Tome Architecture and management of a geological repository - Andra

3 – Design study of a repository in a granite formationC waste and spent fuel thermal characteristics define disposal modules dimensioning and lead to suchlarge footprints that it is not possible to avoid the intersection of a module by a fracture, potentiallywater conducting 7 .Thus, repository architecture must be adapted to two levels of fracturing. Disposal cell size allowsthem to be built in very low permeability granite rock with minimal fracturing. Modules are installedin granite blocks, avoiding fractures which would too water-conducting (the so-called medium-sizedfracturing as mentioned in RFS III.2.f). The large volume of granite rock available underground for arepository between 300 and 1000 metres deep, allows for flexibility in adapting architecture to granitefracturing. General repository architecture can then be designed on one or more levels.Figure 3.3.1Architecture adapted to granite fracturingFrom a functional point of view, such architectural arrangements enable the repository to fulfil aprimary objective of preventing advective water flow within the underground installations. As regardsdisposal cells, this helps to facilitate a diffusion transfer system. Water flow rates are limited in themodule drifts. In addition, the repository is located away from regional faults, safe from major watercirculation.Such architectural arrangements also facilitate other repository functions by limiting radionuclidesrelease by disposal cells and their migration towards the environment.• A disposal process enabling “ongoing” surveying and characterisation of graniteblocks where modules are constructedAdapting repository architecture to fracturing means knowing, accurately enough and without toomuch uncertainty, the granite host rock characteristics.For this purpose, the surveying strategy includes several stages:- Surveying and characterising operations from the surface or from underground structures (geologicalmedium qualification underground facility), in order to define the granite structure where therepository is to be built. An iterative approach between safety analysis and the various phases ofgranite site survey, on the surface and then underground, defines criteria for exclusion of faults (or7 Given the waste volumes and their variations in thermicity, spent fuel disposal requires a larger footprint than C waste.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY43/228