Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite mediumIn Fenno-Scandinavia, the KBS-3 spent fuel disposal concept (copper container) does not include anyredundancy in terms of seal or backfill. Only low permeability swelling clay will backfill drifts.However both provisions are envisaged in the generic context of Andra’s studies.Thus very low permeability long-lasting seals are to be constructed systematically between the waterconductingfaults and the repository modules installed in low permeability granite "blocks".Furthermore, depending of the granite site configuration, limiting direct water arrival from the mostpermeable surface part of the granite, can be achieve through seals installation between the surfaceundergroundaccess structures and the connecting drifts.Backfilling repository drifts are also planned to limit water circulation and contribute to delay anyradionuclides migration towards the environment.By principle, at this generic stage of the studies, the backfill are assumed to be more effective thecloser the drifts are to the waste. In repository modules, drifts are backfilled with low-permeabilitymaterial. Outside repository modules, the permeability level of the backfill emplaced in the drift andsurface-underground access structures is set according with both the conductivity of the fractureslikely to be intersected and the hydraulic properties of the excavation damage zone.• General architecture and dimensioning limiting repository disturbancesModular design of the repository architecture, if it is to be effective in terms of long-term safety, mustensure that the repository itself does not interfere ultimately the proper working of the granite medium,seals and repository modules.Thus the hydro-geological context of a granite site and its fracturing govern the horizontal distancesbetween modules and the vertical distances between levels. Possible interactions between modules arelimited by “buffer” distances of about one hundred metres between modules and water-conductingfaults.By principle, modules with B waste, since systematically including (high pH level) concrete for wasteconditioning, are not located right above or below modules of other repository zones. Thisarrangement avoids risk of interactions between, on one hand, the alkaline concrete water and, on theother hand, C waste and spent fuel repository modules that could be sensitive to it (§ 3.3.3).For similar reasons seal and backfill designs also use materials that by composition tend to limit anypotential disturbances to fractures properties, as fractures are the preferred place for radionuclidestransfer and retention.In particular, the concrete seal abutments may be designed with low pH concrete formulation in orderto limit repository water alkalinity. The same goes for the cementitious material injected into thewater-conducting faults intersected by the connecting structures during excavation work, to limit waterarrival and hydraulic disturbances in granite.From a thermal viewpoint, repository dimensioning for exothermic packages (vitrified C waste andpotentially spent fuel) aims at limiting temperature in repository modules and granite. The maximumtemperature criterion of 100°C in the modules results in a maximum temperature of 50-60° in thegranite, for an original temperature of 25°C at a depth of 500 metres. In the event of multi-levelarchitecture, the distance between two levels is set at about one hundred metres to prevent thistemperature level being exceeded.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY59/228