Effect of smectite illitization on swelling behavior of Gyeongju bentonite

Abstract

Integrity of engineered barrier system (EBS) needs to be secured for the safe isolation of high–level radioactive waste. Mixed clay bentonite, which composes the buffer of EBS, is known to experience the clay synthesis while interacting with heated pore fluid during radioactive decay. This clay synthesis includes the illitization of smectite, where expandable smectite transforms into non–expandable illite. In this study, we experimentally evaluated the changing characteristics of Gyeongju bentonite relevant to the swelling, hydrologic, and chemical behavior, which is used for the Korean disposal systems for high–level radioactive wastes. Hydrothermal reaction experiments of Gyeongju bentonite with 0.01 M, 0.1 M, and 1 M KCl were conducted to observe the illitization of smectite, followed by the analyses of mineral and chemical compositions with X–Ray Diffraction and X–Ray Fluorescence (XRF). Increasing Young's modulus and decreasing swelling pressure were subsequently measured, along with the changing specific surface area, average particle radius, average pore radius, and total pre volume. Inverse modeling of reaction rate constant, activation energy, and specific surface area of dissolving Ca–montmorillonite and precipitating illite was conducted based on the experimental results, as providing the input data for the conceptual forward modeling of THC behavior of Gyeongju bentonite. From the forward modeling, 26.2%–reduction of swelling pressure of bentonite buffer after 10,000 years was forecasted. Forward modeling coupled local sensitivity analysis was subsequently conducted, which provided the relative impacts of reaction parameters on the swelling behavior. They quantitatively predicted the impact of hydrothermal reaction of smectite on swelling behavior, which is expected to be beneficial for the predictive estimation of long–term integrity of bentonite buffer and EBS.