When the atomic nucleus splits, it not only gives off energy, but also throws out two or three more neutrons. 'These new neutrons can, in turn, split two or three other atoms. Ttus way they release more energy and more neutrons, which will split more atoms. In ol her words, once the splitting of the nuclei starts, it becomes self-sustaining. This whole prdccss is calleda chain reaction. If the chain reaction is allowed to go on, it would lead to an explosive release of energy. Control it by absorbing the extra neutrons and you have the slow, smouldering reaction of the "nuclear reactor".