Characterization of Solids in Residual Wastes from Underground Storage Tanks at the Hanford Site, Washington, U.S.A.
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0985-NN03-18
Characterization of Solids in Residual Wastes from Underground Storage Tanks at the Hanford Site, Washington, U.S.A. Kenneth M. Krupka1, William J. Deutsch1, H. Todd Schaef1, Bruce W. Arey1, Steve M. Heald2, Michael J. Lindberg1, and Kirk J. Cantrell1 1 Pacific Northwest National Laboratory, Richland, WA, 99352 2 Argonne National Laboratory, Argonne, IL, 60439
ABSTRACT Solid phase physical and chemical characterization methods have been used in an ongoing study of residual wastes from several single-shell underground waste tanks at the U.S. Department of Energy’s Hanford Site in southeastern Washington State. Because these wastes are highly-radioactive dispersible powders and are chemically-complex assemblages of crystalline and amorphous solids that contain contaminants as discrete phases and/or coprecipitated within oxide phases, their detailed characterization offers an extraordinary technical challenge. X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS) are the two principal methods used, along with a limited series of analyses by synchrotron-based methods, to characterize solid phases and their contaminant associations in these wastes. Depending on the specific tank, numerous solids (e.g., čejkaite; Na2U2O7; clarkeite; gibbsite; böhmite; dawsonite; cancrinite; Fe oxides such as hematite, goethite, and maghemite; rhodochrosite; lindbergite; whewellite; nitratine; and several amorphous phases) have been identified in residual wastes studied to date. Because many contaminants of concern are heavy elements, SEM analysis using the backscattered electron (BSE) signal has proved invaluable in distinguishing phases containing elements, such as U and Hg, within the complex assemblage of particles that make up each waste. XRD, SEM/EDS, and synchrotron-based methods provide different, but complimentary characterization data about the morphologies, crystallinity, particle sizes, surface coatings, and compositions of phases in these wastes. The impact of these techniques is magnified when each is used in an iterative fashion to help interpret the results from the other analysis methods and identify additional, more focused analyses. INTRODUCTION The underground storage tanks at the U.S. Department of Energy’s (DOE) Hanford Site in southeastern Washington State contain waste liquids and solids from the reprocessing of nuclear fuel rods for the production of plutonium. DOE and its contractor CH2M HILL Hanford Group, Inc. (CH2M HILL) are removing as much of the waste material as possible from these tanks, and then evaluating the filling the tanks with sand or other solid material to prevent collapse and covering them to minimize infiltration of and contact with water. Because of the technical limitations of sludge and supernatant removal processes, some material remains in the tanks after the removal campaigns. The residual wastes could represent a potential future risk to
groundwater if infiltrating water leaches contaminants from the waste and transports t
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