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Diffusion of Technetium in Dense Bentonite under Oxidizing and Reducing Conditions

AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada R0E 1L0

*Corresponding author.

ABSTRACT

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 (⁹⁹Tc 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 °C in bentonite compacted to dry bulk densities, _b, ranging from about 0.9 to 1.35 Mg/m³. In a reducing environment, D values were determined at 25 °C and _b 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 °C, D ranged from 2.3 x 10^–10 m²/s at _b 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 °C, D values were two to three times greater than those obtained at 25 °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 TcO^–₄[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 ⁹⁹Tc in the long-term disposal of nuclear fuel waste.

Received for publication August 11, 1990.