Stress corrosion phenomenon affecting nuclear power reactors: ASN considers that EDF’s inspection strategy is appropriate

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Since SC cracking was discovered on Civaux NPP reactor 1 at the end of 2021, EDF has carried out a considerable amount of work to more clearly understand the phenomenon and identify the zones concerned. These investigations notably led to more than 70 assessments being conducted in the laboratory on welds sampled from eight reactors.

These assessments and the analyses carried out by EDF were essential in underpinning its inspection strategy. They were able to identify the geometry of the pipes and the thermomechanical stresses to which they are subjected as being the main factors liable to influence the appearance of SC. EDF identifies the following as being the most susceptible:

• the lines of the safety injection system (RIS) located on the cold leg ^[1] and the intake lines of the residual heat removal system (RRA) for the four reactors of the N4 ^[2] plant series,
• the lines of the RIS injection system located on the cold leg of the reactors of the P’4 plant series ^[3] .

Based on the available knowledge, it would appear that the susceptibility of the reactors of the P4 ^[4] and 900 MWe plant series to the SC phenomenon is low to very low.

EDF intends to inspect all of its reactors by 2025, prioritising inspection of these most susceptible zones of the N4 and P’4 reactors. The inspections will be performed on the reactors using a new, non-destructive ultrasounds process. This process has been developed with the aim of providing reliable detection of SC cracks and being able to estimate their depth. The results currently obtained by EDF, after six months of development, are encouraging and should enable this new inspection method to be deployed as of the second part of 2022.

ASN considers that this strategy meets the need to continue the inspections on the lines considered to be the most susceptible. However, with regard to Belleville NPP reactor 2, ASN considers that inspection of this reactor - scheduled for 2024 - needs to be brought forward.

ASN considers that knowledge of the SC phenomenon will continue to develop and that the inspection programme will have to be adapted if the inspections or analyses bring any new evidence to light.

With the support of IRSN, ASN is continuing to review the data transmitted by EDF. In September, the subject will be presented to the Advisory Committee for Nuclear Pressure Equipment.