- 109 - 3 Flooding Floods are events liable to lead to failures that can impact all the facilities on a site and in particular lead to either a loss of cooling water supply, or a loss of off-site electrical power, or prolonged isolation of the site. Flooding is a risk that is taken into account in the design of the facilities and reassessed on the occasion of the periodic safety reviews or further to certain exceptional events, such as the partial flooding of the Le Blayais nuclear power plant during the storm on 27th December 1999. This reassessment in particular concerns the maximum water level considered in the design of the site protection structures, called the flood safety margin level (CMS), but also all the phenomena and combinations of phenomena that can be the cause of a flood (high river level, storm, rainfall, rising groundwater level, failures of systems and water retention systems and structures, etc.). Analysis of EDF's complementary safety assessments (CSA) shows that the complete review of the way this risk is taken into account in the nuclear power plants, completed in 2007, enables the facilities to be given a high level of protection against the risk of flooding. In order to ensure that this high level of protection is actually reached, ASN will require that EDF: within the time stipulated following the 2007 "flooding" reassessment, and no later than 2014, complete the protective works and measures for the nuclear power plants; improve the volumetric protection16 of the facilities. The ASN inspections revealed that management of volumetric protection needs to be improved on several of the sites inspected; complete the heat sink design review, in particular with regard to prevention of the clogging risks, initiated further to the Cruas incident in 2009; reinforce the protection of the facilities against the flood risk over and above the current baseline safety requirements, for example by increasing the volumetric protection. The CSA in fact revealed the existence of cliff-edge effects (total loss of electrical power supplies) for levels close to those adopted in the safety requirements. Furthermore, following the complementary safety assessments (CSA) of the nuclear facilities, carried out in the wake of the Fukushima accident, ASN considers that nuclear facilities need to be made more robust to highly improbable risks which are not as yet considered in the design of the facilities or following their periodic safety review. This involves providing these facilities with the means to enable them to deal with: a combination of natural phenomena of an exceptional scale and greater than those adopted in the design or during the periodic safety review of the facilities, very long duration loss of electricity sources or heat sinks, capable of affecting all the facilities on a given site. ASN will thus be requiring that the licensees create a "hard core" of reinforced material and organisational measures to guarantee the operational nature of the structures and equipment allowing control of the basic safety functions in these exceptional situations. This subject is covered further in § 8 of this chapter. 16 In a flood situation, the equipment able to guarantee the safety of the reactors must remain operational. Protective devices are thus implemented, whenever necessary, to offer protection against the various unforeseen circumstances that could lead to flooding. This protection is based on several lines of defence (embankments, walls, water drainage networks, etc.), including volumetric protection. Volumetric protection, which encompasses the buildings containing equipment able to guarantee reactor safety, was defined by EDF in such a way as to guarantee that an arrival of water from outside this perimeter does not lead to flooding of the premises inside it. In concrete terms, volumetric protection comprises walls, ceilings and floors. Protection of the existing openings in these walls (doors and others) can constitute possible routes for water entrance in the event of a flood.
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