Architecture and management of a geological repository - Andra

10.3.1.4 Supply of additional geological informationPrior reconnaissance of the rock (i.e. throughout the construction phase) represents the third objectiveof observation and surveillance. This supplements the detailed knowledge already acquired before thestart of the repository construction, that made it possible to establish the characteristics of the Callovo-Oxfordian formation on the site of the underground research laboratory and of the sector under study(see section 2.2).However, despite this acquired knowledge, any additional reconnaissance yields more detailedknowledge of the initial state and properties of the rock at the site of future construction. This initialstate forms a reference for future evolutions, to be monitored and modeled during operation of thestructures.Moreover, this prior reconnaissance allows confirmation of the absence of heterogeneity or anyirregularities. In fact, the dimensioning of the structures and the safety assessment are based on thehomogeneity of the host rock’s properties, i.e. on a relatively low and known level of variability of itsproperties. If the preliminary reconnaissance detects greater variations than those already known, theymust be taken into consideration in the models and, if necessary, make local adjustments to the designof the structures. If necessary, the distribution of monitoring resources mightness have to be adapted tosuch variations (see section 10.3.6).Prior reconnaissance at the site of the structures is thus an essential tool in correct implementation ofrepository monitoring.10.3.2 Parameters to be monitored during repository operationThe parameters to be monitored during the various stages of the disposal process are those whichmake it possible to track and quantify the various phenomena to which any later retrieval operationwould be subject and to maintain the installation in good condition (see section 10.2). Monitoring theevolution of these phenomena provides the operator with the data required to better manage therepository in a reversible context and contributes to operational and long-term safety.10.3.2.1 Monitoring of type B waste disposal cellsIt has been shown previously that the level of reversibility of type B waste disposal cells basicallydepends on the mechanical stability of the cell’s structure. The primary objective of monitoring thesestructures is therefore associated with the assessment of the parameters which indicate this stabilityand which enable its durability to be assessed. To this end, monitoring must concern the level of stressexerted on the liner by the geological structure and any deformation which could affect the liner.Furthermore, given that the level of stress exerted by the geological formation is associated with thelevel of saturation in the immediate environment, this latter parameter must also be monitored.The second objective is associated with the production of gas since any subsequent parcel retrievalrequires knowledge of the composition of the cell’s atmosphere. To this end, the monitoringprogramme must include an analysis of the gases contained in the disposal cell (particularlyhydrogen).Finally, the third objective of monitoring of type B cells is associated with the exothermicity, thoughslight, of certain type B wastes. In fact, knowledge of the temperature is important, not only in order tounderstand the evolution of the various phenomena but also to assess the conditions of any subsequentpackage retrieval operation. Changes in the temperature in the various cell components are thereforealso an element of the monitoring programme for type B waste cells.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM386/495