Diffusion of Iodide in Compacted Bentonite

Abstract

Diffusion coefficients, D, are critical parameters for predicting migration rates and fluxes of contaminants through dense bentonite-based barrier materials used in many waste containment strategies. Values of D were determined for I- (129I is a relatively long-lived radionuclide present in high-level nuclear fuel waste) in saturated bentonite using both transient and steady-state techniques. The bentonite was compacted to dry densities, ρb, ranging from 0.9 to 1.6 Mg m−3, and saturated with a synthetic groundwater solution having an ionic strength of 0.22. Two different D values were determined: an apparent diffusion coefficient, Da, defined as D0τ, and an effective diffusion coefficient, De, defined as Doτne where Do is the diffusion coefficient in pure bulk solution, τ the tortuosity factor, and ne the fraction of the porosity of the saturated clay that is available for diffusion. The value of Da decreased from ≈6 × 10−10 m2 s−1 at ρb ≈ 0.9 Mg m−3 to 1 × 10−10 m2 s−1 at 1.6 Mg m−3. The decrease in Da with increasing ρb is attributed to a decrease in τ as ρb increased. The effect of ρb on De was even greater: De decreased from ≈ 6 × 10−11 m2 s−1 at ρb ≈ 0.9 Mg m−3 to 3 × 10−12 m2 s−1 at 1.6 Mg m−3. In addition to tortuosity effects, this decrease is ascribed to a decrease in ne with increasing ρb. For I-, ne is generally less than n (the total solution-filled porosity of the clay) because of factors such as anion exclusion. Within the ρb range examined, the migration time for 129I to move through the barrier material can be increased somewhat by increasing ρb; relative to the long half-life of 129I, however, the increased migration time is not significant. On the other hand, there appears to be a critical ρb value for this clay of ≈1.4 Mg m−3, beyond which the flux of 129I from the clay can be markedly decreased. This critical value is probably a function of the specific surface area of the clay inasmuch as it influences the magnitude of the anion-exclusion volume. By compacting a bentonitic barrier material to a density greater than the critical value, the potential hazard associated with the long-term disposal of nuclear fuel waste that contains 129I can be decreased.