Closure of the Oak Ridge National Laboratory Hydrofracture Facility: An opportunity to study the fate of radioactive wastes disposed of by subsurface injection

At Oak Ridge National Laboratory, subsurface injection has been used to dispose of liquid low-level nuclear waste for the past two decades. The process consists of mixing the liquid waste with cement and other additives to form a slurry that is injected under pressure through a cased well into a low-permeability shale at a depth of approximately 300 m (1000 ft). The slurry spreads from the well along hydraulic fractures and sets to form irregularly shaped grout sheets of up to 200 m (650 ft) in radius. Closure-related site characterization provides a unique opportunity to study the fate of the injected wastes. A series of monitoring wells are in place to measure groundwater chemistries within the injection strata and within overlying and underlying confining units. Initial results indicate that contaminated groundwater surrounds the grout sheets in the injection zone, extending at least as far as 300 m (1000 ft) from the injection well; contaminated groundwater is largely and perhaps exclusively confined to the host formation; and of the /sup 90/Sr and /sup 137/Cs radionuclides disposed of, only /sup 90/Sr is present in the contaminated groundwater. The illite-rich mineralogy of the injection formation strongly absorbs /sup 137/Cs and greatly retards its migration. Movement of /sup 90/Sr is not as greatly retarded by the injection formation. Geochemical modeling is being used to identify and to evaluate hydrogeological controls on /sup 90/Sr behavior. Preliminary results suggest that the groundwaters within the injection formation are saturated with Sr from natural sources, and that /sup 90/Sr mobility may be lessened by precipitation/dissolution reactions associated with such a saturated condition. 27 refs., 4 figs., 2 tabs.