The modifications, which included repairing a ventilation system and replacing a pump on the 750,000-gallon tank, are part of a plan to close 22 of the site's remaining 49 tanks within eight years.
Originally, there were 51 underground tanks at the site's H and F areas. Two were closed in the 1990s, and the remaining tanks still contain 36 million gallons of decaying waste left behind by decades of Cold War nuclear weapons production.
John Lindsay, a spokesman for liquid waste contractor Savannah River Remediation, said six tanks are in some phase of closure activity.
"Right now we're working on 5 and 6, and those will be followed by 18 and 19 and then we will move to 4 and 12," he said.
The company, with about 1,765 employees, took over management of the liquid waste cleanup program in July.
The waste in the aging tanks includes thick liquids, sludge with a consistency similar to peanut butter and a caustic material that turns to salt.
Twelve of the tanks are leaking, but the leaking areas are contained to prevent material from entering the environment.
Disposal programs include emptying the tanks, processing the waste and then filling in the aging, carbon-steel vats with a specialized grout and leaving the tanks buried forever.
Each tank is 86 feet across and 44 feet deep, and all are buried 10 to 12 feet below ground level.