Architecture and management of a geological repository - Andra

5 – Repository ModulesThe lowest dry density attained during the evolution of the materials is approximately 1.4. Theexperimental results available confirm that MX80 clay has a permeability of less than 10 -12 m/s forsuch a density, even when mixed with 30% sand. This preliminary design of the swelling clay plugtherefore satisfies those requirements.• Concrete retaining plugThe function of the concrete retaining plug is to oppose the deformation of the swelling clay core.During the development of swelling pressure (core resaturation), the concrete plug offers the core asimple geometry favouring homogeneous evolution (deformation and permeability). During thistransitory phase, the concrete plug also slows down possible water ingress susceptible of creatinggutters in the swelling clay prior to swelling. During this phase, it rests on the terrain (friction contact)and on the access drift lining 91In the long term, the material could be subject to chemical degradation, which will decrease itsmechanical properties. The retaining plug will nevertheless remain a link between the swelling clayplug and the access drift backfill, still contributing to the mechanical confinement of the swelling clayand to the preservation of low hydraulic conductivities.5.2.6.3 Constructive provisions for closing a repository moduleClosing a type C waste repository module involves five stages: (i) fitting of the metal radiologicalprotection plug in each cell, (ii) removing of the temporary sleeve, (iii) fitting of the swelling clayplug, (iv) fitting of the retaining plug and (v) back-filling of the access drifts. These various stages aredescribed below.• Fitting of the metal radiological protection plugThe metal plug is fitted using the same equipment as that used to place the disposal package in the cell(see Chapter 9). This operation may be carried out after putting the last package in place.• Removing the temporary sleeveWhen the decision to close the cell has been taken, the removal of the temporary sleeve makes itpossible to obtain direct contact between the swelling clay plug and the rock 92 . A certain number ofprecautions have to be taken in order to avoid damaging the argilite in the sealing zone.In particular, in principle it is preferable not to leave the ground physically unsupported during thisoperation: it is therefore envisaged that the swelling clay core will be placed in position as the sleeve isbeing withdrawn, or very soon afterwards. Additionally, in order to give immediate support, theswelling clay must be placed in contact with the ground: the in-situ compacting of clay pellets or amixture of pellets and powder is therefore now preferred to the fitting of prefabricated blocks whichrequire clearances.As far as radiological protection is concerned, the metal plug provides it in the cell axis and as foroblique radiation, it is stopped by the argilite in the future sealing zone.9192The preliminary design of the concrete plug neglects friction on the terrain, with the entire load transferred by the drift lining. Thispredesign is based on a swelling clay pressure of approximately 4 MPa. In order to integrate the friction in the argillite within the scopeof more complete design calculations for higher loads, the length of the concrete plug has been temporarily fixed at 1 access drift radius.It is recalled that the swelling clay plug must be positioned outside the damaged zone, i.e., at a minimum distance corresponding to halfthe access drift radius.The temporary sleeve may be removed a long time after the metal plug is fitted. If that is the case, a sealing cap is fitted to preventoxygen renewal inside the cell.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM227/495