1. In an uncontrolled reaction, most of the neutrons released from the nuclei go on to hit other nuclei. Many
atoms are split and enormous amount of energy is released. This is what happens in an atomic bomb.
However, to make practical and non-destructive liberation of the energy, we must be able to control the
chain reactions so that our nuclear power station would not turn into an atomic bomb.
There are several types of nuclear reactors in which controlled reactions can take place but they serve
similar purposes and will be discussed in the following section.
Diagram showing the principal components of power station
Nuclear fuels
They are made from uranium oxide [enriched] with extra uranium-235. The fuel is then converted into
small pellets, packed into tubes called pins. The pins are bundled into larger tubes and are ready to be
put into the reactor. One kilogram of this fuel gives as much energy as 55 tons of coal.
Back to diagram
Neutron Moderators
In the reactors, there are always some neutron moderators which slow down the neutrons so that they
have a higher chance of splitting the next nucleus. Materials used for moderators include graphite and
heavy water, containing the isotope deuterium. However, a breeder reactor has no moderator. It depends
on the liquid metal coolant to control the temperature.

2. Back to diagram
Control rods
The core also has neutron-absorbing control rods to control the neutrons released. While the moderator
promotes stronger interaction of the neutrons with the uranium nuclei, the control rods are there to reduce
the concentration of neutrons whenever necessary.
The material used for these control rods is cadmium whose nuclei have a large cross-section for the
capture of neutrons. The cadmium rods are inserted into the nuclear reactor to slow down or shut down
the chain reaction whereas withdrawing the rods restarts or speeds up the reaction.
Back to diagram
Coolants
The temperature of the reactor is very high although the actual temperature depends on the reactor type.
Some reactors operate at temperatures as high as 1500 °C. The coolant flows around the core and
carries heat energy to a steam generator. The coolant materials vary between reactors. They may be
water or carbon dioxide at high pressures. Some reactors such as fast breeders use liquid sodium as
coolant.
Back to diagram
Generation of electricity
The coolant carries the heat energy to a heat exchanger (or steam generator), where it is used to make
steam. The steam then drives the turbine, and the turbine drives the generator that produces electricity.
Waste steam goes to the cooling towers to be [condensed] back into water. Back to diagram
Nuclear Reactor Diagram
Introduction to Nuclear Reactor Diagram
An assembly giving large amount of nuclear energy through fission reaction in a controlled rate
is called a nuclear reactor. It is a system that contains and controls sustained nuclear chain
reactions. The controlled chain reaction and liberation of large amount of energy is the basis of
nuclear reactor. The first fission reaction results in the production of fast moving neutrons. These
neutrons when passed through moderators become thermal neutrons. Controlled fission reaction
can be done by controlling the thermal neutrons used in reaction. Sustained chain reaction with
steady liberation of energy is maintained in a nuclear reactor. Reactors are classified into
different categories based on the fuel used.
Features of Nuclear Reactor Diagram:
Diagram of Nuclear Reactor mainly consists of

3. a) Fuel b) Moderators c) Control rods d) Shielding e) Coolant
Fuel:
The fissionable material used in the reactor is called as fuel. The commonly used fuels are
Uranium, Plutonium or Thorium. It can be U-235, U-238, Pu-236 or Th-232. Uranium is mostly
preferred as it has high melting point.
Moderators:
Only neutrons of a fairly low speed should be used to have controlled chain reaction. To slow
down the speed fast moving neutrons produced during the fission process, moderators are used.
Moderator reduces the speed of the neutron by absorbing its energy but not absorb neutron.
Graphite, Heavy water and Beryllium are common moderators.
Control Rods:
These rods absorb neutrons and stop the chain reaction to proceed further. These are made up of
steel containing a high percentage of material like cadmium or boron which can absorb neutrons.
When control rods are completely inserted into the moderator block then all the neutrons is
absorbed and reaction comes to halt.
Shielding:
Shielding prevents radiations to reach outside the reactor. Lead blocks and concrete enclosure
that is strong enough of several meters thickness are used for shielding.
Coolant:
The coolant is substance in a pipe to the steam generator where water is boiled. This is where
heat-exchange process occurs. Heat is absorbed by the coolant that is produced in the reactor.
Typical coolants are water, carbon dioxide gas or liquid sodium.
Diagram of Nuclear Reactor : Applications

4. Reactors are used for
1) Generating electricity
2) Producing radionuclide for industry and medicine
3) Conducting research tests
4) Military purposes
Nuclear Reactors
A nuclear reactor is a device for obtaining and using the energy from a controlled nuclear chain
reaction. Controlled means the rate or speed of the nuclear fissions can be changed by the
operator. Most reactors are constructed in a similar manner. The six main parts of a nuclear
reactor are fuel, moderator, control rods, coolant, heat exchanger, and safety shields.
The fuel in a nuclear reactor is slightly enriched uranium oxide pellets. It is inserted into long
rods. Nuclear energy is released from the fuel through a chain reaction.
Nuclear Reactor producing heat energy
The moderator is a material that slows the neutrons released during nuclear fission. To keep a
chain reaction going, the speed of the neutrons must be reduced. The neutrons must be moving at
a speed that allows them to be captured by the other nuclei in the fuel. Graphite, a form of
carbon, is one type of moderator. It surrounds the cans containing the fuel. Ordinary water,
heavy water, and beryllium can also be used as moderators. Heavy water contains an isotope of
hydrogen that is heavier than ordinary hydrogen.
Control rods regulate the rate of fission in the fuel. They absorb any excess neutrons not required
to keep the chain reaction going. Control rods are made of boron or cadmium alloyed with

5. aluminium or steel. These materials absorb neutrons. By pushing the rods into the reactor or
pulling them out, the fission rate is controlled. Pushing the rods all the way in stops the fission
reaction.
The coolant removes heat energy from the reactor. Examples of coolant are water, carbon
dioxide, and liquid metals. Liquid sodium is one kind of metal used as a coolant. As coolant is
pumped through spaces in the reactor, it absorbs heat released from fission. The coolant carries
the heat to the heat exchanger.
Safety shields are necessary to protect people from the nuclear radiation produced by a reactor.
Nuclear radiation can produce burns, cancer, loss of hair, vomiting and destruction of blood
cells. An overdose can cause death. Strict safety regulations are enforced to protect workers and
the general public from dangers of radiation. Lead and thick concrete walls are used as shields
for reactors.
A radiation badge is worn by those working with radioactive materials. The film inside blackens
when exposed to gamma rays. The amount of blackening shows the level of radiation to which
the wearer has been exposed.
Nuclear Reactor
A nuclear reactor is an installation where a self-sustaining nuclear fission takes place in a
controlled manner and energy released is used for constructive purposes.
Sub Topics
• Nuclear Fuel
• Moderators
• Coolant

6. Nuclear Fuel
It consists of the fissionable material namely U233 or U235 or Pu239 Generally U235 is piled inside a
sealed aluminum cylinder. Thermal neutrons hit these fuel atoms, fission starts and energy is
released.
Moderators
They slow down the fast moving secondary neutrons. Usually heavy water, graphite, deuterium,
paraffin etc., act as moderators. Control rods have the ability to capture the slow neutrons. Rods
of boron or cadmium are inserted in the holes of the reactor core up to a desirable length for
absorbing the neutrons.
Coolant
Coolants are substances used to remove and transfer the heat produced to the surrounding. It
passes this heat over to water in a heat exchanger.
This results in superheated steam, which drives a turbine coupled to an electric generator. The whole
reactor is protected with concrete walls so that radiations may not produce harmful effects to its
surroundings.