Direct comparison
Nuclear fission vs nuclear fusion
Nuclear fission splits a heavy atomic nucleus into lighter fragments; nuclear fusion joins light nuclei together to form a heavier one. Both release energy, but by opposite routes.
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Side-by-side comparison
| Dimension | Nuclear fission | Nuclear fusion |
|---|---|---|
| What happens | A heavy nucleus splits into two or more lighter nuclei. | Two light nuclei join to form a single heavier nucleus. |
| Typical fuel | Heavy isotopes such as uranium-235 or plutonium-239. | Light isotopes such as deuterium and tritium (hydrogen). |
| Energy per unit mass | Large, but several times less than fusion. | Larger — the most energy-dense reaction known. |
| Conditions needed | Can be triggered by a slow neutron at ordinary temperatures. | Needs extreme temperature and pressure to overcome repulsion. |
| Chain reaction | Released neutrons can split further nuclei, sustaining a chain. | No neutron chain reaction; the plasma must stay confined. |
| Where it occurs | Nuclear power stations and fission weapons. | The Sun and stars; experimental reactors on Earth. |
| Radioactive waste | Produces long-lived radioactive fission products. | Produces far less, shorter-lived radioactivity. |
| Current use for power | Mature: supplies commercial electricity worldwide. | Experimental: not yet a net-energy power source on Earth. |
| Energy source | Mass lost as the fragments form is released per E = mc². | Mass lost as the heavier nucleus forms is released per E = mc². |
Why both release energy
Both reactions release energy because the products weigh very slightly less than the starting materials. That missing mass is converted into energy following Einstein’s equation E = mc², where c is the speed of light. Fusion releases more energy per kilogram of fuel than fission, which is why the Sun, powered by fusion, can shine for billions of years. The dividing line is iron: nuclei lighter than iron release energy by fusing, while nuclei heavier than iron release energy by splitting.
Common questions
FAQ
Which releases more energy, fission or fusion?+
Fusion releases more energy per unit of fuel mass than fission. Joining light hydrogen nuclei liberates several times more energy per kilogram than splitting heavy uranium. However, fusion requires extreme temperatures and pressures to start, which is why it powers stars naturally but is hard to harness on Earth.
Is the Sun powered by fission or fusion?+
The Sun is powered by nuclear fusion. In its core, hydrogen nuclei fuse into helium under immense temperature and pressure, releasing the energy that reaches us as sunlight. Stars run on fusion throughout most of their lives; fission plays no role in their main energy output.
Do nuclear power stations use fission or fusion?+
Today’s commercial nuclear power stations all use fission, splitting heavy nuclei such as uranium-235 in a controlled chain reaction. Fusion power remains experimental: reactors that achieve a sustained net energy gain are still under development, so no fusion station yet supplies the grid.
Going deeper
