- 251 - Assessment of the safety margins To assess the robustness of its facilities to the seismic risk, AREVA considers a seismic intensity where the SSE magnitude has been increased. This increase corresponds to MSSE + 1.5 for the La Hague site. For the La Hague site, AREVA has analysed the earthquake robustness of: the key SSCs identified for pools NPH, C, D and E, and those identified for the fission product storage areas (SPF6 unit only), the Moulinets pond and the West pond that would be used to replenish the backup cooling systems of the tanks and pools (the fixed supply pipes and associated pumps were not analysed), the local security organisation (FLS) building housing the emergency centre and certain emergency management resources, the diesel fuel backup tanks (post-earthquake leak-tightness), the "pendulum-type centrifugal decanters" (DPC) of the R1 and T1 units (robustness analysis of the decanter tanks and their tappingsnozzles, ground attachments of the electric cabinetsranks, decanter rotor, pressurised water tanks). For the above-mentioned pools - and the pond shells in particular - AREVA's analysis of the buildings concludes on high robustness, with the risk of serious damage occurring with earthquakes of magnitude exceeding 6.4 at 15 km (the magnitude of the current SSE for the site is 5.8 at a distance of 15 km). In AREVA's opinion, this level of robustness renders relatively implausible the occurrence of a seismic event that could jeopardise the civil engineering stability and hence the water inventory of the ponds. Likewise, the robustness study of the key SSCs ensuring the cooling functions concludes that their robustness is equivalent or higher than the minimum level of the abovementioned buildings. In the case of fission product storage facilities, AREVA concludes that the robustness of the civil engineering of the buildings corresponds to a risk of significant damage occurring with earthquakes of magnitude exceeding 7.3 at 15 km. Furthermore, the robustness of the key SSCs ensuring the cooling and dilution of hydrogen from radiolysis corresponds to a magnitude of 6.8 at 15 km. According to AREVA, the civil engineering robustness of the facilities that could be affected by feared situations results essentially "from the conservatism of the design-basis methods, due in particular to the design in the linear elastic range which is not required in standard buildings". In the case of the UP2-800 and UP3-A plant units, designed to earthquake design-basis and built between 1982 and 1994, the study methodologies and the work inspection procedures applied by the prime contractor assisted by an approved inspection agency were implemented in globally similar manners. In AREVAs's opinion they should lead to a minimum level of robustness similar to the results evidenced on the buildings mentioned earlier (units BSI, DEDS, T0, T1, T2, T3, T4 and T7 of BNI 116, Extension BST1, R1, R2, R7, SPF5 of BNI II7, STE3A, STE3B, STE3T, MDSA, MDSB and DEEB of BNI 118). For the units built as from the second half of the 1990's, AREVA points out that it had changed its method of integrating margins in the studies in order to obtain greater flexibility in taking potential modifications in account during the project. These changes lead to a higher level of robustness for the units concerned (units ACC, ECC of BNI 116 and R4 of BNI 117), according to AREVA. Likewise, for the mechanical and fabricated equipment in the UP2-800 and UP3-A units, similar in design (operating principles, choice of materials, construction classes, designed to earthquake design-basis by calculation and associated requirements) and production, AREVA carries over the conclusions of its robustness analyses.
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