Principles of nuclear energy Fission reactions Nuclear reactor Nuclear power plants.
Nuclear Power Station · 2020-02-29 · Schematic Layout of Nuclear Power Plant (Continues.):...
Transcript of Nuclear Power Station · 2020-02-29 · Schematic Layout of Nuclear Power Plant (Continues.):...
Nuclear Power Plant Dr. Sikder Sunbeam Islam
Associate Professor
Department of EEE, IIUC.
Introduction The generating station in which nuclear energy is converted into
electrical energy is known as a Nuclear Power Station.
In nuclear power station , heavy elements such as Uranium (U235) or Thorium (Th232) are subjected to nuclear fission in a special apparatus known as a ‘reactor’.
Breaking up of nuclei of heavy atoms into two nearly equal parts with release of huge amount of energy is known as nuclear fission.
The heat energy thus release is utilized in rising steam at high temperature and pressure.
The steam runs the steam turbine which converts steam energy into mechanical energy. The turbine drives the alternator which converts mechanical energy into electrical energy.
Working Principle
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Nuclear Fuels
Fuel of a nuclear reactor should be fissionable material
which can be defined as an element or isotope (that take
part in nuclear fission). It can be one or all of following :
Natural Uranium found in earth crust (outermost solid
shell of our rocky planet) contains isotopes namely U234 ,
U235, U238 and their average percentage is as follows –
U238 99.3% ; U235 0.7% ; U234 Trace
Out of these U235 is most unstable and is capable of
sustaining chain reaction and has been given the name as
primary fuel .
U233 and Pu239 (Plutonium-239) are artificially produced from
Th232 and U238 respectively are called secondary fuel .
Uranium Oxide (UO2) also can be used as nuclear fuel.
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Nuclear Fission & Chain Reaction
The breaking up of nuclei of heavy atoms into two nearly equal parts
with release of huge amount of energy is known as ‘Nuclear fission’.
Nuclear fission is done by bombarding Uranium nuclei with slow
moving neutrons . This splits the uranium nuclei with the release of
huge amount of energy and emission of neutrons which is called
fission neutrons .
These fission neutrons cause further fission. If this process continues,
then in a very short time huge amount of energy will be released
which may cause explosion . This is known as explosive chain
reaction . This process is done in nuclear reactor.
All practical nuclear reactors are started with a startup neutron
source. The original startup source in a typical American light-water
power reactor is californium-252, a heavy synthetic element that has
a significant spontaneous fission (SF) mode.
Cf-252, a californium isotope with a half-life of 2.645 years, is a very
strong neutron source. Californium is a radioactive chemical element
with the symbol Cf and atomic number 98.
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But in a reactor , controlled chain reaction is allowed .
This is done by systematically removing the fission
neutrons from the reactor . The greater the number of
fission neutrons removed , the lesser is the intensity (i.e
fission rate ) of energy released and this controlling is done
by control rod .
However , the neutrons released have a very high velocity of
the order of 1.5 x 107 meters per second .The energy
liberated energy in the chain reaction is according to
Einstein law, E=mc2, Where
E= energy produced, m = mass in grams, c= velocity.
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Nuclear Fission & Chain Reaction (continues.)
Where does the Energy come from ? When an U235 atoms fission , we have the following example
equation,
235U + 1 neutron 2 neutrons + 92 Kr +142 Ba + Energy
You might have wondered , “where does the energy come from? “ . The mass to be some on both sides of the reaction;
235 + 1 = 2 + 92 + 142 = 232
Thus , it seems that no mass is converted into energy . However , this is not entirely correct . The mass of an atom is more than the sum of individual masses of its proton and neutrons .
Extra mass is a result of binding energy that holds the protons and neutrons of the nuclear together . Thus, when the uranium atom is split , some of the energy that held it together is released as radiation in the form of heat . Because, energy and mass are one and the same , the energy released is also mass released. Therefore, the total mass does decrease a tiny bit during the reaction.
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Advantages
The amount of fuel required is quite small.
A nuclear power plant requires less space as compared
to any other type of the same size.
It has low running charges as a small amount of fuel
is used for producing bulk electrical energy.
This type of plant is very economical for producing
bulk electric power.
It ensures reliability of operation.
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The fuel used is expensive.
The capital cost on a nuclear plant is very high as compared to other types of plants.
The fission by-products are generally radioactive and may cause a dangerous amount of radioactive pollution.
Nuclear power plants are not well suited for varying loads as the reactor does not respond to the load fluctuations efficiently.
The disposal of the by-products, which are radioactive, is a big problem. They have either to be disposed off in a deep trench or in a sea away from sea-shore.
Schematic Layout of Nuclear Power Plant
The whole arrangement can be divided into the following main stages :
(i) Nuclear reactor (ii) Heat exchanger (iii) Steam turbine (iv) Alternator.
Disadvantages
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Schematic Arrangement of Nuclear Power Station
Fig.1. The schematic arrangement
of a nuclear power station
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Schematic Layout of Nuclear Power Plant
(Continues.)
Nuclear Reactor
• It is an apparatus in which nuclear fuel (U235) is subjected to nuclear fission.
• It controls the chain reaction that starts once the fission is done. If the chain reaction is not controlled, the result will be an explosion due to the fast increase in the energy released.
Fig.2:Nuclear Reactor
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Schematic Layout of Nuclear Power Plant
(Continues.): Nuclear Reactor
A nuclear reactor is a cylindrical pressure vessel. It houses fuel
rods of Uranium, moderator and control rods .
Fuel Rods: The fuel rods constitute the fission material
and release huge amount of energy when bombarded with slow
moving neutrons.
Moderator : The moderator consists of graphite rods which
enclose the fuel rods. The moderator slows down the
neutrons before they bombard the fuel rods.
Control Rods: The control rods are of cadmium and are
inserted into the reactor. Cadmium is strong neutron
absorber and thus regulates the supply of neutrons for fission.
When the control rods are pushed in deep enough, they absorb
most of fission neutrons and hence few are available for chain
reaction which, therefore, stops. Similarly, withdrawing control
rods increases the intensity of chain reaction (or heat
produced).
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Fig.3
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Therefore, by pulling out the control rods, power of the nuclear reactor is increased, whereas by pushing them in, it is reduced.
In actual practice, the lowering or raising of control rods is accomplished automatically according to the requirement of load.
Coolant: The heat produced in the reactor is removed by the coolant, generally a sodium metal. The coolant carries the heat to the heat exchanger.
Schematic Layout of Nuclear Power Plant
(Continues.): Nuclear Reactor
Fig.4
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In this reactor enriched uranium (enriched uranium contains more
fissionable isotopes’ U235 then naturally occurring percentage 0.7%
) is used as nuclear fuel and water is used as coolant.
Water enters the reactor at the bottom .It takes up the heat
generated due to the fission of fuel and gets converted into steam.
Steam leaves the reactor at the top and flows into the turbine.
Water also serves as moderator .Heavy water, carbon and
beryllium are the best moderators.
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Schematic Layout of Nuclear Power Plant (Continues.):
Nuclear Reactor: Boiling Water Reactor
Fig.5. Boiling Water Reactor
It uses enriched Uranium as fuel .Water is used as coolant and moderator .
Water passed through the reactor core and takes up the heat liberated due to the nuclear fission of fuel .In order that water may not boil (due to its low boiling point 212oF at atmospheric condition) and remain in liquid state.
It is kept under a pressure of about 1200 p.s.i.g by the pressurizer .This enables water to take up more heat from the reactor .From the pressurizer water flows through tubes of steam generator where it passes its heat to the feed water which in turn gets converted into steam.
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Schematic Layout of Nuclear Power Plant (Continues.):
Nuclear Reactor: Pressurized Water Reactor (P.W.R)
Fig.6. Pressurized Water Reactor
In this reactor two liquids metal coolant are used. Liquid sodium (Na) serve as the primary coolant and alloy of sodium potassium (NaK) as secondary coolant.
Liquid sodium as coolant can transfer its heat very easily .The only problem in this system is that sodium becomes sometimes radioactive while passing through the core and reacts chemically with water . So, a secondary coolant NaK is used .
The primary coolant while passing through the tubes of inter mediate heat exchanges(I.H.X) transfer its heat to the secondary coolant .The secondary coolant then flows through the tubes of steam generator and passes its heat to the feed water .Graphite is used as moderator in this reactor.
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Schematic Layout of Nuclear Power Plant (Continues.):
Nuclear Reactor: Sodium Graphite Reactor(SGR)
Fig.7. Sodium Graphite Reactor
In this reactor the core containing U235 is surrounding by a blanket( a layer of fertile material placed outside the core) or fertile material U238 .
In this reactor no moderator is used. The fast moving neutrons liberated due to fission of U235 are absorbed by U238 which gets converted into fissionable material Pu239 which is capable of sustaining chain reaction.
Thus ,this reactor is important because it breeds fissionable material from fertile material U238 available in large quantities.
U238 + neutron Pu239
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Schematic Layout of Nuclear Power Plant (Continues.):
Nuclear Reactor: Fast Breeder Reactor(FBR)
Like, sodium graphite nuclear reactor this reactor also uses two liquid metal coolant circuits. Liquid sodium is used as primary coolant when circulated through the tubes of intermediates heat exchange transfer its heat to secondary coolant sodium potassium alloy .
The secondary coolant while flowing through the tubes of steam generator transfer its heat to feed water.
Fast breeder reactors are better than conventional reactor both from the point of view of safety and thermal efficiency.
Fig.8. Fast Breeder Reactor
The commonly used coolants for FBR are :
• Liquid metal (Na or NaK) (ii) Helium (He) (iii) Co2 .
Sodium has following advantage in this case –
• It has very low absorption .
• It possess good heat transfer properties at high
temperature and low pressure.
• It does not react on any of the structural materials
used in primary circuits.
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Schematic Layout of Nuclear Power Plant (Continues.):
Nuclear Reactor: Fast Breeder Reactor(FBR)
Schematic Layout of Nuclear Power Plant
(Continues.):
• Heat Exchanger: The coolant gives up heat to the heat exchanger which is utilized in raising the steam. After giving up heat, the coolant is again fed to the reactor.
• Steam Turbine: The steam produced in the heat exchanger is led to the steam turbine through a valve. After doing a useful work in the turbine, the steam is exhausted to condenser. The condenser condenses the steam which is fed to the heat exchanger through feed water pump.
• Alternator: The steam turbine drives the alternator which converts mechanical energy into electrical energy. The output from the alternator is delivered to the bus-bars through transformer, circuit breakers and isolators.
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Waste Disposal • Importance: Wastes from atomic energy installation are
radioactive, create radioactive hazard and so, require
strong control to ensure that radioactivity is not released
into the atmosphere to avoid atmospheric pollution.
Nuclear wastes are likely liquid, gas or solid and each is
treated in different manner.
• Liquid Wastes:
• (i) Dilution: Liquid wastes are diluted with proper factor
with large quintiles of water and released into the
ground. The drawback of this method is that there is a
chance of contamination of underground water if dilution
factor is not adequate.
• (ii) Concentration to small volumes and storage: Here
liquid wastes are concentrated to small volume and
stored in underground tanks.
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• Gaseous Waste: Gaseous wastes causes atmospheric pollution. This wastes are passes through filters and then released to atmosphere.
• Solid Wastes: Solid wastes included scrap material or discarded objects contaminated with radioactive matter.
• This waste is burnt if combustible. Non-combustible wastes are always buried deep (around 500 meters deep) in the ground.
• The radioactive matter is mixed with concrete, drummed and shipped for burial.
• High-level waste is delayed for 40-50 years to allow its radioactivity to decay, after which less than one thousandth of its initial radioactivity remains, and it is much easier to handle. Hence containers of vitrified waste, or used fuel assemblies, are stored under water in special ponds, or in dry concrete structures or casks, for at least this length of time.
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Waste Disposal (continues)
Problems
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Examples-1: Calculate the fission rate of U235 for producing power of one watt if 200 MeV of energy is released per fission of U235.
Soln : Given, P = Power = 1 Watt E = Energy released per fission of U235 nucleus = 200 MeV = 200 x 1.6 x 10-13 j
= 3.2 x 10-11 watt. Sec [ `.’ 1 MeV = 1.6 x 10-13 J ]
Fission rate (fission/sec) of producing one watt of Power,
= 𝑃
𝐸 =
1
3.2 × 10−11 = 3.1 x 1010 fissions/ sec ,
Ans.
Examples-2: Determine the energy released by the fission of 1.5 gm of U235 in Kwh , assuming that energy released per fission is 200MeV. .
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Soln :
We know , Avogadro number = 6.025 × 1023
Mass of Uranium = 235 a.m.u. [ 235 gm of U235 contain 6.02×1023 numbers]
n = Number of atoms in 1.5gm of U235
= 1.5 × 6.025 × 1023
235
E1 = Energy released per fission = 200 MeV = 200 × 106 ×
1.6 × 10-19
= 3.2 × 10-11 J
E = Energy released by 1.5 gm of U235
= n × E1 = 1.5 ×6.025 ×1023
235 ×
3.2 ×10−11
103 ×3600 Kwh = 3.42 × 104 Kwh . Ans.
References
1. Principles of Power Systems, V.K.Mehta & Rohit
Mehta
2. Power Plant Engineering, G.R.Nagpal
3. Power Station Engineering &Economy, William
A Vopat