The future of nuclear energy in the country has become a campaign issue as the presidential election approaches this spring, thanks in part to the earthquake and tsunami disaster at Japan's Fukushima plant last year.
But Sarkozy, who lags in the polls, said Thursday while visiting the plant in Fessenheim that it would be a huge mistake and a "scandal" to close it and lay off its workers. He insisted that there was no doubt about the plant's safety.
The plant, which opened in 1978 in northeast France, is the country's oldest. France relies on nuclear energy more than any other nation, with about three-quarters of electricity coming from nuclear reactors.
Socialist presidential candidate Francois Hollande has pledged to close down Fessenheim and reduce nuclear dependence to 50 percent if he's elected president in elections in April and May.
His pledge stemmed from a political pact with France's leading environmental party and signs that the French public is starting to question the safety of nuclear energy.
Hollande leads opinion polls ahead of the vote. Sarkozy has not officially declared his candidacy but is widely expected to seek a second term.
Sarkozy has lobbied hard at home and abroad in favor of nuclear power, a substantial sector of the French economy. France actively exports nuclear energy technology and takes in nuclear plant waste from countries around the world.
"We will not close it, this plant," Sarkozy said, to cheers by workers at Fessenheim. "Why would we close it for political reasons? ... Where else would we go to get (electricity)?"
"It's madness, madness," he said.
France chose to invest heavily in nuclear energy after the oil shocks in the 1970s and governments left and right have stuck to nuclear energy ever since, even when other European countries scaled back after the 1986 accident at the Chernobyl nuclear plant spewed a radioactive cloud over much of the continent.
France's industry minister, Eric Besson, is convening a meeting in Paris on Friday with his counterparts from 15 other European countries that have invested in and remain committed to nuclear energy.