The quality and availability of the resources assigned to the in-depth analyses of significant events are satisfactory on all the sites, which is a positive point. With regard to technical aspects, the analyses reveal that the apparent causes and the root causes are correctly identified and dealt with using appropriate measures. ASN considers that EDF needs to make further progress on evaluating the effectiveness of the corrective measures and the conditions for closure of these measures. Building on lessons learned and sharing OEF remain points to be monitored. With regard to investigation of causes in terms of OHF, ASN considers that the analyses all too often simply focus on human failures without sufficiently investigating the influence of the working situation or the organisational processes involved. Even if improvements have been observed since 2022 in the ESS analyses, the involvement of OHF specialists differs widely and is insufficient on several sites. Notification of significant events by EDF Pursuant to the rules for the notification of significant events (see chapter 3, point 3.3), ASN received 714 Significant Safety Events (ESS) reports from EDF in 2023, along with 140 Significant Radiation Protection Events (ESRs) reports and 46 Significant Environmental Protection Events (ESEs) reports. The number of significant events rose by about 2.4% in 2023 by comparison with the previous year, in particular the ESS (740 in 2020, 762 in 2021, 687 in 2022). Graph 1 presents the trend in the number of significant events notified by EDF and rated on the International Nuclear and Radiological Event Scale (INES), which is graded from 0 to 7 in increasing order of severity, since 2013. Graph 2 shows the trend since 2013 in the number of significant events according to the notification field. Events not rated on the INES scale are also taken into account. Significant events affecting several nuclear reactors are grouped under the term generic significant events. In 2023, 16 events of this type were reported in the field of nuclear safety (26 in 2020, 31 in 2021, 21 in 2022). In 2023, two significant events rated level 2 were notified by EDF. The first event is linked to the discovery of major SC cracks. The second event concerned the external contamination of a worker in the Cattenom NPP (see box page 313). 2.4.5 Protection against internal and external hazards The NPPs must be able to withstand a variety of hazards, originating either inside or outside the installations. The main hazards constituting a risk for safety are detailed below. Fire risks A fire can lead to failure of the equipment needed to control the fundamental safety functions. Steps must thus be taken to protect the sensitive parts of the facility against fire. In the same way as the other BNIs, NPPs are covered by ASN resolution 2014-DC-0417 of 28 January 2014, relating to the rules applicable to BNIs for controlling fire risks. The way the fire risk is taken into account in the NPPs is based on the “Defence in Depth” principle built around three levels, that is the design of the facilities, fire prevention, fire detection and firefighting. Design rules aim to prevent a fire from spreading and mitigate its consequences; they are based primarily on “fire sectorisation”. This involves dividing the facility into sectors and containment areas designed to keep the fire within a given perimeter bounded by items (doors, walls and fire dampers) offering a specified fire resistance duration. The main purpose is to prevent a fire from spreading to two redundant equipment items performing a fundamental safety function. 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 installation. 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.). Seismic risks Although seismic activity in France is moderate, 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, 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. This RFS is supplemented by ASN Guide No. 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 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. The seismic reassessments regularly lead EDF to reinforce the strength of its installations. Heatwave and drought risks During the heat waves in 2003 and 2006, 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. 302 ASN Report on the state of nuclear safety and radiation protection in France in 2023 • 10 • The EDF Nuclear Power Plants
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