The nuclear fission which once started continues till all the
atoms of the fissionable material are disintegrated is known as
In a Chain reaction, the neutrons
keep on multiplying rapidly (in G.P.) during fission till
the whole of the fissionable material is disintegrated. Since
fission of one 92U235 gives rise to
about 3 neutrons, one could except the initiation of chain
reaction in process. Fig. shows how chain reaction in uranium
by a single neutron. When a single neutron initially cause
the fission of 92U235 nucleus, 3 neutrons
are released along with huge amount of energy. These 3 neutrons
in turn cause three more nuclei to split as shown in fig,
thereby liberating a total of 9 neutrons and so on.
The process proceeds very quickly
and in very short time the whole of the uranium undergoes
fission neutrons should be more than the incident ones.
such conditions, the called neutron multiplication factor
(or reproduction factor) is often used in chain reactions.
Neutron multiplication factor,
K = Rate of production of neutrons / Rate of loss of neutrons
for chain reaction to be self-sustaining, the value of K must
be greater than 1.The value of K>1 means that neutrons increase
or multiply with time. For this reason, K is called neutron
multiplication factor or reproduction factor.