Stress corrosion phenomenon: ASN asks EDF for more in-depth analyses

On 11 February 2022, EDF presented ASN with an updated review of the stress corrosion (CSC) phenomenon detected on several of its reactors, its analysis of the corresponding safety implications and its investigation and inspection programme.

Extension of the investigations to the N4 type reactors and to Penly NPP reactor 1 revealed indications that could correspond to CSC on the pipes of the residual heat removal system (RRA) of the main primary system.

As the question concerned the safety of the reactors in operation, EDF provided data to demonstrate the mechanical strength of the pipes concerned and data tending to show that the cracks only propagate through a limited thickness. However, ASN considers that EDF must further demonstrate that the studies conducted cover all possible cases and that certain aspects of its safety analysis need to be examined in greater depth. ASN therefore sent EDF additional requests.

EDF also proposed an inspection strategy for all of its reactors. This programme is notably based on a review of the results of the inspections already carried out and which revealed indications classified as spurious, but which could in fact correspond to CSC. EDF thus identified reactors it believes should be re-inspected in the short term. The inspections on these six priority reactors[1] will be carried out during outages starting no later than the end of April 2022.

Moreover, in order to improve its understanding of the CSC phenomenon, EDF will be carrying out extensive checks on reactors representative of the various models it operates[2]. These reactors are currently shut down.

Finally, EDF is continuing its studies to improve its knowledge of the phenomenon and has initiated the development of new ultrasonic inspection means capable of measuring the crack depths. EDF intends to inspect all of its reactors using these new means, from September 2022 up to the end of 2023.

With the assistance of IRSN, ASN has begun an examination of the data submitted by EDF in order to confirm the pertinence of the safety analyses transmitted, the programme of work proposed and the conditions in which it is carried out. ASN’s Advisory Committee for Nuclear Pressure Equipment will be involved in this process.

In addition to regular technical exchanges with EDF, ASN conducted three inspections on the Civaux NPP and one on the Penly NPP. These inspections concerned the conditions in which the weld checks are carried out, the radiological exposure of the personnel involved and the conditions in which the pipes to be analysed are cut.

[1] This concerns Bugey NPP reactors 3 and 4, Cattenom NPP reactor 3, Chinon NPP reactor B3 and Flamanville NPP reactors 1 and 2.

[2] In addition to Penly NPP reactor 1 and Civaux NPP reactor 1, on which the phenomenon was discovered, this concerns Chinon NPP reactor B3 and Fessenheim NPP reactor 2.

For further information

Published on 31/01/2022 at 11:21

Stress corrosion phenomenon detected on reactors

On 21 October 2021, following ultrasonic checks scheduled during the second ten-yearly outage of Civaux NPP reactor 1 (1450 MWe reactor), EDF informed ASN that it had detected indications on welds on the elbows of the safety injection system piping of the reactor’s main primary system.

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The cut parts will be replaced by new parts. ASN conducted an inspection on the elbow cutting worksites on Civaux reactor 1.
The corresponding follow-up letter is available on the ASN website (in french)

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