Tome Architecture and management of a geological repository - Andra

4 - General architecture of the repository in a granite medium4.4.3.3 Criticality riskThe criticality risk corresponds to an uncontrolled nuclear chain reaction. This is initiated by increasedneutron activity on fissile materials (uranium-235, plutonium-239 and plutonium-241).B and C packages do not contain a sufficient quantity of fissile material (critical mass) for this type ofreaction to occur. Only spent fuel packages are concerned by this risk [34].In the case of spent fuel nuclear surface installations 21 , water ingress must be prevented in spent fuelreception and disposal package conditioning cells to rule out the criticality risk, in line with thepractices in similar existing waste storage installations.In underground installations, package transfer and emplacement must be undertaken in dry conditions.There is no criticality risk associated with this operation.During disposal package transfer via the shaft between the surface and underground installations, thehypothesis of a scenario combining serious damage to the spent fuel package (alteration of its internalgeometry, fuel assemblies breaking and coming closer together, etc.) and ingress of water could leadto a criticality risk.Given the planned risk reduction devices for the risk of falling in a shaft (cf. Figure 4.4.1), damage atsuch a level to a package seems unlikely. However to eliminate this risk altogether, an additionalprecaution would be to ensure that there is no water (or other hydrogenated fluid) in the shaft. Thatwould entail prohibiting the installation of pipes in the package lowering shaft while at the same timeinstalling a water extraction system at the bottom of the shaft.The other dangerous situations envisaged [34] do not appear to induce a criticality risk.4.4.4 ConclusionThe analysis presented in this chapter has highlighted the main risks associated with the disposalprocess for mankind and the environment, and proposed suitable prevention and protection measures.It is based on feedback from existing nuclear installations and lessons learnt from the clay dossierstudies. It is not intended to be exhaustive at this stage of the studies.The analysis has distinguished conventional risks, found in all industrial installations, from the risksassociated with the waste packages. The risks relating to the outer environment of the repository havenot been covered as there is no specific site.The conventional higher and lower level risks have been identified for all the repository activitiescarried out in the surface installations. They are essentially risks of crushing (handled loads dropping,being knocked over by a vehicle, etc.) risks of falling through aerial work, risks of electric shocks, fire,etc. These risks do not call for additional investigations at this stage. Nonetheless, they should bedelved into at later development stages of the studies.In the underground installations, the risk of granite blocks falling during the construction of structures,and to a much lesser degree, that of water ingress, are additional to those risks mentioned above.21The presence of water, that attenuates neutron energy and slows down their speed, makes them more reactive to fissile material andresults in increasing system reactivity. Therefore, the disposal package conditioning processes adopted are dry processes, with noaddition of water.Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY92/228