Diffusion Behavior for Se and Zr in Sodium-Bentonite
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DIFFUSION BEHAVIOR FOR Se AND Zr IN SODIUM-BENTONITE HARUO SATO, MIKAZU YUI, AND HIDEKI YOSHIKAWA Power Reactor and Nuclear Fuel Development Corporation, 4-33 Tokai-mura, Ibaraki-ken 319-11, Japan
Muramatsu,
ABSTRACT Apparent diffusion coefficients for Se and Zr in bentonite were measured by in-diffusion method at room temperature using water-saturated sodium-bentonite. KunigelVl® * containing 50wt% Na-smectite as a major mineral was used as the bentonite material. The experiments were carried out in the dry density range of 400-1800kg/m3 . Bentonite samples were immersed with distilled water and saturated before the experiments. The experiments for Se were carried out under N2 atmospheric condition (02: 2.5ppm). Those for Zr were carried out under aerobic condition. The apparent diffusion coefficients decrease with increasing density of the bentonite. Since dominant species of Se in the pore water is predicted Se0 32 , Se may be retarded by anion-exclusion because of negative charge on the surface of the bentonite and little sorption. The dominant species of Zr in the porewater is predicted Zr(OH)5 or HZrO3 . Distribution coefficient measured for Zr on the bentonite was about 1.0m 3 /kg from batch experiment. Therefore, the retardation may be caused by combination of the sorption and the anion-exclusion. A modelling for the diffusion mechanisms in the bentonite were discussed based on an electric double layer theory. Comparison between the apparent diffusion coefficients predicted by the model and the measured ones shows a good agreement.
INTRODUCTION In the geological isolation research of high level radioactive waste, bentonite has been cosidered as a candidate buffer material. Particularly, sodium-bentonite has been expected as the candidate[l]. The concept of Japanese geological isolation system is shown in Figure 1. The system consists of vitrified waste, overpack, compacted bentonite and bedrock. After the isolation in a deep undergroud, nuclides immobilized in the glass matrix will begin to leach in the groundwater and subsequently migrate through the bentonite by diffusion because of the low hydraulic conductivity. Diffusion coefficient is an important parameter for performance assessment of the geological isolation.
Authors have reported results of measurements of the apparent diffusion
coefficients for H, Tc, Cs, Np and 24 in sodiumbentonite(KunigelVI(*) in the density range of 400-2000kg/m 3[2]. Many laboratory experiments concerning the
diffusivities of radionuclides in bentonite have been carried out so far and reported [3].
Those studies indicate that the apparent diffusion coefficients depend on density of bentonite and porewater chemistry. In addition to that, it is clear that the apparent diffusion coefficients depend on kinds of bentonite. This is considered to be caused by
difference of content of smectite in bentonite which is major component. At present, the repository condition has not been decided in Japan. Therefore, generic approach is required for the application to different cond
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