Carbon nanotubes shown to protect metals against radiation damage
An international team of scientists led by MIT has discovered that adding small amounts of carbon nanotubes to metals makes them much more resistant to radiation damage. Though currently only proven in low-temperature metals like aluminum, the team says that the ability of the nanotubes to slow the breakdown process could improve the operating lifetimes of research and commercial reactors.
Radiation isn't just bad for living things, it's also bad for metals – which is unfortunate because nuclear reactors are full of the stuff. The constant exposure of metals to strong radiation makes them brittle and porous to the point where they crack and fail. Needless to say, this impacts the safety and economy of reactors, so preventing this is a high priority for scientists and engineers.
The problem is that as they are bombarded by radioactive particles the atoms in the metal transmutate and split. This causes tiny bubbles of helium to form in a sort of metallic case of the bends. Just as the bubbles of nitrogen that form in a diver's blood if he ascends too quickly can cause damage, so do these helium bubbles that form along the borders of the crystalline grains that make up the metals. Eventually, the metal becomes porous and brittle and much more prone to fracturing.
The MIT team found that by mixing carbon nanotubes with the metal in quantities of less than two percent by volume during manufacturing, the metal becomes much more resistant to radiation. If the nanotubes are evenly distributed, they produce what the team calls a percolating one-dimensional transport network that channels the helium out of the metal before it can cause damage.
One interesting point is that the casting and forging of the metals destroys the nanotubes by converting them to carbides, but they leave behind their one-dimensional shape in what the scientists compare to insects trapped in amber. This not only allows the helium to leak out, but also repairs the defects by providing a way for the metal to recombine, making it less brittle.
The researchers found that the one-dimensional structure survives up to 70 DPA of radiation damage in the treated metal, DPA being a measure the average of how many times every atom in a crystal lattice is knocked out of its site by radiation. In practical terms, this translates into a five- to ten-fold reduction in embrittlement compared to a control sample.
In addition to protection against radiation, the team says that the addition of the nanotubes improves the strength of the material by half, as well as improving its ductility.
Currently, the technique has only been proven for aluminum, which melts at a low temperatures, but the team is testing it on zirconium and is confident that it could be used generally in higher temperature metals. In the meantime, the carbon nanotubes are already inexpensive, thanks to industrial-scale output in Korea for the car industry, and the improved aluminum could be applied in research reactors, spacecraft, and nuclear waste storage containers.
The research was published in Nano Energy.
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Current reactors are already having their lives extended to 60 or 80 years because the reactor vessel walls are fine (they are shielded from neutrons by the water).