The Hanford nuclear reservation is battling Russian invaders: radioactive tumbleweeds.

Russian thistle is a dead menace here on the windswept desert of south-central Washington. Each winter, the deep tap root on the plant decays and the spiny brown skeleton above ground snaps off and runs with the wind.

"Our dream is that we have this place tumbleweed-free," says Ray Johnson, a biological control manager for radiation protection at Fluor Hanford, the contractor managing the U.S. Department of Energy site.

While less than 1 percent of the tumbleweeds corralled and compacted at Hanford are radioactive, the cost of cleanup can run into millions of dollars.

The most contaminated nuclear site in the country, Hanford was built in 1943 for the top-secret Manhattan Project. For 40 years, Hanford made plutonium for the nation's nuclear arsenal, including one of the two atomic bombs dropped on Japan, ending World War II. The last reactor was shut down in 1986.

Russian thistle roots reach down as far as 20 feet and suck up such nasty elements as strontium and cesium.

A stiff winter wind can push the tumbleweed as far away as four miles, and then "we've lost control of our contamination," Johnson said.

Two years ago, uncontrolled contamination spread by fruit flies made Hanford a national laughingstock, spoofed by columnist Dave Barry and in the syndicated comic strip "Sylvia."

The flies had been attracted to a soil fixative with saccharin in the base that was being sprayed on a contaminated site. They flew to a lunch room, and spread the taint to nearby trash bins, which wound up at the Richland municipal landfill.

The journeys of a few thousand fruit flies cost $2.5 million to clean up.

Riding herd on Hanford's tumbleweeds, and its flying insects, is part of an annual $4 million integrated soil, vegetation and animal control (ISVAC) program run by subcontractor DynCorp for Fluor Hanford.

Radiation control specialists survey the tumbleweeds on the 560-square-mile reservation, using Geiger-Mueller counters that click when radioactivity is present. If contaminated tumbleweeds are found, an ISVAC crew disposes of them.

"The weeds are fairly low danger," says Todd Ponczoch, a radiation control technician, using a counter to scan tumbleweeds along a fence.

A large, three-pound radioactive tumbleweed might measure out at 150 millirads, or about 1/100th of the allowable annual dose of radiation per person at Hanford.

Radioactive tumbleweeds are pitchforked by specially trained and clothed workers into a regulated garbage truck, compacted and disposed of at an on-site low-level waste dump. A trail of paperwork is required.

The sites must be satisfactorily cleaned up and covered with six inches of clean soil or gravel.

The uncontaminated tumbleweeds are dumped in an open pit. Up until five or six years ago, the "clean" tumbleweeds were burned and the ash buried. But the state Department of Health put a halt to that for fear that some radioactive tumbleweeds might find their way into the mix and disperse contamination into the air.

Preventive measures are also part of the control program, and include backpack, roadside and aerial spraying with herbicide to kill the thistle. Sometimes a bio-barrier -- a costly engineered textile -- is laid down to block the formation of thistle roots.

"What you've got to do is make sure your contaminated areas are tumbleweed-free," Johnson said.