Steam generators (boilers) are complex systems that integrate components like furnaces, superheaters, reheaters, boilers, and economizers to generate steam. They can be classified based on application (e.g. utility, industrial), pressure level (subcritical or supercritical), tube movement design (fire tube or water tube), and firing method (externally or internally fired). Modern utility steam generators commonly operate between 130-180 bar pressure to produce superheated steam at 540-560°C with one or two stages of reheating. Pulverized coal is a common fuel that is finely ground and blown into a furnace through burners to facilitate more complete and efficient combustion.

1. STEAM GENERATORS
A steam generator is a complex integration of furnace, superheater, reheater, boiler or evaporator, economizer and air
preheater along with various auxiliaries such as pulverizers, burners, fans, stokers, dust collectors and precipitators,
chimney or stack.
Economisers are basically heat exchangers, their tubes are commonly 45-70 mm in outside diameter and made in vertical
coils of continuous tubes connected between inlet and outlet headers and coils are installed at 45 to 50 mm spacing.
Temperature of feed water is raised to about saturation temperature by means of flue gases.
Superheaters
In modern utility high pressure boilers, more than 40% of heat is absorbed in generation of steam takes place inside
superheater. So, it requires large surface area for superheating of steam.
Superheaters are commonly classified as either convective superheaters, radiant superheaters or combined
superheaters, depending on how heat is transferred from the gases to steam.
Convective superheaters are located in convective zone of furnace, whereas radiant superheaters are located in radiant
zone. Steam leaving the radiant superheaters goes to desuperheater where highly pure water is directly sprayed on to
steam in such a quantity that the temperature of steam after the last stage of superheating in the pendant superheater
(combined super heater) does not exceed the rated value.
Reheaters
The design consideration for reheaters is same for those of superheaters, although the steam outlet temperature are
about the same as in superheater. The steam pressure is about 20-25% of those in superheater. So, the pressure stresses
are therefore lower.
Air-preheaters
The use of hot air makes the combustion process more efficient by making it more stable and lessening the energy losses
due to incomplete combustion and unburnt carbon. It causes a saving in the amount of fuel and consequent increase in
boiler efficiency. The hot air is also used for drying the coal in the pulverizers.
Boilers
Classification of steam generators can be made in different ways.
Depending upon the application, they can be
(a) Utility Steam generators (b) Industrial steam generator(c) Marine steam generators
Depending on whether the pressure of steam is below or above the critical pressure (222.1 bar), they can be either
subcritical or super-critical units.
Subcritical boilers usually operate between 130 and 180 bar steam pressure. Subcritical steam generators are water
tube-drum type.
Supercritical steam generators are drum-less once through type and operate at 240 bar pressure or higher.
Majority of utility steam generators are of the 170-180 bar water tube drum variety, which produce superheated steam
at about 540-560⁰C with one or two stages of reheating. The steam capacities of modern utility steam generators range
from 120 to 1300 kg/s.
Depending upon the movement of gas and water
(a) Fire tube (b) Water tube
Water tube boilers are divided into two types.
(a) Straight tube boilers (b) Bent tube boilers
Bent tube boilers offer few advantages over straight tube boilers. They are
(i) In bent tube boiler, the tubes are so bent that they enter and leave the drums radially.
(ii) They operate at high steam rate.
(iii) They have better accessibility for cleaning, inspection and maintenance.
Depending upon the firing
(a) Externally fired (b) Internally fired

2. 1. Boiler drum 2. Steam generator 3. Boiler feed pump 4. Circulation pump 5. Economizer
6. Evaporator 7. Superheater
 Heat transfer around the wall is mostly by radiation by the flames and less by convection by the flue gases.
 Natural circulations used up to a steam pressure of 180 bar. Boilers with forced circulation is caused by a special
pump, they are suitable up to steam pressure of 200 bar.
 Boilers operating with forced or natural circulation at subcritical pressure are commonly known as drum boilers
 In supercritical boilers no drum is necessary as separation of water and steam does not occur though there is no
recirculation.
 Water enters at bottom of tubes completely transport to steam by the steam by the time they reach the top. Passing
through tube only once, so it’s called drum less boiler or once through boiler.
Fuels and Combustion
Boilers may be coal fired or gas fired.
Coal Analysis
There are two types of coal analysis, (a) proximate (b) ultimate analysis.
 Proximate analysis indicates the behaviour of coal when heated. The percentage of fixed carbon, Volatile matter,
Moisture and Ash are determined from this analysis.
Fixed Carbon + Volatile Matter + Moisture + Ash = 100% by mass
 Ultimate analysis indicates amount of air required for combustion and mainly used to determine constituents in the
products of combustion. This analysis shows the following components on mass basis: carbon, hydrogen, oxygen,
nitrogen, sulphur, moisture and ash.
Coal properties
Swelling index
Some types of coal during and after release of volatile matter become soft and pasty and form agglomerates. These are
called caking coal. Coal that does not cake is called free burning coal.
Caking coals are used to produce coke by heating in a coke oven in the absence of air, which is largely needed in steel
plants. A qualitative evaluation method, called the swelling index has been devised to determine the caking of the coal.
Grindability
This property of coal is measured by the standard Grindability index, which is inversely proportional to the power
required to grind the coal to a specified particle size for burning,
Weatherability
It is a measure of how well coal can be stock piled for long periods of time without crumbling into pieces.
Sulphur content
For high sulphur content coal, operating cost of SO2 removal equipment needs to be considered.
Heating Value
It is the heat transferred when the products of complete combustion of a sample of coal are cooled to the initial
temperature of air and fuel.
Two different heating values are cited for coal. Higher heating value and Lower heating value.
Ash softening temperature
The ash softening temperature is the temperature at which the ash softens and becomes plastic. If the furnace
temperature is greater than ash softening temperature all the ash will melt and would come out of the furnace bottom
continuously as molten slag.

3. Synthetic fuels
Synthetic fuels are gaseous and liquid fuels produced largely from coal in an economical and environmentally acceptable
manner.
Coal gasification
Basically in gasification, coal is blown with steam while also being heated. So during reaction oxygen and water molecules
oxidize the coal and produce a gaseous mixture like CO2, CO, H2O vapor and H2.
3C + H2O + O2 → H2 + 3CO
For low grade coal (brown coal) which generally contains significant amounts of water, there are techniques in which no
steam is required, coal and O2 being only reactants. Some gasification techniques use direct blowing where coal and
oxidizer supply toward each other where as in reverse blowing they are supplied from same side. In all cases oxidizer
supplied is insufficient for complete combustion.
Coal liquefaction
The conversion of coal into liquid fuel requires the addition of hydrogen to the coal. There are three basic modes that
have been used to liquefy coal. These are direct process like hydrogenation and indirect process like catalytic conversion
and hydropyrolysis.
In the hydrogenation process, coal and catalyst are suspended as a slurry, which is reacted with hydrogen at high
pressure and moderate temperature to form liquid hydrocarbons.
In the indirect process, it requires indirect gasification of solid coal to form synthetic gas, which is then converted to
liquid by means of catalyst.
Combustion
In utility steam generators, percentage of excess air required for complete combustion of coal ranges between 15 to 30%.
Combustion of coal may occur in
(a) Fuel bed furnace
(b) Pulverized coal furnace
(c) Cyclone furnace
(d) Fluidized bed furnace
Fuel bed Combustion
A grate is used at the furnace bottom to hold a bed of fuel. There are two ways of feeding coal onto the grate:
(a) Overfeeding (b) Underfeeding
An overfed fuel bed section receives coal fresh coal on its top surface. In underfeeding, coal is fed from below the grate
by a screw conveyor or ram.
In small boilers, the grate is stationary and coal is fed manually by shovels. But for more uniform operating conditions
moving grates or stokers are used. They are power operated coal feeding system.
Pulverized coal firing system
Coal is first ground to dust like size and powdered coal is then carried in a hot stream of air to be fed through burners
into furnace (combustion chamber). The amount of air necessary to complete the combustion (secondary air) is supplied
separately to combustion chamber. The amount of air used to carry the coal and dry it before entering the furnace is
called primary air. The efficiency of combustion depends on size of coal powder.

4. Fluidized bed furnace
When air is passed through a fixed bed of particles, air simply percolates through the interstitial gaps between the
particles. As the airflow rate through the bed is steadily increased, a point is reached at which the pressure drop across
the bed becomes equal to the weight of the particles per unit cross sectional area of the bed. This critical velocity is called
minimum fluidization velocity. At this velocity, solid particles are suspended in gas stream and packed bed becomes
fluidized bed. With further increase in velocity, the bed becomes turbulent and rapid mixing of particles occur. Burning
of fuel in such a stage is called fluidized bed combustion.
Advantages
1. Better heat transfer 2. Cost of crushing coal reduces 3. Low combustion reduces
4. Better pollution control 5. Size of unit reduces.
Fluidized bed combustion
It is the combustion technology for burning solid fuel such as carbon. In its more simplest form, it corresponds to burning
of coal in a heated bed of particles suspended on a gas with an increase in gas velocity particles(bed) behave like a fluid
resulting in rapid mixing of the particles, coal is added to bed and continuous mixing encourages complete combustion.
Bubbling Fluid bed combustion (Stationary)
In BFBC, gas at lower velocity is used and fluidization of solid is relatively stationary with some fine particles being
entering in all types of bubbles.
Circulating Fluid bed combustion
In CFBC, gas velocity is high sufficient to suspend the particles on the bed. Here the large particles are entering from the
bed and entire particles are recirculated back into bed via an external loop. In CFBC, distinction between bed and free
board area is no longer applicable. Bed material (Sand) is used for control of temperature and solvent (limestone) to
precipitate out SO2.
Boiler performance
ηsteam,gen =
Energy utilized
Energy released
=
H. H. V − losses
H. H. V
The various losses have been listed as follows,
1. Energy lost due to unburnt carbon
2. Energy lost due to dry exhaust gases.
3. Energy lost due to moisture in fuel
4. Energy lost due to hydrogen in fuel
5. Energy lost due to moisture in air
6. Energy lost due to convection and radiation loss
7. Energy lost due to ash and slack
Steam generator control
The objective of steam generator control is to provide steam flow required by the turbine at the design temperature and
pressure. The variables that are controlled are fuel firing rate, air flow, gas flow distribution, feed water flow and turbine
valve setting. The key measurements that describe the plant performance are steam flow rate, steam pressure, primary
and secondary air flow rates, fuel firing rate, feed water flow rate and steam drum level and electric power output.
Steam pressure control
The steam pressure control system maintains steam pressure by adjusting the fuel and combustion air flows to meet the
desired pressure.

5. Steam temperature control
Steam temperature control is done by desuperheating and attemperation (reduction in steam temperature by removing
the energy of steam (i.e., portion of steam is taken out). It also done by use of excess air).
By gas recirculation (gas from superheater, reheater outlet as well as from economizer outlet is recirculated back to
furnace).
Temperature is also controlled b adjustable burners.
Ash handling devices
It may be sometime 10-20% of total quantity of coal burnt in a day, handling of ash is a big problem because it is very
hot, dusty and causes irritation. It needs to be quenched before handling, different types of ash quenching devices are
a) Mechanical b) Hydraulic c) Pneumatic d) Steam jet
Dust collector
Products of combustion in coal fired furnaces contains particles of solid metal floating in suspension. It’s mainly fine ash
particles (fly ash). It’s generally intermixed with some quantity of carbon material called sluice. Its removal is generally
done by mechanical dust collector. General types of mechanical dust collectors or electric dust collector. General types
of mechanical dust collectors mainly used are
1. Centrifugal fan screwer
2. Induced spray screwer
Boiler water treatment
It is required for prevention of hard scale formation on heating surface, control of carry over to eliminate deposition on
superheater tubes, prevention of silica deposition and corrosion damage to turbine blades. Treatment can be external or
internal. All water treatment processes are basically aimed at water softening process. Natural water is generally hard
and contains scale forming impurities.

6. Hydroelectric power
Advantages of Water Power
a) Clean energy b) Highly reliable c) less operational and maintenance cost d) easy starting
e) No pollutionf) Long life
Disadvantages of Water power
a) Lot of investment d) long gestation period c) large cost of power transmission
d) Power generation influenced by geographic location
Site selection
a) Availability of waterb) Water storage capacity c) Available water head
d) Accessibility of site e) Distance from load centre f) Type of land of site
Essential elements of a hydroelectric power
1. Catchment area 2. Reservoir 3. Dam 4. Spillways 5. Conduits 6. Surge tanks
7. Draft tubes 8. Power house9. Switch yard for
transmission for power
1. Catchment area
The whole area behind the dam draining into stream or river across which
the dam has been constructed is called the catchment area.
2. Reservoir
a) Natural b) Artificial
Water held in upstream reservoir is called storage, whereas water behind
the dam at the plant is called pondage.
3. Dam
A dam performs the following basic two functions
 It develops a reservoir of the desired capacity to store water.
 It build up a head for power generation.
Dams can be classified into various ways, based on following:
Function: - Based on function dam can be classified into storage dams,
diversion dams and detention dams.
Storage dams are mainly used for storing water and using it subsequently
when required. Diversion dams are constructed to raise the water level
and to divert the river flow in another direction. Detention dams are
primarily constructed to store flood water.
Shape: - a) arch dams b) Trapezoidal dams
Materials of Construction: - Dams can be constructed of earth, rock pieces,
stone masonry, concrete, RCC and even of timber and rubber.
Hydraulic design: - Based on this, dams can be of non-overflow type, in
which water is not allowed to flow over the top of the dam and the
overflow type which allows water to flow over it.
Structural design: - As per structural design there can be gravity dam,
arch dam and buttress dam, where water thrust is resisted by gravity, arch action and buttresses.
4. Spill ways
When the water level in the reservoir basin rises, the stability of the dam structure is endangered. To relieve the
reservoir of this excess water, a structure is provided in the body of a dam or close to dam. This safeguarding structure
is called spill way. Types of spillways are
a) Gravity spillway b) Trough spillway c) Side channel spillway d) Saddle spillway
e) Shaft spillway f) Siphon spillway
5. Conduits
A channel for conveying water or other fluid.
A headrace is a channel which leads water to turbine and tailrace is a channel which carries water from the turbine.
The conduit may be open or closed. Canals and flumes are open, while tunnels, pipelines and penstocks are closed.
6. Surge tank
A surge tank is a small reservoir in which the water level rises or falls to reduce the pressure swings so that they are
not transmitted to the closed conduit. Surge tanks are required for high head power plants where water is taken to
the powerhouse through tunnels and penstocks. Different types of surge tanks are Conical, internal belt mouth
spillway and differential.

7. 7. Draft tube
In turbines like Francis, Kaplan a diffuser tube is installed at the exit of the turbine which is known as draft tube. The
primary function of the draft tube is to reduce the velocity of the discharged water to minimize the loss of kinetic
energy at the outlet. This draft tube at the end of the turbine increases the pressure of the exiting fluid at the expense
of its velocity. It helps in increasing the output and efficiency of turbine.
8. Power house
The equipment provided in the powerhouse includes the following,
Turbines, generator, governor, gate valve, relief valve, water circulation pump, switch board etc...
9. Tailrace
It’s generally an open channel made of concrete, pipe may also be used. Its function is to carry away the water
discharged from the turbine after the production of power.
Classification of Hydro-electric power
According to the availability of head
a. High head power plants (>100m)
b. Medium head power plants (30-100m)
c. Low head power plants (<30m)
According to nature of load
a. Base load plant
b. Peak load plant
According to the quantity of water available
a. Run-of-river plant
b. Hydroelectric plants with storage reservoir
c. Pump storage plants
d. Mini and micro hydel plants
Pump storage – Water after working in turbine is stored in tailrace reservoir and pumped to maintain reservoir when
plant is not in operation,
Hydraulic turbines
Classification based on head available
 Low head → 2-15m (Kaplan/propeller)
 Medium→ 16-70m (Kaplan/Francis)
 High head→ 71-500m (Pelton/Francis)
 Very high head→ >500m (Pelton)
Classification based on axis of turbine
 Horizontal (Pelton)
 Vertical (others
Classification based on flow of water
 Axial (Kaplan)
 Tangential (Pelton)
 Mixed (Francis)
Classification based on action of water on blades
 Impulse
 Reaction
Specific speed
The specific speed is the speed of geometrically similar turbine which produces 1kW of power under the head of 1m.
𝑁𝑠 =
𝑁 ∙ √𝑃
𝐻
5
4
Runner
Specific Speed
Slow Medium Fast
Pelton 5-15 16-30 31-70
Francis 60-150 151-250 251-400
Kaplan 300-450 451-700 701-1100

8. Pelton wheel
It’s a tangential flow impulse turbine. Its runner consists of a large circular disk on the periphery of which a number of
two-lobe ellipsoidal buckets are evenly mounted. Each bucket has a splitter which directs the jet of water into 2 equal
streams. The nozzle (fixed) directs the flow on wheel. It also directs the flows with the help of spear hull, controlled by
the governor. In its simple arrangement there is a single nozzle (jet) but for large discharge there are 6 jets that are
symmetrically arranged.
Vr2 is generally less than Vr1 (because of friction/because of ridge thickness)
Vr2 ≈ Vr1 (by polishing the inside of bucket/ reducing the ridge thickness)
Work done = U(Vw1 + Vw2)
Hydraulic efficiency (ηh) =
Work done on blades
Energy suplied
=
U ∙ (Vw1 + Vw2)
V1
2
For, Vw1 = V1, Vw2 = Vr2 ∙ cos β2 − U,
Vr2
Vr1
= k = blade friction coeff.
For, k = 1, Vr2 = Vr1 = V1 − U
Vw2 = (V1 − U) ∙ cos β2 − U
ηh =
2U(V1 + (V1 − U) cos β2 − U)
V1
2 =
2U((V1 − U) ∙ (1 + cos β2)
V1
2
For a certain value of V1 and ϕ, there is a certain value of U for which hydraulic efficiency is maximum which implies
→
dηh
dU
= 0 ⟹
(1 + cos β2)
V1
2 ∙
d
dU
(2UV1
2
− 2U2) = 0
U =
V1
2
Therefore hydraulic efficiency of a Pelton turbine is max, when the velocity of fluid is half the velocity of the jet of water
at the inlet. Therefore maximum hydraulic efficiency,
ηh Max =
1 + cosβ2
2
Relations
V1 = Cv ∙ √2gH U =
πDN
60
M(Jet ratio) =
Dia of wheel
Dia of jet
=
D
d
No. of buckets in a runner = 15 + (0.5 × M)
Cv → coef. of velocity H → Net head N → speed of wheel

9. Francis turbine
This is an inward mixed flow reaction turbine in which entry is radial and the discharge is axial. Basically it comprises of
Scroll casing, guide vanes, runner vane and draft tubes.
Scroll casing-
It is generally a spiral case ad surrounds the runner and guide
mechanisms. Its basic function is to distribute water over the guide
vanes and prevent the formation of eddies.
Guide vanes
They are of aerofoil shape and are spaced evenly around the periphery
of the runner. It imparts a tangential velocity to water before entering
the runner.
Runner vanes
The runner consists of a series of vanes (12-24). The shape of vane is
such that water enters the vane radially at the outer periphery and
leaves axial direction at the inner periphery.
In Francis turbine, pressure of water at inlet is more than that of outlet, as such water is required to flow in closed conduit.
Unlike Pelton wheel where the water strikes only on a few buckets at a time, in Francis turbine runner is always full of
water.
Work done on the vane = U1 ∙ Vw1 (as Vw2 = 0)
ηh =
work done
energy supplied
=
U1 ∙ Vw1
gH
Speed ratio(ρ) =
U
V1
Flow ratio (ϕ) =
Vf1
V1
𝐃𝐢𝐬𝐜𝐡𝐚𝐫𝐠𝐞 (𝐐) = Area of flow × Flow velocity = πD1B1 × Vf1 = πD2B2 × Vf2
D1 → dia. of runner at inlet B1 → width of vane at inlet Vf1 → flow velocity at inlet
When the thickness of the value is to be considered,
Q = (πD1 − n × t)B1 × Vf1
t → thickness of vane n → no. of vanes attached to runner
Breadth ratio =
B
D
Governing of Hydraulic turbines
Hydraulic turbines are directly coupled to electric generators. Generators are always required at constant speed
irrespective of variation of load. If the load on the generator keeps varying and if the input of the turbine remains same
then the speed of runner tends to increase of the load goes down and speed decreases if the load increases. So, the speed
of generator and its frequency will vary accordingly which is not desired. So the speed of runner is always required to be
maintained at a constant level at all loads. This is done by governor which regulated the quantity of water flowing through
the runner in proportion to the load.

10. In impulse turbine such as Pelton wheel, the flow
through the runner is regulated by the combined action of
the spear and deflector plate. The quantity of water entering
the runner can be increased or decreased by the movement
of the spear towards the left or right respectively. This
movement is automatically controlled by the action of the
governor in conjunction with a well operated servo
mechanism.
The deflector plate is generally placed between
nozzle and the bucket to divert the flow of the water to the
tail race as and when it is required. As per the requirement,
deflector can be brought into action. The governing of
reaction turbine such as Francis turbine is very much similar
to the above, except that, the motion of the piston in servo
motor is used to partially close or open the guide vanes gate,
through which water is supplied to the turbine.
Automatic and electric Hydro-electric plant
An automatic system is safer efficient and reliable that works through governor and voltage regulator. They are of
following types, fully automatic, partly automatic and remote control,
Fully automatic can be controlled by time switch, flood switch and load sensitive device.
Time switch – It can start and stop the power system at pre-set timing.
Flood switch – It works with the change in level of water with the reservoir, so that the water level rises generator output
is increased and vice versa.
Load sensitive device – It’s actuated by the demand for power in the area served by the plant i.e., the increase in the
power demand would increase the generator output. Automatically in case of any trouble, the plant is automatically shut
down.
Partly automatic – Manual starting and synchronization but in case of any fault, automatic shutdown happens.
Remote control – here the control of power station is exercised from a distance, usually a control centre. There the
operator at control point transmits a signal to control station which in turn actuates the automatic system in power
station.
Underground Power station
An underground power station is a type of hydroelectric power station constructed by excavating the major components
(e.g. machine hall, penstocks, and tailrace) from rock, rather than the more common surface-based construction methods.
Advantages
Saving in the cost of land. Stations are well protected from natural calamities. They provide good defence security and
greater uniformity in different climatic conditions. Formation difficulties are difficult to overcome.
Disadvantages
Cost of construction is more, increased cost of lighting, ventilation and Air-conditioning.
Hydro graphs
It’s defined as a graph showing the discharge (run-off) of flowing
water w.r.t time for a specified period. Each hydro-graph has a
reference to a particular river site. Time period from the discharge
hydro-graph may be weekly, hourly, daily or monthly.
Streams of river depend on the catchment area and precipitation of
catchment area. Precipitation may be solid or liquid. Hydrographs
indicate the power available from the stream at different times of
day, week or month.

11. Flow duration curve
It’s another useful form to represent the run-off data _________ data available
for a given time, the curve s plotted between flows of available giving a period
against the fraction of time. The area under the flow duration curve
represents the yield from the stream. B changing the co-ordinate to power
instead of discharge, power deviation curve can be obtained and the area
under the curve would represent the average yield of power, from the hydro-
electric power project.
It might be noted from the figure that Qn is the minimum flow rate that will be available for all times (100% time). Area
under the curve would represent the average yield from the stream, this power is called as primary power.
Additional output available at high water flow is called secondary power. If the flow rate of Qm is required for all times
as indicated by area under the flow duration line (DEF) then it’s possible to make this uniform flow rate or power at all
times, only if the storage is equal to area (BEF) is power. In the absence of storage, net area (BCBE) represents the
secondary power that is available from the river.
Mass Curve
It’s a graph of cumulative values of water quantity against time. It’s an integral curve which expresses the area under
the hydro curve from one time to another. It’s a curvilinear curve to determine the determine the storage requirements.
Nuclear Energy
Atomic number (Z)  No. of Protons Mass number No. of protons + No. of neutrons
Isotopes
Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon
number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each
atom. H1
1 → Protium 1H2
→ Deuterium 1H2
→ Tritium
Naturally occurring Uranium has 3 isotopes 92U235
92U235
92U238
Radioactivity
The phenomena of spontaneous emission of powerful radiations exhibited by heavy elements such as uranium is called
radioactivity. It is essentially a nuclear phenomenon and is a drastic process because the element changes its kind. It is
spontaneous irreversible self-disintegrating activity because the element breaks up for good. This element which
exhibits such activity such called radioactive elements. Radiations emitted by this elements are found as follows,
 α- particle or α rays
 β- particles, γ rays, Neutrons etc..
Chemical and Nuclear reactions
Nuclear reactions involve a change in an atoms nucleus usually producing a different element. Chemical reactions on the
other hand involve only a rearrangement of electrons and do not involve changes in nuclei. Rates of chemical reactions
are influenced are influenced by temperature, pressure catalyst etc.. but the rates of nuclear reaction are unaffected by
this factors.
In nuclear reactions, mass us not strictly conserved. Some of the mass is converted to energy as per the equation E=mc2,
where c is the velocity of the light. Energy changes for nuclear reactions are much larger and this obviously comes from
the destruction of the mass.
Atom
Nucleus
Protons
Nuetrons
Nucleons
Electrons

12. Nuclear fission
It is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller and lighten
nuclei. It can be caused by bombarding with high energy α particles, protons or neutrons. However neutrons are most
suitable as they are electrically neutral and as such requires no high
Kinetic energy to overcome the electrical repulse from the positively
charged nuclei. It can be considered as a process that occurred when a
neutron collides with the nucleus of certain heavy atom causing the
original nucleus to split into two or more unequal fragments which
carry most of the energy of fission as Kinetic Energy. It is accomplished
by the emission of neutrons and γ rays.
It is the representation of fission of Uranium 235 which has been bombarded with neutron. The energy released as a
result of fission is the basis for the nuclear power generation. The release of about 2.5 neutrons per fission makes it
possible to produce sustained fissioning. The fission fragments that result from the fission process are radioactive and
decay by emission of β particles, γ rays and to a lesser extent α particles and neutrons. The neutrons that are emitted
after emission by decay of some fission fractions are called delayed neutrons. This are important as they permit fission
chain reaction to be easily controlled. The fission of uranium 235 yields on average about 193 MeV. The amount of energy
is prompt i.e., released during the fusion process. More energy is however produced due to slow decay of fusion
fragments. The total energy released per fission reaction per atom is about 200 MeV. The complete fission od 1g of U235
thus produces
1 eV = 1.602 × 10−19
J
Avagadro constant
Mass of U − 235 isotope
× 200 Me V =
6.023 × 1023
235.049
× 200 Me V
= 5.126 × 1023
Me V = 8.19 × 1010
J = 2.276 × 1024
kWh = 0.984 MW − day
Thus a nuclear reactor burning 1 gram of uranium 235 generates nearly 1 MW-day of energy. This is referred to by the
term “fuel burnup”.
Nuclear Reactors
It may be considered as a device in which a nuclear fission is produced in the form of a controlled self-sustaining chain
reaction. So it can be looked up on as a sort of nuclear furnace which burns fuels like U235, U233 and (Plutonium)Pu233 etc..
and produces along with other products such as neutrons, radio isotopes etc…
Essential elements of Nuclear reactor
Reactor core
It is that part of the reactor where fission chain reaction is made to occur. It consists of an assembly of fuel element
control rod coolant and moderator. The fuel elements are made of plates or rods of uranium metals. These are usually
clod in a thin sheet of stainless steel or zirconium or aluminium to provide corrosion resistance, retention of radioactivity
and structural support. Enough space is provided between the rods or plates for the passage of coolant.
Reflectors
It is usually placed around the core to reflect back same of the neutrons that leak out from the surface of the core.
Generally made of same material as that of moderator.
Control Mechanism
It is necessary for starting the reaction maintaining the reaction and shutdown the reaction in case of emergency. It
works on the principle of absorbing the access neutrons with the help of control rod made of cadmium strip or boron
steel. By moving the rod up and down, the rate of reaction can be controlled.
Moderator
Its function is to slow down the neutrons from the high velocities and hence high energy level which they have on being
released from fission process. Their function is to slow down the neutrons but not absorb the, heavy water, beryllium,
graphite are commonly used moderators.
Coolant
The function of the coolant is to remove the heat released as a result of fission reaction.
Shielding
Used to protect the walls of the reactor from the radiation damage and protect the operating personal from exposure to
radiation, it may be still lining or thick concrete surrounding.
Types of Nuclear reactor
Reactors can be homogenous or heterogeneous. Heterogeneous reactor has a large number of fuel rods with coolant
circulating around them and carrying away the heat released by the nuclear fission. In homogenous reactor, the fuel and
moderator are mixed e.g. fissionable salt of uranium like uranium sulphate dissolved in the moderator like H2O or D2O..
due to difficulties in the component maintenance, induced radioactivity, erosion and corrosion, homogeneous reactors
are not common. Present day nuclear reactors are the heterogeneous class.
The present day nuclear reactors are of 2 types,

13. Pressurised Water reactor
A PWR power plant is composed of 2 loops in series, the
coolant loop, called the primary loop and the water-steam
or working fluid loop. The coolant picks up heat in the
reactor and transfers it to the working fluid in the steam
generator. The steam is then used in a Rankine type cycle
to produce electricity. In this type the coolant, moderator
and reflector used are generally like heavy water. Fuel used
is slightly enriched uranium in the form of thin rods or
plates. The cladding is either of stainless steel or zircaloy.
Max coolant temperature is 374 OC. In practice temperature
is nearly 300 OC.
The coolant in the PWR primary loop is maintained at a
pressure (about 155 bar) greater than the saturation
pressure corresponding to maximum coolant temperature in the reactor to prevent bulk boiling. Because liquids are
incompressible, small changes in the volume occurs due to change in coolant temperature because to either load
variation or sudden nuclear reactivity insertions cause severe or oscillatory pressure changes, due to which pressure
may increase or decrease. If the pressure increases, some water will flash into steam and it will affect reactor
performance, often leading to burnout. If the pressure decreases, there may be cavitation. So to accommodate changes
in the coolant volume, arising out of variations in the pressure. A surge chamber is used which is called pressuriser.
Advantages Disadvantages
Water can be used as coolant, moderator and reflector. Capital cost is high
Reactor is compact Severe corrosion problem
Fission products are contained in reactor
Boiler Water Reactor (BWR)
A BWR differs from PWR in that the steam flowing to the turbine is
produced directly in the reactor core. Coolant serves the triple function of
coolant, moderator and working fluid. Since the coolant boils in the reactor
itself, its pressure is much less than that in a PWR and it is maintained at
about 70 bar with steam temperature around 285 OC.
Advantages Disadvantages
Heat exchanger circle can be
eliminated
More elaborate safety precaution is
needed
Low pressure can be used Possibility of radioactive contamination
Cycle is more efficient
Gas cooled reactor
In a gas cooled reactor, coolant is generally CO2 or helium. Moderator is generally graphite. It may be of following types.
One is gas cooled graphite moderator (GCGM), other is High temperature Gas cooled reactor (HTGC). The former (GCGM),
uses natural uranium as its fuel, while the latter (HTGC) employs highly enriched uranium carbide mixed with thorium
carbide and cladded graphite coolant. Pressure and temperature in GCGM is about bar and 336 OC and for HTGC, it’s
around 15 – 30 bar and 700 – 800 OC (here the work is large as it handles gas as coolant).
Nuclear Fusion
It’s a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and sub
atomic particles like protons or neutrons. The difference in mass between reactants and products is manifested and
released as energy to cause fusion . It is necessary to accelerate positive charged nuclei to high Kinetic energies, in order
to overcome electrical repulsive forces by raising the temperature to hundreds of millions of degrees resulting in plasma.
There are several possible reactions between the nuclei if light elements that can be the basis for controlled fusion.
Deuterium (1H2), a stable heavy isotope of hydrogen(1H1), present in natural water is the main fuel for fusion reactor.
⟹ 1H2
(D
+ 1H2
D
⟶ 2He3
He3
+ 0n1
n)
3.2 MeV
⟹ 1H2
(D
+ 1H2
D
⟶ 1H3
T
+ 1p1
p)
4.0 MeV
⟹ 1H2
(D
+ 1H3
T
⟶ 2He4
He4
+ 0n1
n)
17.6 MeV
⟹ 1H2
(D
+ 2He3
He3
⟶ 2He4
He4
+ 1p1
p)
18.3 MeV
Nuclear Hazard
There is danger to human health or environment exposed by radiation emanating from atomic nuclei of a given substance
or he possibility of uncontrolled explosion originating from the fission or fusion reaction is called Nuclear hazard. It’s a
natural or potential release of radioactive material from commercial nuclear plant or a transportation accident.

14. Combined Gas power plant
⟶ η1 =
W1
Q1
= 1 −
Q2
Q1
⟶ η2 =
W2
Q2
= 1 −
Q3
Q2
⟶ ηoverall = 1 −
Q3
Q1
= 1 −
(1 − η2)Q2
Q1
= 1 −
(1 − η2)(1 − η1)Q1
Q1
= η1 + η2 − η1η2
The worldwide demand for combined cycle is increasing. In its most basic form a gas
turbine exhausts into a HRSG (Heat Recovery Steam Generator) that supplies steam to
______________ is the most efficient system of ___________
Advantages of this cycle may be summarized as follows
 it has high η
 necessity of small amount of cooling water
 low investment cost
 simplicity in operation
 low environmental impact