Tome Architecture and management of a geological repository - Andra

3 – Design study of a repository in a granite formation• Variability of the hydraulic properties of the fracturesGeometry of large-sized fracturing in granite is a significant element for repository architecturaldesign on a specific site. On a particular site, the design of a repository depends on the majorfracturing geometry. Depending on the tectonic history of the massif, the pattern is more or lessregular and massif splitting is more or less pronounced. Analysis of a large number of French granitesshows that even if the pattern of large-sized fracturing varies between massifs, the distribution ofgranite “blocks” where the repository could be constructed complies with rules relatively common tothe French massifs studied. [8].Small-sized fracturing of granite also has a significant impact for repository design. Rock ability todelay and attenuate radionuclide migration will mainly depend on the characteristics of small-sizedfractures.[9]. Hydraulic conductivity of small-sized fractures is usually low or very low (less than 10 -9 m/s). In the range of low permeability, the values can however vary mainly depending on the granitesand types of fracture [10]. They depend on their geometry, orientation and potential natural cloggingwith minerals (Figure 3.2.3) and the same goes for radionuclide retention properties by fractures.• Considerations relating to the variability of morpho-structural contextsThe morpho-structural context of granite has also been analysed. Site topography and morphologydetermine hydraulic gradients, which are the driving force of underground water flows. Differencesbetween French massifs are significant. Typological analysis has identified three main morphostructuralgranite arrangements which are taken into account in safety analysis: granite massifs intopographical depression compared to the surrounding geological formations, domed massifs andsloping massifs. Each type can correspond to more or less accentuated topographies.[11] (Figure3.2.6).ARCHITECTURE DU MASSIF PAR RAPPORT ASON ENCAISSANT METAMORPHIQUETOPOGRAPHIEMassif affleurant en dépression ougouttièreTopographie peu accentuéeMassif affleurant en dôme ou toitTopographie moyennementaccentuéeMassif affleurant dans un « planincliné »Topographie vigoureuseFigure 3.2.6Role of the morphostructural context• Importance of hydro-geochemical variabilityThe available inventory and analysis of chemical composition of groundwater in French granites showthat different types of water are encountered: so called “alkaline” groundwater and carbo-gaseous one(Table 3.2.1).Carbo-gaseous water is present in the Massif Central and can be linked to the geodynamic context andmore or less old volcanic activity. Alkaline water corresponds to composition close to equilibriumwith the granite medium.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY40/228