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. On this point, EDF transmitted technical data in response to this request and in particular began to develop a new non-destructive examination which should be in use starting in 2026. The Steam Generators For ASN, the SGs remains a point warranting particular attention in 2022. The significant fouling levels observed in certain SGs, liable to impair their operating safety, has led to scheduling of a preventive cleaning programme in 2023 and in the subsequent years. Maintenance in order to guarantee a satisfactory level of cleanness has been insufficient in the past 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. SG replacement operations are scheduled at the rate of one reactor per year over the coming years, starting in 2024. The regular perforation of SG tubes, which are the subject of a multi-year inspection and plugging strategy by EDF, confirms the need to adapt the degree of stringency of the in-service monitoring. In addition, the adoption of repairs in a “thimble” tube of an SG of Nogent-sur-Seine reactor 1, following the detection of a boiler effect, illustrates the need for forward planning of the development of repair processes. Main Primary System auxiliary lines Numerous stress corrosion cracks have been discovered, 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. They led to a very large number of destructive assessments and repairs. An inspection and repair programme is scheduled for the coming years (see “Notable events” in the introduction to this report). 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 Basic Nuclear Installations – BNIs (BNI Order). 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 2014, 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 EDF’s management of the containment function is on the whole relatively satisfactory. ASN however observes occasional but recurring unavailabilities affecting certain equipment participating in the containment function. These unavailabilities notably concern the containment’s leaktightness and monitoring system and the control room ventilation system. These unavailabilities were discussed with EDF in 2022 in order to identify the root causes. These discussions will continue in 2023 in order to verify the pertinence of the measures envisaged by EDF to mitigate these unavailabilities. EDF launched a national action plan in 2014 to ensure that the ventilation flowrates are compliant with the required safety flowrates, and to make appropriate modifications if necessary. The final phase of this national action plan includes a programme to ensure the lasting nature of the adjustments made. An examination of the pertinence of this programme by ASN is ongoing and will lead to a position statement being issued. Single wall containments with an internal metal sealing liner The ten-yearly tests on the 900 MWe reactor containments carried out since 2019 as part of their fourth ten-yearly outages did not bring to light any generic problems liable to compromise their operation. The leaktightness of the Bugey NPP reactor 5 containment was however the subject of particular attention. The containment of this reactor had to be repaired, following damage to the tightness of its metal liner at the lower part of the reactor building, observed in 2015. This containment was tested in 2021 and the results were satisfactory. Double-wall containments The tests on the double-wall containments performed during the first ten-yearly outages of the 1,300 MWe reactors detected a rise in the leak rate from the inner wall of some of them, under the combined effect of concrete deformation and a loss of prestressing of certain tendons, that was greater than anticipated at the design stage. EDF then initiated major work consisting in locally applying a resin sealing coating to the interior and exterior surfaces of the inner wall of the containments of the most severely affected 1,300 MWe reactors, as well as to the 1,450 MWe reactors. For all the reactors on which it was carried out, this work enabled the leak rate criteria to be met during the containment pressure tests. ASN remains vigilant with regard to changes in the leaktightness of these containments and to maintaining the long-term effectiveness of the coatings. 294 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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