The CGT said some 500 staff from EDF's nuclear plants would gather at the firm's Paris headquarters for a protest on Tuesday, and added that power output was expected to be reduced due to the action from Monday night.

Another strike is scheduled for Nov. 29, with the aim of keeping pressure on EDF and the government over salary negotiations, and the future of the utility.

The government is expected to present a new long-term energy policy at the end of the month, which is expected to outline how France will reduce its dependence on nuclear power, boost renewables and curb carbon emissions, and which might indicate how EDF could be restructured.

"We are in a period of uncertainty with regards to French energy policy and this concern is very high among workers in gas and electricity companies," Sebastien Menesplier, Secretary General of CGT's mines and energy branch FMNE, told a news conference.

He added that EDF had offered a 0.3 percent salary increase, which the CGT had rejected because inflation is expected at 2 percent.

To keep the pressure on EDF, the union has been delaying the restart of several reactors during planned maintenance, costing EDF several millions of euros for every day of delay.

Sources have said that there are various scenarios under consideration for a possible restructuring of EDF, which could involve putting its nuclear activities in a separate, fully-state-owned entity.

"Breaking up the EDF model would be a purely capitalistic move, which we are against," Menesplier said.

French Environment Minister Francois de Rugy has said the government was considering all options for EDF, including the status quo.

CGT warned that just as it has done with partially state-owned energy group Engie a former gas monopoly, the government wants to break up EDF so as to get cash from the sale of various units. Such a breakup could be harmful to consumers, workers and France's security of power supply, the union said.

(Reporting by Bate Felix; Editing by Geert De Clercq)

By Bate Felix