NEW YORK, Sept 27 (Reuters) - Storm Helene forced major

U.S. electric utilities to shut or slow power plant operations

on Friday, with Southern Co ( SO ) taking one of its Georgia

nuclear reactors offline and Duke Energy ( DUK ) halting output from two

coal-fired generating units.

Helene, which left more than 3.5 million homes and

businesses without power as it weakened to a tropical storm and

moved north, downed power lines and battered other parts of the

region's electrical grid.

Southern Company ( SO ) also reduced output from another nuclear

reactor at its Edwin I. Hatch Nuclear Power Plant in Georgia

after the storm damaged the area's broader power system, the

company told Reuters on Friday.

The nuclear plant, which is jointly owned by Southern Co ( SO )

subsidiary Georgia Power ( GPJA ), Oglethorpe Power Corporation,

Municipal Electric Authority of Georgia and Dalton Utilities,

was not significantly harmed by the hurricane, the company said.

"However, the power grid experienced significant damage from

the storm and, to maintain grid stability, we adjusted

operations at Plant Hatch," Georgia Power ( GPJA ) spokesperson John

Kraft said.

Unit 1 was taken offline and Unit 2 is running at reduced

power, Kraft said, adding that the company's larger generation

fleet would be able to meet its customers' power demand. The

company did not disclose a timeline for restoring full

operations.

Duke Energy ( DUK ), meanwhile, took two coal-fired power

units offline at its Crystal River energy complex in Florida in

preparation for Helene, the company told Reuters on Friday.

"This will help prevent equipment damage to ensure the units

are available to operate for our customers after the storm,"

company spokesperson Jennifer Garber said.

Tens of thousands of utility workers have been mobilized to

assess damage and make repairs to the southeast electrical

system, which was beset with high winds and flooding.

Duke Energy ( DUK ), which primarily supplies power in the

Carolinas, has said it has been aggressively moving water

through the Catawba-Wateree River Basin in North and South

Carolina earlier this week in anticipation of Helene's arrival.

Even with these preparations, Duke warned that lake levels

are expected to rise sharply and may not be able to store excess

water from the expected rain and increased inflows from

upstream, possibly leading to historic flooding in several

areas.