Superstorm Sandy puts power grids to the test

NEW YORK (CNNMoney) -- Hurricane Sandy wreaked havoc on parts of the power grid Monday, knocking out power lines across the Northeast and forcing two nuclear power plants to shut down.

Some 6.5 million people remain without power as of Tuesday morning, according to CNN estimates.

In Manhattan, a massive explosion at a ConEdison plant along the East River left a big chuck of the island in the dark.

Throughout the region, downed trees and flooding have taken out power lines that serve millions. Utilities say it could take 7 to 10 days to repair the damage, even with extra crews that have been brought in from other parts of the country.

A ConEdison spokesman called it "the worst damage in ConEdison history."

Two nuclear reactors in the New York region closed overnight Monday as Hurricane Sandy disrupted operations at the facilities.

At the Indian Point power plant, located 34 miles north of Midtown Manhattan, one of two reactors automatically shut down at 10:45 p.m. Monday night after equipment detected a problem with the external power grid, according to plant spokesman James Steets.

The reactor was able to obtain power from other outside sources, but shut down as a precaution, said Steets. He said the reactor should be back online within a few days.

In New Jersey, a reactor at the Oyster Creek power plant was manually shut down in the early morning hours after pumps that draw water from the Delaware Bay became unavailable because of the storm, according to a statement from the plant's owner, PSEG Nuclear.

The shutdown occurred without incident and PSEG said the reactor is "stable."

The pumps are used to condense steam on the non-nuclear side of the plant, the statement said. There was no word as to when the reactor may restart. An adjacent reactor at the same plant remains operating at full power, while a third was previously shut for refueling.

Nuclear plants use electricity and electric pumps for many purposes, including circulating the water that cools the reactor core and, in some cases, the spent nuclear fuel.

It was a power failure in Japan caused by massive flooding that ultimately led to the meltdown at the Fukushima Daiichi nuclear power plant.

The Nuclear Regulatory Commission, the federal agency that regulates nuclear safety, sent additional inspectors to nine plants in the storm's path to make sure all safety procedures were followed.