Publications

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An analysis of the Waste Isolation Pilot Plant (WIPP) colloid model constraints and parameter values was performed. The focus of this work was primarily on intrinsic colloids, mineral fragment colloids, and humic substance colloids, with a lesser focus on microbial colloids. Comments by the US Environmental Protection Agency (EPA) concerning intrinsic Th(IV) colloids and Mg-Cl-OH mineral fragment colloids were addressed in detail, assumptions and data used to constrain colloid model calculations were evaluated, and inconsistencies between data and model parameter values were identified. This work resulted in a list of specific conclusions regarding model integrity, model conservatism, and opportunities for improvement related to each of the four colloid types included in the WIPP performance assessment.

Within the Used Fuel Disposition Campaign (UFDC) of the United States Department of Energy Office of Nuclear Energy (DOE-NE), we have investigated used fuel (UF) degradation and radionuclide mobilization (UFD&RM) and implemented/produced a set of models encompassing radiolytic processes, UF matrix degradation, instant release fractions (IRF) of key radionuclides, and first-principles atomistic models for UO2 and its potential corrosion products. The goals of this collaborative effort (among three different national laboratories: Argonne National Laboratory [ANL]; Pacific Northwest National Laboratory [PNNLJ; and Sandia National Laboratories [SNL]) are to enhance the understanding of UF degradation processes and the technical bases for safety analyses in a range of generic disposal environments. In addition to these modeling efforts, integrated experimental studies are being conducted at both ANL and PNNL to evaluate and validate (and ultimately expand) process models for radiolytic phenomena and UF matrix degradation in various geologic disposal conditions. Integration and coupling of these process models into a generic performance assessment model (GPAM) is one focus of SNL efforts within the generic analyses of the Engineered Barrier System (EBS) for various repository environments. As discussed below, the present work has produced a set of models for implementation into the GPAM as an initial step towards an enhanced coupled model of source-term processes.

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The structures of the only known minerals containing peroxide, namely studtite [(UO 2)O 2(H 2O) 4] and metastudtite [(UO 2)O 2(H 2O) 2], have been investigated using density functional theory. The structure of metastudtite crystallizing in the orthorhombic space group Pnma (Z = 4) is reported for the first time at the atomic level and the computed lattice parameters, a = 8.45, b = 8.72, c = 6.75 Å, demonstrate that the unit cell of metastudtite is larger than previously reported dimensions (Z = 2) derived from experimental X-ray powder diffraction data. © 2012 The Royal Society of Chemistry.

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Published results of performance assessments for deep geologic disposal of high-level radioactive waste and spent nuclear fuel provide insight into those aspects of the waste form that are potentially important to the long-term performance of a repository system. Alternative waste forms, such as might result from new technologies for processing spent fuel and advances in nuclear reactor design, have the potential to affect the long-term performance of a geologic repository. This paper reviews relevant results of existing performance assessments for a range of disposal concepts and provides observations about how hypothetical modifications to waste characteristics (e.g., changes in radionuclide inventory, thermal loading, and durability of waste forms) might impact results of the performance assessment models. Disposal concepts considered include geologic repositories in both saturated and unsaturated environments. Specifically, we consider four recent performance assessments as representative of a range of disposal concepts. We examine the extent to which results of these performance assessments are affected by (i) thermal loading of the waste proposed for disposal; (ii) mechanical and chemical lifetime of the waste form; and (iii) radionuclide content of the waste. We find that peak subsurface temperature generally is a constraint that can be met through engineering solutions and that processing of wastes to reduce thermal power may enable more efficient use of repositories rather than improved repository performance. We observe that the rate of radionuclide release is often limited by geologic or chemical processes other than waste form degradation. Thus, the effects on repository performance of extending waste-form lifetime may be relatively small unless the waste form lifetime becomes sufficiently long relative to the period of repository performance. Finally, we find that changes to radionuclide content of waste (e.g., by separation or transmutation processes) do not in general correspond to proportional effects on repository performance. Rather, the effect of changes to radionuclide content depends on the relative mobility of various radionuclides through the repository system, and consequently on repository geology and geochemistry.

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.

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