Cast products ASN is examining the justification for maintaining the cast products of the primary system in service beyond the fourth ten-yearly outage of the 900 MWe reactors. With regard to the cast elbow assemblies on the MPS, and at the request of ASN, the Institute for Radiation Protection and Nuclear Safety (IRSN) assessed the justifications provided by EDF for the most sensitive elbow assemblies (C elbows, which are directly connected to the reactor vessel, certain E elbows that are hard to replace because they are situated in the reactor pit). ASN convened the Advisory Committee of Experts for Nuclear Pressure Equipment (GPESPN) three times on this subject (2016, 2019 and 2023). The methodological principles followed by EDF in its mechanical analyses to justify maintaining these components in service were thus validated by ASN. This examination concluded that virtually all the elbow assemblies (A, B, C, D and E) on the 900 MWe reactors can be kept in service at least until the fifth ten-yearly outage. ASN is at present focusing its review on certain E elbows in the Paluel NPP reactors, for which it is awaiting EDF’s submission of a dossier justifying continued operation beyond the forthcoming ten-yearly outages. To justify maintaining cast elbow assemblies in service beyond the fifth ten-yearly outage, EDF is continuing its justification studies in several areas. It will in particular suggest to ASN that more advanced calculation methods be used to take account of the nature of the defects present in the elbow assemblies. EDF is also developing a new means of non-destructive examination of the inner wall of the E elbow assemblies, as well as a technique for repair of defects on the inner wall, which could be used if EDF is not able to provide justification for maintaining them in service. ASN considers this work to be necessary, as it should allow EDF to define a treatment strategy sufficiently well in advance of the ten-yearly outages. Certain cast branch connections on the primary legs (RIS system nozzles on the cold leg) are for the same reasons also sensitive to thermal ageing phenomena and their potential for being maintained in service was analysed. The methodology was assessed by IRSN (for the 900 and 1,300 MWe reactors). The low toughness of certain nozzles on Paluel NPP reactors 1 and 2, as a result of ageing, led EDF to propose their replacement, with the timeframe currently being examined by ASN. The Steam Generators For ASN, the SGs remain a point meriting particular attention in 2024. The significant fouling and clogging levels detected in certain SGs, liable to alter their operating safety, entails the scheduling of preventive cleaning. Maintenance in order to guarantee satisfactory cleanness has been insufficient in the past and ASN now considers this to be a point warranting particular vigilance. The maintenance strategy regarding clogging and fouling of the secondary part of the SGs deployed by EDF was revised in 2020 to better prevent these situations. ASN is also paying particularly close attention to monitoring the deterioration of the SG feedwater rings on the 1,300 and 1,450 MWe reactors. SG replacement operations are scheduled at the rate of one reactor per year over the coming years. Main Primary System auxiliary lines Numerous stress corrosion or thermal fatigue cracks have been discovered since 2021, in particular on the SIS and RRA lines of the 1,450 MWe and 1,300 MWe type P’4 reactors, in the immediate vicinity of certain welds. This led to numerous checks and repairs being carried out. The investigations will continue in 2025 and 2026, notably on other systems (see box above). 2.3 The containments 2.3.1 The containments The containments, which constitute the third containment barrier, undergo inspection and testing to check their compliance with the safety requirements. More specifically, their mechanical behaviour must guarantee good tightness of the reactor building if the pressure inside it were to exceed atmospheric pressure, which can happen in certain types of accidents. This is why, at the end of construction and then during the ten-yearly outages, these tests include an inner containment pressure rise with leak rate measurement. These tests are required by the creation authorisation decrees of each reactor and by the Order of 7 February 2012, setting the general rules concerning Basic Nuclear Installations (BNIs). Review of auxiliary lines stress corrosion F ollowing the discovery of stress corrosion cracks at the end of 2021 on auxiliary lines of the primary system of some of its reactors, EDF set up a programme to inspect more than 1,200 welds on the RIS and RRA systems. In 2024, EDF continued with the deployment of significant inspection and analysis resources to identify the causes of this degradation. In 2024, EDF completed the scheduled preventive replacements for the most sensitive lines. Five other weld repairs were also made elsewhere, following the discovery of new faults. The inspections carried out since the end of 2021 have so far revealed the presence of 80 cracks more than 2 mm deep on these pipes, which confirms the serious nature of this phenomenon which was not hitherto considered liable to affect these lines. The inspection strategy focused on the welds which had been repaired during the manufacturing process, which are more susceptible to stress corrosion. Inspection of these welds revealed three large cracks which could have led to a loss of integrity of the pipe in the event of high mechanical loading. In addition, the inspection campaign carried out since 2021 discovered ten or so fatigue cracks. This phenomenon was known to be liable to affect these lines, but some of these cracks were discovered on welds which were not subject to any particular monitoring. This should lead EDF to increase the robustness of its in-service monitoring strategy concerning the risk of fatigue cracking. EDF replaced all the lines considered to be the most susceptible to the stress corrosion phenomenon. By the end of the first quarter of 2025, all of the welds on the RIS and RRA lines repaired during manufacturing will have been inspected and, by the end of 2026, EDF will have inspected about 55% of the welds on the RIS and RRA lines liable to be concerned by this phenomenon. ASN considers that this level of inspection will provide a good overview of the stress corrosion susceptibility of these lines and complete the picture regarding the risk of fatigue cracking. EDF is also continuing with spot checks on welds of other stainless steel lines of the reactor primary systems, in order to check that they are not susceptible to stress corrosion. In 2024, ASN continued its monitoring of EDF’s action, by carrying out inspections in the engineering departments, in the NPPs and in the plants of EDF’s subcontractors, with respect to line inspection or replacement operations. ASN also continued its discussions with its foreign counterparts, some of whom carried out specific inspections. At this stage, ASN notes that the foreign PWRs do not seem to be concerned by a phenomenon on such a scale. ASNR will remain mobilised to deal with this issue in 2025, and will closely monitor the results of the inspections carried out by EDF. It will examine any EDF monitoring and inspection strategy changes that could result from this. For the latest information on the subject: asn.fr/l‑asn‑controle/ corrosion‑sous‑contrainte 304 ASN Report on the state of nuclear safety and radiation protection in France in 2024 The EDF Nuclear Power Plants
RkJQdWJsaXNoZXIy NjQ0NzU=