Architecture and management of a geological repository - Andra

5 – Repository Modules• Limiting aqueous alteration of the glassAfter the over-pack deteriorates, environmental conditions must be such as to ensure a slow rate ofglass dissolution. The pH must therefore preferably remain between 7 and 9 (which determines thechoice of materials in the cell). The glass must be kept in chemical equilibrium with the water incontact with it and dissolution of vitrified silica is minimal if the water in contact with the glass issilica saturated [9].When the water reaches the glass it is not silica saturated and the glass therefore dissolves at an initial²rate, V 0 . This leads to silica saturation of the water in contact with the glass. The dissolution rate thengradually drops, V(t), settling at a residual rate, Vr. This rate, determined experimentally, is severalorders of magnitude lower than V 0 .These conditions require that the design ensures no flow and convective transport in the cell (this pointis examined below) and ensures that in the immediate vicinity of the packages, there is a medium inwhich transport is itself limited. In this respect, the argilite must be protected from damage as much aspossible.The need to control damage applies as of the cell construction phase and continues into the laterphases. Convergent deformation of the argilite may occur, owing to creep, in combination withresorption of the cell's internal spaces: minimising the internal spaces is a way of limiting thesedeformations. It should be noted that the expansion of corrosion products can lead to deformation inthe opposite direction [13] [53].5.2.1.3 Controlling water circulation and the transport of radionuclidesWater circulation is a factor in deterioration of the materials, in particular the packages. It thenencourages migration of the species released.Flows through the cell occur differently, depending on whether the rest of the repository is resaturatingor whether resaturation is completed. These two periods therefore have to be examined.Steel corrosion in anoxic conditions produces hydrogen. This process influences the resaturationphase. The fate of the gases in the cell and their evacuation outside are also dealt with.• Limiting water circulation in the cells and modules; general aspectsThe modules must help limit water circulation in the cells. An arrangement in which the cells are in adead end is in this respect beneficial. In a configuration such as this, no difference in the hydraulichead occurs in the cell. Consequently, the only phenomenon affecting the disposal cells is the result ofthese cells intercepting the vertical flows passing through the Callovo-Oxfordian formation (as a resultof the head differences between the Dogger and the Oxfordian). This natural flow, designated by theterm "leakage flow" is also limited by the low permeability of the formation.After resaturation, shaft sealing completes the arrangement by limiting the part of the intercepted flowthat then travels towards the access drifts. The leakage flow thus remains at its natural level. Thislimits the possibility of transport of released substances in the drifts.This limitation must be maintained as far as is possible in the various potentially altered situations(failure of shaft sealing, intrusion, etc.). Hydraulic closure of the modules and cells is thereforerequired: this consists in seals with hydraulic conductivity that is as low as possible.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM191/495