Why does nuclear fission reaction generally not result in an uncontrolled chain reaction

Explanation:

 For the chain reaction in a nuclear fission to sustain , the size of nuclear material must be of some minimum magnitude ( called critical mass ) other wise after some time the chain reaction ceases to continue. The reason is that too many of neutrons that are instrumental in sustaining the reaction starts leaking out of the surface of fissionable material  . This results in the stoppage of the reaction after some time.