- 265 - Evaluation of the safety margins The licensee has considered the cases of an extreme flood of the river Durance, bursting of the dams situated near the Cadarache centre (EDF dam of Serre-Ponçon, dams of Sainte-Croix and Esparron-surVerdon) and bursting of the Canal de Provence. The calculations have shown that the rise in pond water resulting from the most pessimistic dam-break flood wave would correspond to a breach in the Serre-Ponçon dam situated more than 95 km from Cadarache. The dam-break flood wave would result from the gradual but total destruction of the dam caused by deep seepage. It was calculated that this flood wave would reach Cadarache 5 h 40 min after the dam burst and result in a peak flow of the order of 60,000 m3/s with a mid-height width of a few hours. The cases of a simultaneous 100-year flood would represent an additional flow of 5 000 m3/s, which would not significantly change the maximum elevation reached by the flood wave, this being 265 m NGF. The licensee considers that a no nuclear facility on the site would be concerned and that the Centre would remain accessible via the auxiliary gates. The licensee presents the risks of storm water drainage system overflow in the event of rainfall exceeding the design-basis levels. The CEA considers that the storm water runoffs not collected by the drainage system would have no impact on the JHR buildings on account of the small area of the upstream watershed, the access sills which are raised with respect to the roads, or the steep external slopes directing the runoffs downstream to the Ravin de la Bête. With regard to the effects of a gradual rise in the water table beyond the levels considered for the design of the facilities, the licensee highlights either the protection measures that enable - in the same way as for the storm water - the water to be discharged towards the Ravin de la Bête, or the margins before the low points of the sensitive buildings are reached. Measures to improve the safety of the facility - Opinion of ASN Although there is no identified cliff-edge effect, the CEA proposes reinforcing design robustness by taking measures to facilitate pumping of a water build-up in the paraseismic support area following flooding of the containment. ASN considers that the licensee's proposals to increase the flood risk robustness of the JHR facility are satisfactory. It will instruct the licensee to install, in the bottom section of the concrete wall of the paraseismic supports area, means to facilitate management of a containment flood. Phénix Design of the facility The CEA has calculated a flood safety margin level (CMS) of 38.54 m NGF, resulting from bursting of the Douglas dam combined with the 100-year flood of the river Rhone. Level 0 of the Phénix reactor is situated 16 cm above the CMS. The design reference rainfall is the 100-year storm event at the upper bound of the 95% confidence interval. The CEA indicates that the storm water drainage system can evacuate rainfall representing about 100 mm/h, which is comparable with a 100-year storm for the region. The CEA underlines that the drainage system has functioned satisfactorily in the past, notably in the heavy rainfall episode of 2002. The platform would be immersed upwards of the 38.70 m NGF level. Measures to protect the facilities against the flood risk In its CSA report the CEA presents several design measures to protect the Phénix reactor against the flood risk, and notably: the level 0 of the reactor which is situated at elevation 38.70 m NGF, for a CMS of 38.54 m NGF; the various structures of the Compagnie Nationale du Rhône (CNR) that would regulate the flow of the Rhone and lead its overflow towards the left bank. The water level at the facility, which is situated on the right bank, would therefore be limited even though it is not protected by an embankment; the presence of fixed pumping means, notably in the auxiliary building, the inspection-offices building and the steam generator (SG) building.
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