Performance assessment on grouted double-shell tank waste at Hanford

The low-level fraction of liquid waste stored in double-shell tanks at Hanford will be solidified in a cementitious matrix (grout) and disposed in subsurface vaults. This paper discusses activities related to the preparation of a site-specific performance assessment. A draft performance assessment has been prepared for the planned grout disposal system at Hanford using site-specific data. The assessment estimates the incremental increase in the dose to future populations who, after loss of institutional control at the site, use groundwater downgradient of the disposal site. Increases in nonradiological species in water from a hypothetical well are also estimated. Two-dimensional transport models were used to estimate contaminant concentrations in groundwater. Based on diffusional release from the waste package, the projected radiological dose to an individual on a hypothetical farm using water from a well at the disposal facility boundary is estimated at less than one percent of the 25 mrem/yr standard. Technetium accounted for about 95% of the dose. Nitrate was the principle chemical contaminant at 0.3% to 0.5% of apportioned drinking water standards. Sensitivity studies on various parameters are in progress. This performance assessment will be updated as additional data become available. 16 refs., 2 figs., 2 tabs