Architecture and management of a geological repository - Andra

Due to the high thermal conductivity of the metal components, the temperature very quickly reachesthe same level within the sleeve over the entire length of the cell. It increases almost simultaneouslywithin the rock: after a year in disposal, the temperature of the rock wall reaches almost 40°C for typeC0 packages and over 50°C 136 for type C1 to C4 packages. Thermal interaction between adjacent cellsis already marked by a 5°C increase at the mid-distance between type C waste cells. In spent fueldisposal cells, the heat transfer is slowed down by the swelling clay as it has a lower thermalconductivity than that of argilites. Thus the heating of the wall rock is slowed and reduced by the clayring that maintains a temperature difference of around 30°C between its internal and external facesduring the temperature rise.The maximum temperature on the surface of the packages, of between 90°C and 100°C, is due to thedimensioning of the architecture of each repository zone for a given package age (see § 5.2 and 5.3).This maximum temperature is reached after 10 to 20 years for all type C or spent fuel waste packages.At the same time, the argilites or swelling clay in contact with the sleeve reach a temperature ofbetween 80°C and 90°C.This temperature rise within the cells would not be significantly changed by sealing them immediatelyafter they have been filled. Given that the temperature rise is relatively rapid (from 10 to 20 years),maintaining the cell unsealed during this transitional evolutionary phase could be considered in orderto allow the fullest possible monitoring to be carried out without placing major constraints on thetransmission of measurement data (see 10.3.8.3). Furthermore, a sound knowledge of the temperatureof the packages and the cell is needed in order to check that the cell behaves as expected and, inparticular, in order to establish the actual conditions inside the cell in case it is decided to retrieve thepackages.Propagation of heat from the cells leads to heating in the access drifts within the module. Thetemperature of the wall rock remains below 60°C in the vicinity of the cells. Ventilation is maintainedconstantly in the drifts with reduced flow. In the event of intervention by personnel, this flow can beincreased in order to keep the air temperature at a value below regulation limits. The thermomechanicaleffects on the sleeve induced by this heating are taken into consideration whendimensioning and do not reduce the durability of the structures (see sections 5.2, 5.3 and [13]).One of the principal effects induced by the heating of the cell is the expansion of the packages, thesleeve, the swelling clay for spent fuel disposal cells and of the rock. Thanks to the functionalclearances between the packages on the one hand and between the packages and the sleeve on theother 137 , the thermal expansion of the packages, spacers and sleeve do not create notable thermomechanicalstresses within the packages 138 and the deformations induced are sufficiently small so asnot to block the packages or the spacers, at the same time as they reach their maximum amplitude. It istherefore possible to retrieve packages at any time, whatever the amplitude of the expansiondeformation, in the cell.The heating of the rock also accelerates the delayed deformation of the argilites (by a factor of 2 to 3).For type C waste cells, this acceleration encourages the closing of the functional clearance for fittingthe sleeve and accentuates the terrain pressure on the sleeve after closing of this functional clearance.However, the dimensioning of the sleeve makes it possible, as soon as the functional clearance hasclosed, to ensure the durability of the sleeve and the maintenance of the geometry within the sleeve forat least two or three hundred years, and probably longer.136 The temperatures and durations indicated in this section correspond to the reference dimensioning used for the studies (see chapters 5and 6)137 The low level of corrosion during the heating phase does not significantly change the width of these clearances.138 Constructive provisions having been taken in order to limit these stresses locally in the area in which the package touches the sleeve, forexample by placing friction pads between the packages and the sleeve.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM376/495