In a nuclear reactor, the action of atoms splitting is known as what?

The action of atoms splitting in a nuclear reactor is specifically referred to as fission. This process occurs when the nucleus of an atom, often uranium-235 or plutonium-239, absorbs a neutron and becomes unstable, ultimately splitting into smaller nuclei along with the release of energy and more neutrons. This release of neutrons can trigger a chain reaction if those neutrons collide with and split other nearby atomic nuclei, leading to a significant amount of energy production, which is fundamental to the operation of nuclear reactors.

In contrast, fusion refers to the process of light atomic nuclei combining to form a heavier nucleus, typically observed in stars, including the sun. Activation generally refers to the process of making something radioactive, often by bombarding a stable isotope with neutrons, while decay pertains to the natural disintegration of an unstable atomic nucleus into a more stable form, resulting in the emission of radiation. These terms describe processes distinct from the splitting of atoms that characterize fission.