Stress corrosion phenomenon detected on the safety injection system of Civaux NPP reactor 1 – Shutdown of 1450 MWe reactors for inspections

Published on 21/12/2021 at 14:07

Information notice

On 21 October 2021, following ultrasonic inspections performed during the ten-yearly outage inspection of the Civaux NPP, EDF informed ASN that it had detected indications [1] on welds on the elbows of the pipes connecting the safety injection system [2] to the reactor’s main primary system (see image below). In order to identify the origin of these indications, the parts of the pipes concerned were cut for metallurgical analysis in the laboratory.

EDF also decided to shut down Civaux NPP reactor 2 so that these inspections of the areas could be carried out ahead of schedule, as the previous inspections dated from 2012. The preliminary results of these inspections confirmed the presence of indications similar to those of reactor 1.

Pressurised water reactor operating principle.

On 15 December 2021, EDF informed ASN that the initial metallurgical analyses conducted on the parts of the pipes removed from Civaux NPP reactor 1 revealed the presence of cracking resulting from a stress corrosion phenomenon. EDF is continuing its investigations in order to characterise the factors that caused this phenomenon and identify the areas potentially concerned.

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

Given the unexpected origin of the cracks detected, EDF decided to shut down reactors of similar design, as rapidly as possible. The B1 and B2 reactors of the Chooz NPP will therefore be shut down shortly for checks in addition to those carried out in 2019 and 2020 during their ten-yearly outage inspection.

ASN considers that given EDF’s prime responsibility for safety, its decision is an appropriate one for the situation.

With the technical support of IRSN, ASN is closely following the investigations being carried out by EDF, along with the resulting conclusions, notably with regard to in-service monitoring of this equipment. ASN authorises repair work on the equipment concerned and will issue a decision with regard to its return to service.

Photo showing the welds with cracks.

[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 : 07/11/2022