T.T. Vandergraaf, D.J. Drew, and C.J. Hamon, Engineered Barriers and Analysis Branch, Waste Technology Unit, Whiteshell Laboratories, AECL, Pinawa, MB R0E 1L0, Canada,

W.A. Seddon, AECL Technologies Inc., U.S. Department of Energy, Yucca Mountain Site Characterization Office. P.O. Box 364629, North Las Vegas, NV, 89036-8629, USA


The host rock selected for the US spent fuel repository is the Topopah Springs tuff formation in Yucca Mountain. This formation is underlain by the non-welded Calico Hills formation. Transport of any radionuclides leached from the emplaced wastes is expected to be vertically downward from the Topopah Springs formation through the unsaturated zone in the Calico Hills and, below the water table, horizontally in the saturated zone.

To complement migration experiments with non-radioactive tracers in the Busted Butte Test Facility (BBTF) at the Nevada Test Site, migration experiments were performed on a scale of up to 1 m under unsaturated and saturated conditions in a dedicated facility at the Whiteshell Laboratories in blocks of non-welded tuff, excavated from the BBTF and transported to Whitehell. Na-fluorescein, 3H (as tritiated water), 22Na, 60Co, 95mTc and/or 99Tc (as the pertechnetate anion), 137Cs, and 237Np were used as tracers and synthetic Busted Butte pore water as the transport solution. The duration of the experiments was up to 600 days and the volumetric flow rates ranged from 10 to 20 mL/hr. In the experiment performed under saturated conditions, water samples were taken along the flow field over the duration of the experiment. Information on the radionuclide plume in the unsaturated block was obtained by coring into the block.

Results show transport behavior under unsaturated conditions at least qualitatively consistent with measured batch sorption coefficients. However, under saturated conditions, transport of 99Tc was retarded, consistent with the development of chemically reducing conditions in the tuff block.

Subsequent supporting scoping experiments have shown that, under certain conditions, indigenous and unintentionally introduced microbes may play a role in creating these chemically reducing conditions. If it can be shown that these conditions exist in the saturated zone underlying Yucca Mountain, the geological formations downstream from the repository may provide a formidable barrier to the transport of at least some multivalent radionuclides.