Nuclear Reaction
It is reaction in which nucleus of a stable nucleus gets transformed into a nucleus of another element. It is mostly facilitated by bombarding the nucleus by high energy particles.
There are two types of Nuclear Reactions
Nuclear Fission
In nuclear fission, a heavy nucleus when bombarded splits into two smaller nuclei of nearly equal masses.
Nuclear Fusion
In nuclear fusion, two light nuclei combine at very high temperature to form a single heavy nucleus.
Nuclear Chain Reaction
During a nuclear fission reaction, besides the other fission products, this reaction also produces the same particle which are responsible for initiating the reaction. These particles are called neutrons. When a single U(235, 92) nucleus captures a neutron, its fission produces three neutrons. These neutrons facilitates nuclear fission with other atoms of uranium and this reaction keep on happening unless the entire fuel is consumed. This is why this reaction is called nuclear chain reaction.
Uncontrolled Chain Reaction
If the above chain reaction is not controlled them the reaction will accelerate with such a rapid rate that the whole material will explode within a microsecond, liberating a huge amount of energy. Such a reaction is called uncontrolled chain reaction. The working of nuclear bomb is based on uncontrolled chain reaction.
Controlled Chain Reaction
The above chain reaction can be controlled and maintained at a constant rate by absorbing excess of neutrons at each step of the reaction, so that only a single electron should be left for the nuclear fission. Here the energy released does not get out of control.
The working of nuclear reactors is based on controlled chain reaction.
Nuclear Reactor
It is a device in which nuclear chain reaction is initiated, maintained and controlled. It works on the principle of controlled chain reaction and provide energy at a constant rate.
Main Parts of Nuclear Reactor :
1) Nuclear Fuel :
It is the material which can be fissioned by slow moving neutrons. The isotopes like U-235, Th-232 and Pu-239 can be used as the reactor fuel. A certain mass of the fuel is taken in the form of rods, tightly sealed in aluminum containers. The rods are separated by moderators and are placed in the core of the reactor.
2) Moderator
Any substance which is used to slow down fast moving neutrons in a nuclear reactor to thermal energies is called a moderator. The commonly used moderators are Heavy Water (D2O), graphite and deuterium, paraffin etc. can act as a moderator. These moderators are rich in protons.
When fast moving neutrons collided head on with the protons of moderator substances, their energies are interchanged and thus the neutrons are slowed down. Such neutrons are called thermal neutrons which causes fission.
3) Control Rods
These rods are used to start, stop or control the chain reaction. These rods are made up of neutron absorbing material like cadmium or boron. They are inserted into the core of the reactor. The rate of neutron production is controlled by adjusting the depth of the control rods.
4) Coolant
A substance which is used to remove the heat produced and transfer it from the core of nuclear reactor to the surrounding is called coolant. The coolant must have high boiling point and high specific heat. Heavy water and liquid sodium are example of good coolants.
Generally, coolant transfer heat to the water in a heat exchanger. As a result of it, steam is produced which drives a turbine which is connected with an electric generator.
5) Shielding
The entire reactor is protected with 2 to 2.5 metre thick concrete walls, so that radiations coming out from the nuclear reaction may not affect the humans working around the nuclear reactor.
