A type of plutonium used to fuel a space probe to Saturn is ready to make a classified cross-country trip on public roads back here four decades after it was sent to Washington scientists who didn't use it, an federal spokeswoman says.

The plutonium-238, packed in 55-gallon drums, will be tracked by satellite from the Hanford site in Washington to the Savannah River Site near Aiken, said Karen Lutz, an Energy Department spokeswoman at Hanford.

The substance can't be used in nuclear weapons but can still be deadly.

Ingesting a speck can lead to lung cancer or kidney damage. If handled improperly, it can start a fire or trigger an explosion.

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Plutonium-238 is so hazardous terrorists would have a tough time hijacking it, said Robert Alvarez, a former Energy Department senior adviser.

"It's not the kind of stuff you can put in a satchel and sneak into a country," he said.

More than 2,000 miles separate the plutonium-238 from the Savannah River Site. But the shipment to South Carolina isn't quite a done deal.

Hanford was expecting last year to be relieved of the plutonium as early as this spring, Lutz said, but officials are still waiting for Energy Department headquarters to make a decision.

No decision has been made on what to do with the plutonium-238 because a federal committee is deciding whether to further consolidate all the nation's nuclear materials, said Meg Barnett, an Energy Department spokeswoman in Washington, D.C.

If the plutonium does return to South Carolina, the federal government would try to find a use for it. If not, the plutonium would add a small of amount of radioactive material to a site working around the clock to process 27 million gallons of dangerous waste, said Jim Giusti, an Energy Department spokesman at SRS.

It wouldn't be the first time South Carolina ended up with another state's radioactive waste.

Former Gov. Jim Hodges threatened to lie down in front of delivery trucks when the Energy Department decided to send waste from Rocky Flats, Colo., to the Savannah River Site.

"We're not really cleaning up anything," said Dell Isham, executive director of the state's Sierra Club chapter. "We're just transferring the problem to us. Why are we the dumping ground for the country?"

The Energy Department has nixed other options for the plutonium-238, including a plan to send it to New Mexico.

The plutonium-238 shipment would be small compared to the Rocky Flats delivery. And it's a different type of substance. Plutonium-239 is used in nuclear weapons.

The Savannah River Site has years of experience with plutonium-238. The site made the material powering the Cassini space probe, now preparing to orbit Titan, one of Saturn's moons, about 800 million miles from Earth.

In 1966, the site sent a load of plutonium-238 to Hanford for critical mass experiments. Scientists couldn't extract enough of the isotope and never opened the 55-gallon drums.

Hanford buried the containers, along with the waste, in a trench in 1980. A contractor cleaning up the site, Hanford Fluor, dug up the containers last fall, Lutz said.

The workers who unearthed the material may have had the most dangerous part of the job, Alvarez said. "It does run the risk of explosions and fires, if you're not careful."

At SRS, the plutonium would be purified in H-Canyon.

If it can't be used, it would be added to waste in 49 underground tanks before it's poured into 10-foot steel containers.

Huge trucks carry the containers to a building that looks like an empty warehouse, where it's then stored in vaults. The site has filled 2,200 containers out of an expected 5,000. The new shipment wouldn't increase the number of containers.