Thermo-Hydraulic Modelling of the In-Situ HotBENT Experiment: Investigating Bentonite Barrier Behaviour at High Temperature and Hydration

The HotBENT experiment is a joint undertaking of multiple international partners at the Grimsel Test Site operated by NAGRA [1,2]. It was designed to replicate the conditions that occur in a deep underground repository for high-level radioactive waste (see Figure 1). The experiment investigates the behaviour of bentonite buffer subjected to high heat loading (up to 200 °C) from the emplaced waste canisters and hydration from the surrounding host-rock. This gives rise to multiple processes, specifically within the bentonite buffer, that compete and interact in a complex way, including evaporation, induced desaturation and drying  in regions affected by elevated temperatures, and conversely, saturation–induced swelling in the regions which are cooled. While the geometry of the experiment is not overly complex, it is not entirely straightforward due to the presence of multiple components, such as the heaters, bentonite buffer and underlying bases. Combined with the complex material behaviour also contribute to the intricate interaction of water and vapour transport and deformation processes, this introduces significant challenges. Consequently, predicting and assessing the long-term transient behaviour of this system, as observed throughout the HotBENT experiment remains challenging.

Fig. 1. The HotBENT experiment setup.

In this study conducted within the Benterest project, we present the results of our three-dimensional fully coupled thermo-hydraulic simulations of the HotBENT experiment using the computational open-source multi-physics platform OpenGeoSys [3]. Figures 2 and 3 depict the setup we use in the numerical modelling and a solution snapshot, respectively. Vapour diffusion, thermal and hydraulic conductivity, permeability, retention curve of bentonite, granite and concrete are shown to have a significant impact on the evolution of saturation (and desaturation), gas and water pressures. Herein, for the comparison and parameter calibration purposes, we employ the latest experimental data. We also discuss the numerical challenges associated with the parametrization and finite element discretization of the model.

Fig. 2. Numerical setup designated to interpret and simulate the HotBENT experiment (left), along with the prescribed temperature evolution of the heaters defined as Dirichlet boundary conditions (right).

Fig. 3. Simulation snapshot (when the target temperature of all heaters is reached) for the bentonite saturation pattern around the corresponding heaters and along the repository, as well as the spatial temperature distribution in the system.

Our findings will provide insights into the key factors influencing the bentonite buffer’s behaviour, contributing to the understanding of TH processes in engineered barriers under repository-like conditions.

 

References:

[1] https://grimsel.com/gts-projects/hotbent-high-temperature-effects-on-bentonite-buffers/hotbent-introduction

[2] F. Kober, R. Schneeberger, S. Vomvoris, S. Fensterle and B. Lanyon, HotBENT Experiment: objectives, design, emplacement and early transient evolution, Geoenergy, 1,  2023.

[3] https://www.opengeosys.org/