Technicians embarked on the complex restart of France's Superphenix nuclear fast-breeder Thursday despite protests from environmental groups that want the fault-prone reactor closed down forever.

A spokeswoman for Superphenix said checks were being carried out and the reactor in 10 days will start a progressive build-up to the permitted 30 percent of its full 1,240-megawatt capacity.The reactor, authorized to operate for research into recycling nuclear waste, was shut down last December because of a fault in a heat exchanger.

The official safety agency, known as DSIN, earlier this week gave approval for the controversial reactor to be restarted following repairs at a cost of $4 million.

Europeans Against Superphenix, an umbrella organization of 250 environmental groups, called a demonstration for Sept. 6 to protest against the reactor, which operates at a cost of $120 million a year.

"After the ridiculous decision to resume nuclear testing, it is the second time the government . . . gives in to pressure from the military," the group said, referring to France's decision to stage eight underground tests at the Mururoa and Fangataufa atolls in the South Pacific from next month.

The Socialist Party also criticized the restart. "The decisions taken by the president of the Republic and his government are understandably worrying the French people. The Socialists condemn these decisions," it said in a statement.

The 18-year-old reactor at Creys-Malville, near the Alpine town of Grenoble, was closed last Christmas less than three weeks after being restarted following a four-year shutdown caused by a gas leak.