Time-dependent THMC properties and microstructural evolution of damaged rocks in excavation damage zone
Sponsor: Nuclear Energy University Program (NEUP), U.S. Department of Energy
Duration: 10/1/2018 - 9/30/2022
Modeling coupled THMC processes in geomaterials near nuclear waste repositories at various time scales is an extremely challenging task and requires collaborative effort from the field of geomechanics, hydrology, and geochemistry. This project focuses on the geomechanical aspect, addressing the time-dependent evolution of rock microstructure and its coupling with the THC processes that are of first-order importance to the stability and the isolation performance of the repository.
Deep geological disposal has been identified as the most promising solution for high-level nuclear wastes (HLW) management. However, there is no such repository currently in operation across the world, despite the tremendous research effort over the past 50 years. One of the challenges is to assess and control the uncertainties involved in the long-term performance of HLW isolation (one million years required by the U.S. Nuclear Regulatory Commission). To evaluate the performance of geological repository during such a large time span, fundamental knowledge on the coupled thermal-hydrological-mechanical-chemical (THMC) processes in the context of evolving microstructures of host rocks is required. For example, it is well known that the stress state of the host rocks is strongly perturbed during construction of the underground opening, creating the so-called excavation damage zone (EDZ). EDZ are characterized by various degrees of fracturing and fissuring oriented parallel to the opening face which can serve as pathways for rapid fluid flow that jeopardize the integrity of the waste isolation system. Such zone is not permeant. In contrast, its thickness, shape and the extent of damage can be highly dynamic under the co-action of sustained thermal and mechanical loadings. Various time-dependent microscale processes such as subcritical crack propagation (or equivalently stress corrosion), crack healing, pressure solution and brittle creep can alter the macroscopic features of the EDZ. The implications of these delayed processes on the THM response of host rocks are poorly understood and are rarely considered in the numerical models for waste repositories, thus undermining the reliability of the long-term predictions of hazardous species migration. Rooted in this context, this study will focus on quantifying the microstructural alternation under sustained thermomechanical loadings and its impact on the macroscopic transport and creep behavior of damaged rocks, aiming to assess the evolution of EDZ in the coupled temperature, pore pressure and stress field as a function of post-closure time.
Extension of the damaged zone for drifts parallel to the major horizontal stress, after Armand et al. (2014).
A novel directional damage theory to better capture microcrack induced anisotropy in host rocks
Yunping Xi, Ph.D., CUBoulder
Pania Newell, Ph.D., University of Utah
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