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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.

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