This image shows how the researchers created aluminum with carbon nanotubes inside. Image courtesy of the researchers.
This image shows how the researchers created aluminum with carbon nanotubes inside. Image courtesy of the researchers.

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 the Massachusetts Institute of Technology (MIT) and elsewhere has found that, at least in some reactors, adding a tiny quantity of carbon nanotubes (CNTs) to the metal can dramatically slow this breakdown process.

For now, the method has only proved effective for aluminum, limiting 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 a paper in Nano Energy 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 not only used to produce research reactor components but also nuclear batteries and spacecraft, and it has been proposed as a material for storage containers for nuclear waste. So improving its operating lifetime could have significant benefits, says Li, who is a professor of nuclear science and engineering and a professor of materials science and engineering.

The metal with CNTs uniformly dispersed inside "is designed to mitigate radiation damage" for long periods without degrading, says So.

Helium from radiation transmutation can take up residence inside metals, causing 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%, can form a percolating, one dimensional (1D) transport network that provides pathways for the helium to leak back out instead of being trapped within the metal, where it could continue to do damage.

Tests showed that after exposure to radiation, the CNTs within the metal can be chemically altered to carbides, but still retain their slender shape, "almost like insects trapped in amber," 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 1D nanostructures also provides a way for radiation-induced point defects to recombine in the metal, alleviating another process that leads to embrittlement. The researchers showed that the 1D 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).

After radiation exposure, Li says, "we see pores in the control sample, but no pores" in the new material, "and mechanical data shows it has much less embrittlement." For a given amount of exposure to radiation, the tests showed the amount of embrittlement is reduced about five- to tenfold.

The new material needs only tiny quantities of CNTs – about 1% by weight added to the metal – and these are inexpensive to produce and process, the team says. The composite can be manufactured at low cost by common industrial methods and is already being produced by the ton by manufacturers in Korea, for the automotive industry.

Even before exposure to radiation, the addition of this small amount of nanotubes improves the strength of the aluminum by 50% and also improves its tensile ductility – its ability to deform without breaking – the team says.

"This is a proof of principle," says So. The team now plans to run similar tests with zirconium, a metal widely used in high-temperature reactor applications such as the cladding of nuclear fuel pellets. "We think this is a generic property of metal-CNT systems," he says.

"This is a development of considerable significance for nuclear materials science, where composites – particularly oxide dispersion-strengthened steels – have long been considered promising candidate materials for applications involving high temperature and high irradiation dose," says Sergei Dudarev, a professor of materials science at Oxford University in the UK, who was not involved in this work.

Dudarev adds that this new composite material "proves remarkably stable under prolonged irradiation, indicating that the material is able to self-recover and partially retain its original properties after exposure to high irradiation dose at room temperature. The fact that the new material can be produced at relatively low cost is also an advantage."

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.