Comparative studies of sealing capacity of Wyoming MX-80 and BCV bentonites: the HotBENT in-situ experiment and thermo-hydro-mechanical simulations

A set of useful hydro-mechanical (HM) iso-thermal characteristics of bentonite such as swelling under saturation, low hydraulic conductivity and high sorption may significantly be impaired by the thermal (T) effects. Multiple lab studies demonstrate a sharp drop of the swelling pressure in a confined fully saturated bentonite sample once it is subjected to high temperatures, e.g., [1]. Lab-scale evidence of potential bentonite’s deteriorating “sealing capacity” under thermal loading should be a subject of further investigation, especially when addressing large scale engineered barrier systems for radioactive waste repositories.

In our studies, the HotBENT in-situ experiment – a joint undertaking of multiple international partners at the Grimsel Test Site (Switzerland) operated by NAGRA [2,3] – is a point of departure. Geometry-, material-, and THM-process-wise, the experiment is designed to replicate a potential high level waste repository environment: electric heaters embedded in a bentonite buffer heat the buffers up to 200 °C, and the buffer is subjected to hydration from surrounding fractured granite and artificial hydration pipes. This configuration induces complex interactions between bentonite’s swelling, drying, vapour transport, and re-saturation processes. Two types of granular bentonite – Wyoming MX-80 and Bentonite Cerny Vrch (BCV) – are used in two different sections of the experiment as buffer also with compacted bentonite blocks serving as pedestals for the heaters.

For the HotBENT numerical simulations, the OpenGeoSys [4] computational multi-physics platform is used. The overall modeling campaign is multi-step with increasing dimension- and process-complexity.

Initially, simulations are limited to 2D geometry (cross-section of a heater in radial direction) and TH (non-isothermal two-phase two-component flow) response of buffer [5,6]. The available experimental data on temperature, relative humidity, pore pressure fields/distribution within the buffer enable model calibration of liquid and gas hydraulic conductivities, vapour diffusivity, bentonite’s effective thermal conductivity and water retention behaviour. Efforts have also been made to accurately represent heaters and the related heating modes, as well as to properly describe “buffer – host-rock” hydraulic interaction.

In this talk, we present some results of the extended 3D and fully coupled THM analysis. With accurate parametrization of MX-80 and BCV bentonite models at hand, our focus is shifted to comparison of both short- and long-term performance of these bentonite types. This regards the evolution of saturation and pore pressure fields, as well as buffer deformation response.

References:

[1] Najser, J., Mašín, D., (2024). An experimental study on thermal relaxation of BCV bentonite, Applied Clay Science 254, 107374. https://doi.org/10.1016/j.clay.2024.107374

[2] Grimsel Test Site (GTS). HotBENT – High-temperature effects on bentonite buffers: Introduction. https://grimsel.com/gts-projects/hotbent-high-temperature-effects-on-bentonite-buffers/hotbent-introduction

[3] Kober et al. (2023). HotBENT Experiment: objectives, design, emplacement and early transient evolution, Geoenergy, 1.

[4] OpenGeoSys Community. OpenGeoSys – Open-source finite element software for coupled THMC processes. https://www.opengeosys.org/

[5] Gerasimov et al. (2023-2025). Thermal-hydraulic modeling of the HotBENT experiment using the OpenGeoSys, HotBENT Partner Meetings, internal presentations.

[6] Tatomir et al. (2025). Thermo-Hydraulic Modelling of the In-Situ HotBENT Experiment: Investigating Bentonite Barrier Behaviour at High Temperature and Hydration. EGU General Assembly 2025.