Rising water threatened the cooling system at the Oyster Creek nuclear plant, in Toms River, N.J., on Monday night. The plant declared an alert at 8:45 PM, which is the second-lowest level of the four-tier emergency scale established by the Nuclear Regulatory Commission.

The water level was more than six feet above normal. At seven feet, the plant would lose the ability to cool its spent fuel pool in the normal fashion, according to Neil Sheehan, a spokesman for the Nuclear Regulatory Commission.

The plant would probably have to switch to using fire hoses to pump in extra water to make up for evaporation, Mr. Sheehan said, because it could no longer pull water out of Barnegat Bay and circulate it through a heat exchanger, to cool the water in the pool.

Having first come online in 1969, Oyster Creek is the nation's oldest operating nuclear reactor, and is of the same design as those that failed at the Fukushima Daiichi plant. The reactor is located 50 miles east of Philadelphia and 75 miles south of New York City; 4.5 million people live within its 50-mile radius.