Answer: controller.
Justification:
A fission reaction is the split of the nucleus of an atom due to the bombardment with neutrons.
A fission reaction may terminate in a reaction chain, a controlled reaction or extinguish.
When for every 2 or 3 neutrons released, only one neutron is allowed to strike a fissionable nucleus, the reaction is controlled.
When the the ratio is less than one then the reaction will be extinguished.
When the number of reactions grow, this is a reaction chain, where the reaction is out of control, and the consequence is the release of vast amount of energy (heat) is a short time, causing a nuclear explosion.
