Both Sequoyah and Watts Bar had unplanned reactor shutdowns last month.
Both reactors had electrical problems that caused the power plants to scram -- something akin to blowing a safety fuse, said Tennessee Valley Authority spokesman Ray Golden.
In the shutdowns, the reactors -- Unit 1 at Watts Bar and Unit 2 at Sequoyah -- functioned as they were designed to, and neither the plants nor the workers or public were put at risk.
Sequoyah tripped on Aug. 16 following an electrical short in one of the four reactor coolant pumps. It was returned to power on Aug. 18.
Watts Bar shutdown on Aug. 28 due to human error, Golden said. A worker inadvertently caused a fault one of the unit's instrumentation systems by plugging in a testing device that was incorrectly calibrated. The reactor was returned to power on Aug. 30.
But having two local reactors quit in the same short time period is unusual, and unplanned outages can create difficulties for a utility, Golden acknowledged.
While the reactors aren't generating power, even for just a few hours or days, TVA must increase power generation at other plants and sometimes bring idled plants online.
What's more, TVA, with a number of unplanned shutdowns at its nuclear and non-nuclear power plants in the past year, is conscious of its image. Sequoyah's unit 1 had so many that the Nuclear Regulatory Commission last year placed the reactor under its lowest safety concern flag -- a white finding. That reactor had more the three unexpected outages in less than 7,000 operating hours or about a year.
"We're disappointed with having any reactors trips, Golden said. "But it is a complicated machine with a lot of electrical components."
NRC spokesman Joey Ledford said NRC inspectors had no initial safety concerns about the reactor scrams.
"We will make a complete review," he said.
Golden said TVA, too, has made a point of looking for the root causes of the shutdowns and for "lessons" to apply for future prevention.
"But from a morale standpoint," he said, "both of the reactors had been operating for a long time with no problems."
Sequoyah had been operating for 421 days, and Watts Bar had been operating nonstop for 319 days, he said.
Golden said TVA also will have three scheduled reactor outages before this year is out.
"We try to schedule maintenance outages in the spring or fall," normally off-peak seasons, he said.
One began Monday at Watts Bar's Unit 1 -- the plant's only operating reactor.
During that shutdown, plant personnel will replace spent nuclear fuel assemblies, and perform maintenance and system upgrades, as well as make tests and inspections that can only be done when the reactor is off-line.
Later this fall, reactors at Sequoyah and Browns Ferry will undergo similar planned shutdowns for maintenance and upgrades.
By scheduling the outages in one season, TVA is hoping to make use of the same contractors and to economize by streamlining training and worker start-up time, Golden said.