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 structures and equipment. 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. EDF took account of this experience feedback and reassessed the operation of its facilities in air and water temperature conditions more extreme than those initially included in the design. In parallel with development of these “extreme heat” baseline safety requirements, EDF modified its installations (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. During the reactors’ periodic safety reviews, EDF takes account of climate change and is continuing to improve the capacity of its installations to deal with the effects of a heat wave. The capacity of certain cooling systems for equipment required for the nuclear safety case will in particular be improved. EDF has also initiated a climatic monitoring programme to anticipate climate changes which could compromise the temperature hypotheses adopted in its baseline requirements. As with the other hazards, ASN asks EDF to learn the lessons from the various heatwave events, along with their effects on the installations. Consideration of natural hazards of extreme intensity Following the accident that struck the Fukushima Daiichi NPP on 11 March 2011, stress tests led ASN to prescribe the installation of a “hardened safety core” of material and organisational provisions to deal with situations arising from external natural hazards of extreme intensity, the severity of which exceeds that considered hitherto in the baseline safety requirements of each installation. The external natural hazards considered for the design of the “hardened safety core” are the following: earthquake, flooding (including extreme rainfall) and associated phenomena (extreme winds, lightning, hail) as well as tornados. 2.4.6 Assessment of the risk prevention measures relating to hazards ASN checks that risks linked to hazards in the NPPs are taken into account, notably based on the reassessment of the design of the installations during the periodic safety reviews, analysis of the licensee’s baseline safety requirements, examination of significant events and the inspections performed on the sites. The Fukushima Daiichi NPP accident led EDF to reinforce its organisation for the management of risks relating to hazards. More specifically, networks of coordinators were set up in all the NPPs to oversee the implementation of the actions defined to deal with these hazards. Annual reviews are also held to improve this organisation. In general, ASN considers that major efforts are still needed on most of the sites to improve how hazard risks are dealt with, in particular with regard to: ∙the maintenance of equipment (sluice gates, fire doors, sensors, floor drains, etc.); ∙risk assessments during maintenance operations and in the event an equipment malfunction is detected; ∙training of the coordinators and awareness-raising among the EDF and contractor personnel. Reactor outages The nuclear power reactors must be periodically shut down for replacement of the fuel depleted during the electricity production cycle. One third or one quarter of the fuel is thus renewed at each outage. These outages mean that certain parts of the installations which are not accessible during the production phase then become temporarily accessible. They are thus put to good use by EDF to carry out checks, tests and maintenance, as well as to perform works on the facility. These outages can be of several types: • Refuelling Outage and Maintenance Outage: these outages, which last from a few weeks to a few months, are devoted to replacing a part of the fuel and to carrying out a verification and maintenance programme, which is more extensive during a maintenance outage than during a refuelling outage. • Ten-yearly outage: this is an outage involving a programme of in-depth verification and maintenance. This type of outage, which lasts several months and takes place every ten years, enables the licensee to carry out large-scale operations such as the complete inspection and hydraulic testing of the reactor coolant system, hydrotesting of the containment or incorporation of design changes resulting from the periodic safety reviews. These outages are scheduled and prepared by the licensee several months in advance. ASN checks the steps taken by the licensee to ensure the safety of the facility, environmental protection and radiation protection of the workers during the outage, as well as the safety of the reactor for the next production cycle. In the light of the provisions of its resolution 2014-DC-0444 of 15 July 2014 concerning PWR outages and restarts, the monitoring performed by ASN involves spot-checks. These mainly concern the activities with the most significant safety implications, as well as the processing of any unforeseen events. It consists of on-site inspections and documentary checks, throughout the duration of the outage and in particular prior to restart of the reactor. It is after this inspection that ASN may or may not approve reactor restart. 310 ASN Report on the state of nuclear safety and radiation protection in France in 2024 The EDF Nuclear Power Plants
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