Effects of the Orientation of Clay Particles and Ionic Strength on Diffusion and Activation Enthalpies of I - and Cs + I
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Effects of the Orientation of Clay Particles and Ionic Strength on Diffusion and Activation Enthalpies of I– and Cs+ Ions in Compacted Bentonite Haruo SATO, Japan Nuclear Cycle Development Institute (JNC), 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki-ken, 319-1194, JAPAN ABSTRACT Apparent diffusivities (Da) for I– and Cs+ in compacted Na-smectite were studied as a function of smectite’s dry density (0.9–1.4 Mg/m3), ionic strength (IS: [NaCl]=0.01, 0.51 M), temperature (22–60 °C) and diffusion direction to the orientated direction of smectite particles. The Da-values for both ions in parallel direction to the orientated direction showed a tendency to be higher than those in the perpendicular direction at low IS. The Da-values for I– showed different trends depending on diffusion direction and dry density at high IS, but Da-values for Cs+ increased with increasing IS in both diffusion directions. The activation enthalpies (∆Ea) for I–, slightly higher than that of diffusivity in free water (Do) at low IS, similar level to that of Do at high IS, increased with increasing dry density. While, ∆Ea-values for Cs+, clearly higher than that of Do for all conditions, increased with increasing dry density. Since it is known that interlayer distance depends on both dry density and IS, diffusive pathway is considered to differ depending on the charge of diffusion species. INTRODUCTION In safety assessment of the geological disposal of high-level radioactive waste in Japan, a role as a barrier function of buffer material composing multi-barrier system is quite important, and many studies have been therefore reported up to date [e.g. 1]. Particularly, since the diffusion properties of radionuclides (RN) in bentonite directly affect the release rate of RNs from the bentonite, it is regarded as one of the important characteristics in the safety assessment. It is well known from conventional studies that the retardation of RNs in the diffusion process in bentonite receives the effects of the sorption of RNs, porewater chemistry, microstructure, etc. The authors have reported that clay particles orientate in perpendicular direction to the compacted direction of bentonite for a bentonite (Kunipia-F®) which is almost smectite [2–4] and that the diffusivities of tritium (HTO) and deuterium (HDO) are different between parallel and perpendicular directions to the orientated direction of clay particles [2–5]. It has been also reported that the interlayer distance of smectite changes depending on IS [6]. For ∆Ea, ∆Ea-values for Da-values of HTO, Na+, Cs+, Sr2+ and Cl– in compacted Na-montmorillonite have been reported [6–10], and those diffusion mechanisms have been discussed based on the dependencies of the ∆Ea-values on dry density. Moreover, the authors have measured ∆Ea-values for the effective diffusivities of HDO in bentonite samples orientated clay particles [5], and have indicated a possibility that porewater near solid-liquid interface differs from free water because the ∆Ea-values are slightly higher than that of Do. In
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