Haruo Sato1
Okayama University1
Buffer material (compacted bentonite) composing engineered barrier in the geological disposal of a high-level radioactive waste develops swelling stress (pressure) by penetration of groundwater from the surrounding rock mass after disclosure. In the past studies, we measured the activity of water and the Gibbs free energy (relative partial molar Gibbs free energy) of water in Na-montmorillonite which is the major component of Na-bentonite by vapor pressure method, and reported a model to analyze the swelling stress of bentonite based on thermodynamic theory. On the other hand, data for the vapor pressure of water in bentonite are limited. In this study, we determined vapor pressure by measuring relative humidity (RH) and temperature for water in Na-bentonite and Na-montmorillonite, and calculated the activity of water and the Gibbs free energy. In addition, we analyzed the swelling stress of bentonite based on the thermodynamic model and discussed by comparing to data reported so far.<br/>Kunigel-V1<sup>®</sup> and Kunipia-F<sup>®</sup> (provided from Kunimine Industries Co. Ltd.) were used as a Na-bentonite. The montmorillonite contents of both bentonites are approximately 51 % and 99 %, respectively. Bentonite powder which was dried at 105-110 °C for at least 24 hours in an oven was placed in a polyethylene bottle in an amount of 3.00 g each (repeatability n=3), and slowly adsorbed water vapor in a closed container (vacuum chamber) with 100 % RH for about 9 months. The water content of the bentonite was checked by weighing periodically. Next, RH and temperature sensors and moisturized bentonite powder were placed in the vacuum chamber, and the chamber of which inside pressure was reduced to -95 kPa (6.3 kPa with absolute pressure) or less was submerged in a constant temperature water bath at 25 °C. The RH and temperature in the chamber were measured after 24 hours and the weight of the bentonite was measured. The bentonite sample was returned to the chamber again and the water content of the bentonite was reduced by evacuating for a while, and then the chamber was submerged in a constant temperature water bath again. This operation was carried out every about 24 hours. Thus, RH and temperature were measured as a function of water content (ca. 10-100 %), and the activity of water and the Gibbs free energy as a function of water content were determined.<br/>The activities of water and the Gibbs free energies for both bentonites decreased with decreasing water content in water contents lower than approximately 40 %. This trend is the same as the trend obtained in the past studies. On the other hand, the data (both activities of water and the Gibbs free energies) obtained in this study were slightly higher than conventional data. This indicates that the swelling stress of bentonite slightly decreases than conventional analysis results. This is considered to be due to that conventional data were obtained for purified Na-montmorillonite (no impurities). Although the swelling stresses of bentonite calculated using thermodynamic data obtained in this study were slightly lower than conventional analysis results, those were closer to the measured values than the conventional analysis results.