Tome Architecture and management of a geological repository - Andra

6 – C waste repository zoneHence the vertical borehole design, which statistically limits the number of potential minor fractures atthe walls of the disposal cells, has been retained as the reference.A horizontal design could also be envisaged for a site where fracturing distribution turns out to bemainly horizontal. It would be based on the same basic design principles, namely an over-pack and aclay engineered barrier.As short boreholes statistically bisect fewer fractures than longer ones, disposal borehole length isanother element that can be adapted to the fracturing of the granite.The disposal boreholes are dead-end. There is no access from the borehole bottom to the repositorymodule drifts. This arrangement reduces the possibilities of water circulating in the disposal boreholes.Towards the surface they open out onto a handling drift dimensioned for package emplacement in thedisposal boreholes.Thus the option proposed for the C waste cell is a borehole with a limited length of about 12 metres,less than 2 metres in diameter, that enables a clay engineered barrier to be placed between the disposalpackages and the rock (cf. § 6.4).6.2.1.2 Installing repository modules in "blocks" of granite away from water-conductingfaultsThe repository modules are installed in the granite away from the water conductive faults which cannotbe bisected by the module drifts and primarily the handling drift (Figure 6.2.3). These faults generallyextend over several hundred metres or more.The installation of the modules in the granite blocks away from the water-conducting faults does notrule out the presence of minor rock fracturing inside the module. However its hydraulic conductivitymust be low enough not to jeopardise the safety functions.The application of this principle calls for defining criteria concerning the properties of these faults andtheir in situ surveying and characterisation prior to installing the modules in the repository host massif(cf. § 4.3.1).Figure 6.2.2Principles of installing a repository module away from water-conducting faultsDossier 2005 granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY166/228