Diffusion of technetium in dense bentonite under oxidizing and reducing conditions
1991
Sawatsky, N.G. | Oscarson, D.W.
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 unclear 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, ranging from about 0.9 to 1.35 Mg/m3. In a reducing environment, D values were determined at 25 degrees C and dry bulk densities approximately equal 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 st 25 degrees C, D ranged from 2.3 X 10-(10) m2/s at dry bulk densities approximately equal 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 99Tc in the longterm disposal of nuclear fuel waste.
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