France's nuclear safety regulator, the Autorité de Sûreté Nucléaire (ASN), has approved EDF's revised programme to rectify stress corrosion issues at its French nuclear power plants. Under EDF's schedule, all welds identified as a priority will be checked within the next year.
Pipe elbow and adjacent welds which underwent a ten-yearly outage inspection in an N4 plant series (Image: ASN)
In December 2021, maintenance checks on the primary circuit of Civaux 1 revealed corrosion near the welds on pipes of the safety injection system. Checks were then carried out on the same equipment at unit 2, revealing similar defects. EDF decided to replace the affected parts, requiring an extended shutdown of the plant, and also take its two other N4 units at Chooz B offline to carry out similar checks.
In mid-January 2022, EDF announced that similar faults on the safety injection system pipe welds to those discovered at Civaux 1 had been found at Civaux 2 and Chooz B2. In addition, the ten-year in-service inspection at Penly 1 - one of twelve 1300 MWe-class units of the P4 series - also revealed stress corrosion. The discovery of cracking led to the need to carry out checks across much of its French fleet.
In December 2022, EDF submitted to ASN an inspection and repair strategy for all of its reactors with regard to the risk of stress corrosion cracking, for the years 2023-2025. EDF amended this strategy in early March to take into account the discovery of such cracks on welds that had been repaired when the reactors were built. Indeed, the discovery of a defect 23mm-deep on a repaired weld at Penly unit 1 showed that these welds presented an increased risk of the cracking phenomenon.
In mid-March, ASN took note of this amended strategy, which includes tighter checks on repaired welds, while requesting that the technical dialogue continue, in order to ensure the relevance of the timetable envisaged for the checks on priority welds.
"Following these exchanges, ASN considers this timetable to be appropriate," the regulator said on 25 April.
EDF's revised schedule provides in particular for an acceleration of the inspections of the welds of the RIS and RRA systems which were the subject of repairs at the time of the construction of the reactors. EDF said 90% of the repaired welds it identified as a priority due to their repair conditions will be checked before the end of 2023, and all of these welds will be checked by the first quarter of 2024.
"The technical exchanges between ASN and EDF focused more particularly on the reactors which, on two different auxiliary lines, have welds whose inspection is considered a priority," ASN said. "These are unit 1 of the Nogent-sur-Seine power plant and unit 2 of the Cruas power plant".
It noted that EDF has "also implemented additional operating measures to prevent operating situations that would lead to significant stress on these welds, as well as to quickly detect any leaks".
EDF said it still expects output from its French nuclear power plants to be in the range 300-330 TWh in 2023.
