Nuclear energy release follows from changes in binding energy per nucleon.

In fission, a heavy nucleus splits into two or more lighter fragments, usually releasing neutrons and gamma radiation. Because medium-mass nuclei are more tightly bound per nucleon than very heavy nuclei, the products have lower total rest mass and the difference appears as released energy.

A neutron-induced fission event can emit additional neutrons. If, on average, more than one of those neutrons causes another fission, the number of events grows: this is a chain reaction. The effective multiplication factor $k$ summarizes the balance. Roughly, $k<1$ is subcritical, $k=1$ critical and $k>1$ supercritical. Real reactor behavior also depends on neutron energies, leakage, delayed neutrons and material geometry.

In fusion, light nuclei combine into heavier ones. For example, hydrogen isotopes can fuse to form helium products whose binding energy per nucleon is larger. Fusion requires nuclei to approach closely enough for the strong interaction to dominate their electrostatic repulsion. High temperature provides kinetic energy, while quantum tunneling allows reactions even below the classical Coulomb-barrier energy.

Stars sustain fusion because gravity confines hot plasma for enormous times and volumes. Terrestrial fusion devices instead seek sufficient temperature, density and confinement time to make fusion power exceed losses.

Fission and fusion are not opposite mechanisms with identical physics: one exploits instability of heavy nuclei and neutron multiplication, while the other relies on bringing light charged nuclei together. Their common energetic origin is the nuclear binding-energy curve.