Diffusion characteristics of HTO and 99TcO4- in compacted 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 (TcO4-)-Tc-99 on GMZ bentonite of various densities. Diffusion rates were calculated by measuring the diffusion coefficients (D-e, D-a), plotting breakthrough curves and interpreting experiment data. The apparent and effective diffusion coefficients of HTO ranged from (1.68 +/- 0.40) x 10(-11) to (2.80 +/- 0.62) x 10(-11) m(2)/s and from (4.61 +/- 1.28) x 10(-12) to (16.2 +/- 2.50) x 10(-12) m(2)/s, respectively. The apparent and effective diffusion coefficients of (TcO4-)-Tc-99 ranged from (5.26 +/- 0.16) x 10(-12) to (7.78 +/- 0.43) x 10(-12) m(2)/s and from (1.49 +/- 0.002) x 10(-12) to (4.16 +/- 0.07) x 10(-12) m(2)/s, respectively. The distribution coefficients of HTO and (TcO4-)-Tc-99 ranged from (0.70 +/- 0.12) x 10(-2) to (1.36 +/- 0.53) x 10(-2) mL/g and from (1.12 +/- 0.06) x 10(-2) to (5.79 +/- 2.22) x 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 Tc-99 could be considered nonsorbent radionuclides. The data obtained in this study provide a valuable reference for the safety assessment of waste repositories.