Architecture and management of a geological repository - Andra

6 – Overall underground architecture6.3.3.2 Water flows drained into the argillite after return to hydraulic equilibriumWhen resaturation is almost complete, the potential water movement in the argillite occurs againunidirectionally, from the surrounding formation with the strongest hydraulic head towards theformation with the lowest hydraulic head.Water flows that circulate in this way in the argillites are liable to be intercepted by the disposal cellsand by the access and connection drifts. On the scale of the repository, the total flow intercepted isproportional to the accumulated surface area (drifts and cells). It is evaluated 108 at several cubic metersper year.If the overall permeability of the repository is low, close to that of the argillites, the water flowintercepted by each structure (cell containing packages or drift) is restored to the argillite withoutcirculating through the drifts and shafts.The design of the repository aims to come close to this situation. In order to do this, the installation ofseals in the shafts and in the drifts provides considerable hydraulic resistance to the circulation of thewater. Thanks to these seals, the presence of the repository does not significantly modify the hydraulicregime of the geological formation. In the case of an upward gradient, only a small fraction of theflows intercepted by the repository is removed via the drifts and shafts; it is evaluated for a shaft sealpermeability of 10 -11 m/s at several hundred litres per year for the entire repository. This low flow raterenders the rate of the water in the structures negligible. The migration of radionuclides released bythe packages therefore takes place essentially by diffusion and not by advection within the water flow.So these radionuclides will migrate preferentially into the Callovo-Oxfordian layer rather than via thedrifts towards the shafts.In a situation in which the role of the drift and shaft seals is not taken into account (this wouldcorrespond to a situation of total or partial seal failure), the repository would no longer provideresistance (or would provide reduced resistance) to the circulation of the water.In the case of an upward hydraulic head gradient, such a situation would result in an increase in thewater flows that would be evacuated via the drifts and connection shafts rather than via the argillite.Thus, for an upwards gradient of the order of 0.2 and in the absence of seals, the flow evacuated viathe drifts would represent 2 to 3 m 3 /year.Although independently of any other considerations this flow rate remains low, Andra has adopted anarchitectural topology whereby its impact on the release and migration of radionuclides can bereduced: it is a matter of locally limiting the proportion of flow circulating in and near to each disposalcell.The topology adopted is a dead end type tree layout, the principle of which is illustrated by thefollowing figure. The dead ends on different levels of the tree structure are shown in red from theindividual cell to the repository zone. On all levels of the tree structure, access to each repositoryelement is via clusters of a small number of access ways.108 For a vertical hydraulic gradient of 0.2.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM268/495