- 242 - The CEA determined an overall seismic margin factor for each building which, when applied as a multiplication coefficient with respect to the chosen reference MHPE, enables the earthquake level beyond which a cliff-edge effect could occur to be determined. The overall seismic margin factor is defined as the product of the elementary margins induced by the conservatism of the methods and design-basis criteria, the capacity of the structures to dissipate the energy through ductile behaviour, the loading reductions in certain buildings where circuits have been drained and equipment removed, and the valuation of elements not considered in the seismic reassessment and which could contribute to the resistance of the structures. The CEA has examined the primary cooling system, the fuel element storage drum, the ultimate backup cooling system, the sodium primary system auxiliaries (tanks, supports, cold traps), the potential hazards (travelling cranes, steam generator caissons, bridges). The CEA thus obtained an overall seismic margin factor of 4.5 for the primary cooling system, and of up to 7 for the sodium auxiliary systems. The minimum margin is obtained for travelling cranes P2 and P4 (potential hazards) with a factor of 1.8. These values are to be compared with the overall margin factors for the civil engineering which vary from 1.5 to 7. The licensee considered that these margins were satisfactory and did not require the considering of any particular additional measures with respect to the seismic risk. Measures envisaged to reinforce the robustness of the facilities to the seismic risk ASN emphasizes that the risks present in the preparation of decommissioning and during decommissioning result not only from the radioactive inventory of the facility but also from the stored sodium. On completion of the review, the CEA undertook to consolidate the seismic margin evaluation for cranes P2 and P4 and to evaluate the robustness of crane P6 beyond the design-basis earthquake. ASN will require CEA to perform an evaluation of the robustness of crane P6 of the Phénix facility beyond the design-basis earthquake, and to propose reinforcements if necessary. 2.2.2. Reactor operated by the ILL Design of the HFR The High Flux Reactor (HFR) was designed and produced in accordance with the paraseismic rules PS 67 for an earthquake of intensity VIII. The "RFS 1981" spectra were used for the studies carried out from 1994 to 2002. The procedure for evaluating the seismic hazard adopted on completion of the periodic safety review of 2002 was performed in application of RFS 2001-01. It must be pointed out that in 2004, it was decided to consider only the "RFS 2001-01" spectra for the studies that had not yet been started. The "FRS 1981" response spectra established in accordance with RFS I.2.c and validated by ASN in 1997 were used for certain reinforcements. It should be underlined that further to the periodic safety review of 2002, the ILL undertook major seismic reinforcement work on the facility, some of which remains to be finalised. In its CSA report, the ILL indicated that the design basis spectra used correspond to either the "RFS 1981" spectra or the "RFS 2001-01" spectra, depending on the key SSCs. More specifically, the earthquake used for the reactor building is the near earthquake defined by RFS 1981; for the frequencies below 4 Hz however, the "RFS 2001" spectrum is more penalising than the "RFS 1981" near earthquake spectrum. Application of RFS 2001 leads to the defining of a Safe Shutdown Earthquake (SSE) of magnitude 5.7 with its epicentre situated 7 km below the reactor. Measures to protect the facilities against the seismic risk / design-basis earthquake In the event of an earthquake, the licensee has indicated that the shutdown status of the facility corresponds to: emergency shutdown of the reactor; reinforcement of the second barrier containment: closure of all the safety valves and D2O and H2O vents; containment isolation: closure of all the third barrier valves;
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