Question

Cadmium rods are used in a nuclear reactor
(A). To produce neutrons.
(B). To speed up neutrons.
(C). To absorb neutrons.
(D). None of these.

Answer 1

Hint: In a fission reaction, the number of neutrons produced is more than the neutrons required to carry on controlled chain nuclear reactions. A controlled chain nuclear reaction is the basis of a nuclear reactor, otherwise, it will end up into an atomic bomb. Considering a reaction of the fuel Uranium, the average neutrons produced in a single reaction is a bit above 2.5. While for a controlled chain reaction, the neutrons produced should be equal to the neutrons used up for the first cycle of reaction. So we have to reduce the number of neutrons to make it a controllable chain reaction.

Complete step by step answer:
The initial neutron to start the fission reaction is obtained from Californium and the chain begins. After the activation of the fuel, the number of neutrons that are produced has to be regulated to avoid the explosion. The fast neutrons should be absorbed by a material of larger neutron capture cross-section. The element Cadmium has a higher neutron absorption cross-sectional area than nuclear fuels like Uranium or Thorium. Therefore Cadmium rods are inserted into the nuclear reactor chamber which absorbs some high energy neutrons, leaving required neutrons for further reaction. The correct option is hence C.

Additional information:
The Cadmium rods are made of a specific length required for each reactor chamber. The complete length of the rod is not inserted, the number of fast neutrons decides the length required inside the chamber. Heavy water is used in a nuclear reactor as a coolant and as a moderator. The generation factor or the k-factor value is the number of neutrons produced in a generation to the number of neutrons in the preceding generation of neutrons. The k-factor is maintained around 1, with the help of cadmium rods, in a nuclear reactor.

Note: The parts of a reactor and its uses have to be learnt thoroughly. Cadmium element is specifically used as a moderator because of its high cross-section and resistance to the deformation. It has a high affinity to absorb neutrons. Cadmium rods are never used in an uncontrolled chain reaction. Since the fast neutrons are not much effective for energy production in the nuclear reactor. That’s why we are preferring to make the fast-moving neutrons to the slower ones.