Incorporating cross-scale insights into colloid-facilitated radionuclide transport in fractured rocks: A critical review

Driven by the global imperative for the geological disposal of high-level radioactive waste (HLW), notable progress has been made in predicting radionuclide transport within fractured rocks. Current research has recognized that the presence of colloids may significantly influence radionuclide transport. However, challenges remain in understanding and quantifying this process from a multi-scale perspective. This review provides a comprehensive overview of recent developments in understanding the role of colloids in facilitating radionuclide transport in multi-scale fractured rocks. We first revisit the fundamental characteristics and processes controlling the transport of colloids and radionuclides in fractured rocks, including the properties of fractured rocks, colloids, and radionuclides, as well as their complex interactions. Furthermore, we discuss recent advancements in lab- and field-scale experiments and modeling techniques that shed light on the mechanisms controlling colloid-facilitated radionuclide transport. The focus then shifts to scaling issues, including scale-dependent transport processes and parameters, as well as the upscaling theories that bridge the gap between lab-scale experiments and field-scale assessments. Finally, we identify unresolved problems and promising development trends in colloid-facilitated radionuclide transport, which offer new opportunities for enhancing the accuracy of long-term safety assessments in HLW geological repositories.