Chain reactions are modifications in the composition of the atomic nucleus. The nucleus of a uranium atom, for instance, can be split by bombarding with neutrons. A lot of energy is released in the process. The fragments that arise as a result of the initial reaction fly off at a high speed and collide with the neighbouring nuclei, which get split in turn. The whole process takes place as long as the present mass of uranium is split. An enormous amount of heat is released when several nuclei are split at once. This can lead to explosions. For this reason, this chain reaction is initiated in a controlled way in nuclear power plants.
