- 256 - It is noteworthy, as appears clearly in the following summary table of mechanical equipment robustness, that the margin on the backup diesel generator sets is low. Mechanical equipment Minimum Earthquake level without cliff-edge effect (magnitude at distance of 7 km) Storage module STK 7.00 Pellet interim storage areas PSK PSR and PST 7.00 Ventilation system 6.20 Pellet interim storage areas PSA and PSB 6.20 STE storage modules 6.40 STE cooling units 6.6 on ground and 5.8 on flat roof STE recyclers 6.60 Backup ventilation cabinet 6.40 STE chilled water system 6.60 Extraction Storage ventilation duct 6.40 Backup diesel generator sets 5.50 The design-basis earthquake (DBE) is an earthquake of intensity 5.30 at a distance of 7 km. The backup diesel generator sets risk being unusable in the event of an earthquake of intensity of 5.50 or more (at a distance of 7 km). Loss of the backed-up cooling systems (fans, cooling units and recyclers) engages the risks of criticality and loss of confinement. The temperatures that would be reached in the fissile material storage areas, particularly of STE fuel rods, could very rapidly jeopardise the storage area geometry, within time lapses that seem poorly compatible with the intervention times. Measures envisaged to reinforce robustness of the facilities to the seismic risk The review showed that it was necessary for the licensee to study the site effects (lithological and/or geometrical) and evaluate any margin increases to be made to the chosen spectra. The licensee was also recommended to verify the seismic behaviour of the galleries, trenches and loft of building 500 and of the chimney stacks of buildings 500 and 501. The licensee has undertaken to carry out this verification for the gallery of the extension, the loft and the stack of building 500 in order to reinforce the civil engineering robustness. ASN considers for the Mélox facility that it is important to carry out studies to verify the resistance of certain equipment items to earthquake, combined earthquake and fire, or other accident situations beyond the frame of reference, so that the necessary reinforcements can be planned where necessary. It considers that the risk of a cliff-edge effect associated with the loss of the last filtration level (DNF) (further to an earthquake aggravated by a fire in the "powders" unit) must be taken into account in particular. For the seismic detection system and the identified associated controls, ASN considers that provisions must be made to counter the failure of all or part of this system. ASN will issue requests on these various points to the licensee, some of which will be made requirements.
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