UPDATE 1-French energy union to strike over site closures

* Thermal, nuclear power output to be hit

* EDF says will have estimated impact on Tuesday

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PARIS, Oct 8 (Reuters) - France's CGT trade union has called for a 24-hour strike in the energy sector to protest against site closures, an action that could hit power production at the country's thermal and nuclear plants.

The strike is set to start at 1900 GMT on Monday, a CGT spokesman said. The action is part of Tuesday's national day of protest against job cuts and plant closures called by CGT, France's biggest trade union.

"In thermal power stations, we will be at zero (production)," CGT spokesman Laurent Langlard told Reuters. "We are also expecting production reductions in nuclear power."

It was not clear by how much production could be cut. French grid operator RTE was not immediately available for comment.

A spokeswoman for EDF , France's biggest power company said it had received the strike notice by CGT but that it will only be able to provide an estimate for its impact from Tuesday, around 0900 GMT.

The strike comes after President Francois Hollande said he would close France's oldest nuclear power station at Fessenheim near the German border in 2016.

"With the closure of Fessenheim, we are left wondering. We were told it was the oldest nuclear power station, but there is always an oldest one. So which one will be next?" Langlard said.

German power group E.ON also announced last year that it would close four out of seven coal plants that it operates in France. It also has two gas-powered plants in the country.

E.ON's coal and gas plants in France have a combined electricity capacity of 3,000 megawatts, equivalent to about three nuclear reactors.

(Reporting by Michel Rose; Editing by David Goodman)

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Keywords: FRANCE ENERGY/STRIKES