- 103 - ASN considers that the process to search for nonconformities during normal operation, maintenance, conformity reviews and safety reassessments, during the complementary investigations (eventearthquake approach, specific seismic inspections, etc.), and on the occasion of the inspections performed following the Fukushima accident, is satisfactory. The nonconformities identified during the CSAs do not directly compromise the safety of the facilities concerned but they can, in particular if combined, constitute factors such as to weaken the facilities. ASN will therefore require that EDF reinforce the detection and processing of nonconformities. ASN will in particular propose that the regulatory requirements on this topic be strengthened by means of a draft order setting out general rules for basic nuclear installations, especially with regard to an assessment of the cumulative impact of the various nonconformities present in a facility. These requirements will be reinforced by means of ASN prescriptions. Deployment of mobile resources after a DBE The post-earthquake deployment procedures do not require the use of mobile resources up to the design-basis earthquake. The issue of replenishment of consumables (fuel, oil, etc.) is dealt with in the paragraph on site accessibility after an earthquake. 2.2 Evaluation of safety margins On the occasion of the complementary safety assessments, ASN asked EDF: based on the available information, to give an evaluation of the level of earthquake beyond which the loss of fundamental safety functions or fuel damage (in vessel or pool) was inevitable, to identify the weak points and cliff-edge effects, according to the scale of the earthquake to propose measures to prevent these cliff-edge effects and reinforce the robustness of the facility. On the basis of an analysis conducted in a very short period of time, EDF reviewed the seismic strength margins of the structures and equipment important for safety, in order to determine the level of acceleration for which, with a high level of confidence, the facility has a very low probability of failure. EDF supplemented its general study with studies of equipment for which there could be performance discontinuities, based on an analysis performed in a very short period of time, and proposed modifications or reinforcements as applicable. Finally, EDF carried out the seismic inspection of a sample of the equipment needed to operate the unit in the event of total loss of off-site and on-site power supplies, whether or not seismic-classified, for all the nuclear power plants in service. In its overall margin study, EDF identifies three margin sources: Margins between the MHPE and the SSE and between the SSE and the DBE. The response of the structure The design criteria for the structures and equipment. Seismic loading margin: EDF states that using a plant series spectrum for all the reactors of the same series as the design response spectrum, is a conservative approach in that this spectrum is broad-band and designed to cover the characteristics of all the sites. In the frequency ranges of the structures, it therefore considers acceleration levels higher than those which would be transferred to the structures in the case of the spectrum of a site SSE.
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