The process by which the splitting of one atom causes the splitting of additional atoms is called a chain reaction. In the context of nuclear fission, which is the splitting of atomic nuclei, a chain reaction occurs when the products of one nuclear fission event initiate further fission events in neighboring atoms.
In a chain reaction, a fissile material, such as uranium-235 or plutonium-239, undergoes nuclear fission when it absorbs a neutron. This fission event releases a significant amount of energy, as well as two or more additional neutrons. These newly generated neutrons can then go on to collide with other fissile atoms, causing them to undergo fission as well.
Each fission event releases more neutrons, which can induce fission in neighboring atoms. This creates a self-sustaining chain reaction, where the number of fission events multiplies rapidly. If the conditions are favorable, such as having sufficient fissile material and proper neutron moderation, the chain reaction can become a nuclear explosion or, in a controlled setting, a sustained release of energy in a nuclear power reactor.
In order to control a chain reaction, various measures are taken. One approach is to use control rods made of materials, such as boron or cadmium, that can absorb excess neutrons and reduce their availability to induce fission. By adjusting the position of these control rods, the rate of the chain reaction can be controlled.
It's worth noting that chain reactions can occur in other contexts as well, not just in nuclear fission. For example, in a chemical chain reaction, the products of one chemical reaction can initiate subsequent reactions, leading to a self-propagating process.