Answer : When the nuclear reaction produces too many neutrons.
Explanation : In a nuclear fission reactor where the reaction is controlled by regulating the number of neutrons produced by the chain reaction. Production of too many neutrons may cause an accelerated uncontrolled chain reaction which may cause enormous heat to be generated in a very short period of time inside the reactor which may result into an explosion.
For, this purpose control rods, made of neutron absorbing species, like cadmium, are placed in the nuclear reactors to control the excess number of neutrons at any point of reaction when required.