Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger binding energies per nucleon—that is, mass numbers and binding energies per nucleon that are closer to the “peak” of the binding energy graph near 56. Sometimes neutrons are also produced. This decomposition of a large nucleus into smaller pieces is called fission. The breaking is rather random with the formation of a large number of different products. Fission usually does not occur naturally but is induced by bombardment with neutrons.
A tremendous amount of energy is produced by the fission of heavy elements. For instance, when one mole of U-235 undergoes fission, the products weigh about 0.2 grams less than the reactants; this “lost” mass is converted into a very large amount of energy — about 1.8 × 1010 kJ per mole of U-235. Nuclear fission reactions produce incredibly large amounts of energy compared to chemical reactions. The fission of 1 kilogram of uranium-235, for example, produces about 2.5 million times as much energy as is produced by burning 1 kilogram of coal.
When undergoing fission, U-235 produces two “medium-sized” nuclei and two or three neutrons. These neutrons may then cause the fission of other uranium-235 atoms, which in turn provide more neutrons that can cause fission of even more nuclei, and so on. If this occurs, we have a nuclear chain reaction. On the other hand, if too many neutrons escape the bulk material without interacting with a nucleus, then no chain reaction will occur.
Material that can undergo fission as a result of any neutron bombardment is called fissionable; material that can undergo fission as a result of bombardment by slow-moving thermal neutrons is additionally called fissile.
Nuclear fission becomes self-sustaining when the number of neutrons produced by fission equals or exceeds the number of neutrons absorbed by splitting nuclei plus the number that escape into the surroundings. The amount of a fissionable material that will support a self-sustaining chain reaction is a critical mass. An amount of fissionable material that cannot sustain a chain reaction is a subcritical mass. An amount of material in which there is an increasing rate of fission is known as a supercritical mass.
The critical mass depends on the type of material: its purity, the temperature, the shape of the sample, and how the neutron reactions are controlled. Materials typically become less dense at higher temperatures, allowing neutrons to escape more easily. Neutrons starting at the center of a flat object can reach the surface more easily than neutrons starting at the center of a spherical object do. If the material is enclosed in a container made of a neutron-reflecting material such as graphite, then far fewer neutrons can escape, meaning that far less of the fissionable material is required to reach a critical mass.
This text is adapted from Openstax, Chemistry 2e, Section 21.4: Transmutation and Nuclear Energy.