ATLANTA — Federal regulators ordered in-depth inspections today 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.

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.