Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite medium4.2 Underground installations4.2.1 Design principlesWith respect to long-term safety, underground installation design aims at firstly making the most,through architectural arrangements, of the favourable properties of the granite medium: themechanical strength of the rock, its radionuclide retention capacity and its low permeability in"blocks" characterised by rock with little or no fracturing. Making the most of these favourableproperties entails designing repository architecture in modules that can be adapted to the fracturing ofa specific site, since granite is intersected by potentially water-conducting fractures.This modularity underlies the possibility of connecting drifts crossing these water-conductingfractures. With regards to long-term safety this possibility relies on inserting seals in addition to thebackfill to fulfil functions that are redundant with those of the granite.Furthermore the underground installations are dimensioned to limit performance-altering disturbancesto the properties of the granite medium and the seals.Lastly, underground installation design incorporates the reversibility requirement as indicated inchapter 3.• Modular architecture, adapted to granite structure and its fracturingThe principle of modular architecture has been adopted in order to adapt the location of the repositorystructures to the fracturing. Repository structures (drifts and cells) installed in a low-permeabilitygranite "block" at a distance from water-conducting faults (§ 3.3.1) constitute disposal modules.Architecture geometry will therefore depend on "block" distribution in a granite massif.Typological analysis of French granites shows that the largest granite "blocks" generally tend to begrouped by sectors. Thus pre-defining repository architecture in granite medium is impossible, butmust result from both surface exploratory work on a site and underground laboratory studies.Furthermore the layout and geometry details of the repository structures are based as well oncompletion of "on-going" exploratory work during repository construction. This exploratory workconsists of geological surveys and geophysical and hydro-geological measurements performed indrifts and boreholes.Apart from the issue of siting repository structures in fracture-free granite blocks, a working failure ofpart of the repository must have limited impact.Accordingly the architecture is divided into repository zones and modules. Thus distinct repositoryzones accommodate the repository modules of a single broad category of waste: B waste and vitrifiedC waste. A specific repository zone must be constructed if UOX and MOX spent fuel is to be hosted inthe repository.The repository modular character and compartmentalisation (waste of the same type is groupedtogether) contribute to prevent chemical and thermal interactions in particular, between wastepackages of different kinds. For instance, bituminised B wastes are disposed of in specificallydedicated modules away from modules that contain exothermic wastes that are likely to significantlymodify the temperature of the medium.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY57/228