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

Abstract

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, Da, 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, Da, which demonstrated its efficacy for both conservative (HTO and Cl) and sorbing elements. For all the investigated radionuclides, Da values decreased with increasing the clay dry density and increased with increasing temperature. Notably, the highest Da 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 Da (3.6⋅10􀀀 12 m2 s􀀀 1) value was determined for Ba diffusion at 1.65 g cm􀀀 3 and 25 ◦C. The experimental Da 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.