Architecture and management of a geological repository - Andra

2 – General Description• A dimensional design limiting thermal disturbanceThe geometric configuration of the underground installations is designed to limit the temperature inthe repository. In concrete terms, it has been decided to limit the temperature to 90°C in contact withthe rock (or the buffer where the concept includes one). The concepts studied are based on heatdissipation by conduction through the rock. In this framework, the essential dimensional designparameters are the number of waste disposal packages per disposal cell and the space between thecells. The footprint of repository zones for vitrified C waste and spent fuel, depending on the thermalpower given off by the waste packages when they are emplaced, is therefore a direct result of thermalconsiderations.Moreover, we have opted to move B waste, especially bituminised waste, at a sufficient distancefromexothermic waste to protect it from heat.• Multiple sealing of underground installations and a dead-end architectureWhen the repository is closed, the cells, connecting drifts and shafts must be sealed. Lowpermeability,swelling clay plugs are used for this purpose. These seals oppose water circulationsalong these structures. They limit the water flow rate and the circulation velocity of this water.Special systems have been studied to guarantee a continuous seal between the plug and the argillitesand to interrupt, if necessary, the argillite zone fractured in the immediate vicinity of the excavation(see section 7.7).The excavation of the engineered structures can lead to the creation of a damaged zone in theexcavation wall which is expressed in different ways. In this respect, the following is notable:- A fractured area in the immediate vicinity of the engineered structure. This occurs if thebreakdown threshold, which corresponds to the maximum mechanical strength of the rock, isexceeded; It is characterised by the appearance of fractures, which are connected to a greater orlesser extent, and are susceptible to increase the permeability of the rock;- A microfissured zone. This is formed when the fissuring threshold is exceeded either immediatelyin the wall of the structure (if the fractured zone is not formed) or behind the fractured area. It isthe result of mechanical unloading related to the excavation of the engineered structures: theresulting deformations manifest themselves in the form of diffuse micro-cracking which is notvery connected. The fact that the microfissures are not very connected limits the increase inpermeability.Effects accompanying the progress of the excavation may also occur. In this case, oblique shearfracturing to the axis of the drift may appear ahead of the excavation face. As these shear fractures areintersected by the drift as it progresses, only the extremities exist in the wall, forming a network of"chevrons", the length of which is close to that of the microfissured zone.Beyond the microfissured zone is a zone described as being mechanically "influenced" it is the site oflimited modification of the field of constraints and deformations that do not affect the properties of therock (its permeability in particular).To increase seal effectiveness, we have opted to orient the cells and the drift sections designed to besealed parallel to the major principal geomechanical stress. This arrangement reduces the extent of thedamaged argillite zone in the vicinity of the structure [38]. Andra has also sought to minimise thesection of the structures to be sealed. Indeed, the greater the diameter, the greater the extent of thedamaged zone, particularly the fractured part, if any. The excavated diameter of the connecting driftshas been limited to approximately seven metres which is still compatible with the equipment andmaterial transfers and ventilation flows along these drifts (see chapter 7).Furthermore, to limit the possibilities of convection phenomena in the repository, especially in thecase of a seal failure, it has been decided to place all architectural elements in a dead-end. Thisprovision particularly concerns possible water flux from overlying formations. It prevents a circulationfrom being established through the repository under the effect of a horizontal hydraulic head gradient.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM56/495