Nuclear: five minutes to understand the discovery of new cracks in reactors

After corrosion problems in several nuclear reactors, requiring major maintenance work, now EDF has detected “cracks” in a security system of three reactors. These will have to be repaired, extending the shutdown of several production units, and EDF will have to examine its entire fleet.

What is the problem ?

EDF detected a “significant” defect in the reactor number 2 of the Penly nuclear power plant (Seine-Maritime) and in reactor number 3 of the Cattenom power plant (Moselle), the Nuclear Safety Authority (ASN) said on Thursday. According a note she postedclassifying the incident as 1 (“anomaly”) on a scale of 0 (“deviation”) to 7 (“major accident”), these “thermal fatigue cracks” measure 57 mm for the first and 165 mm for the first .

These cracks were detected in the “safety injection circuit” of the reactors. “This is a back-up system that will allow, if you have a loss of water in a reactor, to compensate by adding water via another circuit ”, explains to the Parisian Emmanuelle Galichet, lecturer in nuclear sciences and technologies at the National Conservatory of Arts and Crafts. It is an emergency system which is not used in normal operation, but only in the event of an “accident” (when the water does not come through the primary circuit).

A similar fault, on the same emergency line, had already been detected on reactor number 1 of the Penly nuclear power station, ASN said on Monday. This extends over 155 mm, “that is approximately a quarter of the circumference of the pipe, and its maximum depth is 23 mm, for a pipe thickness of 27 mm”, at the level of a weld which “has is subject to double repair during construction”, indicates the ASN. This incident has been classified as level 2 (“incident”).

What consequences?

“The presence of this crack means that the resistance of the piping is no longer demonstrated”, indicates the Nuclear Safety Authority. This phenomenon is “well known and monitored for a long time under historical preventive maintenance programs”, indicates EDF. But “this line was considered (…) as not susceptible to stress corrosion cracking, in particular because of its geometry”, specifies however the ASN.

“There is no danger in normal operation because it is a system that only works in the event of an accident”, notes Emmanuelle Galichet. And if ever there were to be an accident, and the security system were to break due to a crack, it is duplicated by several other similar systems that would replace it. “There is never just one system,” recalls Emmanuelle Galichet. EDF will however have to repair the system in order to replace the piping in question.

EDF must also submit a revised control strategy to ASN in the coming days. In total, the electrician will have to check 200 welds throughout its fleet, according to ASN. It also “asked EDF to revise its strategy to take account of this new information”, specifies the authority. Already under maintenance due to the discovery, from October 2021, of “stress corrosion” phenomena, several reactors could see their prolonged shutdown.