Architecture and management of a geological repository - Andra

5 – Repository ModulesThe permeability that can be obtained for the plug is lower than or at most equal to that of themicrofissured argillite. A low transmissivity is obtained for the interface due to plug materialdeformation and the swelling pressure developed on the excavation wall. These factors have beenconfirmed in real scale by the TSX test [67]. This test has shown an equivalent permeability of10 11 m/s with a material consisting of Kunigel swelling clay and sand (20%). Andra is consideringMX80 type clay with lower permeability (see section 7.6). In the argillite, the swelling pressureexerted by the plug after resaturation is susceptible of closing the possible fractures.The measures previously indicated allow for obtaining an equivalent permeability lower than that ofthe microfissured argillite. In the current state of knowledge, this corresponds to approximately 10 -11 m/s. An evolution towards a lower value (close to 10 -12 m/s) is probable with the creep phenomenon,bringing the equivalent permeability of the disposal cell seal to a value approaching 10 -12 m/s.5.2.6.2 Description of closed disposal cellThe description that follows covers the following three components (in order of installation): metalplug, clay plug and support structure. It specifies their design with respect to the functions to beensured.Figure 5.2.23Design principle for C waste disposal cell plugs• Metal plugThe metal plug is intended to ensure radiological protection during disposal cell sealing operations,thereby eliminating the need to use the protection transfer cask during the installation of the swellingclay (hence possible with simple and reliable non-nuclear methods).Predesign calculations have been performed to limit the dose rate in the access drift to less than3 μSv/h. The metal plug (see Figure 5.2.24) can consist of a steel cylinder approximately 50 cm long(two thirds devoted to radiological protection and one third adapted for gripping by handling tools),possibly covered with ceramic to prevent adhesion to the sleeve (under the effect of corrosion).DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM223/495