Cementitious Wasteforms for Immobilization of Low-Activity Radioactive Wastes
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Cementitious Wasteforms for Immobilization of Low-Activity Radioactive Wastes Dawn Wellman 1, Chase Bovaird 1, Kent Parker1, Elsa Cordova1, Aaron Davis1, Shas Mattigod1, Laura Powers2, and Marcus Wood 3 1 Pacific Northwest National Laboratory, Richland, WA, U.S.A. 2 Wiss, Janney, Elstner Associates, Inc., Northbrook, IL, U.S.A. 3 CHPRC, Richland, WA, U.S.A ABSTRACT Solidification of low-activity wastes with cementitious materials is a widely accepted technique that contains and isolates waste from the hydrologic environment. The radionuclides I129, Se-75, Tc-99, and U-238 are identified as long-term dose contributors. The anionic nature of these radionuclides in aqueous solutions allows them to readily leach into the subsurface environment. Any failure of concrete encasement may result in water intrusion and consequent mobilization of radionuclides from the waste packages via mass flow and/or diffusion into the surrounding subsurface environment. Assessing the long-term performance of waste grouts for encasement of radionuclides requires understanding the: 1) speciation and interaction of the radionuclides within the concrete wasteform, 2) diffusion of radionuclide species when contacted with vadose zone porewater or groundwater under environmentally relevant conditions, and 3) long-term durability and weathering of concrete waste forms. An improved understanding of the interactions of long-lived radionuclides in cementitious matrices will improve predictions of the long-term fate of these sequestered contaminants. An integrated laboratory investigation has been conducted including a: 1) multifaceted spectroscopic investigation to interrogate the speciation and interaction of radionuclides within concrete wasteforms, 2) solubility tests to quantify the stability of solid phases identified as radionuclide-controlling phases, 3) quantify the diffusion of radionuclides from concrete wasteforms into surrounding subsurface sediment under realistic moisture contents (4%, 7%, and 15% by weight moisture content), 4) quantify the longterm durability of concrete waste forms as a function environmental parameters relevant to depository conditions, and 5) identify the formation of secondary phases or processes (microcracking) that influence radionuclide retention. Data obtained from this investigation provides valuable information for understanding the speciation, behavior, and fate of radionuclides immobilized within concrete wasteforms under vadose zone conditions and underscores the necessity for robust, multi-disciplinary performance assessments for concrete waste forms. INTRODUCTION Cementitious materials are readily accessible, low cost, possess high physical strength, and are easily tailored to afford waste forms for low- and high-level waste streams. Concrete encasement contains and isolates the waste from the hydrologic environment. Any failure of concrete encasement may result in water intrusion and consequent mobilization of radionuclides from the waste packages. The United States Department of Energy plans to dispos
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