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To create a green energy future powered by fusion reactors, scientists need to build machines capable of withstanding some of the harshest conditions in the known universe.
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A new MIT study analyzes ways to develop materials capable of separating super-hot plasma from the energy-generating coolant while also mitigating the damage that occurs along grain boundaries, or defects in a metal’s atomic structure.
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The study discovered that by adding iron silicate to the vacuum vessel, helium atoms created by interactions with high energy neutrons will embed themselves uniformly across the vessel instead of congregating at grain boundaries, which eventually form cracks.
Building a nuclear fusion reactor capable of providing green energy for homes and industry is the goal of many physicists around the world, but many roadblocks stand between our present and this green energy future. While some of those hurdles have been overcome, building robust materials capable of surviving the hellish conditions inside tokamaks is the next frontier.
As engineers construct next-generation fusion reactors, like the International Thermonuclear Experimental Reactor (ITER) in southern France, labs around the world are working on creating exotic materials capable of containing super-hot plasma while also generating electricity. One of those labs is MIT Energy Initiative (MITEI), which is dedicated to finding ways to make future reactors more robust and reliable.
In a new study published in the journal Acta Materiala, MIT’s Ju Li, a senior author on the study, and his colleagues investigate new ways to design a material that can keep the coolant (responsible for creating energy from high-energy neutrons) and the roiling plasma apart while also allowing those neutrons to pass through. The problem is that, compared to fission reactors, the neutrons in fusion reactors are much more kinetic, and that causes some of them to react with the atomic structure of the material itself.
This creates helium atoms that then wreak havoc on the vacuum wall while searching for an area with low “embedding energy” (how much energy it takes for a helium atom to be absorbed). Unfortunately, the best candidate are areas known as “grain boundaries,” which are defects in the crystalline structure of the metal, and as these helium atoms congregate in these areas, they repel each other and create cracks in the material, rending the vessel nonfunctional within as little as six months. That’s not something you want when you’re dealing with plasmas in excess of 150 million degrees Celsius.
“The helium atoms like to go to places with low helium embedding energy,” Li said in a press statement, and explains the problem with a simple analogy. “Babylon is a city of a million people. But the claim is that 100 bad persons can destroy the whole city—if all those bad persons work at city hall.”
So the best way to save the “city” is giving those “bad persons” somewhere else to work.
To do this, Li and his team devised a way to purposefully create other areas in the material with lower “embedding energy” to attract these helium atoms away from potential failure points like grain boundaries. By using a metric known as “atomic-scale free volume,” the researchers examined candidates that have a higher volume and thereby a lower embedding energy. After considering other capabilities that this ceramic would require—mechanically robust, metal compatibility, and resistant to radioactivity with consistent neutron exposure—the team narrowed choices down from 50,000 possibilities to the ultimate winner: iron silicate.
“We want to disperse the ceramic phase uniformly in the bulk metal to ensure that all grain boundary regions are close to the dispersed ceramic phase so it can provide protection to those regions,” Li said in a press statement. “The two phases need to coexist, so the ceramic won’t either clump together or totally dissolve in the iron.”
Li and his team tested the material by implanting iron silicate into an iron sample and took x-ray diffraction images that confirmed that the helium atoms were being stored in the “bulk lattice of the iron silicate,” according to Li.
This doesn’t require a lot of iron silicate, either. The team estimates that only 1 percent of the stuff (by volume) can prevent catastrophic failures because a lot of small bubbles spread across the surface is much better than having them congregate along grain boundaries. Thankfully, this material also isn’t siloed in a lab as Li and fellow postdocs involved in the study have created a startup designed to 3D print these structural materials.
Finding the material solution for nuclear fusion still remains a daunting hurdle, but this new research at MIT offers a significant leg up to help clear a looming obstacle keeping us finally winning the green energy gold medal known as nuclear fusion.
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Source Agencies
