WASHINGTON - The Senate on Thursday agreed to ease cleanup requirements for tanks holding millions of gallons of highly radioactive waste from Cold War-era bomb making.

Senate critics said the change would leave poisonous sludge in underground tanks and risk contamination of groundwater.

An attempt to block the change failed by the narrowest of margins. Senators voted 48-48 on an amendment offered by Sen. Maria Cantwell, D-Wash., that would have stripped the provision from a defense authorization bill.

The provision allows the Energy Department to reclassify radioactive sludge in 51 tanks at a South Carolina nuclear site so it can be left in place and covered by concrete, instead of being entombed in the Nevada desert.

While the plan has been approved by South Carolina officials, it brought sharp criticism from officials in Washington and Idaho who feared the change would put intense pressure on them to agree to a similar cleanup plan at nuclear sites in their states.

The proposal also left South Carolina's two senators sharply divided.

Sen. Lindsey Graham, R-S.C., who had put the provision into the defense bill, said it will quicken waste cleanup at the Savannah River nuclear complex near Aiken, S.C., by 23 years and save $16 billion. He rejected claims the waste would harm the environment.

Sen. Fritz Hollings, D-S.C., said the sludge accounts for more than half of the radioactivity in the tanks of liquid waste and endangers future generations. It's "not harmless sludge we can pour sand over and cover with concrete" as the Energy Department proposes, said Hollings.

The Savannah River tanks contain 34 million gallons of liquid waste. Sludge accounts for about 1 percent of the waste volume.

While supporters of the measure insisted it would apply only to waste at the Savannah River site, opponents said the change in nuclear waste policy would create a "clear precedent" that could force other states - mainly Washington and Idaho where there also are defense waste tanks - to accept less safe cleanup plans.

Cantwell, who led the push to kill the measure, accused the administration of trying to "sneak" the change in cleanup requirements through Congress by tacking it onto a defense measure in closed-door proceedings without hearings.

In an interview, Cantwell said she hasn't given up on getting the provision defeated. "I don't think the issue is over. ... It's too significant of an issue," she said. "We have more amendments." Since the House bill doesn't contain a similar measure, the issue is also likely to come up in final negotiations by a conference.

Graham's provision was put into the $447 billion defense bill during consideration by the Armed Services Committee without hearings. The House panel refused to include the changes in its version of the defense bill and, instead, called on the National Academy of Sciences to examine the Energy Department cleanup proposal.

The White House is trying "to blackmail my state to accept a lower cleanup standard," declared Sen. Patty Murray, D-Wash.

The tanks of nuclear waste are left over from decades of producing plutonium and highly enriched uranium for nuclear weapons. A 1982 law requires that all waste from such reprocessing must be buried at a central repository planned for Nevada.

But the Energy Department argues that the residual sludge should be considered low-level waste and should not have to be removed. Instead, the department wants to cover the sludge with cement-like grout, saying that would be protective for hundreds of years.

Deputy Energy Secretary Kyle McSlarrow said Thursday the proposed treatment of the sludge is a "scientifically sound plan to empty, clean, stabilize and dispose of nuclear waste" in the tanks. He maintained it was "fully protective" of the environment.

Last year a federal judge, acting on a lawsuit by environmentalists, ruled that such an approach violates the 1982 Nuclear Waste Policy Act. To get around the ruling, the department wants to get the law changed.

There are 177 tanks with 53 million gallons of waste at the Hanford nuclear site near Richland, Wash., and 900,000 gallons in tanks at the INEEL facility near Idaho Falls, Idaho.

Environmentalists blasted the Senate action.

It's "a cruel trick that allows the Bush administration to leave a legacy of radioactive pollution that could endanger drinking water for millions of Americans," said Karen Wayland, legislative director of the Natural Resources Defense Council, which filed the lawsuit that successfully challenged the Energy Department plan.

Robert Pregulman, executive director of the Public Interest Research Group in Washington state, said the legislation marks another attempt by the Energy Department "to weasel out of its obligation to properly clean up the radioactive mess it created at Hanford and other sites around the country."