There may soon be a way to produce clean, safe nuclear energy and reduce radioactive waste by using metal-organic frameworks (MOFs) to capture and remove volatile 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.
This technology could apply to nuclear fuel reprocessing or to clean up nuclear reactor accidents. A characteristic of nuclear energy is it is possible to reprocess used fuel 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 you can not burn 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.
“We investigated the structural properties on how they work and translated that into new and improved materials,” Nenoff said.
MOFs are crystalline, porous materials in which a metal center binds to organic molecules by mild self-assembly chemical synthesis. The choice of metal and organic result in a very specific final framework.
The 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 incorporate into glass waste for long-term storage.
The 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, Nenoff said.
Sandia applied for a patent on the pellet technology, which could have commercial applications.