Tome Architecture and management of a geological repository - Andra

6 – C waste repository zoneA value of 10 -10 m/s is the common reference value for all backfill emplaced in the drifts regardless ofwaste type in the modules, (B waste: see section 5.2; C waste and if relevant spent fuel: see chapter 7).In Sweden, dedicated laboratory experiments on backfill emplacement and in situ performanceassessments have been conducted [18]. These experiments have demonstrated the technical feasibilityof using low permeability backfill partly made up of swelling clay in the drifts (cf. § 4.2.2.6).They alsoenhance our understanding of how these performances are acquired as the water returns to thebackfilled drifts and particularly when it comes into contact with the backfill and the drift roof.In situ laboratory experiments have been carried out on very low permeability seal emplacement indrifts in Canada [16]. This programme of experiments has demonstrated the possibility of constructingseals from very low permeability swelling clay (equal to or less than 10 -11 m/s).The precise positioning of the seals will be governed by the geometry of the "blocks" and faultssurrounding them.6.2.2.2 A watertight over-pack during the thermal phaseThe main function of the over-pack is to protect the glass package from water in the cells during thethermal phase. Glass dissolution models demonstrate the importance of temperature on the kinetics ofalteration [47]. Between 50 and 90°C the dissolution rates increase by a factor of 15-30 (depending onthe model and reference package in question).Furthermore, because of our limited knowledge of the thermodynamic data that govern chemicalequilibriums as indicated in chapter 5, control of radionuclide behaviour in the water following releaseby the packages depends on temperature: in practice this behaviour can only be described with amargin of manageable uncertainties for temperatures below 80°C [47].Consequently, the design principle for the over-pack relies on it being watertight for at least the wholeperiod when the temperature is in over about 50°C (cf. § 6.3.2), that is roughly one millennium.The choice of a metallic material guarantees confinement at high temperature and limits the chemicalimpact on the glass (as opposed to concrete whose behaviour is hard to control at temperatures over70-80°C - cf. chapter 5- - whose alkaline pH may alter glass matrix confinement performances in thelong term).When a corrosion "consumable" thickness is incorporated into the over-pack design, steel is a suitablematerial for the desired period of water-tightness. One millennium does not call for the use ofmaterials with greater corrosion resistance (i.e. passive or thermo-dynamically stable such as copper[37]). Additionally, steel provides the disposal package with mechanical strength (cf. 6.3.1).6.2.2.3 A swelling clay engineered barrierThe emplacement of a swelling engineered barrier in the disposal cells is adopted in all the C wasterepository options for a granite medium studied abroad (Figure 6.2.1 and [45]). This is because it canback up the role of the other components in the following ways:- swelling provides a close interface between the granite rock and the engineered barrier, that tendsto sharply reduce the water-conducting properties of the damaged zone created by cell excavation[47];- the swelling clay engineered barrier prevents water circulating in the cells regardless of whether itcomes from the granite or the access drifts; thus transfers of species in solution will be restricted todiffusion;- the swelling clay engineered barrier limits mechanical deformations in the cell that would derivefrom the long-term alteration of the metal components, primarily the over-packs- the swelling clay engineered barrier contributes to maintaining a favourable chemical environmentfor the glass, primarily by buffering chemical interactions firstly between the various cellcomponents and secondly with the granite.Dossier 2005 granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY168/228