UPDATE 1-French power grid operator RTE expects winter electricity demand to be stable

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* RTE says January seen as the month to watch

* Expects several reactors to restart by end-Nov

* France cannot halt all coal, 4 nuclear reactors after 2020 (Adds details, quotes, background)

PARIS, Nov 7 (Reuters) - French electricity demand this winter is expected to be stable, although unplanned nuclear reactor outages and a prolonged cold spell could lead to a tense supply situation, electricity grid operator RTE said in its outlook on Tuesday.

RTE said it was keeping a watchful eye on the situation, particularly on several nuclear reactors operated by utility EDF that are currently offline, but which are expected to resume production by the end of November.

"With the shutdown of some fuel-powered generators in 2017 and 2018, the electricity system is adjusted to French demand," the RTE said in a statement.

In case of a cold spell and nuclear outages, the RTE could be forced to gradually use some of its exceptional measures to guarantee security of supply, it added.

"The forecast for winter 2017-2018 is stable compared with last year, with electricity demand estimated at 221.5 terrawatt-hour in the period from mid November to the end of March," RTE said.

It said peak consumption was expected at 85 gigawatts (GW) if temperatures stayed within what is normal for the season in January, and could rise to 99 GW if there was an episode of a once-in-a-decade cold snap.

The RTE added it could have an additional 2.8 GW of production capacity compared with the same period a year ago, and could also count on 8.1 GW of import capacity from neighbours, a 1 GW increase compared with last year.

In winter last year, peak electricity demand came in at 94 GW as the average temperature fell 7 degrees Celsius below the seasonal norm.

The RTE also said that while France's security of supply could improve after 2020 due to a slight dip in demand, increased renewables, and imports, the situation would not allow for the simultaneous shutdown of France's coal power plants and its four, 40-year old nuclear reactors as expected in its long-term energy plan.

Beyond 2025, RTE said France would have to boost renewables and built new gas fired plants to add about 11 GW of capacity, given the planned shut down of about 24 out of France's 58 nuclear reactors in order to meet its target of reducing the share of nuclear in energy mix to 50 percent. (Reporting by Bate Felix; Editing by Sudip Kar-Gupta)