• Superheated helium bubbles, a fusion reactor’s worst nightmare

    • Image credit: Getty/Monty Rakusen
    • Microscope images and a diagram of the channels created by helium in a nanocomposite material. Image credit: Texas A&M
    Date:16 November 2017 Tags:, , ,

    But scientists may have figured out how to solve the problem of superheated helium bubbles.

    By Jay Bennett

    There is no shortage of challenges to overcome to build an energy-positive fusion reactor. Researchers have been able to spark nuclear fusion reactions for years, heating hydrogen up to the point that it will fuse into helium. However, the reaction typically requires more energy input than it produces – or is a thermonuclear bomb.

    In addition to getting the correct “recipe” of hydrogen ions and deuterium (an isotope of hydrogen) ions and helium-3, finding a way to heat the mixture, and finding a way to suspend the fuel (most commonly with magnetic confinement), fusion researchers will also need to address a more fundamental problem: How do you build a reactor that can stand up to the immense temperatures and pressures of nuclear fusion?

    When hydrogen fuses into helium in a plasma mixture that burns at hundreds of millions of degrees. This superheated helium that is produced can work its way into metal components. When you force helium into a solid material, it forms bubbles similar to carbon dioxide bubbles in a carbonated beverage. These bubbles leave holes and structural weaknesses in reactor components over time. Even if we achieve energy-positive reactions, those reactions would likely rip apart a fusion reactor before the energy production can justify the cost of building the reactor in the first place.

    “Literally, you get these helium bubbles inside of the metal that stay there forever because the metal is solid,” said Michael Demkowicz, associate professor of materials science and engineering at Texas A&M. “As you accumulate more and more helium, the bubbles start to link up and destroy the entire material.”

    Fortunately, Demkowicz and his team at Texas A&M are working on a solution. In a new study published in Science Advances, the materials scientist partnered with researchers at Los Alamos National Laboratory in New Mexico to study the effects of helium bubbles on nanocomposite solids—a thin layer of metal, less than 50 nanometers wide, sandwiched in between thicker layers of a different material. In this case, a thin layer of copper was placed between two thicker layers of vanadium, and helium was then injected into the copper layer.

    Superheated helium bubbles, a fusion reactor’s worst nightmare

    Microscope images and a diagram of the channels created by helium in a nanocomposite material.

    The researchers were surprised to learn that helium created channels spreading through the nanocomposite material, rather than the bubbles they had seen in normal metals. If these channels create stable networks for the helium to pass through, the material might hold up for longer than the metals currently used in fusion experiments.

    “We were blown away by what we saw,” Demkowicz said. “As you put more and more helium inside these nanocomposites, rather than destroying the material, the veins actually start to interconnect, resulting in kind of a vascular system.”

    In addition to producing helium-resistant materials for fusion reactors, Demkowicz believes the findings could have implications across a wide variety of materials science research.

    “Applications to fusion reactors are just the tip of the iceberg,” Demkowicz said. “I think the bigger picture here is in vascularized solids, ones that are kind of like tissues with vascular networks. What else could be transported through such networks? Perhaps heat or electricity or even chemicals that could help the material self-heal.”

    The discovery of stable, naturally occurring networks of veins in structural metals such as this could be one step closer to a cost efficient and energy-positive fusion reactor. And if Demkowicz’s optimism pans out, perhaps those reactors will even have vein networks that we could pump chemicals through so the structure repairs itself. Self-healing fusion reactors—it doesn’t get much more futuristic than that.

    Source: Texas A&M
    From: PM USA

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