Abstract: The safe disposal of high-level radioactive waste (HLW) is a prerequisite for the sustainable development of nuclear power. Due to construction-induced disturbances during early excavation and support operations, coupled with the persistent heat generated from radionuclide decay in the subsequent stages, the surrounding rock of the HLW repository undergoes a sequence of excavation, support installation, heating, and cooling. Consequently, fractures within the surrounding rock are subjected to cyclic shearing under thermal conditions, potentially compromising the sealing efficacy of the repository. This study focuses on Beishan granite, the designated host rock for China’s HLW geological disposal repository, as the research subject. To investigate the shear deformation-seepage behavior of rock fractures under disposal-relevant temperatures, cyclic shear-seepage tests were conducted under constant normal stiffness (CNS) and controlled temperature conditions using a novel thermo-hydro-mechanical (THM) coupled testing system. The mechanism governing the evolution of shear deformation-seepage in granite fractures under CNS, with consideration of temperature effects, was elucidated. The findings are as follows: (1) During the first forward shear process, the fracture deformation-seepage evolution is governed by the dominant roughness structure. The hydraulic aperture experiences a rapid increase concomitant with shear deformation, subsequently stabilizing gradually at an elevated level. In subsequent shear cycles, the hydraulic aperture exhibits no significant variation, while normal deformation continuously demonstrates shear contraction during forward shearing and shear dilation during reverse shearing. (2) Under a low normal boundary stiffness (kn=0.8 and 2 GPa/m), the hydraulic aperture of granite fractures is highly sensitive, exhibiting a sudden increase by up to 6.5 times when the shear behavior switches from contraction to dilation. Conversely, under a higher normal boundary stiffness (kn=4 GPa/m), the residual shear contraction reaches up to 4.12% after each shear cycle, and the hydraulic aperture decreases by up to 9.84% relative to the preceding loading cycle. (3) The impact of rising temperature on the deformation and seepage characteristics of granite fractures is notably pronounced during the shear process. Throughout the initial shear cycle, an increase in temperature leads to a diminished fluctuation in hydraulic aperture. In subsequent shear cycles, at 90 ℃, a pronounced residual shear contraction of up to 26.31% is observed following each shear cycle. Elevated temperatures induce a pronounced reduction in the permeability of granite fractures. Specifically, the hydraulic aperture at 60 ℃ and 90 ℃ decreases to 0.386 and 0.081 times that measured at 25 ℃, respectively. (4) The fracture with higher roughness (JRC=15.53) exhibits a significant reduction in permeability during cyclic shearing compared to its counterpart with lower roughness (JRC=10.31), with the hydraulic aperture decreasing by up to 40.10%. The research results provide important references for the long-term safety evaluation of HLW geological disposal projects. The observed reduction in the permeability of granite fractures induced by heating and cyclic shearing is positive for the tightness of underground engineering projects, such as HLW geological disposal and compressed air energy storage.

Key words: granite fracture, constant normal stiffness, cyclic shear-flow, thermo-hydro-mechanical coupling, high-level radioactive waste geological disposal