The interaction between the nucleus of an atom and a subatomic particle, or the nucleus of another atom. In nuclear energy, nuclear reactions between atomic nuclei and neutrons are important. The collision of a thermal neutron with the nucleus of uranium 235 causes it to split and release a large amount of energy. Collisions of fast neutrons with the nuclei of light atoms are used to slow them down, to moderate them. Through radiative neutron capture, the control rods absorb the excess neutrons and control the fission reaction intensity and reactor power.

In thermonuclear fusion, a nuclear reaction occurs between two nuclei of light atoms, e.g., deuterium and tritium. The process of fusing atomic nuclei also releases large amounts of energy.