Tome Architecture and management of a geological repository - Andra

7 - Spent fuel repository zoneFigure 7.4.2General dimensions of CU cells• The engineered barrierThe swelling clay engineered barrier is expected to have identical properties to those of the C wastecells. Its function is similarly to protect the copper envelope from chemical (site water and workinducedoxygen) and biological (bacterial contamination during the disposal process) attack.According to SKB studies, atmospheric oxygen is rapidly consumed after cell closure [55].Controlling the presence of sulphur in the vicinity of the canister could result in selecting bentonite(such as MX80) with only some traces of pyrite. Likewise bacterial corrosion could be controlled bydefining a criterion for the minimum engineered barrier density above which bacterial life would beunviable [17].The choice of swelling clay and its formulation (as % of sand) can only specified for a specific sitebecause French granite massif groundwaters vary so widely in chemical composition (cf. § 3.2.2.2)The specification could be based on the results of research and experiments carried out internationallyand also for the repository study in the clay medium.The engineered barrier is made up of prefabricated stacked bentonite elements. Its emplacementnecessitates clearances between the borehole wall and the outer surface of the engineered barrier rings.Clearance is also provided between the engineered barrier rings and the disposal package for packagelowering operations.• Disposal cell dimensionsTo maintain favourable long-term environmental conditions for the stability of the cell and the coppercanister an engineered barrier of 35 cm thick around the disposal packages is required.The resulting disposal cell dimensions vary from 1.35-1.85 m in diameter and 7.5-8.25 m deep,depending on the dimensions of the stored package. All these characteristics are summarise Table7.4.1.Dossier 2005 granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY210/228