Architecture and management of a geological repository - Andra

5 – Repository ModulesIn addition, the sealing zone undergoes specific processing to preserve the characteristics of theargillite (see section 5.2.6).The long-term evolution of the drifts is covered in section 7.6.5.2.3.6 Seismic behaviour of disposal cellThe seismic stresses to be considered below the surface have been evaluated [36]. They do not affectthe design of the structures or the long-term behaviour of the argillites. Seismic is very limited underthe surface as compared to on the surface (due to amplification of seismic movements on surface, lowdynamic amplification undergone by buried structures, and small dimensions of structures with respectto wavelength of seismic movement). An analysis of the various possible dynamic effects (seismicwave interaction with isolated cavity, diffraction for all repository cavities) applied to the varioustypes of disposal systems in a repository shows that such effects are negligible.5.2.3.7 Durability of disposal cellHere we consider the sleeve function with regard to maintaining favourable conditions for wastepackage removal during a pluri-secular period (reversibility). This durability essentially depends onthe corrosion taking place and the mechanical resistance of the sleeve. The corrosion has beenestimated. The sleeve design adopted takes into account a corrosion thickness and a mechanicalresistance thickness.The sleeve described is made of the same type of steel as the over-pack. The properties involved are ofthe same type (mechanical, physical and chemical) and the similarity of materials prevents the effectsof galvanic corrosion.The corrosion regimes and rates are identical to those described in chapter 4 for the over-pack (section4.2.3.2). They are divided into three successive periods: an initial period without corrosion, atransitory period of oxidising corrosion and a period of anoxic corrosion during which the corrosionrate becomes very low (ranging from a few tenths of micrometre to a few micrometres per year [53]).Risks of localised corrosion (pitting or crevice corrosion) or specific corrosion (stress corrosion,fragilisation by hydrogen) have been taken into account and their consequences evaluated. Based onthe results of the analysis conducted, generalised corrosion is considered the dominant corrosionmechanism in the medium and long term.Under these conditions, the corrosion allowance of 5 mm for the sleeve will only be consumed afterseveral centuries 87 . This durability forecast is consistent with observations performed on similararchaeological sites.It must also be noted that the sleeve connections can offer a certain degree of impermeability thatcontributes to durability, even though this impermeability has not been defined as a sleeve functionand has not been taken into account in the repository safety assessments.5.2.3.8 Compatibility with disposal package installation methodThe disposal cell and access drift design takes into account operating constraints, particularly thepackage installation method.In this respect, the disposal cell must first and foremost enable packages to be transferred via slidingrunners on the sleeve. As for the access drift, it must be sufficiently wide to allow the vehicle carryingthe transfer cask to manoeuvre and place itself in docking position.Section 9 provides a description of the various systems for transferring packages in access drifts,docking the transfer cask, and positioning packages in a disposal cell (pusher robot). A few importantdetails with regard to the disposal cell and access drift design are indicated below.87The design hypotheses adopted for the corrosion allowance of the lining are reasonable. However, they are less conservative than for theC waste package over-pack. This is explained by the absence of a long-term safety function associated with the lining.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM214/495