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Nuclear fission is a process during which the nucleus of an unstable atom is split into two smaller nuclei, releasing a large amount of energy. When a fissionable nucleus captures a neutron, it causes the nucleus to split into two lighter nuclei, typically releasing several free neutrons and energy in the process. A chain reaction occurs when the neutrons produced during fission cause more fission reactions in other nuclei. This sustained series of fission reactions releases a vast amount of energy and can be controlled or uncontrolled depending on the circumstances.

One of the key aspects that make chain reactions possible is the emission of multiple neutrons with each nuclear fission event. Generally, nuclear fission produces 2 or 3 free neutrons, which can then go on to interact with other fissile nuclei, causing them to undergo fission as well. In this way, each fission event can trigger multiple subsequent fission events, leading to a self-sustaining chain reaction. Another vital aspect is the presence of a sufficient amount of fissile material. This is commonly referred to as the critical mass. When there is sufficient fissile material, a large number of neutrons can interact with nuclei and sustain the chain reaction.

In a nuclear bomb, the chain reaction is an uncontrolled process. The bomb is designed to release a large amount of energy within a short amount of time. This rapid energy release is achieved by bringing together a sub-critical mass of fissile material and quickly compressing it into a critical mass. The result is an exponentially increasing and uncontrolled chain reaction, which leads to an enormous release of energy in the form of a nuclear explosion. On the other hand, a chain reaction in a nuclear power plant is a controlled process. The main objective is to generate a continuous release of energy for producing heat. This heat can be used to convert water into steam, which powers turbines to generate electricity. In a nuclear reactor, the chain reaction is controlled using control rods made of materials that absorb neutrons. These control rods can be inserted or removed to adjust the rate of fission occurring in the reactor core, thus controlling the chain reaction. In summary, a nuclear bomb relies on an uncontrolled chain reaction, resulting in a rapid release of energy and a destructive explosion, while a nuclear power plant utilizes a controlled chain reaction to generate continuous energy for electricity production.