Effect of fracture characteristics on groundwater flow adjacent to high-level radioactive waste repository

In the study, a 2-D numerical model-delineating the high-level radioactive waste repository surrounded by hosting granite-rock was designed to evaluate pressure build-up, elevated temperature, and fracture flow caused by heat generation at the repository. During the early stage (0.1 years), build-up pressure at the repository induced radial groundwater flow. On the other hand, buoyancy-driven vertical flow was dominant during the late stage (2,000 years). The pressure build-up and elevated temperature varied significantly in bentonite, excavation-damaged zone and granite, resulting in different groundwater velocity. Fracture flow in both single and intersecting fractures were also influenced by the pressure and temperature, but their impact varied depending on the geometrical properties (e.g. orientation, elevation, lateral position) of the fracture. In complex discrete fracture network models, build-up pressure in the early stage mainly generated preferential fracture flow through few pathways, while in the late stage, convection-circulating flow induced by the buoyancy was distinct. In the early stage, higher density and connectivity of fracture network did not lead to an increase in average fracture flow, whereas the circulating flow in the late stage became more frequent and larger. Especially, long fractures served as both preferential flow pathways in the early stage and conduits for circulating flow in the late stage. The circulating flow was highly dependent on the spatial distribution of the long fractures, causing a significant uncertainty of average fracture flow in the late stage. The low angle of intersection between fractures resulted in low connectivity that decelerated the average fracture flow in both stages.