In the south of France, ITER is inching towards completion. When it’s finally fully switched on in 2035, the International Thermonuclear Experimental Reactor will be the largest device of its kind ever built, and the flag-bearer for nuclear fusion.
Inside a donut-shaped reaction chamber called a tokamak, two types of hydrogen, called deuterium and tritium, will be smashed together until they fuse in a roiling plasma hotter than the surface of the sun, releasing enough clean energy to power tens of thousands of homes—a limitless source of electricity lifted straight from science fiction.
Or at least, that’s the plan. The problem—the elephant in a room full of potential elephants—is that by the time ITER is ready, there might not be enough fuel left to run it.
Like many of the most prominent experimental nuclear fusion reactors, ITER relies on a steady supply of both deuterium and tritium for its experiments. Deuterium can be extracted from seawater, but tritium—a radioactive isotope of hydrogen—is incredibly rare.
Atmospheric levels peaked in the 1960s, before the ban on testing nuclear weapons, and according to the latest estimates there is less than 20 kg (44 pounds) of tritium on Earth right now. And as ITER drags on, years behind schedule and billions over budget, our best sources of tritium to fuel it and other experimental fusion reactors are slowly disappearing.
Right now, the tritium used in fusion experiments like ITER, and the smaller JET tokamak in the UK, comes from a very specific type of nuclear fission reactor called a heavy-water moderated reactor. But many of these reactors are reaching the end of their working life, and there are fewer than 30 left in operation worldwide—20 in Canada, four in South Korea, and two in Romania, each producing about 100 grams of tritium a year. (India has plans to build more, but it is unlikely to make its tritium available to fusion researchers.)
But this is not a viable long-term solution—the whole point of nuclear fusion is to provide a cleaner and safer alternative to traditional nuclear fission power. “It would be an absurdity to use dirty fission reactors to fuel ‘clean’ fusion reactors,” says Ernesto Mazzucato, a retired physicist who has been an outspoken critic of ITER, and nuclear fusion more generally, despite spending much of his working life studying tokamaks.
The second problem with tritium is that it decays quickly. It has a half-life of 12.3 years, which means that when ITER is ready to start deuterium-tritium operations (in, as it happens, about 12.3 years), half of the tritium available today will have decayed into helium-3. The problem will only get worse after ITER is switched on, when several more deuterium-tritium (D-T) successors are planned.
These twin forces have helped turn tritium from an unwanted byproduct of nuclear fission that had to be carefully disposed of into, by some estimates, the most expensive substance on Earth. It costs $30,000 per gram, and it’s estimated that working fusion reactors will need up to 200 kg of it a year. To make matters worse, tritium is also coveted by nuclear weapons programs, because it helps makes bombs more powerful—although militaries tend to make it themselves, because Canada, which has the bulk of the world’s tritium production capacity, refuses to sell it for nonpeaceful purposes.
