COLUMBIA, S.C. (AP) - Federal officials have made major progress in cleaning up radioactive waste stored in South Carolina, saying Thursday that two underground tanks that previously stored nuclear waste at the Savannah River Site are being cleaned and filled with concrete.

"We've got 99 percent of the radioactive material out of these tanks," U.S. Energy Undersecretary Thomas D'Agostino said during a conference call with reporters. "We are highly confident that this presents no risk at all to the public."

The 310-square-mile Savannah River Site opened in the 1950s on the Georgia-South Carolina border, once producing tritium and plutonium for the nation's nuclear bomb arsenal. Left over from that process were 37 million gallons of radioactive liquid waste that has since been stored in several dozen underground tanks.

The Energy Department and environmental officials have agreed to clean and close 22 of the tanks, pumping out the waste and immobilizing it in glass canisters that officials hope to eventually store off-site. When they are sealed up, the two tanks discussed Thursday will be the first ones closed at Savannah River in 15 years. With each having the capacity to hold two Olympic-sized swimming pools' worth of waste, the tanks are also among the largest the DOE has cleaned up, officials said.

Cleaning these two tanks alone will cost around $50 million, according to David Moody, the Energy Department's top official at Savannah River. A December 2008 contract between Savannah River and the Energy Department estimated costs for the project at $3.2 billion.

But in 2010, a federal audit showed that those initial estimates were "not accurate or comprehensive" and that the project's actual cost had risen more than $1.4 billion, or some 44 percent, to about $4.6 billion.

DOE officials disagreed with the findings, criticizing the audit for only examining parts of the project and noting that the Department actually planned to finish the closures several years ahead of schedule.

Two tanks have already been sealed, and Moody said Thursday that four more should be done by 2015, with the project completed by 2028.

"We're not ignoring any tanks, we're going to get to them all," D'Agostino said. "This is a matter of the tip of the iceberg."