Nuclear Reactors: Working and schematic

Having understood the basic principle behind deriving energy out of nuclear fission let us now look into the working of a nuclear reactor. Broadly speaking any nuclear reactor will have 2 parts as shown in the figure below. One, that extracts energy out of the fissile nucleus and two, that takes this energy and transfers it into a more usable form.

Reactor Core

This is where nuclear fission reaction occurs. This is where the fuel (Fissile material undergoing fission), moderator and control rod are assembled. This part is normally housed in a concrete building so that the radioactive substances do not escape into the environment.

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Coolant System

This is what carries the energy from reactor core to say a generator to produce electricity. This simply has a heat transfer mechanism using carriers of energy like a gas, water, and liquid metal.

Types of Fission Reactors

Depending on how these systems are designed reactors are classified into number of types. In other words, fission reactors are classified based on

  • What fuel is used (accordingly moderator and control rods)
  • How energy is transferred (coolant system)
  • Speed of neutron

Based on the Fuel

  • Natural Uranium Reactors
  • Natural uranium has limited U-235 (0.7%).
  • To increase the probability of neutron hitting the nucleus, moderators are used.
  • Moderators reflect neutrons and don’t absorb them. Heavy water is used as a moderator.
  • CANDU reactors of Canada use heavy water.
  • Thermal or slow neutrons are readily absorbed by U-238 to transform into Pu-239 which can be used as fuel.
  • The energy output of such reactors is low due to low fissile U-235 quantity.

Enriched Uranium

3% U-235, 97% U-238

Plutonium-based reactors

Pu-239 is the fissile material used. Fast neutrons are good but not good enough (only 65% of the neutrons cause fission rest are absorbed by Pu-239). However, Pu-239 is what is used in fast breeder reactors. Fast means neutrons are not slowed down so no moderator used. Breeder means it produces (breeds) more fuel than consumed.

Based on coolant system

The amount of energy produced during fission is about 200 MeV. The energy that is produced in the core must be now transferred to make it more usable. This is done with the help of another element of the reactor called the coolant. The energy produced in the core can be used in two ways

  • to heat a coolant which boils and delivers this energy to a turbine or
  • it can be used to pressurize the coolant keeping it as liquid which in turn transfers the energy to a secondary liquid which is allowed to boil and run the turbine.

Boiling Water Reactors

In these reactors the energy obtained from nuclear fission is used to heat water to its boiling point in the reactor core itself. This steam is then made to pass through a turbine which uses the energy of steam to turn itself thereby generating electricity. The steam is then cooled and sent back to reactor core where it again does the job of carrying energy to the generator.

Pressurised Water Reactors

In these reactors instead of boiling the water it is pressurised in the reactor core. This increases the boiling point of water. This pressurised water in turn transfers its energy to a secondary coolant (water). The secondary coolant upon boiling  becomes steam which then runs the turbine to generate electricity. The advantage of doing this is we can keep the reactor core small and compact.


  • It must be noted that it is the water that acts as energy carrier in these reactors
  • Both light water and heavy water can be used. But when light water is used it may absorb occasional neutron and become radioactive. This is particularly problematic in boiling water reactors. So as a safety measure the reactor core is contained in a containment vessel made of concrete so as to arrest the escape of radioactive materials to the environment.

Based on Moderator Used

Light water reactors

If you have enrichment facility you can use light water as moderator. Otherwise, light water is not used because it absorbs neutrons decreasing the probability of fission reaction. Since U-235 proportion is more in enriched fuel it compensates for loss of neutrons due to water absorption. It saves effort to make heavy water. Most of the reactors in the world are LWRs. (goes on to show everybody wants uranium enrichment)

Heavy water reactors

Heavy water is used as moderator. You do not need to use enriched Uranium because thermal neutrons are absorbed well by U-235. The advantage of heavy water is that being heavy it does not absorb neutron and thus does not become radioactive.

Breeder Reactor

Plutonium-based breeder reactors

  • Breeder reactors produce more fuel than they consume
  • The core has plutonium-239 or natural uranium as fuel, which upon fission gives on an average 2.7 neutrons.
  • The extra neutrons hit a fertile material U-238 encased outside the reactor core, which turns into plutonium-239, producing more fuel.
  • Breeder reactors do not need moderators or control rods.
  • Fast Breeder Reactors (FBRs) use fast-neutrons and liquid/molten metal to transfer energy.
  • FBRs produce energy in a very compact space, requiring an efficient energy transfer system.
  • Liquid sodium is commonly used as the energy carrier in FBRs.

Thorium-based breeder reactors

This is similar to breeder reactor in principle we have seen above. The difference is instead making Pu-239 these reactors are Uranium-233 producing factories.

The advantage of these reactors over Pu-239 reactors is that the neutrons that are emitted upon fission is not absorbed by U-233 in the core. Thus, more neutrons are available for conversion of Thorium-232 into U-233 outside the core. Besides India has vast reserves of thorium which makes it an ideal choice. Thus thorium-based U-233 reactors are the ultimate component of India’s 3-stage nuclear programme.

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