For example, uranium 238 has a mass number of 238, which is the total of its 92 protons and 146 neutrons. Many isotopes contain combinations of neutrons and protons that are unstable. These isotopes will always move toward a stable condition. They do this in a number of ways. Depending on how unstable the isotopes are, they may undergo radioactive decay.
Radioactive decay occurs when isotopes move toward a stable condition by releasing particles.
Both alpha and beta decay change the number of protons in a nucleus resulting in a new daughter element. Occasionally an unstable isotope may fission, that is, break up into two smaller nuclei. It is the process of nuclear fission that releases the binding forces that hold nuclei together. Naturally occurring isotopes that undergo spontaneous fissions are much too rare to produce appreciable amounts of energy. However, several naturally occurring and man made isotopes can be made to absorb a neutron which changes them into very unstable isotopes that do fission.