An alert was declared at the Oyster Creek nuclear power plant in New Jersey because of water exceeding certain high water level criteria in the plant's water intake structure, according to the U.S. Nuclear Regulatory Commission.
An "unusual event" was declared around 7 p.m. on Monday.
The situation was upgraded to an "alert" about two hours later.
Water level was rising in the intake structure due to a combination of a rising tide, wind direction and storm surge, according to the NRC.
The NRC said in a statement that is was continuing to monitor impacts from Hurricane Sandy on nuclear power plants in the northeastern United States.
"All plants remain in a safe condition, with emergency equipment available if needed and NRC inspectors on-site, the NRC's statement said.
The Oyster Creek plant is in Lacey Township, N.J. It gets its cooling water from the Barnegat Bay. It is the nation's oldest nuclear power plant.
Oyster Creek has operated since 1969 and provides about 9% of New Jersey's electricity.