Stress corrosion phenomenon detected on Civaux 1 and 2, Chooz B2 and Penly 1 reactors

Published on 31/01/2022 at 11:21

Information notice

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[1] on welds on the elbows of the safety injection system piping[2] of the reactor’s main primary system (see image below).

As Civaux NPP reactor 2 was the only 1450 MWe reactor not yet to have undergone these types of checks, EDF shut it down in November 2021 so that they could be carried out ahead of the date initially scheduled for its second ten-yearly outage. The checks confirmed the presence of indications similar to those of reactor 1.

On 15 December 2021, EDF informed ASN that the metallurgical analyses conducted on the parts of the pipes removed from Civaux NPP reactor 1 had revealed the presence of cracking resulting from an unexpected stress corrosion phenomenon on the inner face of the piping, close to the weld bead.

The checks carried out during the ten-yearly outages aim to detect the presence of cracking by thermal fatigue, notably by means of ultrasonic inspections. This process is less effective at detecting stress corrosion cracking and may have led to the corresponding indications being classified as spurious. For the additional checks in progress, EDF optimised the process used in order to improve the detection of defects resulting from stress corrosion.

As indications classified as spurious had been detected during the checks previously conducted during the ten-yearly outage on the other two 1450 MWe reactors (Chooz NPP reactors B1 and B2), EDF shut them down in order to carry out additional checks. The initial results indicate the presence on reactor B2 of indications similar to those observed on Civaux. Checks are in progress on reactor B1.

At the same time, the check carried out during the third ten-yearly outage of Penly NPP reactor 1, the design of which is different (1300 MWe reactor), revealed indications on the same pipes. Laboratory analysis of the removed parts also concluded that stress corrosion cracking was present. The results currently available on this reactor show a smaller scale phenomenon than on Civaux NPP reactor 1.

The four 1450 MWe reactors and Penly reactor 1 are currently shut down and are undergoing additional checks to determine which areas and systems are affected by the stress corrosion phenomenon. EDF is also continuing its investigations in order to determine the causes of this corrosion and identify the other areas and reactors that are potentially concerned. EDF is more particularly re-examining the results of the checks previously conducted on all of its reactors, in order to look for possible indications classified as spurious but could correspond to stress corrosion. Following these checks and investigations, EDF will submit a program that will prioritize reactors to be checked, on which ASN will issue a ruling.

Pipe elbow and adjacent welds which underwent a ten-yearly outage inspection (N4 plant series).

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)

Photo showing the welds with cracks.

In addition to regular technical exchanges with EDF, ASN conducted two inspections on the Civaux NPP and one on the Penly NPP. These inspections concerned the conditions for performance of the additional checks, the radiological exposure of the personnel involved and the conditions in which the cutting work of elbows for subsequent analysis   was carried out. During an inspection on the Civaux site, control of the checks procedure was considered to be satisfactory. ASN will be continuing its inspection work.

Finally, ASN authorises cutting up of the systems concerned for subsequent analysis and will rule on any repair procedures and on return to service.

To find out more (in french):

 

[1] An indication is a signal (typically an echo for ultrasound inspections) revealing the possible presence of a defect in the material being inspected.

[2] In the event of an accident causing a major break in the reactor’s primary system, the safety injection system (RIS) enables pressurised borated water to be introduced into it in order to stifle the nuclear reaction and cool the core.

Date of last update : 09/03/2022