Argonne scientists Artem Guelis (left) and Kevin Nichols test their miniaturized apparatus for nuclear recycling research. Photo: Argonne National Laboratory |
Designing
better ways to recycle spent nuclear fuel could make nuclear energy a safer
solution to the global energy problem, but there are a lot of gaps in our
chemical knowledge—and it’s difficult to get those answers when the experiments
involve radioactive material.
Scientists
at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have one
answer: Shrink the whole experiment down—to microliters.
When it
comes out of a reactor, nuclear spent fuel contains a whole glut of different radioactive
isotopes, all mixed together.
For years,
scientists have looked for ways to separate out reusable fuel from the truly
toxic stuff.
The
nations that recycle spent nuclear fuel today use processes based on PUREX, a
technique whose underpinnings date back to the 1940s. Ideally, new processes
would make fuel recycling cheaper, safer, and less complex.
But one
big challenge to creating models that accurately represent fuel reprocessing
lies in determining the rates of reaction in the procedure—essentially, how
quickly different elements move between phases.
Recycling
nuclear fuel is fundamentally a sorting exercise: chemists want to sift out the
useful uranium from the bulk of other byproducts and highly radioactive ones.
The fuel is dissolved in acid and different metals can be separated out using
solvent extraction—a bit like oil collecting on the surface of a bottle of
salad dressing. The rates at which the metals separate is determined by
kinetics, and knowing the rates helps scientists design new and better
techniques.
“From
the chemistry standpoint, if we want to be able to design new and improved
nuclear recycling schemes, you have to be able to understand the
mechanism,” says Argonne chemical
engineer Kevin Nichols, who helped lead the research. “You have to be able
to develop chemical insight, which comes from knowing the kinetics.”
Previous
experiments that looked into the kinetics of these particular classes of
reactions used large volumes of material, which slows the process and leads to less
accurate results. But Nichols and chemist Artem Gelis have built a solution: an
apparatus that miniaturizes the process.
“If
we cut the size down, we can do the same experiment much more quickly, generate
less waste, and get more precise measurements,” Gelis explains.
The
apparatus uses mere drops of radioactive material, rather than liters. This
allows hundreds or even thousands of trials to be performed with just a few
microliters of sample.
The new
process grew out of a combination of solvent extraction research being done at
Argonne and work being done by University
of Chicago professor
Rustem Ismagilov, whose laboratory created a miniaturized apparatus for protein
crystallization. The process generated thousands of aqueous droplets containing
proteins separated by an oil layer, which—as it happens—is similar to the
process for nuclear recycling. Though it had not been tried before, the
researchers decided to modify the technique for nuclear fuel treatment
kinetics.
Next, the
team is planning to adapt the technique for other applications, such as
processes that produce radioactive isotopes for medical use or even rare earth
metal processing.
Rare earth
metals are used in many energy technologies, such as solar panels and compact
fluorescent lightbulbs, but today are primarily mined in China. The U.S. has rare earth metal deposits, but the
popularity of renewable energy has triggered new interest in making U.S. rare earth
metal mining more economical.
The paper, “Toward Mechanistic Understanding of Nuclear Reprocessing
Chemistries by Quantifying Lanthanide Solvent Extraction Kinetics via
Microfluidics with Constant Interfacial Area and Rapid Mixing”, was
published in the Journal of the American Chemical Society.
