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Question 8: What is a nuclear reactor? Give the principle, construction, working and uses of a typical nuclear reactor.


Nuclear Reactor

Nuclear reactor is a device in which controlled fission chain reaction is utilized to produce energy, supply of neutrons and radioisotopes.

Working principle

Principle of nuclear reactor is based upon the controlled fission chain reaction. The controlled fission enables us to maintain energy generation at a constant level.


There are many types of nuclear reactors; however, generally it consists of five parts.

  1. Core of nuclear fuel: It is fissionable substance; usually enriched uranium or plutonium.
  2. Moderator: Moderator is used for slowing down the neutrons.
  3. Control rods: Control rods are Cadmium rods which absorb neutrons.
  4. Coolant: Coolant or heat exchanger is water or some other suitable substance for cooling the fuel rods and moderator.
  5. Shield: This is used for protective measures.


When the neutron is allowed to react with the Uranium nucleus, a fission reaction starts. Energy and more neutrons are released in the process. The neutrons thus released react with other uranium nuclei and a self-sustaining chain reaction is started with the emission of neutrons and a huge amount of energy.

However, the fast moving neutrons released with the reaction cannot be captured by other uranium nuclei, therefore, they must be thremalized (slowed down). For this purpose, the moderator is used. The neutrons when released in the reaction with uranium nuclei are moving randomly and strike with the nuclei of the moderator. Their speeds are slowed down in these collisions and then, when they find the chance to hit a uranium nucleus they are capable of doing the reaction.

As every interaction of neutron with the uranium yields three neutrons, therefore, the number of neutrons increases with a high rate. This increases the number of reactions and hence a huge amount of energy. There is a chance that the heat (energy) liberated in the process may damage the whole system. Therefore, the control rods are inserted in the core of fuel. The neutrons produced during the process are absorbed by the Cadmium rods and the speed of the reaction is slowed down. The temperature also falls down and the core and the whole system are saved. When the reaction needs to be expedited the control rods are drawn out and the number of neutron is increased again which expedites the reaction. 

Coolant has two benefits. It cools the fuel rods and moderator. If they are not cooled, the heat generated can melt or damage them. Second, they carry away the large amount of heat generated in the process to the water provided in the system to convert it to steam. The steam is then directed towards an electric generator. The electricity thus produced is distributed for useful purposes.

The last part of the nuclear reactor is shield, usually a concrete wall few feet in thickness. In the nuclear reaction radiations are produced, which are harmful for life. The concrete wall absorbs these radiations and secure the health of the workers there. of nuclear reactor

Types of nuclear reactors

(1) Thermal Reactors: These reactors use thermal neutrons for fission. Natural or slightly enriched uranium are used in thermal reactors. Pressurized Water Reactor (PWR) is a popular type of thermal reactors. Water is put under high pressure so that it does not boil and the temperature increases above 1000C. This water is used to boil the steam producing water circuit.

(2) Fast Reactor: In fast reactors, 92U238 is used which is 99% of the natural uranium. This isotope of uranium is when fissioned, produces Plutonium. Plutonium can be fissioned with fast neutrons, hence moderator is not used in fast reactors.

Uses of nuclear reactor

  • Nuclear reactors are used for the production of electricity.
  • They are used for the production of radioisotopes used in the scientific work.
  • They are used to produce plutonium used in nuclear weapons.
  • They are used in research work.
  • Nuclear reactors are used to produce atomic energy.

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