Architecture and management of a geological repository - Andra

5 – Repository ModulesFurthermore, for cells containing bituminous waste (type B2 packages), the composition of theconcrete will be adapted to keep the pH in the cell at below 12.5.The phenomena involved in the interaction between the argilites and the concrete lining liable todegrade the retention properties of the argilites primarily concerns the fractured rock zone (extensionestimated at a few decimetres) [48]. The extension of chemical disturbances is therefore slight and hasno impact on the properties of the argilites in the undisturbed thickness of the layer above and belowthe repository.• The other materialsWe attempt to limit the quantity of steel left inside the cell when it is closed, in order to avoid themechanical problems related to corrosion and swelling of corrosion products, and to minimise thelong-term production of hydrogen by corrosion. The handling equipment and the access chamber inparticular comprise large quantities of steel and are recovered before closure of the cell.The concrete lining structural ring works in compression and thus contains no reinforcement. The useof steel in the ground support and lining is thus limited to the protection mesh laid at the face of theworksite, at the same time as the rockbolts and hangers for attaching the filler concrete to the structuralring.It should be noted that stainless steels may be chosen in order to delay and reduce the effects ofcorrosion. Alternatively, materials such as fibreglass - resin composites could conceivably be used inplace of certain steels.5.1.3.7 Criticality-safetyA criticality-safety analysis was conducted in two stages. A preliminary risk analysis for the array of Btype waste packages (i) demonstrated that criticality was not a determining factor for most disposalpackages and B cells and (ii) identified the envelope packages requiring a more detailed criticality riskstudy (B5 packages in particular). A more extensive analysis then confirmed that the criticality aspectwas not determining for cells containing this kind of package [55].5.1.3.8 Gas management and cell ventilationThe vast majority of B packages give off small quantities of hydrogen (a few litres or tens of litres peryear and per package) and gaseous radionuclide traces. Cell ventilation enables these gases to beevacuated during operation, without affecting the personnel, who always work upstream of cellventilation. It should be noted that the production of hydrogen is so low that it would be possible forthe ventilation to be shut down for several weeks with no risk.The studies conducted to date show that with the envisaged spacing between packages, an airflow ofabout 3 m 3 /s would be sufficient to ensure good ventilation of a cell during filling or when filled. Thefresh air supply is in the full section of the drifts from the air inlet shaft to the cell. At the top of thecell, the air passes through the chamber by orifices with baffles (to protect the personnel from theirradiation risk).The air returns through the end of the cell and then through reservations made in the concrete andfinally through a duct which joins the general air return system. The airflow is controlled by theperforated wall at the bottom of the cell and by adjustable valves in the ventilation ducts.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM181/495