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Researchers at the Princeton Plasma Physics Laboratory, a US Department of Energy facility, are using liquid lithium to cool down fusion reactors. In addition to helping maintain the fusion reactor temperature, the liquid metal also protects reactor components from neutron bombardment, a press release sent to Interesting Engineering said.

Fusion reactors recreate conditions on the surface of the Sun to fuse hydrogen atoms and release large amounts of energy. This approach is preferred over nuclear fission since it creates no radioactive waste. However, researchers have only managed limited success with net energy output from these reactions.

Another hurdle in nuclear fusion is controlling the temperature of the reactor itself. For fusion reactions to occur, the reactor temperature must hit 100 million degrees Celsius. However, excess heat is also detrimental since it can damage the interior of the reactor vessel.

Currently, there are no available solid materials that can handle these loads, said Egemen Kolemen, an associate professor of mechanical and aerospace engineering at PrincetonsPrincetons Andlinger Center for Energy and the Environment. Flowing liquid metals have the potential to resolve these materials challenges.

The researchers used slats to facilitate liquid metal flow on the fusion reactors inside edge. The idea of using liquid metal isnt entirely new and has been attempted before in systems called diverters. However, the liquid metal flowed for long periods, risking overheating the reactor vessel and evaporation of the metal.

To avoid this, PPPL researchers used electric current to direct the liquid flow and ensure that it was only briefly exposed to the plasma. The metal then flows down the channel toward the bottom of the device called a divertorlet, where the liquid metal cools down and is then sent back to the top of the slat to be poured down again. This avoids overheating the metal since it is briefly exposed to the plasma and then cooled down soon again.

Lithiums role, however, is not limited to cooling down the system alone. It also performs an additional task of keeping the plasma hot enough by recycling the hydrogen particles. Hydrogen isotopes that leave the plasma typically return at a significantly lower temperature, which cools down the plasma.

If your plasma-facing system is made of lithium, it absorbs and keeps those particles that are colliding against the walls, so your plasma is no longer cooling down at fast rates, added Francisco Saenz, a graduate student at Princetons Department of Mechanical and Aerospace Engineering who was involved in the work.

The research team carried out multiple simulations of this approach and used Galinstan, a mix of gallium, indium, and tin, in their experiments since the mixture has the electrical conductivity of liquid lithium.

We used galinstan in our experiments because it is much easier to work with it given that it is liquid at room temperature, Saenz told IE in an email. Operating with lithium would require a heating system to stay above the melting point of lithium, which is ~ 180 C. The behavior of galinstan should be similar to that of lithium at the reactor scale given that their electrical conductivities are very similar.

The team also experimented with increments in current flow to arrive at a flow uniformity for the liquid metal without splashing inside the reactor vessel. The press release added that the team achieved a flow rate of one meter per second by using 900A of current.

PPPL researchers have also initiated the Lithium Experiment Application Platform to work with larger volumes of liquid lithium and other metals, such as copper and tungsten.

Tungsten is indeed the desired material for a real divertorlets system but it is way harder to build component using tungsten, added Saenz in the email. It would be more advantageous than copper because it would allow the liquid metal to reach higher speeds with smaller power requirements for operation.

The current design of the divertorlet is closed and does not allow liquid lithium to be removed from the reactor vessel. In the future, the researchers are also keen to work with a system in which spent lithium can be removed and new liquid lithium added to cool it.

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Ameya Paleja Ameyais 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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Princeton Lab uses liquid lithium to cool nuclear fusion reactors - Interesting Engineering