nuclear fission

A very heavy nucleus is a barely stable, overstuffed drop, held together by short-range nuclear attraction while its many protons strain to fly apart under mutual electrostatic repulsion. Give it a nudge and it can split into two medium-sized fragments, releasing a burst of energy and a few spare neutrons. That splitting is nuclear fission, discovered by Hahn and Strassmann and interpreted by Meitner and Frisch, and it is the process behind reactors and fission weapons.

The liquid-drop model makes it intuitive. Absorbing a neutron leaves the nucleus excited and wobbling; it deforms, and as it stretches the restoring surface tension competes with the ever-more-effective Coulomb repulsion of the separating charge. Past a critical deformation (the top of the fission barrier, or saddle point) Coulomb wins, the nucleus necks and scissions into two fragments plus typically two or three prompt neutrons. Energetically, the fragments land near the peak of the binding-energy-per-nucleon curve, so about 0.9 MeV per nucleon times ~235 nucleons gives roughly 200 MeV released per fission, most of it as kinetic energy of the fragments. Fission can be induced by neutron capture or occur spontaneously by tunnelling through the barrier.

The freed neutrons are what make fission technologically decisive: they can induce further fissions, sustaining a chain reaction. The fission fragments are neutron-rich and radioactive, undergoing beta-decay chains that produce delayed neutrons (crucial for control) and long-lived waste. Fission of one gram of uranium-235 releases about as much energy as burning three tonnes of coal.