The strike had reduced French power generation by around 4.5 gigawatts (GW) or over 8% of total available generation capacity by 2140 GMT, according to data from EDF and grid operator RTE.

Power station outages will not knock out the grid or hit households, though cuts in power output are costly for EDF, as it has to import any shortfall from overseas.

EDF workers are protesting against plans steered by the French government to restructure and potentially split the heavily-indebted, state-controlled group, with its nuclear power generation business set to one side.

The strike, expected to run until the evening of Sept. 19, could be more disruptive than previous stoppages, with four unions representing a majority of France's energy workers joining forces behind the walkout this time.

It is not yet clear whether job cuts would be involved under the restructuring plan, which is known as "Project Hercule" and was requested by President Emmanuel Macron.

But unions hope to pile pressure on EDF's management and the government to delay the project - the company is due to present a final proposal by the end of the year - and they argue the mooted split would only weaken the group.

EDF's data showed that output was down by a combined 3.7 GW at seven of its 38 available nuclear reactors. The utility operates all 58 French nuclear reactors, which account for around 75 percent of the country's electricity needs.

The other 20 reactors are offline for planned statutory maintenance, refuelling or upgrades. [POWER/FR]

Output at EDF's 1,330-megawatt (MW) Paluel 3 nuclear reactor was down by around 65 percent due to the strike.

Power generation was reduced by around 60 percent at the 900 MW Gravelines 2 nuclear rector, while at the 1,310 MW Nogent 2 reactor, output was reduced to 800 MW. The Blayais 1, St. Laurent 1, Chinon 1 and Cruas 2 nuclear reactors were also hit by the strike.

Electricity production was also curbed at the Blenod 5, and the Martigues Ponteau 5 and 6 gas-fired power plants, EDF data showed.

(Reporting by Paris Newsroom; Editing by Christian Lowe)