Controlling Nuclear Fission

This nuclear fission reaction needs to be controlled, otherwise it will go out of control as a lot of neutrons can be produced very quickly.

In nuclear reactors, where nuclear fission occurs control rods are used to absorb excess neutrons. Every time fission occurs, 2 of the neutrons are absorbed. This means that only 1 neutron is produced for every 1 neutron used. 

If the excess neutrons are not absorbed, then the reaction will go out of control and it will lead to an explosion

This is what happened at the Chernobyl nuclear power station. Since the explosion in the nuclear reactor a protective dome shield has been built over the reactor to prevent the radiation being released. 

In nuclear weapons, the fission reaction is delibrately made to go out of control in order to cause a massive explosion. So, a nuclear bomb is an example of an uncontrolled nuclear fission reaction. The two atomic bombs that were used on Hiroshima and Nagasaki were both nuclear fission bombs.