Florida’s largest electric company shut down a nuclear plant south of Miami for safety reasons today, causing sporadic power outages covering large portions of Central Florida and the state that could last until 6 p.m.

As many as 3 million people are affected, the state says.

The Nuclear Regulatory Commission said that the two Florida Power & Light nuclear reactors at its Turkey Point power plant 30 miles south of Miami automatically shut down. Two other power plants farther north, the Crystal River reactor and St. Lucie twin reactors, in the state continued to operate, although officials at those two facilities noticed the grid disturbance.

The outage was caused by a transmission substation that failed, said Todd Brown, a spokesman with the state Public Service Commission, which regulates investor-owned utilities in Florida. The substation was fed with electricity generated by the nuclear-powered Turkey Point plant owned by Florida Power & Light.

Once the substation stopped working, the two nuclear units at Turkey Point automatically shut down, Brown said. “If there is anything good to come of this, it is that the system worked the way it was supposed to,” Brown said.

The outages have no connection to terrorism, Homeland Security Department spokeswoman Laura Keehner said.

The two Turkey Point units, opened in 1972 and 1973, generate 1,400 megawatts of electricy, enough power to supply the annual needs of more than 450,000 houses. The units are on Biscayne Bay, 24 miles south of Miami, just east of Homestead.

“There are no safety concerns. The reactors shut down as designed,” said Kenneth Clark, a spokesman at the NRC regional office in Atlanta in a telephone interview.

He said both reactors continued to have offsite electric power. He said two coal-burning power plants at Turkey Point also shut down.

Florida’s electric network lost nearly 10 percent of its output thanks to the blown substation in South Florida, said Sarah Rogers, president of the Florida Reliability Coordinating Council, a Tampa association that tracks power production in the state.

That decrease came during a time of reduced consumption because of mild weather, reducing the possibility of more extensive outages, she said.

Utilities throughout the state were cleared to increase production and bring back generating units that tripped off by 3 p.m. The only company experiencing issues was Florida Power & Light, which had to make up for the two nuclear units that remain off line because of the bad substation that triggered the statewide outage.

FP&L is handicapped, Rogers said, because a number of large utility lines feeding its system are not in use because of routine maintenance.

FPL in several media interviews estimated that power should be up statewide within 10 hours. The company did not return repeated calls from The Associated Press or speak to a reporter in the lobby of its Juno Beach headquarters. As many as 800,000 customers are without power today as Florida is experiencing widespread power outages.

There are reports of outages in Orange, Lake, Seminole, Volusia and Brevard counties, as well as both coasts from Miami to Jacksonville and Naples to Tampa Bay.

The power outage in South Florida rippled across Florida as a brownout, or drop in power flowing through the grid that connects all utilities.

The State Emergency Operations Center in Tallahassee has activated to a Level 2, partial activation, in response to the current power outage in the state. Currently there are approximately 3 million persons without power, state officials said.

Locally, schools, businesses and intersections are without power this afternoon.

Orlando Utility Commission spokesman Sheridan Becht said OUC’s generating system automatically began at 1:09 p.m. to shut down 13 circuits at 11 substations across the metro area.

That left 11,438 customers, mostly residential, without power for between two and 20 minutes, Becht said. The utility has about 250,000 customers.