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Washington DC -- Scientists from the United States, United Kingdom, and Japan have pinpointed a group of materials that may safely contain radioactive waste for long-term storage. Their findings are reported in the 4 August issue of Science.
High-level nuclear waste, such as spent fuel from nuclear reactors, is currently stored in containers that hold up for approximately 100 years, according to Kurt Sickafus, of Los Alamos National Laboratory in New Mexico and lead author of the Science study. But stowing nuclear waste for the long run will require containment materials that can resist leaching and radiation damage for thousands of years.
Typically, radioactive emissions jostle the atoms out of their carefully ordered arrangement within the storage material. The material eventually becomes unstable, and thus prone to cracking, swelling, or structural change.
Now Sickafus and his colleagues have hit upon a class of ceramic materials that may fit the bill. The key seems to be that the atoms in the material's structure are relatively disordered and can shift positions with ease, thereby tolerating minute defects caused by radiation. Like an out-of-place object in a messy household, a defect in these materials just isn't that conspicuous.
For several years, researchers looking for better storage materials have had their eye on a class of materials that belong to a larger group of ceramics called "complex oxides." The materials in this class share a basic chemical formula: two different pairs of metallic cations (positively charged ions) and seven oxygen atoms.
Depending on their size, the cation pairs may give these materials either a highly ordered or a somewhat disordered structure.
A material akin to the shiny, brown mineral, pyrochlore, results when the size of the cation pairs differs so much that they can't easily trade places.
A structure more similar to that of the purple mineral, fluorite, occurs when the pairs are close in size. In this case, the cations switch places readily, creating a poorly ordered pattern.
In the past, scientists believed that the materials with the brown pyrochlore structure were promising candidates for use in waste containment, because they would be chemically compatible with the waste constituents. Whether such storage materials would withstand the long-term effects of radiation, however, has been unclear.
Sickafus and his colleagues found that materials with the purple fluorite structure should hold up extraordinarily well under irradiation. In these materials, atoms can shift around to accommodate the defects with little effort, according to computer simulations described in the Science paper.
Radiation-induced defects would cause more commotion in the rigid crystal structure of the pyrochlore group.
"If a material wants to be highly ordered, and the defects are putting atoms where the material doesn't want them, that raises the energy in the structure. Ultimately, the material may have so much energy that it will suffer unwanted structural change," said Sickafus.
The researchers performed some preliminary experiments, irradiating one crystal with a pyrochlore structure, and another with a fluorite structure. As the team had predicted, the highly ordered atoms in the pyrochlore structure changed into an amorphous jumble, while the fluorite structure remained intact.
Both the pyrochlore- and fluorite-type complex oxides are crystalline materials, meaning they consist of units of atoms that, overall, are regularly spaced. Sickafus and his colleagues suspect that other crystalline materials with relatively disordered structures may be resistant to radiation damage as well.
"We think this might be a basic rule that applies to other materials beyond those in this study, but we'll have to do more work to be sure," Sickafus said.
Currently, researchers in many countries, including Japan, Russia, and some in Europe, are studying crystalline materials as candidates for nuclear waste storage containers. In contrast, the United States' policy is to use vitreous, or "glass-like," and cemented materials for storage, according to Sickafus.
"Fluorite-type ceramic materials show promise as safe, radiation-proof materials and should be further developed for containing nuclear wastes," Sickafus said.
The other members of the research team are James A. Valdez, Fuxin Li, Kenneth J. McClellan, and Thomas Hartmann, of Los Alamos National Laboratories, Licia Minervini and Robin W. Grimes, of Imperial College, in London, and Manabu Ishimaru, of Kyushu University, in Fukuoka, Japan. Funding for this work was provided by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences.
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Science