BS Physics (Foundation) Series

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Module # 49
Nuclear Forces & Nuclear Reactors
Atomic Energy
Physicists have provided the modern man with a huge source of
energy called atomic or nuclear energy which is being used for
constructive as well as destructive purposes. For example;
nuclear reactors, nuclear power plants, atomic submarines,
atomic missiles, atomic and hydrogen bombs etc.
Nuclear Energy
The most concentrated form of energy that is available to man is
stored in the nuclei. This energy can be released (or obtained) in
the process of fission (the breaking apart of heavy nuclei) and
fusion (the fusing together of light nuclei).
Fission reactors have been producing electricity all over the world
in commercial quantities for about 55 years. In Pakistan, we have
a nuclear power reactor located at Karachi which generates about
137 MW of power. We have mostly exploited our hydro-electric
sources, while, the production from thermal power plants is also
considerable. To meet our future domestic and industrial needs,
we will have to generate electricity primarily from nuclear power
plants using uranium and plutonium in fission reactions. Pakistan

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has signed different agreements with countries like France, China
etc. under which Pakistan will get more nuclear power reactors
from such countries. So far, the scientists have not succeeded in
developing the fusion reactor. When it happens, heavy hydrogen
(deuterium) will become the principal fuel.
Atomic Bomb
An atomic bomb is a war weapon and its assembly is based on
the principle that if a fission chain reaction is uncontrolled, then,
the energy released will be enormous. Sub-critical masses of
uranium (small quantities of uranium which by themselves cannot
sustain chain reactions) are placed at the two ends of a hollow
tube such that the two masses can be brought suddenly together
by igniting an explosive material. When the two masses combine,
they give rise to a critical mass and uncontrollable fission chain
reaction starts and the bomb explodes.
In an atomic bomb explosion, the heat energy produced by it can
destroy a small city. Explosion of an atomic bomb also produces
shock waves which move with very high speed and while traveling
several miles destroy the buildings which they come across. A
nuclear explosion also releases large quantities of radiations.
These radiations can be very harmful to living beings. The effect
of these radiations remains in the atmosphere for several weeks.

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Hydrogen Bomb
A hydrogen bomb is also a war weapon. The heat energy
released in the explosion of a hydrogen bomb is far greater than
that released from the explosion of an atom bomb. The principle
of the hydrogen bomb assembly is based on the fusion process.
The hydrogen bomb needs a tremendous amount of heat for its
detonation. That is why; a hydrogen bomb is also called a
thermonuclear bomb.
The required heat is obtained by a fission bomb. The first
hydrogen bomb was exploded on an experimental basis in 1952.
This yielded an enormous amount of energy. This was found to
be equivalent to that obtained from 1 million tons of TNT (Tri-
Nitro- Tolune).
Strong Nuclear Force
It is a nuclear force that acts between particles inside the nucleus.
The distance between these particles (elementary particles) is too
small, of the order of 10-15
m. Therefore, strong force acts only
over a very short range. The attraction between the proton and
the neutron inside the nucleus is very strong. Due to this strong
attractive force, the electrostatic repulsive force between protons
inside the nucleus is too small.

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Weak Nuclear Force
During radioactive decay, the emission of -particle is
accompanied with a neutrino. The force between the -particle
and neutrino is weak and too small but it is more than the
gravitational force.
Therefore, we can say as a conclusion that the nuclear force is
the strongest in nature, while, the gravitational force is the
weakest.
Nuclear Reactors
A nuclear fission chain reaction releases a large amount of
energy. This energy is in the form of heat. If the chain reaction is
controlled, then, we can get a steady outflow of heat. This heat
can be used to run a turbine for the generation of electricity. A
system used to obtain a controlled amount of heat from nuclear
fission is called a nuclear reactor.
Nuclear reactors are also used to produce plutonium which is
more fissionable as compared to 92U235.

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Construction and Working of Nuclear Reactor
Fig: Details of a Reactor
The above figure is a schematic diagram of a nuclear reactor. The
fission material in the reactor is uranium 92U235
. This is called the
fuel element. The neutrons released from fission move with high
velocities. These fast moving neutrons are usually lost or
absorbed somewhere in the reactor assembly before producing
further fission. The fast moving neutrons have to be slowed down
before they are able to cause further fission. The process of
slowing down neutrons is called moderation or thermalization of
neutrons. Heavy water is commonly used as a moderator.
When a chain reaction starts, it may produce large number of
neutrons which can cause too much fission. There will be danger
that the huge amount of energy released might melt down the

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nuclear reactor. The rate of a chain reaction is controlled by
inserting some substances that can absorb neutrons. They are
called control-rods. Boron rods are used as control rods. If too
many of the neutrons are absorbed by the control rods, then, the
chain reaction will stop. In order to start the chain reaction, the
control rods are moved out.
Generation of Electricity
Electricity can be generated from the heat energy released in the
nuclear reactor. The following diagram clearly illustrates the
method used for this purpose.
Fig: Generation of Electricity from a Reactor
The heat energy produced due to fission in the nuclear reactor is
carried away by means of the circulating carbon dioxide gas or
pressurized water present around the core of the reactor. This hot
fluid is used to boil water thus producing steam. This steam is in
turn used to drive the turbine of the electric generator for the

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production of electricity. This is how the nuclear fission energy is
utilized for the production of electrical energy.
First Nuclear Reactor
An eminent scientist Fermi proposed the production of controlled
chain reaction in uranium. The first nuclear reactor operated
successfully in Chicago University on Dec. 2, 1942, under the
direction of Fermi.
Slowing Down of Neutrons
Slowing down of fast moving neutrons in a nuclear reactor is a
practical example of elastic collision. A large number of fast
moving neutrons are produced in a nuclear reactor. In order to
slow down these fast moving neutrons, the nuclear fuel is
surrounded by water. Water consists, in part, of protons whose
mass is almost equal to that of neutrons. Thus, when the fast
moving neutrons strike the protons, then, the neutrons practically
come to rest while the protons start moving with speeds equal to
that of the incident neutrons. Very fast moving protons are
stopped over a very short distance by applying opposite electric
field, whereas, neutrons can travel a very large distance before
stopping. There are many reasons for slowing down the fast
neutrons. First, we do not want the neutrons to leave the nuclear
fuel, because, these neutrons are needed to be absorbed by the

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nuclear fuel to produce fission and hence energy. Second, if
these neutrons escape the nuclear reactor, then, they are
dangerous to the workers working around the plant.