Effect of Dry Density on Activation Energy for Diffusion of Strontium in Compacted Sodium Montmorillonite

Abstract

For performance assessments of geological disposal of high-level radioactive waste, activation energies for the diffusion of strontium ions and the basal spacings of compacted sodium montmorillonite in the water-saturated state were determined.Basal spacings determined by XRD indicated changes in the interlamellar space from a three-water layer hydrate state to a two-water layer hydrate state as the dry density of the montmorillonite increased from 1.0 to 1.8 Mg m-3. Activation energies from 17.3 to 30.8 kJ mol-1 for the apparent diffusion coefficients of strontium ions were obtained. The lower activation energies than for diffusion of strontium ions in free water were determined for montmorillonite specimens of lower dry density (1.2 Mg m-3 and below), while the higher activation energies were at higher dry densities (1.4 Mg m-3 and above).These findings cannot be explained by changes in only the geometric parameters, which the pore water diffusion model is based upon. Possible explanations for the dry density dependence of the activation energy are the changes of the temperature dependence of the distribution coefficients and/or of the diffusion process with increasing dry density

Self-Diffusion of Water and Ions in Clay Barriers

“…Relative apparent diffusion coefficients D a / D 0 were calculated using D 0 ¼ 2.30, 2.27, and 2.24 10 À9 m 2 s À1 for H 2 O, HDO, and HTO (Mills, 1973) and 1.33, 2.07, 0.794, 2.03, 2.01, and 2.00 10 À9 m 2 s À1 for Na þ , Cs þ , Sr 2þ , Cl À , Br À , and I À at 298 K, respectively (Li and Gregory, 1974). Kozaki et al (1996Kozaki et al ( , 1997Kozaki et al ( , 1998Kozaki et al ( , 1999bKozaki et al ( , 2001Kozaki et al ( , 2005 Smectite (KP) HTO, Na þ , Cs þ , Ca 2þ 0 (Na/Ca) 288e323 AE 0.1 1.0, 1.8 D a , E a CC Kozaki et al (1999aKozaki et al ( , 2005Kozaki et al ( , 2010 Smectite ( …”

Modeling Cation Diffusion in Compacted Water-Saturated Sodium Bentonite at Low Ionic Strength

“…Kozaki and coworkers used purified Kunipia-F bentonite (11,(25)(26)(27)(28)(29) or mixtures of purified 112…”

Activation energy for diffusion of helium in water-saturated, compacted Na-montmorillonite

“…The activation energy for diffusion of ions in water-saturated, montmorillonite depends on the dry density and is different from that of ions in bulk water (Parsons, 1959). This indicates that the diffusion process of ions in montmorillonite changes with an increase in dry density (Kozaki et al, 1996;Kozaki et al, 1998;Kozaki et al 1997;Kozaki et al 1998b). However, there is little change in the activation energy for the diffusion of helium as is also the case for HTO (Nakazawa et al, 1999), though the activation energy of helium still scatters.…”