Diffusion of14C in dense saturated bentonite under steady-state conditions

Abstract

Diffusion coefficients are critical parameters for predicting migration rates and fluxes of contaminants through clay-based barrier materials used in many waste containment strategies. Cabon-14 is present in high-level nuclear fuel waste and also in many low-level wastes such as those generated from some medical research activities. Diffusion coefficients were measured for14C (in the form of carbonate) in bentonite compacted to a series of dry bulk densities,ρb, ranging from about 0.9 to 1.6 Mg/m3. The clay was saturated with a Na-Ca-Cl-dominated groundwater solution typical of those found deep in plutonic rock on the Canadian Shield. Both effective,De, and apparent,Da, diffusion coefficients were determined.De is defined asD0Τane, where D0 is the diffusion coefficient in pure bulk water,Τa the apparent tortuosity factor, andne the effective porosity available for diffusion; andDa is defined asD0Τane/(ne+ρbKd), where Kd is the solid/liquid distribution coefficient. BothDe andDa decrease with increasingρb:De values range from about 10×10−12 m2/s atρb≃0.9 Mg/m3 to 0.6×10−12 m2/s at 1.6 Mg/m3, andDa values vary from approximately 40×10−12 to 4×10−12 m2/s over the same density range. The decrease inDe andDa is attributed to a decrease in bothΤa andne asρb increases. The data indicate thatne is <10% of the total solution-filled porosity of the clay at all densities.Kd values for14C with the clay range from about 0.3 to <0.1 m3/Mg; this indicates there is a small amount of14C sorbed on the clay and/or some14C is isotopically exchanged with12C in carbonate phases present in the clay. Finally, theDe values for14C are lower than those of other diffusants — I−, Cl−, TcO4−, and Cs+ — that have been measured in this clay and pore-water solution. This is attributed to lower values for bothne andD0 for14C species relative to those of the other diffusants.