Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite medium• The possibility of multi-level architectureA granite massif generally represents a vast volume of rock available for a repository 300 to 1000metres deep. Incorporating the "depth" dimension offers additional flexibility in adapting thearchitecture, not only to granite fracturing but also to the temperature of the medium accordint towaste thermicity. Thus the studies have examined the advantages and limitations of multi-levelrepository architecture to deal with the variability of the characteristics of the French granitesidentified in the typological analysis, in particular their thermal and structuring properties (§ 3.2.2).Firstly, suitable adjustments to massif fracturing through two (or even three) repository levels providea reduction of the underground installations footprint. Furthermore, in the case of French granites thisarrangement enables the knowledge acquired at a given depth to be extrapolated to a volume of rockthat can accommodate two repository levels.There are essentially two limitations to multi-level architecture. From a design viewpoint, A two-levelrepository requires a thick enough massif in order to avoid any The rock that has to separate twosuperposed levels, must be thick enough to avoid the risk of harmful interferences.From an operational viewpoint, the issue of sharing access structures must also be analysed. Howeverthese limitations do not appear to rule out such a two-level concept as the vast volume of availablerock enables the “buffer” space between the two levels to be adjusted, and through construction ofsurface-underground access structures that serve only one repository level.Two-level architecture has likewise been envisaged in Sweden, for the "SR 97" safety assessmentdossier for a theoretical "Aberg" site in a granite medium, modelled from Äspö laboratory data (§6.2).• Surface-underground access structures (shafts or ramps) installed upstream of therepository modules from a hydraulic viewpointIn the interests of long-term safety, the access structures between the surface and the undergroundrepository levels are located upstream of repository modules to limit potential water circulation andradionuclides transfer from the repository to the connecting structures. Likewise, in order to limithydraulic gradients in between structures, access structures are grouped according to siteconfiguration.Shafts or a combination of shafts and ramps may be envisaged to provide the connection between thesurface and the underground repository levels. Dimensioning is adapted to the various transferfunctions assigned to them and the corresponding throughputs: excavated rock removal, ventilation,transfer of repository construction materials, disposal package and personnel transfer.• Multiple seals adapted to repository modularityThe surface-to-underground access workings and repository drifts are likely to intercept waterconductingfractures. Seals and backfill are designed to protect the repository modules from any waterflow resulting from intersected fractures.From this viewpoint, backfill and seal functions are complementary and redundant in long-term safetyterms. The exact specifications depend on the characteristics of a granite site and waste types.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY58/228