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Friday, April 15, 2016

Carbon nanotubes improve metal’s longevity under radiation



One of the main reasons for limiting the operating lifetimes of nuclear reactors is that metals exposed to the strong radiation environment near the reactor core become porous and brittle, which can lead to cracking and failure. Now, a team of researchers at MIT and elsewhere has found that, at least in some reactors, adding a tiny quantity of carbon nanotubes to the metal can dramatically slow this breakdown process.
carbon nanotubes
For now, the method has only proved effective for aluminum, which limits its applications to the lower-temperature environments found in research reactors. But the team says the method may also be usable in the higher-temperature alloys used in commercial reactors.

The findings are described in the journal Nano Energy, in a paper by MIT Professor Ju Li, postdocs Kang Pyo So and Mingda Li, research scientist Akihiro Kushima, and 10 others at MIT, Texas A&M University, and universities in South Korea, Chile, and Argentina.

Aluminum is currently used in not only research reactor components but also nuclear batteries and spacecraft, and it has been proposed as material for storage containers for nuclear waste. So, improving its operating lifetime could have significant benefits, says Ju Li, who is the Battelle Energy Alliance Professor of Nuclear Science and Engineering and a professor of materials science and engineering.

The metal with carbon nanotubes uniformly dispersed inside “is designed to mitigate radiation damage” for long periods without degrading, says Kang Pyo So.

Helium from radiation transmutation takes up residence inside metals and causes the material to become riddled with tiny bubbles along grain boundaries and progressively more brittle, the researchers explain. The nanotubes, despite only making up a small fraction of the volume — less than 2 percent — can form a percolating, one-dimensional transport network, to provide pathways for the helium to leak back out instead of being trapped within the metal, where it could continue to do damage.

Testing showed that after exposure to radiation, the carbon nanotubes within the metal can be chemically altered to carbides, but they still retain their slender shape, “almost like insects trapped in amber,” Ju Li says. “It’s quite amazing — you don’t see a blob; they retain their morphology. It’s still one-dimensional.” The huge total interfacial area of these 1-D nanostructures provides a way for radiation-induced point defects to recombine in the metal, alleviating a process that also leads to embrittlement. The researchers showed that the 1-D structure was able to survive up to 70 DPA of radiation damage. (DPA is a unit that refers to how many times, on average, every atom in the crystal lattice is knocked out of its site by radiation, so 70 DPA means a lot of radiation damage.)

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