Architecture and management of a geological repository - Andra

11 – Operational SafetyVarious data sets were retained for the quantities of radionuclides liable to be freed and released intothe shaft’s atmosphere. The data related to the B5 packages were defined by analogy with theobservations made during the compacting operations (the energy has the same order of magnitude asduring a fall and during a compacting). The data related to the C packages are linked to fall testresults. Regarding the spent fuels, the parametric approach corresponds to assumptions on the extentof deterioration of the assembly, together with a more or less important breakage of the rods andcrushing of the contained fuel pellets.11.7.2.2 Data related to the simulation studiesThe existence of doors at the shaft landing stations (cf. Figure 11.2.3) and the prohibition of access tothe shaft zone during a package transfer operation are measures which should allow preventing anyradiological risk to the persons present in the underground structures. Consequently, no estimation wasmade for the personnel assigned to the underground installations.The radiological consequences of a possible radioactive reject into the environment at the outlet of aventilation shaft were estimated taking into account all the potential exposure paths :- The exposure related to the passing by of the reject plume induces an external exposure and aninternal exposure by inhalation,- The exposure related to the deposits left by the plume induces an external exposure, an internalexposure by inhalation subsequent to the return in suspension and an internal exposure by theingestion of foods.11.7.2.3 ResultsThe simulation studies show that the phenomenon of an instantaneous suspension at the moment of theimpact appears to be preponderant compared to the suspension phenomenon related to a sweeping ofair circulating in the shaft. The released radionuclides would be completely delivered to theenvironment with potential consequences on people at the site limit, which is assumed to be 500 mfrom the reject outlet.The preponderant ways of exposure would be for the three types of wastes the inhalation due to thepassing by of the radionuclide plume and, to a lesser extent, food ingestion.The preliminary results obtained from the simulations would lead to considering measures to reducethe risk, for example, by filtering the exhaust air of the underground installations. With this type ofarrangement 187 , which is commonly used in the existing nuclear surface installations, the totalexposure dose would be on the order of 1 mSv 188 for a person at the site limit.Another imaginable measure would consist of isolating the shaft zone (descent shaft of the packagesand the air exhaust shaft) and then making an assessment of the state of events before taking an actionadapted to the nature, scale and extension of the contaminated zone.11.7.3 Analysis of the risk of criticalityOn the basis of the disposal package concepts defined to date and for the movement of a singledisposal package in the shaft, the accidental fall situations for the B and C waste disposal packagesdon’t present a risk of criticality, regardless of the geometry of the disposal package after the fall. [55].For the spent fuel packages, the damaging of the assemblies (detachment of the rods, breakage of theclads, …) and the moving of them closer together after the cage fall could induce a phenomenon ofcriticality if, in addition, water ingress occurred within the assemblies [55].187 It may be interesting to subordinate the cage lowering movements to the start of the filtering unit to avoid continuous filtering with largeair volumes. This type of operation was retained in the American transuranium waste repository of the WIPP, New Mexico [114], wherethe filtering unit installed on the air exhaust shaft is not started except if an incident occurs.188 This value is to be compared with the fact that specific arrangements are regulatorily planned with respect to the public when accidentalsituations increase doses to more than 10 mSv.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM476/495