A thermodynamic model of effect of temperature on swelling stress of buffer material in geological disposal

Buffer material composing engineered barrier in the geological disposal of a high-level radioactive waste is affected by decay heat from vitrified waste. Swelling stress (pressure) distribution is formed in parallel with the formation of temperature distribution in the buffer material by penetration of groundwater. We have reported a thermodynamic model of swelling stress of Na-bentonite including montmorillonite under standard temperature (25 °C) in previous studies. In the conventional model, swelling stress was calculated based on the difference between the relative partial molar Gibbs free energies of interlayer water and external water of montmorillonite. In this study, we discussed the effect of temperature on the swelling stress of Na-bentonite from the viewpoint of thermodynamics. The effect of temperature on the swelling stress of Na-bentonite was based on Clausius–Clapeyron’s equation. By applying the conventional model to this theory, an advanced thermodynamic model which can calculate the effect of temperature on swelling stress was derived. Key parameter in the model is the relative partial molar enthalpy. The relative partial molar enthalpy of interlayer water of Na-montmorillonite which is the major component of Na-bentonite was approximately 0 in montmorillonite partial densities lower than 1.25 M m⁻³ and decreased in montmorillonite partial densities higher than 1.25 M m⁻³. This indicates that swelling stress gradually decreases with the increase in temperature in montmorillonite partial densities higher than 1.25 M m⁻³. Degree of decrease in swelling stress was 10–20% in the comparison between 25 and 60 °C. Considering design density of buffer material in repository, it is considered that we realistically need not consider the effect of temperature. Graphical abstract: [Figure not available: see fulltext.]