Characterizing Solids in Residual Wastes from Single-Shell Tanks at the Hanford Site.

Solid-phase characterization methods have been used in an ongoing study of residual wastes (i.e., waste remaining after final retrieval operations) from underground single-shell storage tanks 241-C-103, 241 C 106, 241-C-202, 241-C-203, and 241-S-112 at the U.S. Department of Energy’s Hanford Site in Washington State. The results of studies completed to date show variability in the compositions of those residual wastes and the compositions, morphologies, and crystallinities of the individual phases that make up these wastes. These differences undoubtedly result from the various waste types stored and transferred into and out of each tank and the different sluicing and retrieval operations used for waste retrieval. The studies indicate that these residual wastes are chemically-complex assemblages of crystalline and amorphous solids that contain contaminants as discrete phases and/or coprecipitated within oxide/hydroxide phases. Depending on the specific tank, various solids (e.g., gibbsite; bohmite; dawsonite; cancrinite; Fe oxides/hydroxides such as hematite, goethite, and maghemite; rhodochrosite; lindbergite; whewellite; nitratine; and numerous amorphous or poorly crystalline phases) have been identified by X-ray diffraction and scanning electron microscopy/energy dispersive X-ray spectroscopy in residual wastes studied to date. The studies also show that contact of residual wastes with Ca(OH)2- and CaCO3-saturated aqueous solutions, which were used as surrogatesmore » for the compositions of pore-fluid leachants derived from young and aged cements, respectively, may alter the composition of solid phases present in the contacted wastes. Iron oxides/hydroxides have been identified in all residual wastes studied to date. They occur in these wastes as discrete particles, particles intergrown within a matrix of other phases, and surface coatings on other particles or particle aggregates. These Fe oxides/hydroxides typically contain trace concentrations of other transition metals, such Cr, Mn, Ni, Pb, and/or U. Recent analyses of residual waste from 241-C-103 have revealed the presence of Tc-containing Fe oxide/hydroxide particles, which is believed to be the first direct evidence of Tc in solid phases in actual samples of Hanford pre-final retrieval tank waste or post-final retrieval residual waste. The presence of mineralized coatings, such as Fe oxides/hydroxides or reaction products precipitated from contact with cement pore fluids, on contaminant-containing particles could decrease their rate of dissolution, thereby delaying the release of contaminants until the coatings dissolve sufficiently to expose the underlying matrix to infiltrating pore fluids. Certain key cross-cutting geochemical processes and solid phase characteristics important to contaminant waste release are becoming evident, now that residual wastes have been studied from several single-shell tanks. As more residual tank wastes are characterized in terms of composition and contaminant release, it is anticipated that common characteristics with respect to the type of wastes stored in the tanks will become evident. This may allow the grouping of tanks into general categories with certain common chemical features and contaminant release characteristics - an important goal because complete characterization of residual wastes from all 149 single-shell storage tanks is not practical. Additionally, information garnered from residual waste characterization activities can be used to optimize future waste retrieval operations.« less