ANN ARBOR --- An international research team, led by University of Michigan scientists, has found that gadolinium zirconate is much more resistant to radiation than the ceramic currently being considered for disposal of plutonium from dismantled nuclear weapons in the United States. This highly durable material---a zirconate pyrochlore---is calculated to resist radiation damage for up to 30 million years.
The research team performed a systematic study of the radiation resistance of gadolinium titanate, the ceramic currently proposed for plutonium immobilization, and zirconate compositions. Results indicate that the titanate will be damaged by radiation in less than 1,000 years. The zirconate will not sustain damage for periods up to 30 million years. Considering that plutonium is an environmental contaminant with a radioactive half-life of 24,500 years, the multi-million-year calculation of the zirconate's durability makes it a leading candidate for the immobilization of plutonium.
"This is a significant scientific discovery with major environmental impact for future generations," said Dr. Yok Chen, Program Manager in the Office of Basic Energy Sciences at the U.S. Department of Energy, which funded this research at the University of Michigan and Pacific Northwest National Laboratory (PNNL).
Rodney Ewing, professor of nuclear engineering and radiological sciences at the U-M College of Engineering, and William Weber, a senior staff scientist at PNNL, led the team of researchers that included scientists at the Australian Nuclear Science and Technology Organization and the Indira Gandhi Centre for Atomic Research in India.
The team's findings were first published in the December 1999 Journal of Materials Research (JMR). This past week, another international team of researchers at Los Alamos National Laboratory, working independently from the U-M team, announced similar results with an erbium zirconate ceramic.
The safe disposal of plutonium is a relatively new environmental problem. Both the United States and the former Soviet Union have agreed to dismantle nuclear weapons, resulting in 100 metric tons of plutonium, approximately 50 from each side. This plutonium is only a small part of a growing global inventory of plutonium that is already greater than 1,300 metric tons.
"What to do with this plutonium is a science and policy issue of great national and international importance," said Ewing. "Two independent research teams have shown that zirconate-based materials offer an excellent solution to the serious problem of this ever-increasing amount of plutonium. Taken together, these startling results confirm that there are radiation-resistant and chemically durable materials that can safely contain plutonium."
This new material is capable of incorporating a large variety of chemical elements in its structure, including plutonium. The zirconate withstands the radiation that results from the decay of plutonium. The ability to sustain high levels of damage without a disruption of the atomic structure accounts for the radiation stability of this material.
"The currently considered titanate became completely disordered at relatively low exposures to radiation," said Shixin Wang, a U-M postdoctoral fellow and lead author on the JMR article. Wang presented these findings at the Plutonium Futures 2000 conference on July 10 in Sante Fe, N.M.
"The disordered titanate material leads to an increase in the loss of plutonium when the material is in contact with water," added Weber at PNNL.
The team will continue to investigate the chemical durability of gadolinium zirconate by leaching tests. The radiation behavior of the zirconates with high concentrations of impurities will be studied to ensure a complete knowledge of the long-term performance of this material.
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