EDF's French nuclear power capacity was cut by 6.5 GW early Thursday due to a strike by workers at the plants, a company spokesman said.
The strikers were objecting to pay conditions and the action was set to end Thursday night, he said.
National nuclear output was just 39.0 GW at 8:30 am local time (0630 GMT) Thursday, some 5.5 GW lower than the same time on Wednesday, data from grid operator RTE showed.
France switched from exporter to importer Thursday morning, importing around 700 MW at 8:30 am local time.
The capacity cuts were implemented at several nuclear power plant sites, including the Blayais, Bugey, Cattenom and Chinon plants, RTE data showed.
A spokesman for RTE said the strike was called by the CGT union for all for this week and notes published by the grid operator show that action to cut power capacity was implemented around midnight Wednesday.
Separate notes from RTE show the capacity cuts began at 5.3 GW overnight and increased to 6.5 GW by 6:00 am Thursday.
EDF operates all 58 of France's nuclear power reactors, which typically provide around three quarters of the nation's power.
