Influence of temperature and dry density coupled effects on HTO, 36Cl, 85Sr and 133Ba diffusion through compacted bentonite

This study investigated the combined impact of temperature (from 25 to 80 °C) and dry density of bentonite (from 1.2 to 1.65 g·cm − 3 ) on the apparent diffusion coefficient, D a , of four radionuclides (HTO, 36 Cl, 85 Sr and 133 Ba) in a Ca-Mg bentonite. In this study, the different porewater chemistry, present under the specific experimental conditions -- a factor often overlooked in diffusion studies -- was explicitly considered. As a support to diffusion studies, batch sorption tests were carried out at different temperatures (25, 40, 60, and 80 °C) with the sorbing elements ( 85 Sr and 133 Ba). The planar source method was used for the determination of apparent diffusion coefficients, D a , which demonstrated its efficacy for both conservative (HTO and Cl) and sorbing elements. For all the investigated radionuclides, D a values decreased with increasing the clay dry density and increased with increasing temperature. Notably, the highest D a value (1·10 −9 m 2 s −1 ) was attained for HTO diffusion in the clay at 1.2 g cm −3 and 80 °C, while the lowest D a (3.6·10 −12 m 2 s −1 ) value was determined for Ba diffusion at 1.65 g cm −3 and 25 °C. The experimental D a values were analysed employing the Arrhenius law and the Stokes-Einstein equations. Results revealed that diffusion data are in a reasonable agreement with Arrhenius behaviour, but deviations from the Stokes-Einstein equation were observed for compaction densities higher than 1.4 g cm −3 . • The planar source method is an effective method to measure radionuclide diffusion. • The highest D a (1·10 −9 m 2 s −1 ) observed for HTO in the clay at 1.2 g cm −3 and 80 °C. • The lowest D a (3.6·10 −12 m 2 s −1 ) for Ba diffusion at 1.65 g cm −3 and 25 °C. • Reasonable agreement with the Arrhenius law for the D a at different temperatures. • Clay density and temperature directly influences water content and porewater chemistry.