is based on particularly stringent design, manufacturing and in-service inspection provisions in order to guarantee its strength throughout the life of the reactor, including in the event of an accident. ∙During operation, the vessel’s metal slowly becomes brittle, under the effect of the neutrons from the fission reactions in the core. This embrittlement more particularly makes the vessel more susceptible to thermal shocks under pressure, or to sudden pressure rises when cold. This susceptibility is also aggravated by the presence of technological flaws, which is the case for some vessels with manufacturing defects under their stainless steel liner. ∙The vessel is a component for which replacement is not envisaged, owing to both technical feasibility and cost. Cast components of the main primary system The MPS of some of the reactors of the French fleet comprises several austenitic- ferritic stainless steel cast elbow and nozzle assemblies. The ferritic phase of this steel experiences ageing under the effect of temperature. Certain alloy elements present in the material, molybdenum in particular, aggravate this ageing sensitivity, notably on the 900 MWe reactors and the first 1,300 MWe reactors. The result is a deterioration of certain mechanical properties, such as toughness and resistance to ductile tearing(1). The elbows also comprise flaws inherent in the static casting manufacturing method. The effects of thermal ageing lessen the fast fracture resistance margins in the presence of defects. EDF has carried out extensive work to learn more about these materials, their ageing kinetics and to assess the fast fracture margins. These elbows are attentively monitored by ASN, insofar as the continued operation of these components of the main primary system must be justified on the occasion of each periodic safety review, while taking account of this ageing phenomenon. There are particular issues with regard to some cast elbows on the 900 and 1,300 MWe reactors, which are directly connected to the reactor vessel, because they are very hard to replace. The steam generators The SGs comprise two parts, one of which is a part of the MPS and the other a part of the MSS. The integrity of the main components of the SGs is monitored, more specifically the tubes making up the tube bundle. This is because any damage to the tube bundle (corrosion, wear, cracking, etc.) can lead to a primary system leak to the 1. Tearing of a material under the effect of stresses applied to it, following deformation. secondary system. In addition, rupture of one of the tubes in the bundle would lead to bypassing of the reactor containment, which is the third containment barrier. The SGs are the subject of a specific in- service monitoring programme, defined and periodically revised by EDF and examined by ASN. Following the inspections, those tubes which are too badly damaged are plugged, to remove them from service. Over time, the SGs tend to become clogged with corrosion products from the secondary system exchangers. The layer of deposits of corrosion products (fouling) that forms on the tubes reduces the heat exchange capacity. On the tube support plates, the deposits (clogging) prevent the free circulation of the water-steam mixture, which creates a risk of damage to the tubes and the internal structures and which can degrade the overall operation of the SG. Various solutions can be implemented to limit metal deposits in the secondary system: cleaning with hydraulic jets, preventive chemical cleaning, replacement of certain materials used to manufacture the tube bundles (brass replaced by more corrosion-resistant stainless steel or titanium alloy), modification of chemical products used for conditioning systems and pH increase. Some of these operations must be authorised by ASN beforehand, because they imply discharges of some of the dangerous substances. Since the 1990s, EDF has been running a programme to replace the SGs with the most severely degraded tube bundles. The SG replacement campaign concerning 26 reactors with a non-heat-treated Inconel 600 tube bundle has now been completed. It is continuing with replacement of SGs on the reactors in which the tube bundle is made of heat-treated Inconel 600 (21 reactors are still concerned). 2.2.4 Assessment of operation of Nuclear Pressure Equipment Monitoring the operation of reactor main primary and secondary systems ASN considers that EDF’s surveillance of the operation of the MPS and MSS remains a point warranting particular attention. In 2024, EDF more particularly continued its inspections defined after detection of a stress corrosion phenomenon on the auxiliary piping of the primary systems (see box next page). This phenomenon illustrates the possibility that unexpected degradation modes can appear on these systems, including in the absence of French or international OEF. Through a programme of additional investigations on these systems, on the occasion of each periodic safety review, EDF must schedule appropriate checks to take account of the risk of any unexpected degradation phenomena, in addition to the routine monitoring programmes. The implementation of the in-service surveillance programmes on the MPS and MSS, along with their adaptation to take account of changing OEF and knowledge of degradation modes, are thus the subject of particular attention by ASN. In this respect, ASN is attentive to ensuring that EDF uses appropriate non-destructive inspection means for which performance has been qualified. EDF must also be able to reactively deploy checks to ensure that there are no particular risks, for example if a new degradation mode is discovered or suspected. The reactor pressure vessels During the periodic safety reviews, ASN examines the demonstration of the in- service strength of the reactor pressure vessels every ten years. The generic approach implemented by EDF is to use a worst-case approach to verify that all the vessels of a type of reactor offer sufficient resistance to fast fracture, taking account of the loads to which they are subjected in operation (whether during routine, incident or accident operating situations) and their embrittlement under irradiation. During this examination, account is taken of the mechanical properties of each vessel and the presence of a hypothetical flaw positioned in the worst possible place. For vessels with particular flaws, EDF also checks their mechanical strength. Following its review, ASN reached a favourable conclusion regarding the ability of the 900 MWe reactor vessels to continue to function until their fifth ten-yearly outage. In 2024, ASN examined the justifications provided by EDF for the 1,300 MWe reactor vessels and the Authority for Nuclear Safety and Radiation Protection (ASNR) will issue a position statement in 2025. During the ten-yearly outage inspection of each reactor, EDF also carries out checks to ensure that the existing flaws have not developed further, or that prejudicial flaws have not appeared in the steel of the vessels. It also carries out a hydraulic pressure test of the primary system. ASN issues reports following the checks carried out during each ten-yearly outage inspection of the primary system, and in particular with regard to the pressure vessels. In 2024, the results of the checks carried out were satisfactory. ASN Report on the state of nuclear safety and radiation protection in France in 2024 303 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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