DIFFUSION OF TECHNETIUM IN DENSE BENTONITE UNDER OXIDIZING AND REDUCING CONDITIONS

Diffusion is the principal mechanism by which most contaminants migrate through dense bentonite-based barrier materials used in many waste-containment strategies. Diffusion coefficients are, therefore, critical parameters for predicting migration rates of contaminants through these materials. Diffusion coefficients, D, were determined for Tc(Tc-99 is a relatively long-lived radionuclide present in high-level nuclear fuel waste) in compacted, nearly saturated bentonite under both oxidizing and reducing conditions. The D values were measured under oxidizing conditions at 25 or 80-degrees-C in bentonite compacted to dry bulk densities, rho-b, ranging from about 0.9 to 1.35 Mg/m3. In a reducing environment, D values were determined at 25-degrees-C and rho-b congruent-to 1.3 Mg/m3; reducing conditions were established by mixing 1% (w/w) Fe(II)-silicate with the bentonite and by conducting the experiment under a N2 atmosphere. Under oxidizing conditions at 25-degrees-C, D ranged from 2.3 x 10(-10) m2/s at rho-b congruent-to 0.9 Mg/m3 to 0.73 x 10(-10) m2/s at 1.35 Mg/m3. The decrease in D with increasing density is largely attributed to an increase in the tortuosity of the diffusion path as the density increased. At 80-degrees-C, D values were two to three times greater than those obtained at 25-degrees-C at all densities; this is primarily due to a decrease in the viscosity of the saturating solution with increasing temperature. In a reducing environment, D was 6 +/- 3 x 10(-12) m2/s. The lower D values under reducing conditions are attributed to the reduction of TcO4-[Tc(VII)] to a less mobile Tc(IV) species. The results indicate that the migration of Tc through a bentonite-based barrier material will be relatively slow in a reducing environment, such as that expected in a nuclear fuel waste disposal vault located deep in plutonic rock; this will mitigate the potential hazard associated with Tc-99 in the long-term disposal of nuclear fuel waste.