Atlanta (CNN) -- U.S. safety officials have cited Alabama's Browns Ferry nuclear power plant for a failed valve that could have hindered efforts to cool one of the reactors during an emergency, the Nuclear Regulatory Commission announced Tuesday.
The problem in the No. 1 reactor unit has resulted in a top-level "red finding" by the NRC, bringing increased federal oversight of the three-unit plant for some time.
"The public was never endangered because no actual event occurred," the NRC said in announcing the citation. "However, the system is counted on for core cooling during certain accident scenarios and the valve failure left it inoperable, which potentially could have led to core damage had an accident involving a series of unlikely events occurred."
Browns Ferry is located outside Athens, Alabama, about 80 miles north of Birmingham. The Tennessee Valley Authority, which owns the plant, had argued against the finding and may appeal the decision, TVA spokesman Ray Golden told CNN.
"We think we have a safe plant," he said. "We have a number of higher safety measures in place. That said, we can do more and are planning to do more."
The bad valve was part of a residual heat removal system that cools the reactors after they shut down. The problem was discovered when the unit was being refueled in October, said Joey Ledford, a spokesman for the NRC regional office in Atlanta.
"This violation will result in additional regional and headquarters inspectors in the plant doing various other additional inspections," Ledford said. The plant's "safety culture" will be under scrutiny along with its technical systems as a result of the ruling, he said.
The citation is the fifth red finding issued by the NRC under a system that went into effect in 2001, Ledford said. It is the first issued to Browns Ferry or to any of TVA's three nuclear plants, Golden said.
The 3,200-megawatt Browns Ferry plant remains idled following April 27 tornadoes that knocked out external power to the plant. Unit 1 was the scene of a 1975 fire that damaged the control room's electrical systems, an accident that led to new safety procedures at U.S. nuclear plants.
The problem found in October could have led to an increased threat of fire under some conditions, Golden said.
"We acknowledge that we have more work to do in fire protection and are committed to doing that," he said.
The faulty valve had been in place since Browns Ferry was built in the late 1960s, and its failure was the result of a manufacturing defect, he said. But the utility argued that the reactor had multiple backup systems that operated despite the failed valve, and the faulty device eventually responded properly during laboratory tests.
"It became stuck, but we were able to demonstrate in these labs that if you allowed enough time to go by, it would unstick itself," Golden said.
The valve was replaced shortly after the problem was found, and was working at the time of the April tornadoes, said Roger Hannah, an NRC spokesman in Washington. The agency cited TVA for failing to identify the problem during earlier inspections, he said.
"We felt they should have identified this valve as a problem before it was discovered," Hannah said.
CNN's Devon Sayers and Mike Ahlers contributed to this report.