The group has been beset by maintenance problems at its ageing park of reactors over the last year that has forced it to take more than a dozen of them offline for checks and emergency repairs.

The group last month reported the latest "serious corrosion problem" on an emergency cooling system at its Penly 1 plant in northern France which was among the 16 taken offline in the last year.

The report went largely unnoticed until it was covered in French media on Tuesday.

The new crack was 15.5 cm (six inches) long and up to 2.3 cm deep, covering around a quarter of the circumference of the pipe which is 2.7 cm thick, France's Nuclear Safety Authority (ASN) said late on Tuesday.

The regulator ordered EDF to "revise its strategy" of addressing the corrosion problems which could have major financial repercussions for the debt-laden state-owned utility as well as France's energy production capacity.

The country, once a leading electricity exporter in Europe, needed to import power from Germany and other neighbours over the winter because of the problems in its nuclear park, which normally supplies around 70 percent of its energy needs.

The crack at Penly does not pose an immediate danger to the environment or human life, the regulator said, given its location on a pipe system that is designed to be used to cool the reactor only in the event of an emergency.

"What is new... is the depth of the crack," nuclear safety expert Yves Marignac, who is an advisor to the ASN, told AFP.

EDF's debt ballooned to 64.5 billion euros ($68.6 billion) in 2022 while losses totalled 17.9 billion euros.

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EDF - ELECTRICITE DE FRANCE