Ralph Leonard examines the centrifugal contactor critical to the solvent extraction process. This Argonne technology will be used to clean waste at the Savannah River Site in South Carolina.

Solvent extraction process reduces waste

A solvent-extraction process developed jointly at Argonne and Oak Ridge National Laboratory promises to reduce by 15-fold the volume of high-level, liquid waste from nuclear weapons production now stored at the U.S. Department of Energy’s Savannah River Site. Federal agreements call for the tanks holding the liquids to be emptied by 2028.

The process—called “CSSX” for “caustic-side solvent extraction”—combines a highly selective solvent developed at Oak Ridge and a solvent extraction device, called a “centrifugal contactor,” developed in Argonne’s Chemical Technology Division.

This combination will extract radioactive cesium-137 from some 34 million gallons of caustic, liquid high-level waste created at Savannah River as a by-product of plutonium production for national defense. The cesium will be incorporated into a glass waste form for disposal in a geologic repository. The remaining waste can be disposed of as low-level radioactive waste.  

Cesium-137 has a radioactive half-life—the amount of time needed for half a sample to decay—of 30 years. If not isolated from the environment, it can displace potassium in the fluids of living organisms and become more concentrated as it passes up the food chain.

A team from Argonne and Oak Ridge is currently assisting Savannah River in designing a CSSX processing facility.

DOE chose Argonne’s CSSX technology because it extracted all but one part in 150,000 of the original cesium in test samples. This is nearly four times the 40,000 “decontamination factor” required for the project.

The CSSX process uses solvent efficiently. “Our centrifugal contactors need only about one gallon of solvent to clean nearly 3,000 gallons of waste,” said Ralph Leonard, who leads Argonne’s centrifugal contactor team.

Originally developed 30 years ago, Argonne’s centrifugal contactors are used in a continuous process with solvents to extract chemicals from liquid solutions. A centrifugal contactor is essentially a cylindrical rotor surrounded by a mixing bowl. The spinning rotor simultaneously acts as a mixer, a centrifugal settler and a pump.

The liquid waste and the solvent enter the bowl from opposite directions. The high-speed rotor mixes them, allowing the solvent to absorb the material to be removed. Vanes on the bottom of the bowl guide both liquids through a small port into the hollow rotor. Inside the continually spinning rotor, centrifugal forces 100 to 400 times gravity separate the liquids by driving the denser one to the outer wall. The rotor then spins the liquids out the top through separate ports.

Over the years, Argonne’s centrifugal contactors have been used in several DOE facilities, including the Hanford Site, Pacific Northwest National Laboratory, Los Alamos National Laboratory, Oak Ridge, the Oak Ridge Y-12 Plant and the Idaho National Engineering and Environmental Laboratory.

For more information, please contact David Baurac.

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