Europe is building a gigantic fusion reactor to harness solar energy on Earth

In southern France, teams from 35 nations are starting to assemble the core of a huge fusion experiment called International Thermonuclear Experimental Reactor (ITER).

Scientists are building ITER to copy the nuclear reactions that power the Sun. This new phase turns years of design and construction into a single question: can humans control star-level energy inside a machine on Earth?

The work is happening in the hills near Saint Paul lez Durance, in the south of France. The main reactor hall now holds a partly built metal ring that will become the heart of the device.

What happens in this next stretch of assembly will decide whether fusion can move from theory and small experiments to a serious option for future power plants.

Building the ITER fusion machine

The work is led by Pietro Barabaschi, Director General of the ITER Organization. His research focuses on turning large fusion experiments into a reliable guide for the design of future fusion power stations.

ITER uses a type of fusion device called a tokamak. This is a doughnut-shaped chamber that uses magnetic fields to trap extremely hot gas long enough for fusion to occur.

At its center sits a double walled steel vacuum vessel that is about 62 feet (19 meters) wide. Built from nine massive segments, its total mass is nearly 4,500 short tons (4,082 metric tons).

Those sectors are arriving from factories in Europe and South Korea. They are then lifted into place and welded into a single ring.

A U.S. company, Westinghouse, has a contract worth about 180 million dollars to align and weld the pieces while accounting for how thick steel will shift and expand as it heats and cools.

Engineers have almost no room for mistakes as they close the ring. If the super-hot fusion fuel ever touches the inner metal walls instead of staying suspended in the magnetic field, the plasma will quickly cool and the experiment will stop.

How a tokamak mimics the Sun

In the center of the vessel, heated hydrogen gas becomes a plasma. This is a hot soup of free electrons and atomic nuclei that can carry electric current and respond strongly to magnetic fields.

Inside ITER, that plasma will reach temperatures of about 150 million degrees Celsius. This is far hotter than the core of the Sun.

Special forms of hydrogen known as deuterium and tritium will fuel the main experiments. Deuterium has one proton and one neutron in its nucleus, whereas tritium has one proton and two neutrons.

When deuterium and tritium fuse, they form helium. This is accompanied by a burst of energy that is mostly carried by fast moving neutrons.

For fusion to work in a reactor, three conditions must come together at the same time. The plasma must be very hot and dense enough for many collisions to occur.

In addition, it must stay in place for long enough that the fusion reactions create more energy than the systems use to heat the fuel.

ITER will produce about 500 megawatts of fusion power in its plasma from roughly 50 megawatts of heating power. This represents a tenfold gain that physicists call Q equals 10.

That gain would be far higher than any magnetic fusion experiment so far, and would show that fusion can work at a scale relevant for future power plants.

Magnets, metals, and extremes

Around the tokamak, huge superconducting magnets will shape and hold the plasma in place. These magnets involve coils that carry electric current with almost no resistance when cooled near absolute zero.

Together, these coils will weigh about 10,000 metric tons, or around 11,000 short tons, and they must run for many minutes at a time without losing their superconducting state.

Altogether, the machine will use about 10,000 tons of powerful magnets. These will store roughly 51 gigajoules of magnetic energy and operate in fields of up to nearly 12 tesla.

To keep them superconducting, engineers will cool the windings with supercritical helium to about 4 kelvin, which is close to -452 degrees Fahrenheit (-269 degrees Celsius).

Those intense magnetic fields sit just a few feet away from the vacuum vessel that will hold a plasma many times hotter than the Sun.

The gap between the super-cold magnets and the super-hot fuel is filled with complex shielding structures and cooling channels. These have to protect the hardware from both heat and neutron damage.

Inside the vessel, the first surface the plasma sees is called the first wall. It is a lining made from tiles that can survive extreme heating and bombardment by particles.

The ITER team recently chose tungsten for this wall, effectively trading an easier material for one that better matches what future commercial machines are likely to need.

ITER fusion roadmap

The original 2016 plan pushed for an early first plasma, but the pandemic, manufacturing issues in first-of-a-kind parts, and major shipping delays made that timeline impossible.

In response, the ITER Council approved a new baseline in 2024 that begins a full research phase in 2034. The first deuterium-tritium experiments are now due to take place in 2039.

During this early research period the machine will run hydrogen and then deuterium plasmas at full current and magnetic energy.

These runs will test the divertor that manages heat exhaust, the tungsten first wall, and systems that guard the device when the plasma becomes unstable.

This approach gives engineers time to assemble a more complete machine and test the magnets at 4 kelvin before installation.

The later start date comes with a higher chance of meeting the performance targets when the most powerful fuel mix is finally loaded.

Fusion lessons from ITER

Today, most nuclear power plants use fission, which splits heavy elements such as uranium. This process leaves behind long-lived radioactive waste that must be stored for very long periods.

The U.S. Nuclear Regulatory Commission (NRC) explains that fusion joins light hydrogen isotopes, does not depend on a self-sustaining chain reaction, and is expected to produce minimal long-lived radioactive waste.

Fusion machines will still create radioactive material when neutrons from the reaction activate the steel and other structures that surround the plasma.

An analysis of future reactor designs concludes that, per unit of energy produced, fusion plants would create significantly less long-lived radioactive waste than comparable fission plants.

ITER’s designers have those long-term issues in mind as they choose materials and plan how to handle components after the experiment ends.

Every decision about metals, cooling systems, and shielding links the day-to-day work in the French countryside to questions about how future reactors will be built, run, and eventually taken apart.

Fusion research is described as work aimed at developing a safe, abundant, and environmentally responsible energy source.

Supporters of the project say that if ITER can show that star-like plasmas can be created and controlled at will, it will clear the path for DEMO, the first generation of fusion plants designed to send electricity to the grid.

Information from the ITER website.

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