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

摘要

To complement migration experiments with non-radioactive tracers in the Busted Butte experimental facility (BBTF) at the Nevada Test Site, an exploratory migration experiment has been performed under unsaturated conditions in a ～0.3m x ～0.3m x ～0.3m block of tuff. Longer term migration experiments, up to 600 days, under unsaturated and saturated conditions in ～1 m 3 blocks of tuff have recently been completed. Na-fluorescein, 3 H (as tritiated water), 22Na, 60Co, 95mTc and/or 99Tc (as the pertechnetate anion), 137Cs, and 237Np were used as tracers in all three experiments. Under unsaturated conditions, Tc is transported slightly faster than 3 H, while under saturated conditions, the chemical conditions became highly reducing, leading to significant retardation of Tc along the flow field. If chemically reducing conditions can be demonstrated to exist in the saturated zone downstream from the proposed repository, the geological formations underlying the proposed repository horizon can potentially act as a geological barrier to the transport of some multivalent radionuclides.