Architecture and management of a geological repository - Andra

5 – Repository Modules• The geometry and orientation of the cellDefining the diameter of the cell body is based on balancing the need for maximum compactness withthe requirement of constructability of the cell within the constraints of the mechanical behaviour of thegeological formation and any residual uncertainties. The first requirement favours a cell that is as largeas possible, while the second would lead to the adoption of a small structure. As stated earlier, adiameter of 10 to 12 m was regarded as a reasonable compromise. This dimension corresponds to thedimensions frequently encountered in road or rail tunnels beneath a thick rock cover, with a hundredyearlifetime, as for example the Chamoise tunnel excavated in a rock similar to the argillites [59].At this stage in the studies, a useful cell length of the order of 250 m has been adopted. This lengthseems to be a reasonable compromise between the compactness of the repository, the flexibility andprogressiveness of repository management, the adaptability of cells to the wide variety of packages,the constraints of handling and the inclusion of the cells in the argillite layer.The cell has a pseudo-circular cross-section and is aligned parallel to the major horizontal stress. Thisconfiguration creates a quasi-isotropic stress state around the body of the cell and the access driftwhere the future seal will be, and minimizes the mechanical disturbances caused during excavation.The dead-end nature of the cell restricts water circulation within the cell in situations where a seal isdefective (cell seal or other seals).Cells are at least five excavated diameters apart, to minimize any mechanical interference betweencells and to ensure the long-term stability of the whole area. It was further confirmed that this cell andrepository zone geometry was compatible with compliance with temperature criteria (30 °C forreference package B2 and 70 °C for other packages), ignoring the favourable effect of ventilation.• Nature of the cell liningThe cell body is basically characterized by a concrete lining. Choosing concrete meets the need formechanical stability in the structure left without maintenance for a period of at least a hundred years,since concrete is a geomaterial, easy to use and stable over the period in question. Its chemicaldegradation (by hydrolysis and carbonation) is slow under repository conditions [48]. The liningconcrete is not reinforced, so that its durability is not dependent on steel corrosion.Moreover, as stated earlier, concrete has a favourable impact regarding the immobilization of certainradionuclides and regarding the corrosion of metal waste.• Form of the cell liningIn order to restrict mechanical deformation of the geological formation in the long term, the residualspace left after package emplacement has to be reduced as much as possible. A conventional solutionfor limiting these spaces would be to fill the cell’s residual spaces with a granular or cement-basedmaterial after package emplacement. However, this solution has a double drawback: it requires anadditional operation complicated by the irradiating nature of the packages and it increases thedifficulty of possible removal of packages, since they would be blocked in by the fill material.The solution adopted takes into account the need to limit the residual spaces and the need to keep apermanent functional clearance for a period of at least a hundred years (to allow reversible repositorymanagement). The principle adopted during construction consists in adding an additional volume ofconcrete to the lining segment 61 for ensuring mechanical stability, in such a way that a disposalchamber of rectangular cross-section is formed, of a size adapted to the form of the package stack. Thetens of centimetre clearances between packages thus meet the two conditions mentioned above. Theseclearances actually produce an inter-package space of less than 5%, with respect to the excavatedsection [57].61Quasi-circular in shapeDOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM167/495