A Thermodynamic Analysis on the Swelling Stress of Na-Bentonite under Various Solution Conditions
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A Thermodynamic Analysis on the Swelling Stress of Na-Bentonite under Various Solution Conditions 1
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Haruo SATO and Masaki FUKAZAWA Graduate School of Natural Science and Technology, Okayama University, 3−1−1, Tsushima-naka, Kita-ku, Okayama-shi, Okayama 700-8530, Japan 2 Faculty of Engineering, Okayama University, 3−1−1, Tsushima-naka, Kita-ku, Okayama-shi, Okayama 700-8530, Japan 1
ABSTRACT The swelling stress of bentonite which is one of the engineered barriers and backfill materials for radioactive waste disposal is strongly dependent on water chemistry such as saline water. The authors have developed a thermodynamic model for calculating the swelling stress (pressure) of bentonite, based on the thermodynamic data of interlayer water in Na-montmorillonite obtained in earlier studies. In this work, the swelling stress of water-saturated Na-bentonite was calculated for various bentonite dry densities and solution conditions such as sodium chloride concentration and nitrate concentration and compared to the measured data. Swelling stress versus montmorillonite partial density was estimated for solutions containing sodium chloride ([NaCl] = 0−3.4 m, m: molality) and nitrate concentrations ([NaNO3] = 0−6 m) and compared to data measured for bentonites with various montmorillonite contents and silica sand contents. The calculated swelling stresses commonly decreased with increasing [NaCl] and [NaNO3] for the same montmorillonite partial density. The trend of swelling stress versus [NaCl] was in a good agreement with the measured results. The calculated swelling stress versus [NaCl] was also quantitatively in a good agreement within the scattering of the measured data. The trend versus [NaNO3] was also similar to that versus [NaCl]. However, the calculated results were quantitatively different from the measured data ([NaNO3] = 3, 5 m, montmorillonite partial density = 0.76−0.87 Mg/m3). Even though those measurements were conducted under the condition of high ionic strength, the measured data of swelling stresses were almost the same as in the condition of distilled water. Since the measurement periods were quite rather short, it is conceivable that the measurements were not done at equilibrium. INTRODUCTION Na-bentonite will be used as a buffer material composing engineered barrier system (EBS) not only for a high-level radioactive waste (HLW) disposal, but also for a low-level radioactive waste (LLW) disposal [1, 2]. Na-bentonite will be also mixed with surplus soil (muck) as a backfill material for disposal tunnel and access tunnels, etc. In addition, Na-bentonite will be mixed as a backfill material for impermeable layer in the LLW disposal [2]. The dominant clay mineral in a Na-bentonite is usually Na-montmorillonite which develops a swelling stress and a low hydraulic conductivity through hydration process. The swelling property of bentonite is strongly dependent on water chemistry, such as the content of sodium chloride [e.g., 3-5]. A large amount of nitrate is expected to dissolve from salt part of the LLW-T
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