Tome Architecture and management of a geological repository - Andra

3 – Design study of a repository in a granite formationfractures) which may or may not be intersected by i) repository connecting drifts, ii) access drifts tomodules and iii) disposal cells,- On this basis, the process includes in situ characterisation of host-granite blocks for disposal modulesbefore package emplacement. This stage of granite “ongoing” characterisation during the stagedrepository construction finalises module architecture and distribution of disposal cells in the graniteaccording to fracturing.Such a strategy aims at adapting repository architecture as best as possible to granite fracturing and toensure that proposed design concepts fulfil their functions effectively as regards control of watercirculation in the repository.3.3.2 Design of engineered components, complementary and redundant with the granitemedium for long-term safetyAt the scale of both the repository as a whole, and the disposal cells, several arrangements are possibleto ensure complementarity and redundancy between the granite medium and repository engineeredcomponents with respect to long-term safety. They particularly concern repository architecture andchoice of materials for engineered components (disposal packages, engineered barriers, backfills andseals).• Multiple sealing of underground installationsConnecting drifts and access drifts to modules and disposal cells are likely to intersect waterconductingfractures. In order to limit water circulation within the repository, seals are installed atvarious levels of the underground installations.In the case of disposal cells, water may come from drifts serving them. Drifts are likely to beintersected by a more water-conducting fracturing than the cell rock wall one. The ‘dead-end’architecture of cells, their construction in granite rock of very low permeability, and very lowpermeability ‘plugs’ at cell head limit water circulation and aim at establishing a transfer system in thecells governed by diffusion phenomena.At the repository module scale, water circulation is limited by:- Very low permeability seals installed in drifts to cut off modules from water coming from anypossible intersecting faults,- Backfills of sufficiently low permeability in module drifts.Disposal cell seals and plugs are made of swelling clay (bentonite), which are very low permeabilityover long periods of time. Backfills may also incorporate clay materials to ensure sufficiently lowpermeability.At the repository scale, connecting drifts between modules as well as structures between the surfaceand underground are backfilled. Seals are installed in access structures where they intersect waterconductingfaults.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY44/228