Estimation of Effective Diffusivity in Compacted Ben Tonite
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ESTIMATION OF EFFECTIVE DIFFUSIVITY COMPACTED BEN TONITE H.Kato*,
M.Muroi*,
N.Yamada*,
H.Ishida*,
IN
H.Sato**
*Naka Energy Research Center, Mitsubishi Materials Corporation, 1002-14, Mukoyama, Naka-machi, Naka-gun, Ibaraki-ken, 311-01 Japan "**Power Reactor and Nuclear Fuel Development Corporation, 4-33 Muramatsu, Tokaimura, Ibaraki-ken, 319-11 Japan ABSTRACT Effective diffusivities of radioactive nuclides in compacted bentonite were calculated theoretically by using an electric double layer theory. Comparison between calculated diffusivities and measured ones show good agreements. The effective diffusivity is dominated by pore structure and pore diffusivity Dp. The pore structure can be characterized by effective porosity e eff, constrictivity 65, and tortuosity r. The 6 was assumed to be unity. The E eff and the r were determined experimentally. The Dp was estimated by means of the electric double layer theory. In the estimation, smectite interlayer was assumed the space between parallel plane sheets of smectite crystal lattice. Diffusion experiments were carried out by using Cs' for monovalent cation , CIand "I4O4 for monovalent anion, and tritiated water for neutral molecule. The measured and calculated effective diffusivities in different densities showed the same tendency of cation > neutral > anion. The dry density of bentonite became higher, the discrepancy between the estimated and the measured diffusivities became larger. The calculation was limited by the applicability of the electric double layer theory in the near surface region of smectite. INTRODU CTION The apparent diffusivities of radionuclides in bentonite buffer materials have been measured by many researchers. However there are few data for the effective diffusivities, nevertheless it is one of the key parameters to predict the release rate of nuclides from the waste repository. This means that the appropriate parameter set of the effective diffusivities is very important as input data in performance assessments. The effective diffusivity depends on many factors such as type of exchangeable cation, contents of smectite, packing density, kind of diffusing species, and so on. The eventual difficulties exist in compilation of effective diffusivity data covering all influenced conditions. Therefore the mechanistic approach to get a set of the effective diffusivity data for the performance assessment is necessary. In this study, the effective diffusivities in compacted bentonite were estimated by using the model of the pore structure and diffusion mechanism in the pore. In the modeling of the pore structure, the effective porosity, the constrictivity and the tortuosity were taken into account. The effective porosity is the porosity where the species can diffuse through. An electric double layer theory was applied to the modeling of the diffusion behavior in the pore, where the viscosity of interlayer water can be derived from the ionic concentration and charge of smectite surface according to "lbkahashi's method [11 . On the calculation of the
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