Fission vs Fusion
Nuclear fission
Nuclear fission is the splitting of a heavy atomic nucleus into two lighter nuclei, usually triggered when the nucleus absorbs a neutron. The fragments are more tightly bound per nucleon than the original nucleus, so the difference in binding energy is released — along with extra neutrons that can sustain a chain reaction.
Example: Uranium-235 absorbs a neutron and splits into nuclei such as barium-141 and krypton-92 plus neutrons, releasing about 200 MeV per fission in a reactor.
Nuclear fusion
Nuclear fusion is the merging of two light atomic nuclei into a heavier nucleus at temperatures of millions of degrees. The product is more tightly bound per nucleon than the reactants, so binding energy is released — per unit of fuel mass, several times more than fission releases.
Example: In the Sun, hydrogen nuclei ultimately fuse into helium through the proton–proton chain, releasing the light and heat that sustain life on Earth.
Key differences
| Aspect | Nuclear fission | Nuclear fusion |
|---|---|---|
| What happens | A heavy nucleus (e.g. uranium-235) splits into two lighter nuclei when struck by a neutron. | Two light nuclei (e.g. hydrogen isotopes) merge into one heavier nucleus at extreme temperature and pressure. |
| Why energy is released | Releases energy because the two fragments are more tightly bound per nucleon than the original heavy nucleus. | Releases energy because the merged nucleus is more tightly bound per nucleon than the light starting nuclei. |
| Typical fuel | U-235 and Pu-239 — rare heavy elements that must be mined and enriched. | Deuterium and tritium (heavy hydrogen isotopes) — deuterium is abundant in ordinary seawater. |
| Where it happens | Sustained in power reactors by a controlled chain reaction of neutrons; can also be uncontrolled in nuclear weapons. | Powers the Sun and all stars; on Earth it has been achieved in weapons and experimentally in devices like tokamaks. |
| Waste profile | Produces long-lived radioactive fission products and spent fuel that must be stored for thousands of years. | Produces mainly helium; reactor materials become mildly radioactive, but there is no long-lived fission-product waste. |
Both release energy because the products are more tightly bound than the starting materials — the binding-energy curve peaks near iron, so splitting heavy nuclei AND merging light nuclei both move toward that peak. People also confuse fusion with fission waste: fission leaves long-lived radioactive waste, while fusion's main fuel products are harmless helium. And 'cold fusion' claims are unrelated to the high-temperature fusion in stars and reactors.