EDF and nuclear regulator ASN were not immediately available for comment. An IRSN spokesperson said IRSN would not elaborate on the comments made to AFP.

The problem has been discovered on the 1300 MW Penly 1 reactor, which had already been halted for maintenance.

It is the first time this corrosion problem has been spotted on this type of reactor, of which EDF has 11, AFP reported.

The four reactors currently halted because of the corrosion problem are the 1450 MW reactor models in Civaux and Chooz, the youngest models in EDF's fleet of 56 reactors.

EDF said in mid-December it had detected cracks on pipes of a reactor at the Civaux power plant in western France due to corrosion. The faults were detected close to welds on the pipes of the safety injection-system circuit in the two reactors at Civaux.

EDF extended the outage of its Civaux station as a result and also stopped the Chooz plant in eastern France that uses the same technology.

The IRSN had said at the time that checks on other EDF reactors were likely to be necessary.

(Reporting by Benjamin Mallet, writing by Geert De Clercq, editing by Chris Reese and David Gregorio)