Power output in France fell by at least 5 gigawatts after members of the CGT union at utility EDF joined a rolling nationwide strike against planned government reforms, a union official said.
CGT members at 19 nuclear power plants had voted yesterday to join the strike which has already paralysed French businesses and disrupted fuel supplies leading to shortages in some parts of France in the past week.
Laurent Langlard, a CGT spokesman said power output was down by 5,000 MW this morning as reactors reduced production but did not stop running.
State-controlled EDF has rules which require its staff to maintain a minimum output level so as to prevent power cuts during strike action.
The public may not notice a fall in nuclear power output but it incurs costs for EDF, which needs to start up more expensive fossil fuel-fired power plants and import electricity.
EDF was not immediately available to comment.
French power grid operator RTE, a subsidiary of EDF showed on its website that at least 11 nuclear reactors reported unplanned outages after the workers voted yesterday evening.
Prime Minister Manuel Valls is expected to meet representatives of the oil industry on Saturday.
He has taken a tough stance against the CGT union over its protest, which has shut down refineries and disrupted fuel supplies.
Mr Valls has vowed not to withdraw the reforms that make hiring and firing easier and which he says will boost jobs and growth.
He launched a strongly worded attack on the action yesterday saying; "The CGT does not rule this country."
A report on France Inter radio's website cited a CGT delegate saying the union had the ability to cut off a fuel pipeline from the port of Le Havre supplying Charles de Gaulle airport outside Paris.
The SNCF state railway company said many rail connections were less harshly hit than during stoppages last week.
France has also mobilised its strategic oil and fuel reserves to keep fillings stations running.
Laurent Berger, head of the moderate CFDT union which is backing the reforms, today called for a truce.
"The political and industrial relations climate has turned hysterical ... let's calm things down," he said on France Info.