Tome Architecture and management of a geological repository - Andra

5 – B waste repository zone"Dead-end" cell architecture, with one end bound by the granite rock which means without an exit tothe disposal drifts, complements the installation of tunnels in very low permeability granite blocks. Ittends to reduce water renewal in the disposal cells, even in the event of seal failure. This principle isapplied to all the disposal tunnels.5.2.2 Designing engineered components that are complementary and redundant to thegranite mediumMaking the most of the favourable properties of the granite mainly entails installing the repositorymodules in very low permeability granite rock. However the access drifts leading to repositorymodules in granite are likely to intersect water-conducting faults. Thus the various engineered modulecomponents (disposal packages, backfill, seals) must complement the granite by acting as a barrierfirstly to the circulation of advective water in the modules, and secondly to radionuclide release andmigration.These provisions also provide redundancy with the granite in the event of failures or external attacks.5.2.2.1 Multiple sealing of the repository modulesVery low permeability seals are inserted in each access drift to protect the repository modules fromwater flows that could originate from the faults intersected by the access drifts. Their precise locationis governed by the geometry of the "blocks" of granite accommodating the modules and theirsurrounding faults (Figure 5.2.2).Low permeability backfill is placed in the drifts redundant to these seals, as a measure for dealing withseal failure.The repository tunnels are also closed by very low permeability "plugs". These seals isolate thetunnels hydraulically from water flows in the drifts and thus form barriers that limit radionuclidemigration from the waste to the access drifts and into the environment to diffusion.To ensure that seals remain effective in the long term, their dimensioning take into account thepotential disturbance to their properties by the alkaline waters that would result from the concretealteration of the waste package disposed in the tunnels.5.2.2.2 Disposal packages in concreteThe installation of repository tunnels in the very low permeability rock and the emplacement of sealsprotect the waste packages from flows of water once the repository has closed, which slows down thekinetics of package deterioration and thus radionuclide release. Furthermore, underground the granitewater - an element conducive to radionuclide immobilisation - is reducing.The choice of disposal packages in concrete also has a hand in limiting radionuclide release andimmobilising them by providing an alkaline environment (pH level of 7-12.5) inside the repositorytunnels [36].The precise formulation of the concrete will be determined in line with the exact characteristics of thegroundwater in the granite hosting the repository: concrete formulation possibilities mean thatadjustments can be made in the French geological context as dictated by the salinity of the granitewater and their pH level [37].Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY117/228