Architecture and management of a geological repository - Andra

6 – Overall underground architecture6.3.3 Hydraulic behaviour of the repositoryThe limitation of water flows in the repository depends above all on the characteristics of the site (lowpermeability of the argillite, gentle gradients). As indicated above, the architecture studied contributesto this in a complementary way through the general topology and the seals; it can, therefore, reinforcethe robustness of the hydraulic functioning of the repository.This section presents the simple principles of the hydraulic functioning of the repository and illustratesthe contribution of the architecture to the robustness of this functioning.After hydraulic balance is re-established, the following should be distinguished: (i) water flowsdrained by the repository into the argillite (ii) those that could be trapped by the shafts passing throughoverlying formations.Darcy's law, which constitutes an envelope model for argillite formation, has been used here todescribe these flows 106 .6.3.3.1 The hydraulic situation upon closure of the repository and resaturationIn its natural state, the Callovo-Oxfordian layer is saturated with water. The water can move therevertically under the effect of the head difference between the over- and underlying geologicalformations (Oxfordian and Dogger). Its rate of displacement is very slow on account of the very lowpermeability of the formation. For a medium hydraulic head gradient in the Callovo-Oxfordian layerof the order of 0.2 meter per meter, natural vertical displacement of the water takes place 107 at a rate ofa few centimeters per hundred thousand years. The corresponding flows are some hundred litres persquare kilometer of horizontal footprint.During the repository operation and observation phase, the excavation of the structures and theirventilation bring about a drop in hydraulic head that is accompanied by a desaturation of the argillitearound the structures. When the repository is closed, the drifts are progressively backfilled, seals areconstructed all the way through and ventilation is stopped.A repository resaturation phase follows, controlled by the low permeability of the argillite formation:this is what determines the flow of water entering the repository. The influence of gas produced by theanoxic corrosion of the metals present in the repository structures after closure which tends to slowresaturation should also be noted. This reaction between the iron and the water may, in fact, beginbefore the structures are completely resaturated from the moment there is no more gaseous oxygen.The differences between structures in terms of the volume of the empty spaces to be filled with water(cells filled with packages, backfill of drifts, etc.), the different kinetics of gas production anddifferences in the hydrological properties of porous materials (bentonite, concrete, backfill) result indifferences in the resaturation time of the various structures by the argillites [8] [62]. The sealsplanned in the design slow down the hydraulic circulations in the repository (cell plugs and connectiondrift seals); others hydraulically isolate overlying formations (shaft and drift seals in the shaft zone). Inthis way, these seals limit the water flows inside the underground installations. Having taken accountof these factors, resaturation periods can vary from a few decades to several tens of thousands of yearsdepending on the structures.106 This model tends to overestimate the flows for very low permeabilities.107 This drainage rate is lower than the rate of diffusion of the solutes in the waterDOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM267/495