Wiki · Concept
What nuclear fission is
A heavy nucleus splits in two, releasing energy and neutrons, and those neutrons split other nuclei. Everything else follows from that.
When a slow neutron hits a uranium-235 nucleus, the nucleus becomes unstable and splits into two lighter fragments, for example barium and krypton. The split releases about 200 MeV of energy, almost all of it as kinetic energy of the fragments, which slow down in the material and turn into heat. That is the energy that heats the water in a reactor.
Besides the fragments, each fission releases two to three new neutrons. If on average exactly one of them causes another fission, the reaction sustains itself at constant power: that is a critical reactor. If it causes more than one, power rises; if fewer, it falls. All reactor control comes down to keeping that neutron balance.
One gram of fissioned uranium-235 is equivalent to about 2.5 tonnes of burned coal. That million-fold difference in energy density is why a thimble-sized fuel pellet powers a family for months.
Quick facts
| Energy per fission | ≈ 200 MeV, about 3.2 × 10⁻¹¹ J |
|---|---|
| Neutrons per fission | 2.4 on average for U-235 |
| Equivalence | 1 g of U-235 ≈ 2.5 t of coal |
| Discovery | Hahn and Strassmann, December 1938; interpreted by Meitner and Frisch |