Nuclear Chain Reaction

Nuclear Chain Reaction

A nuclear chain reaction is a self-propagating sequence of fission reactions in which neutrons released in fission produce additional fission in at least one other nucleus. The chain reaction can take place only in the proper multiplication environment and only under proper conditions.
The fission process may produce 2, 3, or more free neutrons that are capable of inducing further fissions and so on. This sequence of fission events is known as the fission chain reaction.
We know that U-235 nucleus when hit by a neutron undergoes the reaction-
₉₂U²³⁵ + ₀n¹ → ₅₆Ba¹³⁹ + ₃₆Kr⁹⁴ + 3₀n¹
Each of the three neutrons produced in the reaction strikes another U-235 nucleus, thus causing nine subsequent reactions. These nine reactions, in turn, further give rise to twenty seven reactions. This process of propagation of the reaction by multiplication in threes at each fission, is referred to as a chain reaction.
Heavy unstable isotopes, in general, exhibit a chain reaction by release of two or three neutrons at each fission. It may be defined as-
a fission reaction where the neutrons from a previous step continue to propagate and repeat the reaction. A chain reaction continues till most of the original nuclei in the given sample are fissioned. However, it may be noted that not all the neutrons released in the reaction are used up in propagating the chain reaction. Some of these are lost to the surroundings. Thus for a chain reaction to occur, the sample of the fissionable material should be large enough to capture the neutron internally.
If the sample is too small, most neutrons will escape from its surface, thereby breaking the chain. The minimum mass of fissionable material required to sustain a chain reaction is called critical mass. The critical mass varies for each reaction. For U-235 fission reaction it is about 10 kg. As already stated, even a single fission reaction produces a large amount of energy. A chain reaction that consists of innumerable fission reactions will, therefore, generate many times greater energy.

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