Erosion Properties and Dispersion-Flocculation Behavior of Bentonite Particles

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S. KUROSAWA*, H. KATO*, S. UETA*, K. YOKOYAMA** and H. FUJIHARA*** *Ecosystem and Energy Laboratories, Mitsubishi Materials Corporation, Ibaraki, 311-0102, JAPAN, e-mail: skurosaw @mmc.co.jp "**Research Division of HLW Disposal System, Radioactive Waste Management Center, Tokyo, 105-0001, JAPAN ***Nuclear Fuel Cycle Engineering, Tokyo Electric Power Company, Tokyo, 100-0011, JAPAN

ABSTRACT Experimental and theoretical studies have been performed to clarify the ability of flowing groundwater in contact with bentonite to generate bentonite colloidal particles and disperse such colloids. This information is required to determine (a) the long-term stability of bentonite as a buffer material for borehole disposal of radioactive wastes in deep geologic media and (b) the potential influence of bentonite colloidal particles on radionuclide transport, specifically for use in scenario analyses in the performance assessment of waste disposal. In this study, the minimum groundwater velocity required to erode particles of Nabentonite or Ca-bentonite from a bentonite surface in contact with groundwater was derived from shear strengths of aqueous bentonite gel suspensions, as determined by viscometer tests. The shear strengths were used to estimate the corresponding shear force on bentonite particle-particle bonds, using an estimated value for the number of initial bentonite particle-particle bonds in the experimental systems studied. The derived shear force was converted to corresponding The results groundwater velocity by using Stokes' equation and simplifying assumptions. indicate that groundwater velocities in a range of about 10' to 10' m/s would be required to initiate bentonite erosion. This range is higher than the groundwater flow velocity generally In addition, known groundwater electrolyte found in deep geologic media in Japan. concentrations were compared with theoretical estimates of aqueous electrolyte concentrations required to flocculate colloidal bentonite particles (for example 1 x 10' mol/l Na'). The comparison indicates that, even if erosion of bentonite occurred, the colloidal bentonite particles formed would flocculate. As a result, this study has shown that the effect of bentonite colloids on radionuclide transport is likely to be negligible in the performance assessment of radioactive waste disposal in deep geologic media.

INTRODUCTION Bentonite, used as a radionuclide transport barrier for radioactive waste geologic disposal, can absorb water and expand into fractures in surrounding rock. In a typical disposal environment, the front of the expanding bentonite is a type of clay gel. Groundwater flow contacting this bentonite front could potentially erode bentonite, generating and dispersing

679 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society

colloidal particles of bentonite [1]. Knowledge of the potential extent of colloid generation and dispersion is required (a) to estimate the required stability for disposal borehole buffer material and (b) to estimate the potential influence

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