Diffusion of inert and reactive tracers: Overview of the DIR field experiment in the Callovo-Oxfordian clay rock (France).

The geological disposal of radioactive waste is under investigation in several countries, especially in Europe. In this field, clay-rich sedimentary rocks are promising natural barriers to limit and delay the release of radionuclides toward the biosphere, due to their diffusion-driven transport and retention properties. In addition to laboratory studies, field studies in Underground Research Laboratories (URL) represent a significant value to strengthen safety demonstrations. In this context, the DIR experiment (Diffusion of Inert and Reactive tracers) was developed and implanted in the Meuse/Haute-Marne URL located in the Callovo-Oxfordian clay-rich formation. Over 6 years, several diffusion experiments were performed with radioactive tracers: HTO (tritiated water), 36Cl-, 125I-, 22Na+, 134Cs+, assessing the behaviour of neutral, anionic and cationic species. The evolution of each radiotracer activity was monitored in the injection borehole, from at least 100 days up to 3 years. Overcoring of the rock at the end of the experiments allowed the accurate characterization of radiotracer profiles. An overview of the DIR experiment is provided here, illustrating the large amount of data gathered in 4 specific boreholes at two different depths: -445 and - 495 m, within the Callovo-Oxfordian geological layer. The modelling of experimental results allowed the quantification of diffusion properties parallel to the bedding of the rock for the studied radionuclides. Diffusion parameters for HTO are in agreement with those determined on samples using the conventional through diffusion method. Anions are partially excluded from porosity with quantified values 0.3 < εa(anion)/ε(H2O) < 0.6. On contrary, the diffusion of cations is limited by their adsorption on clay surfaces with the evidence of surface diffusion processes. The sensitivity analysis on adjusted parameters provided additional insights on the local properties, such as homogeneity of the diffusion plume and the limited extension of the damaged zone surrounding boreholes: LEDZ ~ 1-3 cm. Still, further field experiments are necessary to measure diffusion in such EDZ and to improve accuracy on the diffusion anisotropy ratio.