Fissile - Fissile Nuclides

Fissile Nuclides

In general, most actinide isotopes with an odd neutron number are fissile. Most nuclear fuels have an odd atomic mass number (A = the total number of protons and neutrons), and an even atomic number (Z = the number of protons). This implies an odd number of neutrons. Isotopes with an odd number of neutrons gain an extra 1 to 2 MeV of energy from absorbing an extra neutron, from the pairing effect which favors even numbers of both neutrons and protons. This energy is enough to supply the needed extra energy for fission by slower neutrons, which is important for making fissionable isotopes also fissile.

More generally, elements with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others; hence, they are less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else to absorb the neutron but without gaining enough energy from the process to deform the nucleus enough for it to fission. These "even-even" isotopes are also less likely to undergo spontaneous fission, and they also have relatively much longer half-lives for alpha or beta decay. Examples of these elements are uranium-238 and thorium-232. On the other hand, isotopes with an odd number of neutrons and an odd number of protons (odd Z, odd N) are short-lived because they readily decay by beta-particle emission to an isotope with an even number of neutrons and an even number of protons (even Z, even N) becoming much more stable. The physical basis for this phenomenon also comes from the pairing effect in nuclear binding energy, but this time from both proton-proton and neutron-neutron pairing. The short half-life of such odd-odd heavy isotopes means that they are not available in quantity and are highly radioactive.