Researchers at the Massachusetts Institute of Technology (MIT) have turned to nanoparticles of certain ceramic compounds to line the vessels of nuclear fusion reactors to make them more durable and increase their lifetime.
The nanoparticles help absorb helium atoms produced during the fusion reaction, which can degrade the vessel in as little as six months.
As the world pins its hopes on nuclear fusion reaction to meet its energy demands in a clean and green manner, scientists need to figure out solutions to two major hurdles. One is the net energy gain, where the reactor generates more energy output than is put in. While progress has been recently made on this front, the other hurdle is more challenging.
Fusion reaction works at temperatures greater than those on the Sun, but as the reaction continues, neutrons with high kinetic energy are released, causing radiation damage while also generating more heat.
This heat can be transferred out of the reactor through a coolant to generate steam and then electricity. But keeping the coolant and plasma apart has been a challenge. Controlling the flow of neutrons could help solve this.
The problem of helium in fusion reactors
Nuclear fission reactors also generate neutrons, but those released from the fusion reaction are tricky to control. In addition to possessing more kinetic energy, neutrons from fusion reactors penetrate the vacuum vessel walls.
Depending on the material used to construct the vessel, this interaction can lead to the formation of new helium atoms, which can be a hundred times more than those seen in a fission reactor.
Helium atoms need a landing place, which is a place with low embedding energy and typically occurs in grain boundaries of the metal used in fusion reactors. Atoms inside a metal line up in an orderly fashion, also known as grains. In some areas, the atoms do not line up well and have a low helium embedding energy, leading to the congregation of helium atoms.
As helium atoms begin congregating, they repel each other, further pushing the grain boundary open. Over time, this opening of the grain boundary becomes a crack and breaks the vacuum sealing of the reactor vessel.
The black mark on the displayed sample shows where that sample has been subjected to helium implantation. Subsequent imaging under a transmission electron microscope enables the researchers to determine where in the sample’s microstructure helium bubbles have formed. Image credit: Gretchen Ertl/iStock
Dispersing helium atoms within the reactor
A research team led by Ju Li, a material science and engineering professor at MIT, found a relatively simpler solution to this problem. Since the durability of the reactor vessel depends on embedding helium atoms, the researchers introduced materials with lower embedding energy than the grain boundary of the reactor vessel.
Li’s team examined about 50,000 compounds before narrowing down on 750 potential candidates that could play this role based on their mechanical robustness, non-reactive nature with the reactor’s metal, inability to become radioactive by absorbing neutrons, and “free volume” available for helium atoms to be embedded.
MIT researcher dispersing nanoparticles into iron to be dispersed inside a fusion reactor vessel. Image credit: Gretchen Ertl
Based on these factors, iron silicate was selected as a material to disperse along the interior wall of the reactor vessel and then implanted with helium atoms.
Through X-ray diffraction studies and counting the number of helium bubbles on the reactor vessel, the researchers confirmed that iron silicate dispersed helium congregation away from grain boundaries.
The researchers estimate that adding just one percent by volume of iron silicate reduced helium bubbles by half and their diameter by 20 percent. The researchers have also made iron silicate powders compatible with 3D printers to make fusion reactor vessels more sturdy.
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The research findings were published in the journal Acta Materialia.
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Ameya Paleja Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.
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