Architecture and management of a geological repository - Andra

7 – The shafts and the driftsThe connecting and access structures to the repository modules are made up of shafts and drifts. Thepurpose of this chapter is to present the design of these structures, their sizing, and their mode ofconstruction, management and closure.To begin with, this chapter reviews the functions of these structures and the retained design principlesregarding the operational needs and the long-term safety objectives.Then, it describes the shafts and drifts. It shows, in particular, the logic underlying the determinationof the number of shafts. It also mentions the possibility of using a ramp. Regarding the drifts, itdescribes the principles of sizing.This chapter next describes the construction mode of the shafts and drifts. It shows the constructibilityof the access structures in the argillites of the formation studied, as well as when the overlyinggeological formations are crossed through.It indicates how reversibility is taken into account in the sizing and maintenance of the structures.Finally, it gives a description of the way the structures are closed. It explains the backfillingtechniques with a material prepared from the excavating mucks, as well as their sealing by plugs madeof swelling clay of low permeability. It also mentions the demonstration tests on the backfilling andsealing of drifts.7.1 Presentation of the main questionsThe function of the connecting and access structures is to allow the transfer of the flows generated bythe activities of construction, the disposal of the packages and the closure of the undergroundinstallations. Their design is based mainly on the operational needs of these activities. It also integratesthe considerations on reversibility and long-term safety [37].7.1.1 Operational needsThe connecting and access structures to the cells must allow the construction, operation and closure ofthe repository according to the principles defined in chapter 6, that is, progressive construction andseparation of the flows. Chapter 6 also gave the orders of magnitude of the flows generated by thesevarious activities and indicated that the sizing of the drifts was mainly determined by the sizeconstraints of the circulating vehicles and by the ventilation needs.These same factors, as well as the mass of the packages and transfer casks to be transferred, determinethe sizing of the shafts. At this stage in the studies, the maximum weight of the transfer cask andpackage combination is estimated at approximately 100 tons (case of some B waste packages andpossibly spent fuel containers with four type CU1 assemblies).7.1.2 Reversibility and long-term safetyThe repository structures (shafts and drifts) must be able to remain open over a minimum 100-yearduration for reversibility. They must be capable of resisting the pressure of the grounds which may beapplied over this time scale.But these structures must also be able to be closed if desired. They can be closed in successive steps inorder to take advantage of the managerial flexibility which is related to reversibility.Once closed, the structures must contribute to the following safety functions : i) limit the mechanicaldeformations of the argillite over the long term, ii) limit the circulation of water and iii) separate themodules of the repository.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM290/495