NUCLEAR
POWER STATION
nuclear fission
When atoms
are bombarded with neutrons, their nuclei splits into 2 parts which are roughly
equal in size. Nuclear fission in the process whereby a nucleus, with a high
mass number, splits into 2 nuclei which have roughly equal smaller mass
numbers. During nuclear fission, neutrons are released.
Ø There are 2 types of fission that exist:
Ø Spontaneous Fission
Ø Some radioisotopes contain nuclei which are
highly unstable and decay spontaneously by splitting into 2 smaller nuclei.
Ø Such spontaneous decays are accompanied by the
release of neutrons.
Ø Induced Fission
Ø Nuclear fission can be induced by bombarding
atoms with neutrons.
Ø The nuclei
of the atoms then split into 2 equal parts
Ø Induced
fission decays are also accompanied by the release of neutrons.
Ø The
Fission Process
Ø A neutron travels at high speed towards a
uranium-235 nucleus.
Ø The
neutron strikes the nucleus which then captures the neutron.
Ø The
nucleus changes from being uranium-235 to uranium-236 as it has captured a
neutron.
Ø The
uranium-236 nucleus formed is very unstable. It transforms into an elongated
shape for a short time.
Ø It then
splits into 2 fission fragments and releases neutrons.
Ø Energy
from Fission :
Ø Both the
fission fragments and neutrons travel at high speed. The kinetic energy of the
products of fission are far greater than that of the bombarding neutron and
target atom.
Ø Energy is
being released as a result of the fission reaction.
Ø The energy released can be calculated using
the equation E = mc2
Where E =
energy released (J)
m = mass difference (kg)
c = speed of light in a vacuum (3 x
108 ms-1)
Nuclear
fusion:-
Ø In nuclear
fusion, two nuclei with low mass numbers combine to produce a single nucleus
with a higher mass number.
Classification of Nuclear Reactors:-
(1) On The Basis of Neutron of
Energy
(i) Thermal Reactor (ii) Fast Reactors (iii) Intermediate Reactor
(2) On The Basis of Type of
Coolant Used
(i)
Gas (ii)
Light Water (iii) Heavy Water
(iv) Liquid Metal (v) Hydro Carbon
(3) On The Basis of Moderators
Used
(i) Light Water (ii) Heavy Water (iii) Graphite
(4) On The Basis of Fuel &
Moderator Used
(i) Homogeneous (ii) Heterogeneous
(5) On The Basis of Application
(i) Research Reactors (ii) Pu- production Reactors
(iii) Power Reactors (iv) Space Reactors
Types of Nuclear Reactors:-
(1) Pressurized Water Reactor (PWR)
(2) Boiling Water Reactor (BWR)
(3) Fast Breeder Reactor (FBR)
(4) Sodium-Graphite Reactor
(5) Homogeneous Reactor
(6) Gas Cooled Reactor
(7) High Temperature Gas Cooled Reactor
Pressurized Water Reactor:-
Ø PWR is a thermal reactor. It uses uranium
oxide as fuel. Light or heavy water is used as moderator as well as reflectors
Ø Advantages:
§ Cooling system is simple
§ Light or heavy water is used as moderator
which are cheap
§ The reactor is compact in size
§ Fission product remains contained in the
reactor and are not circulated
§ Disadvantages:
§ High pressure requires a costly reactor vessel
and a leak proof container
§ High pressure and high temperature is water
increases corrosion and erosion problem
§ Steam is produced at relatively
low temperature and pressure
Boiling Water Reactor (BWR):-
Ø BWR is a thermal reactor. Light is used as
moderator as well as reflectors. Steam is produced in the reactor itself
instead of the heat exchanger.
Ø Advantages :
§ Overall cost of operation decreases
§ Overall efficiency of the plant increases
§ Thermal efficiency is 30% as compared to
20-22% in PWR
§ The reactor is capable of promptly meeting load fluctuation requirements
Ø Disadvantages :
§ More biological protection is required
§ More elaborated safety precaution is required
Fast Breeder Reactor (FBR):-
Ø It sues highly enriched U-235 as fuel. The
reactor has no moderator. Metal liquid is used as coolant. The U-235 is
surrounded by U-238.
Ø The neutron released from the nuclear fission
of the U-235 is absorbed by U-238, giving rise to Pu 239 which can be used as
fuel. This is known as breeding technology.
Ø Advantages:
§ High breeding gain is possible
§ No moderator is required
§ Low absorption of high energy neutron permits
high fuel burn up
Ø Disadvantages :
§ Reactor need enriched fuel
§ Liquid sodium is highly corrosive
§ Specific
power is not as in thermal reactor
§ Neutron flux is high at the centre of the core
Nuclear Fuels:-
Ø U-235, U-233 and PU-239 are the nuclear fuels
used in the reactors. U-235 is available in nature but U-233 and PU-239 are
obtained from Th-234 and U-238 using breeding technology. Natural uranium
contains 0.006%of U-234, 0.712% of U-235 and 99.28% of U-238
Desirable Properties of Nuclear Fuel:-
§ High tensile strength at high
temperature
§ High conductivity and high ductility
§ It must be corrosion resistive
Ø Enriched Uranium: The enrichment process
increases the concentration of U-235 in natural uranium. Enriched uranium is
used in FBR. Use of enriched fuel reduces the size of the nuclear reactor to be
designed.
Ø Methods of Uranium Enrichment :
(i) Gaseous Diffusion Method (ii) Thermal Diffusion
Method
(iii) Centrifugal Method (iv)
Electro-magnetic Method
Coolants:
Ø Coolants are used to remove the heat generate
in the reactor chamber, which isto be used for steam generation.
Ø Liquid (H2O, D2O), Metal
(Na, Bi, Li, Na-K alloy), Gas (H2, N2, He, CO2)
are used as coolants as per the suitability.
Ø Desirable Properties of Coolants:
§ High melting point
§ High boiling point
§ It must be considerably cheap
§ It should have low corrosion property
§ High co-efficient of heat transfer
Moderators and Reflecting Material
Ø Moderators can be used as reflectors since
both have the same properties
Ø H2O, D2O, He, Be, Graphite can be used as
moderators
Ø Desirable Properties of Moderators :
Ø (i) High slowing down property (ii) High resistance to corrosion
Ø (iii)
Good thermal conductivity (iv)
Chemical & radiation stability
Ø (v) It should be cheap and abundant
Ø (vi) Low melting point and high boiling
point
Control Rod Materials:-
Ø Control rods are used to control the chain
reaction in the process of fission
Ø Boron (B), Cadmium (Cd), Hafnium (Hf) are used
as control rod materials
Ø Desirable Properties of Control Rod Material:
Ø (i) Cheap
(ii) High melting point (iii)
High corrosion resistive
Ø (iv) High chemical and radiation stability
Gas Turbine Power Plant:-
Ø Components : Compressor,
Combustion chamber and Turbine-Generator set
Ø Types of Gas Turbine Plant:
(1) Open Cycle Gas Turbine Plant
(2) Closed Cycle Gas Turbine Plant
Ø Open Cycle Gas Turbine Plant:
The ambient air enters into the compressor and the gas coming out of the
turbine are exhausted into the atmosphere and the working medium has to be
replaced continuously.
- Advantages :
(i) Negligible warm up time (ii) Any hydrocarbon can be
used as fuel
(iii) Less weight and small in size
(iv) Used as peak plant
- Disadvantages :
(i) High loss of heat in exhaust gas (ii) Sensitive to system component
efficiency
(iii) Deposition of carbon and ash on turbine
reduces the turbine efficiency
Ø Closed Cycle Gas Turbine Plant
The working fluid is continuously used in
the system without its change in phase and the required heat is given to the
working fluid in the heat exchanger
- Advantages ;
(i) The machines used are cheaper and smaller
(ii) No corrosion of the turbine blades
(iii) No need of filtration of the incoming
air
(iv) Inferior oil and solid fuel can be used
(v) Maintenance cost is low and reliability is
high due to longer useful life
- Disadvantages :
(i) Large amount of cooling water is required
(ii) Components used are complicated in design
(iii) The response to the variation of load is poor
(iv) It requires a big heat exchanger
Advantages of Nuclear Power Plant:
(i) Reduces the demand for fossil fuels i. e.
coal, gas and oil
(ii) No problem in transportation and storage
of fuel
(iii) Requires less place
(iv) The can produce secondary fuels
(v) Used as base plant
(vi) Operation not affected by weather
condition
Disadvantages of Nuclear Power Plant:
(i)
High initial capital cost
(ii)
Greater technical knowledge is required
(iii)
Radio-active pollution
(iv)
Poor response to the fluctuation
of the load
(v)
The fuel used is rare and
expensive
(vi)
Disposal of nuclear waste is a big problem
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