Radioactive Chain reaction
Consider a neutron causing fission in a uranium
nucleus producing three neutrons. The three neutrons in turn may cause fission
in three uranium nuclei producing nine neutrons. These nine neutrons in turn
may produce twenty seven neutrons and so on. A chain reaction is a self propagating
process in which the number of neutrons goes on multiplying rapidly almost in a
geometrical progression.
Two types of chain reactions are possible. In
the uncontrolled chain reaction, the number of neutrons multiply indefinitely
and the entire amount of energy is released within a fraction of a second. This
type of chain reaction takes place in atom bombs.
In the controlled chain reaction the number of
fission producing neutron is kept constant and is always equal to one. The
reaction is sustained in a controlled manner (Fig). Controlled chain reaction
is taking place in a nuclear reactor.
When a thermal neutron bombards U235 nucleus, it breaks
into two fission fragments and three fast neutrons (Fig). One neutron may
escape and one neutron may be captured by U238 which decays to Np239
and then to Pu239 One neutron is available for carrying out chain
reaction. The chain reaction is possible, only when the loss of neutrons is
less than the neutrons produced.
Critical size
Critical size of a system containing a fissile
material is defined as the minimum size in which atleast one neutron is
available for further fission reaction. The mass of the fissile material at the
critical size is called critical mass. The chain reaction is not possible if
the size is less than the critical size.
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