Tome Architecture and management of a geological repository - Andra

5 – B waste repository zoneThus given the buffering capacities of the rock [38], these disturbances are unlikely to significantly affectneither the long-term prediction of tunnel evolution nor the radionuclide transfers in the surroundinggranite massif and its fractures.5.2.3.2 Managing the mechanical stability of the disposal tunnels over the long termDimensioning manages the mechanical stability of the disposal tunnels over the long term by allowingfor the state of the natural underground stress field [39]. The excavation method is chosen to limit thecreation of a fissuring zone around the disposal tunnels (a "damaged" zone).5.3 Disposal packages in concreteThis section specifies the design options adopted for disposal packages of B waste and succinctlydescribes the associated manufacturing techniques.The options retained for packing in concrete are the same as those considered for the clay medium. Theyhave been studied in conjunction with the French Atomic Energy Commission (CEA) [40]. This is becausethe CEA has a comparable issue to deal with for the study of a long-term storage facility.5.3.1 Selected design principlesThe simplification offered by a disposal package of standardised dimensions containing severalprimary packages enhances the reliability of disposal operations and potential package retrieval inview of the diversity of the shape and size of primary packages.Grouping several primary packages together reduces the throughput of handled packages in theunderground installations. Standardisation of the packages reduces the physical diversity of thehandled objects and simplifies their arrangement in the disposal cell.Compact packing solutions, with no excess thickness to attenuate radiation from the waste have beenadopted to limit the number of disposal cells. The B waste repository zone is thus made up of a lownumber of high-capacity disposal chambers, that each holds several layers of disposal packages instacks (several hundred to a few thousand packages per chamber). These chambers release radiation.They must therefore be automatically or remotely operated, which is additional justification forsimplifying operating processes.• Stackable disposal packagesEach disposal package in the cells must withstand the mechanical load of the masses of packagesstacked on top of it. The mechanical dimensioning of the package must also allow for a potentialvertical “misalignment” that could lead to uneven distribution of efforts. Furthermore, the behaviourof the disposal package must be assessed in accidental situations such as being dropped duringemplacement in the cell. Consequently the deformations induced on primary packages must be keptsufficiently small in order to limit radioactive material dispersion.• CriticalitySome B waste (particularly reference packages B3, B4, B5 and B6) comes from spent fuelreprocessing and therefore contain traces of fissile materials. The repository design must ensure subcriticalityin all circumstances. The B5 reference packages therefore seem to be the most constrainingwith respect to their potential fissile material content. Package design is linked to that of the cell,especially in terms of spacing between packages to maintain sub-criticality.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY119/228