EDF discovers new cracks on other reactors (related to thermal fatigue this time)

EDF discovers new cracks on other reactors (related to thermal fatigue this time)
EDF discovers new cracks on other reactors (related to thermal fatigue this time)

The black series continues for EDF. Barely two days after the discovery of a “significant” crack on the first reactor at Penly (Seine-Maritime), the electrician also has another crack, on the safety injection circuit of Penly 2. This second crack unexpected is, this time, not linked to a stress corrosion phenomenon, but to thermal fatigue. This is a thermomechanical stress in which a part is subjected to significant temperature variations. Its maximum depth is 12 millimeters, specifies EDF. It is therefore smaller than the first crack observed on Penly 1, but remains larger than the microcracks detected on the other reactors. A similar thermal fatigue defect was also spotted on Cattenom 3.

“This event had no impact on staff or the environment. Nevertheless, it affects the safety function related to reactor cooling. Due to its potential consequences and the increased probability of a rupture, the ASN classifies it at level 2 on the INES scale with regard to reactor 1 of the Penly nuclear power plant and at level 1 for the other reactors concerned”, specifies the nuclear safety agency, adding that “more than 150 welds were the subject of laboratory expertise and checks were continuing, with a control program for all the reactors from of 2023.”

Fate is therefore hounding EDF. While the electrician thought that the worst of the crisis was behind him after going through a dark year in 2022, due in particular to a problem of stress corrosion which cost him nearly 30 billion euros, the 2023 could also be undermined by this phenomenon.

Nuclear: blow for EDF after the discovery in Penly of a “significant crack”

Appearing at the end of 2021, this defect has until now materialized by the appearance of microcracks on an emergency circuit located within the radioactive zone. According to the investigation carried out by EDF, 16 reactors are particularly sensitive to it. These are the 16 most recent reactors in the fleet, whose design, that is to say the geometry of the piping, is a little more complex. In detail, these are the reactors of the so-called N4 level (Chooz B1 and B2 and Civaux 1 and 2) as well as the 12 reactors of the P’4 level, i.e. the 1,300 MW reactors more recent.

However, during checks carried out on reactor 1 of the Penly power plant (Normandy) with a power of 1300 MW, the tricolor group detected, at the end of February, a new crack larger than expected. What plunge the electrician, already in great financial difficulty, into a new unknown. The point in six questions.

1- Why is this crack so special?

So far, as part of the stress corrosion phenomenon detected in October 2021, the largest cracks observed on the pipes of the reactors of the P’4 and N4 bearings were 6.5 millimeters over a thickness of a little less 3 centimeters (26 millimeters to be exact). ” These were abnormal cracks, but which did not affect the behavior of the pipes “, explains Karine Herviou, Deputy Director General of the Institute for Radiation Protection and Nuclear Safety (IRSN).

The crack detected on the Penly 1 reactor is significant. It extends over 155 mm, or about a quarter of the circumference of the pipe. It is also 23 millimeters deep and 27 millimeters thick. ” There are only 4 millimeters left before it lets go. EDF can therefore no longer guarantee the integrity of its piping, i.e. the fact that it will not leak “, explains Karine Herviou. In other words, this crack could have led to a leak while water at 150 bars, i.e. a very high pressure, circulates in these pipes.

The only more or less comparable event dates back to 1998.” There was a leak on the cooling circuit of the Civaux power plant and therefore a crack, not due to stress corrosion, but to thermal fatigue “recalls Karine Herviou. ” The crack was through “, she specifies.

Furthermore, if the crack is on the safety injection circuit (RIS), like the cracks previously observed in the context of the stress corrosion phenomenon, it is not located on the same line. The Penly 1 crack formed on the lines which are connected to the hot leg of the injection system, whereas the previous cracks were observed on the lines connected to the cold leg. ” The 23 mm deep crack is located in an area which was considered by EDF to be not very sensitive to the phenomenon of stress corrosion “, points out the Deputy Director General of IRSN.

2 – What is the cause of this crack?

This unforeseen event is largely explained by the causes at the origin of this significant crack. The phenomenon of stress corrosion observed until now was directly linked to the complex geometry of the lines of the safety injection circuit. In the case of Penly 1, EDF considers that the crack is linked to the double repair of the piping which took place during the construction of the plant in the late 80’s.

The first repair consisted of realigning the pipes. It took strain to put the pipes face to face and align them. Which is a very bad practice “, believes Karine Herviou.

There was an approach which is not acceptable, which consisted a little in forcing the pipes to align them to weld them, and there were defects on this weld which led to a second repair “, for his part, explained Wednesday the president of the ASN, Bernard Doroszczuk, in the Senate.

We can therefore wonder about the existence of a design problem. Was the drawing of these lines done incorrectly at the time of design ?” asks Karine Herviou.

As a reminder, only the 32 900 MW reactors are of American design (Westinghouse), the other bearings, including the two bearings affected by the stress corrosion phenomenon, are of French design. Asked about the companies behind the design of these pipes, EDF declined to comment.

3 – Is there a risk of seeing similar cracks on other reactors?

Today, we do not know if other reactors are concerned, but there is a risk that this phenomenon could reproduce elsewhere. », confirms Karine Herviou.

This phenomenon can potentially concern all the reactors, because all of them have repaired welds. The results of future inspections will make it possible to assess the extent “, responds for its part the ASN. ” At this stage, no case similar to that of this welding of Penly 1 on other reactors has been brought to light. “, however, specifies the policeman of the nuclear.

4 – What does the nuclear policeman ask of EDF?

ASN asked EDF to revise its weld inspection strategy. The electrician is currently carrying out an inventory of the welds concerned by repairs on all the reactors. ” In particular, ASN expects EDF to reprioritize its inspections to take account of the greater safety issues concerning repaired welds », Specifies the nuclear policeman. The presentation of this strategy is expected for the next few days. The ASN will then have to take a position, without however seizing the IRSN. In any case, it had not entered it within the framework of the first control strategy presented by EDF.

As a reminder, EDF announced, last September, to carry out systematic repairs on the 16 French reactors that it had identified as sensitive to the phenomenon of stress corrosion. Work is in progress or planned for 2023 for 10 of them.

Nuclear: how EDF seeks to secure a return to normal in 2024

5 – What impact on production forecasts in 2023?

This is the great unknown of this incident. ” Will EDF carry out these checks and potential repairs during the shutdowns already scheduled or will it decide to anticipate the shutdowns? asks Karine Herviou.

This extraordinary crack should probably lead to the extension of stops on other sites, for extensive checks, but it should not ” no massive stops “, told AFP Julien Collet, the deputy director general of the ASN.

At this stage, the production range between 300 and 330 TWh remains unchanged “, declared EDF to The gallery.

A drop in the production range would be very bad news for EDF. In 2022, this industrial crisis had contributed to bringing down the production of nuclear electricity to its lowest historical level, in the midst of the energy crisis, and to widening the losses of the electrician (17.9 billion euros).

6 – What risk does this crack represent for the environment and populations?

The water that circulates in the safety injection circuit is slightly radioactive water because it passes close to the fuel assemblies. However, in the event of a leak, there is no risk for the environment, nor rejection for the populations “says Karine Herviou. Within a reactor, safety is, in fact, ensured by three containment barriers: the sheath that surrounds the fuel pellets, the circuit envelope and, finally, the concrete containment. In the present case, the risk of leakage is only at the level of the envelope around the circuit.

The crack observed at Penly 1 was characterized as a level 2 incident on the Ines scale, set up after the Chernobyl accident. ” Level 2 events occur once or twice a year. It is therefore a significant event, even if it had no consequences on the site.», Comments Karine Herviou. Indeed, Penly 1 has been shut down for almost a year (March 22, 2022) due to its third ten-year inspection. It must be connected to the network again on May 2. In the meantime, the repair of the pipes concerned will be carried out.

The article is in French

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