The good news is that there is a material that works well on the walls of fusion reactors. The bad: it is lithium

We know how the sun works. Another thing is to imitate it. If we got Build a nuclear fusion reactorwe would have clean, safe and practically unlimited energy. But doing so involves incredibly complex engineering challenges.

The wall problem. One of the more colossal challenges In nuclear fusion is to build a container that supports a hottest plasma than the sun’s core. For years, scientists have been experiencing with various materials, from graphite to high resistance metals such as tungsten.

A recent researchthe result of an international collaboration of nine institutions, confirms that we have a star candidate that works spectacularly well for the wall of the reactors: lithium.

A self -refrasinal shield. To understand why lithium is so attractive, you must first visualize the hell that is unleashed inside a tokamak, the most common fusion reactor design. A hydrogen gas, mainly its deuterium and tritium isotopesmore than 100 million degrees Celsius is heated to become a plasma. Magnetic fields potently confine it so that it does not touch anything, but it is impossible to prevent some particles from escaping and violently shocking against the interior walls of the reactor.

This is where lithium shines because it can be used in a liquid state. Instead of eroding and degrading with each impact, it flows and heals himself instantly. This self -referential liquid layer would protect the solid components behind. Moreover, if the reactor walls are hot enough, the lithium can form a steam shield that absorbs much of the impact before it reaches the solid surface.

Goodbye to graphite? Research shows that lithium is not only a passive shield, but an active plasma conditioner. Instead of reflecting the fuel particles that escape, cooling the edge of plasma and destabilizing it, lithium absorbs them. This helps keep heat where it has to be and, therefore, to stabilize the fusion reaction and improve the confinement of plasma.

According to researchers, lithium is a promising candidate to replace graphite, which has a much higher erosion rate. Applied in tungsten walls, it allows to operate the fusion to greater power densities, opening the door to more compact and efficient reactors.

Two ways to apply it. The researchers tested, on the one hand, to cover the lithium walls before lighting the plasma and, on the other, to inject lithium powder directly on the plasma during the reactor operation. The injection was much more effective when creating a uniform and stable temperature profile, one of the sacred conditions for commercial fusion.

All tests were carried out at the Tokamak Diii-D of General Atomics with financing from the United States Department of Energy. The authors of the study, published in the Materials and Energy nuclear magazine, are researchers of the Princeton plasma physics laboratory and his collaborators.

Bad news. In addition to exercising even more pressure on the already tensioning lithium market (Although it does not scarce, it is not extracted to the rhythm that grows its demand), there is a more alarming problem. The lithium is too much Well at work. Catch the tritium with a very high efficiency, preventing it from returning to plasma to be used as fuel.

If the tritio is stuck to the walls, the reactor ends up running out of fuel and the cycle breaks. The accumulation of radioactive tritium in cold areas and difficult to access the reactor also greatly complicates its maintenance and is a safety risk. To top it off, the retention is more significant if the lithium is injected with the reactor in operation, the most efficient application method.

A possible solution. The key is that these experiments were carried out with lithium in solid state, at temperatures below its melting point. In a real reactor, with liquid lithium, The solution could be a “dialysis” system: Instead of bathing the walls by a lithium river and leaving it there, it would be continuously extracted from the reactor, taken to a processing plant to separate the tritium trapped, and pumped back, clean and ready to continue working.

The reactor design would have to adapt to this new proposal. It would be necessary to avoid the cold areas where lithium and tritio could accumulate and stay stagnant, keep the walls at higher and more controlled temperatures, and include the circuit to extract, processes and continuously introduce lithium. A material that solves multiple problems in our mission of simulating the sun, but in return introduces new and also complex.

Image | General Atomics

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