US scientists cool nuclear fusion reactor with liquid lithium breakthrough

Researchers at the Princeton Plasma Physics Laboratory 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 Princeton’sPrinceton’s Andlinger Center for Energy and the Environment. “Flowing liquid metals have the potential to resolve these materials challenges.”

Liquid lithium’s dual role

The researchers used slats to facilitate liquid metal flow on the fusion reactor’s inside edge. The idea of using liquid metal isn’t 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.

Lithium’s 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 Princeton’s Department of Mechanical and Aerospace Engineering who was involved in the work.

Going beyond lithium

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.

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 . 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.