EFFECTS OF BIOTITE DISTRIBUTION ON CESIUM DIFFUSION IN GRANITE

Diffusion coefficients of Cs+ in biotite granite were determined from 5-degrees to 70-degrees-C using an initial solution concentration of < 0.1 ppm. A formation factor of approximately 5.10(-4) was obtained wi th deuterium diffusion experiments. Data fitting with a simple diffusi on equation considering non-linear sorption gave effective diffusion c oefficients of Cs+ from 10(-8) to 5.10(-7) cm2 s-1 at various temperat ures. The complex diffusion profiles with one to three diffusion coeff icients were described by assuming the sorbing mineral biotite had a h eterogeneous distribution. This assumption enabled pore diffusion cont rol to be investigated at concentrations of < 0.1 ppm. The maximum int rinsic surface diffusion coefficient derived from the literature was e stimated to be on the order of 10(-7) cm2 s-1, and established the ran ge 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 determinin g an accurate diffusion coefficient by considering mineral distributio n in the rock. In addition, the Cs+ concentration affects the surface diffusion contribution and must be adequately considered to accurately analyze long-lived Cs-135 transportation in rock formations when the single fracture model is applied to the safety analysis of high-level radioactive waste geological disposal.