It is controlled by the amount of neutrons available.
The rate of fission of heavy element is controlled by the number the number of neutrons. in the fission reaction of Uranium - 235, when one neutron is bombarded on one Uranium 235, three neutrons, small elements along with lot of energy is produced. Now three neutrons bombard three Uranium 235 atoms so that the lot of amount of energy is liberated. it is called uncontrolled chain reaction.
When the number of neutrons are absorbed and only one neutron is available for fission, then energy is in controlled manner and it is called controlled fission reaction. In this reaction, controlled amount of energy is released.
