Modeling of Coupled Thermo-Hydro-Mechanical Processes with Links to Geochemistry Associated with Bentonite-Backfilled Repository Tunnels in Clay Formations

Abstract

This paper presents simulation results related to coupled thermal-hydraulic-mechanical (THM) processes in engineered barrier systems (EBS) and clay host rock, in one case considering a possible link to geochemistry. This study is part of the US DOE Office of Nuclear Energy's used fuel disposition campaign, to investigate current modeling capabilities and to identify issues and knowledge gaps associated with coupled THMC processes and EBSrock interactions associated with repositories hosted in clay rock. In this study, we simulated a generic repository case assuming an EBS design with waste emplacement in horizontal tunnels that are back-filled with bentonite-based swelling clay as a protective buffer and heat load, derived for one type of US reactor spent fuel. We adopted the Barcelona basic model (BBM) for modeling of the geomechanical behavior of the bentonite, using properties corresponding to the FEBEX bentonite, and we used clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland. We present results related to EBS host-rock interactions and geomechanical performance in general, as well as studies related to peak temperature, buffer resaturation and thermally induced pressurization of host rock pore water, and swelling pressure change owing to variation of chemical composition in the EBS. Our initial THM modeling results show strong THM-driven interactions between the bentonite buffer and the lowpermeability host rock. The resaturation of the buffer is delayed as a result of the low rock permeability, and the fluid pressure in the host rock is strongly coupled with the temperature changes, which under certain circumstances could result in a significant increase in pore pressure. Moreover, using the BBM, the bentonite buffer was found to have a rather complex geomechanical behavior that eventually leads to a slightly nonuniform density distribution. Nevertheless, the simulation shows that the swelling of the buffer is functioning to provide an adequate increase in confining stress on the tunnel wall, leading to a stabilization of any failure that may occur during the tunnel excavation. Finally, we describe the application of a possible approach for linking THM processes with chemistry, focusing on the evolution of primary and secondary swelling, in which the secondary swelling is caused by changes in ionic concentration, which in turn is evaluated using a transport simulation model.

Report on THMC Modeling of the Near Field Evolution of a Generic Clay Repository: Model Validation and Demonstration

“…In addition, we now add the material parameters for the BBM, which are given in Table 4-2. The parameters for the BBM used in this initial 3D THM model simulation are those derived for FEBEX bentonite by Gens et al (2009), and also applied in Rutqvist et al (2013) for the modeling of THM processes around a generic nuclear waste repository. A large number of material parameters are required to describe the THM material behavior, and those given in Table 4-2 have been determined from laboratory experiments.…”

“…It was equipped with a linear elastic swelling model (Zheng et al, 2012;Rutqvist et al, 2013) to account for swelling as a result of changes in saturation and pore-water composition. In this study, this linear elastic swelling model is further extended by incorporating the effect of changes in the abundance of swelling clay on stress.…”

Generic Argillite/Shale Disposal Reference Case

“…change in swelling pressure due to porewater chemistry changes) couplings. Coupled THM models have been developed in Rutqvist et al (2009Rutqvist et al ( , 2011Rutqvist et al ( , 2014aRutqvist et al ( , 2014b. The results from the model developed in Rutqvist et al (2014b) show that it takes about 2780 years to fully saturate the EBS bentonite if the interaction between micro-and macrostructures is considered.…”

“…Coupled THM models for argillite have been developed in Rutqvist et al (2014a) and coupled the THMC models have also been developed Zheng et al 2014aZheng et al , 2014b. Because of significant increases in stress due to thermal pressurization , the stress change due to illitization caused by MC coupling Zheng et al 2014a) seems not to be very important.…”

“…These models include the coupled ThermalHydrological-Mechanical (THM) and Thermal-Hydrological-Mechanical-Chemical (THMC) models (e.g. Liu et al 2013;Rutqvist et al 2014a) that focus on THMC processes in the Engineered Barrier System (EBS) bentonite and argillite host hock, the large scale hydrogeologic model (Bianchi et al 2014) that investigates the hydraulic connection between an emplacement drift and surrounding hydrogeological units, and Disposal Systems Evaluation Framework (DSEF) models ) that evaluate thermal evolution in the host rock approximated as a thermal conduction process to facilitate the analysis of design options. However, the assumptions and the properties (parameters) used in these models are different, which not only make inter-model comparisons difficult, but also compromise the applicability of the lessons learned from one model to another model.…”