Effects of biotite distribution on cesium diffusion in granite

Abstract

Diffusion coefficients of Cs + in biotite granite were determined from 5° to 70°C using an initial solution concentration of <0.1 ppm. A formation factor of ∼5·10 −4 was obtained with deuterium diffusion experiments. Data fitting with a simple diffusion equation considering non-linear sorption gave effective diffusion coefficients of Cs + from 10 −8 to 5·10 −7 cm 2 s −1 at various temperatures. The complex diffusion profiles with one to three diffusion coefficients were described by assuming the sorbing mineral biotite had a heterogeneous distribution. This assumption enabled pore diffusion control to be investigated at concentrations of <0.1 ppm. The maximum intrinsic surface diffusion coefficient derived from the literature was estimated to be on the order of 10 −7 cm 2 s −1, and established the range of the diffusion coefficient under various concentration conditions. A simple calculation of Cs + matrix diffusion retardation using a one-dimensional single fracture model showed the importance of determining an accurate diffusion coefficient by considering mineral distribution in the rock. In addition, the Cs + concentration affects the surface diffusion contribution and must be adequately considered to accurately analyze long-lived 135Cs transportation in rock formations when the single fracture model is applied to the safety analysis of high-level radioactive waste geological disposal.