Architecture and management of a geological repository - Andra

12 - SynthesisTo concretely illustrate this, a few possible development perspectives were identified.In terms of an overall architecture of a repository, an installation in the middle of the layer wasretained. In order to optimally use the geological medium’s characteristics, this installation level in thelayer could be specified with the possibility of moving it to a greater depth. This influences thegeotechnical behaviour of the structures, which vary according to installation level (rock resistanceand mechanical stresses), as well as the respective role of the undisturbed argillite thickness betweenthe repository and the over- and underlying geological formations, with respect to the containment ofthe radionuclides released by the packages.Another parameter of the architecture is the adoption of a tree-like structure of dead-end elements.This increases the robustness of the long-term hydraulic behaviour of the respository. The applicationmay be refined by more strongly taking into account certain operational aspects (structureaccessibility, ventilation, management of incidental situations).Prudent assumptions were adopted for the geotechnical sizing of the structures (drifts, B wastedisposal cells, in particular). This sizing may be specified – structure form, support and linerdefinition, excavation sequencing – to adjust it to the local geotechnical characteristics of the rock. Todo this, resorting to a short-term observation (a few years) of the laboratory drifts would allowunderstanding more precisely the deferred in situ behaviour of the rock.In terms of the management strategy of the various structures, we retained minimising maintenanceloads ; in practice, the structures were designed analogously to public road tunnels in whichavailability requirements are particularly high. Depending on the structures, a more subtle strategymay be examined, such as recourse to a periodic examination and an accrued maintenance in theimage of mining practices.For the sizing of C wastes and spent fuels disposal cells, the retained thermal criteria (limittemperatures in the repository) are consistent with respect to those normally considered at theinternational level. Similarly, the studied concepts are based simply on heat dissipation by conductionin the rock without taking into account ventilation. Ventilation may offer a greater flexibility for thethermal design of the repository, but its interest should be examined taking into account the inducedconstraints (more complex architecture).The structure sizes were defined in a reasonable way regarding the technological requirements(excavation, transfer and emplacement of the packages) ; this avoided complicating the feasibilitydemonstration. Optimisations may be studied, allowing in particular a reduction of the total volume ofthe excavations.The study showed the interest of having very small C wastes disposal cells cross sections (in practice,close to that of the disposal packages) : this option limits the excavated rock volume, as well as thedisturbances induced in the geological medium, and favours heat dissipation in the rock.For the spent fuels, if it were decided not to recycle them, it was retained to insert a swelling claybuffer between the packages and the rock by increasing the section of the cells ; this choice resultsfrom the uncertainties on the thermomechanical behaviour of the argillites over the long term (thereduction of heat from the spent fuels is slower than that from the C wastes). As knowledge andobservations in the laboratory progress, the interest in such an option (pertaining to the long-termbehaviour) may be reviewed.These various perspectives for design improvement do not change the relevance of the optionspresented for the repository and a fortiori the conclusions of the study in terms of technical feasibility.From a rationale of optimisation, the study of such perspectives may be accomplished by extendingthe iterative approach of data acquisition, design and safety assessment employed up to now.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM487/495