Residual Waste from Hanford Tanks 241-C-203 and 241-C-204. 1. Solids Characterization

Bulk X-ray diffraction (XRD), synchrotron X-ray microdiffraction (μXRD), and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) were used to characterize solids in residual sludge from single-shell underground waste tanks C-203 and C-204 at the U.S. Department of Energy's Hanford Site in southeastern Washington state. Čejkaite [Na₄(UO₂)(CO₃)₃] was the dominant crystalline phase in the C-203 and C-204 sludges. This is one of the few occurrences of čejkaite reported in the literature and may be the first documented occurrence of this phase in radioactive wastes from DOE sites. Characterization of residual solids from water leach and selective extraction tests indicates that čejkaite has a high solubility and a rapid rate of dissolution in water at ambient temperature and that these sludges may also contain poorly crystalline Na₂U₂O₇ [or clarkeite Na[(UO₂)O(OH)](H₂O)_0-1] as well as nitratine (soda niter, NaNO₃), goethite [α-FeO(OH)], and maghemite (γ-Fe₂O₃). Results of the SEM/EDS analyses indicate that the C-204 sludge also contains a solid that lacks crystalline form and is composed of Na, Al, P, O, and possibly C. Other identified solids include Fe oxides that often also contain Cr and Ni and occur as individual particles, coatings on particles, and botryoidal aggregates; a porous-looking material (or an aggregate of submicrometer particles) that typically contain Al, Cr, Fe, Na, Ni, Si, U, P, O, and C; Si oxide (probably quartz); and Na−Al silicate(s). The latter two solids probably represent minerals from the Hanford sediment, which were introduced into the tank during prior sampling campaigns or other tank operation activities. The surfaces of some Fe-oxide particles in residual solids from the water leach and selective extraction tests appear to have preferential dissolution cavities. If these Fe oxides contain contaminants of concern, then the release of these contaminants into infiltrating water would be limited by the dissolution rates of these Fe oxides, which in general have low to very low solubilities and slow dissolution rates at near neutral to basic pH values under oxic conditions.