Matrix diffusion of 137Cs+, 99TcO4-, Br- and HTO in Beishan granite surrounding rock at wall of TBM-created spiral ramp.

Matrix diffusion is an essential process that controls radionuclide migration in the context of nuclear waste disposal in granite formations. Spiral ramps in underground nuclear repositories are vital for connecting the disposal zone with the surface environment, rendering radionuclide diffusion in the surrounding rock essential for safety evaluation. The excavation process can impact the diffusivity and permeability of granite rock, however, few investigations have been reported on the matrix diffusion of 137Cs+ and 99TcO4- in the granite surrounding rock at wall of TBM-created tunnels. To address this gap, this study investigated the diffusion of 137Cs+, 99TcO4-, Br-, and HTO in two samples (SR-70 and SR-200) collected from the spiral ramp of Beishan underground laboratory. The results indicated that the effective diffusion coefficients (De) for these radioactive substances in the granite samples were similar to those measured in the intact granite. The variability in biotite content and the presence of microfractures significantly influence the diffusion of charged species, such as 137Cs+, 99TcO4- and Br-. SR-70 is characterized by the presence of microfractures and a biotite content of 2.2%, resulting in minimal electrostatic influence on the diffusion of charged species. In contrast, SR-200 lacks microfractures and contains a higher biotite content of 8.2%, which leads to a pronounced cation excess diffusion for Cs+ and an anion exclusion effect for Br- and 99TcO4-. This study clarifies the diffusion behavior of 137Cs+, 99TcO4-, Br- and HTO within the granite surrounding rock of TBM-created tunnels, providing essential data and theoretical insights necessary for evaluating the safety and long-term stability of nuclear waste repositories situated in granite formations.