RADIONUCLIDE MIGRATION EXPERIMENTS AT A SCALE OF UP TO 1 METRE IN TUFF BLOCKS UNDER UNSATURATED CONDITIONS

Results are presented for a migration experiment, performed on a scale of ~0.3 m under unsaturated conditions in a dedicated facility at the Whiteshell Laboratories in support of the US DOE nuclear waste disposal program. Na-fluorescein, H-3 (as tritiated water), Na-22, Co-60, Tc-95m/99 (as the pertechnetate anion), Cs-137, and Np-237 were used as tracers and synthetic Busted Butte pore water as the transport solution. Preliminary results are presented from a similar migration experiment that is currently being performed at a scale of ~1 m with the same suite of tracers in the same facility. The tuff blocks were excavated from the Calico Hills formation in the Busted Butte experimental facility at the Nevada Test Site. After a vertical flow rate of 20 mL/h had been established under unsaturated conditions through the ~0.3 m trial block using synthetic pore water, an 800 mL pulse of tracer-containing solution was added to the top of the block over a 40-hour period. The rate of transport of the tracers through the block was Tc-95m/99~fluorescein>H-3>Np-237. These results agree quantitatively with those predicted from results obtained in static sorption experiments. Both Tc and the dye tracer were eluted slightly ahead of the tritiated water; this has been attributed to the anion exclusion effect. Post-migration radiometric analysis of the tuff in the flow field, using successive removal of layers of tuff and coring into the block, showed that Co-60 was most strongly retained, followed by Cs-137 and Na-22. These results are consistent with measured sorption coefficients obtained under static conditions. The migration experiment at a scale of 1 m was initiated in April 2001. In contrast with the previous experiment, the tracer solution is injected continuously in two locations at the top of the block, at a flow rate of 10 mL/h per injection point. By June 2002, the normalized concentrations of H-3, Tc-99 and the dye tracer in the water collected from the bottom of the block had reached a value of ~0.08. As expected, no evidence of the other tracers has been observed in the eluent from this second block. The results from the smaller scale experiment show that, under chemically oxidizing conditions, Tc moves slightly faster than the transport solution but that Np is retarded by a factor of ~3.2.