Numerical modeling of heating and hydration experiments on bentonite pellets

Abstract

In a common, designed disposal system for high-level radioactive waste, bentonite is used worldwide as a buffer material due to its favorable thermodynamic properties. In-situ experiments at different scales are being conducted in several underground laboratories to investigate the long-term coupled thermo-hydro-mechanical (THM) and chemical behavior of bentonite. Simultaneously, numerous laboratory experiments under well-defined conditions were performed to determine the material properties of bentonite. Since 2012, a laboratory heating and hydration test on MX-80 bentonite has been performed by CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas). The obtained data during the test were analyzed by different research teams within the international DECOVALEX project in order to understand the coupled THM behavior of bentonite and furthermore to determine the thermal-mechanical properties of bentonite. This paper presents a numerical model for fully-coupled THM processes in bentonite based on the finite element method program OpenGeoSys (Kolditz et al., 2012a). In this model, the description of heat conduction is based on Fourier's law, and advection is also considered in the heat transfer. As an important coupling factor from hydraulic process to thermal process, the dependency of thermal conductivity on the water saturation is intensively analyzed. A multiphase flow model considering water evaporation and vapor diffusion is used to describe the hydraulic process. Special attention was also paid on the analysis of the water retention behavior. An elastic constitutive model based on generalized Hook's law was applied to describe the material's mechanical behavior complemented with consideration of the thermally induced strain and the swelling deformation. Good agreement between the calculated and measured data has been achieved concerning temperature, relative humidity, total stress, and water intake. The main coupling processes and bentonite behavior in the experiment could be captured and well analyzed in this model. However, further investigations are also needed, especially for the long-term THM process with respect to water intake.