1.8 The other systems important for safety The other main systems important for safety and required for reactor operation are: ∙ the Component Cooling System (RRI) which cools a certain number of nuclear equipment items. This system functions in a closed loop between the auxiliary and safeguard systems on the one hand, and the systems carrying water from the river or sea (heatsink) on the other; ∙ the Essential Service water System (SEC) which cools the RRI system with water from the river or sea (heatsink). This is a backup system comprising two redundant lines. In certain situations, each of its lines is capable of removing heat from the reactor to the heatsink; ∙ the Reactor Cavity and Spent Fuel Pit Cooling and Treatment System (PTR), which in particular removes residual heat from the fuel elements stored in the fuel building pool. The design of the ultimate water source installed in the wake of the Fukushima Daiichi NPP accident, can also –in an extreme situation– inject water into the fuel building pool, if the PTR system and the water make-up systems are lost; ∙ the ventilation systems, which ensure containment of radioactive materials by creating negative pressure in the rooms and by filtering discharges; ∙ the fire-fighting water systems; ∙ the I&C system, which processes the information received from all the sensors in the NPP. It uses transmission networks and sends orders to the actuators from the control room, through the programmable logic controllers or operator actions. Its main role with regard to reactor safety is to monitor reactivity, control the removal of residual heat to the heatsink and take part in the containment of radioactive substances; ∙ the electrical systems, which comprise sources and electricity distribution. The French NPPs have two external electrical sources: the step-down transformer and the auxiliary transformer. These two external sources are supplemented by two internal electrical sources: the backup diesel generators. In the event of total loss of these external and internal sources, each reactor has another electricity generating set comprising a turbine 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. 2 // Oversight of nuclear safety of the reactors in operation 2.1 Fuel 2.1.1 Fuel and its management in the reactor The leaktightness of the cladding of the fuel rods, tens of thou– sands 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 exam– inations 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 and its management in the reactor EDF’s management of the integrity of the first barrier, that is the fuel rod cladding, was on the whole satisfactory for all the NPPs. ASN notes progress in implementation of the approach to prevent the risk of foreign objects entering the primary system, which could then damage the first containment barrier. This progress however differs from one site to another. In 2021, cladding defects were found on seven reactors. This number is similar to the previous year. ASN will remain 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. As in 2020, few events were reported during fuel handling operations. With regard to the fabrication of fuel pellets, the MOX anomalies encountered in 2017 and 2019 (random presence of large-sized plutonium enriched islands in certain fuel pellets) notably led EDF to implement compensatory measures in the loading plans. CORROSION OF FUEL ASSEMBLIES WITH “M5” CLADDING In February 2021, when unloading fuel from the Chooz B NPP reactor 2, EDF detected traces of white corrosion on several fuel assemblies. This corrosion caused the spalling of several fuel rod claddings, but they were not perforated. EDF subsequently observed the same corrosion phenomenon on other reactors. This only concerns fuel assemblies fabricated by Framatome with cladding made of “M5” alloy. The analyses carried out revealed several parameters liable to explain this phenomenon, notably the iron content of the cladding and the operating power of the reactor. EDF has revised its iron content requirements for the M5 material. Pending the deployment of this modification on all its reactors, EDF can continue to use fuel assemblies with a low iron content, provided that it can be shown that they will not undergo spalling during irradiation and that proportional compensatory measures are taken (power reduction, special core or reactor operating procedures). This strategy led EDF to exclude from the core several fuel assemblies for which the iron content in the M5 alloy was too low. The analyses aiming for improved characterisation of the phenomenon will continue in 2022. 284 ASN Report on the state of nuclear safety and radiation protection in France in 2021 10 – THE EDF NUCLEAR POWER PLANTS
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