LES ACTIVITÉS NUCLÉAIRES : RAYONNEMENTS IONISANTS ET RISQUES POUR LA SANTÉ ET L’ENVIRONNEMENT 1.9 The “hardened safety core” improving resistance to extreme events After the accident at the Fukushima Daiichi NPP, the Authority for Nuclear Safety (ASN) instructed EDF to deploy on each reactor a “hardened safety core” of robust material and organisational measures designed, for the extreme situations that were part of the 2011 stress tests (station blackout, total loss of the heat sink, extreme external events), 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. These provisions are being deployed at each of the sites as part of the periodic safety reviews. In addition, to manage emergency situations, each site shall be equipped with a new local Emergency Response Centre capable of withstanding extreme external hazards. 1.10 The specific features of the Flamanville EPR reactor The Flamanville EPR reactor is a 1,600 MWe third-generation reactor. It has four cooling loops and uses UO2 fuel. Its containment consists of two concrete walls and a metal liner (see diagram previous page) covering the entire internal face of the inner wall. A reinforced concrete “shell” provides a protective cover for the most sensitive buildings: the reactor building, the fuel storage building, the control room and two of the four Safeguard Auxiliary 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”. However, for some systems, each train provides only 50% of the safety function. ∙The electrical power supply sources are independent of each other: main electric power supply, auxiliary electrical power supply, four main emergency generators and two ultimate back-up emergency generators. ∙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 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. During each outage, EDF must look for and identify the assemblies containing leaking rods: the reloading of fuel assemblies containing leaking rods is not permitted. 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 ASNR considers that management of the integrity of the first barrier, that is the fuel rod cladding, was on the whole satisfactory for the majority of the NPPs. In 2025, there were fuel leaktightness defects in three reactors. This low figure confirms the effectiveness of the measures taken in recent years to improve fuel products. The assemblies concerned will be examined to determine the causes. However, management of the risk of foreign material entering the primary circuit and damaging the fuel was not satisfactory in 2025. Foreign objects (waste, tools, various objects) were found in the reactor vessel, a steam generator or the spent fuel pool of several reactors. ASNR will continue to monitor the actions taken by EDF to rectify this situation. 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. In particular, the primary and secondary systems mentioned in point 1.4 of this chapter are formed by the assembly of an NPE: reactor vessel, primary and secondary main piping, steam generators, etc. 286 ASNR Report on the state of nuclear safety and radiation protection in France in 2025
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