As part of the preparation for the fourth periodic safety reviews of the 900 MWe reactors (see point 2.9.3 and “Notable Events” in the introduction to this report), EDF sent ASN a dossier in 2017 substantiating the in-service strength of these reactors up until their fifth periodic safety review. The generic approach adopted by EDF consists in conservatively considering the mechanical properties of the vessel experiencing the worst-case irradiation embrittlement for the 900 MWe reactors. EDF carried out fast fracture resistance studies taking account of the changes in the properties of the materials and will carry out inspections to check there are no prejudicial defects in the steel during the ten-yearly outage of each reactor. This generic approach was submitted to the Advisory Committee for Nuclear Pressure Equipment (GPESPN) for its opinion on 20 November 2018, 15 October 2019 and 8 September 2020. The examination concerned the defects analysed, the esti– mated irradiation ageing of the metal of the vessel, the thermo– mechanical analyses and the studies assessing the margin with respect to fast fracture of the vessels, the classification of small primary break transients and justification of the level of residual stresses in the circumferential welds of the core shells. The studies carried out and the additional information provided at the request of GPESPN lead to a favourable conclusion regarding the ability of the reactor pressure vessels to function for a further ten years, subject to the result of the examinations performed on the occasion of the fourth ten-yearly outages of the reactors concerned. Cast elbow assemblies The dossier produced by EDF was examined by ASN with production of an opinion from the GPESPN on 23 May 2019. Following this analysis, ASN sent EDF requests for additional substantiation of the predicted behaviour of the aged material, identification of the flaws present in the cast elbow assemblies, analysis of the fast fracture margins and in-service monitoring of these components. In 2020, EDF provided substantiating documents for certain types of elbow assemblies and the replacement strategy envisaged for others. The situation of certain elbow assemblies it would be hard to replace has led to technical developments in the fields of non-destructive testing and thermal regeneration. Nickel-based alloy zones In 2018, EDF updated its analysis of the nickel-based alloy zones by reviewing the design, evaluating the risk of initiation of stress corrosion, analysing national and international OEF, reviewing mechanical analyses and safety studies, listing available repair and inspection procedures, and updating its maintenance strategy. This dossier was examined jointly by ASN and IRSN and then presented to the GPESPN during its session of 26 November 2020. The update work carried out by EDF is satisfactory. However, EDF must provide greater guarantees regarding the ability of the non-destructive examinations to detect any damage early on, in particular for the vessel bottom head penetrations. The Steam Generators For ASN, the situation of the second containment barrier remained a point for particular attention in 2021. The significant fouling levels observed in certain SGs, liable to impair their operating reliability, has led to scheduling of a preventive cleaning programme in 2022 and in the subsequent years. Maintenance, in order to guarantee a satisfactory level of cleaness, has been insufficient and must be a priority. The monitoring strategy for the secondary part of the SGs deployed by EDF was revised in mid-2020 to better prevent these situations. ASN notes that the SG replacement operations were able to resume after a year with no work of this type. This work will continue at the rate of one intervention per year in the coming years. The regular perforation of SG tubes, which is the subject of a multi-year tube inspection and plugging strategy by EDF, and the detection of a boiler effect in a “thimble” tube of an SG on Nogent-sur-Seine reactor 1 –repaired in 2021– illustrate the risk of further deterioration associated with the ageing of the installations and confirms the need to adapt the level of in-service monitoring and forward planning for development of repair processes. 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 Order of 7 February 2012, setting the general rules concerning BNIs. Other equipment takes part in the containment function, such as the points of access to the interior of the containment (airlocks and equipment hatch), the circuit depressurising the annulus between the double-wall containments or the control room ventilation system. Since 2016, EDF has also been carrying out an action plan with the aim of guaranteeing that the flowrates in the ventilation systems meet the safety requirements both for the containment and for thermal conditioning of the installations, in the light of the changes made to the reactors since they were built. The action plan is being deployed, reactor by reactor, on all the ventilation systems concerned, and includes an inventory of the condition of the equipment and ducts. As necessary, EDF carries out repairs and improvements and adjusts the ventilation flow rates. 2.3.2 Assessment of the containments Overall management of the containment function ASN observes occasional but recurring unavailabilities affecting certain equipment participating in the containment function. These unavailabilities, already identified in 2020, were the subject of discussions with EDF, which will continue in 2022 in order to verify the pertinence of the measures envisaged by EDF to mitigate these unavailabilities. THE PRINCIPLES OF THE REACTOR VESSELS IN-SERVICE STRENGTH DEMONSTRATION The regulations in force require in particular that the licensee: ཛྷ identify the operating situations with an impact on the vessel; ཛྷ take measures to understand the effect of ageing on the properties of the materials; ཛྷ deploy resources to enable it to ensure sufficiently early detection of defects prejudicial to the integrity of the structure; ཛྷ eliminate all cracks detected or, if this is impossible, provide appropriate specific justification for retaining such a type of defect as-is. 288 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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