ASN Report 2020

example, sectorisation resistance studies, account taken of the effect of smoke). In addition, the new methods adopted identified sectorisation aspects for which correct working is particularly important. For example, the fire doors which are required to be closed were identified and will be subject to specific monitoring. 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. ASN checks these prevention and monitoring measures, paying particular attention to ensuring that the explosion risk is taken into account in EDF’s baseline safety requirements and organisation. ASN also ensures compliance with the “Explosive Atmospheres” (ATEX) regulations to ensure worker protection. Internal flooding risks An internal flood, in other words which comes from within the facility, may lead to failure of equipment necessary for reactor shutdown, fuel cooling and containment of radioactive products. 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.). These measures are regularly inspected by ASN. ASN remains vigilant with regard to the risks of internal flooding as a result of an earthquake, as well as with regard to the integration of operating experience feedback, in particular the processing of deviations affecting certain internal flooding protection measures. 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 com­ pleted 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. For the existing facilities, ASN asked EDF in 2014 to take account of the recommendations of the Guide on all its reactors: ∙ for the 1,300 MWe reactors, ASN asked EDF to give priority to the third periodic safety review; ∙ for the other reactors in service, EDF will give priority to the next periodic safety reviews (fourth reviews for the 900MWe reactors and second reviews for the 1,450 MWe reactors). 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 (see point 2.9). 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 reexamined 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 Basic Safety Rule 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 are thus designed for 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. Changing knowledge leads EDF to reassess the seismic hazard during the periodic safety reviews. Following the Fukushima Daiichi NPP accident, ASN instructed 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, which would be more closely in line with 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 this experience feedback into account 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 298 ASN Report on the state of nuclear safety and radiation protection in France in 2020 10 – THE EDF NUCLEAR POWER PLANTS