UPDATE 4-Irma knocks out power to over 2 million in Florida -utilities

(Updates outages as of 4:36 p.m. local time)

Sept 10 (Reuters) - Hurricane Irma knocked out power to more than 2.4 million homes and businesses in Florida on Sunday, threatening millions more as it crept up the state's west coast, and full restoration of service will take weeks, local electric utilities said.

Much of the state had yet to feel the full brunt of the storm as Irma barreled across the Florida Keys on Sunday morning and rolled up the state's southwest coast. With maximum sustained winds of 120 miles (195 km) per hour, it was a Category 3 storm, the third-worst on the Saffir-Simpson scale.

So far, the brunt of the storm has affected Florida Power & Light's customers in the states' southern and eastern sections.

FPL, the biggest power company in Florida, said more than 2.3 million of its customers were without power by 4:30 p.m. EDT (2030 GMT), mostly in Miami-Dade, Broward and Palm Beach counties. More than 200,000 had electricity restored, mostly by automated devices.

"We will have to rebuild part of our system, particularly in the western part of the state," FPL spokesman Rob Gould said at a news conference on Sunday. "That restoration process will be measured in weeks, not days."

FPL is a unit of Florida energy company NextEra Energy Inc .

Large utilities that serve other parts of the state, including units of Duke Energy Corp, Southern Co and Emera Inc, have had scattered outages.

Duke warned its 1.8 million customers in northern and central Florida that outages could exceed 1 million. Emera's Tampa Electric utility said the storm could affect up to 500,000 of the 730,000 homes and businesses it serves.

The utilities had thousands of workers, some from as far away as California, ready to help restore power once Irma's high winds pass their service areas. About 17,000 were assisting FPL, nearly 8,000 at Duke and more than 1,300 at Emera.

FPL said on Friday that Irma could affect about 4.1 million customers, but that was before the storm track shifted away from the eastern side of the state. Its customers are concentrated in Miami-Dade, Broward and Palm Beach counties.

Last October, Hurricane Matthew knocked out 1.2 million FPL customers as it skirted Florida's east coast without making landfall or damaging infrastructure. It took the utility about two days to restore power.

FPL decided to shut only one of the two reactors at its Turkey Point nuclear plant about 30 miles (48 km) south of Miami on Saturday because the storm track shifted. It plans to leave both reactors in service at the St. Lucie plant about 120 miles north of Miami because hurricane-force winds are no longer expected to hit the sites.

On Sunday, Gould said its nuclear plants were safe.

FPL had planned to shut both units at Turkey Point sometime on Saturday about 24 hours before hurricane-force winds reached the plant.

There is also spent nuclear fuel at Duke's Crystal River plant, about 90 miles north of Tampa. The plant, on Irma's current forecast track, stopped operating in 2009 and was retired in 2013.

In a worst-case scenario, the spent fuel could release radiation if exposed to the air, but a federal nuclear official said that was extremely unlikely.

"There is nothing to indicate there is any concern for the spent fuel stored at Crystal River," said Scott Burnell of the U.S. Nuclear Regulatory Commission. "That fuel is so cold, relatively speaking, it would take weeks before there would be any concern."

Duke was transferring the spent fuel to dry cask storage as part of the work to decommission the plant but suspended the effort temporarily ahead of Irma. (Reporting by Scott DiSavino in New York; additional reporting by Jessica Resnick-Ault; Editing by Jeffrey Benkoe and Lisa Von Ahn)