Finally, the detection of an outbreak of fire and fire-fighting measures should enable a fire to be brought under control and then extinguished within a time compatible with the fire resistance duration of the sectorisation elements. Explosion risks An explosion can damage the items essential for maintaining safety or lead to rupture of the containment and the dispersal of radioactive materials into the facility, or even into the environment. Steps must thus be taken by the licensee to protect the sensitive parts of the facility against explosions. Internal flooding risks An internal flood, that is originating inside the facility, can lead to failure of the equipment needed to control the fundamental safety functions. Flooding may in particular be caused by an earthquake. Steps are therefore taken to prevent internal flooding (maintenance of piping carrying water, etc.), or mitigate its consequences (presence of floor drains and water extraction pumps, installation of sills or leaktight doors to prevent the flood from spreading, etc.). External flooding risks Following the partial flooding of the Blayais NPP in December 1999, the licensees, under the supervision of ASN, reassessed the safety of their facilities in the face of this risk, in conditions that were more severe than before, and made numerous safety improvements, according to a schedule defined according to the risks. In accordance with the ASN requirements, EDF completed the required work on all its nuclear power reactors in 2014. At the same time, to ensure more exhaustive and more robust integration of the flooding risk, as of the facilities design stage, ASN published Guide No. 13 in 2013 concerning BNI protection against external flooding. Following the stress tests performed in the wake of the Fukushima Daiichi NPP accident, ASN considered that with regard to flooding protection, the requirements resulting from the complete reassessment carried out following the flooding of the Blayais NPP in 1999 would be able to provide the NPPs with a high level of protection against the external flooding risk. However, ASN issued several resolutions in June 2012 asking the licensees: ∙ to reinforce NPP protection against certain hazards, such as intense rainfall and earthquake-induced flooding; ∙ to define and implement a “hardened safety core” of material and organisational measures to control the fundamental safety functions in extreme situations and in particular in the case of flooding beyond the design-basis safety requirements. Seismic risks Although seismic activity in France is moderate or slight, EDF’s inclusion of this risk in the safety case for its nuclear power reactors is the subject of constant attention on the part of ASN, given the potential consequences for the safety of the facilities. Seismic protection measures are designed into the facilities. They are periodically re-examined in the light of changing knowledge and changes to the regulations, on the occasion of the periodic safety reviews. Basic Safety Rule (RFS) 2001-01 of 31 May 2001 defines the methodology used to determine the seismic risk for surface BNIs (except for radioactive waste long-term disposal facilities). This RFS is supplemented by ASN Guide 2/01 of May 2006 which defines acceptable calculation methods for a study of the seismic behaviour of nuclear buildings and particular structures such as embankments, tunnels and underground pipes, supports or tanks. The design of the buildings and the equipment important for safety in the NPPs must thus enable them to withstand earthquakes of an intensity greater than the strongest earthquakes that have occurred in the region. EDF’s NPPs must thus be able to withstand seismic levels incorporating the local geological features specific to each one. As part of the periodic safety reviews, the seismic reassessment consists in verifying the adequacy of the seismic design of the facility, taking account of changing knowledge about seismic activity in the region of the site or about the methods for assessing the seismic behaviour of elements of the facility. The lessons learned from international experience feedback are also analysed and integrated into this framework. Following the Fukushima Daiichi NPP accident, ASN asked EDF to define and implement a “hardened safety core” of material and organisational measures to control the fundamental safety functions in extreme situations comparable, in the French context, to that which occurred in Japan on 11 March 2011. This “hardened safety core” shall notably be designed to withstand an earthquake of an exceptional level, exceeding those adopted in the design or periodic safety review of the installations. In order to define this exceptional level earthquake, ASN asked EDF to supplement the deterministic approach to defining the seismic hazard with a probabilistic approach, in order to take account of international best practices. Heatwave and drought risks During the heat waves in recent decades, some of the watercourses used to cool NPPs experienced a reduction in their flow rate and significant warming. Significant temperature rises were also observed in certain NPP premises housing heat-sensitive equipment. EDF took account of this OEF and initiated reassessments of the operation of its facilities in air and water temperature conditions more severe than those initially included in the design. In parallel with development of these “extreme heat” baseline safety requirements, EDF initiated the deployment of a number of priority modifications (such as the increase in the capacity of certain heat exchangers) and implemented operating practices optimising the cooling capacity of the equipment and improving the resistance of equipment susceptible to high temperatures. For the periodic safety review of its reactors, EDF has initiated a modifications programme on its facilities designed to provide protection against heat wave situations. The capacity of certain cooling systems for equipment required for the nuclear safety case will in particular be improved. CAMPAIGN OF MAINTENANCE-RELATED INSPECTIONS In 2021 and 2022, ASN ran a campaign of 14 inspections on the organisation of the maintenance departments in the NPPs, in order to check EDF’s deployment of its new maintenance strategy. The inspections revealed an organisation of the departments in charge of maintenance that was on the whole satisfactory, with a good level of deployment of the new tools. This is particularly the case with the performance of the function reviews, which enable each NPP to identify the technical problems – with an integrated vision per function – that could affect equipment and systems, along with the associated areas for improvement, and monitoring of trends, with the aim to be able to detect equipment performance degradation early on. 298 ASN Report on the state of nuclear safety and radiation protection in France in 2022 • 10 • The EDF Nuclear Power Plants 10
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