24.1 Fission Reactions and Energy Release

In induced fission, a heavy nucleus captures a neutron and then splits into lighter nuclei plus additional neutrons. Uranium-235 is the standard example because thermal neutrons can trigger the process efficiently. Spontaneous fission can also occur in some very heavy nuclei without incoming neutrons, but power reactors depend on induced fission because it can be controlled through neutron economy.

Not every fission event yields the same daughter nuclei. There is a distribution of fission fragments, but the bookkeeping logic is always the same: nucleon number and charge are conserved, and a small mass deficit appears between reactants and products. That deficit becomes released energy.

Typical fission Q-values are roughly 170-210 MeV per event. That scale is tiny in everyday units per nucleus, but enormous once multiplied by Avogadro-scale counts of nuclei in reactor fuel. This is why mass-energy conversion in nuclei supports high power output from relatively small fuel mass compared with chemical combustion.

Fission products often lie closer to the peak of the binding-energy-per-nucleon curve than the original heavy nucleus. That means nucleons are on average more tightly bound in the products, so the final nuclear configuration has lower mass-energy. The released difference appears mostly as kinetic energy of fragments and neutrons, and eventually as thermal energy in reactor materials.