A multi-physics numerical model was developed to predict the fluid flow and mass transport [(Re2.1)] behavior of rock fracture [(Re2.1)] under coupled thermal-hydraulic-mechanical-chemical (THMC) conditions. In particular, the model was employed for the purpose of describing the evolution of permeability and reactive transport behavior within rock fractures by taking into account the geochemical processes of the free-face dissolution and the pressure dissolution. In order to examine the capability of the developed

When considering the safe isolation of high-level radioactive wastes, the long-term evolution of the hydraulic and transport behavior of the rocks of interest should be predicted prior to its operation because coupled thermal-hydraulic-mechanical-chemical (THMC) processes should be significantly active in such situations where relatively high ground pressure and temperature are induced. In this study, a coupled THMC numerical model has been developed to examine the long-term change in permeability of the porous sedimentary rocks that are assumed to be composed purely of quartz. Specifically, the chemo-mechanical process of the pressure solution was incorporated into the model. The developed model was validated by replicating the existing experimental measurements of the porosity reduction and the evolving silica concentration. Subsequently, by simulating the burial of high-level radioactive wastes in the deep subsurface, namely, by applying the simulated confining pressure and temperature conditions, the long-term evolution of the rock permeability was predicted. The model predictions clearly showed a significant influence of the pressure dissolution on the change in permeability with time. The predicted permeability of the rocks close to the wastes decreased by one order of magnitude in 10 4 years when considering the pressure dissolution, while the permeability changed little during the same period when the pressure dissolution was not considered. This reduction should delay the dispersion of the radioactive materials dissolved in the groundwater.

The coupled THMC model, Interface for Pressure Solution Analysis under Coupled Conditions, IPSACC, that was proposed by the authors and can describe the long-term evolution in rock permeability due to mineral reactions (i.e., pressure solution and free-face dissolution/precipitation) within rock fractures, was upgraded in the present study by incorporating the processes of fracture initiation/propagation. The remarkable characteristic of the proposed model is its ability to simulate the generation of fractures and the mineral reactions within the generated fractures as well as the subsequent changes in permeability. The proposed model was applied to predictions of the long-term changes in the permeability of rock located near high-level radioactive waste within a geological repository. The predicted results revealed that fractures were generated near the disposal cavity and that the permeability of the damaged zone increased significantly more than that of the intact rock during the excavation, while the permeability in almost the entire damaged zone decreased by about one order of magnitude due to pressure solution at the contacting asperities within the rock fractures after setting virtual radioactive waste into the disposal cavity. Overall, it was clarified that the proposed model is capable of calculating the permeability evolution of rock through fracture generation and subsequent sealing due to mineral reactions at the actual field scale. Thus, the potential for using the proposed model to examine the long-term performance of natural barriers for delaying the transport of radionuclides has been shown.

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