Architecture and management of a geological repository - Andra

6 – Overall underground architectureThe purpose of this chapter is to describe the logic behind the layout of the repository module in theclay formation studied and that of the shafts and drifts giving access to it.The introduction recaps the main questions to be taken into consideration in the overall architectureconcerning long-term safety, reversibility and operational aspects.It then describes the design principles adopted to meet these requirements. In particular, theseprinciples include a modular tree-structure for the repository zones, both for long-term safety reasonsand to provide management flexibility. It is also shown how the architecture allows the orientation ofthe structures to be adapted to the distribution of the natural mechanical stresses in the formation.This chapter then goes on to detail the main dimensioning factors of the overall architecture. The geomechanicalstability defines the distances to be observed between excavations The thermal criteriaaffect the area of the footprint of heat-generating waste repository zones; in particular, the thermalindependence of repository zones is checked. Finally, this chapter details why, at this stage, thegrouping of surface/bottom connecting structures and a principle of dead-end elements increase theeffectiveness of the control over long-term water circulation.The procedure for progressively constructing repository modules, as part of a phased approach, isdescribed. Material, waste package and ventilation flows determine the number and dimensions ofconnecting drifts making it possible to construct then use the repository modulesFinally, this chapter outlines a few orders of magnitude concerning the underground footprint of arepository corresponding to the package production scenarios introduced in Chapter 3. It alsodescribes the sensitivity of the footprint to the pre-disposal storage time of high-level waste.6.1 Presentation of main questions6.1.1 Long-term safetyChapter 2 introduced the long-term safety functions of a repository. The architecture described hereincorporates provisions that favour the fulfilment of these functions. A first point concerns thelimitation of the ability of water to circulate within the repository; in fact, the architecture conditionsthe expected overall hydraulic behaviour. The layout of the structures in the clay formation also affectsthe ability of the repository to delay or reduce the migration of the radioactive nuclides released by thewaste packages. Finally, the overall architecture must contribute to the limitation of mechanical,thermal and chemical disturbance of the host medium and repository modules.6.1.1.1 Control of the repository hydraulic behaviourThe search to limit the water circulation concerns several spatial scales within the repository: theimmediate vicinity of waste packages, disposal cells and the entire repositoryAt disposal cell level, limiting the renewal of water makes it possible to reduce the alteration of wastepackages and the release of the radioactive nuclides they contain; Chapter 5 indicated that a very slowwater circulation rate, making transport by convection negligible, particularly favours the durability ofthe glass which forms type C waste and of the spent fuel ceramic pellets. At repository level, limitingflows in drifts and shafts delays the migration of radioactive nuclides that have been released.The characteristics of the formation studied (low permeability, small gradients) naturally limit waterflows liable to be drained in the argilite. However, the possibility of reducing these flows even furtherin each type of structure, by means of the repository architecture, was investigated, notably byminimising the differences in hydraulic head within it. As far as this function is concerned, thearchitecture and seals of the drifts and shafts can play complementary roles, strengthening the concept.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM252/495