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  Geotimes - March 2007 - Mineral crumbles under nuclear heat
Science and Society
Mineral crumbles under nuclear heat

When it comes to storing nuclear waste, it turns out that zircon can’t take the heat. A new, high-resolution look at the mineral — previously thought to be a model material for storing nuclear waste — reveals that it is quick to succumb to radiation damage.

Researchers have long sought viable ways to store the radioactive components separated out from spent nuclear fuel. One method involves locking the radioactive element inside the crystal structure of minerals, such as zircon. As such, the waste becomes integrated into a hard, durable zircon storage medium, ready to be shipped off for storage in a repository. But Ian Farnan, a materials physicist at the University of Cambridge is calling zircon’s containment prowess into question: Probing zircon’s structure down to microscopic levels, Farnan turned up radiation damage in zircon up to five times greater than researchers previously thought, he and colleagues reported Jan. 11 in Nature.

Zircon survives in Earth’s crust, sometimes for billions of years, enduring extreme geological change. Because the mineral naturally contains radioactive elements and has demonstrated extreme endurance, researchers naturally gravitated toward it as a potential storage medium for spent nuclear waste. But researchers remained uncertain as to how zircon would hold up through time after being artificially infused with radioactive elements, particularly at levels higher than those found in nature.

Previously, researchers tried to quantify damage to synthetic, plutonium-enriched zircon by using x-ray diffraction: When x-rays bounce off organized, equally separated atoms in the crystal, they diffract and produce an interference pattern, but where atoms are damaged and scrambled after being bombarded by radiation, no such pattern turns up. By analyzing how the diffraction pattern has changed, researchers can estimate the number of damaged atoms. The signal, however, can be “difficult to characterize,” Farnan says, making it hard to determine the true extent of the damage.

Farnan’s team turned instead to a technique that employed nuclear magnetic resonance to probe all of the silicon atoms of the plutonium-containing zircon crystal and get a high-resolution look at the damage. Farnan’s results showed that every atom would be permanently displaced in their sample within 1,400 years, which falls short of the minimum 10,000-year regulatory compliance period for geologic repositories under review.

The results, however, are not all “gloom and doom,” Farnan says, adding that the method better constrains how zircon behaves out to 1,000-year timescales. “That’s what we need to really reduce a lot of the uncertainties about disposing of nuclear waste over these very long timescales,” he says.

The technique will not likely apply to nuclear waste disposal in the United States, however, says Allison Macfarlane, a geologist and policy specialist at George Mason University. The United States does not separate out plutonium from spent nuclear fuel for recycling like the United Kingdom and other countries do, and excess plutonium from weapons production will likely be blended with reprocessed uranium to fuel thermal nuclear reactors, Macfarlane says.

Still, “the authors’ approach seems to be a solid one and this technique is promising in helping to identify the best possible waste forms for this material,” Macfarlane says. “One should focus on both the waste form and the geology of the repository — that way, you have much more assurance that your radioactivity will stay put.”

Kathryn Hansen

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