Diffusion of Cs, Np, Am and Co in compacted sand-bentonite mixtures; Evidence for surface diffusion of Cs cations

Sawaguchi, Takuma ; Yamaguchi, Tetsuji ; Iida, Yoshihisa ; Tanaka, Tadao ; Kitagawa, Isamu

Diffusive transport of Cs in compacted sand-bentonite mixtures was studied by a through diffusion method. Experiments were performed under variable aqueous compositions. Effective diffusivity () values of 5.2E-10 5.9E-9 m s were obtained. The variation was somewhat large in the values. Apparent diffusivity () values, on the other hand, were 2.0E-12 6.2E-12 m s, which shows small variation. The results indicate that, in applying Fick's 1st law of diffusion, diffusive flux is proportional to the apparent concentration gradient of Cs in the sand-bentonite mixture rather than the gradient of Cs concentration in pore water. Since the apparent concentration gradient in sand-bentonite mixtures is nearly equal to the gradient of adsorbed Cs, diffusion of Cs under adsorbed state would be the main mechanism of diffusion of Cs in sand-bentonite mixtures.