Process Chemistry and Engineering

Chemical engineer Candido Pereira examines a sample of solvent from an experiment using a centrifugal contactor that is being tested for processing spent nuclear fuel as part of the Advanced Fuel Cycle Initiative. Photo by George Joch

Contactor Is Key The workhorse behind the solvent extraction research is an Argonne-designed centrifugal contactor designed more than 30 years ago. The device is a cylindrical rotor surrounded by a mixing bowl. The spinning rotor acts as a mixer, a centrifugal settler and a pump. The liquid waste and solvent enter the bowl from opposite directions, and the rotor mixes them, allowing the solvent to extract the material to be removed. The liquids enter the hollow spinning rotor, and centrifugal forces 100 to 400 times gravity separate the liquids, which leave through separate ports at the rotor’s top. The centrifugal contactors are efficient. They use only about one gallon of solvent per 3,000 gallons of waste. The contactor has been used at many DOE facilities including Idaho, Los Alamos and Oak Ridge national laboratories, the Oak Ridge Y-12 Plant, and the Hanford Site.

Resources DOE Advanced Fuel Cycle Initiative (Program Offices) DOE Office of Civilian Radioactive Waste Management Global Nuclear Energy Partnership DOE RERTR Program Argonne's RERTR Program HEU Transparency Program at Argonne Argonne Nuclear Engineering Division

The Process Chemistry and Engineering Department conducts research that supports closing the nuclear fuel cycle and eliminating the use of highly enriched uranium in civil research programs throughout the world.

Closing the Nuclear Fuel Cycle

• Spent Nuclear Fuel Generation and Accumulation (poster, .pdf, 4MB)
• Spent Nuclear Fuel Management Options (poster, .pdf, 1 MB)
• UREX+ Process for Recovering Key Radionuclides from Commercial Spent Nuclear Fuel

Spent nuclear fuel and high-level radioactive waste are materials from nuclear power plants and government defense programs. These materials contain highly radioactive elements, such as cesium, strontium, technetium, plutonium, and neptunium. Some of these elements will remain radioactive for a few years, while others will be radioactive for millions of years. Scientists worldwide agree that the safest way to manage these materials is to dispose of them deep underground in what is called a geologic repository.

Currently, spent nuclear fuel is stored in specially designed pools at individual reactor sites around the country and high-level radioactive waste is stored at government facilities. Before such wastes can be stored in the proposed Yucca Mountain geologic repository, they will require further treatment to reduce overall toxicity, fissile content and volume. In addition, the behavior of waste materials following their disposal in the repository needs to be understood.

Nearly all the risk from spent fuel comes from about 1 percent of its content—primarily the transuranics plutonium, neptunium, americium, and curium, and the long-lived isotopes of iodine and technetium. With these elements removed, the remaining 99 percent of the waste needs only about 1,000 years before its toxicity drops below that of natural uranium ore. Removing strontium and cesium with these wastes also reduces decay heat from the final waste form, which means that waste packages can be stored closer together, effectively expanding the repository's capacity. Transuranics will be separated from spent fuel and destroyed in advanced reactors.

Under the Department of Energy's Advanced Fuel Cycle Initiative (AFCI), Argonne is leading development of the UREX+ aqueous separations, a multi-step process for separating out the high-risk elements of spent nuclear fuel. Argonne has successfully demonstrated the entire process in hot cells and glove boxes and is preparing for scale-up demonstration.

DOE is preparing a license application for submittal to the Nuclear Regulatory Commission to construct the Yucca Mountain geological repository for disposal of the nation’s spent nuclear fuel and other highly radioactive waste materials. We are providing support to DOE by using scientific understanding and data developed through experimental studies (both here at Argonne and elsewhere) to develop and refine computer models that DOE can use to assess the repository’s long-term performance.

Waste from Government Defense Programs

The Savannah River Site (SRS) in South Carolina has 34 million gallons of high-level waste (HLW), stored in underground tanks, that must be decontaminated. A very small fraction of the waste is in the form of harmful radionuclides, including cesium-137. Separating out this fraction can reduce overall disposal cost while concentrating the radionuclide into a more manageable volume that can be incorporated into a glass waste form for disposal in a geologic repository.

The SRS uses several different waste separation processes, depending on the radionuclide involved. In the selection of a process to be used for the removal of cesium, the design goal was set as a decontamination factor of 40,000 for all tank wastes.

	Technician Mark Clark (left) remotely handles a radioactive sample using a manipulator in Argonne National Laboratory's shielded-cell facility as chemical engineer Argentina Leyva confers with technician Lohman Hafenrichter in preparation for another remote test. Photo by George Joch.

For cesium removal, one of the candidate processes was a caustic-side solvent extraction process called CSSX. Our scientists showed that CSSX is extremely effective in separating the cesium from the highly saline liquid present in the underground tanks. The volume of HLW containing cesium can be reduced 15-fold while the cesium is removed from the bulk of the HLW with decontamination factors of 40,000 or higher. These goals are met using a relatively small volume of solvent, an achievement made possible by using multistage centrifugal contactors and the new, highly selective solvent. Since the solvent is expensive, the small volume is important in holding down process costs.

The CSSX process was successfully demonstrated first at Argonne with synthetic waste and then at SRS with real waste. After our initial success, we worked with our partners at Savannah River National Laboratory (SRNL, formerly the Savannah River Technical Center) and Oak Ridge National Laboratory to increase the robustness of the process and develop an appropriate flowsheet (a diagram of the sequence of operations) for the CSSX process at the SRS. At the conclusion of this work, the Department of Energy (DOE) chose this solvent extraction technology to process the highly radioactive salt waste at SRS. DOE chose Parsons Corporation as the contractor to build the Salt Waste Processing Facility (SWPF), which will use the CSSX process to remove the radioactive cesium-137.

The use of the Argonne-designed multistage centrifugal contactor in the demonstrations at Argonne and on real waste at SRNL is an important element in the success of this effort, and in the development of the UREX+.processes (see above). The centrifugal contactor will also be used in the SWPF plant.

Eliminating the Use of Highly Enriched Uranium

The U.S. Department of Energy initiated the Reduced Enrichment for Research and Test Reactors (RERTR) Program in 1978 with the mission of developing the technologies necessary to convert research and test reactors from the use of fuels and targets containing highly-enriched uranium (HEU, 20% or more U-235) to the use of fuels and targets containing low enriched uranium (LEU, less than 20% U-235). This mission is consistent with the U.S. nonproliferation policy goal of minimizing and, to the extent possible, eliminating the use of highly enriched uranium in civil programs worldwide.

Argonnne's Chemical Engineering Division supports the RERTR Program at Argonne by developing new processes that allow the production of molybdenum-99 (the most commonly used medical isotope in the world) from low-enriched uranium targets. The Division's Remote Handling Mockup Facility is a valuable tool in our research, allowing us to carry out and refine planned handling of radioactive isotopes in preparation for hot-cell experiments, saving considerable time and expense. (More on the RERTR Program at Argonne)

Publications, Presentations, and Patents

For More Information

Monica C. Regalbuto, Head
Process Chemistry and Engineering Department
Chemical Engineering Division
Argonne National Laboratory
9700 S. Cass Ave.
Argonne, IL 60439
Phone: 630-252-1540
Fax 630-972-4495
regalbuto@cmt.anl.gov