Fusion and fission compared

Fusion and fission both release energy from atomic nuclei, and both are called nuclear. They work in opposite directions, use different fuels, and stand at very different stages of development. This guide sets them side by side. It does not argue for either.

The physics

The nuclei in the middle of the periodic table, around iron and nickel, are the most tightly bound. Moving towards them from either end releases energy.

• Fission splits a heavy nucleus, such as uranium-235, into two middle-sized fragments. One fission releases about 193 MeV.
• Fusion joins two light nuclei into a heavier one. The deuterium-tritium (D-T) reaction, the easiest to achieve, produces a helium nucleus and a neutron and releases 17.6 MeV.

A single fission releases about eleven times as much energy as a single D-T fusion. Per kilogram of fuel the order reverses, because fusion fuel is so light. Dividing each energy by the mass of the nuclei involved: D-T gives 17.6 MeV over 5 atomic mass units, about 3.5 MeV per unit; uranium-235 gives 193 MeV over 236 units, about 0.8 MeV per unit. D-T therefore releases a little over four times as much energy per kilogram, which matches the World Nuclear Association’s figure.

Starting and stopping

The most important practical difference is how each reaction is sustained.

Fission sustains itself. Each fission releases two or three neutrons that can cause more fissions. Gather enough fissile material in the right arrangement, with a moderator if needed, and a chain reaction starts on its own. The engineering challenge is to hold it steady and to stop it reliably. See how fission reactors work.

Fusion has to be forced. Nuclei repel each other, and D-T fuel must be heated to around 150 million °C, about ten times the temperature of the Sun’s core, and held together long enough to react. There is no chain reaction. If the conditions are lost, the reaction stops within seconds or less. The engineering challenge is to keep it going at all. See how fusion works.

Fuels

	Fission	Fusion (D-T)
Fuel	Uranium, usually enriched; sometimes plutonium or thorium	Deuterium and tritium
Where it comes from	Mined, converted, enriched and fabricated into fuel	Deuterium from water; tritium must be made from lithium, mostly inside the plant
Main supply question	Enrichment capacity, especially for HALEU	Whether a plant can breed enough tritium to fuel itself

Deuterium occurs naturally in seawater. Tritium does not exist in useful quantities in nature: it is radioactive, with a half-life of 12.32 years, and a D-T power plant would have to breed it by letting its neutrons strike lithium in a surrounding blanket. No device has yet done this at the rate a power plant would need. The nuclear fuel guide covers the fission side.

Neutrons

Both reactions produce neutrons, and neutrons are where most of the engineering difficulty lies.

• Fission neutrons are born with energies of a few MeV.
• D-T neutrons carry 14.1 MeV each, about 80% of the energy of each reaction.

The higher energy matters for the materials around the reaction. A 14.1 MeV neutron knocks atoms out of place in a metal more violently, and it can drive nuclear reactions that release helium inside the metal, which makes it brittle over time. The neutron flux that fission reactors produce has a different spectrum, so materials cannot be fully qualified for fusion using fission test reactors alone. A comparison of “neutron flux” between the two that ignores this spectrum difference misses the main problem.

Waste and activation

Fission leaves used fuel that contains fission products and heavy elements such as plutonium and americium, some of them radioactive for many thousands of years. Used fuel also keeps generating heat: about 10 kW per tonne a year after removal, and about 1 kW per tonne after ten years. It must be cooled, then stored, and eventually disposed of.

Fusion produces no fission products and no long-lived heavy elements of this kind. The World Nuclear Association states that fusion produces “no long-lived radioactive products”. That is true of the reaction itself. It is not true of the machine. Neutrons make the surrounding structure radioactive, a process called activation, and that material has to be handled and disposed of as radioactive waste. The World Nuclear Association describes its long-term radiotoxicity as “considerably lower” than fission waste. How quickly it decays depends on the materials chosen, which is why fusion programmes develop special low-activation steels. Tritium, which is radioactive and moves easily through metals, must also be contained.

“Fusion produces no radioactive waste” is therefore false. “Fusion produces no long-lived fission products or actinides, but does produce activated structural material” is accurate.

Waste comparisons also need to say whether they mean volume or activity. A fusion plant may produce a larger volume of activated steel than a fission plant produces used fuel, while that material is far less radioactive in the long term.

Safety

The two safety cases rest on different physics.

• Fission reactors must control a self-sustaining reaction, and must remove decay heat after shutdown: about 6.5% of previous power at the moment of shutdown, falling over hours and days. The accidents at Three Mile Island and Fukushima Daiichi both came from losing that cooling, not from the chain reaction itself.
• Fusion has no chain reaction to run away. The World Nuclear Association states that a runaway fusion reaction “is intrinsically impossible”, because any malfunction ends the reaction. The hazards that remain are the tritium inventory, the activated structure, the very large stored energies in magnets or pulsed-power systems, and, in some designs, liquid lithium or other chemically reactive materials. Activated structure also produces some decay heat after shutdown; how much, and whether it needs active cooling, depends on the design.

Proliferation also differs. A fission plant uses enriched uranium and produces plutonium in its fuel. A fusion plant holds no fissile material in normal operation, although its neutrons could in principle be used to make some, so international safeguards are not irrelevant to it.

Maturity

This is where the two differ most.

Fission is a mature industry. The World Nuclear Association counted 441 operable power reactors with about 404 GWe of capacity on 5 August 2026. The technology has operated commercially for decades, and the questions about it are mainly about cost, construction schedules, fuel supply and waste policy.

Fusion has not yet supplied electricity to a grid, and no fusion device has produced more electricity than it used. The only experiments to release more fusion energy than was delivered to the fuel target are at the National Ignition Facility, which in December 2022 first released 3.1 MJ of fusion energy from 2.05 MJ of laser light delivered to the target, a target gain of about 1.5. This dataset records a wall-plug gain for the same shot of about 0.0078, counted against the roughly 400 MJ of electrical energy the facility stores to fire its lasers for each shot; that figure is a claim derived from the facility’s stated energy use rather than a measurement. The fusion gain guide explains why both numbers are true. No fusion device has yet bred its own tritium, and no fusion power plant has been built. The benchmark records what each fusion device has actually measured, and the gain page shows why the denominator matters.

Side by side

	Fission	Fusion
Reaction	Splits heavy nuclei	Joins light nuclei
Energy per reaction	about 193 MeV (uranium-235)	17.6 MeV (D-T)
Energy per kilogram of fuel	lower	about four times higher (D-T vs uranium-235)
Chain reaction	Yes, self-sustaining	No
Main neutron energy	a few MeV	14.1 MeV (D-T)
Long-lived waste	Used fuel with fission products and actinides	Activated structure; no fission products or actinides
Decay heat after shutdown	about 6.5% of previous power, falling	Smaller; design-specific
Electricity supplied to a grid	Yes, 441 operable reactors	None

Further reading on this site

• Fission segment and fusion segment
• How fission reactors work and how fusion works
• Fusion fuels and neutrons
• National Ignition Facility
• Glossary: fission, fusion, deuterium-tritium

Sources

• World Nuclear Association, Nuclear Fusion Power (D-T energy, per-mass comparison, lithium, waste, runaway)
• World Nuclear Association, Nuclear Power Reactors (operable reactor table, updated 5 August 2026)
• ITER Organization, What is fusion? (150 million °C)
• Wikipedia, Nuclear chain reaction (energy and neutrons per fission)
• Wikipedia, Decay heat (reactor and used-fuel decay heat)
• Wikipedia, Tritium
• National Ignition Facility figures (3.1 MJ, 2.05 MJ, target gain 1.5, wall-plug gain 0.0078) are from this dataset’s NIF record, where each carries its own sources.