Tome Architecture and management of a geological repository - Andra

3 – Design study of a repository in a granite formation• Design limiting mechanical and thermal disturbancesGranite is a mechanically resistant rock. The structures (drifts and cells) are dimensioned to ensuremechanical stability in the long term.Heat released by C waste and, if need arises, also by spent fuel means a temperature rise in thedisposal cells and surrounding granite. In order to control the thermal phenomena incurred, the aim isto keep the temperature in the cells lower than 100°C (and therefore in the rock). In practical terms, amaximum temperature of 90°C has been adopted for the hottest point in the swelling clay buffers for Cwaste cells and at the surface of spent fuel copper containers.The essential parameters for repository architecture dimensioning in order to limit the temperature areon one hand the number of disposal packages per cell and on the other hand the spacing in betweendisposal cells. The C waste and spent fuel repository zones footprint is mainly subordinate to thesethermal considerations, which depend on the thermal power released by the packages once emplacedin the repository.• Disposal process limiting hydrogeological and hydrogeochemical disturbance of thegranite hosting the underground installationsExcavation of underground installations drains off the granite groundwater and disturbs initialhydrogeology. As granite is only slightly permeable, this disturbance mainly affects the most waterconductingfaults and fractures. In order to limit water drainage from the granite and by extension thequantities of pumped water while excavating the underground installations, it can be envisaged to callfor to injection techniques for the most water-conducting faults and fractures intersected by thestructures.After a transient phase of disturbance related to excavation of underground installations, equilibriumbetween water drainage and re-supply is established within the granite massif.Managing appropriately, according to granite hydrogeological context, excavation of repository zones,their operation and then their closure constitutes a mean of limiting hydrogeological andhydrogeochemical disturbance of the granite.3.3.4 Adaptation of design arrangements to long-term safety functionsThe various options proposed contribute to one and/or another of the main functions of a repository, assummarised in Table 3.3.1:- the function “preventing water circulation in the repository” is mainly fulfilled by architecturaland sealing arrangements. Repository architecture is adapted to granite fracturing,- the function “limiting radionuclides release and immobilising them within the repository” ismainly fulfilled by systems implemented near the packages in order to permanently ensurefavourable environmental conditions to the protection of waste and immobilisation ofradionuclides released,- the function “delaying and attenuating radionuclides migration” makes the most of alltechnical arrangements adopted within the design options: structure dimensioning, choice ofstructure and packages materials.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY46/228