Application of Scanning Electron Microscopy to Characterization of Radioactive Solid Wastes from Underground Storage Tanks

Abstract

Performance assessments will be completed to evaluate the long-term health risks associated with closure of 177 single-and double-shell carbon steel-shell underground storage tanks containing residual radioactive and toxic wastes at the U.S. Department of Energy's (DOE) Hanford Site in southeastern Washington. The primary contaminants of concern for the Hanford Site are typically 99 Tc, 238 U, 29 I, and Cr because of their mobility in the environment and long half-lives. PNNL is currently developing source term models that describe the release of contaminants as infiltrating water contacts the residual solids. To simulate the geochemical interactions between the leachant phase and contaminant-containing solids, these models must be based on detailed characterization of the residual waste. Because these sludges are highly radioactive, dispersible powders and are chemically-complex assemblages of crystalline and amorphous solids that contain contaminants as discrete phases and/or sequestered within oxide phases, their detailed characterization offers an extraordinary challenge to electron microscopists, geochemists, and laboratory-safety engineers.Scanning electron microscopy/energy dispersive spectrometry (SEM/EDS) and X-ray diffraction (XRD) are the two principal methods being used to characterize solid phases and their contaminant associations in these sludges. SEM/EDS and XRD have been completed on as-received (unleached) samples as well as residual solids obtained from water leach studies and selective extractions designed to isolate the principal phases that control the release of each contaminant of interest. 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 sludge. Figure 1 shows micrographs for sludge samples from Hanford tanks C-203 and C-204 [1]. Because these images were obtained using the BSE signal, the hexagonal needlelike crystals containing Na-U-O in Figure 1A are clearly distinguishable from the Na-Al-P-O and Fe-Cr-Ni oxide phases that make up the sludge matrix. The morphological features and EDS data for these needlelike crystals were consistent with the XRD identification of cejkaite [Na 4 (UO 2 )(CO 3 ) 3 ] in this sludge. Figure 1B shows the same sludge material after a 2-week water leach. This BSE micrograph also allows quick identification of the U-containing solids relative to the rest of the sample matrix. It also shows that the cejkaite needles dissolved during the water leach, but a second, less soluble, amorphous-looking U-Na-O phase [possibly Na 2 U 2 O 7 or clarkeite Na[(UO 2 )O(OH)](H 2 O) 0-1 ] was present in the residual sludge and will impact the long-term release of U from this residual waste. SEM/EDS and XRD also provide different, but complimentary characterization data, which help determine the morphologies, particle sizes, surface textures, and compositions of phase...

Contaminant Release Data Package for Residual Waste in Single-Shell Hanford Tanks

“…Total chromium measured in tank C-106 residual waste was below the EQL at (896) μg/g-dry wt (Table 2. 4;Deutsch et al 2007a). No crystalline chromium mineral phases were detected by XRD analysis.…”

“…Nitrate was not measured above its detection limit of 70 μg/g-dry wt in tank C-106 residual waste (Table 2. 4;Deutsch et al 2007a). No crystalline nitrate mineral phases were detected by XRD analysis.…”

“…Technetium-99 was measured at a total concentration of 1.16 μg/g-dry wt (from fusion method) in tank C-106 residual waste (Table 2. 4;Deutsch et al 2007a). This concentration is too low to allow for detecting crystalline technetium mineral phases by XRD analysis.…”