Long-Term Density-Dependent Groundwater Flow Analysis and Its Effect on Nuclide Migration for Safety Assessment of High-Level Radioactive Waste Disposal with Consideration of Interaction between Fractures and Matrix of Rock Formation in Coastal Crystalline Groundwater Systems

This study presents the long-term evolution of coastal groundwater environments in fractured rocks for the underground disposal of high level radioactive waste. A conceptual model is designed to include the significant mass of the terrestrial land and subsurface under the seafloor to consider multiple cycles of over 100 m sea level fluctuations. A dual porosity conceptualization is applied to simulate density-driven flow and transport to assess the long-term impacts of fracture network permeability and matrix diffusion characteristics, terrestrial land slope (as a main driving force for freshwater flow), and sea level fluctuations on groundwater flow velocity and salinity evolution over time at repository target depth. In the near-field block scale, a methodology is suggested where the regional flow is mapped onto a three dimensional block of a discretely fractured dual permeability model to approximate the long term effects of sea level fluctuation. The near-field block scale model can then be used to evaluate the details of nuclide migration characteristics near the repository. Regional analysis indicates that groundwater velocity at target depth for repositories can be influenced by the transgression and regression patterns, the rate of uplift and erosion as well as the fracture network permeability and effective matrix diffusion. A dual porosity dual permeability conceptualization approach for fractured rock formation presented in this study provides a basis for an effective and efficient safety analysis methodology for high-level radioactive waste disposal to assess the long-term evolution of coastal groundwater environments in the fractured rock and to evaluate the nuclide migration in the changing coastal groundwater condition.