The new method measured the number of atoms dislodged on average in a sample of zircon every time it was struck by an alpha particle spewed by radioactive plutonium. Every time one of these particles slams into a mineral such as zircon, it knocks thousands of atoms loose, breaking down the material's regularly spaced crystal structure and making it more prone to leak. But researchers have had trouble pinpointing the exact rate of destruction. "We actually come up with quite a hard number and a date," says minerals physicist Ian Farnan of the University of Cambridge. "That sort of predictability has been lacking in materials that could be used to store nuclear waste."
Experts pondering how to deal with radioactive waste (particularly long-lived plutonium shed from dismantled weapons and reprocessed reactor fuel) a decade ago proposed storing it underground mixed with the durable mineral zircon. They soon discovered that zircon expands by up to 18 percent when irradiated, making it less attractive as a potential containment medium.
In an attempt to better gauge zircon's staying power, Farnan and his colleagues at the Pacific Northwest National Laboratory spun small samples of synthetic zircon mixed with plutonium 239 in a type of magnetic resonance imaging (MRI) device. The instrument keyed in on changes in the orientation of silicon 29 isotopes in the zircon. Specifically, in response to a magnetic pulse from the MRI, the isotopes would vibrate at a particular frequency depending on how many were crystalline and how many were disordered. (The spinning, at 200,000 revolutions per minute, enhanced the instrument's resolution.)
The team found that each alpha particle displaced about 5,000 atoms, according to a report in this week's Nature. Prior estimates had put the number at between 1,000 and 2,000, which translated to about 6,000 years of durability for zircon, a mere sliver of the hundreds of thousands of years that underground facilities are expected to be able to contain nuclear waste without releasing any radiation.
"What's exciting to me," says materials scientist Rodney Ewing of the University of Michigan, "is the possibility of applying this technique to other materials," such as pyrochlore, the current mineral of choice for storing plutonium.
Farnan says the method might also help evaluate the durability of the proposed waste repository in Yucca Mountain, Nev., progress on which has been stymied for years because of safety concerns.