Reactive transport mechanisms and homogenization for americium in rough-walled fractures

Americium (Am) is a typical long‑lived radionuclide of major concern in deep geological disposal, whose transport behavior in fractured media is one of the critical issues in repository safety assessment. Given the lack of systematic understanding, this study develops a comprehensive framework for numerical simulation and parameter homogenization in rough‑walled fractures, integrating fracture characterization, precipitation kinetic modeling, reactive transport simulation, two‑stage homogenization and sensitivity analysis. Results show that Am precipitation in open‑flow fractures cannot reach instantaneous equilibrium, and thermodynamic equilibrium models tend to underestimate dissolved Am concentrations at the core scale; a kinetic model is therefore essential for such systems. Fracture heterogeneity and precipitation processes jointly govern Am transport, with preferential channels dominating solute breakthrough and precipitation distribution. To quantify the precipitation‑induced retardation effect, the theoretical evolution law of the equivalent retardation factor R eq is derived, and an adaptive feature‑point selection strategy is proposed accordingly. The established homogenization method achieves both high accuracy and efficiency across various scenarios, and the obtained equivalent parameters can elucidate the mechanism of Am transport. Furthermore, the homogenization method shows stable performance for extrapolation across large spatiotemporal scales, and the developed stepwise upscaling strategy further markedly enhances homogenization efficiency. This study also systematically clarifies how fracture geometric characteristics, boundary conditions, chemical reactions, and transport parameters govern Am transport.