It is absolutely necessary to control the fission reaction in nuclear power plants.

Explanation

(i) Nuclear power plants work on the principle of controlled chain reaction.
(ii) When slow moving neutrons strike the uranium nucleus, it splits into Barium and Krypton with release of three neutrons and tremendous amount of energy (200 MeV).
(iii) Three neutrons strike on three more uranium nucleus and fission continues creating tremendous energy which is harmful.
(iv) Reaction is controlled by using neutron absorbent material like boron steel and cadmium rods which absorb excess neutrons and heat generated can be used for welfare of mankind.
(v) Hence, it is absolutely necessary to control the fission reaction in nuclear power plants.