Architecture and management of a geological repository - Andra

2 – General Description2.4.2.3 Waste disposal packages and cells for spent fuelThe disposal packages designed for spent fuel comprise a cylindrical envelope of non-alloy steel. Thethickness of this envelope must guarantee leaktightness during the thermal phase. For fuel types CU1(UOX) and CU2 (MOX) discharged from electricity-generating pressurised water reactors (PWR), aminimum leaktightness period of the order of 10 000 years has been adopted. The thickness has beenconservatively determined (approximately 110 mm for container CU1).The number of assemblies in each package depends on the type of spent fuel and, in particular, theheat released – see section 4.3. For PWR fuels, the packages studied contain one (in the case of MOX– package type CU2) or four assemblies (in the case of UOX – package type CU1). Moreover, thedisposal package design ensures that the risk of criticality is controlled over the various time scales.Figure 2.4.8 (inset) illustrates the largest disposal package studied (43 tonnes, outer diameterapproximately 1.3 metres, length 5.4 metres).Like C waste disposal cells, the spent fuel cell design is dependent on thermal design criteria. Thesecells are horizontal tunnels, some 45 metres long, in which the disposal packages are emplaced – seesection 5.3. In the case of PWR fuels releasing considerable heat (1000 to 1500 watts per packageafter pre-disposal storage of 60 to 90 years), the disposal packages are spaced apart with spacers andeach tunnel contains three of four packages.For the design of spent PWR fuel disposal cells, the insertion of a swelling clay buffer (engineeredbarrier) between the packages and the geological formation has been adopted. The aim is to safeguardagainst uncertainties in the thermomechanical behaviour of the cell, caused by a relatively slowdecrease in the heat released from the spent fuel. Where water resaturates the cell, this option enables acontinuous, low-permeability medium to be formed around the packages by exploiting the capabilityof certain clays to swell considerably in the presence of water, and accept a high deformation rate.This barrier will limit, locally, the transport of dissolved species and thereby favour the control thephysical and chemical environment of the fuel. It makes it possible to manage the uncertaintiesexisting at this stage regarding the thermo-hydro-mechanical evolution of the argillites located inproximity during the thermal phase. The excavated diameter of the disposal cell corresponding to thisconfiguration is around 3 metres. The engineered swelling clay barrier is provided with an axialinternal sleeve to enable the introduction and possible future withdrawal of the packages.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM63/495