Carbon nanotubes

ONE of the main reasons for limiting the operating lifetime 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. Scientists at the Massachusetts Institute of Technology (MIT), U.S., and collaborators from Texas A&M University, and universities in South Korea, Chile and Argentina, have found that 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 aluminium, which limits its applications to the lower-temperature environments found in research reactors. The team said the method might also be usable in the higher-temperature alloys used in commercial reactors. The findings were described in a recent issue of Nano Energy.

Aluminium is currently used in not only research reactor components but also nuclear batteries and spacecraft and has been proposed as a material for storage containers for nuclear waste. So, improving its operating lifetime could have significant benefits, said Ju Li of MIT, the lead author of the work. The metal with CNTs uniformly dispersed inside “is designed to mitigate radiation damage” for long periods without degrading, said Kang Pyo So of MIT, co-author of the paper. 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 explained. The nanotubes, despite only making up a small fraction of the volume—less than 2 per cent and about 1 per cent by weight—can form a percolating, one-dimensional transport network that provides pathways for the helium to leak out instead of being trapped within the metal, where it can continue to do damage, they pointed out. The huge total interfacial area of these one-dimensional nanostructures provides a way for radiation-induced point defects to recombine in the metal, alleviating a process that also leads to embrittlement. For a given amount of exposure to radiation, tests have shown the amount of embrittlement is reduced about five- to tenfold. The composite can be manufactured at low cost by common industrial methods and is already being produced by the tonne by manufacturers in South Korea for the automotive industry.

While the material used for testing was aluminium, the team plans to run similar tests with zirconium, a metal widely used for high-temperature reactor applications such as the cladding of nuclear fuel pellets. “We think this is a generic property of metal-CNT systems,” he said.

Compiled by R. Ramachandran