From the fission of one uranium nucleus by a single neutron, on an average, 2 to 3 neutrons are released which induce fission in more uranium nuclei. This sets up a self-sustaining chain of reaction called nuclear chain reaction. Following precautions have to be taken in carrying out such a reaction.
( 1 ) The neutrons released during the fission are quite fast having kinetic energy of about 2 MeV. They have to be slowed down to the level of thermal neutrons ( kinetic energy 0.04 MeV ) so that they do not escape the fission material and induce further fission. To slow down the neutrons, reflectors and moderators like heavy water ( D2O ), carbon in the form of graphite, Beryllium and ordinary water are used in nuclear reactors. Moreover, the core region of the reactor is kept large to prevent the neutrons leakage
from the surface.
( 2 ) Large amount of energy released in nuclear reaction can raise the temperature to about 105 K. Hence to cool the fission material and moderators, coolants like suitable gases, water, liquid sodium, etc. are used.
( 3 ) The ratio of number of neutrons produced to the number of neutrons incident at any stage of the chain reaction is called the multiplication factor ( k ). For controlled chain reaction, this ratio should be kept nearly one. With higher value of k, chain reaction can go out of control and with lower value of k, it may tend to stop. For this, some controlling rods of neutron absorbing materials like Boron and Cadmium are inserted in the fission material with automatic control device. Rods move further inside the fission material if k increases beyond one and come out if k reduces below one.
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