powerstationorpowerplant

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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 heatengine, 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 steamturbine generator.

Fossil fuel powered plants may also use a steam turbine generator orin the case of Natural gas fired plants may use a combustion turbine.

Geothermal power plants use steam extracted from hot undergroundrocks.


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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 concentratedenough to use for power generation, usually in a steam boiler and

turbine.

By prime mover

Steam turbine plants use the pressure generated by expanding steamto turn the blades of a turbine.

Gas turbine plants use the heat from gases to directly operate theturbine. 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 powerfor 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 reciprocatingengines 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 :


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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 ofsmoke 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


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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 station2. A set of transformers to raise the generated power to the high

voltages used on the transmission lines

3. The transmission lines4. The substations at which the power is stepped down to the

voltage on the distribution lines5. The distribution lines6. 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 worlds 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


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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 frequentlyused. 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


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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


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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 conditionof 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


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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 majortransmission 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 statesConnecticut, Massachusetts, Michigan, New Jersey, New York,

Ohio, Pennsylvania, and Vermontand the Canadian provinces of Ontario

and Qubec 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. FromNovember 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


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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 Californias 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 pleadedguilty 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


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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 throughouta large organizations 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 organizations 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.


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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 Mnsterland - 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.


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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 ofsouthern 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 sabotagedby 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 ofMoscow 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.


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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 :


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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 powercapability, 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 canfind it in the dark.

Do not use candles for lighting. Candles are in the top three causes ofhousehold fires.

Turn off all but one light or a radio so that you'll know when the powerreturns.

Check that the stove, ovens, electric kettles, irons, air conditionersand (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 themfrom damage when the electricity returns, in case of power surges.

Listen to local radio and television for updated information. (Thereason for having a battery powered (ie. portable) radio.)

Keep refrigerator and freezer doors closed. A full modern freezer willstay 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


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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. Ifit 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 across breeze in the absence of air conditioning and electric fans.

In summer: close blinds, curtains, drapes, windows and doors on thesunny side of your home to block out the heat from the sun.

In winter: open blinds, curtains and drapes during the day on thesunny 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 youhave 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 anInverter with built-in battery.

Do not use propane or other combustion-type heaters indoors due tothe probability of toxic carbon monoxide buildup.

Other notes:

Water pressure may drop and even stop above a certain height inhigh-rise buildings due to their water pumps losing power.


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Remember that electrical devices such as elevator will not work. Youcan 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 maybe 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.


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The above diagram shopower plant.

Main parts of the plant

1. Coal conveyor 2. Sto

preheater 7. Electrosta

Condenser 11. Transfo

13. Generator 14. High

Basic Operation :A th

Coal conveyor : This i

transported from coal s

boiler.

Stoker : The coal whic

furnance for combustio

to a furnace.

s the simplest arrangement of Coa

re

ker 3. Pulverizer 4. Boiler 5. Coal a

ic precipitator 8. Smoke stack 9. T

mers 12. Cooling towers

- votge power lines

rmal power plant basically works o

a belt type of arrangement.With t

orage place in power plant to the p

is brought near by boiler has to pu

.This stoker is a mechanical device

l fired (Thermal)

sh 6. Air

rbine 10.

Rankine cycle.

is coal is

lace near by

t in boiler

for feeding coal


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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, mixedwith 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


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may be converted to el

used for these purpose

economy and suitable t

Classification of Boilers

Bolilers are classified as

Fire tube boilers : In fir

tubes and water surrou

rugged in construction.

horizontal these are fur

boilers.In this since the

they can't meet quickly

steam are not possible,

17.5kg/sq cm.Due to la

time for steam raising.

low pressures.The outu

Water tube boilers : In

ctrical energy. Although other fluid

, water is by far the most common

ermodynamic characteristics.

tube boilers hot gases are passed

ds these tubes. These are simple,c

epending on whether the tubes ar

her classified as vertical and horizo

water volume is more,circulation wi

the changes in steam demand.High

maximum pressure that can be atta

rge quantity of water in the drain it

he steam attained is generally wet,

of the boiler is also limited.

hese boilers water is inside the tub

are sometimes

because of its

through the

ompact and

vertical or

ntal tube

ll be poor.So

pressures of

ined is about

requires more

economical for

s and hot gases


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are outside the tubes.T

tubes.They may contain

fig).Feed water enters t

boiler).This water circul

drums.Hot gases which

in to steam.This steam

the drum since it is of li(upper drum).The entir

from there (see in laout

high, so rate of heat tra

efficiency.They produce

quickly to changes in st

vertical,horizontal and i

tubes.These are of less

surfaces can be obtaine

pressure as high as 125

centigrade.

Superheater : Most of t

reheater arrangement.

unit in which steam, aft

saturation temperature.

influenced by the locati

ey consists of drums and

any number of drums (you can se

he boiler to one drum (here it is dru

tes through the tubes connected e

surrounds these tubes wil convert t

is passed through tubes and collect

ght weight.So the drums store steasteam is collected in one drum an

fig).As the movement of water in t

nsfer also becomes high resulting i

high pressure , easily accessible an

am demand.These are also classifi

nclined tube depending on the arra

weight and less liable to explosion.

d by use of large number of tubes.

kg/sq cm and temperatures from

e modern boliers are having super

Superheater is a component of a st

er it has left the boiler drum, is hea

The amount of superheat added to

n, arrangement, and amount of su

2 drums in

m below the

ternal to

he water in tubes

d at the top of

m and waterit is taken out

e water tubes is

greater

d can respond

d as

gement of the

arge heating

e can attain

15 to 575

eater and

am-generating

ed above its

the steam is

erheater surface


21/85

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 mechanicalenergy.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 steamgenerator 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


22/85

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 airaround tubes in an air-cooled condenser).

Condensers are classified as (i) Jet condensers or contact condensers (ii)

Surface condensers.

Injet 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 passedto 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


23/85

(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-transferprocess 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


24/85

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 arearequired 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)


25/85

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 smokefrom 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


26/85

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 reactordeveloped 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


27/85

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 ofmoderator and

coolant, as well. In this case the steam is generted in the reactor it self.Asyou 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 afterpreheating. 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


28/85

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 decreasingthe 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 downcomerregion, 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.


29/85

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 quicklyremoved 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 TurbinesSteam 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


30/85

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 ensurethat 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.


31/85

Comparison with oth

Light water is ordinary

reactor types use heav

hydrogen replaces the

(D2O instead of H2O,The Pressurized Water

reactor developed beca

civilian motivation for t

through design simplific

reactors, BWR designs

quietness. The descripti

which the same water u

cycle turbine generator

primary and secondary

In contrast to the press

secondary loop, in civili

the electrical generator

generators or heat exch

in which the water is at

pressure) compared to

reactor is designed to o

r reactors

ater. In comparison, some other

water. In heavy water, the deuteri

ommon hydrogen atoms in the wat

olecular weight 20 instead of 18).eactor (PWR) was the first type of

se of its application to submarine

e BWR is reducing costs for comm

ation and lower pressure componen

re used when natural circulation is

on of BWRs below describes civilian

sed for reactor cooling is also used

. A Naval BWR is designed like a P

loops.

urized water reactors that utilize a

n BWRs the steam going to the tur

is produced in the reactor core rath

angers. There is just a single circui

lower pressure (about 75 times at

PWR so that it boils in the core at

perate with steam comprising 121

ater-cooled

um isotope of

r molecules

light-water

ropulsion. The

rcial applications

ts. In naval

specified for its

reactor plants in

in the Rankine

R that has both

rimary and

bine that powers

er than in steam

in a civilian BWR

ospheric

about 285C. The

% of the volume


32/85

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 asubstantially 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 aPWR, 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 comparedto 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 bothnormal and emergency operations, which results in easy and intuitive

assessment of emergency conditions.

Can operate at lower core power density levels using naturalcirculation without forced flow.

A BWR may be designed to operate using only natural circulation sothat recirculation pumps are eliminated entirely. (The new ESBWR

design uses natural circulation.)

Disadvantages

Complex operational calculations for managing the utilization of thenuclear 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


33/85

of a factor with modern computers). More incore nuclear

instrumentation is required.

Much larger pressure vessel than for a PWR of similar power, withcorrespondingly 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 aPWR.

Control rods are inserted from below for current BWR designs. Thereare 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 topentry 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 severalmethods, 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


34/85

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


35/85

Fluid fueled

Gas Fueled

The Nuclear Fuel Cycle

The Nuclear Fuel Cycle

The nuclear fuel cycle is the series of industrial processes whichinvolve the production of electricity from uranium in nuclear power

reactors.

Uranium is a relatively common element that is found throughout theworld. 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 reactorto produce steam and drive a turbine connected to a generator.

Fuel removed from a reactor, after it has reached the end of its usefullife, 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.


36/85

There are a number of

uranium in the ground i

nuclear fuel is economi

below figure represents

Uranium Mining

Both excavation and in

Excavation may be und

In general, open pit mi

and underground minin

120 m deep. Open pit

the size of the ore depo

prevent collapse. As a r

in order to access the o

small surface disturban

to access the ore is con

An increasing proportio

leaching (ISL), where o

porous orebody to diss

reas around the world where the c

s sufficiently high that extraction of

ally feasible. Such concentrations a

various stages in Nuclear Fuel cycl

situ techniques are used to recover

rground and open pit mining.

ing is used where deposits are clos

is used for deep deposits, typicall

ines require large holes on the sur

sit, since the walls of the pit must b

sult, the quantity of material that

re may be large. Underground mine

e and the quantity of material that

iderably less than in the case of an

of the world's uranium now comes

ygenated groundwater is circulate

lve the uranium and bring it to the

ncentration of

it for use as

re called ore.The

uranium ore.

to the surface

greater than

ace, larger than

e sloped to

ust be removed

s have relatively

must be removed

open pit mine.

from in situ

through a very

surface. ISL may


37/85

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 andprecipitated. 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.


38/85

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 formedfrom pressed uranium oxide which is sintered (baked) at a high temperature

(over 1400C). 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


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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 fuelWith 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 electricalpower 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


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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, andthe 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


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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 fromreprocessing, 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


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energy in the bonds that hold atoms together.This binding energy can be

calculated from the Einstein relationship: mass-energy equivalence formula

E = mc, 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 spontaneouslyinto a lighter nucleus by emitting a particle;

Endothermic nuclear reactions where two nuclei merge to produce twodifferent 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


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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 fissionalso known as atomic fissionis 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,


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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


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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 ene

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