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One of Xcel Energy's two Prairie Island nuclear generators remains shut down after the company reported an "unusual event" Saturday to federal nuclear regulators.

Multiple fire alarms — which were not verified as false alarms within the required 15 minutes — went off in a reactor containment building, triggering the unusual event declaration.

There was no fire, and no threat to the public or plant workers, Xcel said in a notification to the U.S. Nuclear Regulatory Commission (NRC).

An unusual event is the lowest of four NRC emergency classifications. It indicates a potential reduction in a nuclear plant's safety level. Xcel last declared an unusual event at its nuclear operations in 2015.

Minneapolis-based Xcel said in a statement that Prairie Island's plant operators "were able to quickly determine that no fire existed and formally ended the unusual event."

The fire alarms went off at Prairie Island's Unit 2 after an external transformer — which steps up power flowing from the generator to the grid — malfunctioned.

The transformer's failure triggered a shutdown of the plant's turbine, which then signaled the reactor to shut down. The system worked as it was designed to do, Xcel said.

The company said it plans to power up Unit 2 later this week after finishing work on the transformer and completing standard procedures that must be done after a shutdown.

Prairie Island's Unit 1 has continued operating, and Xcel said electric service won't be affected by the Unit 2 shutdown.

Xcel also operates a nuclear plant in Monticello that the utility shut down for a few days in March to fix a pipe leaking tritium, a mildly radioactive form of hydrogen.

The pipe broke in November and Xcel initially fixed it a few weeks later but not before more than 400,000 gallons of contaminated water seeped into the ground. The first fix didn't hold, prompting a second fix in March.

Xcel said last week that it has recovered 53% of the leaked tritium, which is in an underground plume confined to Xcel's property. The company and state health and pollution regulators have said the leak does not pose a risk to people or the environment.

From a safety perspective, the tritium leak did not merit the declaration of an unusual event. Xcel did have to report it to the NRC and the state. Xcel and state regulators were criticized for not announcing the leak until mid-March.