Stress corrosion: update following the ASN hearing

[Updated on May 20th]. EDF has submitted to the French Nuclear Safety Authority (ASN) its analysis report and investigation program on the phenomenon of stress corrosion affecting some of its nuclear reactors. This program is currently being examined, but the chairman of the ASN, at a parliamentary hearing, gave the first details of the state of play and the actions in progress concerning this unprecedented phenomenon which is not linked to the aging of the fleet.  EDF, for its part, considers that there will be no further early shutdowns of reactors beyond the twelve concerned.

On Tuesday May 17th, the ASN was heard by the Parliamentary Office for the Evaluation of Scientific and Technological Choices (OPECST) on the safety of nuclear installations in 2021. On this occasion, the President of the ASN, Bernard Doroszczuk, was questioned about the stress corrosion cracking (SCC), phenomenon detected in certain reactors in the nuclear fleet. He confirmed that on May 13th EDF submitted an updated version of its analysis and its investigation, control and remediation programs. The latter confirmed that at this stage, 12 reactors are subject to an ” in-depth assessment”, namely four N4 reactors, five 1300 reactors and three 900 reactors. Régis Clément, Deputy Director of EDF’s Nuclear Generation Division, explains “that, today, beyond the 12 shutdown reactors, it is not necessary to anticipate new shutdowns for additional checks”. Dedicates checks will be carried out during the partial ten-yearly inspections and for refuelling in 2022 and 2023.

 Phenomenon and its consequences on safety

Indications of stress corrosion cracks in the piping of the RIS safety injection circuits were detected between November 2021 and January 2022, during non-destructive testing on the N4 nuclear reactors (Civaux, Chooz B) and on reactor 1 of the Penly power plant (1300MW). On three of them (Civaux 1, Chooz B2, Penly 1), parts of RIS piping were removed and subjected to metallurgical expertise, which confirmed the presence of cracks. The cracks assessed on reactors other than Civaux 1 are smaller (maximum 2 mm). Such a phenomenon, which concerns a common stainless steel, is extremely rare in pressurized water reactors. To this date, only one similar example has been recorded worldwide, notably in Japan.

An information note published by the ASN on February 24th indicated that EDF had provided, during a presentation on February 11th  “justifications for the mechanical strength of the pipes concerned and elements tending to show that the cracks only spread over a limited thickness”. The ASN also requested additional checks on other parts of the auxiliary circuits of the primary circuit, in particular the shutdown reactor cooling circuits (RRA) and the reactor chemical and volumetric control circuits (RCV). In a conference organised by EDF on May 19th, it was stated that only four reactors are concerned by proven stress corrosions. On the one hand, there are Civaux 1, Chooz 1, Penly 1 on the RIS and RRA circuits. On the other hand, at Chinon B3, a stress corrosion was found on the RRA; it is linked to a 300-micron defect on a weld.

Source: IRSN, the components of a reactor in detail

 The ASN also asked EDF for additional demonstrations, which led to the initiation of numerous works:

• on mechanical calculations (size of critical defects, according to conventional methods and realistic methods, for the different lines and bends);
• numerical simulation of welding, in order to estimate the work hardening rates, and therefore the sensitivity to SCC, of the different bends
• residual stress calculations, to assess how far the defects could propagate (in the absence of residual stresses, SCC stops, and this appears to be the case in the central part of the pipe thickness).

At this stage of the examinations, the ASN President indicates that the N4 reactors are the most affected. He explained that “the analysis carried out on the deposited pipes seems to show that the phenomenon comes from the geometry of the lines”, which is different between the 900 MW levels and the other reactors in the fleet. Bernard Doroszczuk therefore specifies that “this is not a phenomenon linked to aging. The geometry of the lines favors thermal stratification of the fluids. This generates thermomechanical stresses. The making of welds seems to be a second-order cause today.

On the inspection program for nuclear reactors

In its note of 24 February, ASN sets out the elements of EDF’s inspection strategy for all its reactors. Thus, the operator reexamined the results of the last inspections carried out on all the reactors to look for indications, considered at the time to be parasitic, that could correspond to CCS. New inspections are therefore under way, in addition to the first five reactors, on Bugey 3 and 4, Cattenom 3, Chinon B3 and Flamanville 1 and 2.

For reactors falling into the category with the strongest presumptions of defects: either a unit outage was already scheduled in the first half of the year, and this is used to carry out a new inspection, or the unit outage is scheduled in the second half of the year and a specific inspection outage is carried out in the first half. A test unit was defined for each level: Civaux 1 for the N4 level, Penly 1 for the 1300 level and Chinon B3 for the 900 level, and Bugey for the 900 CP0 level. The Chinon B3 reactor is currently undergoing an in-depth inspection (cutting of pipes and metallurgical analyses).

“For the time being, non-destructive testing does not allow us to characterize the size (of the cracks). To do this, the pipes must be cut out,” explains the president of the ASN. The latter specifies that a non-destructive measurement method is being developed by EDF and will be available in the second half of 2022. Between now and then, several dismantling operations will be carried out for assessment. In addition to the 35 already carried out, 105 more are expected by the end of June 2022. ■