DIFFUSION OF IODIDE IN COMPACTED BENTONITE

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- (I-129 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, rho(b), 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, D(e), defined as D(o)tau, and an effective diffusion coefficient, D(e) defined as D(o)taun(e) where D(o) is the diffusion coefficient in pure bulk solution, tau the tortuosity factor, and n(e) the fraction of the porosity of the saturated clay that is available for diffusion. The value of D(a) decreased from almost-equal-to 6 x 10(-10) m2 s-1 at rho(b) almost-equal-to 0.9 Mg m-3 to 1 x 10(-10) m2 s-1 at 1.6 Mg m-3. The decrease in D(a) with increasing rho(b) is attributed to a decrease in tau as rho(b) increased. The effect of rho(b) on D(e) was even greater: D(e) decreased from almost-equal-to 6 x 10(-11) m2 s-1 at rho(b) almost-equal-to 0.9 Mg m-3 to 3 x 10(-12) m2 s-1 at 1.6 Mg m-3. In addition to tortuosity effects, this decrease is ascribed to a decrease in n(e) with increasing rho(b). For I-, n(e) is generally less than n (the total solution-filled porosity of the clay) because of factors such as anion exclusion. Within the rho(b) range examined, the migration time for I-129 to move through the barrier material can be increased somewhat by increasing rho(b); relative to the long half-life of I-129, however, the increased migration time is not significant. On the other hand, there appears to be a critical rho(b) value for this clay of almost-equal-to 1.4 Mg m-3, beyond which the flux of I-129 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 I-129 can be decreased.