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 EBS- rock 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 hor- izontal 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 geo- mechanical 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 ini- tial THM modeling results show strong THM-driven interactions between the bentonite buffer and the low- permeability 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 distribu- tion. 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 stabil- ization of any failure that may occur during the tunnel excavation. Finally, we describe the application of a pos- sible 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.

Highlights

  • Clay/shale has been considered as potential host rock for geological disposal of high-level nuclear waste throughout the world, because of its low permeability, low diffusion coefficient, high retention capacity for radionuclides, and capability for self-sealing fractures induced by tunnel excavation

  • 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

  • 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

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Summary

Introduction

Clay/shale has been considered as potential host rock for geological disposal of high-level nuclear waste throughout the world, because of its low permeability, low diffusion coefficient, high retention capacity for radionuclides, and capability for self-sealing fractures induced by tunnel excavation. This paper presents simulation results related to coupled thermal, hydrogeological, mechanical, and chemical (THMC) processes in the engineered barrier system (EBS) and near-field rock associated with high-level radioactive waste repositories hosted in clay formations. We use clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland (Gens et al 2007) and adopt a heat load developed within the UFD as a generic disposal system environment (GDSE) for pressurized water reactor (PWR) used nuclear fuel. The BBM was recently successfully applied to model the coupled thermal–hydrological–mechanical (THM) behavior of unsaturated bentonite clay associated with the FEBEX in situ heater test at the Grimsel test site, Switzerland (Gens et al 2009). After equipped with THMC capability, we consider the coupling of chemical and mechanical processes with a simple coupling relation to evaluate the effect of concentration changes on stress and we limit our chemical analysis to conservative transport

Thermal-Elasto-Plastic BBM in TOUGH-FLAC
Three-Dimensional Yield Surface
Stress State
Elastic Strain
Plastic Strain
Ks ds ð23Þ in which Ks the suction bulk modulus
THM Model Setup and Simulation Sequence
Base-Case THM Simulation Results
Thermal Management Study and Peak Temperature
Buffer Resaturation Time
Thermal Pressurization
Modeling THMC Processes Using Elastic CM Coupling
Findings
Concluding Remarks
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