Federal regulators ordered in-depth inspections Tuesday at a nuclear power plant run by the Tennessee Valley Authority in northern Alabama after deciding the failure of an emergency cooling system there could have been a serious safety problem.

The U.S. Nuclear Regulatory Commission issued a rare red finding against the Browns Ferry nuclear power plant near Athens, Ala., after it investigated how a valve on a residual heat removal system became stuck shut. Safety regulators said only five red findings — the most severe ranking the agency gives to problems uncovered in its inspections — have been issued nationwide in the past decade.

In an emergency, the failure of the valve could have meant that one of the plant’s emergency cooling systems would not have worked as designed. The problem, which was identified as the plant was being refueled in October 2010, was fixed before the reactor was returned to service.

“The valve was repaired prior to returning the unit to service and Browns Ferry continued to operate safely,” said Victor McCree, the NRC’s Region II administrator. “However, significant problems involving key safety systems warrant more extensive NRC inspection and oversight.”

It was not immediately clear whether TVA officials would appeal the finding from federal regulators. Officials at the TVA had attributed the valve to a manufacturer’s defect and said it inspected all similar valves in the facility to catch any problems.

NRC officials were critical of the utility for not identifying the problem sooner through routine inspections and testing. The valve failed sometime after March 2009 but wasn’t discovered until more than a year later.

As part of the upcoming inspections, the NRC said it will review the plant’s performance, its safety culture and its organization.

“The results of this inspection will aid the NRC in deciding whether additional regulatory actions are necessary to assure public health and safety,” McCree said in a letter to TVA officials.

Past problems at the plant have led to increased scrutiny. The Browns Ferry Plant is known in the industry as the site where a worker using a candle to check for air leaks in 1974 started a fire that disabled safety systems. It is similar in design to the reactors that malfunctioned at the Fukushima Dai-ichi nuclear plant in Japan after a massive earthquake and tsunami earlier this year.

TVA, the county’s largest public utility, supplies power to about 9 million people in Tennessee, Alabama, Mississippi, Kentucky, Georgia, North Carolina and Tennessee.