ABOUT THE PLANT
• Construction began in 1966, and the plant began operating in 1974.
• In March 2002, Brown's Ferry Unit 3 finished the longest unbroken operational run of a TVA nuclear unit and the second longest among all U.S. plants. It ran nonstop for 669 days before going offline for a scheduled refueling outage, generating a world-record 18 million megawatt hours during that time.
Source: Tennessee Valley Authority
One of the Browns Ferry Nuclear Plant's shutdown reactors lost cooling for about 40 minutes Thursday evening, triggering another mandatory notification of the Nuclear Regulatory Commission.
TVA spokesman Ray Golden said the new problem was akin to an electrical short that occurred as maintenance employees were working on electrical relays.
The problem caused a Unit 3 cooling pump to lose power. When the problem was discovered, the system was reset, much as a breaker switch would be flipped, he said.
The reactor cooling water temperature rose about 10 degrees during that 40 minutes - from 112.5 degrees to 122 degrees, according to the report TVA filed with the NRC.
The ferocious April 27 tornadoes took out all of the Tennessee Valley Authority's power grid in Alabama and much of the grid in Mississippi. All three reactors at Browns Ferry in Athens, Ala., were forced into automatic shutdown.
With transmission lines downed, the nuclear plant was not able to ship its excess power to distributors, and the plant shut down to stop producing electricity.
Because Browns Ferry is self-powered, the plant automatically switched to diesel generator power. TVA was able to supplement the diesel power with an outside electrical line to operate lights in the plant, but diesel generators powered the plant's shutdown cooling until May 2 when offsite power supplies to the plant were restored.
NRC spokesman Roger Hannah confirmed Friday that an electrical issue "unexpectedly affected additional equipment" at the nearly 40-year-old nuclear plant.
"Our inspectors believe the operators and maintenance staff dealt with the issue effectively, and it has been entered into the plant's corrective action program for further evaluation. Our resident inspectors will follow that evaluation," Hannah said.
Earlier this week, the NRC, which regulates all 104 U.S. commercial reactors, red-flagged Browns Ferry for safety problems with a stuck valve last fall and placed the plant on "red" or "high safety significance" status.
The NRC's finding will result in increased NRC inspection and oversight - something the commission has ordered four other times in the past decade.
On Oct. 23, the faulty valve failed to open when operators shut down the Unit 1 reactor for refueling. To overcome the problem, TVA used another cooling water line that was supposed to be dedicated to fire safety - a design adopted as a result of a fire at the plant in 1975 that damaged reactor control cables.
The 1975 fire was started by a worker using a candle to search for air leaks, according to TVA and NRC reports.
TVA later determined that the last time the faulty valve definitely had worked as required was on March 12, 2009.
In announcing the NRC "red" finding, Hannah said the public was not endangered when the valve failed because plant operators were able to use the alternative shutdown method.
"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," he said.
Golden has said TVA may appeal NRC's decision.
The Browns Ferry plant has three nuclear reactors with a similar design to those at the Fukushima Dai-ichi nuclear plant in Japan. That plant was thrown into nuclear crisis after a 9.0-magnitude earthquake and tsunami left its six reactors without cooling water.