- 104 - For each site, EDF proposes a table of margin factors between the reassessed site SSE and the DBE, between 1 and 6 or 10 Hz (because this is the frequency of interest for the structures). It considers it an unnecessary penalty to take account of the rest of the unfiltered seismic signal. EDF adopts margin values between 1 and 1.7 depending on the sites and the buildings considered. Margin on the structure response: EDF mentions an attenuation on the structures of the free-field signal measured, owing to their significant foundation depth, the interaction between the soil and the structures and inertial effects conservatively incorporated into the models. Margin on the design criteria of structures and equipment: EDF states that the design of the facilities and their construction are based on codified or standardized methods and that these codes or standards comprise considerable safety margins in that the design rules remain within the linear elastic domains, for a fraction of the elastic domain. In addition, EDF regularly conducts multipartite design and R&D actions to characterise the behaviour of structures in the post-elastic domains. As part of its ageing projects and the "operating lifetime" project, EDF carries out R&D work on the conformity criteria and the implicit margins usable. EDF has also carried out or taken part in destructive tests on components and structures comprising defects, in order to study the margins and phenomenology of collapse mechanisms. According to EDF, seismic operating experience feedback (for flexible lines and light cableways) or tests on a vibrating table (equipment or structure mock-ups) or on anchor pull-out, show considerable margins. EDF focused in particular on the behaviour of: large components, flat-bottomed tanks, pipes, supports, ventilation ducts, relay cabinets and I&C switchboards, cableways. The margin factors identified by EDF are higher than 2, except for the tanks and relay and I&C cabinets (where there is enough strength but not the functionality in the event of an earthquake greater than the design-basis earthquake). In any case, there is margin in relation to the design-basis earthquake. Performance of specific studies or inspections dedicated to evaluating the seismic robustness of the facilities: For the Tricastin site, as a means of assimilating the method, the licensee carried out an SMA (Seismic Margin Assessment). This method was developed by the American electricity utilities and their safety regulator and aims to study the robustness of the facility to an earthquake larger than the design-basis earthquake. This evaluation concerning the deterministic study of the strength of equipment, systems and structures necessary for shutdown of the unit to a safe state, considering as standard a small RCS break and a loss of offsite power. It is performed using hypotheses different from those of the safety case (earthquake larger than design-basis, conformity criteria based on "average" behaviour of the equipment).
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