What Is Nuclear Fission?

Nuclear fission is a natural phenomenon where atomic nuclei split into smaller ‘daughter’ nuclei, releasing large amounts of energy in the process. Many atomic nuclei are fissionable, meaning that they can be split by being hit with neutrons. Out of these nuclei, three are of special interest to humanity because the manner in which they split releases enough free neutrons for a self-sustaining chain reaction to occur among neighbouring atoms of the same isotope. This makes it possible to build nuclear reactors for practical power production. The nuclei in question are two uranium isotopes, U-233 and U-235, and one plutonium isotope, Pu-239. These are called fissile isotopes.

Out of the three fissile isotopes, only U-235 occurs naturally on Earth, where it composes 0.7% of natural uranium. The other two can be produced artificially by exposing certain other nuclei to neutron radiation to transmute them into other isotopes which then undergo radioactive decay to become the desired fissile isotopes. This process of irradiating non-fissile nuclei in order to convert them into fissile nuclei is called breeding. Non-fissile nuclei which can be bred into fissile nuclei are called fertile. The precursor fertile isotopes are:

1) Thorium-232, which can be transmuted to Th-233, decaying to protactinium-233, then to U-233.

2) Uranium-238, which can be transmuted to uranium-239, which will then decay to neptunium-239, which ultimately decays to Pu-239.

Uranium-238 composes over 99% of natural uranium, which is roughly as abundant as tin in the earth's crust. Thorium is roughly four times as common as uranium, and consists of 100% Th-232.

Nuclear fission can be further classified according to the velocity of the neutrons initiating the process. Neutrons emitted from an atomic nucleus typically have a velocity of around 14,000 km/s, about 5% of the speed of light. These are known as fast neutrons, and nuclear reactors which utilise them are known as fast reactors. The chief advantages of fast reactors is that they liberate more neutrons per collision in the fission process, which enables more efficient fuel breeding, and the energy of the collision is high enough to fission most actinides such as U-238, rather than just U-235 as usual. The chief disadvantage is that fast neutrons have a very small capture cross-section for their target nuclei, thus reducing the likelihood of fission-inducing collisions.

It is possible to slow neutrons down by exposing them to elements with light nuclei. These absorb most of the momentum of the collisions until the neutrons are travelling at only a couple of kilometres per second. This velocity corresponds to room temperature. These are known as thermal neutrons, and nuclear reactors designed to make use of them are known as thermal reactors. The vast majority of current nuclear power plants are thermal reactors. The chief advantage of thermal fission reactions is that they have a much larger capture cross-section with their target nuclei than fast reactions, thereby increasing the likelihood of a fission event. The disadvantages are that they cannot initiate fission in non-fissile nuclei such as U-238, and they release insufficient neutrons to achieve a break-even breeding rate for converting U-238 to Pu-239. Thermal neutrons can achieve break-even breeding of U-233 from Th-232, but this property has not yet been widely exploited.