Fission is the splitting of a heavy atom into smaller pieces. Fusion is the joining of light atoms into a heavier one. Both processes convert a tiny amount of matter into a large amount of energy, but they work in opposite directions and have very different requirements, fuels, and consequences.
How Fission Works
Fission starts with a heavy, unstable atom, most commonly uranium-235. This atom has 92 protons and 143 neutrons packed into its nucleus, and that arrangement is inherently wobbly. When a free neutron strikes the nucleus, it absorbs the extra neutron and quickly splits into two smaller atoms, releasing energy and two or three additional neutrons in the process.
Those newly freed neutrons can then strike other uranium-235 atoms nearby, causing them to split as well. Each split releases more neutrons, which cause more splits. This is a chain reaction. In a nuclear reactor, engineers control how many neutrons are available at any given moment so the reaction stays steady and predictable. In a nuclear weapon, the chain reaction is deliberately left uncontrolled, releasing enormous energy in a fraction of a second.
A single fission event releases roughly 200 million electron volts of energy. For comparison, burning a single atom of coal releases just a few electron volts. That millionfold difference is why a small amount of nuclear fuel can power a city for months.
How Fusion Works
Fusion goes the other direction. Instead of splitting a large atom, it forces two small atoms together until they merge into a new, heavier atom. The most studied version uses two forms of hydrogen: deuterium (one proton, one neutron) and tritium (one proton, two neutrons). When these two nuclei collide with enough force, they fuse into a helium nucleus and release a spare neutron along with a burst of energy.
The challenge is getting those nuclei close enough. Both carry a positive charge, so they repel each other the way two magnets push apart when you hold them the wrong way. Overcoming that repulsion requires temperatures of tens of millions of degrees, hotter than the core of the sun. At those temperatures, matter exists as plasma, a superheated state where electrons are stripped away from atoms entirely. Containing and controlling that plasma is the central engineering problem of fusion energy.
A single deuterium-tritium fusion event releases about 17.6 million electron volts. That’s roughly one-tenth the energy of a single fission event. But fusion fuel is far lighter per atom, so pound for pound, fusion releases several times more energy than fission.
Where the Fuel Comes From
Fission relies on uranium, a metal that must be mined and then enriched to increase the concentration of the uranium-235 isotope. Natural uranium is mostly uranium-238, which doesn’t sustain a chain reaction easily. The mining, processing, and enrichment steps are expensive and politically sensitive because the same enrichment technology can produce material for weapons.
Fusion fuel is a different story. Deuterium is naturally present in seawater at a ratio of about 1 in every 6,500 hydrogen atoms. Given the volume of Earth’s oceans, the supply is essentially limitless. Tritium is the harder part. It’s radioactive with a half-life of just 12 years, so almost none exists in nature. Instead, it has to be manufactured by exposing lithium to energetic neutrons. Future fusion reactors would need to breed their own tritium internally using lithium blankets surrounding the reactor core.
The lithium supply is substantial. The U.S. Geological Survey identified up to 98 million tons of lithium resources worldwide as of 2023. However, only about 7.5% of natural lithium is the specific isotope (lithium-6) needed for tritium production, so scalable separation methods are still being developed.
Radioactive Waste
Fission produces significant radioactive waste. When uranium atoms split, they create lighter radioactive elements like cesium-137 and strontium-90, both with half-lives of about 30 years. These fission products generate most of the heat and dangerous radiation in spent fuel. On top of that, some uranium atoms don’t split but instead absorb neutrons and transform into heavier elements like plutonium-239, which has a half-life of 24,000 years. These transuranic elements account for most of the long-term radioactive hazard, which is why spent fuel must be stored securely for thousands of years.
Fusion produces far less problematic waste. The reaction itself creates helium, which is completely stable and non-radioactive. The neutrons released during fusion do make the reactor’s structural materials radioactive over time, but those materials generally have much shorter radioactive lifespans than fission waste. A fusion reactor would not produce plutonium or the long-lived transuranic elements that make fission waste so difficult to manage.
Where Each Technology Stands
Fission is mature. The first commercial nuclear power plants began operating in the 1950s, and today hundreds of reactors around the world generate electricity. Fission provides roughly 10% of global electricity. The technology works, though it comes with ongoing challenges around waste storage, high construction costs, and public concern about accidents.
Fusion is still in development. The largest international effort, ITER in southern France, was originally expected to produce its first plasma by 2025, but the project has faced repeated delays and cost overruns. Once operational, ITER aims to demonstrate that a fusion reactor can produce 10 times more energy than it consumes, using short bursts of deuterium-tritium fuel. Dozens of private companies are now pursuing alternative fusion designs, with some targeting prototype power plants by the mid-2030s. No fusion reactor has yet produced sustained, net-positive energy that could be converted to electricity for the grid.
Fission vs. Fusion at a Glance
- Direction: Fission splits heavy atoms apart. Fusion joins light atoms together.
- Fuel: Fission uses uranium or plutonium, which must be mined and processed. Fusion uses hydrogen isotopes, with deuterium extracted from seawater.
- Energy per event: A single fission reaction releases about 200 MeV. A single fusion reaction releases about 17.6 MeV, but fusion fuel is much lighter, so energy per unit of mass is higher.
- Temperature: Fission operates at relatively modest temperatures (a few hundred degrees in a reactor’s coolant). Fusion requires tens of millions of degrees to force nuclei together.
- Waste: Fission creates long-lived radioactive waste, including elements with half-lives of tens of thousands of years. Fusion’s primary byproduct is helium, with only short-lived radioactivity in reactor materials.
- Availability: Fission has powered grids since the 1950s. Fusion has not yet been commercialized.