Fission vs Fusion — Two Ways to Release Nuclear Energy
Nuclear Fission
A heavy nucleus (like uranium-235) splits into lighter nuclei when struck by a neutron, releasing energy plus more neutrons that can sustain a chain reaction. One U-235 fission releases about 200 MeV.
Unit: mega-electronvolt (MeV)
Example: Today's nuclear power plants fission uranium-235: each split releases neutrons that split further nuclei, and the heat makes steam for turbines.
Nuclear Fusion
Light nuclei (like deuterium and tritium) combine into a heavier nucleus, releasing energy. The Sun does this — fusing ~600 million tonnes of hydrogen every second. A D–T fusion releases about 17.6 MeV.
Unit: mega-electronvolt (MeV)
Example: Experimental reactors like ITER heat hydrogen isotopes past 100 million °C, trying to make fusion release more energy than it consumes.
Key differences
| Aspect | Nuclear Fission | Nuclear Fusion |
|---|---|---|
| What happens | Heavy nucleus splits into lighter ones | Light nuclei merge into a heavier one |
| Fuel | Uranium-235, plutonium-239 (rare, mined) | Hydrogen isotopes (abundant in seawater) |
| Conditions | Room temperature; needs neutron moderation and control rods | Over 100 million °C plasma confined by magnets or lasers |
| Waste | Long-lived radioactive fission products | Mostly helium; reactor parts become mildly radioactive |
| Energy per reaction | ~200 MeV per U-235 split | ~17.6 MeV per D–T fusion (but far more reactions per kg of fuel) |
| Status | Commercial power since the 1950s | Still experimental — net energy gain only just demonstrated |
Both are 'nuclear energy' but they are opposites on the binding-energy curve: fission climbs down from the heavy end, fusion climbs up from the light end, and iron sits at the peak where neither releases energy.