The hardline CGT union sought to tighten its grip on France's energy supplies and transport network on Thursday as its stand-off with the government over planned labor reforms continued into a day of nationwide protests.
Signals were mixed, though, over how much success it was having in its bid to kill off reform plans it fears remove too much worker protection.
French nuclear power capacity was cut by at least 4 gigawatts (GW) as part of a rolling nationwide strike, grid operator RTE showed on its website. That is equivalent to about 6 percent of the country's total production capacity.
A report on France Inter radio's web site 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.
Power industry experts have said the nuclear plant strike is unlikely to provoke blackouts due to legal limits on strike action in the nuclear industry and power imports from abroad.
France has also mobilized its strategic oil and fuel reserves to keep fillings stations running as the strike action disrupts refinery production and deliveries from depots.
Prime Minister Manuel 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 on Wednesday saying; "The CGT does not rule this country."
On Thursday, Laurent Berger, head of the moderate CFDT union which is backing the reforms, called for a truce.
"The political and industrial relations climate has turned hysterical ... let's calm things down," he said on France Info.