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 .
NEW : The plant 's owner may challenge the NRC citation . The designation indicates a problem of `` high safety significance '' The NRC hits Browns Ferry with a `` red finding '' over a faulty valve . New York City Mayor Michael Bloomberg has ordered an investigation into whether Entergy Corp. and its parent company Exelon