- 130 - Snow With regard to snow, EDF did not feel that there was any need to consider it in the CSA. On this point, other licensees concerned by the ASN decision of 5th May 2011 included snow in the extreme natural phenomena and there are thus disparities between licensees of nearby sites. ASN will be asking EDF to present studies taking snow into account. Combinations of extreme climatic phenomena EDF considers that the event combinations considered can generate a risk capable of creating a common mode failure, in other words a risk of the unavailability of functionally redundant equipment or systems. A situation such as this is liable to lead to total loss of the heat sink (situation referred to as H1), or a loss of off-site power supplies (LOOP) on all the units of an NPP. These situations are presented in § 5 of this chapter. 4.2 Evaluation of safety margins 4.2.1 Estimation of margins in extreme meteorological conditions Wind EDF considers that the design of the buildings for the off-site explosion risk guarantees their robustness to extreme winds. EDF evaluated the existing margin by comparison with this event. EDF concludes that for all its sites, all the buildings designed for the "off-site explosion" risk are thus robust to extreme winds, with significant margins. For buildings not covered by the "off-site explosion" design, EDF considers that the loads associated with extreme winds are not liable to have consequences for reactor safety. Concerning the direct effects of wind on the equipment necessary in an H3, H1 or severe accident situation and situated outside the buildings (ASG steam generator auxiliary feedwater system piping and demineralised water distribution tanks for the conventional SER parts), EDF concludes that the loads associated with extreme winds do not compromise their strength. ASN considers that the profiles of the two situations ("off-site explosion" and "extreme wind") are not the same: there is a single load on the structures from an explosion, whereas gusting wind leads to several loadings. ASN also considers that the wind speed to be included in these studies should be consolidated. EDF simply analyses the behaviour of its facilities and the possible cliff-edge effects for a wind speed value of about 200 km/h. This value is close to that of the amended 1999 Snow and Wind 65 rules (which give speeds varying on the whole between 150 km/h and 200 km/h for the NPPs). ASN then considers that the value used by EDF to study the cliff-edge effects does not go far enough beyond the scenarios used for the design of the facilities. Moreover, ASN considers that a speed of 200 km/h is one that is rarely observed in metropolitan France but is not the maximum speed recorded within the past thirty years (storm of 16th October 1987: observed wind speeds of 216 km/h). On the occasion of the examination in preparation for the meeting of the advisory committees in November 2011, EDF made an undertaking to "transmit a statistical study within 6 months, allowing verification of the limited behaviour of exceptional wind speeds and confirmation of the maximum wind speed to be considered when evaluating any cliff-edge effects. The values adopted for each site will be compared with the maximum speed recorded by Météo France's metropolitan weather stations representative of each site." This undertaking is also a partial response to ASN's request. ASN considers that EDF needs to conduct studies which also take account of the specific nature of gusting winds and will send EDF a request accordingly. ASN considers that the conclusions on the direct effects of wind are also valid for the indirect effects of wind: ASN considers that the wind speed value to be used in these studies needs to be consolidated. ASN also considers that, for wind speeds of about 200 km/h, EDF should check that the only projectiles to be taken into account are indeed cladding sheets which are not liable to damage the outdoor IPS equipment, owing to their lack of rigidity.
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