Effect of Fracture Geometry on Heat-Induced Groundwater Flow near High-Level Radioactive Waste Repository

The high-level radioactive waste (HLW) repository was designed to isolate the HLW from the groundwater system by using artificial and natural barriers, to prevent the migration of the radionuclides in HLW into the biosphere via groundwater. Granite formations are considered as the great natural barrier for the HLW repository in various countries such as Sweden, Canada, and Korea due to their low porosity and permeability. However, granite is known to be a highly fractured rock which can likely act as conduits for groundwater and radionuclides. Through these fractures, potential heat-induced groundwater flow can occur due to the significant amount of decay heat generated by HLW. Since the heat-induced groundwater flow can occur in a different direction, magnitude, and lasting time depending on the fracture geometry, the effect of fracture geometry on the groundwater flow around the repository should be analyzed. In this study, groundwater systems were simulated with the difference in fracture slopes and intersection points to provide an evaluation of the effect of fracture geometry on the heat-induced groundwater flow.In all models, the pressure around the repository peaked at 0.1 years after the disposal and only lasted for a short period. In contrast, the temperature lasted for 10,000 years after the disposal, and so as the convective groundwater flow. Single fracture models with different orientations were conducted to evaluate the variations in groundwater velocities around the repository depending on the fracture slope. At the horizontal fracture, the groundwater flow initially showed the fastest velocity but quickly decreased over time. In contrast, the groundwater flow through a vertical fracture initially had a slower velocity than that of horizontal fracture but decreased at a noticeably slower rate. In double fracture models, inclined fractures were added to the horizontal fracture with different intersection points. Initially, the flow through the horizontal fracture controlled the groundwater flow along with the fracture network, but eventually, the effect on flow through the vertical fracture became dominant, modifying the flow along the horizontal fracture. These results implied that the slope of fracture or how the fracture intersected could affect the heat-induced groundwater flow.