Malfunction shuts down Browns Ferry Unit 1 for more testing
By Eric Fleischauer
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Unit 1 at Browns Ferry Nuclear Plant hit another glitch Saturday, briefly causing an increase in pressure within the reactor vessel, according to Nuclear Regulatory Commission records.
The event, coming after Tennessee Valley Authority hooked Unit 1 to the power grid for the first time in 22 years and after making it to 99-percent power, was a slight setback in the re-start.
According to a report TVA filed with the NRC, a turbine malfunction forced TVA to drop power levels to 0 percent Saturday at 11 a.m. Unit 1 had been at 99-percent power Saturday morning. The report filed with the NRC said it was at 79-percent power at the time of the shutdown.
"We were doing power ascension testing. Part of that is to see what problems you
can find," Browns Ferry spokesman Terry Johnson said. "The plant hasn't been run in a long time, so there's all types of opportunities for things not to go quite as expected."
The malfunction occurred when sensors indicated increasing water levels in one of the steam-fed turbines. The turbine shut down automatically to avoid damage from excess water.
The turbines, shaped like fans, rotate at 1,800 revolutions per minute, said Johnson. At such high speeds, any water droplets in the steam can cause damage. For that reason, the sensors detect when the droplets have not been separated from the steam before entering the turbines.
Steam from the reactor vessel feeds the turbines. When the turbines shut down, not all of the steam produced in the reactor could escape.
"It was pretty insignificant," Johnson said.
Unit 1's steam pipes have 11 safety relief valves to prevent a dangerous build-up of reactor pressure, according to Dave Lochbaum, formerly a Unit 1 reactor engineer. The relief valves operated properly, dropping the reactor pressure.
Lochbaum, now director of nuclear safety at the Union of Concerned Scientists, called the event a "bump" in the re-start process, but "not a big bump."
He said the severity of the issue depends on the result of TVA's ongoing investigation into the cause of the turbine malfunction.
TVA investigators also are studying why one system involved in the automatic shutdown of the reactor did not work properly. The faulty system could, in some scenarios, permit water to leak from the reactor.
Called a "hot shutdown" or scram, the drop in Unit 1's power levels did not require removal of the fuel rods. Rather, TVA fully inserted neutron absorbers into the reactor vessel, halting the energy-producing fission within the reactor.
At full power, Unit 1 generates 1.2 billion watts of electricity, enough to supply 650,000 homes.
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