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 (Japan) 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” Dieselgenerator Set (DUS) for each reactor. 2. Oversight of nuclear safety of the reactors in operation The year 2022 was marked by the prolonged outage of a significant number of reactors, notably as a result of stress corrosion being discovered on certain lines connected to the primary system (see “Notable events” in the introduction to this report). These unusual prolonged outages had an impact on the regulation and the assessment of the safety of the reactors, as certain topics could not be inspected in the same way on the reactors shut down and those in operation. The trends and developments highlighted in this chapter take account of this context. 2.1 Fuel 2.1.1 Fuel and its management 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 and its management in the reactor In 2022, all the NPPs satisfactorily managed the integrity of the first barrier, that is the fuel rod cladding. The number of reactors with fuel leakage faults was lower than in 2021. This improvement is notably the result of the gradual incorporation of fuel assemblies fabricated by Framatome, for which the mixing grid springs have been heat treated, thereby increasing their strength. The technical discussions on the subject of the generalised corrosion of certain M5 alloy fuel claddings detected in February 2021, enabled the operational compensatory measures defined by EDF and implemented on the 900 and 1,300 MWe reactors, to be lifted. An investigation is currently under way to determine whether or not to lift some or all of these measures for the 1,450 MWe reactors. In addition, since 2022, in order to control the corrosion risk, the iron content of the M5 alloy fabrication specification has been increased. The industrial manufacturing capability with increased iron contents was inspected by ASN, which made no subsequent requests. Finally, the production difficulties encountered in the Melox plant once again led EDF in 2022 repeatedly to refuel with fewer MOX assemblies than usual for its 900 MWe reactors. For the same reactor, ASN thus authorised EDF to repeat refuelling without fresh MOX fuel or to use a series of atypical refuelling loads consecutively. In 2022, under EDF surveillance, Orano qualified a MOX fuel fabrication process which should eventually restore a level of production quality enabling standard MOX fuel refuelling loads to be obtained. 290 ASN Report on the state of nuclear safety and radiation protection in France in 2022 • 10 • The EDF Nuclear Power Plants 10
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