When evaluating the stability of the geological disposal system for the high-level radioactive wastes (HLW), it is necessary to estimate the long-term permeability change of rock fractures that control the groundwater flow within natural barrier by numerical simulation. In the previous study, it is indicated that mineral reactions within rock fractures such as pressure dissolution alter fracture permeability over a long duration. In the actual environment where HLW are disposed, mineral reactions may result in the change of chemical property in groundwater such as a change in pH due to the inflow of the highly alkaline cement solution from an artificial barrier. However, numerical models that can simulate the influence of change in pH of groundwater on rock permeability change driven by mineral reactions observed in the experiment have not been developed well. In this study, a THMC coupled model considering multi-mineral reactions depending on pH condition is presented. Then, the proposed model was applied to replicate the results obtained from flow-through experiments using granite samples with a single fracture under the various pH conditions of permeable water. The model predictions could reproduce the permeability change observed in the experiment under neutral conditions, but couldn't simulate well it under alkaline conditions. The predicted effluent element concentrations showed a relatively good agreement with experimental measurements in several elements under both neutral and alkaline conditions.

1. Introduction

When discussing the performance of the natural barrier within the geological disposal facilitiy of high-level radioactive waste (HLW), it is essential to predict the permeability evolution of rock fractures due to coupled thermal-hydraulic-mechanical-chemical phenomena. Espercially, mineral reactions such as pressure dissolution have significant influence on change of fracture permeability with time. During disposal period, mineral reactions may induce the change of chemical conditions such as pH of ground water driven by inflow of alkaline cement solution from an artificial barrier (Acker J. G. and Bricker O. P., 1992; Amrhein C. and Suarez D. L., 1992; Gautier J.-M. et al., 1994; Hellman R., 1994a; Knauss K. G. and Wolery T. J., 1988). However, to date, actual permeability evolution due to mineral reactions depending on pH condition has not been well simulated by existing coupled numerical models.

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