Effective Diffusivities of Iodine, Chlorine, and Carbon in Bentonite Buffer Material
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'Energy & Ecosystem laboratories, Central Research Institute, Mitsubishi Materials Corp., 1002-14, Mukohyama, Naka-machi, Ibaraki-ken, 311-0102, Japan. ; Fax::+81-29-298-1964, E-mail:hiro((s naka.mmc.co.jp Fuel Cycle Technology Center, Mitsubishi Materials Corporation, 1-3-25, Koishikawa, Bunkyo-ku, Tokyo, 112-0002,
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* Nuclear
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Research Division of Geological Repository. Radioactive Waste Management Center, 2-8-10, Toranomon. Minato-ku, Tokyo,
105-0001. Japan Nuclear Fuel Cycle Engineering, Tokyo Electric Powver Company, 1-1-3. Uchisaiwaicho, chiyoda-ku, Tokyo. 100-0011, Japan
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ABSTRACT Effective diffusivities of iodine, chlorine, and carbon in mixtures of bentonite and sand were determined by measuring the effective diffusivities of common chemical compounds labeled with radioactive isotopes of these elements. For carbon, both inorganic and organic carbon compounds were used in order to consider the variety of chemical forms of carbon possible in a radioactive waste repository. The bentonite content and dry density of the bentonite-sand mixture were varied. Two chemically different aqueous solutions, representing concrete pore water and bentonite pore water, were used to represent different conditions that could affect diffusivity in bentonite buffer material in a hypothetical radioactive waste disposal situation. The effective diffusivities of iodine, chlorine, and carbon tended to decrease with increasing bentonite content and dry density of the mixture. In the presence of simulated concrete pore water, the effective diffusivities for iodine, chlorine, and carbon in the bentonite mixtures were not higher than those obtained when simulated bentonite pore water was used. Except for some organic compounds, the measured effective diffusivities were lower than that of tritiated water under the same experimental conditions. This was attributed primarily to exclusion of anions from the bentonite pores. The effective diffusivity of carbon depended on its chemical form. The effective diffusivity of the anionic forms of organic carbon tested (carboxylic acids) was as low as that of inorganic anionic carbon. Measured effective diffusivities were compared with those calculated using a model based on electrical double layer theory. The theory was applied to calculate distributions of electrolyte ions and diffusing ions in the bentonite pores. The calculated effective diffusivities showed good agreement with the measured values.
INTRODUCTION 1-129, CI-36 and C-14 are key nuclides in the safety assessment of radioactive waste disposal in Japan. These nuclides have comparatively long half-lives and low sorption properties in engineered barriers and natural geologic media. It is therefore important to quantitatively evaluate the capability of bentonite buffer materials to restrain diffusion of these radionuclides. 1-129, Cl36, and C-14 exist mainly in anionic form in bentonite pore water, therefore low effective diffusivities would be expected if an anion exclusion mechanism existed for bentonite pores. Many
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