Nuclear Fission vs Fusion: What's the Difference?

KEY TAKEAWAYS

Fusion combines atoms, fission splits them.
Fusion releases about 4x more energy than fission.
Fission produces long-lived radioactive waste and carries meltdown risk; fusion does not.
Fusion’s fuel supply is virtually limitless; fission depends on uranium, a rare and finite element.
All current commercial nuclear plants use fission reactions, but scientists are making great advances with fusion.

What’s the difference between nuclear fusion and nuclear fission?

Nuclear fusion occurs when two atoms fuse together, or combine, into a larger atom. This fusion process releases a great amount of energy.
Nuclear fission occurs when one atom is ripped apart into smaller atoms. Fission release a great amount of energy, but less than fusion.

Fission and fusion both release a great deal of energy, but fusion releases about four times more.

	FUSION	FISSION
Reaction	Atoms fusing together	Atoms splitting apart
Energy Released	4X more than fission	4X less than fusion
Radioactive Waste	Does not produce long-lived waste	Produces long-lived radioactive waste
Electricity Generation	Not used commercially	Used commercially
Fuel Source	Mix of deuterium and tritium, heavy hydrogen isotopes	Uranium
Safety	Safer: Meltdowns are impossible, no long-lived waste	Less safe: Meltdowns possible, produces radioactive waste
Fuel Abundance	Vast, virtually limitless supply	Finite supply of rare elements

How does nuclear fusion work?

Nuclear fusion works when extremely large amounts of pressure force two atoms to fuse together, a process that releases immense energy as a result.

Nuclear fusion in space

If you look up at the sun, you’re looking at the largest fusion reactor in the solar system. The sun is so massive that the force of its own gravity crushes hydrogen atoms together in its core.

The crushing force, or pressure, in the sun’s core forces the hydrogen atoms to fuse together. Under normal circumstances, hydrogen atoms repel each other because they both have one proton and positive charges.

The strength of the sun’s extreme heat and pressure forces the atoms together anyway. The two atoms join together, ultimately forming helium.

This fusion process releases a massive burst of energy. In the sun, the energy takes the form of heat and light which travel through space and reach Earth eight minutes later.

Nuclear fusion on Earth

On Earth, we cannot recreate the sun’s tremendous gravity. Instead, scientists and researchers attempt to use heat to force hydrogen to fuse. Fusion experiments have reached temperatures north of 150 million degrees Celsius.

The problem with nuclear fusion on Earth lies with the difficulty of creating, containing, and sustaining these insanely hot conditions long enough for it to be useful.

Fortunately, our planet’s nuclear scientists and researchers are making progress.

December 2022: U.S. Department of Energy scientists at the National Ignition Facility (NIF) were able to produce more energy than they put into a nuclear fusion reactor for just a few seconds. They were successful again in August 2023.
February 2025: Scientists at the WEST tokamak reactor in France set a new world record by sustaining a 50-million-degree-Celsius plasma for over 22 minutes.
July 2025: NIF scientists achieved ignition (i.e. generating more energy than the amount put in) with the new fuel container called TOR (Thinned Hohlraum Optimization for Radflow).

How does nuclear fission work?

Nuclear fission works when a neutron strikes a heavy atom with enough force to split it apart, a process that releases immense energy as a result.

Nuclear fission in a reactor

Every operating commercial nuclear power plant today runs using fission. It begins with uranium, a heavy element with large and unstable atoms.

Nuclear fission plant operators fire neutrons at uranium atoms at incredibly high speeds. The impact is so strong that the uranium atom’s nucleus splits into two, releasing a burst of energy.

That’s not all. That split releases even more neutrons, which go on to strike neighboring uranium atoms. Those atoms are split as well, releasing even more neutrons. This is called a chain reaction.

Controlling nuclear fission’s chain reaction

Without controls, the chain reaction can accelerate to the point where we lose control. In that scenario, the reactor melts down, releasing a dangerous amount of radiation.

To prevent a meltdown, reactors use materials that absorb excess neutrons, which enables operators to control the process.

The energy released from the fission reactions takes the form of heat, which is used to convert water into steam. The steam rotates a turbine that generates electricity.

Learn more about nuclear energy

Written by Graham Lumley

Graham Lumley is the Growth Product Manager at BKV Energy, a retail electricity provider in Texas' deregulated market, where he has worked since 2023. He analyzes customer usage and billing data, helps develop BKV's fixed-rate plans, and creates tools to simplify Texas electricity shopping. He has written over 200 articles on Texas electricity rates, delivery charges, plan types, energy generation, and more to help Texans understand their bills and pay less. With over 10 years of experience in consumer products, marketing, and energy, he explains the Texas electricity market in plain language.