Tome Architecture and management of a geological repository - Andra

3 – Design study of a repository in a granite formation3.2.2.2 Main analysis information: variability of French granites properties for therepository design and safety analysis• Variability of mechanical propertiesMechanical resistance of granite rock differs according to the types of granite, mainly depending ontheir hydrothermal alteration. However, generally these variations are not likely to cause anysignificant differences in the granite response to structure excavation. Differences for repositoryarchitectural design are thus minor and only affect detailed dimensioning of undergroundstructures.[6].• Considerations relating to the variability of thermal propertiesThe initial temperature of underground granite cannot be directly measured without a borehole.Modelling was thus based on thermal flux mapping in France and thermal conductivity of rocksdepending on their quartz content. Estimations made for a depth of 500 metres have led touncertainties concerning initial temperatures of between ± 2.8°C and ± 3.5°C depending on themassifs [7].The graph (Figure 3.2.5) illustrates the relatively contrasting situations between the different types ofFrench granite on this issue. Initial temperatures at a depth of 500 metres vary between 17 and 30°C.Rock conductivity has a value of between 2.4 and 3.8 W/m.K.Figure 3.2.5Correlation between thermal conductivity and temperature for granite massifsin FranceVariability in the thermal properties of French granites influences the dimensioning of undergroundrepository installations for highly exothermic waste (C waste and spent fuel). From this standpoint, thedifference is significant with respect to Fenno-Scandinavian granites where underground temperatureis lower by around 10°C.For the least exothermic waste such as B5 and B1, this variability has little influence on repositorydimensioning.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY39/228