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Material might be safer grave for nuclear waste

07.08.2000  |  127× přečteno      vytisknout článek


WASHINGTON - A new, crystalline material that can withstand pounding radiation might provide a safer grave for high-level nuclear waste, an international team of scientists said yesterday. The scientists hope they can fine-tune the material into a form that can be used to contain waste safely for tens of thousands of years. "If this work points the way towards finding the absolute best radiation-tolerant material, and it is then used as an encapsulation material, then this is phenomenally important," said Robin Grimes of Londons Imperial College of Science, Technology and Medicine, who worked on the study. The secret of the material, called erbium zirconate, is its ability to put up with a little disorder, Grimes said. Currently, high-level nuclear waste such as spent fuel from nuclear reactors is stored in containers that may last for only about 100 years. These are put into geologically stable places such as disused salt mines, or buried very deep in the earth, but if the containers rupture, the radiation could escape into the environment. The materials currently used in containers are glass-like chemicals. When these are bombarded by constant radiation, the carefully arranged atoms get jostled out of place. The result can be eventual cracking, swelling and instability. "Glasses absolutely get completely screwed up," Grimes said in a telephone interview. "They are very, very good, cheap ways for shortish-term storage ... but what we are looking for is material that will last tens of thousands of years." Grimes and graduate student Lisia Minervini ran a series of computer models aimed at predicting the kind of materials that would tolerate having their atoms shifted around a bit. They came up with a kind of map, with erbium zirconate at one end and erbium titanate at the other. Kurt Sickafus and colleagues at Los Alamos National Laboratory in New Mexico created the two compounds and ran tests in which they bombarded them with xenon gas. The experiments confirmed what Grimes had predicted - that erbium zirconate was the best at resisting the bombardment. MATERIAL ABSORBS RADIATION LIKE A PUNCHING BAG "This material is on the edge between being a pyrochlore and being a fluorite," Grimes said. "What this means is that when the radiation damage screws up the lattice a little bit, first the oxygen ions rearrange themselves back as they were. Second, although the cations, positive ions, are a bit disordered, the structure doesnt care. It is quite happy," he added. He described it as being like a punching bag that can absorb a punch and then slowly regain its shape. Grimes and Sickafus said they believe that other disordered crystalline materials may work even better. "We think this might be a basic rule that applies to other materials beyond those in this study, but well have to do more work to be sure," Sickafus said in a statement. Grimes said the new material, a ceramic with properties resembling sapphire, could be physically combined with the nuclear waste to create a more stable storage system. "The radioactive material is actually embedded within the crystal lattice on the atomic level," he said. Conventional ceramic processing, which is cheap and easy to do, can be used for this step, Grimes said. This makes it more stable over a geological time scale, Grimes said, because in time even the most stable cave will deteriorate. "Over tens of thousands of years bits of the ceiling fall. It slowly crumbles. So the metal canisters are going to break. You cant do anything about that," Grimes said. "The nice thing about our proposal is the material is still contained within the crystal lattice. It doesnt come out." There is one hitch. "One of the problems is we dont yet know how this material will stand up to water," Grimes said. He said his team may return to the computer to find a material that will not leach radioactive material in water. Congress has been battling with the problem of how to dispose of 400,000 tons of spent fuel from 80 reactors in 40 U.S. states, much of it held at sites not intended for long-term storage. In April, President Bill Clinton vetoed a bill that would have created a site in Nevada to store the waste. Story by Maggie Fox, Health and Science Correspondent REUTERS NEWS SERVICE

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