Architecture and management of a geological repository - Andra

11 – Operational SafetyHydrogen emitted by the waste disposal packages dilutes homogeneously in the free space below thedisposal packages 165 and it should be noted that ventilation remains in operation until the time when acell is sealed.Estimates were based on the pessimistic hypothesis of a constant outgassing rate over the duration ofthe operation and closure activity of the repository.11.3.2 Analysis during the operational phaseThis analysis is described in accordance with the logic of the cycle followed by the packages: storageof primary packages, storage of disposal packages, transfer of waste disposal packages to undergroundfacilities, emplacement of waste disposal packages in disposal cells.11.3.2.1 Surface storage (primary and disposal packages)A simulation, carried out based on the assumptions previously described, indicates that the hydrogencontent in the atmosphere in the storage area is negligible under normal ventilation conditions (withthe content varying from 10 -6 to 10 -7 in the various surface storage areas), and that the time taken toreach the explosive limit of 4.10 -2 (4 %) in the event of a ventilation failure is several decades. There istherefore no risk of an explosion in these areas.11.3.2.2 Transfer of waste disposal packagesHydrogen is also produced inside the transfer transfer cask during the transfer of the packages fromsurface installations to the disposal cell. The existence of the transfer cask door constructionclearances and, where applicable, the presence of a vent, will allow the hydrogen produced by thepackages to be diluted in the atmosphere of the spaces through which the transfer cask passes andthere will be no risk of an explosion, given the low hydrogen emission rate compared to the ventilationflow rates in the various installations.11.3.2.3 Disposal cellsThe simulation was carried out with a ventilation failure in a 250 m long disposal cell filled with typeB5.1 packages. Hydrogen was then concentrated in the top 15 cm of the disposal cell, above the wastedisposal packages.Under these conditions, the time taken to reach the 4% explosive limit is around 30 days. There istherefore no risk of explosion, even assuming a temporary ventilation failure.11.3.3 Analysis during the closing phaseAs far as the risk of an explosion is concerned, this phase can be sub-divided as follows.11.3.3.1 Disposal cell sealing phaseThe sealing process would begin by fitting a radiological protection shield consisting of concreteblocks fitted with pipes for maintaining the ventilation in the cell (see 5.1.6). This stage is followed bythe construction of a concrete retaining plug which fills the top of the cell and thus isolates the wastedisposal packages from the access drift. Under these conditions, the release of hydrogen into theaccess drift would be very slight and could not be the source of an explosion, all the more so since thisdrift will be ventilated throughout the fitting of the seal swelling clay core.11.3.3.2 Post disposal cell-sealing phaseThe backfilling work may be carried out in two stages: backfilling of the type B waste repository zoneinfrastructure then, at a later date, backfilling of the remainder of the infrastructure before completeclosing of the repository.165 Hydrogen, being lighter than air, will tend to migrate above the packages towards the top of the cell.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM455/495