Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite mediumThroughout this period, which could last several centuries, the reversible disposal system can bemanaged as a storage facility. Underground structures are continuously observed, which providesunderstanding of the phenomena concerning the installation, and solid knowledge of the state of thecells and their waste packages. Retrieving the waste packages in this configuration, if so decided, iseasy.The gradual nature of repository development also means making the most of the experience andknowledge acquired during the construction and operation of the first cells in order to adapt anddevelop the design of future structures.At the end of an observations phase, which in principle has no set duration, the decision to close therepository cells is the starting point of the closure process. This is also gradually implemented througha series of successive stages, which can be separated by observation periods. Would package retrievalbe decided at the start of the closure process, seal deconstruction and backfill removal would call forconventional civil engineering techniques; indeed, granite mechanical stability does not requirecomplex technologies aiming at emplacing ground support while excavating.Thus the staged nature of the repository process enhances the management flexibility offered by themodular design of the underground installations, combined with adapting the architecture to thefracturing of the host massif.These elements ensure that at each stage:- the installation can be kept as such and can be observed;- going backward is possible, up to the retrieval of all or some of the emplaced packages;- or keeping on with the disposal process is possible by deciding to move on to the following stagecharacterised by a greater passivity of the installation.Likewise, while surveying and characterisation activities are going on, this capacity for action meanspursuing, reorienting, suspending or abandoning the investigations of part of a repository zone, oreven giving up the installation of disposal cells in an identified granite block.4.3.1 On-going exploration and characterisationOn-going exploration and characterisation of the geological medium belong fully to the repositoryprocess in the granite context. They are performed according to the staged approach as described inthe introduction.The first two stages carried out from the surface and then in the laboratory, aim mainly at defining thegeneral structure of the granite and identifying the criteria for excluding faults and fractures that mayor may not be intersected in the connecting drifts and disposal cells. This identification results from aniterative approach phased by safety analyses.The last stage aims at surveying and characterising the host blocks in detail prior to repository moduleconstruction, followed by the validation of the repository cell layout within each constructed module.The small dimension geological objects addressed by this stage can only be detected using small-scaleexploration techniques (geophysical tomography, geological correlations between core samplings,borehole logging, hydro-geological tests, etc.). Implementation of these exploration techniques callsfor the construction of drifts and core samplings in the volumes of the massif which, on the basis ofprevious exploration phases appear to be suitable for accommodating repository structures.The technical feasibility of exploration drifts and cored boreholes does not raise any particular issuewith regards to construction feasibility of other repository infrastructures.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY78/228