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News

Historic research division continues to push nuclear frontiers

DOE/Argonne National Laboratory : 14 March, 2007  (Technical Article)
The building housing Argonne's Chemical Engineering Division was named a Nuclear Historic Landmark this summer by the American Nuclear Society. The award recognizes the division's significant contributions to the development, implementation and peaceful use of nuclear technology.
“Nuclear energy for the power industry started here at Argonne,” said David Lewis, CMT director. “We're part of that.”

Since Argonne's opening 58 years ago, CMT has been developing processes for reducing nuclear waste volume and recovering valuable elements from spent reactor fuel. Some techniques allow parts of the fuel to be reused.

The division continues its key role in the nation's energy future through its research into nuclear waste separation techniques. CMT engineers and chemists have developed both aqueous and non-aqueous methods to separate and reduce waste created in nuclear reactors.

Current work is part of the Advanced Fuel Cycle Initiative, which aims to reduce the nation's nuclear waste and extend the capacity of the Yucca Mountain repository. The goal is to delay or even avoid the multi-billion dollar cost of a second repository.
“This waste is an international concern,” said CMT's Jim Laidler, who serves as a technical director for the U.S. Department of Energy's chemical separations program. “Separating the elements of spent nuclear fuel reduces the amount of high-level waste requiring long-term storage in a geologic repository.”

Aqueous separations
CMT's first foray into radionuclide separation was aqueous processing to separate uranium and plutonium from dissolved irradiated fuel. The REDOX process Argonne helped to design replaced an expensive, waste-intensive operation at DOE's Hanford Site in Washington State. Cost savings from REDOX paid for a new $50-million REDOX plant.

CMT also played a key role in developing next-generation solvent-extraction methods, including PUREX and TRUEX.

Key contactor
The workhorse behind the division’s solvent extraction research is a 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 Pacific Northwest, Los Alamos and Oak Ridge national laboratories, the Oak Ridge Y-12 Plant, the Hanford Site and the Idaho National Engineering and Environmental Laboratory.

More recently, CMT, in cooperation with teams at Oak Ridge and Savannah River national laboratories and the Savannah River Site, developed a process called caustic-side solvent extraction. CSSX combines CMT's centrifugal contactor (see sidebar) with a highly selective solvent developed at Oak Ridge. CSSX has been chosen by DOE to reduce by 15-fold the volume of radioactive cesium-137 from tank waste created as a by-product of defense activities. The cesium will then be incorporated into a glass waste form for disposal in a geologic repository. The remaining material can be disposed of as low-level radioactive waste.

In tests at Argonne, CSSX extracted all but one part in 150,000 of the original cesium in test samples, which is better than DOE required.

UREX+ is CMT's current aqueous research success. UREX+ is a four-step process CMT researchers designed and demonstrated to separate the components of light-water reactor fuel created by commercial reactors. The first step removes uranium (about 96 percent of the waste mass) for disposal as low-level waste. The second step separates cesium and strontium from for decay storage. In the third step, plutonium and neptunium are removed for recycling into light-water-reactor fuel (they are destroyed as they burn, making them unavailable for weapons use). The fourth step removes americium and curium for storage and eventual destruction in advanced reactors.

Pyrochemical separations
While UREX+ can process spent commercial fuel and recycle some of it into fuel for today's commercial light-water reactors, pyrochemical separation processes are being developed to support the future U.S. nuclear enterprise.

In the late 1950s, chemical engineers began developing pyrochemical methods to separate nuclear waste products created by Argonne's Experimental Breeder Reactor-II. These early methods relied on extraction techniques similar to those used in aqueous processing but were conducted using molten salts and liquid metals. A melt-refining technique was developed and used to treat approximately five tonnes of EBR-II fuel, and research on pyrochemical processes has continued to evolve over the years as part of Argonne's advanced reactor programs.

Recent developments rely on electrochemical rather than extraction methods that use chemical reagents to separate materials from irradiated fuel. This avoids the additional waste stream associated with using chemical reagents. Electrorefining, which recovers uranium from spent fuel, was developed to close the fuel cycle for the Integral Fast Reactor, an advanced fast reactor technology Argonne developed in the 1984-1994 period, and is now used to condition spent fuel from EBR-II.

Advancement of electrorefining technology continues with the development of the Planar Electrode ElectoRefiner. PEER's design eases materials handling requirements (which is important in a remotely operated facility), facilitates scaling to achieve fuel throughput goals, and provides a means for electrorefining to transition from a batch process to a continuous process.

Like the aqueous UREX+ process, electrorefining separates uranium into one separate stream. But electrorefining keeps the plutonium and neptunium with the americium and curium, which are recovered in a subsequent pyroprocess, recycled into advanced reactor fuel, and destroyed as they produce electricity. In addition, keeping the plutonium with the other elements eases nuclear proliferation concerns by making it unsuitable for direct use in nuclear weapons. Pyrochemical processing may also reduce the cost of treating spent fuel and provide greater flexibility in the deployment of fuel treatment facilities.

Argonne's aqueous and non-aqueous processes both have attracted international attention from such nations as France, Japan, Russia, South Korea and the United Kingdom, which have growing amounts of spent fuel for disposal.
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