Variability of effective diffusivity in fractured and mineralized metamorphic host rock from Bukov URF, Bohemian Massif (CZ)

Crystalline rocks are considered host rocks for high-level radioactive waste (HLW) disposal because of their mechanical, thermal, and chemical properties. However, fractures and associated mineral precipitates, e.g. calcite, introduce significant heterogeneity that complicates predictions of radionuclide (RN) transport in fractured crystalline rock systems. This study combines positron emission tomography (PET) imaging and transport modeling to investigate the effects of calcite-filled fractures on predominantly diffusive transport in crystalline rocks. Diffusion experiments using a conservative iodine tracer (¹²⁴I) were performed on rocks from the BUKOV underground research facility (Czech Republic), and CT-based calcite-filled fracture geometries were incorporated into RTM simulations. The results show two types of transport modifications due to calcite precipitation in fractures: enhanced transport in porous calcite generations and suppressed transport in low-porosity calcite fracture fillings. The spatial distribution of fracture-filling calcite, its porosity, and the porosity of the surrounding matrix materials can lead to heterogeneous or anisotropic diffusion behavior. This suggests that important transport characteristics in heterogeneous systems may be overlooked by conventional modeling using homogenized diffusion coefficients. The apparent diffusivity of the fractured and variously sealed BUKOV rock ranges from 10⁻¹³ to 10⁻¹⁰ m²/s. We conclude that exploiting the variability and anisotropy of diffusive flux behavior is beneficial for improving the accuracy of RN migration predictions in fractured and mineralized crystalline rock systems. This work demonstrates the value of integrating advanced imaging and modeling techniques to better characterize solute transport in fractured crystalline host rocks.