Three US nuclear power reactors remained shut down and a fourth on alert Tuesday after Hurricane Sandy wreaked havoc with transmission networks and high waters threatened cooling systems.

Amid worries that waters could overwhelm the reactors as happened in Japan's Fukushima nuclear emergency last year, authorities said there were no risks to the public.

New Jersey's main power company PSEG Nuclear shut down its Salem 1 unit at Hancocks Bridge on the Delaware river, saying most of its water circulation pumps had been rendered unusable "due to weather impacts".

The Nine Mile Point unit 1 in Scriba, New York, and the Indian Point reactor 3 in Buchanan, New York were also halted.

Power company Entergy said the Indian Point plant was shut down "due to external electrical grid issues."

Separately, the country's oldest nuclear plant, the 43-year-old Oyster Creek plant in Lacey Township, New Jersey, was under an "alert" designation because of high waters in its water intake structure due to the huge storm surge.

Nuclear regulators said they had been monitoring all the plants during the storm and that there were no current dangers of meltdowns.

"Everything appears to be under control at this point. There was no infrastructure damage to any of the plants," Neil Sheehan, spokesman for the Nuclear Regulatory Commission, told AFP.

"Overall, considering that this was a historic storm, the plants have by and large fared well."

"We will continue to work to make sure that it's safe to bring those units back on line, there are still some great stability issues that we'll have to work through," he added.

Many other nuclear power units continued to work throughout the storm, though some were at reduced levels to adjust to demand changes in the power system and grid outages.

Power outages across the northeast left more than eight million people and businesses without electricity Tuesday, with warnings that many would not regain power for several days.

The Oyster Creek plant, which was in a scheduled outage when the storm hit, set off an alert when floodwaters exceeded a threshold set for its water intake facilities.

The NRC said late Tuesday that water levels were subsiding to more normal levels, reducing the threat.

"The plant remains in an Alert status until there is enough confidence levels will remain at more normal levels. Offsite power at the plant is in the process of being restored," it said in a statement.