5. Types of fission reactions
There are two types of fission reactions :
1) Uncontrolled fission reaction
2) Controlled fission reaction
Uncontrolled fission reaction
In a fission of 23592U by slow neutrons, three more neutrons are created.

external image Nuclear-Fission-Fusion_01e.gifexternal image Nuclear-Fission-Fusion_01f.gif

The emitted neutrons can create further fission reactions. So the second stage fissioning of 23592U will give rise to nine more neutrons. These 9 neutrons can further cause fissioning giving rise to 27 neutrons. This is the chain reaction. Each fission reaction gives out an enormous amount of energy. Now you can see how a chain reaction can give rise to explosive amount of heat energy.
A fission reaction whereby the reaction is allowed to proceed without any moderation (by removal of neutrons) is called an uncontrolled fission reaction.
An uncontrolled fission reaction is used for nuclear bombs.
Controlled fission reaction
In a fission reaction, if we can remove the emitted neutrons from the fissionable material, then we can control the rate at which the chain reaction proceeds.

external image Nuclear-Fission-Fusion_01e.gifexternal image Nuclear-Fission-Fusion_01f.gif

In the nuclear fission reaction shown above, if we remove two of the three neutrons then, instead of having three more 23592U to fission, we will have only one more of such a reaction. In this way, the rate of reaction can be controlled.
The neutrons can be removed by what are called moderators which are made up of materials which like to absorb neutrons. Cadmium (Cd) is one such material.
A nuclear fission reaction, whereby the rate of reaction can be moderated externally by allowing just enough number of neutrons to keep the fission reaction rate constant is called a controlled fission reaction.
Allowing the moderators to absorb just enough number of neutrons to keep the chain reaction continuing can also control energy emitted during the fission reaction. This is the principle behind nuclear reactors that produce power and generate electricity.

Controlled Nuclear fission is used in nuclear reactors and harnessed for electricity where uncontrolled nuclear fission is used for war and nuclear weapons.

The advantages and disadvantages of having a controlled nuclear fission i.e. fission in nuclear reactors are:


  • Relatively little fuel is needed to produce electricity and the fuel is relatively inexpensive and available in trace amounts around the world.
  • Fission is not believed to contribute to global warming or other pollution effects associated with fossil fuel combustion


  • Possibility of nuclear meltdown from uncontrolled reaction--leads to nuclear fallout with potentially harmful effects on civilians
  • Waste products can be used to manufacture weapons
  • High initial cost because plant requires containment safeguards

Here are some animations which show you the process by which nuclear fission is made use of in nuclear power plants...

Boiling water reactor
Neat animation

However with uncontrolled nuclear fission, these controls don't apply and the chain reaction is left to carry on, thereby spanning out of control resulting in collossal release of energy and mass destruction.

The use of such weapons is considered immoral by the populous as a whole and unethical too.

Here is a link everybody should visit which gives the viewer's a perspective from a survivor of nuclear energy. It talks of the chernobyl disaster and revisits the places where it happened where the aftermaths of the accident are still being felt. Imagine what it would be like to do such a thing on purpose...


Sources: http://home.att.net/~cat4a/nuclear_IV.htm