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, ³⁶Cl, ⁸⁵Sr and ¹³³Ba) 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 (⁸⁵Sr and ¹³³Ba).

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 m² 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 m² 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.