A US nuclear plant exited "alert" status Wednesday after the storm surge from Hurricane Sandy receded, ending the flood threat at New Jersey's Oyster Creek plant, though it remained offline.

Three other reactors were still shut down in the wake of the storm, which damaged transmission lines and other power infrastructure in the US northeast, officials said.

And one of the halted units, the New Jersey power company PSEG's Salem 1 reactor, reported having had to vent an amount of steam that possibly had small amounts of radioactive tritium.

"When there is a steam dump (non radioactive) there are possibly trace amounts of tritium," PSEG spokesman Joseph Delmar told AFP via email.

"It is closely monitored and well below any permit limitations," he said.

Officials ended the alert at the 43-year-old Oyster Creek reactor in Lacey Township, New Jersey, the country's oldest nuclear power plant, early Wednesday, saying the danger to the plant from rising waters had ended.

The 636-megawatt plant, operated by power company Exelon, had already been shut down for maintenance ahead of the huge storm on Monday.

But an alert situation -- the second lowest of four emergency classifications -- was declared as the storm knocked out the 636-megawatt plant's main power supply, forcing it to rely on generators, and surging waters caused by the storm threatened to top the banks of canals used to supply cooling water.

The alert was cancelled after waters fell to normal levels early Wednesday and power supplies from the main electricity grid resumed, the Nuclear Regulatory Commission said.

"Station employees responded quickly and appropriately to the storm's challenges and all plant safety systems, including used fuel cooling, operated as designed," the NRC said in a statement.

"Water levels were never high enough to top the intake canal banks or impact operation of plant equipment."

"Additional workers are expected to return to the plant today to resume a refueling and maintenance outage that began October 22 and was temporarily delayed by the storm."

The three other reactor units in the region still offline were the Salem 1 unit near Salem, New Jersey; the Nine Mile Point unit 1 in Scriba, New York; and the Indian Point reactor 3 in Buchanan, New York.

The companies did not say when they would resume generating power.