Architecture and management of a geological repository - Andra

10.2.4 Conclusion concerning the minimum duration of reversible management of thedisposal processThroughout the disposal process, the mechanical stability of disposal cells affects, to a greater or lesserextent, the difficulty of any package retrieval operations.The design of the liner of type B waste cells and the sleeve of type C waste (or spent fuel) cells givesthem mechanical stability for 200 to 300 years without special maintenance, and practicallyindependently of going through the closing stages. Given the dimensioning margins, the cells shouldin fact be stable for an even longer period. Monitoring of these structures would enable their lifetimeto be regularly re-assessed. In order to extend the period during which it would be possible to retrievethe packages, special work (for example additional maintenance) would be required, that may includetemporary recovery of the packages disposed therein. The complexity of this work increases as eachclosing stage is completed, as gaining access to the cells then involves re-opening sealed sections.As for the stability of the access structure liner (drifts and shafts), it can be increased by maintenancework as long as the structures are accessible. As for the cells, this is at least several centuries. The insitumeasurements made while they remain open and, where applicable, after they were closed, wouldenable this period to be more accurately predicted.It is considered that the ultimate end of reversible management of the disposal process will be themechanical failure of the cell lining. In fact, beyond this milestone, retrieving packages blocked by thegeological formation would require the simultaneous use of mining and nuclear resources. Althoughtechnically possible (based on experience feedback from certain uranium-rich deposits), their userepresents a low level of reversibility.A period of two or three centuries therefore represents the minimum period during which reversiblemanagement by stages can be implemented without requiring technically complex operations. Inconcrete terms, that means that, whatever the level of repository closing, it would be possible toretrieve packages from a cell once access to the cell has been regained, using handling facilitiessimilar to those used at the time of package placement. In order to extend this period, from a technicalpoint of view, it would be necessary to adopt specific measures (increased maintenance, strengtheningof the structure, reconstruction etc.).10.3 Observation and surveillanceThis section firstly describes the main motivations for observing and surveilling a repository, andgives a brief overview of thinking on the subject at international level. It underlines the contributionthat observation makes to reversible management. It then identifies the main constraints specific to therepository. It also sets out the industrial experience feedback from the monitoring of major structuresand available technologies. Finally, it proposes monitoring systems adapted to the disposal ofradioactive waste, which will be enriched at a later date by knowledge acquired from theMeuse/Haute-Marne underground laboratory [87].10.3.1 Why carry out observation and surveillance?The main reason for conducting observation and surveillance is to provide tools for controlling thedisposal process. In fact, monitoring the evolution of the structures and host rock will, in the longterm,improve our knowledge and strengthen confidence in the mastering of the process.Under no circumstances is this monitoring aimed at replacing the preliminary knowledge acquisitionstages, notably by conducting experiments in the underground laboratory. On the contrary,implementing monitoring inside the repository could benefit from the feedback of experience acquiredin the underground laboratory. Nor can the existence of an observation and surveillance programmeduring repository operation be used to compensate for a lack of any data concerning the safetyassessments that would precede any disposal authorisation.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM384/495