Architecture and management of a geological repository - Andra

5 – Repository ModulesSensitivity to uncertainties in the input data.Apart from the design elements aimed at helping dissipate heat (package and steel sleeve), thefollowing main parameters influence the temperature peak:- the thermal properties of the geological medium (Callovo-Oxfordian argilites),- the presence or absence of a functional clearance between the sleeve and the geological medium 78 .Uncertainties regarding these parameters were approximated through various sensitivity calculations.These show an increase or reduction in the temperature peak of about +/- 10°C. Adopting 90°Cdimensionning ensures that the rock temperature does not exceed 100°C taking the uncertainties intoaccount.The consequences of these deviations can be assessed in terms of the impact on the packagepreliminary storage period. The resulting storage period variation for "earliest possible" disposal isabout +/- 10 years.• Conclusion concerning thermal designThe dimensions of the C waste repository modules are primarily the result of the thermal design. Thisdetermines the number of packages per cell and the inter-axial distance between cells. However, thereis room for optimisation if the feasibility of longer cells can be demonstrated, both in terms of theirconstruction and from the package handling viewpoint. This avenue of optimisation has yet to beexplored at this stage in the studies.5.2.3.4 Mechanical and thermomechanical design of the access drift - disposal cell systemThe mechanical and thermomechanical design is intended to ensure the integrity of the access drift –disposal cell system structure and functional clearances during a minimum secular period. It concernsthe thickness of the disposal cell sleeve and the characteristics of the access drift concrete lining.• Disposal cell sleeve calculationsThe following phenomena may affect the sleeve's integrity in the long term:- Argillite creep filling the annular cavity on the sleeve extrados and producing radial compressionstress around the sleeve. The radial compression stress value at a given data depends on theargillite creep rate and the initial dimensions of the annular space. The creep rate is itselfdependent on the argillite temperature.- Terrain expansion (due to the exothermicity of the packages), producing a supplementary radialcompression stress in the absence of clearance. This stress increases during the first decades andthen decreases once the temperature peak has been passed.- Expansion differential between steel and argillite (due to the exothermicity of the waste) which,combined with the lateral friction stresses due to argillite creep, induces an axial compressionstress in the sleeve. This phenomenon undergoes a time evolution similar to terrain expansion.Sleeve steel corrosion (its thickness includes a 5 mm corrosion allowance).Quantification of radial load stressesGiven the uncertainties concerning the initial dimensions of the annular space and the kinetics of thevarious phenomena involved, it has been decided to dimension the sleeve thickness for a 12 MPa load,corresponding to the initial geostatic stress of the geological medium.78The thickness of the functional clearances is not as important as the number of clearances located between the package and thegeological medium. The difference in temperature caused by the air gap between two cylinders is hardly affected by the thickness ofthis gap between one and several cmsDOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM211/495