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 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 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, potentially forcing it to use emergency water supplies from the in-house fire suppression system to keep the rods from overheating.