The Joint European Torus, a large fusion reactor in the United Kingdom, received five steel drums lined with cork in 2020. A steel cylinder the size of a Coke can held a small amount of hydrogen gas or the weight of a few sheets of paper.

This wasn't ordinary hydrogen, it was a rare radioactive isotope of tritium. The price is worth it for fusion researchers. Tritium and deuterium can burn like the Sun at high temperatures. As soon as fusion scientists figure out how to spark it, the reaction could give abundant clean energy.

The Canadian tritium fueled experiment at JET showed fusion research is approaching an important threshold: producing more energy than goes into the reactions. JET offered reassurance that ITER, a reactor twice the size of JET under construction in France, will bust past breakeven when it begins burns, by getting to one third of the breakeven point. Fernanda Rimini is JET's Plasma operations expert.

fusion scientists are aware of the possibility of a Pyrrhic victory. Most of the world's tritium is expected to be consumed by ITER.

The fuel for a fusion reactor will be cheap. One in 5000 hydrogen atoms in the ocean is deuterium, and it sells for $13 a gram. Tritium can only be found in trace amounts in the upper atmosphere, which was bombarded by the Cosmic Ray. Nuclear reactor produce small amounts but few harvest them.

Most fusion scientists don't care about the problem as long as they can breed tritium. If the reactor wall is lined with metal, the high-energy neutrons released in fusion reactions can split the metal into two parts. Demand for it in electric car batteries isn't as high as it could be.

If you want to breed tritium you need a fusion reactor that works, and there may not be enough to start the first generation of power plants. Half of the 19 Canada Deuterium Uranium (CANDU) nuclear reactor are due to retire in the next 10 years. The available tritium will peak before the end of the decade and begin a decline as it is sold off and decays.

CANDU plants in Canada and South Korea produce a small amount of tritium. When ITER begins burning tritium, supplies will peak this decade, then begin a decline that will accelerate.

Graph of projected tritium supplies from 2000 to 2060. Supplies peak around 28kg before 2030. Decline accelerates in late 2030s as ITER burns 0.9 kg of tritium per year, dropping to under 5 kg by 2050. Even without ITER,supplies would decline because of CANDU retirement, tritium decay, and other sales. In early 2050s, inventory is boosted by about 1 kg as decommissioned ITER returns unused tritium.
K. Franklin/Science

No net energy will be produced by the first experiments of ITER. The reactor is expected to consume up to 1 kilogram of tritium annually once it begins energy- producing D-T shots. He says it will consume a lot of what's available. It is possible that ITER already drank their milkshake when fusion scientists wanted to fire up their reactor.

Tritium breeding, which has never been tested in a fusion reactor, may not be up to the job, according to some. In a best-case scenario, a power-produced reactor could only produce slightly more tritium than it needs to fuel itself. The narrow margin will be eaten away by tritium leaks or maintenance shutdowns.

The field needs to learn to deal with fitful operations, turbulent bursts of plasma, and neutron damage, as well as scarce tritium. The tritium issue looms large for Daniel Jassby, a retired physicist and critic of fusion energy. He says it could be fatal. Deuterium-tritium fusion reactor is impossible due to this.

D-T fusion would not be possible without the CANDU reactor. CANDU produce tritium as a byproduct and it's the luckiest thing to happen for fusion reactor. Nuclear Reactors use ordinary water to cool the core and moderate the chain reaction so they are more likely to start a fire. Deuterium takes the place of hydrogen in the CANDU reactor because it absorbs less neutrons. Occasionally, a deuterium nucleus can transform into tritium.

If too much tritium builds up in the heavy water it can be a radiation hazard so every so often operators send their heavy water to the utility company. Most of the tritium is sold as a medical radioisotope and for glow-in-the-dark watch dials and emergency signs. Ian Castillo of Canadian Nuclear Laboratories says that it is a nice waste to product story.

The demand will be greatly increased by fusion reactor. When ITER and other fusion startups start burning tritium, James Van Wart expects to ship up to 2 kilograms annually. He says that their position is to get all they can.

Many of the CANDUs are 50 years old or older and will reduce the supply. Things have only gotten worse since researchers realized more than 20 years ago that fusion's "tritium window" would slam shut. The ITER was supposed to burn D-T in the early-2010s. The start of ITER has been delayed because of the safety checks demanded by French regulators. The tritium supply will have dwindled by the time ITER burns D-T.

According to the ITER projections, 5 kilograms or less of tritium will remain. Federici concedes that there is insufficient tritium to satisfy the fusion demand after ITER.

A segment of a huge donut-shaped reactor vessel, suspended in a circular room.
In May, engineers began to assemble ITER’s reactor vessel. The first tritium burns are scheduled for 2035.© ITER Organization

Smaller fusion reactor designs that use less tritium are being designed by private companies. Commonwealth Fusion Systems, a startup in Massachusetts, says it has secured tritium supplies for its compact prototype and early demonstration reactor, which are expected to need less than one kilogram of the isotope during development.

China, South Korea, and the United States will need several kilograms each to build their test reactor. A monster of a machine called DEMO will be needed to start up Eurofusion's successor to ITER. It is expected to be up to 50% larger than ITER and will supply 500megawatts of electricity.

The right conditions for fusion only occur in the hottest part of the ionized gases, so a large startup tritium supply is needed. Most of the tritium in the tokamak gets burned. The rest of the injected tritium will diffuse out to the edge of the tokamak and be swept into a recycling system, leaving a mixture of D-T. The radioactive material is fed back into the reactor. It can take a long time.

DEMO's designers are trying to reduce its startup needs. Christian Day is the project leader in the design of the fuel cycle. It's a problem if you need 20 kilogram to fill it.

If you want to tame the demand, you can fire frozen fuel pellets deeper into the reactor. It is possible to cut recycling time to 20 minutes by using metal foils as filters to remove impurities quickly and also by feeding the hydrogen isotopes straight back into the machine without separating them. Day says that it will be close enough for a working reactor.

C. Bickel/Science

The appetite is likely to be large. The D-T fuel cycle was modeled by him and his colleagues. The time it takes to recycle unburnt fuel and the fraction of time the reactor will operate were estimated. In a paper published in 2021, the team concludes that DEMO alone will require between 5 and 14 kilograms of tritium to begin, more than is likely to be available when the reactor is expected to fire up in the future.

If tritium breeding doesn't work, fusion won't have a future. The output of hundreds of CANDU will be generated by a fusion reactor that burns 167 kilograms of tritium a year.

The challenge for breeding is that fusion doesn't produce enough neutrons. A single tritium nucleus can be created with fusion. Because breeding systems can't catch all the neutrons, they need help from a neutron multipliers. The engineers plan to mix the two materials in a way that makes them work together.

ITER will be the first fusion reactor to experiment. Liquid blankets and solid pebbles beds will be included in the tests. The 600- square-meter reactor interior will only have 4 square meters of breeder systems. After ITER, fusion reactor will need to cover as much of the surface as possible in order to satisfy their tritium needs.

The tritium can be taken continuously or during scheduled shutdowns, but the breeding must be constant. The blankets absorb power from the neutrons and turn it into heat. The hot blankets will pick up the heat and make steam that drives the electricity- generating machines. Mario Merola is the head of engineering design at ITER. The challenge is an engineering one.

It is more than a challenge for the group of people. According to their analysis, breeding blankets could only produce 15% more tritium than a reactor consumes. The figure is more likely to be 5% according to the study.

When tritium breeding stops but the isotope continues to decay, it's called reactor downtime. If the reactor runs more than 50% of the time, it is impossible for an experimental reactor such as ITER to be sustainable. Time between failures is likely to be hours or days, and repairs are likely to take months. Future reactor could struggle to run more than 5% of the time, according to him.

Operators will need to control tritium leaks. Jassby thinks this is the real culprit. The metal walls of a reactor can be eaten by tritium. The analysis assumed a small loss rate. Jassby doesn't believe that's realistic. Think of all the places tritium has to go as it moves through the reprocessing system. If you lose any tritium, you can't afford it.

Two private fusion efforts have stopped using tritium fuel. California startup TAE Technologies plans to use plain hydrogen and Boron, while Washington state startup Helion plans to use deuterium and Helion. The companies think that higher temperatures are worth the extra cost to avoid tritium hassles. The existence of the company is due to the fact that tritium is not plentiful.

The D-T approach threatens the material damage and radioactivity that the alternative fusion reactions can avoid. The absence of neutrons should allow the reactor to last at least 40 years. It takes 1 billion degrees to make hydrogen and boron.

The fuel of deuterium and Helion is compressed with magnetic fields and burns at a very low temperature. It's nearly as hard to get tritium as it is to get helium 3. Commercial sources of it depend on the decay of tritium. Helion CEO David Kirtley claims that his team can create D-D fusion reactions by adding extra deuterium to the fuel mix. He says it is a much lower cost system.

Proponents of conventional D-T fusion believe that more tritium could be built. Militaries around the world use tritium to increase the yield of nuclear weapons and have built up their own tritium stockpiles.

The commercial reactorWatts Bar Units 1 and 2 are operated by the Tennessee Valley Authority and used by the DOE. Occasionally, the rods are removed and processed. When the lab had a D-T burning reactor, the DOE provided tritium. Federici doesn't think the agency, or militaries around the world, will sell the isotope. He says that defense inventories of tritium are not likely to be shared.

There could be a renaissance of the CANDU technology. South Korea does not sell tritium commercially. A tritium facility is being worked on byRomania. India and China both have a few CANDUs. They could increase their tritium production by adding rods to their core. Michael Kovari of the Culham Centre for Fusion Energy and colleagues wrote a paper in Nuclear fusion in which they argued that modifications to the reactor would likely face regulatory barriers.

fusion reactor could create their own tritium if they ran on deuterium alone. If D-D reactions were to be used, they would consume a lot of electricity. The D-D tritium breeding could cost $2 billion per kilogram. The solutions pose significant economic and regulatory difficulties.

The goal of fusion research has been to reach the breakeven point. Jassby says they looked at other issues, such as acquiring enough tritium. Nuclear engineers like Abdou say it's time to worry about engineering details that are far from trivial as the reactor approaches breakeven. It would be wrong to leave them until later.

Related story By Daniel Clery

It was a physics challenge for a long time. Practical obstacles to fusion power plants are coming into focus as the ITER megaproject gears up to demonstrate fusion's potential as an energy source. There is a shortage of fuel. It is necessary for fusion to provide a constant baseload to complement intermittent solar and wind power.

The design of the tokamak reactor has some of fusion's fitfulness in it. The charged particles themselves, as they flow around the vessel, generate the magnetic field. Each pulse of electrical current in a coil of wire in the doughnut's hole lasts a few minutes. The tokamak operations are interrupted when the magnetic field goes down. The mechanical stresses generated by the reactor's powerful magnetic fields could eventually tear it apart.

The current can be driven by the beams of particles and microwaves. The bootstrap effect is a quirk of physics. The particles spiral in a way that they interfere with each other and push themselves around the ring.

Researchers at ITER think they can run an hourlong run using a combination of beams and bootstraps. The bootstrap effect is best at high pressures and can cause the reactor to be damaged.

A turbulent outburst of plasma was caught on camera at the Mega Ampere Spherical Tokamak, a small fusion reactor in the United Kingdom.UK Atomic Energy Authority

The metal that can be found off the vessel's inner wall is a problem for reactor operators. At the Joint European Torus, a U.K.-based tokamak with a reactor wall made of beryllium and tungsten, an automated protection system injects gas into the plasma to stop the bursts. Fernanda Rimini is JET's Plasma operations expert.

ITER operators are hoping to quell the disruptions by applying an additional magnetic field. The edge of the plasma should be slightly less damaged by the two measures that have been put in place.

There is a threat from the flood of high-energy neutrons. Andy London is a materials scientist with the UK Atomic Energy Authority. On the other hand, they dump heat in the reactor wall that will eventually generate electricity, and they can bombard the battery. They can lodge in the reactor walls and weaken the material by knocking atoms out of position. Nuclear particles in the structures can cause further damage. The nickel in steel can be turned into a form that gives off helium, which can cause the steel to swell. London said that the metal turned into a sponge.

It is difficult to find tougher materials because we don't have a fusion reactor to test them in. The world's most intense neutron beam is supposed to be used to test fusion materials. Construction hasn't begun.

It will take a long time to fix damaged reactor components. Repairs will be made using remote handling arms that can navigate the narrow access ports of a tokamak. According to a nuclear engineer at the University of California, Los Angeles, a future reactor may only operate 5% of the time.

He says to compare this with the current reactor. Even if individual fuel rods fail, they can continue to run. The fuel rods can be swapped out by the cranes in a few days. The availability can be high. It will be very difficult to achieve something like fusion.