2. A nuclear reactor is similar to the furnace of a steam power plant or
combustion chamber of a gas turbine plant.
Main Components of a Nuclear Reactor
1. Reactor core
2. Moderator
3. Control Rods
4. Reflector
5. Cooling System
6. Reactor Vessel
7. Biological Shielding
What is nuclear reactor ?
4. Reactor Core
It consists of
1. Nuclear Fuel
2. Neutron Moderator
3. Space for Coolant
The nuclear fuel is an element whose nucleus undergoes nuclear fission
by nuclear bombardment and it produes chain reation.
Nuclear fuel may be U233,U235 and Pu239
A reactor core generally has a approximately right circular cylinder shape.
It’s diameter ranges from 0.5m to 15m.
These plates are usually coated with a thin sheet of stainless steel,
Zirconium or Aluminium to provide corrosion resistance.
5. Moderator
The process of slowing down the neutrons from high velocity
without capturing them is known as moderation.
Moderator is a material which is used to slow down the
neutrons from high velocity without capturing them.
The fast moving neutrons are far less effective.
They try to escape from the reactor.
Thus, the speed of fast moving neutron is reduced by
introducing moderators.
Heavy water (D2O), Water (H2O), Helium (He) etc,. are
commonly used moderators.
6. A good moderator should possess the following properties
It is lighter in weight.
It has high resistance to corrosion.
It has high thermal conductivity.
It has high melting point for solids and low melting point for
liquids.
It has good stability under heat and radiation.
It is abundantly available in pure form.
7. Control Rods
The functions of control rod are as follows.
To control the rate of fission.
To start the nuclear chain reaction when the reactor is
started from cold.
To shut down the reactor under emergency conditions.
To maintain the chain reaction at a steady state.
To prevent melting of fuel rods.
8. Boron, Cadmium and Hafnium are mostly used as control rods.
These control rods are used to absorb neutrons there by reducing the
chain reaction.
The control rods must be able to absorb excess neutrons.
The position of these rods is regulated by an electronic or electro
mechanical device.
Control rods should possess the following properties.
It should have good stability under heat and radiation.
It should have adequate heat transfer properties.
It should have better corriosion resistance.
9. Reflector
Reflector material is placed around the core to reflect some of the
neutrons back that they leak out from the surface ofthe core.
The reflected neutrons cause more fission and they improve the
neutrons economy of the reactor.
The reflector is generally made of same material as moderators.
Water, Carbon, Graphite, beryllium are generally used as
reflectors.
10. Cooling System
The coolants are used to carry away heat produced inside the
reactor to the heat exchanger.
From the heat exchanger, the heat is transferred to another
working medium for further utilization of power generation.
Various coolants used are given below.
Light or Heavy Water
Gases such as Helium, Hydrogen, Air or CO2 etc,.
Liquid metals such as Sodium and Organic liquids.
11. The desirable properties of a reactor coolant are as follows.
It should have low melting point.
It should have high boiling point.
It should be non-corrosiveness.
It should be non-toxicity.
It should have high chemical and radiation stability.
It should have high density.
12. Reactor Vessel
The reactor vessel encloses
The reactor core
Moderator
Reflector
Shield
Control rods.
It is a string walled container to withstand high pressure.
At the top of the vessel, holes are provided to insert control rods.
At the bottom of the vessel, the reactor core is placed.
13. Biological Shielding
Shielding is necessary to protect the walls of the reactor vessel
from radiation damage.
It protects the operating personnel from exposure to radiation.
Thick layers of lead, concrete or steel are provided all around the
reactor.
These layers absorb the gamma rays, neutrons etc,.
14. A good shielding material should have the following properties.
It should absorb α, β and ϒ radiations efficiently.
It should have uniform density.
It should not be decomposed by radiation.
It should be fire resistant.
15. Factors controlling the selection of a particular type of a reactor
The following factors control the selection of a particular type of a reactor.
1. Neutrons ennergy
2. Type of fuel
3. Type of coolant
4. Type of moderators
5. Construction of core
16. Classification of Nuclear Reactors
The nuclear reactors are classified according to the
following characteristics.
i. According to the neutrons energy.
ii. According to fuel used.
iii. According to the type of coolant used.
iv. According to the type of moderators used.
v. According to the construction of core.
17. According to the neutrons energy
a. Fast reactors in which fast fission is caused by high-energy neutrons.
b. Intermediate or epithermal.
c. Low energy or thermal reactors in which fission is due to slow moving
neutrons.
According to fuel used
a. Natural fuel reactor in which natural Uranium is used as fuel.
b. Enriched Uranium reactor in which Uranium used in this reactor
contains 5% to 10% of U235
18. According to the type of coolant used
a. Water cooled reactors in which ordinary or heavy water is used as
coolants.
b. Gas cooled reactors in which CO2, He, N2 air etc,. are used as
coolants.
c. Liquid metal cooled reactors in which liquid metals such as sodium,
bismuth and lead are used as coolants.
According to the type of moderators used
a. Graphite moderator reactor.
b. Beryllium moderator reactor.
c. Water moderator reactor.
19. According to the construction of core
a. Cubical core reactor.
b. Cylindircal core reactor.
c. Spherical core reactor.
d. Annulus core reactor.
e. Slab core reactor.
20. Boiling water reactor Pressurised water reactor
Advantages:
Overall thermal efficiency is high.
Water is cheap.
Thermal efficiency of BWR plant is
more (30%) than a PWR plant (20%)
Disadvantages:
Reactor must be shut down to unload
and reload core.
The condenser leak may cause a
serious trouble.
Advantages:
Number of control required is less.
Easily available natural uranium is
used as a fuel.
Steam is not contaminated by
radiation.
Disadvantages:
Fuel element fabrication is
expensive.
Reprocessing of fuel is difficult since
it is affected by radiation.
