Architecture and management of a geological repository - Andra

12 - SynthesisThis reversibility is not paradoxical with respect to the operational safety of a repository or its safetyover a longer term. On the one hand, the search for durability in the architecture to favour reversibilitydoes not introduce any new risk and a priori moves in the direction of safety. On the other hand, theobservation and surveillance of the repository, which are associated with the implementation ofreversibility, allow managing the disposal process in an informed manner. For the technical optionsstudied, we also checked that the possibility of extending - within the 100-year limit previouslymentioned - each stage in the disposal process did not have any significant impact on the long-termevolution of the facility.The reversible management of the packages and the disposal process implies human decisions andactions up to the complete closure : maintenance, observation and surveillance, closure work. Thefacility will only become fully passive afterwards. This active management supposes a continuity inthe social and technical systems. But it also makes much less likely a gradual forgetting and anabandoning of the facility before its complete closure.Another factor was underscored by the study : the progressive construction of the repository facilitiesby successive modules. This also allows for a flexibility in the management of the repositorydevelopment. It also allows a certain “reversibility in the repository design″ : the design of the newstructures may be modified to take into account in particular the experience acquired by theimplementation and observation of the previous structures.12.3 Safety in the architectureThe presented architecture was designed to respond to the expected safety functions of a repository.This traditionally concerns operational safety, but also, specifically in the context of a repository, thesafety after the closure. This latter is based on the objectives laid down in the basic safety rule no.III.2-f.One should note, without being exhaustive, the modular architecture and tree-like structure of deadendelements, the arrangements taken in terms of structure dimensions and choice of materials to limitdisturbances (thermal, mechanical, chemical, in particular), the special role of the high-level wastecontainers, the systems constructed during the closure stages (backfills and seals).The robustness of the presented options, in light of the uncertainties of knowledge, participatesstrongly in the repository safety approach. The simplicity of these options contributes to theirdemonstrability with respect to the safety objectives.Arrangements were also introduced in the architecture in order to increase the repository’sperformance in the case of abnormal operations. As a result of this, the underground installations weresubdivided into independent units. Systems which complement each other or are redundant wereadopted (for example, architecture and seals to prevent potential water convection).It should be recalled that the safety aspect in a repository’s architecture was taken into account bymeans of an iterative approach such as recommended by the basic safety rule. These iterative steps ledto interim appointments with the Safety Authority. Note also the international review of the Dossier2001 Clay under the aegis of the OECD’s Nuclear Energy Agency, whose main instructions weretaken into account in the study.Finally, recall that the presented architecture was covered by a study on the long-term behaviour andevolution and a safety assessment given, respectively, in the two other volumes of the Dossier 2005Clay.12.4 OutlookThe presented options contribute to establishing from an engineering viewpoint the feasibility of areversible and reliable repository. Nevertheless, they must not be considered as frozen and final. Infact, they are not aimed at this stage at a technical or economic optimisation.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM486/495