The phenomenon of stress corrosion, detected in October 2021, is weighing on French nuclear power assets. Another crack (this time a large one) has been detected in the safety injection circuit of reactor number one at the Penly power plant (Seine-Maritime, in the North of France). The plant has a capacity of 1,300 megawatts (MW), but is currently shut down. EDF had thought, at the end of 2022, that the crisis over stress corrosion was behind it, and that it had entered “a phase of industrialization and standardization” of repairs. But on Tuesday, March 7 the French Nuclear Safety Authority (ASN) asked the electric utility to “revise its strategy.”
This crack had been reported in a note published by EDF on February 24, but had gone unnoticed until it was revealed on March 7 by the specialized site Context. Unlike the microcracks detected on other reactors (such as those of Chooz, in the Ardennes, and Civaux, in Vienne, which at 1,450 MW is both the most powerful and the most recent French reactor), the defect found at Penly is described as particularly significant. The ASN described a crack extending over 155 millimeters (mm), “or about a quarter of the circumference of the piping.” The nuclear watchdog has added that its maximum depth is 23 mm, for a pipe thickness of 27 mm.
Although the Penly 1 reactor had already been identified as one of the most sensitive to stress corrosion, this particular portion of the circuit was considered “non-sensitive” by EDF, because of its geometry. The operator, like the ASN, considers that the presence of corrosion could be explained by the double repair of the piping during the construction of the reactor.
‘Increased probability of rupture’
According to the ASN, the presence of this crack means that “the strength of the piping is no longer proven” and it “affects the safety function related to the cooling of the reactor.” The safety injection circuit is an essential element. It is this backup system that allows water to be injected into the main primary circuit to cool the reactor core in the event of a breach.
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As a result of the “potential consequences and the increased probability of rupture,” the ASN classified this incident as level 2 on the international scale of nuclear and radiological events. The severity scale goes from 0 to 7, but level 2 events are very rare.
Since the end of 2021, the welding repair of pipes affected by corrosion has involved a huge amount of work, involving 500 people as well as a hundred or so North American welders and pipe fitters. According to EDF, of the 16 reactors identified as the most sensitive, 10 still need to be inspected and repaired in 2023. The rest of the assets should be inspected in 2024 and 2025.
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