- 106 - Seals between buildings: some seals between buildings are filled with materials such as expanded polystyrene, which no longer corresponds to current practice in paraseismic engineering. A large part of these materials is removed during the ten-yearly outages. If it is to be retained, an assessment of the impact of interaction between buildings at 1.5 SSE will be performed. Venting-filtration system for the containment in the event of a severe accident: this equipment is currently not covered by any seismic resistance requirement. EDF is initiating a complementary analysis to assess the seismic resistance of this equipment. EDF will be studying additional measures necessary for unit safe shutdown in the event of a loss of offsite power caused by an earthquake larger than the design-basis (which requires a study of the adequacy of the steam generator backup system water inventory and the speed of connection to the residual heat removal system). EDF envisages speeding up the conformity work on the RRI (CCWS) section that is not seismicqualified. EDF conclusions concerning the seismic margins Based on all the margins studied (seismic loading, structural response, design criteria for structures and equipment) and the seismic inspections it carried out, EDF concludes that the seismic capacity of the containment and of the structures and equipment which, in the event of failure, would compromise the safety functions, is 1.5 times greater than the spectrum corresponding to the SSE. EDF considers that this level easily exceeds the seismic context of the sites, up to hazard values that are implausible for these sites. ASN position statement: The licensee's assessment did not identify the level of earthquake leading to the gradual loss of the various basic safety functions on the basis of a hazard increasing progressively beyond the DBE. EDF studied the consequences of an earthquake with a value of 1.5 times the SSE, which it does not consider to be plausible and which enabled it, within the allotted time, to use seismic verification methods according to the industrial state of the art and not requiring any lengthy studies or research. ASN considers that, within the allotted time, the principle of studying the consequences of an earthquake significantly larger than the design-basis earthquake allows robustness studies to be conducted to identify the weakest points beyond the design-basis earthquake. ASN considers that EDF's performance of targeted inspections on the seismic behaviour of equipment for a hazard level higher than that used in the design, and EDF's commitment to performing a seismic behaviour review of the equipment necessary in loss of heat sink or loss of electrical power supply situations are sufficient. ASN considers that the margin review supplemented by inspections, enabled equipment modifications or reinforcements to be defined for an earthquake larger than the facility's design-basis earthquake and beyond the initial design hypotheses. ASN considers that the modifications and reinforcements identified (strengthening of tanks and anchors, limiting interactions, additional seismic qualification studies, etc.) can be performed rapidly. ASN considers that these studies complement the periodic review approach for the seismic part, which hitherto did not exceed the design-basis and only concerned the conformity of the equipment and structures as described in the safety case. However, although ASN does not question the general approach adopted in identifying the various conservative values, ASN does believe that the margin values presented and evaluated on the basis of an analysis performed within a very short period of time, are inadequately justified. ASN considers that some of the margins proposed by the licensee correspond to provisions used in the design to offer protection against the uncertainty and variability of the seismic hazard, in the same way as the variability of the behaviour of materials or uncertainties linked to modelling or construction. Consequently, ASN considers
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