Explain the principle and working of a nuclear reactor with the help of a labeled diagram.

Principle : A nuclear reactor works on the principle of achieving controlled chain reaction in natural uranium U²³⁸ enriched with U²³⁵, consequently generating large amounts of heat.
A nuclear reactor consist of
1. Fuel. 
2. Moderator
3. Control rods.
4. Radiation shielding.
5. Coolant.

1) Fuel : In reactor the nuclear fuel is fabricated in the form of thin and long cylindrical rods. These groups of rods treated as a fuel assembly. These rods are surrounded by coolant, which is used to absorb and transfer of heat immediately produced in them. A part of the nuclear reactor which is used to store the nuclear fuel is called the core of the reactor. Natural uranium, enriched uranium, plutonium, and uranium-233 are used as nuclear fuels.
2) Moderator :For slowing down fast neutrons we use heavy water or graphite as moderators.
3. Control rods : These are used for controlling chain reaction by absorbing the neutrons. Cadmium or boron rods are used as control rods.
4. Shielding : During fission reaction beta and gamma rays are emitted which are harmful to human beings and are absorbed by concrete shield, steel or lead sheets.
5. Coolant : The heat generated in fuel elements is removed by using a suitable coolant like water or molten sodium etc.
Working : Uranium fuel rods are placed in the aluminum cylinders. The graphite moderator is placed between fuel cylinders.
When a few U²³⁵ nuclei undergo fission fast neutrons are liberated. These neutrons pass through the surrounding graphite moderator and lose their energy to become thermal neutrons. These thermal neutrons are captured by U²³⁵ to carry out the chain reaction. By proper arrangement of control rods, the fission events are suitably controlled. The heat generated is used to convert water into steam, made to rotate steam turbine and there by driving generators for production of electricity.