Architecture and management of a geological repository - Andra

4 – Waste disposal PackagesThe YAG Laser (Yttrium Aluminium Garnet) process is a recent process using filler metal. Itsapplication to thick steel is still in the development stage 37 . On the other hand, this process hasindustrial references for welding thin steel up to 10 millimetres (Tokaï Muraï factory in Japan, MOXworkshop in Cadarache, Atalante installation at Marcoule).The friction welding process is a process without filler metal, currently being developed in Sweden bySKB for welding copper-based containers. At present, this process does not appear to be directlytransposable for welding thick steel.Electron beam (EB) welding is performed in a vacuum, without filler metal. This process has manyadvantages. At first, it is a process which has been industrially tested on thick metals, up to200 millimetres. Concerning the quality of the welds produced, the thermally affected zone (TAZ) ofmetal by the heat of welding is of limited extent.This process also displays metallurgical qualities ofthe TAZ close to those of the base material, which is favourable with respect to corrosion. Vacuumwelding particularly limits the risk of cold-cracking. Concerning operational implementation, thepossibility of welding with a single pass limits the time needed and the lack of filler metal isfavourable for application in an irradiation cell. Finally, the process can easily be automated.In conclusion, of all these processes, at this stage Andra prefers the electron beam (EB) process inview of the advantages listed above. This preference does not exclude reconsideration of this choice ata later date.• Number of primary packages per over-pack.It can technically be envisaged to place one or more primary packages in the over-pack, beforelowering the disposal package into the underground installations. However, the thermal couplingwhich would result from the installation of two primary packages side by side, would limit flexibilityin terms of managing heat emissions. Indeed, for disposal of C1 to C4 type packages (see chapter 3)after a storage period of 60 to 70 years, it seems that it would be necessary to redistribute the heatsources as evenly as possible by distancing the primary packages from each other (this point isdeveloped in chapter 5). Finally, increasing the number of primary packages per over-pack would leadto more complex operations during emplacement in the waste disposal cells 38 .In conclusion, Andra prefers the over-pack option at this stage, notably for the flexibility it providesin terms of managing heat emissions from the waste.4.2.2.3 The solution selectedThe solution selected for its simplicity and robustness in the light of current knowledge andtechniques, is that of an individual over-pack made of 55 mm thick non-alloy steel [50]. The over-packconsists of a body and a lid made of the same material. After inserting the primary packing into thebody, the lid is welded onto it using the electron beam method. The whole unit can then be transferredto the underground repository.The thickness selected for the body and lid results from the double considerations of durable watertightnessand mechanical stresses. Concerning corrosion, it includes a "consumable" part which is thethickness affected by generalised corrosion during the period under consideration.3738This development takes place within the terms of the CLFA (Franco-German Laser Cooperation); it particularly focusses on welding ofpipelines and the "vacuum vessel" in the ITER reactor project; it is also being considered by the CEA for producing long-term storagecontainers.An alternative over-pack would involve making the disposal cell lining durably water-tight (see chapter 5), the basic function of thislining being to provide mechanical support. This concept, called a "collective over-pack" would involve producing sections of lining ofthe necessary thickness and welding them (as well as the end parts) in situ, through the entire thickness. This option provides lessguarantee of the quality of the object produced than would the option of producing the over-pack in a surface workshop. Furthermore, inthe configuration in which disposal packages are separated from each other for thermal reasons, this solution would require a greatervolume of steel than individual over-packs.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM132/495