Architecture and management of a geological repository - Andra

5 – Repository ModulesSleeve thickness calculationBased on knowledge of the load applied on the sleeve, it is possible to calculate the thickness requiredto not exceed a given stress level in the steel. It amounts to 20 mm (not including corrosion allowance)for S235 grade steel [65], chosen for its compatibility with the over-pack but showing low thermalcharacteristics.This design includes safety margins, since the sleeve thickness required varies significantly with theradial load considered and the 12 MPa load taken into account is particularly high. Moreover, it mustbe noted that a lower thickness could be considered in the case of a more resistant steel grade.Design verification with respect to axial load thermal stresses and risk of bucklingWhen the temperature increases, the sleeve tends to expand radially and axially.Its thermal expansion (free displacement) is centimetric 79 . This increase in length will be theoreticallyabsorbed at the two ends of the sleeve, which are left free. If the terrain quickly closes in on the sleeve,the tube deformation could be negatively affected, leading to an axial stress inside the tube close to theelastic limit of S235 steel at 100°C. Given the capacity of argillites to accompany the sleevedeformation, the appearance of plastic deformations in the sleeve is not foreseen.The risk of buckling 80 has been analysed according to the above factors, in terms of both radial stress(risk of tube collapse) and axial stress. The buckling resistance of the sleeve has been verified for athickness of 20 mm (not including corrosion allowance) 81 .• Access drift concrete lining designThe access drift lining is designed to withstand the effects of the heat released by the waste (radialexpansion of concrete and argillite) and of argillite creep (itself accelerated by the thermal release).Argillite desaturation that rigidifies the argillite and reduces its creep rate has been taken into accountfor drifts ventilated in the operating phase.The concrete lining is placed in contact with the rock (no annular cavity) after a deconfinement phaseallowing a certain degree of stress relaxation. A period of 9 months to lay the lining has beenhypothetically considered. The radial compression stress due to the weight of the soil on the lining isin the order of 6 MPa 82 after one century 83 . This value integrates creep acceleration due to increasedtemperature and takes into account the moderative effect of desaturation.The thermomechanical stress in the concrete can be assimilated with a radial load of approximately2 MPa at peak temperature.The total equivalent radial load applied on the concrete lining is in the order of 8 MPa (not includingponderation coefficients), to be compared with the 12 MPa of isostatic stress theoretically applied afteran infinite time period.The calculation performed yields a lining thickness of approximately one metre 84 for B60 typeconcrete. The thermal component amounts to approximately 40% of this thickness.The acquisition of better knowledge of creep rates in unsaturated conditions could reduce the concretethicknesses.798081828384For the S235 steel considered and a lining length of approximately 40 m, a maximum temperature increase of 78°C induces a lininglength increase of 4 cm.Buckling is the rupture (and significant deformation) of a part under compressive stress. It occurs when a critical stress level is exceeded.The presence of the terrain around the lining limits lateral deformations, thereby significantly increasing the critical stress level. Thiseffect has been taken into account in the verification performed.Average value based on access drift orientation (anisotropic stress).In case of extended use of the drift, reinforcement work remains a possibility.The anisotropic stress regime and the shape of the drift lead to higher thicknesses in the cell (1.20 m) and raft (1.40 m)DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM212/495