Sandia chemist Tina Nenoff heads a team of researchers focused on removal of radioactive iodine from spent nuclear fuel. They identified a metal-organic framework that captures and holds the volatile gas, a discovery that could be used for nuclear fuel reprocessing and other applications. Photo: Randy Montoya
by a team of Sandia National Laboratories chemists could impact worldwide
efforts to produce clean, safe nuclear energy and reduce radioactive waste.
Sandia researchers have used metal-organic frameworks (MOFs) to capture and
remove volatile radioactive gas from spent nuclear fuel. “This is one of the
first attempts to use a MOF for iodine capture,” said chemist Tina Nenoff of
Sandia’s Surface and Interface Sciences Department.
discovery could be applied to nuclear fuel reprocessing or to clean up nuclear
reactor accidents. A characteristic of nuclear energy is that used fuel can be
reprocessed to recover fissile materials and provide fresh fuel for nuclear
power plants. Countries such as France,
Russia, and India are
reprocessing spent fuel.
process also reduces the volume of high-level wastes, a key concern of the
Sandia researchers. “The goal is to find a methodology for highly selective separations
that result in less waste being interred,” Nenoff said.
of the challenge of reprocessing is to separate and isolate radioactive
components that can’t be burned as fuel. The Sandia team focused on removing
iodine, whose isotopes have a half-life of 16 million years, from spent fuel.
studied known materials, including silver-loaded zeolite, a crystalline, porous
mineral with regular pore openings; high surface area; and high mechanical,
thermal, and chemical stability. Various zeolite frameworks can trap and remove
iodine from a stream of spent nuclear fuel, but need added silver to work well.
attracts iodine to form silver iodide,” Nenoff said. “The zeolite holds the
silver in its pores and then reacts with iodine to trap silver iodide.”
silver is expensive and poses environmental problems, so the team set out to
engineer materials without silver that would work like zeolites but have higher
capacity for the gas molecules. They explored why and how zeolite absorbs
iodine, and used the critical components discovered to find the best MOF, named
investigated the structural properties on how they work and translated that
into new and improved materials,” Nenoff said.
are crystalline, porous materials in which a metal center is bound to organic
molecules by mild self-assembly chemical synthesis. The choice of metal and
organic result in a very specific final framework.
This illustration of a metal-organic framework, or MOF, shows the metal center bound to organic molecules. Each MOF has a specific framework determined by the choice of metal and organic. Sandia chemists identified a MOF whose pore size and high surface area can separate and trap radioactive iodine molecules from a stream of spent nuclear fuel. Image: Sandia National Laboratories
trick was to find a MOF highly selective for iodine. The Sandia researchers took
the best elements of the zeolite Mordenite—its pores, high surface area,
stability, and chemical absorption—and identified a MOF that can separate one
molecule, in this case iodine, from a stream of molecules. The MOF and
pore-trapped iodine gas can then be incorporated into glass waste for long-term
Sandia team also fabricated MOFs, made of commercially available products, into
durable pellets. The as-made MOF is a white powder with a tendency to blow
around. The pellets provide a stable form to use without loss of surface area,
has applied for a patent on the pellet technology, which could have commercial
Sandia researchers are part of the Off-Gas Sigma Team, which is led by Oak
Ridge National Laboratory and studies waste-form capture of volatile gasses
associated with nuclear fuel reprocessing. Other team members—Pacific
Northwest, Argonne, and Idaho
national laboratories—are studying other volatile gases such as krypton,
tritium, and carbon.
project began six years ago and the Sigma Team was formalized in 2009. It is
funded by the U.S. Department of Energy Office of Nuclear Energy.
iodine and MOFs research was featured in two articles in the Journal of the American Chemical Society.
most important thing we did was introduce a new class of materials to nuclear
waste remediation,” said Sava, postdoctoral
appointee on the project.
said another paper in Industrial &
Engineering Chemistry Research shows a one-step process that incorporates
MOFs with iodine in a low-temperature, glass waste form. “We have a volatile
off-gas capture using a MOF and we have a durable waste form,” Nenoff said.
and her colleagues are continuing their research into new and optimized MOFs
for enhanced volatile gas separation and capture.
shown that MOFs have the capacity to capture and, more importantly, retain many
times more iodine than current materials technologies,” said Argonne’s