Explain the function of moderator, control rods and coolant in a nuclear reactor.

Explanation

Nuclear reactor. Uncontrolled chain reaction causes a lot of damage. If the chain reaction is controlled in such a way that on the average, one neutron from each fission is left to excite further fission, the number of fissions occurring per second thus remains constant. In such cases, energy will not get out of control. Such a system in which the arrangement for absorption of desired number of neutrons is provided so that the chain reaction may proceed in a controlled manner is called nuclear reactor.
Construction. It consists of thick blocks of graphite in which long cylindrical holes have been drilled . Fissionable material is inserted through these holes.
Fuel. It consists of certain mass of fissionable material taken in the form of rod of few centimetres in diameter. It may consist of enriched uranium which is 3.5 % uranium -235 and 96.5% of uranium-238.
Moderator. To slow down the neutrons, they must suffer collision against atoms of material of moderator. Suitable material used as moderator are heavy water and graphite.

Controlling rods. Cadmium rods are used as controlling rods. The rods are inserted in the reactor and the chain reaction is controlled by careful adjustment of their length inside the carbon blocks. The movement of a cadmium rod by a little (say one cm distance) causes a significant difference in the performance of the reactor.
Working. Neutrons produced by the action of `alpha`- particle on polonium or beryllium are slowed down and are used initially to bring about fission of say `U^(235)` nuclei. The emitted neutrons are slowed down by the passage through the moderator to split further `U^(235)` nucleus. The chain reaction is said to be steady so that effective multiplication factor `K_(e)` is 1.
`K_(e)=("Rate of emission of neutrons")/("Rate of loss of neutrons")`
Some lost neutrons are absorbed by `""_(92)U^(238)` present in enriched uranium which gives rise to `""_(94)U^(239)` as shown below:
`""_(0)n^(1)+""_(92)U^(238)rarr ""_(92)U^(239)rarr""_(-1)beta^(0)+""_(93)Np^(239)`
`""_(93)Np^(239) rarr ""_(-1)beta^(0)+""_(94)Pu^(239)`
`""_(94)Pu^(239)` is produced as a bye-product and can be used for fission purpose. India used `""_(94)Pu^(239)` in nuclear device which was exploded on 18th May 1974 and later on May 11 and 13, 1998 at Pokhran.
Uses. A nuclear reactor is used for generation of electricity, preparation of radio-isotopes. Most of energy set free is in the form of heat which through heat exchange is made to furnish steam to operate conventional turbined and electric generators.