Numerical analyses of coupled thermal-hydraulic-mechanical-chemical processes for estimating permeability change in fractured rock induced by alkaline solution

Abstract

The chemical condition of groundwater, such as the pH, affects the rate of the mineral-groundwaterreactions which may alter the permeability of rock masses with time. A numerical model that can address the permeability evolution of rock masses, containing newly generated fractures due to geochemical processes depending on the pH, is proposed by upgrading our coupled THMC model, IPSACC. Subsequently, the proposed model is utilized to estimate the long-term permeability evolution of a natural barrier composed of granite within a geological repository under subsurface conditions, considering the inflow of the alkaline cement solution from an artificial barrier which is virtually installed in a disposal cavity of HLW. In particular, a quantitative evaluation of the impact of the inflow of the alkaline cement solution on the change in permeability in the fractured rock is the novelty of this work. The computed predictions show that the fractures generated during the cavity excavation drastically increase the rock permeability near the cavity and that the rapid permeability reduction within the several shear-induced fractures is the result of the pressure solution at the contacting asperities of the fractures after the disposal of the radioactive waste into the disposal cavity. The reduction of the fracture permeability is enhanced by the increase in the pressure solution rate due to the spreading of the alkaline solution only close to the disposal cavity, while there was no enhancement in the fractures, except for close to the disposal cavity, because the alkaline solution cannot reach the fractures before the pressure solution reaches the equilibrium state within an early period after the disposal of the HLW. From these results, in the geological environment assumed in this study, it is expected that the performance of a natural barrier for delaying the migration of radionuclides over the long duration may be almost unaffected by the inflow of the alkaline solution from an artificial barrier because its impact on the permeability evolution of a natural barrier is spatiotemporally limited.