Diffusion characteristics of HTO and 99TcO4 Gaomiaozi (GMZ) bentonite

The characteristics of diffusion are essential to the transport of radionuclides through buffer/backfill materials, such as bentonite, which are commonly found in waste repositories. This study used through-diffusion techniques to investigate the diffusion behavior of HTO and 99TcO4 - on GMZ bentonite of various densities. Diffusion rates were calculated by measuring the diffusion coefficients (De, Da), plotting breakthrough curves and interpreting experiment data. The apparent and effective diffusion coefficients of HTO ranged from (1.68 ± 0.40) 9 10-11 to (2.80 ± 0.62) 9 10-11 m2/s and from (4.61 ± 1.28) 9 10-12 to (16.2 ± 2.50) 9 10-12 m2/s, respectively. The apparent and effective diffusion coefficients of 99TcO4 ranged from (5.26 ± 0.16) 9 10-12 to (7.78 ± 0.43) 9 10-12 m2/s and from (1.49 ± 0.002) 9 10-12 to (4.16 ± 0.07) 9 10-12 m2/s, respectively. The distribution coefficients of HTO and 99TcO4 - ranged from (0.70 ± 0.12) 9 10-2 to (1.36 ± 0.53) 9 10-2 mL/g and from (1.12 ± 0.06) 9 10-2 to (5.79 ± 2.22) 9 10-2 mL/g, respectively. The De and Kd values were shown to decrease with an increase in the bulk dry density of compacted bentonite. Our results show that HTO and 99Tc could be considered nonsorbent radionuclides. The data obtained in this study provide a valuable reference for the safety assessment of waste repositories.