Introduction The construction and the operational phases of a high-level radioactive waste repository include the ventilation of the underground drifts since excavation until the waste emplacement and their subsequent backfilling and sealing. This ventilation that will occur over a significant period of time will induce a rock desaturation that may have an important impact in the near field of the clay barrier. Microstructure of argillaceous rocks is related to the spatial distribution of minerals (mainly swelling clay minerals) and pores and consequently with that of water. The effects of desaturation on the microstructures of the Opalinus clay were investigated by visualizing and quantifying the organization of minerals and porosity of core samples taken under assumed saturated and unsaturated conditions. Experimental concept Two boreholes were drilled from the microtunnel of the Mont Terri URL dedicated to the Ventilation Experiment phase II framed in the NF-PRO RTD Component 4, WP4.3 EC project. The two boreholes about 1.2 m long each were air core-drilled, 30° upward and subparallel to the bedding at a distance of about 20cm from each other along the same beddings. The first borehole (BVE-96) was assumed to be representative of the saturated conditions as it was realized at the end of a 16 months natural resaturation period without ventilation (VE phases I + II; Mayor et al., 2005). The second borehole (BVE-103) was drilled at the end of the second desaturation period (VE phase II) after one year of a high flowrate (60m3/h) and dry air (1<RH%<3a) ventilation and was therefore assumed to be representative of unsaturated conditions. For each borehole cores were sawed for obtaining 3x15cm samples in the very first 45cm and a 15cm sample at the borehole end (between 105 and 120cm). In addition of a drillcore mapping a combination of techniques was applied for each sample among which, petrophysical determinations by water content measurements at 150°C, SEM observations, XRD, autoradiographs and high resolution X-ray tomography. Results and interpretation Values of degree of saturation (Figure 1) estimated from water content and volume measurements indicate a water loss in the rock mass for the first borehole (giving assumed initial conditions) with values of saturation circa 70% and attributed either to an artifact due to an evaporation process during drilling and the subsequent handling during mapping or to a residue of desaturation due to the former VE experiment. Samples taken from the 2nd borehole at the end of the desaturation period (final conditions) show a much bigger desaturation with values reaching 50% in the very first 20cm and a resaturation trend between 80% and 100% further. For both boreholes the combination of techniques has revealed the occurrence of micrometric cracks parallel to the bedding. These cracks have a frequency that decreases with the distance from the borehole head and are quantitatively well correlated to the degree of saturation especially for the BVE-103 borehole. Theses cracks are therefore attributed to desaturation cracks. Autoradiographs obtained after rock fragments impregnation with a 14C-radioactive resin enabled us to quantify the role of fracturation in total porosity by comparing matrix porosity to total porosity. The matrix porosity decreases up to about 40cm then slightly increases in the borehole end. This reduction of matrix porosity was also observed around drifts excavated in the well compacted Tournemire argillite submitted to a natural dry-air ventilation and was attributed to the reduction of volume by water loss (Altinier, 2006; Matray et al;, 2007). High resolution X-rays tomography was performed with the aim of visualizing the organization of porosity in 3D at a micrometric scale and quantifying porosity and minerals. The proportions of minerals estimated by this method are consistent with the known mineralogy of Opalinus Clay (Gautschi, 2006). It also enabled us to verify the occurrence of desaturation cracks and to quantify their contribution to the total porosity that appeared in good agreement with estimates of fracture and matrix porosity deduced from autoradiographs.
Figure 1:Degrees of saturation obtained at 150°C from boreholes BVE-96 and BE-103 representing the initial and final conditions of a 1year dry air ventilation