ABSTRACT
We developed a coupled THMC simulator employing fracture generation and a mineral dissolution/precipitation scheme for generated fractures based on the pH value of groundwater by improving our previously presented simulator, IPSACC. Then, the developed simulator was applied to evaluate the long-term evolution of rock permeability within a geological disposal facility of HLW under subsurface conditions considering the inflow of the alkaline cement solution from an artificial barrier. The predicted results indicated that the permeability of a damaged zone would eventually decrease to a value close to that of an undamaged zone due to the pressure solution at the contacting asperities within the fractures enhanced by an increase in the pH value due to the inflow of the alkaline cement solution from the artificial barrier. From these results, it is suggested that the impact of the pH alteration, brought about by the inflow of the alkaline cement solution, should be considered accurately for estimating a more realistic scenario of rock permeability evolution.
In order to investigate the performance of a geological repository of high-level radioactive waste (HLW) for delaying the transport of radionuclides, it is essential to predict the long-term permeability evolution of fractured rocks that work as a natural barrier under the coupled thermo-hydro-mechanical-chemical (THMC) conditions. Specifically, in the near field of a repository, the complex physical/chemical phenomena interact with each other by means of heat transfer from the waste body, the mass transport by groundwater, fracture generation, and geochemical reactions between the rock minerals and the groundwater. In particular, among the coupled phenomena, the occurrence of the geochemical reactions of the free-face dissolution/precipitation and pressure solution within the rock fractures generated during the cavity excavation may have a non-negligible impact on the long-term evolution of rock permeability. They depend on the chemical condition of the groundwater (e.g., pH value), and thus, should be influenced by the inflow of the alkaline cement solution from an artificial barrier in the geological repository of the HLW. Although many coupled numerical models have previously been proposed [1–3], coupled THMC simulations that consider the long-term permeability evolution of fractured rocks due to geochemical reactions with an alteration in the chemical condition of the groundwater, such as the pH value, have not been performed well. For example, although Liu et al. [1] presented a coupled THMC model employing the rock damage enhanced by hydraulic-chemical erosion and depending on the pH values, the pressure solution was not described. Ogata et al. [3] developed a coupled THMC simulator, IPSACC, that can describe the changes in rock permeability through fracture generation and subsequent sealing by the pressure solution and free-face precipitation. However, this simulator does not take into account the influence of the pH values of the groundwater.
