At the end of 2021, EDF informed ASN that it had discovered cracks resulting from a stress corrosion phenomenon on the lines of the Safety Injection System (SIS) of the main primary system of Civaux NPP reactor 1 (N4 type reactor), and then on that of Penly reactor 1 (P’4 type reactor). Although stress corrosion is a known phenomenon and one that had already occurred on other components of the French NPP fleet, this type of crack had not been expected on these lines. This is because they are made of stainless steel, which is considered to be resistant to this type of damage. By leading to cracking of the material on the inner wall of the lines, this phenomenon weakens them mechanically. It is thus liable to lead to rupture of the SIS or Residual Heat Removal System (RHRS) in the event of major mechanical stresses. As a result of this situation, EDF shut down its four N4 type reactors, which were considered to be the most susceptible, and brought forward the outage of several reactors in order to carry out inspections. These cracks are particularly hard to detect. In 2022, EDF developed a new inspection process, notably using ultrasounds, capable of measuring the depth of the cracks. During the first half of 2022, EDF initiated an in-depth programme of inspections and expert assessments(1) on the various types of reactors(2). This programme enabled it to identify the geometry of the lines and the thermomechanical stresses to which they are subjected as being the main factors liable to influence the appearance of stress corrosion and thus identify the reactors most susceptible to the phenomenon. EDF defined an inspection strategy, which was validated by ASN on 26 July 2022. Stress corrosion phenomenon affecting the French nuclear power reactors Civaux Blayais Golfech Tricastin Cruas-Meysse Saint-Alban Bugey Nogent-sur-Seine Cattenom Chooz Gravelines Penly Dampierre-en-Burly Belleville-sur-Loire Flamanville Paluel Chinon Saint-Laurent-des-Eaux 2 2 4 2 4 2 1 6 4 2 4 2 4 2 2 900 MWe Reactor capacity Number of reactors per site 1,300 MWe 1,450 MWe 1,600 MWe P4 P4 P’4 CPY CPY P’4 P’4 P’4 P4 2 P’4 CP0 CPY CPY 4 CPY 4 CPY 4 CPY N4 N4 X LOCATIONS OF NPP REACTORS IN FRANCE IN 2023 Civaux Blayais Golfech Tricastin Cruas-Meysse Saint-Alban Bugey Nogent-sur-Seine Cattenom Chooz Gravelines Penly Dampierre-en-Burly Belleville-sur-Loire Flamanville Paluel Chinon Saint-Laurent-des-Eaux 2 2 4 2 4 2 1 6 4 2 4 2 4 2 2 900 MWe Reactor capacity Number of reactors per site 1,300 MWe 1,450 MWe 1,600 MWe P4 P4 P’4 CPY CPY P’4 P’4 P’4 P4 2 P’4 CP0 CPY CPY 4 CPY 4 CPY 4 CPY N4 N4 X he presence of stress corrosion cracking was the major event encountered by the French Nuclear Power Plant (NPP) fleet in 2022. This unexpected event led EDF to mobilise significant resources to identify the causes and carry out repairs. ASN considers that EDF was reactive and responsible in response to this event with significant safety implications and affecting a large percentage of its reactors. This situation illustrates the electricity supply problems that could arise in the event of a generic problem concerning several reactors on the same time. As early as 2013, ASN had underlined the need for sufficient margins in the electrical system in order to deal with an event of this type. T 12 ABSTRACTS – ASN Report on the state of nuclear safety and radiation protection in France in 2022 Notable events 2022
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