Standing next to one of the four turbine shafts inside the Tennessee Valley Authority's Raccoon Mountain Pumped Storage Facility, Ken Cornett talks about the need for draining the 500-acre reservoir to complete repairs and updates on Friday.plant manager. TVA spokesman Scott Fielder, left, listens to the explanation by Cornett some 1,100 feet underground.

Backup Power Generation

› TVA’s Raccoon Mountain Pumped Water Storage Plant is one of the agency’s most unusual sites. Water is pumped from Nickajack Lake on the Tennessee River to a 528-acre storage reservoir at the top of Raccoon Mountain. The dam is 230 feet high and 5,800 feet long — the largest rock-fill dam ever built by TVA. But what really makes the facility unusual is the tunnel drilled through the mountain. During periods of high electric demand, water is released through the tunnel, driving generators in an underground hydroelectric plant at the mountain’s base.

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The intake structure at the Tennessee Valley Authority's Raccoon Mountain Pumped Storage Facility is seen as draining continues Friday at the 500-acre reservoir atop Raccoon Mountain in Marion County.

Raccoon Mountain Reservoir, TVA's backup for peak power demand periods, is full again just in time for winter's worst.

The mountain houses a pumped storage power plant and a 700-million gallon reservoir, which is flushed down a massive tunnel to generate hydropower during excessive cold or warm spells. Those are the times when TVA's power system is most likely to be strained, and having Raccoon Mountain online can save TVA from having to buy more expensive power from an outside source.

TVA first shut down the Raccoon Mountain plant in March 2012 because of four cracked rotors in turbines that spin in a magnetic field, creating electricity. The German firm that designed the units alerted TVA to check for cracks after rotors in a similar plant in Austria splintered in late 2009. It took about three years to get the plant fully back online, with all four generators working.

TVA spokesman Scott Fiedler said the 2015 maintenance check, that began on Oct. 16, was part of an annual safety program designed to ensure the facility operates smoothly.

"The facility got a clean health check," he said. "Everything has been inspected and put back together and ready to generate power."

Among changes, workers installed cables that are 15 pounds lighter per foot, replaced a 400,000-pound transformer, thoroughly inspected the four generating units, and drained the 528-acre reservoir, which took about 30 hours to refill after beginning Tuesday afternoon, Fiedler said.

Those details were but mere technicalities to some who frequent the mountain. What mattered to them was that the 22 miles of bike trails, which had been off limits for about five weeks during the best of the fall riding season, were accessible again as of Nov. 21.

"It broke a few hearts," said Kevin Smith of the closure. Smith is vice president of the Southern Off-Road Bicycle Association, which helps maintain the riding trails.

Although he hasn't returned since Raccoon Mountain reopened, Smith said he expected to squeeze in a Thanksgiving-morning ride — a two-going-on-three-year tradition — to justify his caloric intake.

It didn't take a holiday meal or bicycles to get Jeff Clay, and his two children, Alex and Sarah, out. The family was huddled along the railing at the reservoir lookout, watching the rising water move under the morning sun.

"We just had breakfast, and she's home from college," Clay said. "We were just looking for something to do."

The autumn wind rattled the leaves, while below, the water rippled in a dazzling display of silver and gold.

The Clay family watched for a moment, enraptured, silent, then packed into their car and went home.

Contact staff writer Zack Peterson at zpeterson or 423-757-6347. Follow @zackpeterson918.