Abstract:
This paper is the result of geological survey engineering.
Objective In evaluating high-level radioactive waste disposal sites, investigating and assessing the sealing integrity of fracture zones is critical for evaluating the long-term safety of disposal systems over a millennial timescale.
Methods This study systematically investigated the geological, mineralogical, and geochemical characteristics of the F34-1 fissure zone, a representative fissure zone within the Beishan Underground Laboratory site, through well and tunnel excavation, in-situ sample collection, and laboratory analysis. The integrity of the fissure zone was examined using these methods.
Results F34-1 trends northeast, with its scale diminishing markedly with depth. Most fractures exhibit closed geometries. Groundwater infiltration rates within the zone decrease rapidly over short periods, suggesting low hydraulic connectivity to regional groundwater systems. Filling minerals within the zone are dominated by calcite, quartz, and clay minerals, with most minerals cemented into fractures, contributing to their closed state. The major element characteristics of the fill material exhibit high silica and alumina content with low iron and calcium, enhancing the filling minerals' nuclide retention and adsorption capacity. Trace element content may indicate that deep fractures underwent multiple episodes of hydrothermal alteration and mineral precipitation, further reducing fracture permeability and restricting radionuclide migration within the fractures.
Conclusion Comprehensive analysis suggests that the F34-1 fracture zone exhibits excellent sealing properties, effectively slowing the outward migration of groundwater and radionuclides. This finding provides crucial scientific basis for evaluating the natural barrier performance of the Xin-Chang rock mass and assessing site suitability.
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