Describe the working of nuclear reactor with a block diagram.

Explanation

Nuclear reactor :
Nuclear reactor is a system in which the nuclear fission takes place in a self -sustained controlled manner and the energy produced is used either for research purpose or for power generation.
The main parts of a nuclear reactor are fuel, moderator and control rods. In addition to this, there is a cooling system which is connected with power generation set up.
Fuel :
The fuel is fissionable material usually uranium or plutonium. Naturally occuring uranium contains only `0.7 %` of `""_(92)^(235)"U"` and `99.3%` are only `""_(92)^(235)"U"`. So the `""_(92)^(238)"U"` must be enriched such that it contains at least 2 to 4% of `""_(92)^(235)"U"`.
In addition to this, a neutron source is required to initiate the chain reaction for the first time. A mixture of beryllium with plutonium or polonium is used as the neutron source.During fission of `""_(92)^(235)"U"`, only fast neutrons are emitted but the probability of initiating fission by it in another nucleus is very low.Therefore , slow neutrons. Usually the moderators are chosen in such a way that it must be veruy lightnucleus having mass comparable to that of neutrons.Hene , these light nuclei undergo collision with fats neutrons and the speed of the neutron is reduced.
Most of the reactors use water , heavy water `(D_(2)O)` and graphite as moderators. The blocks or uranium stacked together with blocks of graphite (the moderator) to form a large pile.

Control rods:
The control rods are used to adjust the reaction rate. During each fission, on an average 2.5 neutrons are emitted and in order to have the controlled chain reactions. Only one neutron is allowed to cause another fission and the remaining neutrons are absorbed by the control rods.
Usually cadmium or boron acts as control rod material and these rods are inserted into the uranium blocks. Depending on the insertion depth of control rod into the uranium, the average number of neutrons produced per fission is set to be equal to one or greater than one.
If the average number of neutrons produced per fission is equal to one, then reactor is said to be in critical state. In the fact, all the nuclear reactors are maintained in critical state by suitable adjustment of control rods. If it is greater than one , then reactor is said to be in super - critical and it may explode sooner or may cause massive destruction.
Shielding :
For a protection against harmful radiations, the nuclear reactor is surrounded by a concrete wall of thickness of about 2 to 2.5 m.
Cooling system :
The cooling system removes the heat generated in the reactor core. Ordinary water,heavy water and liquid sodium are used as coolant since they have very high specific heat capacity and have large boiling point under high pressure.
Thsi coolant passes through the fuel block and carries away the heat to the steam generator through heat exchanger. The steam runs the turbines which produces electricity in power reactors.