Los Alamos researchers Chris Stanek (right) and Blas Uberuaga are learning how to predict the performance of materials designed to hold nuclear waste.
Nuclear Energy Is Here. Again.
February / March 2009
Volume 7 Number 1
After a prolonged absence, the word "nuclear" has returned to the lexicon of sustainable domestic energy resources. Due in no small part to its demonstrated reliability and clean, carbon-neutral production of electricity, nuclear power is poised to play a greater role in the nation's energy future and contribute even more to our energy security. For scientists, this nuclear revival presents an opportunity to inject new technologies into the industry, technologies that can help maximize nuclear energy's many benefits.
"By developing new options for waste management and exploiting innovative materials to make key technological advances, we can significantly impact the use of nuclear energy in our future energy mix," says Chris Stanek, a materials scientist at Los Alamos National Laboratory. Stanek approaches the big technology challenges by considering how the smallest bits of matter behave. He and his colleagues are using cutting-edge atomic-scale computer simulations to address a difficult aspect of nuclear waste—€”predicting its behavior far into the future. Their research is part of a broader, coordinated effort on the part of the laboratory to use its considerable experimental, theoretical, and computational capabilities to explore advanced materials central not only to waste issues, but to nuclear fuels as well.
Nuclear energy is released when a large, heavy nucleus, such as uranium, fissions and splits into two pieces. Referred to as fission products, the pieces are themselves atomic nucleus, and tend to be highly radioactive. Used nuclear fuel that has spent time in a nuclear reactor consists of un-fissioned uranium, other actinide elements (plutonium, neptunium, and americium), and a potpourri of fission products. It is the radioactive fission products that are the immediate concern surrounding used fuel.
Currently, the used fuel is treated as waste, but the fuel could be more easily managed if the small amount of fission products were separated from the large amount of uranium and other actinides. Incorporated into a stable material, or waste form, the fission products can be safely disposed of in a repository. Fission products have been entrained in smoky, obsidian-like glasses, various metals, or ceramics.
Stanek is exploring the possibility of using optimized ceramic waste forms to capture specific fission products. The problem is that the entrained radioactive atoms decay into new atoms that frequently have different chemical properties than their parents.
"Until now, we have not understood how the decay of a fission product to its daughter product might affect the chemical stability of a ceramic waste form," says Stanek. "But by understanding the chemical changes and exploiting them, we actually have the opportunity to design more robust waste forms that can be tailored to contain specific waste products."
Recent theoretical predictions by Stanek, Chao Jiang, Kurt Sickafus, and Blas Uberuaga, all from Los Alamos, and Nigel Marks from Curtin University of Technology, may have unlocked the door to understanding this "radio-paragenesis," or the formation of new atomic structures due to the radioactive decay of one or more of the original structure's constituents.
The researchers ran atomic-scale simulations for a trial waste form, cesium chloride, to which was added a radioactive cesium isotope. The isotope decays to barium, so as time passes the barium content inside the waste form increases. The outstanding question was what would happen to the atomic structure of the cesium chloride as more barium evolved.
Based on simple bonding arguments, one would expect a metal compound to form. But to everyone's surprise, the simulations predicted that another stable structure would be formed. This structure has never been observed and defies conventional thinking about barium compounds. The calculations also revealed that as cesium chloride converts to the new structure, the volume decreases by more than 20 percent. Thus, cesium chloride would be a poor choice for a waste form, as the large volume change would likely cause cracks to develop. Because the cesium isotope decays with a half life of 30 years, it would have taken a prohibitively long time to reach that conclusion experimentally.
"Our research shows that properties that govern waste form performance are likely altered by the formation of new material phases," says Stanek. "But the ability to do atomistic simulations allows us to design a waste form that will accommodate the decay product and actually become more stable over time."
The research may also have exciting, far-reaching implications. The team, and Nigel Marks in particular, realized that radioactive decay might be a route to synthesize metastable compounds. Armed with a table of isotopes and atomistic simulation, they may be able to predict and then synthesize specialty materials for a range of industrial applications. So by tackling the nuclear waste challenge, the Los Alamos researchers may have discovered a new way to synthesize materials with novel properties.
Other strategies for effectively managing nuclear waste involve recovering the uranium and other actinides from the used fuel, mixing them with fresh uranium, and burning the new "actinide-bearing" fuel in an advanced nuclear reactor.
The laboratory's Ken McClellan and his team of specialists have already produced so-called minor-actinide mixed oxide (MAMOX) fuel pellets made from a mixture of uranium oxide, plutonium oxide, neptunium oxide, and americium oxide. The fuel is currently being irradiated in the advanced test reactor at Idaho National Laboratory, in part to elucidate the role of the initial oxygen content on the fuel's performance.
"The oxygen content of an oxide fuel is critical because it governs many important properties relating to fuel performance," says McClellan. "For example, adding a mere 10 percent more oxygen to uranium dioxide drops its melting point by nearly 1,000 degrees centigrade."
Unfortunately, the initial oxygen content is affected by any impurities in the fuel mixture, and the feedstock for the actinide-bearing fuels, obtained from used fuel, will almost certainly have impurities.
McClellan joined forces with Stanek to apply atomic-scale modeling to help understand how the impurities affect fuel synthesis and performance. The question centered around how a fission product would fit into the fuel's crystal lattice. There were two possibilities; one affected the oxygen content, the other did not. The simulations predicted how much the lattice would either expand or shrink for the respective possibilities, predictions that the researchers verified by measuring the size of the lattice via x-ray diffraction.
"The experimental data fit the theoretical data perfectly," says McClellan. "The implication is that we can now fundamentally understand the affect of these impurities on oxide fuel, which will help us to design fuel that should behave predictably during synthesis and in the reactor."
Production of actinide-bearing fuels would require a highly efficient and cost effective separations strategy. As part of a laboratory-directed research and development project, Los Alamos scientists are studying how to design a revolutionary nuclear fuel form that, exhibits robust and predictable behavior when burned in a reactor, but is easy to separate after it is removed.
The so-called "dispersion fuel form" would have two-components—€”an insoluble "kernel" that contains the fissionable actinides, and a soluble "matrix" that surrounds the kernel. Fission products are born within the kernel with so much kinetic energy that they pass right through it into the matrix, where they stop.
Separation of the matrix from the kernel thus separates the fission products from the actinides. Such a fuel would drastically reduce separation costs and substantially mitigate proliferation risks, as the actinides would remain within the insoluble kernel.
"Our goal is to develop the underpinning science that will help the nuclear power community rapidly evaluate and, hopefully, accept new fuel forms," says Kurt Sickafus, the project co-leader with Gordon Jarvinen. "By advancing the science, we can derive maximum benefit from this remarkable energy source."
Jay Schecker is the editor of 1663, The Los Alamos Science and Technology Magazine.
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