EDF, which found premature wear in back-up diesel generators in 8 of its 19 nuclear power plants, said it had started repairs which can be done whilst reactors were in action.
The French Nuclear Safety Authority ASN ranked the incident at level 1 of the international nuclear event scale that goes from zero to 7 on seven reactors and at level 2 for 1 reactor.
The flaw was detected in some of EDF's oldest plants: Blayais, Bugey, Chinon, Cruas, Dampierre, Gravelines and Saint-Laurent and Tricastin.
"The anomaly is caused by premature wear," EDF said in a statement. "It is of no consequence to the operation and safety of the facilities in question."
Each nuclear power unit has three back-up diesel generators.
EDF said it has detected a technical flaw in 34 of its 58 nuclear reactors that could have an impact on safety.