Architecture and management of a geological repository - Andra

10.2.3 Behaviour of access structures during the “Post repository zone-closing” phaseThe closing of secondary connecting drifts (inside the repository zones) reduces the level ofreversibility. In fact, access to packages becomes more and more difficult, even if technical solutionsexist for clearing the back-fill from the drifts (see section 10.4). In type B waste repository zones,clearing the back-fill from secondary connecting drifts only slightly reduces accessibility since thevolume to be cleared is around 20,000 to 30,000 m 3 . However, closing type C waste (or spent fuel)repository zones considerably reduces the accessibility of the packages. The volume to be cleared togain access to the cells increases by a factor of 3 to 5 when compared to the previous stage.The evolution of the cells and repository modules is influenced very little by closing a repository zone.The evolution of back-filled and sealed connecting drifts in these zones is very slow, as previouslydescribed for access drifts to type C waste (or spent fuel) cells, or even slower as they are subjected toless heating. The seals placed in drifts behave like type B waste cell seals. They are not re-saturated,even around the periphery, for over a thousand years. Any re-saturation of the rock in contact withthese seals and clay grooves is also very slow and all the more slow (one to several thousand years) ifthe ventilation has been maintained over a long period [8]. This slow re-saturation also limits thechemical breakdown of the lining, particularly by corrosion or hydrolysis. The absence of any notablechange to the structures after having been closed for several decades, or even centuries, thus facilitatesany reversal to dismantle a seal or retrieve packages.The main connecting drifts still accessible can be left as they are for an unlimited period, thanks to thedesign of their liner, maintaining the ventilation and, in particular, thanks to being able to maintainthese installations. This maintenance does not significantly increase the disturbances induced in therock at the location of the future seals. In fact, the area of micro-fissured or potentially fractured rocknear the disposal cell wall due to excavation does not extend. On the contrary, the fissures tend to becompressed by delayed deformation of the argilites blocked by the liner [66]. The rock is de-saturatednotably by the dry ventilated air in the micro-fissured or fractured areas. Maintaining the ventilationfor a century or more only induces a very low level of de-saturation beyond that in the non-fissuredrock (saturation rate over 95%), without any impact on the state of the rock, but contributies toslowing the later re-saturation (Figure 10.2.10).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM382/495