Chemist finds new material (ZIF-8): Remove radioactive gas from spent nuclear fuel

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 by Randy Montoya)

The 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.

The 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.

The 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.

Part 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.

They 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.

“Silver 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.”

But 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 ZIF-8.

Silver found in lunar rocks

Silver was found in small amounts in lunar rocks collected by the Apollo astronauts, and now its spectral signature may have been spotted by NASA's LCROSS mission (Image: T. A. Rector/I. P. Dell'Antonio/NOAO/AURA/NSF)

The Cloak of Invisibility

When J. K. Rowling described Harry Potter's invisibility cloak as "fluid and silvery", she probably wasn't thinking specifically about silver-plated nanoparticles suspended in water. But a team of theorists believe that using such a set-up would make the first soft, tunable metamaterial – the "active ingredient" in an invisibility device.

The fluid proposed by Ji-Ping Huang of Fudan University in Shanghai, China, and colleagues, contains magnetite balls 10 nanometres in diameter, coated with a 5-nanometre-thick layer of silver, possibly with polymer chains attached to keep them from clumping.

In the absence of a magnetic field, such nanoparticles would simply float around in the water, but if a field were introduced, the particles wouldself-assemble into chains whose lengths depend on the strength of the field, and which can also attract one another to form thicker columns.

The chains and columns would lie along the direction of the magnetic field. If they were oriented vertically in a pool of water, light striking the surface would refract negatively – bent in way that no natural material can manage.

This property could be exploited for invisibility devices, directing light around an object so that it appears as if nothing is there, or be put to use in lenses that could capture finer details than any optical microscope.

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