Updated September 2026.
Fusion is the process that powers the Sun and every other star: light atomic nuclei slam together and merge into a heavier one, releasing energy. Recreating it on Earth would give us a power source with abundant fuel, no carbon emissions and no risk of a runaway meltdown. It is also one of the hardest engineering problems ever attempted. Here is how it works, and why it is so difficult.
Fusion vs. fission
Today’s nuclear power plants use fission: they split heavy uranium atoms. Fusion does the opposite, joining light atoms. Both release energy because the products weigh very slightly less than what went in, and that missing mass becomes energy (Einstein’s E = mc²).
The fuel
The easiest reaction fuses two forms of hydrogen:
- Deuterium is plentiful: roughly one in every 6,500 hydrogen atoms in seawater.
- Tritium is not. It is radioactive, with a half-life of about 12 years, and barely exists in nature. Only a few tens of kilograms exist worldwide, mostly produced in Canadian reactors.
A fusion power plant would have to breed its own tritium, by lining the reactor with lithium that turns into tritium when struck by fusion neutrons. That technology hasn’t yet been proven at scale.
Why it’s so hard
Atomic nuclei are positively charged and repel each other strongly. To make them fuse, the fuel must be heated to over 100 million °C, several times hotter than the Sun’s core, to form a plasma. It must be held dense enough, for long enough, that more energy comes out than goes in. No material can touch something that hot, so there are two main ways to hold it.
- Magnetic confinement. Powerful magnets hold the plasma in a doughnut shape. Tokamaks (like ITER and JET) and stellarators (like Germany’s Wendelstein 7-X) are the main designs. This is the approach most private companies follow, using new superconducting magnets.
- Inertial confinement. Lasers or other drivers crush a tiny fuel pellet so fast that it fuses before it can fly apart. The National Ignition Facility uses 192 lasers this way.
The other problems
- Materials. Fusion neutrons are very energetic. They damage and activate the reactor walls, which then need replacing and must be handled as radioactive waste. That waste is much shorter-lived than fission waste.
- Energy balance. Beating the energy that goes into the plasma isn’t enough. A plant must make more electricity than the whole facility uses, including magnets, lasers, heating and cooling.
- Cost. Even if it works, fusion power has to compete on price with solar, wind, batteries and fission.
Where things stand
Scientific gain was reached in 2022, and records for fusion energy, plasma duration and performance have fallen in the past few years (see The Latest Breakthroughs in Fusion). But no machine has yet produced net electricity. Most experts expect the first real fusion power plants in the 2030s at the earliest, with a meaningful share of the world’s electricity unlikely before mid-century.
