Nuclear Fission

Nuclear fission exploits the DECREASING part of the binding-energy curve (Fig.15.6) on its heavy-nucleus side: a very heavy nucleus, with mass number around A = 230, has a lower binding energy per nucleon than nuclei of roughly half that mass. So if such a heavy nucleus were to split into two mid-mass fragments (each with A somewhere between 50 and 150), the resulting fragments would be MORE tightly bound, individually, than the original heavy nucleus was -- meaning their combined mass is LESS than the original nucleus's mass, with the difference released as kinetic energy of the fragments. This splitting process, with its accompanying release of energy, is called nuclear fission.

Uranium is the key practical fuel for fission-based energy generation. Natural uranium is overwhelmingly (more than 99%) the isotope $^{238}U$, with less than 1% being the much more useful isotope $^{235}U$. When $^{235}U$ absorbs a slow-moving neutron, it forms the compound nucleus $^{236}U$, which is highly unstable and undergoes fission almost immediately, splitting in various different ways into different pairs of daughter fragments, releasing on average about 2.7 additional neutrons per fission event along with a large amount of energy (of order 200 MeV, appearing mainly as the fragments' kinetic energy, i.e. as heat). …