Tome Architecture and management of a geological repository - Andra

3 – Design study of a repository in a granite formation3.2 The granite mediumRepository design initially aims to make the most of geological formation properties beneficial tounderground disposal so that it fulfils the various safety functions assigned to it.In the absence of a specific study site, the repository design study cannot be based on the descriptionof a specific granite massif. Design principles adopted by Andra are mainly based on propertiescommon to all granites (cf. § 3.2.1).However, French granites have specific characteristics which the design studies must consider in orderto propose relevant options. Andra has therefore drawn up a reference document on French granitedetails to identify the granite characteristics which could affect repository design. (cf. § 3.2.2).3.2.1 Granite: its properties for radioactive waste disposalThe word granite has two meanings for a repository study: it designates both a rock and a geologicalformation. Generally granite, the geological formation, is organised into massifs 5 . Thus, thepossibility of a repository in granite medium depends on rock properties and characteristics andgeological context of the massif studied.3.2.1.1 Granite rock: a hard, resistant rockThe common perception of granite as a stone used for a long time as a lasting ornament is that of ahard rock, of very low porosity and very low permeability.Rock mechanical resistance is naturally of interest for construction of underground structures. Thisrock can be excavated without any significant ground support being required, over volumescompatible with repository dimensions and depth. This mechanical resistance is due to rock texturecomposed of quartz (crystallised silica) and feldspars (aluminium silicates). Quartz also contributes tothe usually high thermal conductivity of the rock, which makes it a formation likely to easily dissipateheat emitted by radioactive waste.Granite rock contains very little water: its water porosity is usually less than 0.5%. Rock permeabilityis very low and can be on the limit of accessibility to in situ measurements.These characteristics constitute interesting properties a priori for a radioactive waste repository.5Unlike sedimentary geological formations which are usually arranged in superimposed layers (for example: the clay formation of theCallovo-Oxfordian of the Meuse/Haute-Marne site), magmatic formations such as granites often have bulkier geometries (threedimensional)than planar. In the case of granites the word massif is generic and applies to most of the arrangements likely to be met .Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY33/228