1. Power Station or Power Plant and classification
Power Station or Power Plant :
A power station or power plant is a facility for the generation of electric
power. 'Power plant' is also used to refer to the engine in ships, aircraft and
other large vehicles. Some prefer to use the term energy center because it
more accurately describes what the plants do, which is the conversion of
other forms of energy, like chemical energy, gravitational potential energy or
heat energy into electrical energy. However, power plant is the most
common term in the U.S., while elsewhere power station and power plant
are both widely used, power station prevailing in many Commonwealth
countries and especially in the United Kingdom.
At the center of nearly all power stations is a generator, a rotating machine
that converts mechanical energy into electrical energy by creating relative
motion between a magnetic field and a conductor. The energy source
harnessed to turn the generator varies widely. It depends chiefly on what
fuels are easily available and the types of technology that the power
company has access to.
Classification of Power plants :
Power plants are classified by the type of fuel and the type of prime mover
installed.
By fuel
• In Thermal power stations, mechanical power is produced by a heat
engine, which transforms thermal energy, often from combustion of a
fuel, into rotational energy
• Nuclear power plants use a nuclear reactor's heat to operate a steam
turbine generator.
• Fossil fuel powered plants may also use a steam turbine generator or
in the case of Natural gas fired plants may use a combustion turbine.
• Geothermal power plants use steam extracted from hot underground
rocks.

2. • Renewable energy plants may be fuelled by waste from sugar cane,
municipal solid waste, landfill methane, or other forms of biomass.
• In integrated steel mills, blast furnace exhaust gas is a low-cost,
although low-energy-density, fuel.
• Waste heat from industrial processes is occasionally concentrated
enough to use for power generation, usually in a steam boiler and
turbine.
By prime mover
• Steam turbine plants use the pressure generated by expanding steam
to turn the blades of a turbine.
• Gas turbine plants use the heat from gases to directly operate the
turbine. Natural-gas fuelled turbine plants can start rapidly and so are
used to supply "peak" energy during periods of high demand, though
at higher cost than base-loaded plants.
• Combined cycle plants have both a gas turbine fired by natural gas,
and a steam boiler and steam turbine which use the exhaust gas from
the gas turbine to produce electricity. This greatly increases the
overall efficiency of the plant, and most new baseload power plants
are combined cycle plants fired by natural gas.
• Internal combustion Reciprocating engines are used to provide power
for isolated communities and are frequently used for small
cogeneration plants. Hospitals, office buildings, industrial plants, and
other critical facilities also use them to provide backup power in case
of a power outage. These are usually fuelled by diesel oil, heavy oil,
natural gas and landfill gas.
• Microturbines, Stirling engine and internal combustion reciprocating
engines are low cost solutions for using opportunity fuels, such as
landfill gas, digester gas from water treatment plants and waste gas
from oil production.
Other sources of energy :

3. Other power stations use the energy from wave or tidal motion, wind,
sunlight or the energy of falling water, hydroelectricity. These types of
energy sources are called renewable energy.
Thermal power plant,Advantages and Disadvantages
Thermal power plant or Steam power plant :
A generating station which converts heat energy of coal combustion in to
electrical energy is known as Thermal power plant or Steam power plant.
Some of its advantages and disadvantages are given below.
Advantages
1. The fuel used is quite cheap.
2. Less initial cost as compared to other generating plants.
3. It can beinstalled at any place iirespective of the existence of
coal. The coal can be transported to the site of the plant by rail or
road.
4. It require less space as compared to Hydro power plants.
5. Cost of generation is less than that of diesel power plants.
Disadvantages
1. It pollutes the atmosphere due to production of large amount of
smoke and fumes.
2. It is costlier in running cost as compared to Hydro electric plants.
Electric Power Systems and its components
Electric Power Systems :
Electric Power Systems, components that transform other types of energy
into electrical energy and transmit this energy to a consumer. The
production and transmission of electricity is relatively efficient and
inexpensive, although unlike other forms of energy, electricity is not easily

4. stored and thus must generally be used as it is being produced.
Components of an Electric Power System
A modern electric power system consists of six main components:
1. The power station
2. A set of transformers to raise the generated power to the high
voltages used on the transmission lines
3. The transmission lines
4. The substations at which the power is stepped down to the
voltage on the distribution lines
5. The distribution lines
6. the transformers that lower the distribution voltage to the level
used by the consumer's equipment.
Power Station
The power station of a power system consists of a prime mover, such as a
turbine driven by water, steam, or combustion gases that operate a system
of electric motors and generators. Most of the world's electric power is
generated in steam plants driven by coal, oil, nuclear energy, or gas. A
smaller percentage of the world’s electric power is generated by
hydroelectric (waterpower), diesel, and internal-combustion plants.
Transformers
Modern electric power systems use transformers to convert electricity into
different voltages. With transformers, each stage of the system can be
operated at an appropriate voltage. In a typical system, the generators at
the power station deliver a voltage of from 1,000 to 26,000 volts (V).
Transformers step this voltage up to values ranging from 138,000 to
765,000 V for the long-distance primary transmission line because higher
voltages can be transmitted more efficiently over long distances. At the
substation the voltage may be transformed down to levels of 69,000 to
138,000 V for further transfer on the distribution system. Another set of
transformers step the voltage down again to a distribution level such as
2,400 or 4,160 V or 15, 27, or 33 kilovolts (kV). Finally the voltage is

5. transformed once again at the distribution transformer near the point of use
to 240 or 120 V.
Transmission Lines
The lines of high-voltage transmission systems are usually composed of
wires of copper, aluminum, or copper-clad or aluminum-clad steel, which are
suspended from tall latticework towers of steel by strings of porcelain
insulators. By the use of clad steel wires and high towers, the distance
between towers can be increased, and the cost of the transmission line thus
reduced. In modern installations with essentially straight paths, high-voltage
lines may be built with as few as six towers to the kilometer. In some areas
high-voltage lines are suspended from tall wooden poles spaced more closely
together. For lower voltage distribution lines, wooden poles are generally
used rather than steel towers. In cities and other areas where open lines
create a safety hazard or are considered unattractive, insulated underground
cables are used for distribution. Some of these cables have a hollow core
through which oil circulates under low pressure. The oil provides temporary
protection from water damage to the enclosed wires should the cable
develop a leak. Pipe-type cables in which three cables are enclosed in a pipe
filled with oil under high pressure (14 kg per sq cm/200 psi) are frequently
used. These cables are used for transmission of current at voltages as high
as 345,000 V (or 345 kV).
Supplementary Equipment
Any electric-distribution system involves a large amount of supplementary
equipment to protect the generators, transformers, and the transmission
lines themselves. The system often includes devices designed to regulate the
voltage or other characteristics of power delivered to consumers.
To protect all elements of a power system from short circuits and overloads,
and for normal switching operations, circuit breakers are employed. These
breakers are large switches that are activated automatically in the event of a
short circuit or other condition that produces a sudden rise of current.
Because a current forms across the terminals of the circuit breaker at the
moment when the current is interrupted, some large breakers (such as those
used to protect a generator or a section of primary transmission line) are

6. immersed in a liquid that is a poor conductor of electricity, such as oil, to
quench the current. In large air-type circuit breakers, as well as in oil
breakers, magnetic fields are used to break up the current. Small air-circuit
breakers are used for protection in shops, factories, and in modern home
installations. In residential electric wiring, fuses were once commonly
employed for the same purpose. A fuse consists of a piece of alloy with a low
melting point, inserted in the circuit, which melts, breaking the circuit if the
current rises above a certain value. Most residences now use air-circuit
breakers.
Power Failures,Protection from outages and Restoration
Power Failures :
A power outage (Also power cut, power failure or power loss) is the loss of
the electricity supply to an area.
The reasons for a power failure can for instance be a defect in a power
station, damage to a power line or other part of the distribution system, a
short circuit, or the overloading of electricity mains. While the developed
countries enjoy a highly uninterrupted supply of electric power all the time,
many developing countries have acute power shortage as compared to the
demand. Countries such as Pakistan have several hours of daily power-cuts
in almost all cities and villages except the metropolitan cities and the state
capitals. Wealthier people in these countries may use a power-inverter or a
diesel-run electric generator at their homes during the power-cut.
A power outage may be referred to as a blackout if power is lost completely,
or as a brownout if the voltage level is below the normal minimum level
specified for the system, or sometimes referred to as a short circuit when
the loss of power occurs over a short time (usually seconds). Systems
supplied with three-phase electric power also suffer brownouts if one or
more phases are absent, at reduced voltage, or incorrectly phased. Such
malfunctions are particularly damaging to electric motors. Some brownouts,
called voltage reductions, are made intentionally to prevent a full power
outage. 'Load shedding' is a common term for a controlled way of rotating
available generation capacity between various districts or customers, thus

7. avoiding total wide area blackouts.
Power failures are particularly critical for hospitals, since many life-critical
medical devices and tasks require power. For this reason hospitals, just like
many enterprises (notably colocation facilities and other datacenters), have
emergency power generators which are typically powered by diesel fuel and
configured to start automatically, as soon as a power failure occurs. In most
third world countries, power cuts go unnoticed by most citizens of upscale
means, as maintaining an uninterruptible power supply is often considered
an essential facility of a home.
Power outage may also be the cause of sanitary sewer overflow, a condition
of discharging raw sewage into the environment. Other life-critical systems
such as telecommunications are also required to have emergency power.
Telephone exchange rooms usually have arrays of lead-acid batteries for
backup and also a socket for connecting a diesel generator during extended
periods of outage.
Power outages may also be caused by terrorism (attacking power plants or
electricity pylons) in developing countries. The Shining Path movement was
the first to copy this tactic from Mao Zedong.
Live Examples of breakdown in interconnected grid system
In most parts of the world, local or national electric utilities have joined in
grid systems. The linking grids allow electricity generated in one area to be
shared with others. Each utility that agrees to share gains an increased
reserve capacity, use of larger, more efficient generators, and the ability to
respond to local power failures by obtaining energy from a linking grid.
These interconnected grids are large, complex systems that contain
elements operated by different groups. These systems offer the opportunity
for economic savings and improve overall reliability but can create a risk of
widespread failure. For example, a major grid-system breakdown occurred
on November 9, 1965, in eastern North America, when an automatic control
device that regulates and directs current flow failed in Queenston, Ontario,
causing a circuit breaker to remain open. A surge of excess current was
transmitted through the northeastern United States. Generator safety

8. switches from Rochester, New York, to Boston, Massachusetts, were
automatically tripped, cutting generators out of the system to protect them
from damage. Power generated by more southerly plants rushed to fill the
vacuum and overloaded these plants, which automatically shut themselves
off. The power failure enveloped an area of more than 200,000 sq km
(80,000 sq mi), including the cities of Boston; Buffalo, New York; Rochester,
New York; and New York City.
Similar grid failures, usually on a smaller scale, have troubled systems in
North America and elsewhere. On July 13, 1977, about 9 million people in
the New York City area were once again without power when major
transmission lines failed. In some areas the outage lasted 25 hours as
restored high voltage burned out equipment. These major failures are
termed blackouts.
The worst blackout in the history of the United States and Canada occurred
August 14, 2003, when 61,800 megawatts of electrical power was lost in an
area covering 50 million people. (One megawatt of electricity is roughly the
amount needed to power 750 residential homes.) The blackout affected such
major cities as Cleveland, Detroit, New York, Ottawa, and Toronto. Parts of
eight states—Connecticut, Massachusetts, Michigan, New Jersey, New York,
Ohio, Pennsylvania, and Vermont—and the Canadian provinces of Ontario
and Québec were affected. The blackout prompted calls to replace aging
equipment and raised questions about the reliability of the national power
grid.
The term brownout is often used for partial shutdowns of power, usually
deliberate, either to save electricity or as a wartime security measure. From
November 2000 through May 2001 California experienced a series of
planned brownouts to groups of customers, for a limited duration, in order to
reduce total system load and avoid a blackout due to alleged electrical
shortages. However, an investigation by the California Public Utilities
Commission into the alleged shortages later revealed that five energy
companies withheld electricity they could have produced. In 2002 the
commission concluded that the withholding of electricity contributed to an
“unconscionable, unjust, and unreasonable electricity price spike.” California

9. state utilities paid $20 billion more for energy in 2000 than in 1999 as a
result, the head of the commission found.
The commission also cited the role of the Enron Corporation in the California
brownouts. In June 2003 the Federal Energy Regulatory Commission (FERC)
barred Enron from selling electricity and natural gas in the United States
after conducting a probe into charges that Enron manipulated electricity
prices during California’s energy crisis. In the same month the Federal
Bureau of Investigation arrested an Enron executive on charges of
manipulating the price of electricity in California. Two other Enron
employees, known as traders because they sold electricity, had pleaded
guilty to similar charges. See also Enron Scandal.
Despite the potential for rare widespread problems, the interconnected grid
system provides necessary backup and alternate paths for power flow,
resulting in much higher overall reliability than is possible with isolated
systems. National or regional grids can also cope with unexpected outages
such as those caused by storms, earthquakes, landslides, and forest fires, or
due to human error or deliberate acts of sabotage.
Protecting the power system from outages
In power supply networks, the power generation and the electrical load
(demand) must be very close to equal every second to avoid overloading of
network components, which can severely damage them. In order to prevent
this, parts of the system will automatically disconnect themselves from the
rest of the system, or shut themselves down to avoid damage. This is
analogous to the role of relays and fuses in households.
Under certain conditions, a network component shutting down can cause
current fluctuations in neighboring segments of the network, though this is
unlikely, leading to a cascading failure of a larger section of the network.
This may range from a building, to a block, to an entire city, to the entire
electrical grid.
Modern power systems are designed to be resistant to this sort of cascading
failure, but it may be unavoidable (see below). Moreover, since there is no

10. short-term economic benefit to preventing rare large-scale failures, some
observers have expressed concern that there is a tendency to erode the
resilience of the network over time, which is only corrected after a major
failure occurs. It has been claimed that reducing the likelihood of small
outages only increases the likelihood of larger ones. In that case, the short-
term economic benefit of keeping the individual customer happy increases
the likelihood of large-scale blackouts.
Power Analytics
Power Analytics is the term used to describe the management of electrical
power distribution, consumption, and preventative maintenance throughout
a large organization’s facilities, particularly organizations with high electrical
power requirements. For such facilities, electrical power problems – including
the worst-case scenario, a full power outage – could have a devastating
serious impact. Additionally, it could jeopardize the health and safety of
individuals within the facility or in the surrounding community.
Power Analytics use complex mathematical algorithms to detect variations
within an organization’s power infrastructure (measurements such as
voltage, current, power factor, etc.). Such variations could be early
indications of longer-term power problems; when a Power Analytics system
detects such variations, it will begin to diagnose the source of the variation,
surrounding components, and then the complete electrical power
infrastructure. Such systems will – after fully assessing the location and
potential magnitude of the problem – predict when and where the potential
problem will occur, as well as recommend the preventative maintenance
required preempting the problem from occurring.
Restoring power after a wide-area outage
Restoring power after a wide-area outage can be difficult, as power stations
need to be brought back on-line. Normally, this is done with the help of
power from the rest of the grid. In the absence of grid power, a so-called
black start needs to be performed to bootstrap the power grid into
operation.

11. Latest Power Outages,Causes and factors contributing to it
Latest Power Outages :
Electricity Blackout in Germany on November 4th 2006 -even France, Italy,
Spain and other countries were affected.
One of the worst and most dramatic power failures in three decades plunged
millions of Europeans into darkness over the weekend, halting trains,
trapping dozens in lifts and prompting calls for a central European power
authority. The blackout, which originated in north-western Germany, also
struck Paris and 15 French regions, and its effects were felt in Austria,
Belgium, Italy and Spain. In Germany, around 100 trains were delayed.
Additional Power Outages
09/24/2006 On September 24th afternoon 1.30pm Pakistan was hit by a
nationwide blackout. Millions of homes across Pakistan were left without
power for several hours. Power has been restored in capital Islamabad after
over a two-hour breakdown. The outage was caused due to a fault that
occurred during maintenance of a high-tension transmission line.
07/12/2006 Electricity Blackout in Auckland (New Zealand) - 700,000 people
without electricity for up to 10 hours. An earth wire, which snapped in high
winds, fell into Transpower's Otahuhu substation, damaging 110 kilovolt
supply lines. The cause - a simple metal shackle.
11/25/2005 Electricity Blackout in Münsterland - 250,000 people without
electricity for up to six days. Ice and storm had caused serious damage to
the network , leading to the blackout.
10/24/2005 -11/11/2005 Hurricane Wilma caused loss of power for most of
South Florida and Southwest Florida, with hundreds of thousands of
customers still powerless a week later, and full restoration not complete.
09/12/2005 A blackout in Los Angeles affected millions in California.

12. 08/29/2005 Millions of Louisiana, Mississippi and Alabama residents lost
power after a stronger Hurricane Katrina badly damaged the power grid.
08/26/2005 On 1.3 Million People in South Florida lost power due to downed
trees and power lines caused by the then minimal Hurricane Katrina. Most
customers affected were without power for four days, and some customers
had no power for up to one week.
08/22/2005 All of southern and central Iraq, including parts of the capital
Baghdad, all of the second largest city Basra and the only port Umm Qasr
went out of power for more than 7 hours after a feeder line was sabotaged
by insurgents, causing a cascading effect shutting down multiple power
plants.
08/18/2005 Almost 100 million people on Java Island, the main island of
Indonesia which the capital Jakarta is on, and the isle of Bali, lost power for
7 hours. In terms of population affected, the 2005 Java-Bali Blackout was
the biggest in history.
05/25/2005 On most part of Moscow was without power from 11:00 MSK
(+0300 UTC). Approximately ten million people were affected. Power was
restored within 24 hours.
09/04/2004 On five million people in Florida were without power at one point
due to Hurricane Frances, one of the most widespread outages ever due to a
hurricane.
12/20/2003 Apower failure hit San Francisco, affecting 120,000 people.
09/27/2003- 09/28/2003 Italy blackout - a power failure affected all of Italy
except Sardinia, cutting service to more than 56 million people.
09/23/2003 A power failure affected 5 million people in Denmark and
southern Sweden.
09/02/2003 A power failure affected 5 states (out of 13) in Malaysia
(including the capital Kuala Lumpur) for 5 hours starting at 10 am local time.

13. 08/28/2003 There was a 2003 London blackout on which won worldwide
headlines such as "Power cut cripples London" but in fact only affected
500,000 people.
Direct Causes and Contributing Factors to power outage:
• Failure to maintain adequate reactive power support
• Failure to ensure operation within secure limits
• Inadequate vegetation management
• Inadequate operator training
• Failure to identify emergency conditions and communicate that status
to neighboring systems
• Inadequate regional-scale visibility over the bulk power system.
Conclusions and Recommendations:
• Conductors contacting trees
• Ineffective visualization of power system conditions and lack of
situational awareness
• Ineffective communications
• Lack of training in recognizing and responding to emergencies
System Enhancement & Elimination of Bottlenecks
• Insufficient static and dynamic reactive power supply: FACTS
• Need to improve relay protection schemes and coordination
• On-Line Monitoring and Real-Time Security Assessment
• Increase of Reserve Capacity : HVDC / Generation
Electricity Power Blackout and Outage tips
Electricity Power Blackout and Outage tips :

14. • Assemble an emergency kit with:
(i) plenty of water (in general a minimum of 4 litres per person per
day is needed);Water can be partially supplemented with canned or
tetra pak juices.
(ii) ready-to-eat foods that do not need refridgeration.. Don't forget
the manually operated can opener;
(iii) flashlights;
(iv) portable radio;
(v) alkaline batteries, stored separately from electronic equipment
(such as radios) in case of battery leakage."Heavy duty batteries" are
not recommended for emergency use, as they have much less power
capability, a shorter shelf life and are much more prone to leaking.
(vi) money. Remember bank machines will not operate during a
blackout. You may want to keep a small amount of cash ready for this
situation.
• Place the emergency kit in a pre-designated location so that you can
find it in the dark.
• Do not use candles for lighting. Candles are in the top three causes of
household fires.
• Turn off all but one light or a radio so that you'll know when the power
returns.
• Check that the stove, ovens, electric kettles, irons, air conditioners
and (non-wall or ceiling mounted) lights are off. This can be serious
safety issues if you forget you have left some of these devices on.
Also by keeping them turned off will prevent heavy start-up loads
which could cause a second blackout when the utilities restart the
power.
• Turn off or unplug home electronics and computers to protect them
from damage when the electricity returns, in case of power surges.
• Listen to local radio and television for updated information. (The
reason for having a battery powered (ie. portable) radio.)
• Keep refrigerator and freezer doors closed. A full modern freezer will
stay frozen for up to 48 hours; partially full freezers for 24 hours.
Most food in the fridge will last 24 hours except dairy products, which

15. should be discarded after six hours. These estimates decrease each
time the refrigerator door is opened.
• Do not ration water (or juice). If you are thirsty you need the fluids. If
it is hot you need to drink plenty of fluids even if you do not feel
thirsty.
• Remember to provide plenty of fresh, cool water for your pets.
• Keep off the telephone unless it is an emergency, or for short periods
if it is for an important purpose such as checking up on your loved
ones, particularly people who have disabilities or infirmaties.
• In summer: open windows at opposing ends of a room to create a
cross breeze in the absence of air conditioning and electric fans.
• In summer: close blinds, curtains, drapes, windows and doors on the
sunny side of your home to block out the heat from the sun.
• In winter: open blinds, curtains and drapes during the day on the
sunny side of your home to let sunlight and its heat during the sunny
days, and close during the night. Otherwise keep them closed to keep
the heat in. You may also want to use window insulation kits or plastic
sheeting to add extra insulation to keep the heat in.
• In winter: make sure you have extra blankets. Also make sure you
have a bucket and a wet mop to soak up any water from frozen and
burst water pipes.
• While generally unnecessary and expensive, if you are using a gas-
powered generator, run it in a well-ventilated area and not in a closed
areas such as a room or garage. They can give off deadly carbon
monoxide fumes. And do not hook up the generator to your local
wiring, instead plug in the items you want or need into the generator.
For short-term use a much safer and cheaper alternative is an
Inverter with built-in battery.
• Do not use propane or other combustion-type heaters indoors due to
the probability of toxic carbon monoxide buildup.
Other notes:
• Water pressure may drop and even stop above a certain height in
high-rise buildings due to their water pumps losing power.

16. • Remember that electrical devices such as elevator will not work. You
can not predict when a blackout will strike to make a choice about
using elevators, but if a blackout does strike, check the elevators of
any of the building you are in to hear if there are people stuck; in
which case call up the fire department to get the people out.
• Electrically operated garage doors will not work. While landlords may
be able to hoist the heavy door up manually, some may not want to
do so for security purposes or because it volates the conditions of
their insurance policies.
Thermal Power Plant Layout and Operation
Thermal Power Plant Lay out :
The above diagram is the lay out of a simplified thermal power plant and the
below is also diagram of a thermal power plant.

17. The above diagram shows the simplest arrangement of Coal fired (Thermal)
power plant.
Main parts of the plant are
1. Coal conveyor 2. Stoker 3. Pulverizer 4. Boiler 5. Coal ash 6. Air
. .
preheater 7. Electrostatic precipitator 8. Smoke stack 9. Turbine 10.
. .
Condenser 11. Transformers 12. Cooling towers
.
13. Generator 14. High - votge power lines
.
Basic Operation :A thermal power plant basically works on Rankine cycle.
A
Coal conveyor : This is a belt type of arrangement.With this coal is
transported from coal storage place in power plant to the place near by
boiler.
Stoker : The coal which is brought near by boiler has to put in boiler
furnance for combustion.This stoker is a mechanical device for feeding coal
to a furnace.

18. Pulverizer : The coal is put in the boiler after pulverization.For this
pulverizer is used.A pulverizer is a device for grinding coal for combustion in
a furnace in a power plant.
Types of Pulverizers
Ball and Tube Mill
Ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to
three diameters in length, containing a charge of tumbling or cascading steel
balls, pebbles, or rods.
Tube mill is a revolving cylinder of up to five diameters in length used for
fine pulverization of ore, rock, and other such materials; the material, mixed
with water, is fed into the chamber from one end, and passes out the other
end as slime.
Ring and Ball
This type consists of two rings separated by a series of large balls. The lower
ring rotates, while the upper ring presses down on the balls via a set of
spring and adjuster assemblies. Coal is introduced into the center or side of
the pulverizer (depending on the design) and is ground as the lower ring
rotates causing the balls to orbit between the upper and lower rings. The
coal is carried out of the mill by the flow of air moving through it. The size of
the coal particals released from the grinding section of the mill is determined
by a classifer separator. These mills are typically produced by B&W (Babcock
and Wilcox).
Boiler : Now that pulverized coal is put in boiler furnance.Boiler is an
enclosed vessel in which water is heated and circulated until the water is
turned in to steam at the required pressure.
Coal is burned inside the combustion chamber of boiler.The products of
combustion are nothing but gases.These gases which are at high
temperature vaporize the water inside the boiler to steam.Some times this
steam is further heated in a superheater as higher the steam pressure and
temperature the greater efficiency the engine will have in converting the
heat in steam in to mechanical work. This steam at high pressure and
tempeture is used directly as a heating medium, or as the working fluid in a
prime mover to convert thermal energy to mechanical work, which in turn

19. may be converted to electrical energy. Although other fluids are sometimes
used for these purposes, water is by far the most common because of its
economy and suitable thermodynamic characteristics.
y
Classification of Boilers
Bolilers are classified as
Fire tube boilers : In fire tube boilers hot gases are passed through the
tubes and water surrounds these tubes. These are simple,compact and
rugged in construction.Depending on whether the tubes are vertical or
horizontal these are further classified as vertical and horizontal tube
boilers.In this since the water volume is more,circulation will be poor.So
they can't meet quickly the changes in steam demand.High pressures of
demand.High
steam are not possible,maximum pressure that can be attained is about
17.5kg/sq cm.Due to large quantity of water in the drain it requires more
time for steam raising.The steam attained is generally wet,economical for
low pressures.The outut of the boiler is also limited.
Water tube boilers : In these boilers water is inside the tubes and hot gases

20. are outside the tubes.They consists of drums and
tubes.They may contain any number of drums (you can see 2 drums in
fig).Feed water enters the boiler to one drum (here it is drum below the
s
boiler).This water circulates through the tubes connected external to
drums.Hot gases which surrounds these tubes wil convert the water in tubes
in to steam.This steam is passed through tubes and collected at the top of
collecte
the drum since it is of light weight.So the drums store steam and water
(upper drum).The entire steam is collected in one drum and it is taken out
from there (see in laout fig).As the movement of water in the water tubes is
high, so rate of heat transfer also becomes high resulting in greater
efficiency.They produce high pressure , easily accessible and can respond
quickly to changes in steam demand.These are also classified as
vertical,horizontal and inclined tube depending on the arrangement of the
arrangeme
tubes.These are of less weight and less liable to explosion.Large heating
surfaces can be obtained by use of large number of tubes.We can attain
pressure as high as 125 kg/sq cm and temperatures from 315 to 575
centigrade.
Superheater : Most of the modern boliers are having superheater and
reheater arrangement. Superheater is a component of a steam-generating
steam
unit in which steam, after it has left the boiler drum, is heated above its
saturation temperature. The amount of superheat added to the s steam is
influenced by the location, arrangement, and amount of superheater surface

21. installed, as well as the rating of the boiler. The superheater may consist of
one or more stages of tube banks arranged to effectively transfer heat from
the products of combustion.Superheaters are classified as convection ,
radiant or combination of these.
Reheater : Some of the heat of superheated steam is used to rotate the
turbine where it loses some of its energy.Reheater is also steam boiler
component in which heat is added to this intermediate-pressure steam,
which has given up some of its energy in expansion through the high-
pressure turbine. The steam after reheating is used to rotate the second
steam turbine (see Layout fig) where the heat is converted to mechanical
energy.This mechanical energy is used to run the alternator, which is
coupled to turbine , there by generating elecrical energy.
Condenser : Steam after rotating staem turbine comes to
condenser.Condenser refers here to the shell and tube heat exchanger (or
surface condenser) installed at the outlet of every steam turbine in Thermal
power stations of utility companies generally. These condensers are heat
exchangers which convert steam from its gaseous to its liquid state, also
known as phase transition. In so doing, the latent heat of steam is given out
inside the condenser. Where water is in short supply an air cooled condenser
is often used. An air cooled condenser is however significantly more
expensive and cannot achieve as low a steam turbine backpressure (and
therefore less efficient) as a surface condenser.
The purpose is to condense the outlet (or exhaust) steam from steam
turbine to obtain maximum efficiency and also to get the condensed steam
in the form of pure water, otherwise known as condensate, back to steam
generator or (boiler) as boiler feed water.
Why it is required ?
The steam turbine itself is a device to convert the heat in steam to
mechanical power. The difference between the heat of steam per unit weight
at the inlet to turbine and the heat of steam per unit weight at the outlet to
turbine represents the heat given out (or heat drop) in the steam turbine
which is converted to mechanical power. The heat drop per unit weight of

22. steam is also measured by the word enthalpy drop. Therefore the more the
conversion of heat per pound (or kilogram) of steam to mechanical power in
the turbine, the better is its performance or otherwise known as efficiency.
By condensing the exhaust steam of turbine, the exhaust pressure is
brought down below atmospheric pressure from above atmospheric
pressure, increasing the steam pressure drop between inlet and exhaust of
steam turbine. This further reduction in exhaust pressure gives out more
heat per unit weight of steam input to the steam turbine, for conversion to
mechanical power. Most of the heat liberated due to condensing, i.e., latent
heat of steam, is carried away by the cooling medium. (water inside tubes in
a surface condenser, or droplets in a spray condenser (Heller system) or air
around tubes in an air-cooled condenser).
Condensers are classified as (i) Jet condensers or contact condensers (ii)
Surface condensers.
In jet condensers the steam to be condensed mixes with the cooling water
and the temperature of the condensate and the cooling water is same when
leaving the condenser; and the condensate can't be recovered for use as
feed water to the boiler; heat transfer is by direct conduction.
In surface condensers there is no direct contact between the steam to be
condensed and the circulating cooling water. There is a wall interposed
between them through heat must be convectively transferred.The
temperature of the condensate may be higher than the temperature of the
cooling water at outlet and the condnsate is recovered as feed water to the
boiler.Both the cooling water and the condensate are separetely with
drawn.Because of this advantage surface condensers are used in thermal
power plants.Final output of condenser is water at low temperature is passed
to high pressure feed water heater,it is heated and again passed as feed
water to the boiler.Since we are passing water at high temperature as feed
water the temperature inside the boiler does not dcrease and boiler efficincy
also maintained.
Cooling Towers :The condensate (water) formed in the condeser after
condensation is initially at high temperature.This hot water is passed to
cooling towers.It is a tower- or building-like device in which atmospheric air

23. (the heat receiver) circulates in direct or indirect contact with warmer water
(the heat source) and the water is thereby cooled (see illustration). A cooling
tower may serve as the heat sink in a conventional thermodynamic process,
such as refrigeration or steam power generation, and when it is convenient
or desirable to make final heat rejection to atmospheric air. Water, acting as
the heat-transfer fluid, gives up heat to atmospheric air, and thus cooled, is
recirculated through the system, affording economical operation of the
process.
Two basic types of cooling towers are commonly used. One transfers the
heat from warmer water to cooler air mainly by an evaporation heat-transfer
process and is known as the evaporative or wet cooling tower.
Evaporative cooling towers are classified according to the means employed
for producing air circulation through them: atmospheric, natural draft, and
mechanical draft. The other transfers the heat from warmer water to cooler
air by a sensible heat-transfer process and is known as the nonevaporative
or dry cooling tower.
Nonevaporative cooling towers are classified as air-cooled condensers and as
air-cooled heat exchangers, and are further classified by the means used for
producing air circulation through them. These two basic types are sometimes
combined, with the two cooling processes generally used in parallel or
separately, and are then known as wet-dry cooling towers.
Evaluation of cooling tower performance is based on cooling of a specified
quantity of water through a given range and to a specified temperature
approach to the wet-bulb or dry-bulb temperature for which the tower is
designed. Because exact design conditions are rarely experienced in

24. operation, estimated performance curves are frequently prepared for a
specific installation, and provide a means for comparing the measured
performance with design conditions.
Economiser : Flue gases coming out of the boiler carry lot of heat.Function
of economiser is to recover some of the heat from the heat carried away in
the flue gases up the chimney and utilize for heating the feed water to the
boiler.It is placed in the passage of flue gases in between the exit from the
boiler and the entry to the chimney.The use of economiser results in saving
in coal consumption , increase in steaming rate and high boiler efficiency but
needs extra investment and increase in maintenance costs and floor area
required for the plant.This is used in all modern plants.In this a large
number of small diameter thin walled tubes are placed between two
headers.Feed water enters the tube through one header and leaves through
the other.The flue gases flow out side the tubes usually in counter flow.
Air preheater : The remaining heat of flue gases is utilised by air
preheater.It is a device used in steam boilers to transfer heat from the flue
gases to the combustion air before the air enters the furnace. Also known as
air heater; air-heating system. It is not shown in the lay out.But it is kept at
a place near by where the air enters in to the boiler.
The purpose of the air preheater is to recover the heat from the flue gas
from the boiler to improve boiler efficiency by burning warm air which
increases combustion efficiency, and reducing useful heat lost from the flue.
As a consequence, the gases are also sent to the chimney or stack at a lower
temperature, allowing simplified design of the ducting and stack. It also
allows control over the temperature of gases leaving the stack (to meet
emissions regulations, for example).After extracting heat flue gases are
passed to elctrostatic precipitator.
Electrostatic precipitator : It is a device which removes dust or other
finely divided particles from flue gases by charging the particles inductively
with an electric field, then attracting them to highly charged collector plates.
Also known as precipitator. The process depends on two steps. In the first
step the suspension passes through an electric discharge (corona discharge)

25. area where ionization of the gas occurs. The ions produced collide with the
suspended particles and confer on them an electric charge. The charged
particles drift toward an electrode of opposite sign and are deposited on the
electrode where their electric charge is neutralized. The phenomenon would
be more correctly designated as electrodeposition from the gas phase.
The use of electrostatic precipitators has become common in numerous
industrial applications. Among the advantages of the electrostatic
precipitator are its ability to handle large volumes of gas, at elevated
temperatures if necessary, with a reasonably small pressure drop, and the
removal of particles in the micrometer range. Some of the usual applications
are: (1) removal of dirt from flue gases in steam plants; (2) cleaning of air
to remove fungi and bacteria in establishments producing antibiotics and
other drugs, and in operating rooms; (3) cleaning of air in ventilation and air
conditioning systems; (4) removal of oil mists in machine shops and acid
mists in chemical process plants; (5) cleaning of blast furnace gases; (6)
recovery of valuable materials such as oxides of copper, lead, and tin; and
(7) separation of rutile from zirconium sand.
Smoke stack :A chimney is a system for venting hot flue gases or smoke
from a boiler, stove, furnace or fireplace to the outside atmosphere. They
are typically almost vertical to ensure that the hot gases flow smoothly,
drawing air into the combustion through the chimney effect (also known as
the stack effect). The space inside a chimney is called a flue. Chimneys may
be found in buildings, steam locomotives and ships. In the US, the term
smokestack (colloquially, stack) is also used when referring to locomotive
chimneys. The term funnel is generally used for ship chimneys and
sometimes used to refer to locomotive chimneys.Chimneys are tall to
increase their draw of air for combustion and to disperse pollutants in the
flue gases over a greater area so as to reduce the pollutant concentrations in
compliance with regulatory or other limits.
Generator : An alternator is an electromechanical device that converts
mechanical energy to alternating current electrical energy. Most alternators
use a rotating magnetic field. Different geometries - such as a linear
alternator for use with stirling engines - are also occasionally used. In

26. principle, any AC generator can be called an alternator, but usually the word
refers to small rotating machines driven by automotive and other internal
combustion engines.
Transformers :It is a device that transfers electric energy from one
alternating-current circuit to one or more other circuits, either increasing
(stepping up) or reducing (stepping down) the voltage. Uses for
transformers include reducing the line voltage to operate low-voltage
devices (doorbells or toy electric trains) and raising the voltage from electric
generators so that electric power can be transmitted over long distances.
Transformers act through electromagnetic induction; current in the primary
coil induces current in the secondary coil. The secondary voltage is
calculated by multiplying the primary voltage by the ratio of the number of
turns in the secondary coil to that in the primary.
Boiling Water Reactor (BWR) - Advantages and Disadvantages
Boiling Water Reactor (BWR)
A boiling water reactor (BWR) is a type of light-water nuclear reactor
developed by the General Electric Company in the mid 1950s.
1.Reactor pressure vessel 2.Fuel rods 3. Control rod 4.Circulating pump
5.Control rod drive 6.Fresh steam 7. Feedwater 8.High pressure turbine
9.Low pressure turbine 10.Generator 11.Exciter 12.Condenser 13.Cooling
water 14.Preheater 15.Feedwater pump 16. Cooling water pump
17.Concrete shield

27. The above diagram shows BWR and its main parts.The BWR is characterized
by two-phase fluid flow (water and steam) in the upper part of the reactor
core. Light water (i.e., common distilled water) is the working fluid used to
conduct heat away from the nuclear fuel. The water around the fuel
elements also "thermalizes" neutrons, i.e., reduces their kinetic energy,
which is necessary to improve the probability of fission of fissile fuel. Fissile
fuel material, such as the U-235 and Pu-239 isotopes, have large capture
cross sections for thermal neutrons.
In a boling water reactor, light water (H2O) plays the role of moderator and
coolant, as well. In this case the steam is generted in the reactor it self.As
you can see in the diagrm feed water enters the reactor pressure vessel at
the bottom and takes up the heat generated due to fission of fuel (fuel rods)
and gets converted in to steam.
Part of the water boils away in the reactor pressure vessel, thus a mixture of
water and steam leaves the reactor core. The so generated steam directly
goes to the turbine, therefore steam and moisture must be separated (water
drops in steam can damage the turbine blades). Steam leaving the turbine is
condensed in the condenser and then fed back to the reactor after
preheating. Water that has not evaporated in the reactor vessel accumulates
at the bottom of the vessel and mixes with the pumped back feedwater.
Since boiling in the reactor is allowed, the pressure is lower than that of the
PWRs: it is about 60 to 70 bars. The fuel is usually uranium dioxide.
Enrichment of the fresh fuel is normally somewhat lower than that in a PWR.
The advantage of this type is that - since this type has the simplest
construction - the building costs are comparatively low. 22.5% of the total
power of presently operating nuclear power plants is given by BWRs.
Feedwater
Inside of a BWR reactor pressure vessel (RPV), feedwater enters through
nozzles high on the vessel, well above the top of the nuclear fuel assemblies
(these nuclear fuel assemblies constitute the "core") but below the water
level. The feedwater is pumped into the RPV from the condensers located
underneath the low pressure turbines and after going through feedwater

28. heaters that raise its temperature using extraction steam from various
turbine stages.
The feedwater enters into the downcomer region and combines with water
exiting the water separators. The feedwater subcools the saturated water
from the steam separators. This water now flows down the downcomer
region, which is separated from the core by a tall shroud. The water then
goes through either jet pumps or internal recirculation pumps that provide
additional pumping power (hydraulic head). The water now makes a 180
degree turn and moves up through the lower core plate into the nuclear core
where the fuel elements heat the water. When the flow moves out of the
core through the upper core plate, about 12 to 15% of the flow by volume is
saturated steam.
The heating from the core creates a thermal head that assists the
recirculation pumps in recirculating the water inside of the RPV. A BWR can
be designed with no recirculation pumps and rely entirely on the thermal
head to recirculate the water inside of the RPV. The forced recirculation head
from the recirculation pumps is very useful in controlling power, however.
The thermal power level is easily varied by simply increasing or decreasing
the speed of the recirculation pumps.
The two phase fluid (water and steam) above the core enters the riser area,
which is the upper region contained inside of the shroud. The height of this
region may be increased to increase the thermal natural recirculation
pumping head. At the top of the riser area is the water separator. By
swirling the two phase flow in cyclone separators, the steam is separated
and rises upwards towards the steam dryer while the water remains behind
and flows horizontally out into the downcomer region. In the downcomer
region, it combines with the feedwater flow and the cycle repeats.
The saturated steam that rises above the separator is dried by a chevron
dryer structure. The steam then exists the RPV through four main steam
lines and goes to the turbine.

29. Control systems
Reactor power is controlled via two methods: by inserting or withdrawing
control rods and by changing the water flow through the reactor core.
Positioning (withdrawing or inserting) control rods is the normal method for
controlling power when starting up a BWR. As control rods are withdrawn,
neutron absorption decreases in the control material and increases in the
fuel, so reactor power increases. As control rods are inserted, neutron
absorption increases in the control material and decreases in the fuel, so
reactor power decreases. Some early BWRs and the proposed ESBWR
designs use only natural ciculation with control rod positioning to control
power from zero to 100% because they do not have reactor recirculation
systems.
Changing (increasing or decreasing) the flow of water through the core is
the normal and convenient method for controlling power. When operating on
the so-called "100% rod line," power may be varied from approximately
70% to 100% of rated power by changing the reactor recirculation system
flow by varying the speed of the recirculation pumps. As flow of water
through the core is increased, steam bubbles ("voids") are more quickly
removed from the core, the amount of liquid water in the core increases,
neutron moderation increases, more neutrons are slowed down to be
absorbed by the fuel, and reactor power increases. As flow of water through
the core is decreased, steam voids remain longer in the core, the amount of
liquid water in the core decreases, neutron moderation decreases, fewer
neutrons are slowed down to be absorbed by the fuel, and reactor power
decreases.
Steam Turbines
Steam produced in the reactor core passes through steam separators and
dryer plates above the core and then directly to the turbine, which is part of
the reactor circuit. Because the water around the core of a reactor is always
contaminated with traces of radionuclides, the turbine must be shielded
during normal operation, and radiological protection must be provided during
maintenance. The increased cost related to operation and maintenance of a
BWR tends to balance the savings due to the simpler design and greater

30. thermal efficiency of a BWR when compared with a PWR. Most of the
radioactivity in the water is very short-lived (mostly N-16, with a 7 second
half life), so the turbine hall can be entered soon after the reactor is shut
down.
Safety
Like the pressurized water reactor, the BWR reactor core continues to
produce heat from radioactive decay after the fission reactions have
stopped, making nuclear meltdown possible in the event that all safety
systems have failed and the core does not receive coolant. Also like the
pressurized water reactor, a boiling-water reactor has a negative void
coefficient, that is, the thermal output decreases as the proportion of steam
to liquid water increases inside the reactor. However, unlike a pressurized
water reactor which contains no steam in the reactor core, a sudden
increase in BWR steam pressure (caused, for example, by a blockage of
steam flow from the reactor) will result in a sudden decrease in the
proportion of steam to liquid water inside the reactor. The increased ratio of
water to steam will lead to increased neutron moderation, which in turn will
cause an increase in the power output of the reactor. Because of this effect
in BWRs, operating components and safety systems are designed to ensure
that no credible, postulated failure can cause a pressure and power increase
that exceeds the safety systems' capability to quickly shutdown the reactor
before damage to the fuel or to components containing the reactor coolant
can occur.
In the event of an emergency that disables all of the safety systems, each
reactor is surrounded by a containment building designed to seal off the
reactor from the environment.

31. Comparison with other reactors
Light water is ordinary water. In comparison, some other water-cooled
water
reactor types use heavy water. In heavy water, the deuterium isotope of
hydrogen replaces the common hydrogen atoms in the water molecules
(D2O instead of H2O, molecular weight 20 instea of 18).
instead
The Pressurized Water Reactor (PWR) was the first type of light-water
light
reactor developed because of its application to submarine propulsion. The
civilian motivation for the BWR is reducing costs for commercial applications
through design simplification and lower pressure components. In naval
cation
reactors, BWR designs are used when natural circulation is specified for its
quietness. The description of BWRs below describes civilian reactor plants in
which the same water used for reactor cooling is also used in the Rankine
cycle turbine generators. A Naval BWR is designed like a PWR that has both
primary and secondary loops.
In contrast to the pressurized water reactors that utilize a primary and
secondary loop, in civilian BWRs the steam going to the turbine that powers
turbine
the electrical generator is produced in the reactor core rather than in steam
generators or heat exchangers. There is just a single circuit in a civilian BWR
in which the water is at lower pressure (about 75 times atmospheric
pressure) compared to a PWR so that it boils in the core at about 285°C. The
mpared
reactor is designed to operate with steam comprising 12–15% of the volume
12 15%

32. of the two-phase coolant flow (the "void fraction") in the top part of the
core, resulting in less moderation, lower neutron efficiency and lower power
density than in the bottom part of the core. In comparison, there is no
significant boiling allowed in a PWR because of the high pressure maintained
in its primary loop (about 158 times atmospheric pressure).
Advantages
• The reactor vessel and associated components operate at a
substantially lower pressure (about 75 times atmospheric pressure)
compared to a PWR (about 158 times atmospheric pressure).
• Pressure vessel is subject to significantly less irradiation compared to a
PWR, and so does not become as brittle with age.
• Operates at a lower nuclear fuel temperature.
• Fewer components due to no steam generators and no pressurizer
vessel. (Older BWRs have external recirculation loops, but even this
piping is eliminated in modern BWRs, such as the ABWR.)
• Lower risk (probability) of a rupture causing loss of coolant compared
to a PWR, and lower risk of a severe accident should such a rupture
occur. This is due to fewer pipes, fewer large diameter pipes, fewer
welds and no steam generator tubes.
• Measuring the water level in the pressure vessel is the same for both
normal and emergency operations, which results in easy and intuitive
assessment of emergency conditions.
• Can operate at lower core power density levels using natural
circulation without forced flow.
• A BWR may be designed to operate using only natural circulation so
that recirculation pumps are eliminated entirely. (The new ESBWR
design uses natural circulation.)
Disadvantages
• Complex operational calculations for managing the utilization of the
nuclear fuel in the fuel elements during power production due to "two
phase fluid flow" (water and steam) in the upper part of the core (less

33. of a factor with modern computers). More incore nuclear
instrumentation is required.
• Much larger pressure vessel than for a PWR of similar power, with
correspondingly higher cost. (However, the overall cost is reduced
because a modern BWR has no main steam generators and associated
piping.)
• Contamination of the turbine by fission products.
• Shielding and access control around the steam turbine are required
during normal operations due to the radiation levels arising from the
steam entering directly from the reactor core. Additional precautions
are required during turbine maintenance activities compared to a
PWR.
• Control rods are inserted from below for current BWR designs. There
are two available hydraulic power sources that can drive the control
rods into the core for a BWR under emergency conditions. There is a
dedicated high pressure hydraulic accumulator and also the pressure
inside of the reactor pressure vessel available to each control rod.
Either the dedicated accumulator (one per rod) or reactor pressure is
capable of fully inserting each rod. Most other reactor types use top
entry control rods that are held up in the withdrawn position by
electromagnets, causing them to fall into the reactor by gravity if
power is lost.
Classification of Nuclear Reactors
Classification of Nuclear Reactors
Nuclear Reactors, specifically fission reacors, are classified by several
methods, a brief outline of these classification schemes is given below.
Classification by use
Research reactors : Typically reactors used for research and training,
materials testing, or the production of radioisotopes for medicine and
industry. These are much smaller than power reactors or those propelling
ships, and many are on university campuses. There are about 280 such
reactors operating, in 56 countries. Some operate with high-enriched

34. uranium fuel, and international efforts are underway to substitute low-
enriched fuel.
Production reactors
Power reactors
Propulsion reactors
Classification by moderator material
Graphite moderated reactors
water moderated reactors
• Light water moderated reactors (LWRs)
• Heavy Water moderated reactors
Classification by coolant
Gas cooled reactor
Liquid metal cooled reactor
Water cooled reactor
• Pressure water reactor
• Boiling water reactor
Classification by type of nuclear reaction
Fast Reactors
Thermal reactors
Classification by role in the fuel cycle
Breeder reactors
burner reactors
Classification by Generation
Generation II reactor
Generation III reactor
Generation IV reactor
Classification by phase of fuel
Solid fueled

35. Fluid fueled
Gas Fueled
The Nuclear Fuel Cycle
The Nuclear Fuel Cycle
• The nuclear fuel cycle is the series of industrial processes which
involve the production of electricity from uranium in nuclear power
reactors.
• Uranium is a relatively common element that is found throughout the
world. It is mined in a number of countries and must be processed
before it can be used as fuel for a nuclear reactor.
• Electricity is created by using the heat generated in a nuclear reactor
to produce steam and drive a turbine connected to a generator.
• Fuel removed from a reactor, after it has reached the end of its useful
life, can be reprocessed to produce new fuel.
The various activities associated with the production of electricity from
nuclear reactions are referred to collectively as the nuclear fuel cycle. The
nuclear fuel cycle starts with the mining of uranium and ends with the
disposal of nuclear waste. With the reprocessing of used fuel as an option for
nuclear energy, the stages form a true cycle.
Uranium
Uranium is a slightly radioactive metal that occurs throughout the earth's
crust. It is about 500 times more abundant than gold and about as common
as tin. It is present in most rocks and soils as well as in many rivers and in
sea water. It is, for example, found in concentrations of about four parts per
million (ppm) in granite, which makes up 60% of the earth's crust. In
fertilisers, uranium concentration can be as high as 400 ppm (0.04%), and
some coal deposits contain uranium at concentrations greater than 100 ppm
(0.01%). Most of the radioactivity associated with uranium in nature is in
fact due to other minerals derived from it by radioactive decay processes,
and which are left behind in mining and milling.

36. There are a number of areas around the world where the concentration of
uranium in the ground is sufficiently high that extraction of it for use as
nuclear fuel is economically feasible. Such concentrations are called ore.The
below figure represents various stages in Nuclear Fuel cycle
Uranium Mining
Both excavation and in situ techniques are used to recover uranium ore.
techniques
Excavation may be underground and open pit mining.
In general, open pit mining is used where deposits are close to the surface
and underground mining is used for deep deposits, typically greater than
120 m deep. Open pit mines require large holes on the surface, larger than
the size of the ore deposit, since the walls of the pit must be sloped to
prevent collapse. As a result, the quantity of material that must be removed
in order to access the ore may be large. Underground mines have relatively
Underground
small surface disturbance and the quantity of material that must be removed
to access the ore is considerably less than in the case of an open pit mine.
An increasing proportion of the world's uranium now comes from in situ
leaching (ISL), where oxygenated groundwater is circulated through a very
aching
porous orebody to dissolve the uranium and bring it to the surface. ISL may

37. be with slightly acid or with alkaline solutions to keep the uranium in
solution. The uranium is then recovered from the solution as in a
conventional mill.
The decision as to which mining method to use for a particular deposit is
governed by the nature of the orebody, safety and economic considerations.
In the case of underground uranium mines, special precautions, consisting
primarily of increased ventilation, are required to protect against airborne
radiation exposure.
Uranium Milling
Milling, which is generally carried out close to a uranium mine, extracts the
uranium from the ore. Most mining facilities include a mill, although where
mines are close together, one mill may process the ore from several mines.
Milling produces a uranium oxide concentrate which is shipped from the mill.
It is sometimes referred to as 'yellowcake' and generally contains more than
80% uranium. The original ore may contains as little as 0.1% uranium.
In a mill, uranium is extracted from the crushed and ground-up ore by
leaching, in which either a strong acid or a strong alkaline solution is used to
dissolve the uranium. The uranium is then removed from this solution and
precipitated. After drying and usually heating it is packed in 200-litre drums
as a concentrate.
The remainder of the ore, containing most of the radioactivity and nearly all
the rock material, becomes tailings, which are emplaced in engineered
facilities near the mine (often in mined out pit). Tailings contain long-lived
radioactive materials in low concentrations and toxic materials such as heavy
metals; however, the total quantity of radioactive elements is less than in
the original ore, and their collective radioactivity will be much shorter-lived.
These materials need to be isolated from the environment.
Conversion
The product of a uranium mill is not directly usable as a fuel for a nuclear
reactor. Additional processing, generally referred to as enrichment, is
required for most kinds of reactors. This process requires uranium to be in
gaseous form and the way this is achieved is to convert it to uranium
hexafluoride, which is a gas at relatively low temperatures.

38. At a conversion facility, uranium is first refined to uranium dioxide, which
can be used as the fuel for those types of reactors that do not require
enriched uranium. Most is then converted into uranium hexafluoride, ready
for the enrichment plant. It is shipped in strong metal containers. The main
hazard of this stage of the fuel cycle is the use of hydrogen fluoride.
Enrichment
Natural uranium consists, primarily, of a mixture of two isotopes (atomic
forms) of uranium. Only 0.7% of natural uranium is "fissile", or capable of
undergoing fission, the process by which energy is produced in a nuclear
reactor. The fissile isotope of uranium is uranium 235 (U-235). The
remainder is uranium 238 (U-238).
In the most common types of nuclear reactors, a higher than natural
concentration of U-235 is required. The enrichment process produces this
higher concentration, typically between 3.5% and 5% U-235, by removing
over 85% of the U-238. This is done by separating gaseous uranium
hexafluoride into two streams, one being enriched to the required level and
known as low-enriched uranium. The other stream is progressively depleted
in U-235 and is called 'tails'.
There are two enrichment processes in large scale commercial use, each of
which uses uranium hexafluoride as feed: gaseous diffusion and gas
centrifuge. They both use the physical properties of molecules, specifically
the 1% mass difference, to separate the isotopes. The product of this stage
of the nuclear fuel cycle is enriched uranium hexafluoride, which is
reconverted to produce enriched uranium oxide.
Fuel fabrication
Reactor fuel is generally in the form of ceramic pellets. These are formed
from pressed uranium oxide which is sintered (baked) at a high temperature
(over 1400°C). The pellets are then encased in metal tubes to form fuel
rods, which are arranged into a fuel assembly ready for introduction into a
reactor. The dimensions of the fuel pellets and other components of the fuel
assembly are precisely controlled to ensure consistency in the characteristics
of fuel bundles.
In a fuel fabrication plant great care is taken with the size and shape of

39. processing vessels to avoid criticality (a limited chain reaction releasing
radiation). With low-enriched fuel criticality is most unlikely, but in plants
handling special fuels for research reactors this is a vital consideration.
Power generation
Inside a nuclear reactor the nuclei of U-235 atoms split (fission) and, in the
process, release energy. This energy is used to heat water and turn it into
steam. The steam is used to drive a turbine connected to a generator which
produces electricity. Some of the U-238 in the fuel is turned into plutonium
in the reactor core. The main plutonium isotope is also fissile and it yields
about one third of the energy in a typical nuclear reactor. The fissioning of
uranium is used as a source of heat in a nuclear power station in the same
way that the burning of coal, gas or oil is used as a source of heat in a fossil
fuel power plant.
As with as a coal-fired power station about two thirds of the heat is dumped,
either to a large volume of water (from the sea or large river, heating it a
few degrees) or to a relatively smaller volume of water in cooling towers,
using evaporative cooling (latent heat of vapourisation).
Used fuel
With time, the concentration of fission fragments and heavy elements
formed in the same way as plutonium in a fuel bundle will increase to the
point where it is no longer practical to continue to use the fuel. So after 12-
24 months the 'spent fuel' is removed from the reactor. The amount of
energy that is produced from a fuel bundle varies with the type of reactor
and the policy of the reactor operator.
Typically, some 36 million kilowatt-hours of electricity are produced from
one tonne of natural uranium. The production of this amount of electrical
power from fossil fuels would require the burning of over 20,000 tonnes of
black coal or 8.5 million cubic metres of gas.
Used fuel storage
When removed from a reactor, a fuel bundle will be emitting both radiation,
principally from the fission fragments, and heat. Used fuel is unloaded into a
storage pond immediately adjacent to the reactor to allow the radiation

40. levels to decrease. In the ponds the water shields the radiation and absorbs
the heat. Used fuel is held in such pools for several months to several years.
Depending on policies in particular countries, some used fuel may be
transferred to central storage facilities. Ultimately, used fuel must either be
reprocessed or prepared for permanent disposal.
Reprocessing
Used fuel is about 95% U-238 but it also contains about 1% U-235 that has
not fissioned, about 1% plutonium and 3% fission products, which are highly
radioactive, with other transuranic elements formed in the reactor. In a
reprocessing facility the used fuel is separated into its three components:
uranium, plutonium and waste, containing fission products. Reprocessing
enables recycling of the uranium and plutonium into fresh fuel, and produces
a significantly reduced amount of waste (compared with treating all used
fuel as waste).
Uranium and Plutonium Recycling
The uranium from reprocessing, which typically contains a slightly higher
concentration of U-235 than occurs in nature, can be reused as fuel after
conversion and enrichment, if necessary. The plutonium can be directly
made into mixed oxide (MOX) fuel, in which uranium and plutonium oxides
are combined.
In reactors that use MOX fuel, plutonium substitutes for the U-235 in normal
uranium oxide fuel.
Used fuel disposal
At the present time, there are no disposal facilities (as opposed to storage
facilities) in operation in which used fuel, not destined for reprocessing, and
the waste from reprocessing can be placed. Although technical issues related
to disposal have been addressed, there is currently no pressing technical
need to establish such facilities, as the total volume of such wastes is
relatively small. Further, the longer it is stored the easier it is to handle, due
to the progressive diminution of radioactivity. There is also a reluctance to
dispose of used fuel because it represents a significant energy resource
which could be reprocessed at a later date to allow recycling of the uranium

41. and plutonium. (There is a proposal to use it in Candu reactors directly as
fuel.)
A number of countries are carrying out studies to determine the optimum
approach to the disposal of spent fuel and wastes from reprocessing. The
general consensus favours its placement into deep geological repositories,
initially recoverable.
Wastes
Wastes from the nuclear fuel cycle are categorised as high-, medium- or
low-level wastes by the amount of radiation that they emit. These wastes
come from a number of sources and include:
• low-level waste produced at all stages of the fuel cycle;
• intermediate-level waste produced during reactor operation and by
reprocessing;
• high-level waste, which is waste containing fission products from
reprocessing, and in many countries, the used fuel itself.
The enrichment process leads to the production of much 'depleted' uranium,
in which the concentration of U-235 is significantly less than the 0.7% found
in nature. Small quantities of this material, which is primarily U-238, are
used in applications where high density material is required, including
radiation shielding and some is used in the production of MOX fuel. While U-
238 is not fissile it is a low specific activity radioactive material and some
precautions must, therefore, be taken in its storage or disposal.
Nuclear Energy,Nuclear Fuels
Nuclear Energy
Nuclei are made up of protons and neutron, but the mass of a nucleus is
always less than the sum of the individual masses of the protons and
neutrons which constitute it. The difference is a measure of the nuclear
binding energy which holds the nucleus together.
Nuclear energy is energy released from the atomic nucleus. Atoms are tiny
particles that make up every object in the universe. There is enormous

42. energy in the bonds that hold atoms together.This binding energy can be
calculated from the Einstein relationship: mass-energy equivalence formula
E = mc², in which E = energy, m = mass, and c = the speed of light in a
vacuum (a physical constant).The alpha particle gives binding energy of 28.3
MeV
Nuclear energy is released by several processes:
• Radioactive decay, where a radioactive nucleus decays spontaneously
into a lighter nucleus by emitting a particle;
• Endothermic nuclear reactions where two nuclei merge to produce two
different nuclei. The following two processes are particular examples:
• Fusion, two atomic nuclei fuse together to form a heavier nucleus;
• Fission, the breaking of a heavy nucleus into two nearly equal parts.
Nuclear Fuels
Nuclear fuel is any material that can be consumed to derive nuclear energy,
by analogy to chemical fuel that is burned to derive energy. By far the most
common type of nuclear fuel is heavy fissile elements that can be made to
undergo nuclear fission chain reactions in a nuclear fission reactor; nuclear
fuel can refer to the material or to physical objects (for example fuel bundles
composed of fuel rods) composed of the fuel material, perhaps mixed with
structural, neutron moderating, or neutron reflecting materials.
Not all nuclear fuels are used in fission chain reactions. For example, 238Pu
and some other elements are used to produce small amounts of nuclear
power by radioactive decay in radiothermal generators, and other atomic
batteries. Light isotopes such as 3H (tritium) are used as fuel for nuclear
fusion. If one looks at binding energy of specific isotopes, there can be an
energy gain from fusing most elements with a lower atomic number than
iron, and fissioning isotopes with a higher atomic number than iron.
The most common fissile nuclear fuels are natural urnium,enriched
uranium,plutonium and 233U.Natural uranium is the parent material.The
materials 235U,233U and 239Pu are called fissionable materials.The only
fissionable nuclear fuel occuring in nature is uraium of which 99.3% is 238U
and 0.7% is 235U and 234U is only a trace.Out of these isotopes only 235U

43. will fission in a chain reaction.The other two fissionable materials can be
produced artificially from 238U and 232Th which occur in nature are called
fertile materials.Out of the three fissionable materials 235U has some
advantages over the other two due to its higher fission
percentage.Fissionable materials 239Pu and 233U are formed in the nuclear
reactors during fission process from 238U and 232Th respectively due to
absorption of neutrons with out fission.Getting 239Pu process is called
conversion and getting 233U is called breeding.
Nuclear Fission
Nuclear Fission
Nuclear fission—also known as atomic fission—is a process in nuclear physics
and nuclear chemistry in which the nucleus of an atom splits into two or
more smaller nuclei as fission products, and usually some by-product
particles, Hence, fission is a form of elemental transmutation. The by-
products include free neutrons, photons usually in the form gamma rays,
and other nuclear fragments such as beta particles and alpha particles.
Fission of heavy elements is an exothermic reaction and can release
substantial amounts of useful energy both as gamma rays and as kinetic
energy of the fragments (heating the bulk material where fission takes
place).
Nuclear fission produces energy for nuclear power and to drive explosion of
nuclear weapons. Fission is useful as a power source because some
materials, called nuclear fuels, generate neutrons as part of the fission
process and undergo triggered fission when impacted by a free neutron.
Nuclear fuels can be part of a self-sustaining chain reaction that releases
energy at a controlled rate in a nuclear reactor or at a very rapid
uncontrolled rate in a nuclear weapon.
The amount of free energy contained in nuclear fuel is millions of times the
amount of free energy contained in a similar mass of chemical fuel such as
gasoline, making nuclear fission a very tempting source of energy; however,

44. the byproducts of nuclear fission are highly radioactive and remain so for
millennia, giving rise to a nuclear waste problem.
Splitting the Uranium Atom:
Uranium is the principle element used in nuclear reactors and in certain
types of atomic bombs. The specific isotope used is 235U. When a stray
neutron strikes a 235U nucleus, it is at first absorbed into it. This creates
236U. 236U is unstable and this causes the atom to fission. The fissioning of
236U can produce over twenty different products. However, the products'
masses always add up to 236. The following two equations are examples of
the different products that can be produced when 235U fissions:
235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY
235U + 1 neutron 2 neutrons + 92Sr + 140Xe + ENERGY
Let's discuss those reactions. In each of the
above reactions, 1 neutron splits the atom. When the atom is split, 1
additional neutron is released. This is how a chain reaction works. If more
235U is present, those 2 neutrons can cause 2 more atoms to split. Each of
those atoms releases 1 more neutron bringing the total neutrons to 4. Those
4 neutrons can strike 4 more 235U atoms, releasing even more neutrons.
The chain reaction will continue until all the 235U fuel is spent. This is
roughly what happens in an atomic bomb. It is called a runaway nuclear
reaction.
Where Does the Energy Come From?
In the section above we described what happens when an 235U atom
fissions. We gave the following equation as an example:
235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY
You might have been wondering, "Where does the energy come from?". The

45. mass seems to be the same on both sides of the reaction:
235 + 1 = 2 + 92 + 142 = 236
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 the individual
masses of its protons and neutrons, which is what those numbers represent.
Extra mass is a result of the binding energy that holds the protons and
neutrons of the nucleus 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.
Fission in Nuclear Reactors
To make large-scale use of the energy released in fission, one fission event
must trigger another, so that the process spreads thoughout the nuclear fuel
as in a set of dominos. The fact that more neutrons are produced in fission
than are consumed raises the possibility of a chain reaction. Such a reaction
can be either rapid (as in an atomic bomb) or controlled (as in a reactor).
In a nuclear reactor, control rods made of cadmium or graphite or some
other neutron-absorbing material are used to regulate the number of
neutrons. The more exposed control rods, the less neutrons and vice versa.
This also controls the multiplication factor k which is the ratio of the number
of neutrons present at the beginning of a particular generation to the
number present at the beginning of the next generation. For k=1, the
operation of the reactor is said to be exactly critical, which is what we wish it
to be for steady-power operation. Reactors are designed so that they are
inherently supercritical (k>1); the multiplication factor is then adjusted to
the critical operation by inserting the control rods.
An unavoidable feature of reactor operation is the accumulation of
radioactive wastes, including both fission products and heavy "transuranic"
nuclides such as plutonium and americium.
Nuclear Power

46. Nuclear Power
Nuclear power is the controlled use of nuclear reactions to release energy for
work including propulsion, heat, and the generation of electricity. Use of
nuclear power to do significant useful work is currently limited to nuclear
fission and radioactive decay. Nuclear energy is produced when a fissile
material, such as uranium-235 (235U), is concentrated such that nuclear
fission takes place in a controlled chain reaction and creates heat — which is
used to boil water, produce steam, and drive a steam turbine. The turbine
can be used for mechanical work and also to generate electricity. Nuclear
power provides 7% of the world's energy and 15.7% of the world's
electricity and is used to power most military submarines and aircraft
carriers.
The United States produces the most nuclear energy, with nuclear power
providing 20% of the electricity it consumes, while France produces the
highest percentage of its electrical energy from nuclear reactors—80% as of
2006. In the European Union as a whole, nuclear energy provides 30% of
the electricity.Nuclear energy policy differs between countries, and some
countries such as Austria, Australia and Ireland have no nuclear power
stations.
Concerns about nuclear power
The use of nuclear power is controversial because of the problem of storing
radioactive waste for indefinite periods, the potential for possibly severe
radioactive contamination by accident or sabotage, and the possibility that
its use in some countries could lead to the proliferation of nuclear weapons.
Proponents believe that these risks are small and can be further reduced by
the technology in the new reactors. They further claim that the safety record
is already good when compared to other fossil-fuel plants, that it releases
much less radioactive waste than coal power, and that nuclear power is a
sustainable energy source. Critics, including most major environmental
groups, claim nuclear power is an uneconomic and potentially dangerous
energy source with a limited fuel supply, especially compared to renewable
energy, and dispute whether the costs and risks can be reduced through
new technology.

47. There is concern in some countries over North Korea and Iran operating
research reactors and fuel enrichment plants, since those countries refuse
adequate IAEA oversight and are believed to be trying to develop nuclear
weapons. North Korea admits that it is developing nuclear weapons, while
the Iranian government vehemently denies the claims against Iran.
Several concerns about nuclear power have been expressed, and these
include:
• Concerns about nuclear reactor accidents, such as the Chernobyl
disaster
• Vulnerability of plants to attack or sabotage
• Use of nuclear waste as a weapon
• Health effects of nuclear power plants
• Nuclear proliferation
Nuclear Power Plant,Types, Advantages and Disadvantages
Nuclear Power Plant
Nuclear power is generated using Uranium, which is a metal mined in
various parts of the world.
The structure of a nuclear power plant in many aspects resembles to that of
a conventional thermal power station, since in both cases the heat produced
in the boiler (or reactor) is transported by some coolant and used to
generate steam. The steam then goes to the blades of a turbine and by
rotating it, the connected generator will produce electric energy. The steam
goes to the condenser, where it condenses, i.e. becomes liquid again. The
cooled down water afterwards gets back to the boiler or reactor, or in the
case of PWRs to the steam generator.

48. The great difference between a conventional and a nuclear power plant is
how heat is produced. In a fossile plant, oil, gas or coal is fired in the boiler,
which means that the chemical energy of the fuel is converted into heat. In a
nuclear power plant, however, energy that comes from fission reactions is
utilized.
How it works
• Nuclear power stations work in pretty much the same way as fossil
fuel-burning stations, except that a "chain reaction" inside a nuclear
burning
reactor makes the heat instead.
• The reactor uses Uranium rods as fuel, and the heat is generated by
nuclear fission. Neutrons smash into the nucleus of the uranium
atoms, which split roughly in half and release energy in the form of
heat.
• Carbon dioxide gas is pumped through the reactor to take the heat
away, and the hot gas then heats water to make steam.
• The steam drives turbines which drive generators. Modern nuclear
ves
power stations use the same type of turbines and generators as
conventional power stations.

49. In Britain, nuclear power stations are built on the coast, and use sea water
for cooling the steam ready to be pumped round again. This means that they
don't have the huge "cooling towers" seen at other power stations.
The reactor is controlled with "control rods", made of boron, which absorb
neutrons. When the rods are lowered into the reactor, they absorb more
neutrons and the fission process slows down. To generate more power, the
rods are raised and more neutrons can crash into uranium atoms.
Nuclear Power Plant Types
Several nuclear power plant (NPP) types are used for energy generation in
the world. The different types are usually classified based on the main
features of the reactor applied in them. The most widespread power plant
reactor types are:
• Light water reactors: both the moderator and coolant are light water
(H2O). To this category belong the pressurized water reactors (PWR)
and boiling water reactors (BWR).
• Heavy water reactors (CANDU): both the coolant and moderator are
heavy water (D2O).
• Graphite moderated reactors: in this category there are gas cooled
reactors (GCR) and light water cooled reactors (RBMK).
• Exotic reactors (fast breeder reactors and other experimental
installations).
• New generation reactors: reactors of the future.
Advantages
• Nuclear power costs about the same as coal, so it's not expensive to
make.
• The amount of fuel required is quite small ,therfore there is no
problem of transportation, storage etc.
• Does not produce smoke or carbon dioxide, so it does not contribute to
the greenhouse effect.
• Produces huge amounts of energy from small amounts of fuel.
• Produces small amounts of waste.

50. • The output control is most flexible.
• Nuclear power is reliable.
Disadvantages
• The fuel used is expensive and is difficult to recover.
• The fission by-products are generally radio active and may cause a
dangerous amount of radio active pollution.
• Although not much waste is produced, it is very, very dangerous. It
must be sealed up and buried for many years to allow the radioactivity
to die away.
• The initial capital cost is very high as compared to other power plants.
• Nuclear power is reliable, but a lot of money has to be spent on safety
- if it does go wrong, a nuclear accident can be a major disaster.
People are increasingly concerned about this - in the 1990's nuclear
power was the fastest-growing source of power in much of the world.
In 2005 it was the second slowest-growing.
• The cooling water requirements of a nuclear power plant are very
heavy.
Pelton Wheel
Pelton Wheel
A Pelton wheel, also called a Pelton turbine, is one of the most efficient types
of water turbines. It was invented by Lester Allan Pelton (1829-1908) in the
1870s, and is an impulse machine, meaning that it uses Newton's second
law to extract energy from a jet of fluid.

51. The pelton wheel turbine is a tangential flow impulse turbine, water flows
along the tangent to the path of the runner. Nozzles direct forceful streams
of water against a series of spoon-shaped buckets mounted around the edge
spoon shaped
of a wheel. Each bucket reverses the flow of water, leaving it with
water,
diminished energy. The resulting impulse spins the turbine. The buckets are
mounted in pairs, to keep the forces on the wheel balanced, as well as to
ensure smooth, efficient momentum transfer of the fluid jet to the wheel.
The Pelton wheel is most efficient in high head applications.
Since water is not a compressible fluid, almost all of the available energy is
extracted in the first stage of the turbine. Therefore, Pelton wheels have
only one wheel, unlike turbines that operate with compressible fluids.
Applications
Peltons are the turbine of choice for high head, low flow sites. However,
Pelton wheels are made in all sizes. There are multi-ton Pelton wheels
multi ton
mounted on vertical oil pad bearings in the generator houses of hydroelectric
plants. The largest units can be up to 200 megawatts. The smallest Pelton
wheels, only a few inches across, are used with household plumbing fixtures
to tap power from mountain streams with a few gallons per minute of flow,
untain
but these small units must have thirty meters or more of head. Depending
on water flow and design, Pelton wheels can operate with heads as small as
15 meters and as high as 1,800 meters.
In general, as the height of fall increases, less volume of water can generate
a bit more power. Energy is force times distance, in the instance of fluid flow
power is expressed as P = Constant x Pressure x Volume/t. The power P

52. grows linearly with flow rate and grows with f(Pressure^3/2.) Thus it is
usually best to seek as much head or pressure as possible in hydro designs
then go for flow rate.
Kaplan Turbine
Kaplan Turbine
The Kaplan turbine is a propeller-type water turbine that has adjustable
blades. It was developed in 1913 by the Austrian professor Viktor Kaplan.
The Kaplan turbine was an evolution of the Francis turbine. Its invention
allowed efficient power production in low head applications that was not
possible with Francis turbines.
Kaplan turbines are now widely used throughout the world in high-flow, low-
head power production.
The Kaplan turbine is an inward flow reaction turbine, which means that the
working fluid changes pressure as it moves through the turbine and gives up
its energy. The design combines radial and axial features.

53. The above figures shows flow in a Kaplan turbine. In the picture, pressure on
runner blades and hub surface is shown using colormapping (red = high,
blue = low).
The diameter of the runner of such machines is typically 5 to 8 meters.
The inlet is a scroll-shaped tube that wraps around the turbine's wicket gate.
Water is directed tangentially, through the wicket gate, and spirals on to a
propeller shaped runner, causing it to spin.
The outlet is a specially shaped draft tube that helps decelerate the water
and recover kinetic energy.
The turbine does not need to be at the lowest point of water flow, as long as
the draft tube remains full of water. A higher turbine location, however,
increases the suction that is imparted on the turbine blades by the draft
tube. The resulting pressure drop may lead to cavitation.
Variable geometry of the wicket gate and turbine blades allow efficient
operation for a range of flow conditions. Kaplan turbine efficiencies are
typically over 90%, but may be lower in very low head applications.
Applications
Kaplan turbines are widely used throughout the world for electrical power
production. They cover the lowest head hydro sites and are especially suited
for high flow conditions.
Inexpensive micro turbines are manufactured for individual power production
with as little as two feet of head.
Large Kaplan turbines are individually designed for each site to operate at
the highest possible efficiency, typically over 90%. They are very expensive
to design, manufacture and install, but operate for decades.
Variations
The Kaplan turbine is the most widely used of the propeller-type turbines,
but several other variations exist:

54. Propeller turbines have non-adjustable propeller vanes. They are used in low
cost, small installations. Commercial products exist for producing several
hundred
watts from only a few feet of head.
Bulb or Tubular turbines are designed into the water delivery tube. A large
bulb is centered in the water pipe which holds the generator, wicket gate
and runner. Tubular turbines are a fully axial design, whereas Kaplan
turbines have a radial wicket gate. Pit turbines are bulb turbines with a gear
box. This allows for a smaller generator and bulb.
Straflo turbines are axial turbines with the generator outside of the water
channel, connected to the periphery of the runner.
S- turbines eliminate the need for a bulb housing by placing the generator
outside of the water channel. This is accomplished with a jog in the water
channel and a shaft connecting the runner and generator.
Tyson turbines are a fixed propeller turbine designed to be immersed in a
fast flowing river, either permanently anchored in the river bed, or attached
to a boat or barge.
Francis Turbine
Francis Turbine
The Francis turbine is a type of water turbine that was developed by James
B. Francis. It is an inward flow reaction turbine that combines radial and
axial flow concepts.
Francis turbines are the most common water turbine in use today. They
operate in a head range of ten meters to several hundred meters and are
primarily used for electrical power production.

55. The Francis turbine is a reaction
turbine, which means that the working fluid changes pressure as it moves
through the turbine, giving up its energy. A casement is needed to contain
the water flow. The turbine is located between the high pressure water
source and the low pressure water exit, usually at the base of a dam.
The inlet is spiral shaped. Guide vanes direct the water tangentially to the
runner. This radial flow acts on the runner vanes, causing the runner to spin.
The guide vanes (or wicket gate) may be adjustable to allow efficient turbine
operation for a range of water flow conditions.
As the water moves through the runner its spinning radius decreases,
further acting on the runner. Imagine swinging a ball on a string around in a
circle. If the string is pulled short, the ball spins faster. This property, in
addition to the water's pressure, helps inward flow turbines harness water
energy.At the exit, water acts on cup shaped runner features, leaving with
no swirl and very little kinetic or potential energy. The turbine's exit tube is
shaped to help decelerate the water flow and recover the pressure.
Application
Large Francis turbines are individually designed for each site to operate at
the highest possible efficiency, typically over 90%.
Francis type units cover a wide head range, from 20 meters to 700 meters
and their output varies from a few kilowatt to 1000 megawatt. Their size
varies from a few hundred millimeters to about 10 meters.
In addition to electrical production, they may also be used for pumped
storage; where a reservoir is filled by the turbine (acting as a pump) during
low power demand, and then reversed and used to generate power during
peak demand.
Francis turbines may be designed for a wide range of heads and flows. This,
along with their high efficiency, has made them the most widely used
turbine in the world.