Neng-Chuan Tien
This study develops a triple-continuum model to systematically investigate coupled fracture skin effects, colloid-associated transport, and radioactive decay chains on radionuclide migration in fractured rock. The model explicitly resolves distinct physicochemical properties of fracture skin, mobile/deposited colloid dynamics, and multi-member decay chain complexities. Using a four-quadrant framework classifying skin permeability (semi-permeable/permeable) and colloid state (mobile/deposited), we comprehensively evaluate individual and combined impacts on key radionuclides for high-level waste disposal. Crucially, for both the 4N + 1 and 4N + 2 decay chains, the breakthrough of Np-237 and Ra-226 under conditions of mobile colloids and permeable skin (Simulation D) was observed to exceed traditional conservative estimates (Simulation A). This counterintuitive result is attributed to the accumulation of parent radionuclides, Am-241 and Th-230, within the skin and matrix. Their subsequent in-situ decay leads to the production of daughter radionuclides (Np-237 and Ra-226) in these storage domains at concentrations that can surpass those in the fracture. This reduces the concentration gradient at the fracture-skin interface, suppressing diffusive loss from the fracture. In certain cases, it can even drive back-diffusion of daughter radionuclides from the matrix/skin into the fracture, particularly pronounced for weakly sorbing Ra-226. By integrating fracture skin, colloid-associated transport, and decay chains, this work establishes an efficient and rigorous framework for radionuclide migration modeling. The findings conclusively demonstrate that omitting any of these coupled mechanisms can lead to non-conservative or overly conservative predictions, thereby establishing a robust scientific basis that is critical for significantly enhancing the reliability of long-term safety assessments for high-level radioactive waste disposal in geological systems.