus weapons-usable plutonium. In this paper, we use a linear free energy relationship to predict the Gibbs free energies of formation of pyrochlore phases (CaMTi2O7). The Pu-pyrochlore phase is predicted to be stable with respect to PuO2, CaTiO3, and TiO2 at room temperatures. Pu-pyrochlore is expected to be stable in a geologic repository where silica and carbonate components are absent or limited. We suggest that a repository in a salt formation be an ideal environment for disposal of high level, pyrochlore-based ceramic wastes. In such environment, adding CaO as a backfill will make pyrochlore minerals thermodynamically stable and therefore effectively prevents actinide release from these mineral phases.

Uranium and its fission product Tc in aerobic environment will be in the forms of UO22+ and TcO4-. Reduced forms of tetravalent U and Tc are sparingly soluble. As determined by transmission electron microscopy, the reduction of uranyl acetate by immobilized cells of Desulfovibrio desulfuricans results in the production of black uraninite nanocrystals precipitated outside the cell. Some nanocrystals are associated with outer membranes of the cell as revealed from cross sections of these metabolic active sulfate-reducing bacteria. The nanocrystals have an average diameter of 5 nm and have anhedral shape. The reduction of Re7+ by cells of Desulfovibrio desulfuricans is fast in the media containing H2 electron donor, and slow in the media containing lactic acid. It is proposed that cytochrome in these cells has an important role in the reduction of uranyl and Re7+ that is a chemical analogue for one uranium fission product Tc7+ through transferring electron from molecular hydrogen or lactic acid to the oxyions of UO22+ and ReO4-. © 2000 Materials Research Society.