Comparative modelling of the coupled thermal–hydraulic-mechanical (THM) processes in a heated bentonite pellet column with hydration

Abstract

For the deep geological disposal of high-level radioactive waste in argillaceous rocks, the heat production of the waste is an important driver for thermal-hydraulic-mechanical (THM)-coupled processes. These THM processes influence the properties and conditions of the near field that in many repository designs contains bentonite as a clay buffer. One task in the DECOVALEX-2015 (DEvelopment of COupled models and their VALidation against Experiments) project was the modelling of a heated bentonite column (Villar et al. in Long-term THM tests reports: THM cells for the HE-E test: update of results until February 2014. Deliverable-no: D2.2-7.3. CIEMAT Technical Report IEMAT/DMA/2G210/03/2014, 2014) in preparation for the in situ heater experiment HE-E at the underground rock laboratory Mont Terri. DECOVALEX is an international cooperative project that focuses on the development and validation of mathematical models for simulating such coupled processes associated with disposal in deep geological repositories. Eight modelling teams developed their own THM-coupled models for the bentonite column experiment, using six different simulation codes. Each of the teams individually calibrated the THM parameters for the bentonite material. The eight resulting parameter sets agree well and allow a satisfactory reproduction of the TH measurements by all models. The modelling results for the evolution of temperature and relative humidity over time at three sensors in the bentonite column are in good agreement between the teams and with the measured data. Also, changes of the temperature due to modifications of the insulation and the adjustment of the heating power during the course of the experiment are well reproduced. The models were thus able to reproduce the main physical processes of the experiment, both for vapour-dominated diffusion during the heating phase and combined liquid and vapour transport during a subsequent heating and hydration phase. Based on the parameter sets, the teams predict a penetration of the water infiltration front in the 48-cm column filled with bentonite pellets to a depth between 25 and 35 cm over the 15,000 h (i.e. over 20 months) of the hydration phase of the experiment.

DECOVALEX-2015: an international collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems

“…This task is summarized in three synthesis papers for each of the three sub-tasks, with Garitte et al (2017a) focusing on a comparative analysis of THM models simulating the HE-D test, Graupner et al (2018) evaluating collaborative THM modelling of the heated bentonite pellet column, and Garitte et al (2017b) summarizing the predictive capability of THM models to simulate a complex heated bentonite-clay system as observed in the HE-E experiment. Individual team contributions are given in Pan et al (2017), Ofoegbu et al (2017), and Wang et al (2016), respectively.…”

The Modeling of Laboratory Experiments with COMSOL Multiphysics Using Simplified Hydromechanical Model

“…A broad range of aspects related to thermo-hydro-mechanicalchemical processes was investigated in numerous studies focusing on high temperature effect [13,14], liquid/gas flow properties [15][16][17], mechanical [18,19] or chemical [20,21] aspects. While much research has been conducted on compacted bentonite material, comparativelyfewer investigations have been performed on other forms of bentonite (granular [22], pellets of different sizes and forms [23][24][25][26], pellet-powder mixtures [6,[27][28][29]).…”

The hydration of bentonite buffer material revealed by modeling analysis of a long-term in situ test

“…Hydration of unsaturated bentonite has been extensively studied by experiments and models for laboratory column tests (e.g Åkesson et al, 2009;Chijimatsu et al, 2009;Tong et al, 2010;Graupner et al, 2018) and field tests Rutqvist et al, 2001) at all kinds of scales (e.g. Lloret and Villar 2007;Villar et al, 2018) and different model approaches.…”