WASHINGTON — The U.S. Department of Energy should postpone plans to leave nuclear waste in storage tanks at the Savannah River Site, a new study commissioned by Congress recommends.

Environmentalists are praising the findings by the National Academy of Sciences, but they predict the Energy Department won’t accept a key recommendation.

“We’re vindicated; this is what we’ve been saying all along,” said Dell Isham, director of the S.C. chapter of the Sierra Club. “But it may be a hollow victory because they may do whatever they want anyway.”

The Energy Department — which owns SRS and wants to leave up to 5 percent of the waste at the site in some of the 49 tanks and mix it with grout — confirmed Isham’s prediction.

“We believe that for human as well as environmental health, the wisest course of action is to proceed with tank closure,” department spokesman Mike Waldrin said.

“Doing otherwise puts the cleanup in the position of always waiting for the next technological development to come along and would hamstring tank closure without providing a clear benefit.”

But Isham and other environmentalists question whether the tanks will hold up. They say all waste should be removed and sent to a deep nuclear waste vault — such as the one at yet-to-open, controversial Yucca Mountain in Nevada.

They point to the report’s conclusion that new technologies developed during the next five to 10 years could make it easier and cheaper to remove all waste from the tanks, so grouting and sealing should be delayed.

The Department of Energy has indicated the report likely won’t change its tank closing schedule. Two tanks already have been sealed, and the department wants the remaining 49 closed by 2022.

SRS, the 310-square-mile nuclear campus near Aiken that produced much of the nuclear fuel for the nation’s Cold War arsenal, is now primarily a nuclear waste reprocessing, research and storage facility.

The nuclear waste addressed in the study sits in carbon steel tanks buried a few feet below the ground. They can hold 36.4 million gallons of waste.

From each, the bulk of the waste can be removed and “vitrified” — turned into glass logs for burial at Yucca Mountain.

The disagreement is over the fraction of waste that lies at the bottom of the tanks — the hardened “heel” of the sludge, which is more difficult to remove.

The Department of Energy estimates it could cost $500 million to remove this sludge and argues that it is safer to leave it and seal the tanks.

Environmentalists in 2003 won a lawsuit that demanded complete removal. But subsequent federal legislation allows the Energy Department to reclassify the sludge as low-level waste — meaning it could stay in the tanks.

U.S. Rep. John Spratt, D-York, inserted language into a defense bill earlier this year directing the independent, Washington-based National Academy of Sciences to take a year and $1.5 million to study the storage of such waste at three sites, including SRS.

The just-released study, which focuses only on SRS, is the Academy’s interim report.

“The NAS panel suggests that we can have our cake and eat it, too — that we can allow research into other technologies to ensure that we are removing as much waste as feasible,” Spratt said.

Mal McKibben, executive director of Aiken-based Citizens for Nuclear Technology Awareness, and a former SRS scientist, said the Energy Department and SRS officials are willing to consider new technologies that could remove even more waste from the tanks — but must balance that with the need to seal the tanks.

“The longer we take to close those tanks, the greater the possibility of having leaks,” he said.

Reach Markoe at (202) 383-6023 or lmarkoe@krwashington.com.