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Nuclear Reactors

The general chemical reactions takes place by the interaction between valence electrons around nucleus of atom. While the nuclear reactions occur by alter the particles of atom and in atom also. Some nuclear reaction takes place naturally while some are stimulated artificially like nuclear fission and nuclear fission.

Nuclear energy generated from the conversion of mass to energy in various nuclear reaction. Nuclear energy can produce in different ways either by splitting of large nuclei to release energy or by combining small nuclei for generating energy.


Nuclear Power Reactors

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The basic principle of nuclear power reactors are to produce electricity. The nuclear power reactor involves the use of released energy as heat to make steam for generating electricity. The main common components of nuclear power reactors are described as below.

1. Fuel

Generally Uranium-235 is used as the basic fuel. The uranium oxide (UO2) pallets are arranged in tubes which work as fuel rods. The rods are further arranged into fuel assemblies in the reactor core.

2. Moderator

This is core material which uses to slow down the speed of released neutrons that can be undergo other fission. Usually water or heavy water or graphite is used for this purpose.

3. Control Rods

They work as a controller to control the rate of nuclear reaction to avoid the nuclear explosion. These rods are made with neutron-absorbing material like cadmium, hafnium or boron etc. They can be inserted or withdrawn from the core according to the requirement for control or speed up the rate of reaction respectively.

4. Coolant

Generally water is used as coolant. This is a liquid or gas that circulates from the core to
transfer the heat from it.

5. Pressure Vessel or Tubes

This is a robust steel vessel that contains the reactor core and moderator or coolant. If the tubes are used then these are series of tubes that holds the fuel.

6. Steam Generator

The steam generator is the part of the cooling system. In the steam generator, the heat from the reactor comes from the primary coolant and this heat is used to make steam for the

7. Containment

The containment works to protect the reactor form outside intrusion and the effects of radiation. This is a meter thick concrete and steel structure around the reactor core.

How Nuclear Reactors Work?

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In the nuclear fission power plant, the heat generates by a nuclear fission process is used to turns a steam turbine which produce electric power. The physical process is common for all power plant but they differ only the way by which the nuclear reaction is controlled.


  1. When neutron is absorbed by the atom of nuclear fuel uranium, the fission reaction takes place which cause the splitting of uranium into two smaller atoms waste.
  2. In the nuclear reactor, the energy produced by enriched uranium must be control and allows it to heat water into steam.
  3. The uranium pellets are arranged as long rods which are collected together to form bundles.
  4. The bundles are put in water inside a pressure vessel.
  5. The water is used as coolant.
  6. The control rods are used to prevent the overheating.
  7. The material of control rods absorbs neutrons.
  8. Thus the rate of nuclear reaction can be controlled by them.
  9. If more energy is needed then the rods are lifted out from the bundle to absorb few electrons and for reducing heat, control rods are lowered into the uranium bundle.
  10. The heat of uranium bundle turns the water in to steam by heating it.
  11. This produced steam drives a turbine and thus generator spins and produces the electric energy.
Nuclear Reactor

Some important factors should be take care during process like control the reaction, safety, addition of nuclear fuel (refueling), production of waste, efficiency of generating heat etc.

Fast Reactors

In the fission of uranium, high energy neutrons are emitted. In the case of fast reactor, these high energy neutrons are directly absorbed by the other uranium atom. A problem is that the isotope Uranium-238 has high probability to absorb the fast moving neutrons than Uranium-235. So a much larger fraction of U-235 is used as fuel in fast reactors.

Slow Reactors

They are also called thermal reactor. In these reactors, the speed of produced neutrons is in control so that neutrons are absorbed by the correct isotope of uranium-235.

List of Nuclear Reactors

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The various type of nuclear reactors are listed below.

Type of reactor Fuel
Capacity in thousands of megawatts
Pressurized water reactor (PWR) Enriched uranium oxide UO2 251.6
Boiling water reactor (BWR)
Enriched uranium oxide UO2 86.4
Advanced gas-cooled reactor
Natural uranium or enriched uranium oxide 10.8
Pressurized heavy water reactor (PHWR)
Natural uranium oxide UO2
Light water graphite reactor (RBMK)
Enriched uranium oxide UO2 12.3
Fast neutron reactor (FBR)
PuO2 and UO2 1.0

Types of Nuclear Reactors

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There are various types of nuclear reactors which are given as below.

  1. Pressurized Water Reactor
  2. Boiling Water Reactor
  3. Pressurized Heavy Water Reactor
  4. Advanced Gas-cooled Reactor
  5. Light Water Graphite Reactor
  6. Fast Neutron Reactor

1. Pressurized Water Reactor

This is one of the most common types of power generation reactor in which ordinary water is used as coolant and moderator. It is designed as a submarine power plant. It has a primary cooling circuit and a secondary circuit.

The primary circuit flows around the reactor core under very high pressure. The steam is generated in a secondary circuit to drive the turbine. These are also known as WER reactors in Russia that means water-moderated and cooled.

Nuclear Reactors Structure
In PWR, the fuel assemblies (approx 200-300 rods each) are vertically arranged in the core. In the case of a large reactor, approximately 150 to 250 fuel assemblies are used which contain 80 to 100 tons of uranium as reactor fuel. The temperature of water of the reactor core can be hike up to 325°C so it is controlled by keeping it under atmospheric pressure (about 150 times) to prevent it boiling. This pressure is maintained in pressurize by steam.

In the primary cooling circuit, the water works as both the coolant and the moderator. So if the water turns to steam then it would slow down the fission reaction. Due to this limitation, boron is added to the primary circuit when the secondary system shutdown. While the secondary circuit works under less pressure and the water generate steam by boiling in the heat exchangers. The steam turns the turbine to generate electricity. After this processing, the condensed steam returns to the heat exchangers which attach with the primary circuit.

2. Boiling Water Reactor

  1. The boiling water reactors are very similar to PWR but there is only one difference is that only single circuit is used in BWR.
  2. The water in single circuit is at very low pressure than in PWR.
  3. It is approx 75 times atmospheric pressure.
  4. So the water generates steam by boiling in the core at about 285°C.
  5. The design of reactor makes it more efficient than PWR.
  6. They can operate with 12-15% of the water in the top part of the core as steam which makes them more efficient and less moderating. The steam passes from drier plates of steam separators situated above the core.
  7. Then the steam passes through the turbines which are part of the reactor circuit.
  8. The turbine must have radiological protection and shielded because the water gets always contaminated with radio nuclides traces.
  9. Due to its simple design and short lived water radioactivity, the turbine hall can be entered just after the reactor is shut down.
  10. The fuel assembly of BWR contains approx 90 to 100 fuel rods.
  11. So there are up to 750 assemblies in a reactor core which have 140 tons of uranium.
  12. The secondary control system restricts the flow of water through the core to reduce moderation.
Boiling Water Reactor

3. Pressurized Heavy Water Reactor

  1. It is also known as PHWR or CANDU. It developed in Canada. As natural uranium oxide is used as fuel which contains 0.7% U-235, so it requires a more efficient moderator.
  2. Thus the heavy water (D2O) is used as moderator.
  3. The moderator presents in a large tank.
  4. This large tank is called a calandria which contains several hundred horizontal pressure tubes.
  5. These pressure tubes form channels for the fuel.
  6. The cooling process is done by flow of heavy water under high pressure in the primary cooling circuit.
  7. The temperature reaches up to 290°C. Thus the primary coolant produces steam in a secondary circuit to turn the turbines like in PWR.
  8. The reactor can be refueled easily by isolating individual pressure tubes from the cooling circuit without shutdown the reactor.
  9. In the CANDU, the fuel assembly contains a bundle of long fuel rods about 37 half meter.
  10. These are made of ceramic fuel pellets in zircaloy tubes.
  11. There are also 12 bundles lying end in a fuel channel as support channel.
  12. The control rods penetrate the calandria in vertical direction and a secondary shutdown system involves the addition of gadolinium in the moderator.
  13. The heavy water moderator which circulates from the body of the calandria vessel also generates some amount of heat.
Newly designed PHWR are like the Advanced Candu Reactor contains light water cooling and enriched fuel.

Pressurized Heavy Water Reactor
CANDU reactors can be processes with recycled uranium from reprocessing fuel of Light Water Reactor and depleted uranium of enrichment plants.

4. Advanced Gas-Cooled Reactor

  1. These are advanced British gas-cooled reactors. The graphite and carbon dioxide is used as moderator and coolant respectively.
  2. Uranium Oxide pellets are used as fuel present in stainless steel tubes which enriched up to 2.5 to 3.5%.
  3. The carbon dioxide passes around the core and then through the steam generator tubes.
  4. The temperature reaches up to 650°C.
  5. The moderator is penetrated from control rods and due to secondary shutdown system nitrogen is injected to the coolant.
  6. The AGR was generated from the Magnox reactor in which graphite and CO2 are used as moderator and coolant respectively, natural uranium as fuel and water as secondary coolant. These types of reactors are still operated in UK.
Advanced Gas Cooled Reactor

5. Light water graphite-moderated reactor

  1. These are also known as RBMK.
  2. They are developed from plutonium production reactors.
  3. They are designed with vertical pressure tubes approx 7 meter long, graphite as moderator, and water as coolant.
  4. Water is boiled in the core at temperature 290°C as in a BWR.
  5. Low-enriched uranium oxide is used as fuel. The fuel assemblies are approx 3.5 meters long.
  6. The fixed graphite and excess boiling is the cause of moderation as it reduces the cooling and absorption of neutron occurs frequently.
  7. So they are less used and particularly at outside area.

6. Fast neutron reactors (FNR)

These types of reactors do not contain moderator and fast neutrons are used to generate power. Plutonium or Uranium-238 is used as fuel which produces 60 times more energy than as the original uranium in the normal reactors but they are commercially not viable as they are expensive reactors.

More topics in Nuclear Reactors
Nuclear Fuel Cycle
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