nonfissionable

Nonfissionable refers to materials that cannot sustain a nuclear fission chain reaction. This is a critical characteristic in nuclear engineering and materials science. Fission occurs when an atomic nucleus absorbs a neutron, becomes unstable, and splits into lighter nuclei, releasing energy and more neutrons. A chain reaction is possible when, on average, at least one neutron from each fission event goes on to cause another fission.

Materials are considered nonfissionable if they do not possess the necessary properties to facilitate this process. This can be due to several factors. Some isotopes are simply not fissile, meaning they do not readily undergo fission when bombarded with neutrons, particularly thermal neutrons. Other materials might absorb neutrons without undergoing fission, effectively removing them from the chain reaction process, which is a characteristic of neutron poisons.

In the context of nuclear reactors, nonfissionable materials are often used as moderators or reflectors. Moderators, such as water or graphite, slow down fast neutrons released during fission to thermal energies, where they are more likely to cause fission in fissile materials like uranium-235. Reflectors, also often made of nonfissionable materials, surround the reactor core and bounce stray neutrons back into the core, improving neutron economy and reducing fuel requirements. Control rods, typically made of neutron-absorbing materials like cadmium or boron, are also nonfissionable in the sense that they are used to absorb excess neutrons and control the rate of the fission chain reaction. Understanding the nonfissionable nature of materials is paramount for the safe and efficient design and operation of nuclear facilities.