Tome Architecture and management of a geological repository - Andra

3 – Design study of a repository in a granite formation3.3.1 Making the most of favourable granite propertiesThe granite medium is characterised by rock of extremely low permeability, with high capacity forradionuclides retention and mechanically resistant. As granite is intersected with fractures liable toconduct water, making the most of its beneficial properties entails adapting repository architecture tothis fracturing.3.3.1.1 A compartmentalised architecture, adapted to the granite fracturingRepository architecture is organised into different zones by major categories of packages: B waste, Cwaste and spent fuel. These zones are sufficiently far apart to avoid interaction between different typesof waste, particularly from a thermal or chemical view point. Module separation in each repositoryzone also reduces quantities of waste and radioelements which would be affected in cases of systemfailure or intrusion.Apart from these design principles, repository architecture and module separation is imposed bygranite fracturing.• Construction away from the faultsAt repository scale, repository zones for the various categories of waste are constructed away frommajor faults of the granite massif.Each repository zone is divided into repository modules grouping together a series of cavities thedisposal cells) for the same type of waste. Modules are located in granite “blocks” not intersected bylarge- or medium-sized faults, considered as significant water conducting. One of the basic principlesof a repository in a granite medium is to construct disposal cells in very low permeability rock. Thisdoes not mean that there can be no fracturing whatsoever in the rock but that small-sized fractureswhich may exist in cell walls do not conduct ground water or in small quantities. Therefore waterwhich might come into contact with packages is minimal.Disposal cells are of a “dead-end” type (therefore with only one access to repository drift), thuslimiting possibilities of circulation of water coming from the drifts.These principles regarding repository location and architecture meet the requirements laid down inBasic Safety Rule III.2.f, which states:”Repositories in geological formations must be located- in crystalline mediums, within a host-block exempt from major faults, as the latter arelikely to be preferential pathways of hydraulic movement. Disposal modules must beprotected from medium-sized fracturing, although this may be intersected by accessstructures”.• Construction of modules with respect to waste thermicityAt the module scale, the principle of adapting architecture to fracturing works differently for each typeof waste. The footprint required for different types of packages may necessitate implementation ofdifferent options (Figure 3.3.1).The inexistent (or low) heat transfer by B waste makes possible a design with compact disposal cellsand therefore requiring only a low volume of granite. B waste disposal thus requires a small footprintand adjustment of architecture to granite fracturing is then eased. Disposal modules are built so as toavoid fractures likely to cause water circulation.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY42/228