Architecture and management of a geological repository - Andra

12 - SynthesisThe existence of uncertainties over the scientific data was taken into account during the studies,whether the uncertainities relate to the knowledge acquired at this stage or whether they are inevitablyinherent to the large time scales implied by the long-term safety functions of a repository. Faced withthese uncertainties, the choice of robust options was systematically favoured.For example, considering the mechanical behaviour of argillites, prudent work assumptions wereretained at this stage, which probably led to the sizing of the underground structures linings with somemargins. At the location of the seals, ways were proposed and studied to hydraulically interrupt theargillite ring which could be fractured in the vicinity of the excavations.The simplicity and robustness of the presented options render them also realistic from an industrialand economic viewpoint. The feasibility of realising the various components of a repository waschecked based on proven processes in industry (excavating of underground cavities, installing groundsupports and liners, etc.). The existence or the possibility of designing resources to operate theinstallations was also shown based on today’s available technologies (receipt, preparation andemplacing of packages, ventilation and maintenance of the installations). Generally, the technologicalfeasibility of the presented options is based on feedback from industry, favouring analogiesparticularly in the field of underground work and nuclear installations. On some specific aspects, thiswas completed by tests : building of disposal container demonstrators, development of tests related tobackfilling and sealing of underground structures.12.2 The reversibility rationale taken into account in the architectureThe demand for reversibility was integrated in the proposed architecture. Concretely, this concerns thedurability conferred to the structures and to the disposal packages, the modular structuring of theinstallations and the choice of simple processes for disposal which are easily reversible.Thus, a reversible disposal is evidenced, first of all, by a flexibility of management of the emplacedpackages similar to that of a surface storage. The waste packages can be retrieved as easily as theywere emplaced without damaging the structures and the packages themselves. The studies showed thatthis possibility applied as a minimum to 100-year durations (one to several centuries) without anyparticular maintenance. If desired, longer durations could be planned, but requiring special work on amore extensive scale (work on the packages, reinforced maintenance and cell strengthening,reconstruction…)Contrary to surface storage, a repository can also be closed to passively ensure the containment of thewastes and long-term protection of man and the environment. In a search for flexibility in the controlof the disposal process, the proposed options offer the possibility of a progressive closure.To do this, several successive stages were identified rendering the repository more and more passive.At each stage, free choice is offered for the process’ control, with the possibility, here again fordurations on the order of a century, to wait before moving ahead to the next stage, or to go back, untilthe packages are retrieved. To support the disposal process management, it is technically possible tomonitor the behaviour and evolution of the repository during the stages. It amounts to identifying, ifnecessary, the need for an action, such as a structure or a piece of equipment requiring maintenance, orincreasing confidence in knowledge and scientific models. Within this framework, the possibility ofmaking ″control″ structures was examined ; these could be more extensively instrumented.We checked that the repository’s level of reversibility, comparable to a surface storage in the initialstages, reduces progressively as the stages are passed through down to the lowest level, whichcorresponds to the complete closure of the facility. Nevertheless, after this closure, it is still technicallypossible to return to the repository structures and remove the disposed wastes.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM485/495