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Diffusion of14C in dense saturated bentonite under steady-state conditions

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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,ρ b, 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,D e, and apparent,D a, diffusion coefficients were determined.D e is defined asD 0 Τ a n e, where D0 is the diffusion coefficient in pure bulk water,Τ a the apparent tortuosity factor, andn e the effective porosity available for diffusion; andD a is defined asD 0 Τ a n e/(n e b K d ), where Kd is the solid/liquid distribution coefficient. BothD e andD a decrease with increasingρ b:D e values range from about 10×10−12 m2/s atρ b≃0.9 Mg/m3 to 0.6×10−12 m2/s at 1.6 Mg/m3, andD a values vary from approximately 40×10−12 to 4×10−12 m2/s over the same density range. The decrease inD e andD a is attributed to a decrease in bothΤ a andn e asρ b increases. The data indicate thatn e is <10% of the total solution-filled porosity of the clay at all densities.K d 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, theD e 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 bothn e andD 0 for14C species relative to those of the other diffusants.

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Oscarson, D.W., Hume, H.B. Diffusion of14C in dense saturated bentonite under steady-state conditions. Transp Porous Med 14, 73–84 (1994). https://doi.org/10.1007/BF00617028

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Key words

  • Clay barriers
  • carbonate
  • diffusion
  • sorption
  • mass transport
  • nuclear fuel waste management