Architecture and management of a geological repository - Andra

5 – Repository ModulesA concept without an swelling clay buffer has considerable advantages in terms of the simplicity andcompactness of the repository. The absence of swelling clay buffer limits the diameter of theexcavation and facilitates construction, thereby disturbing the rock less. A cell without a swelling claybuffer, of a more simple design than a cell with a swelling clay buffer, would also limit the number ofinterfaces between the packages and the ground; the quantity of steel is reduced; the cross-section ofthe cell seal retaining plug is smaller thus limiting the amount of concrete used. As the space betweenthe cells is above all dictated by thermal considerations, using an swelling clay buffer would be suchas to increase the distance between the cells owing to the introduction of additional clearances ormaterials which conduct heat less (swelling clay buffer partially saturated at installation). This leads toa larger footprint for the repository.Analysis of these two options led Andra to prefer a solution without swelling clay buffer for the Cwaste cells. This type of configuration is made possible by the compactness of the damaged zoneexpected around the cells, owing to their small diameter, the geotechnical properties of the rock andthe envisaged excavation method. The studies have shown:- the absence of any fractured zone around the cell at a depth of 500 m at construction of the cell,- the creation of a small micro-fissure area, the permeability of which does not modify the diffusiontransport conditions in the near field around the cell,- long-term evolution tending to close the micro-fissures around the cell (phenomena of creep onthe support and clogging),The uncertainties concerning the geotechnical characteristics, rheological models, or even the longtermevolution prior to seating, may lead to a fractured zone around the cell. However, this would inany case be limited to a very small thickness of argilite (a few centimetres) and would not warrantadopting an swelling clay buffer. A cell variant with swelling clay buffer would in principle be similarto the spent fuel disposal cell (see section 5.2.3.5).5.2.2.2 The chosen solutionThe reference concept considered at this stage for the C waste is a dead-end horizontal tunnel withoutswelling clay buffer. This tunnel is about 700 mm in diameter and has a metal sleeve. At this stage inthe studies, the length of cells was limited to make the technological feasibility demonstration easier.The design work primarily concerned the following aspects: defining the spacing between packagesand cells, for thermal reasons; control of damage to the argilite by minimising empty spaces in thecell; sealing of the cell and the resulting provisions for the cell head.The cell comprises a "useful part" intended for disposal of the packages, and a cell head designed toseal the cell. The heat produced by the waste is evacuated solely by passive conduction in thegeological formation; no ventilation is required for this process. To adhere to the temperature criteriamentioned earlier, some packages could only be placed side by side after a very long storage period.These packages could be placed in the repository earlier provided that they are spaced in order toreduce the mean heat flux. One solution would be to place inert spacers between the packages, thelength of which is adjusted according to the heat rating of the packages concerned.After the packages are emplaced, the cell head is sealed by a swelling clay core. This swelling core ismechanically confined by a concrete plug.The disposal cells are arranged on either side of an access drift. They are spaced between 8 and 13metres apart, depending on the heat given off by the waste.The diagrams in Figure 5.2.2 illustrate this design.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM195/495