Chain Reaction:

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".