Architecture and management of a geological repository - Andra

The monitoring of infrastructures requires us to select certain sections of drifts or shafts to beinstrumented. These instrumented zones are spread throughout all the infrastructures. The distancebetween these zones is not specified in theory. The density of the instrumented zones in relation withoperational safety, and in particular for the monitoring of the mechanical strength of the structures,will be based on the experience feedback built up in civil engineering and especially in theunderground laboratory. In order to limit the number of instrumented zones, we will attempt tocombine the measurements designed to observe the repository with those specifically for operationalmonitoring.For the closure structures (backfill or seals), monitoring is designed to achieve objectives relating tolong-term safety. However, the complete resaturation of these structures necessitates excessively longperiods to permit comprehensive monitoring as part of a programme of observation and surveillance.The monitoring of these structures is thus limited to their short-term evolution after installation(settling and fluid exchange in the walls). The first seals of B cells and drifts and also sections of thefirst backfilled drifts will undergo thorough monitoring. The monitoring of the following structureswill then be adapted according to the results obtained on the first structures.The basis for the distribution of the control cells and the instrumented drift sections at the repositorylevel, is illustrated in Figure 10.3.9. This distribution shows that a moderate number of structuresobserved allow monitoring at repository level. However, this illustration does not represent a frozenview of the monitoring programme in as far as the experience feedback from the first monitoredstructures will make it possible to adjust the choice and the distribution of the measurementequipment.10.3.6.2 Anticipation of changes triggered by management stages.The installation of monitoring equipment takes place at the construction phase of the structureconcerned. This equipment must allow monitoring of the phenomena which will occur during thedifferent repository stages, and in particular those which will occur after the packages are put in place,then again after the cells are sealed. The design of the monitoring system takes account of an expectedsensor failure rate, according to the monitoring period envisaged. If the structure becomes inaccessible(for example after a cell is filled or after a drift is backfilled), these potential losses can becompensated by adapting sensor redundancy. Finally, when the use of transmission by cables or fiberoptic becomes unfavourable (especially to avoid passing through a seal), it is possible to use a wirelesstransmission system.10.3.7 Observation of B waste repository modulesThe observation of B waste disposal cells is essentially focused on the phenomena which characterisethese structures during the different stages of the repository process, i.e.: the evolution of stressesexerted by the geological formation on the lining, the evolution of the level of saturation of thestructure's near environment and the production of gas by certain waste material. Moreover, for cellscontaining exothermic waste, the evolution of the temperature is also an important phenomenon.10.3.7.1 The system of observation of a control moduleThe system enabling observation of a "B control cell" is made up of 6 instrumented sections providingmeasurements at certain specific points and of fiber optic systems providing measurements distributedaxially along the whole length of the cell. This system is complemented by an instrumented bore-holeat the bottom of the cell and by the monitoring of the ventilation air (Figure 10.3.10). The entiresystem, conceived to provide physical measurements, is fitted during the construction of the structure.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM402/495