NEW YORK (Reuters) - Exelon Corp declared an "alert" at its New Jersey Oyster Creek nuclear power plant due to a record storm surge, the Nuclear Regulatory Commission said, warning that a further water rise could force the country's oldest working plant to use emergency water supplies to cool spent uranium fuel rods.

The alert -- the second lowest of four NRC action levels -- came after water levels at the plant rose by more than 6.5 feet, potentially affecting the pumps that circulate water through the plant, an NRC spokesman said late on Monday.

Those pumps are not essential since the 43-year-old plant was shut for planned refueling since October 22. However, a further rise to 7 feet could submerge the service water pump motor that is used to cool the water in the spent fuel pool.

Exelon said in a statement that there was no danger to equipment and no threat to public health or safety.

The incident at Oyster Creek, which is about 60 miles east of Philadelphia on the New Jersey Coast, came as Sandy made landfall as the largest Atlantic storm ever, bringing up to 90 mile per hour (mph) winds and 13-foot storm surges in the biggest test of the industry's emergency preparedness since the Fukushima disaster in Japan a year and a half ago.

Although such alerts are considered serious events in the industry -- with only about a dozen such instances in the past four years, according to NRC press releases -- flood waters should be receding at the plant following high tide, reducing the risk of emergency action.

Sandy had been expected to force the closure of at least two other nuclear plants in New Jersey, although the NRC said none of the country's other nuclear reactors had been shut by the storm.

The NRC spokesman said the company could use water from a fire suppression system to cool the pool if necessary. The used uranium rods in the pool could cause the water to boil within 25 hours without additional coolant; in an extreme scenario the rods could overheat, risking the eventual release of radiation.

Exelon spokesman David Tillman said the plant has "multiple and redundant" sources of cooling for the spent fuel pool. He said he did not know whether the service water system was operational at the moment.

MONITORING NUCLEAR POWER PLANTS

Constellation Energy Nuclear Group's 630-MW Nine Mile Point 1 nuclear power reactor in upstate New York did shut down due to a problem putting power onto the grid, although it was not clear whether the trouble was related to the storm, the NRC spokesman said.

The relatively small 636-megawatt Oyster Creek plant also experienced a "power disruption" at its switch yard, causing two backup diesel generators to kick in and maintain a stable source of power, Exelon said.

Tillman said another Exelon reactor at the Limerick facility in Pennsylvania was reduced to 91 percent power after Sandy caused a problem with the condenser.

An alert-level incident means there is a "potential substantial degradation in the level of safety" at a reactor.

"Given the breadth and intensity of this historic storm, the NRC is keeping a close watch on all of the nuclear power plants that could be impacted," NRC Chairman Allison Macfarlane said.

The concerns over the status of the spent fuel pool at Oyster Creek was reminiscent of the fears that followed the Fukushima disaster last year, when helicopters and fire hoses were enlisted to ensure the pools remained filled with fresh, cool water. The nuclear industry has said that the spent fuel rods at Fukushima were never exposed to the air.

Nuclear plants must store the spent uranium fuel rods for at least five years in order to cool them sufficiently before they can be moved to dry cask storage containers.