The diffusion and adsorption behaviors of sodium and cesium were investigated in the Callovo-Oxfordian claystones (France) under unsaturated conditions. Through-, out-, and in-diffusion laboratory experiments were performed on intact and compacted samples. These samples were partially saturated using an osmotic method for imposed suction up to 9 MPa. This specific technique enabled us to obtain water saturation degree ranging from 81% to 100% for intact samples and from 70% to 100% for compacted materials. The results showed a very low impact of water saturation on the extent of adsorption for 22 Na and cesium, onto intact and compacted materials. Such observations suggest that the saturation degrees were not low enough to limit the access of cations to adsorption sites on clay surfaces. At full saturation, enhanced diffusion for 22 Na and cesium was clearly evidenced onto intact and compacted samples. Under unsaturated conditions, the diffusion behavior for Cs and 22 Na was not only slower but also distinct as compared to fully saturated samples. For the intact rock and under suction of 1.9 MPa, the Cs diffusivity was reduced by a factor of 17, whereas for sodium, it was reduced by a factor of 5. Explanation was then proposed to explain such a difference.

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This work presents a novel approach to study tritiated water (HTO) diffusion through unsaturated cement-based materials (hardened CEM V/A cement pastes). The diffusion studies were performed under hydric conditions covering those that can prevail in near-surface disposal facilities for short-lived nuclear waste. In order to investigate the required large range of suction, two complementary techniques were developed. The first technique was based on a through-diffusion method where suction (up to 9 MPa) was controlled using osmosis process. In the second technique diffusion experiments were performed in humidity chambers controlled by under-saturated saline solutions (suction up to 170 MPa). Tritiated water (HTO) was added in the saline solution and diffused through the studied material in gas form. This latter method showed that the extent of irreversible HTO uptake on cement-based materials increased with desaturation. Moreover, diffusive data acquired by means of the two techniques revealed a decrease of effective diffusion coefficient by a factor of 10 from 100% to 23% of saturation degree. Comparison with dihydrogen (HT) diffusion results suggests that HTO diffuses through unsaturated cement-based materials at rates more than 4 orders of magnitude lower.