Fission vs Fusion – What Is the Difference?

Fission vs Fusion
Fission breaks apart larger nuclei, while fusion combines smaller nuclei.

Nuclear fission and fusion are two fundamental processes that release vast amounts of energy, significantly impacting society, especially in the production of electricity. They hold the promise to solve many of the world’s energy problems, offering high-energy outputs with reduced carbon footprints. However, harnessing these processes comes with challenges and potential dangers. Here is a look at the intricacies of both fission and fusion, examining their similarities, differences, applications, and potential pitfalls.

Commonalities Between Nuclear Fission and Fusion

At their core, both nuclear fission and fusion are nuclear reactions that release energy due to high-powered bonds between particles in the atomic nucleus. They share certain things in common:

  1. Release of Energy: Both processes release tremendous amounts of energy, primarily because of Einstein’s equation E=mc2, where a small amount of mass is converted directly into energy.
  2. Binding Energy: Both fission and fusion involve changes in the binding energy between nuclear particles. The energy comes from the rearrangement of nucleons in the nuclear reactions.
  3. Chain Reactions: Both can initiate chain reactions. In fission, one event can cause subsequent fissions, amplifying the energy release. Similarly, fusion of certain nuclei can induce further fusion events. Fusion chain reactions occur in the Sun, but they are difficult to sustain on Earth.

Differences Between Fission and Fusion

While fission and fusion are both nuclear reaction, they are essentially opposite processes of one another:

  1. Nature of Reactions: Fission is the splitting of a large atomic nucleus into smaller particles. Fusion, on the other hand, is the process where two light atomic nuclei combine to form a heavier nucleus.
  2. Energy Required: Fission can often be initiated with little or no energy input, especially for certain isotopes like uranium-235. In uranium, spontaneous fission sometimes occurs. Fusion, conversely, requires extremely high temperatures and pressures to overcome the electrostatic repulsion between the two positively charged nuclei.
  3. By-products: Fission generally produces radioactive waste, which poses storage and contamination challenges. Fusion’s primary by-products are typically harmless; for example, the fusion of hydrogen nuclei produces helium and energy. That being said, fusion releases neutrons, which can make other materials radioactive. Alternatively, the neutrons could treat nuclear waste to make it less problematic.

Nuclear Fission

Fission involves a large atomic nucleus being split into smaller particles. A neutron collides with the nucleus of an atom, which then splits into two or more smaller nuclei, along with the release of a significant amount of energy. This process often releases additional neutrons, which can then cause further fission reactions.

Example Reaction:

Uranium-235 fission is a good example reaction:

  • A neutron (10​n) collides with a Uranium-235 nucleus (23592U).
  • This results in the production of a Barium-139 nucleus (13956​Ba), a Krypton-94 nucleus (9436​Kr), three neutrons (10​n), and a release of energy.

Written out, it looks like: 1 neutron + Uranium-235 → Barium-139 + Krypton-94 + 3 neutrons + energy


  • Power Production: Many nuclear power plants in operation today use nuclear fission to produce electricity.
  • Weapons: The atomic bomb, dropped during World War II, utilized nuclear fission to produce a devastating explosion.

Potential Negatives:

  • Radioactive Waste: Fission produces radioactive by-products that can remain hazardous for thousands of years.
  • Nuclear Meltdown: Improperly managed or malfunctioning reactors can result in catastrophic meltdowns, like the Chernobyl or Fukushima incidents.

Nuclear Fusion

Fusion involves two light atomic nuclei coming together to form a heavier nucleus. This process is responsible for the vast energy produced by the sun and stars. Fusion on Earth requires temperatures of millions of degrees to overcome the repulsion between the positively charged nuclei.

Example Reaction:

An example of a fusion reaction occurs between deuterium and tritium, which are both isotopes of hydrogen:

  • A deuterium nucleus (21​H) fuses with a tritium nucleus (31​H).
  • This results in the production of a helium-4 nucleus (42​He), a neutron (10​n), and a release of energy.

Written out, it looks like: Deuterium + Tritium → Helium-4 + 1 neutron + energy


  • Power Production: Fusion power plants, like the ITER project in France, are in development, promising cleaner, and more abundant energy than fission reactors.
  • Weapons: Hydrogen bombs or thermonuclear weapons utilize fusion reactions and are much more powerful than atomic bombs.

Potential Negatives:

  • Technical Challenges: Achieving and maintaining the conditions necessary for controlled fusion on Earth is challenging.
  • Neutron Activation: Fusion reactions, especially using tritium, produce fast neutrons that can activate structural materials, making them radioactive.

Fission vs Fusion FAQs

Which is stronger – fission or fusion? What is the energy output?

Generally, fusion reactions produces several times more energy than fission reactions. Fusion powers stars, after all! The International Atomic Energy Agency (IAEA) estimates fusion potentially generals four times more energy per kilogram of fuel than fusion. This is around four million times more energy than burning fossil fuels release.

Do nuclear reactors use fission or fusion?

All nuclear power plants use fission. Most of them use uranium.

Which is safer?

Right now, containing a fusion reaction is an issue. But, ultimately fusion is safer than nuclear fission because it does not form long-lived radioactive waste. Also, a reactor meltdown or runaway reaction is not an issue with fusion, which can be shut down in seconds.

Do fission and fusion always produce more energy than they require?

No. Most fission reaction, like the fission of 235U or 239Pu, are exergonic. They release more energy than it takes to start them. But, some fission reactions are endergonic and require more energy than they release.

Similarly, the fusion of deuterium and tritium is exergonic. It releases more energy than the reaction requires. However, the challenge of fusion on Earth is that it requires high temperatures and pressures. Achieving these conditions often takes more energy than we get from the reaction. Further, some fusion reactions (like fusing two iron nuclei together) require a higher energy input than the resulting energy output.

Fission vs Fusion Interesting Facts

  1. Stellar Powerhouse: Fusion powers stars. The Sun fuses about 620 million metric tons of hydrogen every second, converting it into helium and releasing vast amounts of energy in the process.
  2. First Controlled Reaction: The first controlled nuclear fission reaction was conducted by Enrico Fermi in 1942 beneath the bleachers of Stagg Field at the University of Chicago. This experiment was a pivotal step in the development of nuclear reactors and the atomic bomb.
  3. Hottest Places on Earth: Fusion experiments on Earth, such as those conducted at the Tokamak Fusion Test Reactor in Princeton or JET in the UK, have achieved temperatures over five times hotter than the core of the sun.
  4. Cold Fusion Controversy: In 1989, two electrochemists claimed to have achieved nuclear fusion in a laboratory setting at room temperatures, dubbed “cold fusion.” While initially met with excitement, further investigations found their results to be inconclusive. “Cold fusion” remains a topic of debate and skepticism.
  5. Hydrogen’s Role: The most common fuel for fusion reactions is a combination of isotopes of hydrogen – deuterium and tritium. Deuterium comes from seawater, so it is a potentially abundant fusion fuel.
  6. Fission Waste Longevity: Some by-products of nuclear fission remain radioactive and hazardous for thousands of years. For instance, Plutonium-239 has a half-life of 24,100 years.
  7. ITER Project: The International Thermonuclear Experimental Reactor (ITER) in France is one of the most ambitious fusion projects. It aims to prove the feasibility of using fusion as a large-scale and carbon-free source of energy.
  8. Fusion Bombs: The most powerful bomb ever detonated, the Tsar Bomba by the Soviet Union in 1961, was a hydrogen bomb (fusion-based) with an estimated yield of 50 megatons, over 3,000 times more powerful than the atomic bomb dropped on Hiroshima.
  9. Fission in Space: Radioisotope thermoelectric generators (RTGs), which use the heat from the decay of radioactive isotopes (a form of spontaneous fission) to generate electricity, power some space missions, including the Voyager probes and Mars rovers.
  10. Break-Even Point: In fusion research, the “break-even” point refers to the moment when the energy produced by a fusion reaction equals the energy put into it. Achieving and surpassing break-even is a crucial milestone for making fusion a viable energy source.


While both fission and fusion hold immense promise for power generation, they come with challenges. The radioactive waste from fission and the technical hurdles for fusion are significant concerns. Nonetheless, mastering these processes holds the keys for unlocking clean, sustainable energy.


  • Arora, M. G.; Singh, M. (1994). Nuclear Chemistry. Anmol Publications. ISBN 81-261-1763-X.
  • Byrne, J. (2011). Neutrons, Nuclei, and Matter. Mineola, NY: Dover Publications. ISBN 978-0-486-48238-5.
  • Kikuchi, M.; Lackner, K.; Tran, M. Q. (2012). Fusion Physics. International Atomic Energy Agency. ISBN 9789201304100.
  • Sowerby, M.G. (2008). “Nuclear fission and fusion, and neutron interactions“. National Physics Laboratory.