Stress Corrosions: The ASN validates EDF’s control program

No new shutdowns are required on the French nuclear power plants. The program of non-destructive ultrasonic tests proposed by EDF to monitor the phenomenon of stress corrosion in the French nuclear fleet has been validated by the French Nuclear Safety Authority (ASN). All the reactors will be checked by 2025.

It is good news from the French Nuclear Safety Authority (ASN). The atomic watchdog has validated EDF’s monitoring program to follow the stress corrosion cracking (SCC) phenomenon, discovered at the end of 2021 in four French reactors. The agency published a press release on 27 July entitled “Stress corrosion phenomenon affecting nuclear power reactors: the ASN considers that EDF’s monitoring strategy is appropriate.”

At the end of 2021, this unexpected SCC phenomenon was discovered in four reactors. As a result, EDF stopped or extended the shutdown of 12 reactors. The ASN testifies to the company’s considerable work: “EDF has carried out several works to deepen the understanding of the phenomenon and identify the areas concerned. These investigations have led to nearly 70 laboratory assessments of welds taken from eight reactors”.

The N4 and P’4 levels
This work carried out by the electrician, thanks to the removal of numerous pipes, made it possible to draw up a list of the infrastructures most sensitive to the CCS phenomenon:
  • the safety injection circuit (RIS) lines located in the cold branch and the shutdown reactor cooling circuit (RRA) suction lines. This concerns the four N4 reactors (1450 MWe) of the Chooz B and Civaux nuclear power plants.
  • the RIS injection circuit lines are located in the cold branch of the P’4 reactors. The twelve 1300 MWe reactors of the Belleville, Cattenom, Golfech, Nogent-sur-Seine, and Penly nuclear power plants.

For their part, “given the knowledge available, the P4 and 900 MWe reactors appear to be little or very little sensitive to the CCS phenomenon”, explains the ASN.

Inspections by 2025

“EDF plans to inspect all its reactors by 2025, prioritizing inspecting the most sensitive areas of the N4 and P’4 reactors. The inspections will be carried out on the reactors using a new non-destructive testing process using ultrasound. This process has been developed to reliably detect SCC cracks and estimate their depth”, validates the ASN. The only downside is that the agency considers that the inspection of reactor 2 at the Belleville plant, scheduled for 2024, is too late.

On Linkedin, Cédric Lewandowski, director of EDF’s nuclear fleet, is delighted with the ASN’s opinion. He comments: “This position validates eight months of intense work by our teams to qualify, understand and deal with a phenomenon as complex as it is unexpected.

The level of knowledge acquired in less than a year is remarkable. We can now tell where the phenomenon is developing, which circuits are the most sensitive, and how we will monitor it as part of our preventive maintenance programs.

During the presentation of EDF’s half-yearly results, the company announced that, with the opinion of the ASN, it was maintaining its production forecast in 2022 at between 280 and 300 terawatt hours. ■

Published on 29 July 2022

By Ludovic Dupin (Sfen)

Photo : ©ASN