Long term in situ tracer diffusion tests in the Callovo-Oxfordian clay: results and modelling

Document available in extended abstract form only. Argillaceous formations are considered as potential host rocks for deep geological disposal of radioactive waste, mainly due to their low hydraulic conductivity, highly favorable to a diffusive transport, and their good sorption properties. In this framework, the French National Agency for radioactive waste management (Andra) is studying the Callovo-Oxfordian argillite in the Meuse/Haute-Marne URL. Seven field migration tests are performed to estimate the diffusion parameters (effective diffusion coefficient De, accessible porosity φ, and retardation factor Kd) of safety-relevant radionuclides (tritiated water, iodide-36, chloride-125, sodium-22, selenium-75, strontium-85 and caesium-134) at various lithological levels of the host rock (Illite/Smectite type R0 and Illite/Smectite type R1). Six short boreholes (15 m deep) were performed from the drifts of the URL and one deep borehole (EST208: 542 m deep) was carried out from the surface. Radionuclide activity monitoring of the traced fluids circulating in boreholes is useful to provide a first estimation of diffusion parameters in the clay before the overcoring phase. In several boreholes, this monitoring phase began 4 years ago. The results show the need to take into account: - a disturbed zone around the injection borehole in addition to the pristine rock. This damaged zone is characterized by a higher porosity due to unloading and a partial de-saturation of the rock during the drilling. Thus, 5 diffusion parameters (De in the damaged zone, De in the pristine rock, φ of the damaged zone, φ of the pristine rock and thickness of the damaged zone) are required to fit the data; - a dilution effect due to the sampling methods. Despite some uncertainties (model not constrained enough), but using assumptions, modelling results are consistent with parameters measured on drill core samples of a few millimetres thickness. The six short boreholes were dismantled and the rock around the injection borehole was over-cored. The radionuclide activities in the rock profiles are running. The diffusion parameters values estimated from fluids monitoring can now be compared to parameters obtained from tracers' distribution profiles within the rock. These two kinds of data sets offer a good opportunity to test models specifically developed by CEA and Subatech teams. (authors)