- 110 - 3.1 Design of the facilities 3.1.1 Floods for which the facilities are designed In its specifications, ASN asked EDF to give: the characteristics of the flood for which the facility is designed (in particular the water level considered), their justification, as well as the values of these parameters taken into account for the facility's initial authorisation decree; the methodology selected for evaluating the characteristics of the flood for which the facility is designed (return period, past events considered, their location and the reasons for this choice, the margins added, etc.); flooding sources considered (tsunami, tide, storm, dam burst, etc.); validity of historical data. ASN also asked the licensee to state its position regarding whether the facility flood level design is adequate. For the design of the flood protections, the sites use basic safety rule RFS I.2.e of 12th April 1984 ("Consideration of the off-site flood risk"). This text in particular defines a method for determining the water levels to be considered when designing the facilities. This method is based on the definition of the flood safety margin level (CMS) and differentiates between three cases: 1. For coastal sites, the CMS corresponds to the combination of the maximum calculated tide (coefficient 120) and the thousand year storm surge. 2. For river sites, the CMS (or CBMS) is the highest of the following two levels: a. Level reached by a river whose discharge is obtained by increasing the thousand year flood level by 15%; b. Level reached by a combination of the highest known flood waves, or the hundred-year flood level if higher, and collapse of the most prejudicial retaining structure. 3. For estuary sites, the CMS is the highest of the following three levels: a. Level reached by a combination of the thousand year river flood level and the tide of coefficient 120; b. Level reached by the combination defined in 2.b and a tide of coefficient 70; c. Level reached by the combination of the thousand-year marine surge and the tide of coefficient 120. Following the partial flooding of the Le Blayais nuclear power plant in December 1999, EDF updated its CMS evaluation of all the sites and systematically took account of other hazards liable to cause flooding: 1. For all the sites: The deterioration of a water storage structure (pipeline, air cooling tower ponds, water storage ponds, etc.) close to the site, for which the waterline is higher than the platform of this site; The intumescence ?17; High intensity rainfall (hundred-year return period) and regular and continuous rainfall (maximum hundred-year averages over 24 hours); A rise in the ground water level; Failure of a system or equipment item. 2. For river sites: Influence of the wind on the river or the chop (determined for a hundred-year wind). 3. For coastal sites Wave swell 17 Free surface deformation wave caused by a sudden variation in the speed of (discharge) flow. Phenomenon comparable to fluid "hammers" in a pipe. Known as "positive" intumescence when there is a sudden reduction in speed, and conversely "negative" intumescence when there is a sudden increase in speed. It can be observed at sudden stoppage/startup of the units on a run of river hydroelectric plant, or CRF pumps on a once-through PWR nuclear power plant intake channel.
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