Architecture and management of a geological repository - Andra

5 – Repository Modules5.2.2 Design principles adopted5.2.2.1 Several solutions envisagedThe options envisaged start from a certain number of common characteristics or principles. Theydiffer in the size or layout of the repository cavities, the materials employed and the emplacementmethods.• Common principlesThe importance of controlling heat release with respect to damage to the argilite was underlined in thepresentation of the main issues. One general principle of repository design is to build its long-termsecurity on passive evolution (with no human intervention). After closure, the heat produced by thepackages dissipates by conduction through the geological medium. Ventilation is not used to dissipateheat during the operating phase. This basic choice means that the closure decisions can be dissociatedfrom the thermal phase management arrangements.All the cell solutions considered are of the dead-end type. This arrangement minimises watercirculation after closure, even in altered situations.Limiting the migration of released substances requires the preservation of undisturbed argilitethickness around the repository. This goal leads to the cells and their access drifts being placed in themiddle of the argilite formation. The geometry of the formation entails a primarily horizontal footprintof little thickness. On this basis, vertical (shaft) and horizontal (tunnel) cell solutions were examined.Finally, immobilisation of the radionuclides in the repository requires control of the flow and transportconditions in the immediate vicinity of the packages. This goal implies limiting damage to theimmediately surrounding argilite.• Horizontal or vertical cellsHorizontal cell have less impact on the geological formation in terms of excavated volume andrepository footprint. Tunnel-based architectures require significantly less excavated volume than shaftarchitectures. The gain is due to the length of the access drifts, as one access drift can serve two rowsof horizontal cells located on either side of it, whereas it could only serve one row of vertical cells.The same length of access drift thus serves twice as many horizontal cells as a vertical shaft. Figure5.2.1 illustrates this aspect. The horizontal arrangement also maximises the thickness of theundisturbed argilite.Using horizontal cells offers optimisation possibilities, as for a constant total cumulative cell length(i.e. for the same disposal capacity), increasing the unit length of the cells enables their number to bereduced, consequently reducing the length of access drift required. In the clay formation studied, thistype of optimisation would only be possible in a horizontal configuration as preservation of the argilitebuffer zones limits vertical extension of the cells and consequently rules out deep shafts.Package handling would at first glance appear more complex with a horizontal configuration than witha vertical one. However, section 9, which deals with operating systems, demonstrates the feasibility ofpackage disposal using a horizontal configuration. This relative drawback does not outweigh theadvantages with respect to the other criteria, in particular those concerning repository confinement.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM193/495