Corrosion under stress: EDF, ASN, and IRSN present initial feedback to parliamentarians

EDF discovered an unexpected anomaly in some reactors a year ago. In record time, thanks to a massive technical mobilization, the experts were able to identify the phenomenon of stress corrosion, define its origin, propose a means of repair and develop an appropriate non-destructive testing tool. In front of the parliamentarians, a first complete assessment was made of this event.

On Thursday, October 27, the Parliamentary Office for the Evaluation of Scientific and Technological Choices (OPECST) heard from EDF, ASN, IRSN, and associations representatives. They presented initial feedback on this unexpected phenomenon that occurred in the fleet’s primary circuit of several reactors. Cédric Lewandowski, EDF’s director in charge of the nuclear and thermal power plants, began by recalling the facts.

A year to understand

During the ten-yearly inspection of the Civaux 1 reactor, “an abnormal and atypical signal” was detected on the safety injection circuit (RIS), directly connected to the primary circuit. The ultrasonic inspection operators were looking for thermal fatigue but noticed a different indication. After cutting the pipe section, the metallurgical expertise in a “hot laboratory” revealed stress corrosion cracking (SCC). This phenomenon was complicated to detect with the non-destructive testing methods available at the time. Cédric Lewandowski describes this first defect as: “a crack 5 to 6 mm deep around the circumference of the pipe, near a weld”. The cracks detected later will be much smaller. Such a phenomenon of SCC of this type of material (stainless steel) is rare in PWRs and was not expected in this area. It had been observed, for example, once in the 1980s on a reactor at Bugey, but it was then linked to a chemical pollution phenomenon.

EDF discovered that it was a generic problem on N4 and P’4 reactors. In the end, 12 reactors were shut down in the winter of 2021-2022, and portions of their piping were dismantled for expertise (the non-destructive testing methods available did not allow this type of defect to be characterized). To date, 115 welds and 230 samples have been assessed. It has been shown that “900MW and P4 reactors are little or very little sense to the CCS phenomenon”. On the other hand, “P’4 or N4 reactors are sensitive to CCS,” explains Cédric Lewandowski.

A phenomenon linked to reactor design

CCS is a progressive cracking based on the interaction between a material (in this case, stainless steel), water, and mechanical stress, explains EDF. The stainless steel in question is not usually sensitive to it unless it has undergone work hardening (deformation that increases its hardness) during manufacture, in this case, welding. The mechanical stresses, for their part, would come from a phenomenon of thermal stratification in these pipes connected to the primary circuit (“dead arms”). EDF considers that “the geometry of the RIS lines is the main factor explaining the CCS.” Once again, all the participants exclude any link with aging.

In the end, 16 reactors (the 12 P’4 reactors and the four N4) were defined as priorities by EDF. The ASN validated the monitoring program and repair method proposed by the operator during the summer. Currently, 10 sites have been opened. Six have been completed. One is expected to be finished in November, and the other three before the end of the year. The entire fleet will have to be checked by the end of 2024, including the 900 MWe reactors that are not very sensitive to the phenomenon.

During this period, EDF teams have also developed an inspection method capable of detecting and characterizing this SCC without systematically removing the pipes. Cédric Lewandowski explains: “We had to develop a new ultrasonic inspection technique that is a real feat. It is not yet perfect, but we hope to be able to take full advantage of it next year,”.

Learning for safety

According to Bernard Doroszczuk, president of the French Nuclear Safety Authority (ASN), “CCS cracking is a serious subject that has been treated seriously by EDF. He assures that in case of a breach, we would be in an accident situation with a lasting loss of the reactor and limited external releases. The IRSN, through the voice of its president Jean-Christophe Niel, adds that EDF’s calculations on the performance of the reactor are valid: the loss of two RIS lines would not present a risk of reactor meltdown. The president of the ASN emphasized the difficulty of dealing with this subject because the EDF operator had no feedback. Bernard Doroszczuk does not hesitate to say that for six months, “we were blind.”

The ASN draws two significant lessons from this event, whose understanding is still “evolving.” On the one hand, it calls for developing “the ability of operators to see things that are not expected.” On the other hand, it again insists on the need to have a margin of production capacity in France to cope with a simultaneous shutdown of several reactors in the event of a generic anomaly.

No trade-off with safety

On the one hand, some accused them of being overzealous in shutting down 12 reactors very quickly, while others accused them of not doing enough. EDF categorically states that “there is no trade-off between reactor safety and security of electricity supply. The ASN supports this position.

EDF has consulted international experts, and an alternative solution to dismantling and replacing sections has been discussed. Cédric Lewandowski said that American experts have recommended using “overlay,” which can be translated into French as “manchonnage.” This “manchonnage” involves covering the portions of the pipes affected by CCS with a second layer of steel. This simple and rapid solution cannot be applied in France, as it is not qualified. It is a procedure that would have required months or years of investigation. In the end, the strategy chosen by EDF was the quickest way to restore the availability of the reactors.

Labour for repairs

Restoring reactors whose circuit sections had been dismantled required EDF to call on foreign welders and suppliers. This requirement led to criticism of the resources of the industry. From the point of view of pipes supply, Cédric Lewandowski explains that EDF has called on Italian smelters and blacksmiths who are already working with the electric utility in the major refurbishment program. However, he said that it took six months to supply the equipment and that he regretted that no French supplier was able to meet EDF’s needs.

Regarding human resources, EDF explains that multiple personnel, controllers, welders, and pipe fitters, are needed because the operations are carried out in a dosing environment. The company called on its partners: Monteiro, Onet, Endel, Framatome, and Westinghouse. He confirms that it was necessary to bring in employees from abroad, but Cédric Lewandowski puts this into perspective by assuring that it represents only 10% of the workforce mobilized. ■

By: Ludovic Dupin

Photo : OPECST Twitter