Crews are working to determine what caused a transformer fire Tuesday evening at the Watts Bar Nuclear Plant.

The Tennessee Valley Authority said the fire at an onsite switchyard was reported around 9:11 p.m. Tuesday.

When the transformer failed at the Spring City site, Watts Bar Unit 2 was automatically taken offline, according to a post on TVA's website.

Unit 2 is the first new nuclear reactor in the U.S. in the 21st century. It's been undergoing testing to get it ready for full-power operation.

While plant personnel requested assistance from three local fire departments, onsite personnel were able to put the fire out before the additional help was necessary.

Crews fully extinguished the fire about an hour after it was first reported.

TVA stressed "there was never a danger to the general public" and Watts Bar Unit 1 was unaffected by the fire and remains online.

"Our current focus remains on ensuring the safety of the public and our employees as we work to determine the cause of the fire and make necessary repairs to allow Unit 2 to resume its power ascension testing," TVA explained.