Architecture and management of a geological repository - Andra

4 – Waste disposal Packages4.1.4.2 Enhanced retention capacityFor durable mobilisation of the concrete retention capacity, a concrete must be defined which has verygood intrinsic properties in terms of density, and the state of cracking linked with conditions ofprefabrication and package closure must be controlled.In terms of material, the considered concrete mix airs at high performances (permeability less than2.10 -13 m/s, diffusion coefficient less than 2.10 -13 m²/s, and porosity of 10 % at most). A thickness of150 mm has been selected.Particular attention must be paid to the junction of each housing between the body of the container andthe concrete cast during the second phase. Design provisions have been studied to provide a goodbody/lid connection: limiting the diameter of the cast lids, reducing any shrinkage which couldgenerate microfissures, a circular shape avoiding particular zones, choice of profile for the interfacebased on feedback from existing packages (packages of waste showing low and medium activitydisposed at the Aube Centre). A programme of tests on this junction is being performed in 2005 tomake sure that there are no weak points.4.1.4.3 Mechanical designCompared with the standard package, removing all reinforcement reduces tensile strength, whichbecomes the most restrictive characteristic for design. It is therefore a good idea to limit tensile stressliable to create cracks or propagate them, and to reinforce the intrinsic characteristics of the fibrousconcrete by increasing the quantity of fibres (55 kg/m 3 to 90 kg/m 3 depending on the type of fibreschosen) and the compactness of the concrete. A minimum resistance of 5 MPa in traction/deflection isthe target.The principle of housings with separate cast lids provides an adequate structure which limits tensilestress in the concrete (limiting amplitude and the extent of the zone subject to tensile forces above thepackage housing).Mechanical design is based on a pressure load to be applied uniformly to the container. Waste disposalcontainers will be loaded gradually in the long term, depending on the evolution of the waste disposalcell sleeve and argillite creep. An isotropic pressure of 12 MPa has been set for package design. Thisvalue is considered to set the limit of uniform loading which may be applied in the long term 29 .Design calculation was conducted according to Eurocode 2 regulations [45], in particular as pertains toselecting penalising safety factors used to take into account geometric faults and operating tolerancesfor the containers, plus an extra safety ageing factor which incorporates deterioration of the concrete inthe long term[41].The application of these very severe conditions for compression and traction stresses means that a highperformance concrete (HPC) with a compression strength of 90 MPa is preferred, to ensure that thetraction-compression criterion is met at every point with an additional safety margin. The high valueof dimensioning stress used (12MPa) means increasing the thickness of inner partitions (220 mm) andouter walls (150 mm already specified to delay radionuclides) relative to the standard disposal package(inner partitions and side walls around 110 mm thick) to reinforce the mechanical strength of thestructure. This element favours the choice of separate lids which will reduce the space taken by lidsabove the housings.29For this load, the stresses engendered when the packages are emplaced, due to faulty alignment by 20 mm noted in the previous section,appear negligible relative to the 12 MPa exerted in the long term.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM120/495