Stress Corrosion or “SCC” due to its small name. Since December 2021, not a week has passed without a phenomenon mentioned in the news, significantly weakening the combat capability of the French nuclear fleet. And for good reason: Of the 27 EDF reactors currently shut down, 12 are due to corrosion problems.
But what exactly are we talking about? To What does stress corrosion look like? Where is this anomaly located inside the reactor? How can we detect it? How is it developing? What about its impact on security? And, above all, how can we fix it? This Thursday, March 19, EDF presented to the press its conclusions to date regarding this extremely unfortunate phenomenon. Game state.
What does this anomaly look like?
“Contrary to what common sense might suggest, the phenomenon of stress corrosion does not correspond to the appearance of rust, but to microcracks on a part of the pipeline., explains Régis Clément, Deputy Director of EDF’s Atomic Energy Production Division. In this case, the cracks are located on the surface of the pipeline, and their size ranges from only one hundred microns to about 5 millimeters at the Civaux 1 reactor.
Where are mini-cracks located?
These cracks are on the auxiliary circuits. They are directly connected to the main primary circuit, which surrounds the reactor and which circulates water at a temperature of 320 degrees. These are, roughly speaking, steel pipes, the diameter of which varies from 25 to 30 centimeters, and the thickness is 3 centimeters. “Medium pipes for the nuclear industry”, says Régis Clement. These pipes are not straight, but flow-through, with vertical and horizontal sections and bends. They are tortuous.
Today, cracks have been identified, in particular, on two auxiliary circuits. First, the emergency injection circuit (RIS), which plays a role in protecting the reactor in the event of an accident by directing water to the main primary circuit. “Normally [d’une centrale, ndlr], it is stopped. Water does not circulate Regis Clement says.
This anomaly also affects the Shutdown Reactor Cooling Loop (RRA). This scheme “plays a role in normal operation. It is used each time the reactor is shut down to remove the residual heat generated by the fuel.”Régis Clément explains.
EDF is preparing to test whether this corrosion problem exists in two other auxiliary circuits: the chemical control circuit and the pressurizer expansion line. An electrician who has already carried out the first analyzes is confident that these circuits will not be affected.
How many reactors were affected by these cracks?
EDF has identified 12 reactors affected or potentially affected by the stress corrosion phenomenon. To Evaluations to date have confirmed this anomaly at four reactors: Civaux 1, Chooz 1, Penly 1 and Chinon B2.
What are they for?
According to an investigation initiated by the nuclear plant operator, the occurrence of this anomaly is due to two factors that create physical restrictions on the pipeline: the design, i.e. the geometry of the pipeline, and the design of the welding process for pipe assembly on site. sections. “We have a clear belief that design is becoming the predominant cause” Régis Clément explained. This information is good news, as it means that the stress corrosion phenomenon is not related to reactor aging.
On the other hand, according to the types of reactors, the design of auxiliary pipelines is not the same. At the 900 MW reactors (the most numerous and oldest in the tricolor park), they are quite short and not very tortuous. On the other hand, on later 1300 MW and N4 reactors, they are longer and more tortuous.
Specifically, hot water from the primary circuit enters the RIS circuit, where the stagnant water is colder. In the pipeline, water licks the metal at different temperatures and, thus, creates alternating stresses responsible for the appearance of microcracks. EDF noted that the longer the pipelines of the corresponding circuits and with significant horizontal sections, the more the corrosion phenomenon increases.
Then, during welding, the temperature of the material rises and falls. And, according to EDF, the first pass of welding, which in some way allows you to create the basis of the weld, is highly correlated with the appearance of the phenomenon of stress corrosion.
Will the design of the respective pipes be reviewed?
EDF is not planning a new cut pipe design. On the other hand, areas subject to thermal and mechanical stress are subject to more thorough inspection. “We are going to rebuild the pipe lines just as we know them, but the way we supervise the repairs will be especially different.”said Regis Clement.
How to find these cracks?
To conduct an investigation, EDF uses two approaches. First, do the checks. This non-invasive method allows you to examine the pipeline from the outside using ultrasound, a technology similar to ultrasound in medicine. “This technique does not allow us to know the nature of the defect, its size and depth”, however notes Régis Clément. The second approach is considered destructive because it involves cutting out part of the pipeline and placing it in the laboratory for “deeply explore matter.” Therefore, a lot of work is required. To To date, 35 sections and laboratory examination have been carried out. Another 105 should be delivered by the end of June. In the future, EDF hopes to use advanced ultrasound technology to perform these measurements non-invasively. The tool should be available by the end of the second half of 2022.
How does this anomaly affect security?
EDF has calculated the height of the maximum allowable defect in terms of safety requirements. The electrician then compared the actual height of the cut pipes with the results of his digital simulation. None of these cracks appear to have exceeded the maximum level. In addition, EDF noted that the corrosion phenomenon cannot extend to the entire thickness of the pipe, but is limited to its surface. Welding actually creates areas of compression where the grains of the material fit very tightly together. This cohesion of the steel forms a kind of front that stress corrosion cannot cross. Analyzes also show that this phenomenon developed slowly. “Incubation can take decades, during which nothing happens” Regis Clement says.
How long will the immobilization of 12 respective reactors last?
The twelve currently shut down reactors are scheduled to return to service between August 7 and February 18, 2023, according to data published on the REMIT website. cutbacks that subsequently inevitably lead to refurbishment lead to a shutdown of more than 10 weeks.
However, repairs can only begin with the consent of the Office of Nuclear Safety (ASN), which is currently investigating the EDF case filed May 12. The nuclear police officer must, in particular, decide on the conditions of welding and radiation protection under which this work will be carried out. “In terms of our planning project, we will need this green light at the turn of summer,” – trusted the deputy director of the nuclear production shop.
Will other reactors have to be shut down?
“Apart from 12 reactors, there is no need to wait for new shutdowns to conduct analyzes related to the corrosion phenomenon under control”Regis Clement said. Thus, the research work will be timed to the rhythm of shutdowns planned for 2023. These planned shutdowns are related, for example, to loading fuel or a ten-year inspection. Today, neither the EDF nor the nuclear cop comment on the duration of the impact of this corrosion problem on the availability of the nuclear fleet. The electrician nevertheless clarifies that when “At this stage, and until the completion of inspections and repairs, the assessment of nuclear energy production for 2023, i.e. 300-330 TWh, does not change.