NEW YORK, Oct 30 (Reuters) - Hurricane Sandy slowed or shut a half-dozen U.S. nuclear power plants, while the nation's oldest facility declared a rare "alert" after the record storm surge pushed flood waters high enough to endanger a key cooling system.

Exelon Corp's 43-year-old New Jersey Oyster Creek plant remains on "alert" status, the U.S. Nuclear Regulatory Commission (NRC) said early Tuesday. It is only the third time this year that the second-lowest of four emergency action levels was triggered.

The alert came after water levels at the plant rose by more than 6.5 feet (2 meters) above normal, potentially affecting the "water intake structure" that pumps cooling water through the plant, an NRC spokesman said.

Those pumps are not essential since the reactor has been shut for planned refuelling since Oct. 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, potentially forcing it to use emergency water supplies from the in-house fire suppression system to keep the rods from overheating.

On Tuesday, an NRC spokesman said the levels reached a peak of 7.4 feet -- apparently above the threshold. As of 6:10 a.m. EDT waters were at 6.5 feet, with the next high tide at 11:45 a.m. He said the company had moved a portable pump to the water intake structure as a precaution, but has not needed to use it.

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

"Right now there's no imminent threat of releases. There's no protective actions around the plant," Federal Emergency Management Agency Administrator Craig Fugate said on the Today Show.

"Some of these reporting requirements are due though to the severity of the storms. That they have to make these notifications based upon conditions, that does not mean that they are in an imminent threat at the plant," Fugate said.

The incident at Oyster Creek, which is about 60 miles (95 km) 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.

Despite the alert -- which is a serious but not catastrophic event that signals a "potential substantial degradation in the level of safety" -- the U.S. nuclear industry was broadly seen having passed the test. About a dozen alerts have been issued in the past four years, according to NRC press releases.

On Tuesday morning, the NRC said that Entergy Corp's Indian Point 3 automatically tripped offline at about 10:41 p.m. last night due to fluctuations in the power grid caused by the storm, while Public Service Enterprise Group Inc's Salem Unit 1 was manually shut down at 1:10 a.m. due to a loss of "condenser circulators" due to the storm surge and debris.

SPENT FUEL

The relatively small 636-megawatt (MW) Oyster Creek plant earlier 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.

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 in about 25 hours without additional coolant; in an extreme scenario the rods could overheat, risking the eventual release of radiation.

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.

Exelon spokesman David Tillman said Monday night 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 last night.

The plant uses pumps to take in external water that circulates through a heat exchanger used to cool the internal water that surrounds the rods, keeping them from overheating.

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

In addition, Sandy caused power reductions at both units at Exelon's Limerick nuclear plant in Pennsylvania and one unit at Dominion's Millstone plant in Connecticut.

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