The implementation of the in-service surveillance programmes on these systems, 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 test means for which performance has been qualified, and that it carries out reactive inspections, which may not necessarily be qualified, to obtain greater knowledge of particular risks. 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 phase, 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 the mechanical strength of these flaws. 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. It is currently examining the justifications provided by EDF for the pressure vessels of the 1,300 MWe reactors. 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 2023, the results of the checks carried out were satisfactory. Cast elbow assemblies The files produced by EDF to justify maintaining the cast elbow assemblies and nozzles of the primary system in service beyond the fourth ten-yearly outage of the 900 MWe and 1,300 MWe reactors were reviewed by ASN. In order to support its position regarding this approach, ASN convened the GPESPN Advisory Committee on two occasions. The review concluded that virtually all the elbow assemblies can be kept in service at least until the fifth ten-yearly outage. It focused in particular on the case of a few elbow assemblies that would be hard to replace. Only the case of one elbow assembly on Paluel NPP reactor 2 needs to be examined further. EDF must propose a strategy to justify continued operation of this elbow assembly. Following this analysis, ASN asked EDF to continue with its investigations regarding the possibility of removing, repairing and performing non-destructive test on the most sensitive elbow assemblies, in order to define a strategy sufficiently well in advance of the fifth ten-yearly outages for the elbow assemblies which could not be justified for operation beyond that date. The Steam Generators For ASN, the situation of the SGs remains a point for particular attention in 2023. The significant fouling 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 should now be considered a priority. The monitoring strategy for the secondary part of the SGs deployed by EDF was revised in mid2020 to better prevent these situations. In 2023, EDF continued to deploy the action plan defined following the discovery of SC cracks at the end of 2021. EDF continued to mobilise significant resources to identify the causes and carry out checks on the reactors and repair the lines affected. EDF thus continued to implement the in-depth inspection and appraisal programme which had been started in 2022 on the different types of reactors. Part of this programme is devoted to welds which were repaired during manufacturing and which are liable to be subject to greater risks. 301 welds were therefore inspected in 2023, and 170 were repaired. Inspection of these welds revealed four large cracksxwhich could have led to a risk of leaks in the event of high mechanical loading. In parallel with this inspection programme, the lines felt to be most susceptible to the SC phenomenon were replaced. Thus, the cold leg safety injection lines on the P’4 type reactors underwent this work in 2023. In 2023, the inspection programme also revealed the presence of a few thermal fatigue cracks on the same types of lines. EDF drew up an investigation programme in order to improve the understanding of the SC phenomenon itself, but also the competition between this phenomenon and thermal fatigue. The inspection programme is compatible with the detection of the two phenomena. ASN continued its monitoring of the action taken by EDF. By the end of 2023, it had carried out more than 65 inspections devoted to this issue since the end of 2021. These inspections notably took place as part of the lines verification or replacement operations in the EDF engineering departments, in the NPPs and at its subcontractors. ASN also continued its discussions with its foreign counterparts, some of whom carried out inspections on this same subject. By the end of 2025, EDF will have checked the RIS and RRA systems lines on all of its reactors. In 2024, it will expand its inspections to the repaired welds on the other lines connected to the MPS. With the technical support of the French Institute for Radiation Protection and Nuclear Safety (IRSN), ASN will remain focused on this issue in 2024 and will closely monitor the results of the checks carried out by EDF. It will examine any EDF strategy changes that could result from this. The latest information on the subject is available on asn.fr. RESULTS OF EDF ANALYSES AND REPAIRS AND ASN IN-DEPTH INSPECTIONS SINCE THE DISCOVERY OF STRESS CORROSION CRACKS ASN Report on the state of nuclear safety and radiation protection in France in 2023 297 • 10 • The EDF Nuclear Power Plants 10 05 15 08 11 04 14 06 07 13 AP 03 02 09 12 01
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