Workers at French energy companies including EDF carried out stoppages on Thursday after several labour unions called for protests against looming restructurings that could hit jobs.
The CGT union, alongside CFE-CGC, FCE-CFDT and Forces Ouvrieres (FO), had urged staff at EDF to strike for at least an hour, even those working remotely.
About 40% of EDF staff had taken part in the industrial action halfway through the day, the CGT said. EDF said it could not immediately confirm of the numbers.
The French government is working on a plan to ringfence EDF’s nuclear arm from the rest of the power giant and is in talks with Brussels on the matter, which has raised concern among unions over the fallout for employees.
Unions said they were also protesting against a reorganisation of utility Engie.
France’s nuclear output was down sharply on Thursday morning, according to data from grid operator RTE, though it did not detail how much of that was because of strike action.
Nuclear output stood at 42,557 megawatts (MW) at 1245 GMT, down from 46,954 MW at the same time on Wednesday, a drop equivalent to the power produced by four reactors.
Hydroelectric energy production was up 8.6% from the same time a day earlier.