Answer:
It happens when a heavy nucleus is bombarded with neutrons, so it becomes unstable and decomposes into two nuclei.
Step-by-step explanation:
This is known by the term nuclear fission, which is a reaction by which a heavy nucleus, when bombarded with neutrons, becomes unstable and divides into two nuclei, of sizes of the same order of magnitude, there is a high release of energy and emits two or three neutrons. These neutrons can lead to more fissions by interacting with other fissile nuclei.