generator and each NPP has an ultimate backup source, the nature of which varies according to the plant in question. Finally, following the Fukushima Daiichi NPP accident, these resources were supplemented by an “Ultimate back-up” Diesel generator set (DUS) for each reactor. 1.9 The “hardened safety core” improving resistance to extreme events After the accident at the Fukushima Daiichi NPP, ASN instructed EDF to deploy on each reactor a “hardened safety core” of robust material and organisational measures designed, for the extreme situations studied in the 2011 stress tests, to: ∙prevent an accident with fuel melt, or limit its progression; ∙avoid large-scale radioactive releases; ∙enable the licensee to carry out its emergency management duties. For each reactor, the “hardened safety core” mainly consists of: ∙a DUS; ∙an ultimate heat sink; ∙a new means of injecting borated water into the primary system when it is at high pressure; ∙a system for heat removal by the SGs; ∙a means for topping up the fuel storage pool from the ultimate heatsink; ∙an extra fuel pool cooling system, partly reliant on mobile means; ∙an ultimate cooling system for the containment, which relies in part on mobile means; ∙measures to stabilise the corium on the basemat, in the event of an accident with core melt and reactor vessel melt-through; ∙an ultimate I&C system, electrical distribution and the necessary instrumentation. In addition, to manage emergency situations, each site shall be equipped with a new local emergency centre capable of withstanding extreme external hazards. These provisions are being deployed as part of the periodic safety reviews. 1.10 The specific features of the Flamanville EPR reactor The Flamanville EPR is a 1,600 MWe reactor third-generation reactor. It has four cooling loops and uses UO2 fuel. Its containment consists of two concrete walls and a metal liner (see previous page) covering the entire internal face of the inner wall. A reinforced concrete “shell” covers the most sensitive buildings: reactor building, fuel storage building, control room and two of the four safeguard auxiliaries buildings. As for the other reactors, it has safety-important systems needed for reactor operation, the main specific features of which are as follows: ∙most of the safeguard systems have four redundant “trains”; ∙the electrical power supply sources are independent of each other: main electrical power supply, auxiliary electrical power supply, four main back-up generator sets and two ultimate back-up generator sets; ∙the IRWST (In Containment Refueling Water System Tank) is a tank located inside the containment, containing a large quantity of borated water which can be injected into the primary system in the event of an accident; ∙to mitigate the consequences of a core melt which could lead to rupture of the vessel and molten materials leaking from it, a very thick concrete recovery device designed to collect the molten fuel and cool it, is located under the reactor vessel; ∙the fuel storage pool is cooled by two redundant cooling systems supplemented by a third diversified system. 2 Monitoring the nuclear safety of the reactors in operation 2.1 Fuel 2.1.1 Fuel in the reactor The leaktightness of the cladding of the fuel rods, tens of thousands of which are present in each core and which constitute the first containment barrier, receives particularly close attention. In normal operation, leaktightness is monitored by EDF through permanent measurement of the activity of the radionuclides contained in the primary system. Any significant increase in this activity is a sign of a loss of leaktightness in the fuel assemblies. If the activity of the primary system exceeds a predetermined threshold, the General Operating Rules (RGEs) require shutdown of the reactor before the end of its normal cycle. At each outage, EDF is required to search for and identify the assemblies containing leaking rods: reloading of fuel assemblies containing leaking rods is not authorised. EDF conducts examinations of leaking rods in order to determine the origin of the failures and prevent them from reoccurring. The preventive and corrective measures may concern the design of the rods and assemblies, their manufacture or the reactor operating conditions. The conditions of fuel assembly handling, of core loading and unloading, as well as prevention of the presence of foreign objects in the systems and pools are also covered by operating specifications, in order to prevent the risks of fuel rods leaking. 2.1.2 Assessment of the condition of the fuel in the reactor In 2024, there were fuel leaktightness defects in six reactors. Despite a rise in this indicator, ASN considers that management of the integrity of the first barrier, that is the fuel rod cladding, was on the whole satisfactory for all the NPPs. ASN remains attentive to the investigations carried out by EDF on the fuel assemblies concerned, in order to determine the origin of these defects and identify the necessary corrective measures. With regard to handling the obsolescence of the sipping test container used by EDF on its sites to detect fuel assembly tightness defects, it in 2024 presented a mobile unit project, the performance of which is currently being tested on a mock-up. 2.2 Nuclear pressure equipment 2.2.1 Design and manufacturing of Nuclear Pressure Equipment Nuclear Pressure Equipment (NPE) is Pressure Equipment (PE) that confines radioactive fluids. Some of this equipment is essential for reactor safety. The primary and secondary systems mentioned in point 1.4 of this chapter in particular consist of NPE. In the light of the potential risks from the failure of this equipment, both for reactor safety and the safety of the personnel operating them, there are regulations on their design, manufacture and operation, requiring that the manufacturer comply with essential safety requirements designed to preclude their failure. These requirements for example concern the strength of the materials used, the design calculations to be performed, or the in-service inspectibility of the equipment. The regulations also state that an NPS manufacturer shall take account of the particular requirements of the reactor’s safety case, which are liable to have an impact on the design of the equipment. ASN Report on the state of nuclear safety and radiation protection in France in 2024 301 01 The EDF Nuclear Power Plants 10 02 03 04 05 06 07 08 09 11 12 13 14 15 AP
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