Nuclear fission and fusion

Fission and fusion sit on opposite sides of the same binding-energy-per-nucleon curve, and both release energy for the same underlying reason: they move the resulting nuclei closer to the peak of that curve, near iron, where nucleons are most tightly bound. The mechanisms by which each is triggered and sustained, however, are quite different.

In fission, a heavy nucleus such as uranium-235 splits into two lighter fragments, typically after absorbing a neutron. Because heavy nuclei have a higher neutron-to-proton ratio than the medium-mass fragments they split into, fission releases several free neutrons along with the fragments and a large amount of energy. Those released neutrons can go on to trigger fission in nearby fissile nuclei, and if enough fissile material is present — a critical mass — this becomes a self-sustaining chain reaction, releasing neutrons and energy at each generation. Below critical mass, too many neutrons escape without causing further fissions and the reaction dies out; this critical-mass threshold is why controlled reactors and fission weapons both depend sensitively on the geometry and quantity of fissile material present.

Fusion combines two light nuclei, such as isotopes of hydrogen, into a heavier one, releasing energy in the process — this is the process that powers the Sun and other stars. Fusion faces a different obstacle: nuclei are positively charged, and bringing two of them close enough for the short-range strong nuclear force to bind them requires overcoming a substantial electrostatic (Coulomb) repulsion first. Achieving this on a useful scale requires extreme temperature (tens of millions of kelvin) and pressure, so that nuclei collide with enough kinetic energy to get close enough for fusion to occur despite the repulsion — conditions found naturally in stellar cores, and pursued with great difficulty in fusion reactor research on Earth. Unlike fission, fusion has no equivalent "critical mass" threshold; instead it is limited by achieving and sustaining the necessary temperature and confinement.