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 (II) Haruo Sato, Japan Atomic Energy Agency (JAEA), 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan ABSTRACT The apparent diffusivities (Da) and activation enthalpies (∆Ea) for I− and Cs+ ions in compacted Na-smectite were obtained in the parallel and perpendicular directions to the orientation of smectite particles as a function of smectite’s dry density (0.9-1.4 Mg/m3), salinity ([NaCl]=0.01, 0.51 M) and temperature (295-333 K). The Da-values for both ions tended to be higher in the parallel direction than in the perpendicular direction to the orientation of smectite particles. The Da-values of I− ions in the parallel direction decreased with increasing salinity only at low dry density, but those of Cs+ ions increased with increasing salinity for all conditions. Based on this, it is interpreted that I− ions predominantly diffuse in external pores and Cs+ ions diffuse in both interlayer and external pores. The ∆Ea-values for I− ions are at similar levels as those for the ionic diffusivity in free water (Do) of I− ions at low dry density and increased with increasing dry density. The ∆Ea-values for Cs+ ions are higher than those for the Do of Cs+ ions even at low dry density and increased with increasing dry density. Such high ∆Ea-values for Cs+ ions are considered to be due to the combined effects of the ion exchange enthalpy between Cs+ and Na+ ions in smectite and the lowering in the activity of porewater. INTRODUCTION In the safety assessment (SA) of the geological disposal for high-level radioactive waste in Japan, the role as a barrier function of the bentonite buffer is important to restrict the release of radionuclides (RNs) from repository, and therefore many related studies have been reported so far [e.g., 1, 2]. Particularly, because the diffusion properties of RNs in compacted bentonite directly control the release of RNs from the bentonite buffer to the geosphere, it is regarded as one of the important characteristics in the SA. It is well known from conventional studies that the retardation in the diffusion process of RNs in compacted bentonite is affected by various physico-chemical properties such as porosity, bentonite’s dry density [1, 3-6], sorption properties, exchangeable cations in the interlayer of smectite [7], porewater chemistry [8, 9], additives to bentonite (e.g., silica sand) [10], initial bentonite grain size [10, 11], temperature [5, 12-17] etc. The author has reported so far that clay particles oriented in the perpendicular direction to the compacted direction of bentonite, of which the smectite content is high, such as Kunipia-F® which is almost 100 wt.% smectite [18-20] and that the effective diffusivities (De) of tritium (HTO) and deuterium (HDO) are different between parallel and perpendicular directions to the orientation of clay particles [18-21]. It has also been reported that the basal spacing of smectite changes dependi
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