Nuclear fission
The splitting of a heavy atomic nucleus such as uranium-235 or plutonium-239 into lighter nuclei after absorbing a neutron, releasing about 200 MeV of energy and further neutrons.
When a U-235 nucleus absorbs a slow (thermal) neutron it becomes unstable and splits, typically into two fragments such as barium and krypton, plus 2–3 fast neutrons. About 200 MeV is released per fission, more than 80% as kinetic energy of fragments that heats the fuel.
A sustained chain reaction requires the effective multiplication factor k to equal 1. In light-water reactors a moderator (water) slows neutrons to raise the fission probability; control rods (boron, cadmium, hafnium) and soluble boron absorb neutrons to regulate power. Reactor control is possible because about 0.65% of U-235 fission neutrons are delayed by seconds, emitted from decaying fragments.
Power reactors use low-enriched uranium (see uranium enrichment). Fertile U-238 also captures neutrons to form Pu-239, which contributes about one-third of the energy late in a fuel cycle. Fast reactors without a moderator can breed more fuel than they consume.
About 415–440 reactors with roughly 375–400 GW were operating in 2025, generating around 2,800 TWh, about 9% of world electricity, with lifecycle emissions of about 5–12 g CO2e/kWh (UNECE 2021).
Related terms
All terms →- Uranium enrichment
The process of increasing the share of fissile U-235 in uranium above its natural 0.711%, to make reactor fuel or, at very high levels, weapons material.
- Pressurised water reactor (PWR)
The most common nuclear reactor type, in which water under high pressure cools the core without boiling and transfers heat to a separate steam circuit.
- Boiling water reactor (BWR)
A light-water nuclear reactor in which coolant boils in the core and the resulting steam drives the turbine directly, without steam generators.
- Burnup
The thermal energy extracted from nuclear fuel per unit mass of initial heavy metal, usually expressed in gigawatt-days per tonne (GWd/tHM).
- Nuclear fusion
The joining of light nuclei, such as deuterium and tritium, into a heavier nucleus, releasing energy; the process that powers the Sun and an experimental route to electricity.
- Spent nuclear fuel
Fuel assemblies removed from a reactor after use, highly radioactive and thermally hot, containing uranium, plutonium, fission products and minor actinides.