- 263 - The CEA also indicates that no cliff-edge effect occurs and no essential element is threatened as long as the -4 m level is not flooded to a depth exceeding 60 cm of water, which would represent a volume of more than 5000 m3 in the facility basements. The CEA considers that flooding of the facility cannot lead to the failure of an essential equipment item and that the main flood risk comes from heavy rainfall. The storm water drainage systems, whether for the facility or the whole Centre, were designed for a 10-year storm event. According to the abovementioned study of 2005, a 100-year storm would produce minor drainage system overflows, which would run towards the Corbeville stream situated to the south of the Centre. The CEA thus considers that there is no risk of a cliff-edge effect. It states moreover that to lose the nuclear ventilation and the electrical cabinets situated at the 4 m level, 5000 m3 of water would have to infiltrate the facility, a volume that vastly exceeds that expected in the event of drainage system overflow further to a 100-year storm (less than 250 m3). Measures envisaged to reinforce the robustness of the facilities to the flood risk The CEA envisages the following improvements: extend the creation of low walls around all the passageways giving access to the -4 m level from the 0 m level; isolate a gallery linking the Pierre Süe laboratory (external to the facility, not specifically protected against flooding) to the annular space of the Osiris reactor (gallery situated around the basemat); implement provisions preventing the lifting of the tanks of acid, sodium hydroxide and fuel, or preventing the water from entering their bunkers; protect the pulsed ventilation suction inlet situated on the exterior of the facility at the 0 m level, against water ingress. ASN has no particular remarks concerning the licensee's proposals to reinforce the robustness of the Osiris facility to the risk of flooding. JHR Design of the facility When designing the Cadarache site facilities, the CEA calculated a flood safety margin level of 265 m NGF (French datum level system) based on the bursting of the Serre-Poncon dam. The Cadarache site is moreover crossed by the Ravin de la Bête stream, which is at an elevation 297 m NGF at the JHR, then channelled at the ATPu in an oval drain dimensioned to discharge the 100-year storm flow estimated for this part of the watershed. Given the margins available between the levels of the platforms and of the two watercourses (which can reach several tens of meters), or between the extreme flow of the stream and the discharge capacity of its channel, the CEA has ruled out flooding of the Cadarache facilities by the River Durance or overflowing of the Ravin de la Bête stream. The reference rainfall levels considered for the design are the 100-year storm events, as defined in the experience feedback (REX) from the Blayais site. As regards an extreme rise in the water table, the CEA considered a level corresponding to the 100-year flood for the JHR, where the extreme level of the water table would reach the 316 m NGF elevation at the JHR nuclear reactor. Measures aiming to protect the facilities against the flood risk The following design measures were taken: The JHR storm water drainage system is designed to cope with a 100-year storm to which a safety factor of 1.5 has been applied. In addition to the storm water drainage system, measures have been taken to prevent the ingress of runoff water into the structures: As regards water ingress into the nuclear unit (UN): The access doors (truck airlock, new systems airlock, emergency door) are on a gradient so that the flow is directed towards the exterior;
RkJQdWJsaXNoZXIy NjQ0NzU=