In nuclear fission, a neutron plays a crucial role by initiating the reaction through its collision with a heavy nucleus, resulting in the nucleus splitting into smaller fragments.

Specifically, nuclear fission is a type of nuclear reaction where a heavy nucleus, such as uranium-235 ($^{235}\text{U}$) or plutonium-239 ($^{239}\text{Pu}$), divides into two or more lighter nuclei, accompanied by the release of a significant amount of energy. The process begins when a neutron collides with the heavy nucleus, causing it to become unstable and ultimately split apart.

The neutron that triggers this fission is commonly known as a ‘slow’ or ‘thermal’ neutron. This designation arises from the fact that it has been slowed down to thermal energies, which increases the probability of it being absorbed by the heavy nucleus. Upon absorption, the neutron increases the mass of the nucleus, pushing it into an unstable state that leads to its fission into smaller nuclei.

Additionally, the fission process releases several more neutrons. These newly generated neutrons can initiate further fission reactions, resulting in a self-sustaining chain reaction. This chain reaction is foundational to both nuclear power generation and the detonation of atomic bombs. In a nuclear power plant, the chain reaction is meticulously controlled to ensure a steady output of energy. Conversely, in an atomic bomb, the chain reaction is uncontrolled, leading to a rapid and explosive release of energy.

In summary, the neutron is essential to the process of nuclear fission. It initiates the reaction by colliding with and being absorbed by a heavy nucleus, which becomes unstable and splits. The fission process not only produces additional neutrons that can trigger further reactions but also establishes the basis for a chain reaction.