Architecture and management of a geological repository - Andra

4 – Waste disposal PackagesWhen the sleeve loses its strength, it deforms and the thrust of the terrain is then transmitted to thedisposal packages. After 1 000 years, and conservatively not taking into account the partial dissipationof terrain thrust, the total radial pressure applied to the packages is more than 12 MPa (weight of theground at 500 metres).In an axial direction, the natural stress, which is stronger than the radial stress, accentuates the effectof deferred deformation at the disposal cell end. However, this effect is localised and covered by themargins allowed for the determination of radial pressure.Andra has therefore decided to dimension the over-pack to withstand a pressure of 12 MPa applieduniformly to the walls after a loss of 27 millimetres of metal corresponding to generalised corrosion(see below).The allowable material stresses (P235 steel) are 360 MPa for the tensile limit and 200 and 185 MPa ofelastic limit for temperatures of 20°C and 55°C respectively. Thicknesses of 28 millimetres in the walland about 50 millimetres for the base will respect these acceptable stresses. In fact, dimensioncalculations show that the tensile stresses exerted on the shell are lower (around 100 MPa [50]).• Duration of watertightnessGeneralised corrosion is the main process to be taken into account in the corrosion of non-alloy steels[53].In this context, three successive periods can be distinguished during the life of a package in a disposalsituation: an initial period with no corrosion, a transient period of corrosion under oxidizing conditionsand, finally, a period of corrosion under anoxic conditions.Initial periodInitially, the relative humidity in the cell is low because of the exothermicity of the packages, so that afilm of water does not form on the packages and corrosion is not triggered. However, the temperatureis sufficiently low (less than 100°C) for corrosion by oxygen gas 42 to be ignored.Transient period (oxidation and/or resaturation)As soon as the relative humidity of the atmosphere in the cell exceeds a critical value 43 , a film of wateris formed in contact with the package and corrosion may begin. The predominant oxidating speciesduring this phase is oxygen dissolved in water.Oxygen renewal is limited by constructive provisions (fast installation of a leaktight cover at the headof the cell). It is also preferentially consumed by the corrosion of metallic elements encountered beforethe package, in particular the sleeve. Therefore, this phase will be of short duration, if it occurs at all(around ten years) [53]. During this phase, the speed of progress of the corrosion of disposal packagesdepends on the kinetics of the phenomena 44 and takes into account the protective role of the layer ofcorrosion products. On the basis of corrosion tests performed in a clay environment, a semi-empiricallaw gives generalised corrosion speeds of several tens of µm/year at most.Anoxic periodOnce the oxygen is consumed, the environment becomes anoxic once more, as it was at first, andaqueous corrosion (by water reduction) in an anoxic medium becomes predominant.The thickness of the over-pack limits radiolytic corrosion due to oxidating species, such as H 2 O 2("oxygenated water") which would be created by water radiolysis under the effect of radiation.424344Several hundreths of microns per year at mostAqueous corrosion cannot start as long as the atmospheric humidity in the cell remains below a "critical relative humidity" between 40%and 70%, depending on the composition of the atmosphere.Redox reaction and oxygen transport linked to resaturation.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM137/495