Quick Revision ( Article:3, Science): Type Of Nuclear Reactors






Fuel: Usually pellets of uranium oxide (UO2) arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core


  • Moderator: This is material which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.


  • Control rods: These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. (Secondary shutdown systems involve adding other neutron absorbers, usually as a fluid, to the system.)


  • Coolant: A liquid or gas circulating through the core so as to transfer the heat from it. In light water reactors the moderator functions also as coolant.


  • Pressure vessel or pressure tubes: Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the moderator.


  • Steam generator: Part of the cooling system where the heat from the reactor is used to make steam for the turbine.


  • Containment system: The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any malfunction inside. It is typically a metre-thick concrete and steel structure.




Most of the nuclear reactors work on the principle of nuclear fission. The energy released in the reaction is used to turn water into steam. The steam is used to run the turbine.

Diff types of nuclear reactor:

Type of reactorCharacteristics
Light water reactor1.   Boiling water reactor

·        Reactor core heats water which turns to steam and then drives a steam turbine.

2.   Pressurised water reactor:

·        Reactor core heats water which does not boil. Pressurised water can reach upto higher temperature. This water energy is transferred to secondary low pressure water system.

Heavy water reactorHeavy water is used as a moderator. Heavy water does not readily absorb the neutrons as readily as water. The use of heavy water increases the probability of fission in U-235. It increases the probability of sustained chain reaction.
Gas cooled reactorsHigh thermal efficiency and can operate at very high temperature.
Fast reactorsThey can operate on fast neutrons. The  need of moderator is not there. But to sustain the chain reaction, fission material needs to be highly enriched . High amount of fuel is also required and this is a costly affair. They use liquid sodium as coolant.
Breeder reactorA breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile fuel from fertile material like U-238 or thorium-232.


For every 1kg of fuel used in the rod, 1.1 kg comes out of the reaction.

Thorium ReactorsThorium 232 is a fertile material.


Th-232 converts into Th-233. Th-233 converts into Protactinium-233. Pa-233 converts into U-233. U-233 releases energy via nuclear fission.



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