Diffusion of Technetium in Dense Bentonite under Oxidizing and Reducing Conditions

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, ρb, ranging from about 0.9 to 1.35 Mg/m3. In a reducing environment, D values were determined at 25 °C and ρb ≃ 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 × 10−10 m2/s at ρb ≃ 0.9 Mg/m3 to 0.73 × 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 × 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.

Surface Diffusion: Is It an Important Transport Mechanism in Compacted Clays?

“…The in-diffusion method (also called a halfcell or back-to-back technique) was used to determine D, values for Sr 2+ in Avonlea bentonite and Lake Agassiz clay at 0 = 1.25 Mg/m 3. The diffusion cells are described in detail by Hume (1993), and the experimental procedure by Sawatsky and Oscarson (1991). The method involves the measurement of the amount of a diffusant that migrates from a tagged to an untagged clay plug within a given time (for Sr 2+, the diffusion time was 22 d).…”

Summary report on Phase 1 feasibility study of in-drift diffusion

“…(In future work, we will determine an actual value of the diffusion coefficient by reducing the diffusion time.) If we use the TcO 4 -aqueous diffusion coefficient for ReO 4 -with a value of 1.48 × 10 -9 m 2 /s (Sawatsky and Oscarson, 1991), about 92% of the source concentration will exist at a distance of 1.5 cm (our cube side distance) by diffusion after 87 days. This extent of diffusion is observed in surface profiling (with the sampling depth about 9 µm).…”

Comparison of measured and calculated diffusion coefficients for iodide in compacted clays

“…The half-cell method used to measure diffusion coefficients in compacted clays is described in detail by Sawatsky & Oscarson (1991). A diagram of the diffusion cell is shown in Fig.…”