Setback for safe storage of nuclear waste
A material that promised to lock up nuclear waste for hundreds of thousands of years may not be up to the job. At present high-level waste is "vitrified" by combining it with liquid borosilicate glass and solidifying the mixture. This makes the waste safer as it delays leakage of the radioactive material. The glass is not ideal, though, because geological activity can break it up, so researchers are on the lookout for more robust "immobilisation" materials.
Minerals such as zircon (ZrSiO4) are believed to have kept naturally occurring radioactive uranium and thorium locked in the Earth's crust for up to 4.4 billion years, surviving earthquakes and volcanic eruptions. As a result researchers have argued that zircon, or similar synthetic ceramics, could trap nuclear waste within their crystalline structures for at least 241,000 years, the time plutonium-239 takes to become relatively safe.
Now a study shows that this is unlikely. It turns out that alpha particles released as plutonium decays knock the atoms in zircon out of position faster than originally predicted, impairing the material's ability to immobilise waste (Nature, vol 445, p 190).
Ian Farnan of the University of Cambridge and colleagues at the Pacific Northwest National Laboratory in Richland, Washington, added plutonium to zircon and used nuclear magnetic resonance spectroscopy to distinguish between crystalline zircon and its leaky, damaged form.
The researchers found five times as many damaged zircon atoms as estimated by computer simulations. They conclude that radioactive plutonium trapped in zircon would start leaching out after just 210 years and lose its crystal structure entirely after 1400 years.
The result could dash hopes for ceramics similar to zircon under consideration in Australia, Russia and the US. Farnan believes, however, that it is still possible to develop synthetic ceramics that don't lose their crystalline structure as quickly as zircon. "We have demonstrated a method that will allow us to be more confident about the storage of waste in the future," he says.
NUCLEAR WASTE: With Plutonium, Even Ceramics May Slump
Crystalline ceramics such as zircon, a promising idea for immobilizing nuclear waste, may not be so solid after all. A new study reveals that alpha radiation could break down this ceramic's structure more rapidly than assumed.
"A zircon mix containing 10% plutonium-239, for example, could become amorphous in just 1400 years - far short of the U.S. containment target of 210,000 years. This experimental finding, experts say, points to a need for more research on alternative forms of waste storage."
"Vitification is a 'completely unstable' method of storing wastes," says Kurt Sickafus of the Materials Science and Technology Division at LANL. He argues that ceramic forms can be made 'highly stable,' but not the silicate-based forms such as zircon. He suggests fluorite crystal structure instead because their amorphousness lies somewhere between that of glass and the rigid silicates. This makes them able to tolerate radiation-induced defects without severe disruption of the crystal lattice, he says."