A deep geological repository for disposal of high-level radioactive waste (HLW) consists of the canister, buffer material, and natural rock. If radionuclides leak from a disposal container, it can pass through buffer materials and rock, and move into the biosphere. Transport and migration of radionuclides in the rock differently were affected by the fracture type, filling minerals in the fracture, and the chemical and hydraulic properties of the groundwater. In this study, aperture distribution in fractured granite block was investigated by hydraulic test and CFD analysis. The fractured rock block (1 m × 0.6 m × 0.6 m), which is simulated as natural barrier, was prepared from Iksan, Jeollabuk-do. 9 test holes were drilled and packer system was installed to perform hydraulic test at the surface of fracture. 3D model simulated for aperture distribution of rock block was made using results of hydraulic test. And then, CFD analysis was performed to evaluate the co-relation between experiment results and analysis results using FLUENT code.

A disposal research program for HLW has been carried out since 1997 with the aim of establishing the preliminary concept of geological disposal in Korea. The preliminary studies were conducted by conducting manufacture and installation of an in-situ nuclide migration system in KAERI Underground Research Tunnel (KURT). Nuclides could be released from a deep underground disposal facility due to thermal and physicochemical changes into the surrounding environments. Understanding on the migration and retardation processes of nuclides in a fractured rock is very important in the safety assessment for the radioactive waste disposal. In this study, we evaluated fracture filling minerals and aperture distribution (3D map) along the fracture surfaces under the controlled conditions. The fractured granite block which has a single natural fracture of 1 m scale was sampled in a domestic quarry (Iksan), which groundwater had been flowed through. This rock has an interconnected porosity of 0.36 with the specific gravity of 2.57. The experimental set-up with the granite block with dimensions of 100×60×60 (cm). A flow of de-ionized water through the fracture between pairs of boreholes was initiated and the pressure required to maintain a steady flow was measured. In additions, fracture filling minerals were sampled and examined by mineralogical and chemical analyses. There are phyllosilicate minerals such as illite, kaolinite, and chlorite including calcite, which are fracture filling minerals. The illite and kaolinite usually coexist in the fracture, where their content ratio is different according to which mineral is predominant. For the evaluation of fracture, surface was divided into an imaginary matrix of 20×20 sub-squares as schematically. The calculated results are expressed as a two dimensional contour and a three dimensional surface plot for the aperture distribution in the fracture. The aperture value is distributed between 0.075 and 0.114 mm and the mean aperture value is 0.095 mm. The fracture volume is about 55 ml. Also the 137Cs sorption (batch test) distribution coefficients increased to Kd = 800~860 mL/g in the fractured rock because of the presence of secondary minerals formed by weathering processes, compared to the bedrock (Kd = 750~830 mL/g). These results will be very useful for the evaluation of environmental factor affecting the nuclides migration and retardation.

In this study, we evaluated fracture filling minerals and aperture distribution along the fracture surfaces under the controlled conditions. The fractured granite block which has a single natural fracture of 1 m scale was sampled in a domestic quarry (Iksan), which groundwater had been flowed through. This rock has an interconnected porosity of 0.36 with the specific gravity of 2.57. The experimental setup with the granite block with dimensions of 100×60×60 (cm). The fracture is sealed with rock silicone rubbers when it intersects the outer surfaces of the block and the outer surfaces are coated with the silicone to prevent loss of water by evaporation. Nine boreholes were drilled of orthogonal direction at the fracture surface. A flow of de-ionized water through the fracture between pairs of boreholes was initiated and the pressure required to maintain a steady flow was measured. In additions, fracture filling minerals were sampled and examined by mineralogical and chemical analyses. There are phyllosilicate minerals such as illite, kaolinite, and chlorite including calcite, which are fracture filling minerals. The illite and kaolinite usually coexist in the fracture, where their content ratio is different according to which mineral is predominant. For the evaluation of fracture, surface was divided into an imaginary matrix of 20×20 sub-squares as schematically. The calculated results are expressed as a two dimensional contour and a three dimensional surface plot for the aperture distribution in the fracture. The aperture value is distributed between 0.075 and 0.114 mm and the mean aperture value is 0.082 mm. The fracture volume is about 49 ml. These results will be very useful for the evaluation of environmental factor affecting the nuclides migration and retardation.