- 272 - The licensee has estimated that the rates of water rise before reaching the mean low level of the units is 10 days if there is no ground water pumping, which is not guaranteed in an earthquake situation. When designing the plants, AREVA used the 10-year return rainfall (precipitation of 2.1 mm per for 10 minutes) as the design flood hazard. Conformity Although the licensee has not performed a proper conformity review, it has assessed the capacity of the storm water drainage systems to evacuate the volumes of water created by a 100-year return storm and any possible alternative run-off routes the water would take. Of the fourteen buildings examined, seven have drainage systems that are too small to evacuate the volumes of water considered (24 mm rainfall in 6 min). Furthermore, the risk of the storm water drainage system being unavailable is not retained by the licensee, who considers that this can be avoided through regular maintenance of the network. Measures to protect the facilities against the flood risk The site has a pumping system that lowers the water table levels. Evaluation of the safety margins The majority of the La Hague site is situated at the summit of the Jobourg plateau, more than one hundred meters above sea level. To the south, below the plateau and close to the sea, the site also has a connecting area comprising the Moulinets dam in particular. The base of the dam and the adjacent facilities are situated at an elevation of about 28 m NGF, i.e. 33 m above the marine charts 0 datum level. These facilities are situated more than 10 m above the highest envisageable waters (high water of spring tide + thousand-year storm surge + strong swell). Consequently the licensee considers that any risk of flooding associated with a tsunami combined with an earthquake can be excluded, even in an extreme scenario. The licensee presented an assessment of the flood risk associated with water table rises. Considering the dynamics of the water table risk phenomenon to be sufficiently slow, the licensee ruled out the risk associated with loss of the pumping that lowers the water table levels. The licensee also presented an assessment of the flood risk associated with maximum high intensity rainfall (PFI). The PFI values used were evaluated referring to the "REX Blayais" (Blayais site experience feedback) methodology and the impact assessment was modelled taking account of the storm water drainage system evacuation capacities, the predicted overflows and the possible paths taken by surface runoff water. The water heights estimated using this procedure were between 0 and 13 cm at the access points to the buildings containing the key SSCs. Measures envisaged to reinforce the robustness of the facilities to the flood risk The licensee concluded that it was necessary to raise the access sills of two of the 14 buildings studied in the flood risk assessment associated with the PFI. Proposed studies to supplement the procedure For the La Hague facilities, ASN considers that the licensee must supplement its procedure by analysing at least the risks of flooding of the key SSCs with respect to the design-basis storm scenarios and perform a sensitivity study with respect to partial blocking of the pipes and the risks of multiple breaks in systems and structures further to an earthquake, taking account of the site configuration and the location of access points to the buildings containing equipment to be protected. It will make requests in this respect.
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