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Chain Reaction

In nuclear fission, three neutrons are produced along with the release of large energy (Fig. 7). Under favorable conditions, these neutrons can cause further fission of other nuclei, producing large number of neutrons. Thus, a chain of nuclear fissions is established which continues until the whole of the uranium is consumed.
Fig. 7
In the chain reaction, the number of nuclei undergoing fission increases very fast. So, the energy produced takes a tremendous magnitude very soon.

Difficulties in chain reaction

In a chain reaction, following difficulties are observed:
Absorption of neutrons by U238 The major part in natural uranium is the isotope U238 (99.3%); the isotope U235 is very little (0.7%). It is found that U238 is fissionable with fast neutrons, whereas U235 is fissionable with slow neutrons. Due to the large percentage of U238there is more possibility of collision of neutrons with U238. It is found that the neutrons get slowed on coliding with U238. As a result of it further fission of U238 is not possible (because they are slow and they are absorbed by U238). This stops the chain reaction.
Removal To sustain chain reaction 92U235 is separated from the ordinary uranium. Uranium so obtained 92U235 is known as enriched uranium, which is fissionable with the fast and slow neutrons and hence chain reaction can be sustained.
If neutrons are slowed down by any method to an energy of about 0.3 eV, then the probability of their absorption by U238 becomes very low, while the probability of their fissioning U235 becomes high. This job is done by moderators, which reduce the speed of neutron rapidly. Graphite and heavy water are the example of moderators.
Fig. 8
Critical size The neutrons emitted during fission are very fast and they travel a large distance before being slowed down. If the size of the fissionable material is small, the neutrons emitted will escape the fissionable material before they are slowed down. Hence, chain reaction cannot be sustained.
Removal The size of the fissionable material should be larger than a critical size.
The chain reaction once started will remain steady, accelerate or retard depending upon a factor called neutron reproduction factor (k). It is defined as follows:
If k = 1, the chain reaction will be steady. The size of the fissionable material used is said to be the critical size and its mass, the critical mass.
If k > 1, the chain reaction accelerates, resulting in an explosion. The size of the material in this case is super critical (atom bomb).
If k < 1, the chain reaction gradually comes to a halt. The size of the material used is said to be sub-critical.
Table 1: Types of Chain Reactions
Controlled chain reaction
Uncontrolled chain reaction
Controlled by artificial method.
No control over this type of nuclear reaction.
All neurons are absorbed except one.
More than one neutron takes part into reaction.
Its rate is slow.
Fast rate.
Reproduction factor k = 1.
Reproduction factor k > 1.
Energy liberated in this type of reaction is always less than explosive energy.
A large amount of energy is liberated in this type of reaction.
Chain reaction is the principle of nuclear reactors.
Uncontrolled chain reaction is the principle of atom bomb.

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