The Tennessee Valley Authority today is trying to determine the cause of a fire in the switch yard of its Watts Bar Nuclear Power Plant that started around 9:11 p.m. Tuesday night and took TVA fire crews about an hour to extinguish.

The blaze in one of the main bank transformers connected to the new Unit 2 reactor triggered notice of “an unusual event” — the lowest of four emergency classifications for problems at a nuclear power plant. Although the fire did not affect any nuclear or generation equipment in either the reactor or turbine buildings at Watts Bar, it did damage a transformer and required TVA to shut down its Unit 2 reactor indefinitely, TVA spokesman Jim Hopson said today.

“Nobody was injured and there was no risk to the public from the fire, but we are still evaluating the cause and assessing the damage,” Hopson said.

TVA declared the unusual event Monday night but exited the notice by 11:40 p.m. after the fire was extinguished about 10:20 p.m. and TVA was later able to verify that all parts of the plant performed as expected during and after the reactor trip.

Because the transformers are cooled by oil and operate in a switch yard dispatching up to 2,200 megawatts of electricity when the units are at full power, TVA called upon three volunteer fire departments in Rhea and Meigs counties to help with the fire response. TVA fire crews put out the fire, Hopson said, and the Unit 1 reactor at Watts Bar was not affected by the fire.

But the transformer fire and the unusual event notification triggered an alert to both state emergency agencies and the U.S. Nuclear Regulatory Commission, which is now reviewing the incident.

TVA was in the process of bringing the Unit 2 reactor up to 100 percent power for the first time when the fire broke out Monday night. The unit 2 reactor at Watts Bar is the first new nuclear unit to be added to America’s electric grid in more than 20 years and began limited power generation in May.

TVA had originally budgeted to have the unit 2 online at full power by the middle of 2016, but that schedule has slipped and TVA had to increase its capital budget for the work at the plant from $4.5 billion to $4.7 billion earlier this year.

TVA brought the Unit 2 reactor up to 75 percent power in early August and last week idled the unit for what officials had hoped would be the last time before the unit was brought to full power as part of a series of power ascension tests for the new reactor.

The main bank transformers take high-voltage power generated by the reactors and convert the power for distribution into the plant switchyard, where the electricity can then be dispatched across TVA’s transmission lines to local power companies across TVA’s 7-state region.

When the fire erupted, the reactor had to be shut down and the unit will remain idle until a new transformer is installed and the cause of the fire is determined.