Impact of fracture connectivity and sequence on radionuclide migration in series-connected fractured rock: A triple-continuum model study

This study investigates the transport of Th-230 and Ra-226 in series-connected fractured rock using a triple-continuum framework that represents fracture, skin, and matrix domains coupled with colloid-facilitated transport and radioactive decay chains. The analyses build on previous single-fracture studies showing that local retention structures can give rise to non-intuitive migration patterns, and examine how these behaviors may extend to fracture networks. The results suggest that radionuclide transport in series-connected systems is non-additive and can be strongly influenced by the sequence in which fracture segments are arranged. A fracture type characterized by deposited colloids and a permeable skin (Fracture C) is found to exert particular control: when positioned midstream (D–C–A), it behaves as a generation and release hotspot for Ra-226, whereas when located at the downstream end (D–A–C) it functions as a barrier that retains both Th-230 and Ra-226. These sequence-dependent behaviors indicate that representing fractured host rocks by simple superposition of single-fracture responses may lead to inaccurate estimates of breakthrough timing and peak concentrations for safety-relevant radionuclides. In addition, comparisons between flowing-fracture simulations and idealized closed-sink formulations imply that closed-system monitoring concepts can under- or overestimate radionuclide contributions in advective systems, underscoring the importance of explicitly considering fracture connectivity and boundary conditions in long-term safety assessments for deep geological repositories.