- 267 - The CEA has moreover undertaken to: Evaluate the margin on the river Rhone flow rate before reaching the Phénix platform; Supplement its study to take into account scenarios of rainfall combined with the risk of unserviceability of the storm water drainage system, areas of local water build-up resulting from the site configuration and the location of the accesses to buildings containing equipment that must be protected; Specify the measures adopted to protect the sensitive premises in the event of flooding of the platform caused by a flood of the river Rhone and the schedule for implementing these measures. The main risk the Phénix reactor presents with respect to the extreme situations studied in the CSA is linked to flooding, in view of the risk of sodium / water interaction. The ASN will issue requirements relative to the necessary to work to increase the facility's robustness to this risk, and the sensitive premises in particular. 3.2.2. Reactor operated by the ILL Design of the facility The high flux reactor is situated upstream of the confluence of the Isère and the Drac, on the right bank of the Drac and the left bank of the Isère. These two rivers have relatively different flow regimens, the first being supplied with rainwater associated with oceanic disturbances, the second by rainfall associated with Mediterranean disturbances. The result is that it is relatively rare for the two rivers to become flooded simultaneously. In its CSA report, ILL indicates that the HFR is designed to withstand a flood at the maximum elevation of 210.50 m NGF, which covers the elevation of all the floods of the Isère and the Drac, but not that resulting from bursting of the Monteynard dam. Measures to protect the facilities against the flood risk The first protection measures against flooding of the Isère and the Drac are the ILL site embankments. Anti-flood devices are also provided in the cooling system water intakes and by the installation of flood gates. A system protecting against bursting of the Monteynard dam, which would result in a flood wavefront on the site, can also be deployed with respect to the external metallic containment which risks not resisting such a load. The principle of this system would be to depressurise the annular space by opening it at two points to let the water rise on either side of the enclosure thereby preventing any damaging load build-up. Conformity of the facilities to the current frame of reference The ILL indicates in its CSA report that the design of the HFR does not comply with RFS I.2.e. This is because: as the thousand-year return flood is not known, the safety margin corresponding to this flood + 15% is not defined; the design does not take into account the loss of the most constraining retention structure (Monteynard dam) combined with the highest flood in history or the 100-year return flood if this is higher. Bringing into compliance is planned as part of the next update of the safety report planned for 2012. The ILL indicates that this level is nevertheless already taken into account and integrated in the emergency management procedures for site flooding. As regards the key SCCs, the licensee has identified the necessary reinforcements (see below). Evaluation of the safety margins The ILL has examined a scenario involving the bursting of the four dams situated on the river Drac further to an earthquake. It considers this scenario extremely improbable and emphasizes the penalising natures of the calculated elevations (216.2 m) and after adding the safety margin (218 m). The reference level taken to evaluate the robustness of the facility is the 216.2 m NGF level. This choice is explained by the conservative
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