On Tuesday, the Nuclear Regulatory commission staff announced that a valve that got stuck last October at the Browns Ferry nuclear plant, near Athens, Ala., posed a safety threat that fell into the "red" category, the most serious on its four-color scale.
It is only the fifth time since the scale was established in 2001 that the commission has put a problem into that category.
The valve, at Browns Ferry Unit 1, is located in the reactor's residual heat removal system, which enables the reactor to cool after it has shut down. The failure was discovered when operators intentionally shut the reactor down for refueling, one of the few times that the heat removal system is used. It would also be used if there were an accident that required the reactor to shut down.
The failure of the residual heat removal system is a cause of the meltdowns at the Fukushima reactors, which are of a similar design to those at Browns Ferry.
The valve consists of a flat metal disk that sits inside the pipe and is moved by a rod. The rod had become disconnected from the disk, apparently some time weeks or months earlier. The residual heat removal system consists of two separate sets of pumps, valves and piping, and one set would not have worked, Mr. Hannah said.
"However, the system is counted on for core cooling during certain accident scenarios, and the valve failure left it inoperable, which could have led to core damage."
Such an accident would nonetheless involve "a series of unlikely events," the commission said.
The Tennessee Valley Authority told the commission staff that the valve had a manufacturing defect but that it would still have opened if needed; the commission staff disagreed.
In an unrelated event, General Electric notified Oyster Creek on May 4 and Nine Mile Point on May 6 of four separate calculation errors it had made in determining the peak cladding temperature of a new fuel that the plants were using in their reactor cores, operators said.
In some cases the errors caused G.E. to underestimate the peak temperature, and in others, overestimate it. The calculation is important because it helps determine the maximum power level at which a reactor can safely run.
The mistakes also involved the calculation of how much of the cladding would be damaged in an accident.
Scott Burnell, a spokesman for the Nuclear Regulatory Commission in Washington, said that when the calculations were corrected, each plant was still legal to operate but had less of a safety margin than the operators believed.