Nuclear Fission and Fusion - Part IV

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5. Types of fission reactions, difference between nuclear fusion and 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.

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.

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.

Both, nuclear fusion and nuclear fission reactions lead to the production of new nuclei, there are some basic differences between the two.

  • In nuclear fusion reactions, two light nuclei  combine to form a heavy nuclei.
 In nuclear fission reactions a heavy nucleus breaks up to form light nuclei.
  • Fusion reaction can never be spontaneous.
 Fission reactions can be spontaneous.
  • No chain reaction is present.
 Chain reaction can sustain the reaction, once started.
  • Fusion reaction can be started by increasing the temperature of the nuclei to be fused. The temperature required is very high.
Fission reaction can be started by bombarding one nuclei with high energy on the other nuclei.
  • Nuclear fusion has not been sustained in the laboratory conditions.
Nuclear fission can be sustained and controlled in practical situations.
  • Nuclear fusion as yet has been used for making hydrogen bombs only.
Nuclear fission has been used for bombs as well as power generation.

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