- 121 - The consequences of the reference flood augmentation scenarios vary widely. The nuclear island platforms of some sites would remain above water level. For the others, the flooding could reach up to about two metres on the nuclear island platforms. For a certain number of riverside sites, EDF considers that the water height estimates, based on extrapolations from existing studies or models, would need to be consolidated. The consequences of each of the two rainfall scenarios are on the centimetre scale. Depending on the site, EDF considers that the volumes of water associated with each of the two maximum rainfall scenarios are either contained by the roadways or liable to cause a layer of water a few centimetres high on the nuclear island platforms. With regard to the flood scenarios induced by an earthquake beyond design-basis, the water levels obtained are of the centimetre or decimetre scale, in certain cases. However, depending on the sites, EDF estimates that: either the risk of flooding can be ruled out because the platform on which the failed structure is situated is well below the nuclear island platform, or the associated water volumes are contained by the roadways, or the associated water volumes are liable to create a layer of water a few centimetres high on the nuclear island platform. EDF was unable to issue a final statement for all the sites concerning the consequences of this type of hazard in the situations considered. Further studies are still required. Evaluation of the water heights induced by these three scenarios is based on the principle of calculating the spreading of the volume of rainwater not evacuated by the network. ASN considers that certain hypotheses need to be checked (hydraulic, topographical hypotheses) and that the studies are not sufficiently conservative to cover the dynamic flow effects. Additional data would seem to be necessary to justify the spreading hypotheses as well as the hydraulic hypotheses utilised in the studies, in particular those concerning blockage of the drains24. For certain sites, EDF considers that the volumes of water induced by these three scenarios will be contained by the roadways of the site platforms. For the others, the water elevation is compared with the building access thresholds. In the event of an H1, LOOP or H3 risk, EDF proposes studying the plausibility of a water layer risk on the nuclear island platforms and, as applicable, the TA/TS transformers. During the investigation, EDF specified that these studies will retain the water layer spreading hypothesis, but will be enable the conservative nature of the current evaluations to be reduced. However, ASN considers that the uncertainties surrounding the hydraulic and spreading hypotheses adopted by EDF can lead to flood heights in excess of those presented, therefore the margins should not be calculated down to the nearest centimetre. At the meeting of the advisory committees in November 2011, EDF made the following undertaking, which offers a satisfactory response to the specifications: "The influential parameters listed (duration of precipitation, absorption and drainage capacity) are considered beyond the baseline safety standards with a view to verifying protection of the "Hard Core" equipment". In order to initiate the studies announced in the RECS, aimed at providing a more detailed characterisation of the layers of water induced by the "PFIx2", "PFI+SEO blockage", and "flooding induced by an earthquake bigger than design safety standards" scenarios, EDF intends to define and justify the various hypotheses utilised (land absorption capacity, evacuation flows to off-site land, spreading hypotheses, consideration of dynamic effects, consideration of topographical data). Furthermore, concerning the maximum scenario "PFIx2", a vulnerability study concerning the duration of precipitation greater than the network concentration time will be performed". 24 System primarily designed to collect run-off water and channel it to the sewer network.
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