Architecture and management of a geological repository - Andra

5 – Repository Modules• Orientation of cells and architecture of the disposal zoneThe cells are oriented parallel to the major horizontal stress. This configuration leads to an isotropicstress field in a plane perpendicular to the main axis of the cell and thus helps minimise mechanicaldisturbance of the rock around the cell (no fractured zone created around the cell at a depth of 500 m).The disposal cells are grouped into modules. A disposal module consists of several tens to severalhundreds of cells (depending on the type of waste) served by three interconnected and parallel accessdrifts (see Figure 5.2.17).• The sleeveThe cell contains a low-alloy steel metal sleeve. This solution was considered to meet three mainrequirements: the sleeve mechanically supports the cell for emplacement of the packages; it mitigatesin favour of reversibility by ensuring long-term preservation of the internal functional clearances forup to several centuries; in chemical terms, it is compatible with the disposal packages.The sleeve comprises two parts: a permanent sleeve is placed in the useful part in which the packagesare to be emplaced, and a temporary sleeve is placed on the cell head and is then removed when thecell is sealed.• The plugThe cells are sealed by a swelling clay plug installed in the cell head. This plug, together with theargilite, constitutes a low-permeability continuous medium: it thus provides hydraulic and physicalchemicalclosure of the cell.The clay plug is located outside the damaged zone of the access drift. It is mechanically confined by aconcrete retaining plug which limits volumetric expansion of the clay while it is resaturating. A lengthof 3 m was considered for the swelling clay plug. This length was chosen for reasons of robustness:aspect ratio higher than one and relaxation compatible with maintaining a minimum swelling pressure(this point is examined in detail in § 5.2.6.2). This is sufficient given the extension of the alkalinedisturbance caused by the neighbouring concrete (in the retaining plug and the drift sleeve): the studyof alkaline disturbance propagation led to an estimate of this extension in the clay plug of about 0.6 mafter 100,000 years and 1.8 m after one million years.• The spacersInside the cell, C1 to C4 disposal packages are separated by spacers. A spacer consists of a metalenvelope containing a material chemically compatible with the vitrified waste (glass frit, silica, etc.).The presence of spacers creates thermal separation between the packages. It reduces the mean thermalflux density applied to the sleeve and avoids temperature peaks at the packages by ensuring greaterthermal uniformity. This arrangement enables the number of packages per cell to be increased withoutincreasing the thermal peak value. It thus enables the number of disposal cells to be reduced bymaking them longer (in relation to a configuration in which the packages are placed end to end in shortcells). It thus makes the repository more compact.In the case of type C0 waste, spacers are not needed owing to the moderate heat rating.• Summary of requirements and solutions adoptedThe various cell components described above, and their associated functions, are summarised in Table5.2.1 [7] [22].DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM197/495