Colloid-driven radionuclide migration in granite: Experimental evidence and safety assessment Relevance

Colloids are increasingly recognized as important vectors for radionuclide transport in fractured crystalline rocks, raising critical concerns for the long-term safety of geological disposal of high-level radioactive waste (HLW). This study aims to clarify how europium (Eu), a trivalent analogue of actinides, diffuses into granite when associated with bentonite colloids. The diffusion analysis is based on the assumption of a semi-infinite medium with constant surface concentration, allowing application of a complementary error function model to describe transport behavior. A multi-technique experimental approach, including Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (SEM/EDS), Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectrometry (SIMS), and spectroscopic methods (XPS and XANES), was employed to characterize colloid–Eu interactions and quantify diffusion. Results show that colloid-associated Eu exhibits diffusion coefficients up to 3.7–4.5 times higher than free Eu ions in two cases, while one dataset shows comparable values, highlighting both the enhancement effect and variability. Overall, the findings demonstrate that colloids can facilitate radionuclide penetration into granite matrices while still being retained within the rock, thereby influencing repository safety assessments. This work provides mechanistic insights and robust experimental data that strengthen predictive models of radionuclide migration in crystalline host rocks.