What is chain reaction? Classify the chain reaction.

Explanation

Chain reaction: When one `""_(92)^(235)U` nucleus undergoes fission, the energy released might be small. But from each fission reaction, three neutrons are released. These three neutrons cause further fission in another three `""_(92)^(235)U` nuclei which in turn produce nine neutrons. These nine neutrons initiate fission in another 27 `""_(92)^(235)U` nuclei and so on. This is called a chain reaction and the number of neutrons goes on increasing almost in gerometric progression.
There are two kinds of chain reactions:
(i) uncontrolled chain reaction
(ii) controlled chain reaction.
(i) uncontrolled chain reaction : In an uncontrolled chain reaction, the number of neutrons multiply indefinitely and the entire amount of energy released in a fraction of second.
(ii) The atom bomb is an example of nuclear fission in which uncontrolled chain reaction occurs. Atom bombs produce massive destruction for mankind.
(iii) During World War II, in the year 1946 August 6 and 9, USA dropped two atom bombs in two places of Japan, Hiroshima and Nagasaki. As a result, lakhs of people were killed and the two cities were completely destroyed.
(iv) Even now the people who are living in those places have side effects caused by the explosion of atom bombs.
(v) It is possible to calculate the typical energy released in a chain reaction.
(vi) In the first step, one neutron initiates the fission of one nucleus by producing three neutrons and energy of anout 200 MeV.
(vii) In the second step, three nuclei undergo fission, in third step nine nuclei undergo fission, in fourth step 27 nucleus undergo fission and so on. In the `100^(th)` step, the number of nuclei which undergoes fission is around `2.5xx10^(40)`.
(viii) The total energy released after `100^(th)` step is `2.5xx10^(40)xx200MeV=8xx10^(29)J`. It is really an enormous amount of energy which is equivalent to electrical enrgy required in Tamilnadu for several years.
(i) controlled chain reaction : If the chain reaction is controllable, then we can harvest an enormous amount of energy for our needs.
(ii) It is achieved in a controlled chain reaction. In the controlled chain reaction, the average number of neutron released in each stage is kept as one such that it is possible to store the released energy.
(iii) In nuclear reactors, the controlled chain reaction is achieved and the produced energy is used for power generation or for research purpose.