A nuclear chain reaction occurs when a single nuclear reaction causes a series of additional reactions. This process typically involves the splitting of an atomic nucleus, known as fission, which releases energy and more neutrons. These released neutrons can then collide with other nuclei, causing them to split and release even more neutrons, creating a self-sustaining cycle. Nuclear chain reactions are fundamental to the operation of nuclear reactors and the development of nuclear weapons. In reactors, the chain reaction is carefully controlled to produce energy for electricity, while in weapons, it is designed to release a massive amount of energy in a very short time.