Architecture and management of a geological repository - Andra

4 – Waste disposal PackagesConcerning the method of assembly, the need for water-tightness means that full-penetration weldingmust be used. From this point of view, various welding processes have been envisaged. They are splitinto two families: welding with filler metal (TIG "Tungsten Electrode with Inert Gas", MIG, MAG"Metal Electrode with Inert or Active Gas", YAG / Laser processes) and welding without filler metal(Electron Beam and Friction). It will be noted that at the current state of knowledge, sealing byinsertable joints does not appear to be compatible with the objective of higt durability.Concerning the number of primary packages per over-pack, one or two primary packages can beconsidered in view of considerations of mass and handling in the access shaft and underground drifts.4.2.2.2 Comparison of the options envisaged and justification of choice• Choice of material constituting the over-pack"Noble" metals such as copper or titanium are thermodynamically stable under the reducing chemicalconditions of the repository. Chromium, nickel or manganese based alloys are capable of passivationthrough the surface formation of a fine corroded layer which protects the rest of the metal. Forthousand-year durability, Andra has performed particularly in-depth studies of passivable alloys fromamong these materials. However, these high-performance materials appear to be sensitive toenvironmental conditions (hyperoxidating conditions generated by radiolysis of water, temperature,the presence of aggressive species, particularly chlorides), and could suffer corrosion by pitting, wichwould require a probabilistic approach to resistance against localised corrosion.Non-alloy steels have the disadvantage of being corrodable under repository conditions. However, theprocess involved in the corrosion of non-alloy steel is now well understood. Concerning aqueouscorrosion, a set of experimental results and models show that generalised corrosion is the dominantmechanism in the medium and long term. The speed of generalised corrosion, reflecting the thicknessof corroded metal over time, can be quantified on the basis of models which have been validatedexperimentally. Localised corrosion by pitting or increvices, can be seen in these materials,particularly in the presence of oxygen, but experimentally, the speed of localised corrosion observed inthe short term falls faster than that of generalised corrosion so that its relative importance decreasesover time. Finally, the risks of specific corrosion such as corrosion under stress or hydrogenembrittlement, remain secondary relative to other corrosion mechanisms. Archeological analogues iniron dating back more than 2000 years support the evaluation of corrosive processes and provide atemporal reference as to the durability of steels over a significant time scale [47].Moreover, models of non-alloy steel corrosion are tolerant to water chemistry, so that they require lessprecision in environmental chemical conditions [47]. They are also tolerant with respect tocomposition of the metal, its structural status and surface condition. This element considerably limitsthe risk that consequential faults will deteriorate the tightness and durability of the object, in particularalong the welds.Finally, non-alloy steels display very good weldability and provide excellent feedback about weldingtechniques, tested on thick samples.In conclusion, non-alloy steel was preferred at this stage, because of the controlled, predictable natureof its corrosion process and because it is easy to weld.• Method used for welding over-pack componentsProcesses using filler metal such as TIG (Tungsten Inert Gas), MIG (Metal Inert Gas) and MAG(Metal Active Gas) require several successive welding passes. Using these methods leads to longwelding times, making these processes unsuitable for welding very thick parts. They also engenderconsiderable extension of the heat-affected zone.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM131/495