Moisture Adsorption and Volume Change of Partially Saturated Bentonite Buffer Materials
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MOISTURE ADSORPTION AND VOLUME CHANGE OF PARTIALLY SATURATED BENTONITE BUFFER MATERIALS TAKESHI KANNO" AND HISAO WAKAMATSU" *Ishikawajima-Harima Heavy Industries Co., Ltd., Research Institute, 1 Shin-Nakahara-cho, Isogo-ku, Yokohama 235, Japan **Ishikawajima-Harima Heavy Industries Co., Ltd., Nuclear Power Division, 3-2-16 Toyosu, Koto-ku, Tokyo 135, Japan ABSTRACT Water vapor adsorption characteristics of buffer materials to be used for the geologic disposal of high-level radioactive waste have been investigated. Highly compacted blocks of Japanese 3Na bentonite were used as the buffer material. Initial dry density of the blocks was 1.8 g/cm . Volume changes and water contents of the blocks in equilibrium with surrounding moist air were measured. The humidity of the surrounding air was kept constant, and the suction of the block at equilibrium has been determined by the humidity. The temperature of the surrounding air was varied between 20'C and 60'C. The volumetric strain was observed to vary approximately between -3% and 13% depending on the humidity of 0.6% to 95%. Some analytical expressions derived from these experiments on moisture adsorption are presented in this paper. Water vapor adsorption isotherm of the bentonite blocks was also obtained. The swelling pressures calculated from the analytical expressions are compared with literature data for this bentonite. INTRODUCTION Bentonite clay, in a highly compacted form, is considered to be one of the most promising candidate buffer material mainly because of its low hydraulic conductivity and high adsorption capacity of radionuclides. Fundamental properties for various kinds of bentonite, in both mechanical and chemical aspects, have been investigated in many countries [1]-[4]. In a mechanical aspect, most attention has been focused on the measurement of properties such as hydraulic conductivity, water diffusivity, thermal conductivity, and swelling pressure of highly compacted bentonite. However, in order to evaluate the performance of the buffer material, more detailed properties should be taken into account. A physical and numerical model describing fully coupled thermo-hydro-mechanical behaviors of partially saturated
buffer materials is thought to be essential for the prediction on phenomena in an actual
repository for high-level radioactive waste. Only one model of the fully coupled behaviors has been applied to the highly compacted bentonite [5]. One of reasons for this scarcity is lack of the information about analytical expressions for the highly compacted bentonite on state surfaces including suction, void ratio, volumetric water content, applied stress, and temperature. In the case of common soils, for example, Alonso et al. carried out coupled flow-stress-strain analysis of partially saturated expansive claystone by using analytical expressions of void ratio, degree of saturation, applied stress and suction [6]. Moreover, thermodynamic consideration to energy states of water in the compacted bentonite is also required for the modeling. Thermodyna
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