"We will not close this station, it's out of the question," Sarkozy told cheering workers during a visit to the plant, in Alsace in northeastern France.

Ahead of a two-round presidential election in April and May, Socialist frontrunner Francois Hollande has promised to reduce France's reliance on nuclear energy from 75 percent to 50 percent by shutting down 24 reactors by 2025.

The Socialist plan includes shutting down the two reactors at the Fessenheim plant, which dates from 1977.

Without mentioning Hollande by name, Sarkozy said the closure of the plant would come "at the cost of jobs in the nuclear industry, the cost of our industrial competitiveness and the cost of our energy independence.

"I will never accept the closing of the Fessenheim station for political reasons," Sarkozy said. "Wanting to close Fessenheim is a scandal, because it would mean sacrificing your jobs for backward political thinking."

About 30 anti-nuclear protesters demonstrated outside the plant during the visit and an aide to Sarkozy met with anti-nuclear activists.

France, the world's most nuclear-dependent country, operates 58 reactors and has been a leading international proponent of atomic energy.

But the country's reliance on nuclear power has been called into question since the Fukushima disaster in Japan, which prompted Germany to announce plans to shut all of its reactors by the end of 2022.

"Due to persisting heavy winter conditions in the country we will stop electricity exports as of 1:00 am Friday (2300 GMT Thursday)," the ministry said in a statement.

"The decision is necessitated by the shortage of about 400 megawatts per hour for the local market," it added.

Bulgaria exports an average 1,000 megawatts of electricity per hour to neighbouring Greece, Serbia, Macedonia and Turkey.

But the ministry said it had already switched on all its reserve capacities to cover increased domestic demand as temperatures dipped to minus 28.6 degrees Celsius (-19.5 Fahrenheit) in the northwestern town of Vidin early Thursday.

The situation was aggraveted by problems with production at the US-owned AES Galabovo coal-fired plant in southern Bulgaria, it added without elaborating.

Severe flooding had disrupted coal supplies to all four plants in the Maritza East complex near Galabovo this week as miners could not get down to work in the three open pit mines in the region.

"Electricity exports will resume when we recover the reserves necessary to guarantee safe and uninterrupted home supply," the ministry said Thursday.

Bulgaria is a key electricity exporter on the Balkans sending abroad 10.5 billion kilowatt hours last year.