# What is Chain Reaction ? Describe the ideal conditions to support a chain reaction.

ANS:



* If in a reaction the products of one step result in producing the subsequent step of the reaction then that reaction is called 'Chain Reaction'.

* During the fission of 92U235, two or three neutrons are released. If the energy of these neutrons is utilised to cause fission of other nuclei, then again two or three neutrons are released per nuclei. Thus, the process of fission continues as chain reaction.

* The reaction mentioned above will go beyond control if all the neutrons released in one step are utilised to cause fission in the subsequent step. Because the number of neutrons released will go on increasing, the amount of energy produced will also go on increasing and reach an uncontrollable stage.

* The following processes can take place after neutrons are produced in the fission reaction.:

1. At least one of the neutrons (released during previous step) hits the other 92U235 to cause fission to release more neutrons ( & energy).

2. The neutrons may be captured by the uranium nucleus (parent nucleus) without fission to take place.

3. The neutrons may be captured by the other nucleus (daughter nuclei) either in fission material or in the surrounding containers even without causing fission.

4. The neutrons may not interact with one another and the nucleus and may escape from the system.


* The fission reaction would stop if the last three processes dominate. However, the fission reaction will continue if the first process is repeated at least once.

* Thus, the ideal condition to support the chain reaction will be to lower the energies of released neutrons to thermal neutron energy range of 0.04 eV.

* This can be achieved by using special substances called moderators like graphite and heavy water which take out some of the energy of the neutron.

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