EDF's nuclear reactors will struggle to reach 400 TWH annual output levels again

France's nuclear fleet will struggle to reach previous annual output levels of 400 terrawatt hours over the years to come, the head of the country's power grid operator RTE told lawmakers on Thursday.

"The French nuclear fleet is ageing and, in a way, we can be almost certain that it will have difficulty reproducing 400 terawatt-hours per year in the coming years", said Xavier Piechaczyk.

France has been on alert for possible rolling blackouts as several reactors, run by power giant EDF , were shut down for maintenance and repairs.