Architecture and management of a geological repository - Andra

6 – Overall underground architectureIn the B waste zone, each module comprises one cell and one drift that provides access to it andcorresponds to an operating unit. In C waste (or spent fuel) repository zones, each module comprisesseveral dozens of cells. In the architecture presented here, one module is subdivided into twooperating units.Operating units are constructed and operated as repository space is required at a rate of one unit every1 to 2 years depending on the zone. When the construction or operating periods exceed thisperiodicity, several operating units can be under construction or in operation simultaneously in thesame zone.Details of the layout of the various repository zones is given in Sections 6.2.5 and 6.2.6 below.In order to ensure compartmentization, each module is isolated from the rest of the undergroundinstallations by one or more seals at the time of closure.6.2.3 Access to repository zones6.2.3.1 Repository access shaftsThe surface installations are connected to the underground installations by means of a group of shaftscommon to all repository zones. The layout used facilitates control of the long-term hydraulicfunctioning of the repository:- The shafts are all located together in the same zone, which has a radius of approximately200 meters and is offset from the repository zones;- The shaft zone is connected to the repository connecting infrastructures by a group of drifts thatrun parallel to the main geotechnical stress. These parallel drifts shall be sealed when therepository is closed.A detailed description of the shafts is given in Section 7.3.6.2.3.2 Connecting driftsThe repository modules are connected to the shafts by connection infrastructure made up of clusters ofparallel connection drifts that vary in number depending on the size of the flow to be managed. Theseare all hierarchised into main connection drifts, which connect the shaft zone to the various repositorysub-zones, and secondary connection drifts, inside each sub-zone. This hierarchisation makes theconstruction and operation of the repository more flexible and means that the various sub-zones areindependent from each other. Distinguishing between dedicated package transfer drifts, constructiondrifts and, depending on requirements, air exhaust drifts facilitates the management of coactivitybetween nuclear activities and civil engineering activities.A detailed description and design of the connection drifts can be found in Section 7.4.6.2.4 General repository layout6.2.4.1 Positioning in the clay formationAt this point, Andra used the layout of all the repository modules in the median part of the Callovo-Oxfordian formation. This option ensures the same undisturbed argillite thickness between therepository and the over- and underlying carbonate formations 105 . There is no significant verticalvariation in the parameters that govern the retention and solubility of the radionuclides between thedifferent lithofacies of the formation; the Callovo-Oxfordian conceptual model therefore adoptsidentical values to these parameters for different lithological horizons [75] and [76] – Chapter 6).105 From an optimisation viewpoint, this option could be reviewed taking into account (i) the geomechanical behaviour of the differentlithofacies of the formation, and (ii) the migration times of toxic elements into the surrounding formations.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM260/495