Architecture and management of a geological repository - Andra

4 – Waste disposal PackagesFinally, since the phenomena described above are linked to the presence of water in contact with thewaste, we should investigate the time at which water can actually reach the glass and dissolve theradionuclides. The presence of condensed water from the argillites in the cells, cannot be excluded inthe long term [8]. On contact with the primary container, this water would be likely to undergoradiolysis due to the high radioactivity of the waste, leading to a high potential for oxidation. Underthese conditions, a risk of penetration of the primary container by corrosion cannot be excluded on ascale of several tens of years. If water comes into contact with the glass at an early stage, this couldlead to initiation of radionuclide dissemination.In conclusion, the sensitivity of glass alterability to temperature, the relative uncertainty ofradionuclide behaviour above 80°C, and the risks of the stainless steel envelope being penetrated bycorrosion have led Andra to prefer the option of an over-pack to prevent water from reaching the glassat least during the thermal phase, i.e. the phase during which the temperature in the core of the glass ishigher than about 50°C.4.2.1.2 Other needs relative to over-packs linked to safety functionsWhen the over-pack has lost its sealing function, it should also be ensured that the packageenvironment helps to limit dissolution of the glass and the release of radionuclides.In practice, it must be verified that chemical interactions between the materials constituting the overpack,glass constituents 36 and surrounding clay materials can be understood.Furthermore, it must be verified that there is no criticality risk linked to traces of fissile materials leftin C waste packages by reprocessing (C0, C1, C2, C3) or to the presence of fissile materials (C4).4.2.1.3 Operational needs relative to over-packsManagement of C waste exothermicity, with the aim of making good use of the underground area, hasled to Andra preferring a repository architecture which uses horizontal tunnels, as indicated in chapter2. The over-pack must be designed to be compatible with emplacement operations in horizontaltunnels. Within the terms of reversible repository management, it should also be ensured that it can beremoved for at least a century.4.2.2 Selected design principles4.2.2.1 Options consideredSeveral key parameters govern the design of the over-pack: the type of materials, method ofassembling the over-pack components and the number of primary packages per over-pack.Concerning the choice of materials, a metallic envelope is used to guarantee water-tightness during along-term thermal phase (on the scale of a thousand years).Several families of metallic materials have been envisaged. A "noble" metal such as titanium or apassivable alloy (based on nickel, chromium or manganese) could be used in thin layers (20 to 30mm). Copper, for which behavioural data on corrosion under deep water conditions are available [49],may also be considered. However, copper could only be envisaged in association with anothermetallic material such as steel or cast iron to ensure the mechanical strength of the disposal package,as was the case of the container studied in Sweden by SKB [49] for spent fuel. Finally, thinck (around50 to 60 mm) non-alloy or low-alloy steel provides another solution. Non-alloy steel is a strong metalroutinely used, for which general corrosion processes are well understood.36This particularly concerns silica sorption by over-pack alteration products and their effects on the change from initial speed to residualspeed of glass dissolution (See § 4.2.1.1).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM130/495