The Watts Bar unit 2 nuclear reactor brought online earlier this year — 42 years after construction began — has been shut down following a fire.

Unit 2 at the Watts Bar facility between Knoxville and Chattanooga, Tennessee, went offline automatically Tuesday evening because of a transformer fire in the adjacent switch yard, with plant operator Tennessee Valley Authority (TVA) personnel and three local fire departments extinguishing the fire within an hour, without injury.

The fire is the latest obstacle for a plant that took more than four decades to be built amid competition from cheap fossil fuels and required safety upgrades in the wake of the Fukushima disaster in Japan.  Low-cost natural gas is squeezing nuclear generators’ profits, leading power producers to announce plant closures or seek state subsidies, as is the case in New York.

“Our current focus remains on the safety of our personnel and the public as we determine the cause of the fire and any repairs necessary,” TVA spokesman Jim Hopson said. There is no timeline for returning the unit to service.

Unit 2 connected to the regional power grid earlier this year after receiving a license from the U.S. Nuclear Regulatory Commission in late 2015. Power levels at the reactor have fluctuated in recent months as TVA conducts required testing to begin full commercial operation.

Watts Bar 2 was the first new U.S. reactor to connect to the grid since TVA, the largest public power utility in the U.S., started running Unit 1 in 1996. Unit 2 has a maximum summer capacity of 1,150 megawatts.