Abstract

The French National Radioactive Waste Management Agency (Andra) is in charge of studying the disposal of high-level and intermediate-level long-lived waste (HLW and ILW-LL) in a deep geological repository (Cigéo project) within the host formation is the Callovo-Oxfordian claystone (COx). The heat emitted from waste packages induces a thermo-hydro-mechanical (THM) coupling within the structural elements and the host rock. This study focuses on the behavior of the concrete lining of an ILW-LL cell subjected to THM loading during its construction and operational phases. The mechanical behavior of the host rock is represented by an elasto-visco-plastic model taking into account the anisotropies in stiffness and strength. The coupled THM formulation is based on the Biot theory. Different simulations including full THM coupling and HM coupling with or without creep behavior of COx claystone have been performed to show the effect of the thermal load (generated by the waste packages), of the water seepage and of the creep strain of the host rock on the stress evolution in the concrete liner. The results show the preponderant role of the creep strain of COx claystone on the stress state of the liner during the operational phase, while the effect of the heat loading is moderate and that of the seepage is not significant.

Introduction

Within the context of deep geological radioactive waste disposal, the French National Radioactive Waste Management Agency (Andra) has been conducting a research program to demonstrate the feasibility of constructing and operating a High Level Waste (HLW) and Intermediate-Level Long-Lived (ILW-LL) disposal within the Callovo-Oxfordian (COx) claystone in the Meuse and Haute Marne departments, nearly 300 km East of Paris. This program includes scientific and technological in-situ experiments at the Meuse/Haute-Marne Underground Research Laboratory (MHM URL), laboratory tests on the sample scale, theoretical and numerical analysis. Numerous laboratory tests have been performed on the sample scale for 20 years to understand and characterize the hydromechanical behavior of COx claystone. Under a triaxial loading path, the instantaneous mechanical behavior of COx material exhibits the following main features: (a) linear elasticity under low deviatoric stress; (b) plastic hardening before the peak representing rather diffuse damage; (c) strain softening describing the post-peak behavior where macroracks are generally observed on the sample; (d) a residual stage controlled rather by the friction of macrocracks; (e) dependence of the mechanical response on the confining pressure with a transition from a brittle towards a ductile material (at confining stress about 20 MPa); (f) a slight anisotropy in Young’s modulus; and (g) a dependence of the compressive strength on the angle between the axial loading direction and the bedding plan [1]. Continuous monitoring has been also carried out at the MHM URL to understand the hydromechanical responses of COx claystone around drifts during and after their excavation. Particular attention has been made to the excavation induced damaged zone [2]; pore pressure distribution [3]; permeability change [4]; and rock deformation [5]. Moreover, COx claystone exhibits also time-dependent behavior, which is clearly observed by both laboratory tests and in-situ observation at the MHM URL [1]. Many constitutive models have been proposed, among which some models have successfully described several features of COx claystone at both sample and field scales, such as: local and non-local anisotropic elasto-visco-plastic models [6][7][8][9]; double phase field model [10]; etc.

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