TRANSPORTNUMBEROFSODIUMIONS.PDF

P/MTPM/45 TRANSPORT NUMBER OF SODIUM IONS IN WATER-SATURATED, COMPACTED NA-MONTMORILLONITE Tomohiro Higashihara *, Kumiko Kinoshita, Yorimasa Akagi, Seichi Sato, Tamotsu Kozaki Division of Energy and Environmental Systems, Graduate School of Engineering, Hokkaido University, Kita 13 Nishi 8, Sapporo 060-8628, Japan * Present address, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-shi, Chibaken 270-1194, Japan INTRODUCTION In geologic disposal of both high-level radioactive wastes and a part of TRU wastes, compacted bentonite is a promising material to be used for a buffer material. Radionuclides are not migrated by advection but by diffusion, because of very low hydraulic conductivity of the bentonite. It is important to clarify the diffusion process of the radionuclide as a part of a safety assessment of the geologic disposal. In the buffer material, the diffusion of the radionuclides may be affected by other ions in the groundwater, because the nuclides as ions diffuse in electrically neutral condition by coupling with counter ions. However, there are quite few reports in which the contribution of these counter ions to diffusion of ions is examined. Nakazawa et al. (1999) suggested that the charge carrier of electrical conduction in the Na- montmorillonite is hydrogen ions rather than Na+ ions. However, the major charge carrier in the compacted bentonite has not been clarified sufficiently, because their conclusion was based on the indirect method such as measurement of electrical conductivities. Measurement of transport number is one of direct methods and is one of the most basic electrochemical properties of materials. The transport number suggests the contribution of counter ions to the migration of radionuclides. The