Power station or power plant and classificationDocument Transcript
Power Station or Power Plant and classificationPower Station or Power Plant :A power station or power plant is a facility for the generation of electricpower. Power plant is also used to refer to the engine in ships, aircraft andother large vehicles. Some prefer to use the term energy center because itmore accurately describes what the plants do, which is the conversion ofother forms of energy, like chemical energy, gravitational potential energy orheat energy into electrical energy. However, power plant is the mostcommon term in the U.S., while elsewhere power station and power plantare both widely used, power station prevailing in many Commonwealthcountries and especially in the United Kingdom.At the center of nearly all power stations is a generator, a rotating machinethat converts mechanical energy into electrical energy by creating relativemotion between a magnetic field and a conductor. The energy sourceharnessed to turn the generator varies widely. It depends chiefly on whatfuels are easily available and the types of technology that the powercompany has access to.Classification of Power plants :Power plants are classified by the type of fuel and the type of prime moverinstalled.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 reactors 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.
• 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 :
Other power stations use the energy from wave or tidal motion, wind,sunlight or the energy of falling water, hydroelectricity. These types ofenergy sources are called renewable energy.Thermal power plant,Advantages and DisadvantagesThermal power plant or Steam power plant :A generating station which converts heat energy of coal combustion in toelectrical 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 componentsElectric Power Systems :Electric Power Systems, components that transform other types of energyinto electrical energy and transmit this energy to a consumer. Theproduction and transmission of electricity is relatively efficient andinexpensive, although unlike other forms of energy, electricity is not easily
stored and thus must generally be used as it is being produced.Components of an Electric Power SystemA 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 consumers equipment.Power StationThe power station of a power system consists of a prime mover, such as aturbine driven by water, steam, or combustion gases that operate a systemof electric motors and generators. Most of the worlds electric power isgenerated in steam plants driven by coal, oil, nuclear energy, or gas. Asmaller percentage of the world’s electric power is generated byhydroelectric (waterpower), diesel, and internal-combustion plants.TransformersModern electric power systems use transformers to convert electricity intodifferent voltages. With transformers, each stage of the system can beoperated at an appropriate voltage. In a typical system, the generators atthe 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 to765,000 V for the long-distance primary transmission line because highervoltages can be transmitted more efficiently over long distances. At thesubstation the voltage may be transformed down to levels of 69,000 to138,000 V for further transfer on the distribution system. Another set oftransformers step the voltage down again to a distribution level such as2,400 or 4,160 V or 15, 27, or 33 kilovolts (kV). Finally the voltage is
transformed once again at the distribution transformer near the point of useto 240 or 120 V.Transmission LinesThe lines of high-voltage transmission systems are usually composed ofwires of copper, aluminum, or copper-clad or aluminum-clad steel, which aresuspended from tall latticework towers of steel by strings of porcelaininsulators. By the use of clad steel wires and high towers, the distancebetween towers can be increased, and the cost of the transmission line thusreduced. In modern installations with essentially straight paths, high-voltagelines may be built with as few as six towers to the kilometer. In some areashigh-voltage lines are suspended from tall wooden poles spaced more closelytogether. For lower voltage distribution lines, wooden poles are generallyused rather than steel towers. In cities and other areas where open linescreate a safety hazard or are considered unattractive, insulated undergroundcables are used for distribution. Some of these cables have a hollow corethrough which oil circulates under low pressure. The oil provides temporaryprotection from water damage to the enclosed wires should the cabledevelop a leak. Pipe-type cables in which three cables are enclosed in a pipefilled 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 highas 345,000 V (or 345 kV).Supplementary EquipmentAny electric-distribution system involves a large amount of supplementaryequipment to protect the generators, transformers, and the transmissionlines themselves. The system often includes devices designed to regulate thevoltage 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. Thesebreakers are large switches that are activated automatically in the event of ashort circuit or other condition that produces a sudden rise of current.Because a current forms across the terminals of the circuit breaker at themoment when the current is interrupted, some large breakers (such as thoseused to protect a generator or a section of primary transmission line) are
immersed in a liquid that is a poor conductor of electricity, such as oil, toquench the current. In large air-type circuit breakers, as well as in oilbreakers, magnetic fields are used to break up the current. Small air-circuitbreakers are used for protection in shops, factories, and in modern homeinstallations. In residential electric wiring, fuses were once commonlyemployed for the same purpose. A fuse consists of a piece of alloy with a lowmelting point, inserted in the circuit, which melts, breaking the circuit if thecurrent rises above a certain value. Most residences now use air-circuitbreakers.Power Failures,Protection from outages and RestorationPower Failures :A power outage (Also power cut, power failure or power loss) is the loss ofthe electricity supply to an area.The reasons for a power failure can for instance be a defect in a powerstation, damage to a power line or other part of the distribution system, ashort circuit, or the overloading of electricity mains. While the developedcountries enjoy a highly uninterrupted supply of electric power all the time,many developing countries have acute power shortage as compared to thedemand. Countries such as Pakistan have several hours of daily power-cutsin almost all cities and villages except the metropolitan cities and the statecapitals. Wealthier people in these countries may use a power-inverter or adiesel-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 levelspecified for the system, or sometimes referred to as a short circuit whenthe loss of power occurs over a short time (usually seconds). Systemssupplied with three-phase electric power also suffer brownouts if one ormore phases are absent, at reduced voltage, or incorrectly phased. Suchmalfunctions are particularly damaging to electric motors. Some brownouts,called voltage reductions, are made intentionally to prevent a full poweroutage. Load shedding is a common term for a controlled way of rotatingavailable generation capacity between various districts or customers, thus
avoiding total wide area blackouts.Power failures are particularly critical for hospitals, since many life-criticalmedical devices and tasks require power. For this reason hospitals, just likemany enterprises (notably colocation facilities and other datacenters), haveemergency power generators which are typically powered by diesel fuel andconfigured to start automatically, as soon as a power failure occurs. In mostthird world countries, power cuts go unnoticed by most citizens of upscalemeans, as maintaining an uninterruptible power supply is often consideredan 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 systemssuch as telecommunications are also required to have emergency power.Telephone exchange rooms usually have arrays of lead-acid batteries forbackup and also a socket for connecting a diesel generator during extendedperiods of outage.Power outages may also be caused by terrorism (attacking power plants orelectricity pylons) in developing countries. The Shining Path movement wasthe first to copy this tactic from Mao Zedong.Live Examples of breakdown in interconnected grid systemIn most parts of the world, local or national electric utilities have joined ingrid systems. The linking grids allow electricity generated in one area to beshared with others. Each utility that agrees to share gains an increasedreserve capacity, use of larger, more efficient generators, and the ability torespond to local power failures by obtaining energy from a linking grid.These interconnected grids are large, complex systems that containelements operated by different groups. These systems offer the opportunityfor economic savings and improve overall reliability but can create a risk ofwidespread failure. For example, a major grid-system breakdown occurredon November 9, 1965, in eastern North America, when an automatic controldevice that regulates and directs current flow failed in Queenston, Ontario,causing a circuit breaker to remain open. A surge of excess current wastransmitted through the northeastern United States. Generator safety
switches from Rochester, New York, to Boston, Massachusetts, wereautomatically tripped, cutting generators out of the system to protect themfrom damage. Power generated by more southerly plants rushed to fill thevacuum and overloaded these plants, which automatically shut themselvesoff. 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 inNorth America and elsewhere. On July 13, 1977, about 9 million people inthe New York City area were once again without power when majortransmission lines failed. In some areas the outage lasted 25 hours asrestored high voltage burned out equipment. These major failures aretermed blackouts.The worst blackout in the history of the United States and Canada occurredAugust 14, 2003, when 61,800 megawatts of electrical power was lost in anarea covering 50 million people. (One megawatt of electricity is roughly theamount needed to power 750 residential homes.) The blackout affected suchmajor cities as Cleveland, Detroit, New York, Ottawa, and Toronto. Parts ofeight states—Connecticut, Massachusetts, Michigan, New Jersey, New York,Ohio, Pennsylvania, and Vermont—and the Canadian provinces of Ontarioand Québec were affected. The blackout prompted calls to replace agingequipment and raised questions about the reliability of the national powergrid.The term brownout is often used for partial shutdowns of power, usuallydeliberate, either to save electricity or as a wartime security measure. FromNovember 2000 through May 2001 California experienced a series ofplanned brownouts to groups of customers, for a limited duration, in order toreduce total system load and avoid a blackout due to alleged electricalshortages. However, an investigation by the California Public UtilitiesCommission into the alleged shortages later revealed that five energycompanies withheld electricity they could have produced. In 2002 thecommission concluded that the withholding of electricity contributed to an“unconscionable, unjust, and unreasonable electricity price spike.” California
state utilities paid $20 billion more for energy in 2000 than in 1999 as aresult, the head of the commission found.The commission also cited the role of the Enron Corporation in the Californiabrownouts. In June 2003 the Federal Energy Regulatory Commission (FERC)barred Enron from selling electricity and natural gas in the United Statesafter conducting a probe into charges that Enron manipulated electricityprices during California’s energy crisis. In the same month the FederalBureau of Investigation arrested an Enron executive on charges ofmanipulating the price of electricity in California. Two other Enronemployees, 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 gridsystem provides necessary backup and alternate paths for power flow,resulting in much higher overall reliability than is possible with isolatedsystems. National or regional grids can also cope with unexpected outagessuch as those caused by storms, earthquakes, landslides, and forest fires, ordue to human error or deliberate acts of sabotage.Protecting the power system from outagesIn power supply networks, the power generation and the electrical load(demand) must be very close to equal every second to avoid overloading ofnetwork components, which can severely damage them. In order to preventthis, parts of the system will automatically disconnect themselves from therest of the system, or shut themselves down to avoid damage. This isanalogous to the role of relays and fuses in households.Under certain conditions, a network component shutting down can causecurrent fluctuations in neighboring segments of the network, though this isunlikely, 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 entireelectrical grid.Modern power systems are designed to be resistant to this sort of cascadingfailure, but it may be unavoidable (see below). Moreover, since there is no
short-term economic benefit to preventing rare large-scale failures, someobservers have expressed concern that there is a tendency to erode theresilience of the network over time, which is only corrected after a majorfailure occurs. It has been claimed that reducing the likelihood of smalloutages only increases the likelihood of larger ones. In that case, the short-term economic benefit of keeping the individual customer happy increasesthe likelihood of large-scale blackouts.Power AnalyticsPower Analytics is the term used to describe the management of electricalpower distribution, consumption, and preventative maintenance throughouta large organization’s facilities, particularly organizations with high electricalpower requirements. For such facilities, electrical power problems – includingthe worst-case scenario, a full power outage – could have a devastatingserious impact. Additionally, it could jeopardize the health and safety ofindividuals within the facility or in the surrounding community.Power Analytics use complex mathematical algorithms to detect variationswithin an organization’s power infrastructure (measurements such asvoltage, current, power factor, etc.). Such variations could be earlyindications of longer-term power problems; when a Power Analytics systemdetects such variations, it will begin to diagnose the source of the variation,surrounding components, and then the complete electrical powerinfrastructure. Such systems will – after fully assessing the location andpotential magnitude of the problem – predict when and where the potentialproblem will occur, as well as recommend the preventative maintenancerequired preempting the problem from occurring.Restoring power after a wide-area outageRestoring power after a wide-area outage can be difficult, as power stationsneed to be brought back on-line. Normally, this is done with the help ofpower from the rest of the grid. In the absence of grid power, a so-calledblack start needs to be performed to bootstrap the power grid intooperation.
Latest Power Outages,Causes and factors contributing to itLatest 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 plungedmillions of Europeans into darkness over the weekend, halting trains,trapping dozens in lifts and prompting calls for a central European powerauthority. The blackout, which originated in north-western Germany, alsostruck 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 Outages09/24/2006 On September 24th afternoon 1.30pm Pakistan was hit by anationwide blackout. Millions of homes across Pakistan were left withoutpower for several hours. Power has been restored in capital Islamabad afterover a two-hour breakdown. The outage was caused due to a fault thatoccurred during maintenance of a high-tension transmission line.07/12/2006 Electricity Blackout in Auckland (New Zealand) - 700,000 peoplewithout electricity for up to 10 hours. An earth wire, which snapped in highwinds, fell into Transpowers Otahuhu substation, damaging 110 kilovoltsupply lines. The cause - a simple metal shackle.11/25/2005 Electricity Blackout in Münsterland - 250,000 people withoutelectricity for up to six days. Ice and storm had caused serious damage tothe network , leading to the blackout.10/24/2005 -11/11/2005 Hurricane Wilma caused loss of power for most ofSouth Florida and Southwest Florida, with hundreds of thousands ofcustomers still powerless a week later, and full restoration not complete.09/12/2005 A blackout in Los Angeles affected millions in California.
08/29/2005 Millions of Louisiana, Mississippi and Alabama residents lostpower 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 downedtrees and power lines caused by the then minimal Hurricane Katrina. Mostcustomers affected were without power for four days, and some customershad no power for up to one week.08/22/2005 All of southern and central Iraq, including parts of the capitalBaghdad, all of the second largest city Basra and the only port Umm Qasrwent out of power for more than 7 hours after a feeder line was sabotagedby insurgents, causing a cascading effect shutting down multiple powerplants.08/18/2005 Almost 100 million people on Java Island, the main island ofIndonesia which the capital Jakarta is on, and the isle of Bali, lost power for7 hours. In terms of population affected, the 2005 Java-Bali Blackout wasthe 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 wasrestored within 24 hours.09/04/2004 On five million people in Florida were without power at one pointdue to Hurricane Frances, one of the most widespread outages ever due to ahurricane.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 Italyexcept Sardinia, cutting service to more than 56 million people.09/23/2003 A power failure affected 5 million people in Denmark andsouthern 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.
08/28/2003 There was a 2003 London blackout on which won worldwideheadlines such as "Power cut cripples London" but in fact only affected500,000 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 emergenciesSystem 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 / GenerationElectricity Power Blackout and Outage tipsElectricity Power Blackout and Outage tips :
• 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.. Dont 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 youll 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
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.
• 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 OperationThermal Power Plant Lay out :The above diagram is the lay out of a simplified thermal power plant and thebelow is also diagram of a thermal power plant.
The above diagram shows the simplest arrangement of Coal fired (Thermal)power plant.Main parts of the plant are1. 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. ACoal conveyor : This is a belt type of arrangement.With this coal istransported from coal storage place in power plant to the place near byboiler.Stoker : The coal which is brought near by boiler has to put in boilerfurnance for combustion.This stoker is a mechanical device for feeding coalto a furnace.
Pulverizer : The coal is put in the boiler after pulverization.For thispulverizer is used.A pulverizer is a device for grinding coal for combustion ina furnace in a power plant.Types of PulverizersBall and Tube MillBall mill is a pulverizer that consists of a horizontal rotating cylinder, up tothree diameters in length, containing a charge of tumbling or cascading steelballs, pebbles, or rods.Tube mill is a revolving cylinder of up to five diameters in length used forfine pulverization of ore, rock, and other such materials; the material, mixedwith water, is fed into the chamber from one end, and passes out the otherend as slime.Ring and BallThis type consists of two rings separated by a series of large balls. The lowerring rotates, while the upper ring presses down on the balls via a set ofspring and adjuster assemblies. Coal is introduced into the center or side ofthe pulverizer (depending on the design) and is ground as the lower ringrotates causing the balls to orbit between the upper and lower rings. Thecoal is carried out of the mill by the flow of air moving through it. The size ofthe coal particals released from the grinding section of the mill is determinedby a classifer separator. These mills are typically produced by B&W (Babcockand Wilcox).Boiler : Now that pulverized coal is put in boiler furnance.Boiler is anenclosed vessel in which water is heated and circulated until the water isturned in to steam at the required pressure.Coal is burned inside the combustion chamber of boiler.The products ofcombustion are nothing but gases.These gases which are at hightemperature vaporize the water inside the boiler to steam.Some times thissteam is further heated in a superheater as higher the steam pressure andtemperature the greater efficiency the engine will have in converting theheat in steam in to mechanical work. This steam at high pressure andtempeture is used directly as a heating medium, or as the working fluid in aprime mover to convert thermal energy to mechanical work, which in turn
may be converted to electrical energy. Although other fluids are sometimesused for these purposes, water is by far the most common because of itseconomy and suitable thermodynamic characteristics. yClassification of BoilersBolilers are classified asFire tube boilers : In fire tube boilers hot gases are passed through thetubes and water surrounds these tubes. These are simple,compact andrugged in construction.Depending on whether the tubes are vertical orhorizontal these are further classified as vertical and horizontal tubeboilers.In this since the water volume is more,circulation will be poor.Sothey cant meet quickly the changes in steam demand.High pressures of demand.Highsteam are not possible,maximum pressure that can be attained is about17.5kg/sq cm.Due to large quantity of water in the drain it requires moretime for steam raising.The steam attained is generally wet,economical forlow pressures.The outut of the boiler is also limited.Water tube boilers : In these boilers water is inside the tubes and hot gases
are outside the tubes.They consists of drums andtubes.They may contain any number of drums (you can see 2 drums infig).Feed water enters the boiler to one drum (here it is drum below the sboiler).This water circulates through the tubes connected external todrums.Hot gases which surrounds these tubes wil convert the water in tubesin to steam.This steam is passed through tubes and collected at the top of collectethe 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 outfrom there (see in laout fig).As the movement of water in the water tubes ishigh, so rate of heat transfer also becomes high resulting in greaterefficiency.They produce high pressure , easily accessible and can respondquickly to changes in steam demand.These are also classified asvertical,horizontal and inclined tube depending on the arrangement of the arrangemetubes.These are of less weight and less liable to explosion.Large heatingsurfaces can be obtained by use of large number of tubes.We can attainpressure as high as 125 kg/sq cm and temperatures from 315 to 575centigrade.Superheater : Most of the modern boliers are having superheater andreheater arrangement. Superheater is a component of a steam-generating steamunit in which steam, after it has left the boiler drum, is heated above itssaturation temperature. The amount of superheat added to the s steam isinfluenced by the location, arrangement, and amount of superheater surface
installed, as well as the rating of the boiler. The superheater may consist ofone or more stages of tube banks arranged to effectively transfer heat fromthe 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 theturbine where it loses some of its energy.Reheater is also steam boilercomponent 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 secondsteam turbine (see Layout fig) where the heat is converted to mechanicalenergy.This mechanical energy is used to run the alternator, which iscoupled to turbine , there by generating elecrical energy.Condenser : Steam after rotating staem turbine comes tocondenser.Condenser refers here to the shell and tube heat exchanger (orsurface condenser) installed at the outlet of every steam turbine in Thermalpower stations of utility companies generally. These condensers are heatexchangers which convert steam from its gaseous to its liquid state, alsoknown as phase transition. In so doing, the latent heat of steam is given outinside the condenser. Where water is in short supply an air cooled condenseris often used. An air cooled condenser is however significantly moreexpensive and cannot achieve as low a steam turbine backpressure (andtherefore less efficient) as a surface condenser.The purpose is to condense the outlet (or exhaust) steam from steamturbine to obtain maximum efficiency and also to get the condensed steamin 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 tomechanical power. The difference between the heat of steam per unit weightat the inlet to turbine and the heat of steam per unit weight at the outlet toturbine represents the heat given out (or heat drop) in the steam turbinewhich is converted to mechanical power. The heat drop per unit weight of
steam is also measured by the word enthalpy drop. Therefore the more theconversion of heat per pound (or kilogram) of steam to mechanical power inthe turbine, the better is its performance or otherwise known as efficiency.By condensing the exhaust steam of turbine, the exhaust pressure isbrought down below atmospheric pressure from above atmosphericpressure, increasing the steam pressure drop between inlet and exhaust ofsteam turbine. This further reduction in exhaust pressure gives out moreheat per unit weight of steam input to the steam turbine, for conversion tomechanical power. Most of the heat liberated due to condensing, i.e., latentheat of steam, is carried away by the cooling medium. (water inside tubes ina 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.In jet condensers the steam to be condensed mixes with the cooling waterand the temperature of the condensate and the cooling water is same whenleaving the condenser; and the condensate cant be recovered for use asfeed water to the boiler; heat transfer is by direct conduction.In surface condensers there is no direct contact between the steam to becondensed and the circulating cooling water. There is a wall interposedbetween them through heat must be convectively transferred.Thetemperature of the condensate may be higher than the temperature of thecooling water at outlet and the condnsate is recovered as feed water to theboiler.Both the cooling water and the condensate are separetely withdrawn.Because of this advantage surface condensers are used in thermalpower plants.Final output of condenser is water at low temperature is passedto high pressure feed water heater,it is heated and again passed as feedwater to the boiler.Since we are passing water at high temperature as feedwater the temperature inside the boiler does not dcrease and boiler efficincyalso maintained.Cooling Towers :The condensate (water) formed in the condeser aftercondensation is initially at high temperature.This hot water is passed tocooling towers.It is a tower- or building-like device in which atmospheric air
(the heat receiver) circulates in direct or indirect contact with warmer water(the heat source) and the water is thereby cooled (see illustration). A coolingtower may serve as the heat sink in a conventional thermodynamic process,such as refrigeration or steam power generation, and when it is convenientor desirable to make final heat rejection to atmospheric air. Water, acting asthe heat-transfer fluid, gives up heat to atmospheric air, and thus cooled, isrecirculated through the system, affording economical operation of theprocess.Two basic types of cooling towers are commonly used. One transfers theheat 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 employedfor producing air circulation through them: atmospheric, natural draft, andmechanical draft. The other transfers the heat from warmer water to coolerair by a sensible heat-transfer process and is known as the nonevaporativeor dry cooling tower.Nonevaporative cooling towers are classified as air-cooled condensers and asair-cooled heat exchangers, and are further classified by the means used forproducing air circulation through them. These two basic types are sometimescombined, with the two cooling processes generally used in parallel orseparately, and are then known as wet-dry cooling towers.Evaluation of cooling tower performance is based on cooling of a specifiedquantity of water through a given range and to a specified temperatureapproach to the wet-bulb or dry-bulb temperature for which the tower isdesigned. Because exact design conditions are rarely experienced in
operation, estimated performance curves are frequently prepared for aspecific installation, and provide a means for comparing the measuredperformance with design conditions.Economiser : Flue gases coming out of the boiler carry lot of heat.Functionof economiser is to recover some of the heat from the heat carried away inthe flue gases up the chimney and utilize for heating the feed water to theboiler.It is placed in the passage of flue gases in between the exit from theboiler and the entry to the chimney.The use of economiser results in savingin coal consumption , increase in steaming rate and high boiler efficiency butneeds extra investment and increase in maintenance costs and floor arearequired for the plant.This is used in all modern plants.In this a largenumber of small diameter thin walled tubes are placed between twoheaders.Feed water enters the tube through one header and leaves throughthe other.The flue gases flow out side the tubes usually in counter flow.Air preheater : The remaining heat of flue gases is utilised by airpreheater.It is a device used in steam boilers to transfer heat from the fluegases to the combustion air before the air enters the furnace. Also known asair heater; air-heating system. It is not shown in the lay out.But it is kept ata 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 gasfrom the boiler to improve boiler efficiency by burning warm air whichincreases 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 lowertemperature, allowing simplified design of the ducting and stack. It alsoallows control over the temperature of gases leaving the stack (to meetemissions regulations, for example).After extracting heat flue gases arepassed to elctrostatic precipitator.Electrostatic precipitator : It is a device which removes dust or otherfinely divided particles from flue gases by charging the particles inductivelywith an electric field, then attracting them to highly charged collector plates.Also known as precipitator. The process depends on two steps. In the firststep the suspension passes through an electric discharge (corona discharge)
area where ionization of the gas occurs. The ions produced collide with thesuspended particles and confer on them an electric charge. The chargedparticles drift toward an electrode of opposite sign and are deposited on theelectrode where their electric charge is neutralized. The phenomenon wouldbe more correctly designated as electrodeposition from the gas phase.The use of electrostatic precipitators has become common in numerousindustrial applications. Among the advantages of the electrostaticprecipitator are its ability to handle large volumes of gas, at elevatedtemperatures if necessary, with a reasonably small pressure drop, and theremoval of particles in the micrometer range. Some of the usual applicationsare: (1) removal of dirt from flue gases in steam plants; (2) cleaning of airto remove fungi and bacteria in establishments producing antibiotics andother drugs, and in operating rooms; (3) cleaning of air in ventilation and airconditioning systems; (4) removal of oil mists in machine shops and acidmists 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. Theyare typically almost vertical to ensure that the hot gases flow smoothly,drawing air into the combustion through the chimney effect (also known asthe stack effect). The space inside a chimney is called a flue. Chimneys maybe found in buildings, steam locomotives and ships. In the US, the termsmokestack (colloquially, stack) is also used when referring to locomotivechimneys. The term funnel is generally used for ship chimneys andsometimes used to refer to locomotive chimneys.Chimneys are tall toincrease their draw of air for combustion and to disperse pollutants in theflue gases over a greater area so as to reduce the pollutant concentrations incompliance with regulatory or other limits.Generator : An alternator is an electromechanical device that convertsmechanical energy to alternating current electrical energy. Most alternatorsuse a rotating magnetic field. Different geometries - such as a linearalternator for use with stirling engines - are also occasionally used. In
principle, any AC generator can be called an alternator, but usually the wordrefers to small rotating machines driven by automotive and other internalcombustion engines.Transformers :It is a device that transfers electric energy from onealternating-current circuit to one or more other circuits, either increasing(stepping up) or reducing (stepping down) the voltage. Uses fortransformers include reducing the line voltage to operate low-voltagedevices (doorbells or toy electric trains) and raising the voltage from electricgenerators so that electric power can be transmitted over long distances.Transformers act through electromagnetic induction; current in the primarycoil induces current in the secondary coil. The secondary voltage iscalculated by multiplying the primary voltage by the ratio of the number ofturns in the secondary coil to that in the primary.Boiling Water Reactor (BWR) - Advantages and DisadvantagesBoiling 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 pump5.Control rod drive 6.Fresh steam 7. Feedwater 8.High pressure turbine9.Low pressure turbine 10.Generator 11.Exciter 12.Condenser 13.Coolingwater 14.Preheater 15.Feedwater pump 16. Cooling water pump17.Concrete shield
The above diagram shows BWR and its main parts.The BWR is characterizedby two-phase fluid flow (water and steam) in the upper part of the reactorcore. Light water (i.e., common distilled water) is the working fluid used toconduct heat away from the nuclear fuel. The water around the fuelelements also "thermalizes" neutrons, i.e., reduces their kinetic energy,which is necessary to improve the probability of fission of fissile fuel. Fissilefuel material, such as the U-235 and Pu-239 isotopes, have large capturecross sections for thermal neutrons.In a boling water reactor, light water (H2O) plays the role of moderator andcoolant, 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 atthe 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 ofwater and steam leaves the reactor core. The so generated steam directlygoes to the turbine, therefore steam and moisture must be separated (waterdrops in steam can damage the turbine blades). Steam leaving the turbine iscondensed in the condenser and then fed back to the reactor afterpreheating. Water that has not evaporated in the reactor vessel accumulatesat 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 thePWRs: 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 simplestconstruction - the building costs are comparatively low. 22.5% of the totalpower of presently operating nuclear power plants is given by BWRs.FeedwaterInside of a BWR reactor pressure vessel (RPV), feedwater enters throughnozzles high on the vessel, well above the top of the nuclear fuel assemblies(these nuclear fuel assemblies constitute the "core") but below the waterlevel. The feedwater is pumped into the RPV from the condensers locatedunderneath the low pressure turbines and after going through feedwater
heaters that raise its temperature using extraction steam from variousturbine stages.The feedwater enters into the downcomer region and combines with waterexiting the water separators. The feedwater subcools the saturated waterfrom the steam separators. This water now flows down the downcomerregion, which is separated from the core by a tall shroud. The water thengoes through either jet pumps or internal recirculation pumps that provideadditional pumping power (hydraulic head). The water now makes a 180degree turn and moves up through the lower core plate into the nuclear corewhere the fuel elements heat the water. When the flow moves out of thecore through the upper core plate, about 12 to 15% of the flow by volume issaturated steam.The heating from the core creates a thermal head that assists therecirculation pumps in recirculating the water inside of the RPV. A BWR canbe designed with no recirculation pumps and rely entirely on the thermalhead to recirculate the water inside of the RPV. The forced recirculation headfrom 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 thisregion may be increased to increase the thermal natural recirculationpumping head. At the top of the riser area is the water separator. Byswirling the two phase flow in cyclone separators, the steam is separatedand rises upwards towards the steam dryer while the water remains behindand 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 chevrondryer structure. The steam then exists the RPV through four main steamlines and goes to the turbine.
Control systemsReactor power is controlled via two methods: by inserting or withdrawingcontrol rods and by changing the water flow through the reactor core.Positioning (withdrawing or inserting) control rods is the normal method forcontrolling power when starting up a BWR. As control rods are withdrawn,neutron absorption decreases in the control material and increases in thefuel, so reactor power increases. As control rods are inserted, neutronabsorption increases in the control material and decreases in the fuel, soreactor power decreases. Some early BWRs and the proposed ESBWRdesigns use only natural ciculation with control rod positioning to controlpower from zero to 100% because they do not have reactor recirculationsystems.Changing (increasing or decreasing) the flow of water through the core isthe normal and convenient method for controlling power. When operating onthe so-called "100% rod line," power may be varied from approximately70% to 100% of rated power by changing the reactor recirculation systemflow by varying the speed of the recirculation pumps. As flow of waterthrough 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 beabsorbed by the fuel, and reactor power increases. As flow of water throughthe core is decreased, steam voids remain longer in the core, the amount ofliquid water in the core decreases, neutron moderation decreases, fewerneutrons are slowed down to be absorbed by the fuel, and reactor powerdecreases.Steam TurbinesSteam produced in the reactor core passes through steam separators anddryer plates above the core and then directly to the turbine, which is part ofthe reactor circuit. Because the water around the core of a reactor is alwayscontaminated with traces of radionuclides, the turbine must be shieldedduring normal operation, and radiological protection must be provided duringmaintenance. The increased cost related to operation and maintenance of aBWR tends to balance the savings due to the simpler design and greater
thermal efficiency of a BWR when compared with a PWR. Most of theradioactivity in the water is very short-lived (mostly N-16, with a 7 secondhalf life), so the turbine hall can be entered soon after the reactor is shutdown.SafetyLike the pressurized water reactor, the BWR reactor core continues toproduce heat from radioactive decay after the fission reactions havestopped, making nuclear meltdown possible in the event that all safetysystems have failed and the core does not receive coolant. Also like thepressurized water reactor, a boiling-water reactor has a negative voidcoefficient, that is, the thermal output decreases as the proportion of steamto liquid water increases inside the reactor. However, unlike a pressurizedwater reactor which contains no steam in the reactor core, a suddenincrease in BWR steam pressure (caused, for example, by a blockage ofsteam flow from the reactor) will result in a sudden decrease in theproportion of steam to liquid water inside the reactor. The increased ratio ofwater to steam will lead to increased neutron moderation, which in turn willcause an increase in the power output of the reactor. Because of this effectin BWRs, operating components and safety systems are designed to ensurethat no credible, postulated failure can cause a pressure and power increasethat exceeds the safety systems capability to quickly shutdown the reactorbefore damage to the fuel or to components containing the reactor coolantcan occur.In the event of an emergency that disables all of the safety systems, eachreactor is surrounded by a containment building designed to seal off thereactor from the environment.
Comparison with other reactorsLight water is ordinary water. In comparison, some other water-cooled waterreactor types use heavy water. In heavy water, the deuterium isotope ofhydrogen replaces the common hydrogen atoms in the water molecules(D2O instead of H2O, molecular weight 20 instea of 18). insteadThe Pressurized Water Reactor (PWR) was the first type of light-water lightreactor developed because of its application to submarine propulsion. Thecivilian motivation for the BWR is reducing costs for commercial applicationsthrough design simplification and lower pressure components. In naval cationreactors, BWR designs are used when natural circulation is specified for itsquietness. The description of BWRs below describes civilian reactor plants inwhich the same water used for reactor cooling is also used in the Rankinecycle turbine generators. A Naval BWR is designed like a PWR that has bothprimary and secondary loops.In contrast to the pressurized water reactors that utilize a primary andsecondary loop, in civilian BWRs the steam going to the turbine that powers turbinethe electrical generator is produced in the reactor core rather than in steamgenerators or heat exchangers. There is just a single circuit in a civilian BWRin which the water is at lower pressure (about 75 times atmosphericpressure) compared to a PWR so that it boils in the core at about 285°C. The mparedreactor is designed to operate with steam comprising 12–15% of the volume 12 15%
of the two-phase coolant flow (the "void fraction") in the top part of thecore, resulting in less moderation, lower neutron efficiency and lower powerdensity than in the bottom part of the core. In comparison, there is nosignificant boiling allowed in a PWR because of the high pressure maintainedin 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
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 ReactorsClassification of Nuclear ReactorsNuclear Reactors, specifically fission reacors, are classified by severalmethods, a brief outline of these classification schemes is given below.Classification by useResearch reactors : Typically reactors used for research and training,materials testing, or the production of radioisotopes for medicine andindustry. These are much smaller than power reactors or those propellingships, and many are on university campuses. There are about 280 suchreactors operating, in 56 countries. Some operate with high-enriched
uranium fuel, and international efforts are underway to substitute low-enriched fuel.Production reactorsPower reactorsPropulsion reactorsClassification by moderator materialGraphite moderated reactorswater moderated reactors • Light water moderated reactors (LWRs) • Heavy Water moderated reactorsClassification by coolantGas cooled reactorLiquid metal cooled reactorWater cooled reactor • Pressure water reactor • Boiling water reactorClassification by type of nuclear reactionFast ReactorsThermal reactorsClassification by role in the fuel cycleBreeder reactorsburner reactorsClassification by GenerationGeneration II reactorGeneration III reactorGeneration IV reactorClassification by phase of fuelSolid fueled
Fluid fueledGas FueledThe Nuclear Fuel CycleThe 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 fromnuclear reactions are referred to collectively as the nuclear fuel cycle. Thenuclear fuel cycle starts with the mining of uranium and ends with thedisposal of nuclear waste. With the reprocessing of used fuel as an option fornuclear energy, the stages form a true cycle.UraniumUranium is a slightly radioactive metal that occurs throughout the earthscrust. It is about 500 times more abundant than gold and about as commonas tin. It is present in most rocks and soils as well as in many rivers and insea water. It is, for example, found in concentrations of about four parts permillion (ppm) in granite, which makes up 60% of the earths crust. Infertilisers, uranium concentration can be as high as 400 ppm (0.04%), andsome coal deposits contain uranium at concentrations greater than 100 ppm(0.01%). Most of the radioactivity associated with uranium in nature is infact due to other minerals derived from it by radioactive decay processes,and which are left behind in mining and milling.
There are a number of areas around the world where the concentration ofuranium in the ground is sufficiently high that extraction of it for use asnuclear fuel is economically feasible. Such concentrations are called ore.Thebelow figure represents various stages in Nuclear Fuel cycleUranium MiningBoth excavation and in situ techniques are used to recover uranium ore. techniquesExcavation may be underground and open pit mining.In general, open pit mining is used where deposits are close to the surfaceand underground mining is used for deep deposits, typically greater than120 m deep. Open pit mines require large holes on the surface, larger thanthe size of the ore deposit, since the walls of the pit must be sloped toprevent collapse. As a result, the quantity of material that must be removedin order to access the ore may be large. Underground mines have relatively Undergroundsmall surface disturbance and the quantity of material that must be removedto access the ore is considerably less than in the case of an open pit mine.An increasing proportion of the worlds uranium now comes from in situleaching (ISL), where oxygenated groundwater is circulated through a very achingporous orebody to dissolve the uranium and bring it to the surface. ISL may
be with slightly acid or with alkaline solutions to keep the uranium insolution. The uranium is then recovered from the solution as in aconventional mill.The decision as to which mining method to use for a particular deposit isgoverned by the nature of the orebody, safety and economic considerations.In the case of underground uranium mines, special precautions, consistingprimarily of increased ventilation, are required to protect against airborneradiation exposure.Uranium MillingMilling, which is generally carried out close to a uranium mine, extracts theuranium from the ore. Most mining facilities include a mill, although wheremines 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 than80% 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 byleaching, in which either a strong acid or a strong alkaline solution is used todissolve the uranium. The uranium is then removed from this solution andprecipitated. After drying and usually heating it is packed in 200-litre drumsas a concentrate.The remainder of the ore, containing most of the radioactivity and nearly allthe rock material, becomes tailings, which are emplaced in engineeredfacilities near the mine (often in mined out pit). Tailings contain long-livedradioactive materials in low concentrations and toxic materials such as heavymetals; however, the total quantity of radioactive elements is less than inthe original ore, and their collective radioactivity will be much shorter-lived.These materials need to be isolated from the environment.ConversionThe product of a uranium mill is not directly usable as a fuel for a nuclearreactor. Additional processing, generally referred to as enrichment, isrequired for most kinds of reactors. This process requires uranium to be ingaseous form and the way this is achieved is to convert it to uraniumhexafluoride, which is a gas at relatively low temperatures.
At a conversion facility, uranium is first refined to uranium dioxide, whichcan be used as the fuel for those types of reactors that do not requireenriched uranium. Most is then converted into uranium hexafluoride, readyfor the enrichment plant. It is shipped in strong metal containers. The mainhazard of this stage of the fuel cycle is the use of hydrogen fluoride.EnrichmentNatural uranium consists, primarily, of a mixture of two isotopes (atomicforms) of uranium. Only 0.7% of natural uranium is "fissile", or capable ofundergoing fission, the process by which energy is produced in a nuclearreactor. The fissile isotope of uranium is uranium 235 (U-235). Theremainder is uranium 238 (U-238).In the most common types of nuclear reactors, a higher than naturalconcentration of U-235 is required. The enrichment process produces thishigher concentration, typically between 3.5% and 5% U-235, by removingover 85% of the U-238. This is done by separating gaseous uraniumhexafluoride into two streams, one being enriched to the required level andknown as low-enriched uranium. The other stream is progressively depletedin U-235 and is called tails.There are two enrichment processes in large scale commercial use, each ofwhich uses uranium hexafluoride as feed: gaseous diffusion and gascentrifuge. They both use the physical properties of molecules, specificallythe 1% mass difference, to separate the isotopes. The product of this stageof the nuclear fuel cycle is enriched uranium hexafluoride, which isreconverted to produce enriched uranium oxide.Fuel fabricationReactor fuel is generally in the form of ceramic pellets. These are formedfrom pressed uranium oxide which is sintered (baked) at a high temperature(over 1400°C). The pellets are then encased in metal tubes to form fuelrods, which are arranged into a fuel assembly ready for introduction into areactor. The dimensions of the fuel pellets and other components of the fuelassembly are precisely controlled to ensure consistency in the characteristicsof fuel bundles.In a fuel fabrication plant great care is taken with the size and shape of
processing vessels to avoid criticality (a limited chain reaction releasingradiation). With low-enriched fuel criticality is most unlikely, but in plantshandling special fuels for research reactors this is a vital consideration.Power generationInside a nuclear reactor the nuclei of U-235 atoms split (fission) and, in theprocess, release energy. This energy is used to heat water and turn it intosteam. The steam is used to drive a turbine connected to a generator whichproduces electricity. Some of the U-238 in the fuel is turned into plutoniumin the reactor core. The main plutonium isotope is also fissile and it yieldsabout one third of the energy in a typical nuclear reactor. The fissioning ofuranium is used as a source of heat in a nuclear power station in the sameway that the burning of coal, gas or oil is used as a source of heat in a fossilfuel 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 afew 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 elementsformed in the same way as plutonium in a fuel bundle will increase to thepoint 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 ofenergy that is produced from a fuel bundle varies with the type of reactorand the policy of the reactor operator.Typically, some 36 million kilowatt-hours of electricity are produced fromone tonne of natural uranium. The production of this amount of electricalpower from fossil fuels would require the burning of over 20,000 tonnes ofblack coal or 8.5 million cubic metres of gas.Used fuel storageWhen removed from a reactor, a fuel bundle will be emitting both radiation,principally from the fission fragments, and heat. Used fuel is unloaded into astorage pond immediately adjacent to the reactor to allow the radiation
levels to decrease. In the ponds the water shields the radiation and absorbsthe heat. Used fuel is held in such pools for several months to several years.Depending on policies in particular countries, some used fuel may betransferred to central storage facilities. Ultimately, used fuel must either bereprocessed or prepared for permanent disposal.ReprocessingUsed fuel is about 95% U-238 but it also contains about 1% U-235 that hasnot fissioned, about 1% plutonium and 3% fission products, which are highlyradioactive, with other transuranic elements formed in the reactor. In areprocessing facility the used fuel is separated into its three components:uranium, plutonium and waste, containing fission products. Reprocessingenables recycling of the uranium and plutonium into fresh fuel, and producesa significantly reduced amount of waste (compared with treating all usedfuel as waste).Uranium and Plutonium RecyclingThe uranium from reprocessing, which typically contains a slightly higherconcentration of U-235 than occurs in nature, can be reused as fuel afterconversion and enrichment, if necessary. The plutonium can be directlymade into mixed oxide (MOX) fuel, in which uranium and plutonium oxidesare combined.In reactors that use MOX fuel, plutonium substitutes for the U-235 in normaluranium oxide fuel.Used fuel disposalAt the present time, there are no disposal facilities (as opposed to storagefacilities) in operation in which used fuel, not destined for reprocessing, andthe waste from reprocessing can be placed. Although technical issues relatedto disposal have been addressed, there is currently no pressing technicalneed to establish such facilities, as the total volume of such wastes isrelatively small. Further, the longer it is stored the easier it is to handle, dueto the progressive diminution of radioactivity. There is also a reluctance todispose of used fuel because it represents a significant energy resourcewhich could be reprocessed at a later date to allow recycling of the uranium
and plutonium. (There is a proposal to use it in Candu reactors directly asfuel.)A number of countries are carrying out studies to determine the optimumapproach to the disposal of spent fuel and wastes from reprocessing. Thegeneral consensus favours its placement into deep geological repositories,initially recoverable.WastesWastes from the nuclear fuel cycle are categorised as high-, medium- orlow-level wastes by the amount of radiation that they emit. These wastescome 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% foundin nature. Small quantities of this material, which is primarily U-238, areused in applications where high density material is required, includingradiation 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 someprecautions must, therefore, be taken in its storage or disposal.Nuclear Energy,Nuclear FuelsNuclear EnergyNuclei are made up of protons and neutron, but the mass of a nucleus isalways less than the sum of the individual masses of the protons andneutrons which constitute it. The difference is a measure of the nuclearbinding energy which holds the nucleus together.Nuclear energy is energy released from the atomic nucleus. Atoms are tinyparticles that make up every object in the universe. There is enormous
energy in the bonds that hold atoms together.This binding energy can becalculated from the Einstein relationship: mass-energy equivalence formulaE = mc², in which E = energy, m = mass, and c = the speed of light in avacuum (a physical constant).The alpha particle gives binding energy of 28.3MeVNuclear 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 FuelsNuclear 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 mostcommon type of nuclear fuel is heavy fissile elements that can be made toundergo nuclear fission chain reactions in a nuclear fission reactor; nuclearfuel can refer to the material or to physical objects (for example fuel bundlescomposed of fuel rods) composed of the fuel material, perhaps mixed withstructural, neutron moderating, or neutron reflecting materials.Not all nuclear fuels are used in fission chain reactions. For example, 238Puand some other elements are used to produce small amounts of nuclearpower by radioactive decay in radiothermal generators, and other atomicbatteries. Light isotopes such as 3H (tritium) are used as fuel for nuclearfusion. If one looks at binding energy of specific isotopes, there can be anenergy gain from fusing most elements with a lower atomic number thaniron, and fissioning isotopes with a higher atomic number than iron.The most common fissile nuclear fuels are natural urnium,enricheduranium,plutonium and 233U.Natural uranium is the parent material.Thematerials 235U,233U and 239Pu are called fissionable materials.The onlyfissionable nuclear fuel occuring in nature is uraium of which 99.3% is 238Uand 0.7% is 235U and 234U is only a trace.Out of these isotopes only 235U
will fission in a chain reaction.The other two fissionable materials can beproduced artificially from 238U and 232Th which occur in nature are calledfertile materials.Out of the three fissionable materials 235U has someadvantages over the other two due to its higher fissionpercentage.Fissionable materials 239Pu and 233U are formed in the nuclearreactors during fission process from 238U and 232Th respectively due toabsorption of neutrons with out fission.Getting 239Pu process is calledconversion and getting 233U is called breeding.Nuclear FissionNuclear FissionNuclear fission—also known as atomic fission—is a process in nuclear physicsand nuclear chemistry in which the nucleus of an atom splits into two ormore smaller nuclei as fission products, and usually some by-productparticles, 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 releasesubstantial amounts of useful energy both as gamma rays and as kineticenergy of the fragments (heating the bulk material where fission takesplace).Nuclear fission produces energy for nuclear power and to drive explosion ofnuclear weapons. Fission is useful as a power source because somematerials, called nuclear fuels, generate neutrons as part of the fissionprocess and undergo triggered fission when impacted by a free neutron.Nuclear fuels can be part of a self-sustaining chain reaction that releasesenergy at a controlled rate in a nuclear reactor or at a very rapiduncontrolled rate in a nuclear weapon.The amount of free energy contained in nuclear fuel is millions of times theamount of free energy contained in a similar mass of chemical fuel such asgasoline, making nuclear fission a very tempting source of energy; however,
the byproducts of nuclear fission are highly radioactive and remain so formillennia, giving rise to a nuclear waste problem.Splitting the Uranium Atom:Uranium is the principle element used in nuclear reactors and in certaintypes of atomic bombs. The specific isotope used is 235U. When a strayneutron strikes a 235U nucleus, it is at first absorbed into it. This creates236U. 236U is unstable and this causes the atom to fission. The fissioning of236U can produce over twenty different products. However, the productsmasses always add up to 236. The following two equations are examples ofthe different products that can be produced when 235U fissions:235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY235U + 1 neutron 2 neutrons + 92Sr + 140Xe + ENERGY Lets discuss those reactions. In each of theabove reactions, 1 neutron splits the atom. When the atom is split, 1additional neutron is released. This is how a chain reaction works. If more235U is present, those 2 neutrons can cause 2 more atoms to split. Each ofthose atoms releases 1 more neutron bringing the total neutrons to 4. Those4 neutrons can strike 4 more 235U atoms, releasing even more neutrons.The chain reaction will continue until all the 235U fuel is spent. This isroughly what happens in an atomic bomb. It is called a runaway nuclearreaction.Where Does the Energy Come From?In the section above we described what happens when an 235U atomfissions. We gave the following equation as an example:235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGYYou might have been wondering, "Where does the energy come from?". The
mass seems to be the same on both sides of the reaction:235 + 1 = 2 + 92 + 142 = 236Thus, it seems that no mass is converted into energy. However, this is notentirely correct. The mass of an atom is more than the sum of the individualmasses of its protons and neutrons, which is what those numbers represent.Extra mass is a result of the binding energy that holds the protons andneutrons 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 formof heat. Because energy and mass are one and the same, the energyreleased is also mass released. Therefore, the total mass does decrease atiny bit during the reaction.Fission in Nuclear ReactorsTo make large-scale use of the energy released in fission, one fission eventmust trigger another, so that the process spreads thoughout the nuclear fuelas in a set of dominos. The fact that more neutrons are produced in fissionthan are consumed raises the possibility of a chain reaction. Such a reactioncan 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 someother neutron-absorbing material are used to regulate the number ofneutrons. The more exposed control rods, the less neutrons and vice versa.This also controls the multiplication factor k which is the ratio of the numberof neutrons present at the beginning of a particular generation to thenumber present at the beginning of the next generation. For k=1, theoperation of the reactor is said to be exactly critical, which is what we wish itto be for steady-power operation. Reactors are designed so that they areinherently supercritical (k>1); the multiplication factor is then adjusted tothe critical operation by inserting the control rods.An unavoidable feature of reactor operation is the accumulation ofradioactive wastes, including both fission products and heavy "transuranic"nuclides such as plutonium and americium.Nuclear Power
Nuclear PowerNuclear power is the controlled use of nuclear reactions to release energy forwork including propulsion, heat, and the generation of electricity. Use ofnuclear power to do significant useful work is currently limited to nuclearfission and radioactive decay. Nuclear energy is produced when a fissilematerial, such as uranium-235 (235U), is concentrated such that nuclearfission takes place in a controlled chain reaction and creates heat — which isused to boil water, produce steam, and drive a steam turbine. The turbinecan be used for mechanical work and also to generate electricity. Nuclearpower provides 7% of the worlds energy and 15.7% of the worldselectricity and is used to power most military submarines and aircraftcarriers.The United States produces the most nuclear energy, with nuclear powerproviding 20% of the electricity it consumes, while France produces thehighest percentage of its electrical energy from nuclear reactors—80% as of2006. In the European Union as a whole, nuclear energy provides 30% ofthe electricity.Nuclear energy policy differs between countries, and somecountries such as Austria, Australia and Ireland have no nuclear powerstations.Concerns about nuclear powerThe use of nuclear power is controversial because of the problem of storingradioactive waste for indefinite periods, the potential for possibly severeradioactive contamination by accident or sabotage, and the possibility thatits use in some countries could lead to the proliferation of nuclear weapons.Proponents believe that these risks are small and can be further reduced bythe technology in the new reactors. They further claim that the safety recordis already good when compared to other fossil-fuel plants, that it releasesmuch less radioactive waste than coal power, and that nuclear power is asustainable energy source. Critics, including most major environmentalgroups, claim nuclear power is an uneconomic and potentially dangerousenergy source with a limited fuel supply, especially compared to renewableenergy, and dispute whether the costs and risks can be reduced throughnew technology.
There is concern in some countries over North Korea and Iran operatingresearch reactors and fuel enrichment plants, since those countries refuseadequate IAEA oversight and are believed to be trying to develop nuclearweapons. North Korea admits that it is developing nuclear weapons, whilethe Iranian government vehemently denies the claims against Iran.Several concerns about nuclear power have been expressed, and theseinclude: • 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 proliferationNuclear Power Plant,Types, Advantages and DisadvantagesNuclear Power PlantNuclear power is generated using Uranium, which is a metal mined invarious parts of the world.The structure of a nuclear power plant in many aspects resembles to that ofa conventional thermal power station, since in both cases the heat producedin the boiler (or reactor) is transported by some coolant and used togenerate steam. The steam then goes to the blades of a turbine and byrotating it, the connected generator will produce electric energy. The steamgoes to the condenser, where it condenses, i.e. becomes liquid again. Thecooled down water afterwards gets back to the boiler or reactor, or in thecase of PWRs to the steam generator.
The great difference between a conventional and a nuclear power plant ishow 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 anuclear power plant, however, energy that comes from fission reactions isutilized.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.
In Britain, nuclear power stations are built on the coast, and use sea waterfor cooling the steam ready to be pumped round again. This means that theydont have the huge "cooling towers" seen at other power stations.The reactor is controlled with "control rods", made of boron, which absorbneutrons. When the rods are lowered into the reactor, they absorb moreneutrons and the fission process slows down. To generate more power, therods are raised and more neutrons can crash into uranium atoms.Nuclear Power Plant TypesSeveral nuclear power plant (NPP) types are used for energy generation inthe world. The different types are usually classified based on the mainfeatures of the reactor applied in them. The most widespread power plantreactor 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 its 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.
• 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 1990s 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 WheelPelton WheelA Pelton wheel, also called a Pelton turbine, is one of the most efficient typesof water turbines. It was invented by Lester Allan Pelton (1829-1908) in the1870s, and is an impulse machine, meaning that it uses Newtons secondlaw to extract energy from a jet of fluid.
The pelton wheel turbine is a tangential flow impulse turbine, water flowsalong the tangent to the path of the runner. Nozzles direct forceful streamsof water against a series of spoon-shaped buckets mounted around the edge spoon shapedof a wheel. Each bucket reverses the flow of water, leaving it with water,diminished energy. The resulting impulse spins the turbine. The buckets aremounted in pairs, to keep the forces on the wheel balanced, as well as toensure 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 isextracted in the first stage of the turbine. Therefore, Pelton wheels haveonly one wheel, unlike turbines that operate with compressible fluids.ApplicationsPeltons 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 tonmounted on vertical oil pad bearings in the generator houses of hydroelectricplants. The largest units can be up to 200 megawatts. The smallest Peltonwheels, only a few inches across, are used with household plumbing fixturesto tap power from mountain streams with a few gallons per minute of flow, untainbut these small units must have thirty meters or more of head. Dependingon water flow and design, Pelton wheels can operate with heads as small as15 meters and as high as 1,800 meters.In general, as the height of fall increases, less volume of water can generatea bit more power. Energy is force times distance, in the instance of fluid flowpower is expressed as P = Constant x Pressure x Volume/t. The power P
grows linearly with flow rate and grows with f(Pressure^3/2.) Thus it isusually best to seek as much head or pressure as possible in hydro designsthen go for flow rate.Kaplan TurbineKaplan TurbineThe Kaplan turbine is a propeller-type water turbine that has adjustableblades. It was developed in 1913 by the Austrian professor Viktor Kaplan.The Kaplan turbine was an evolution of the Francis turbine. Its inventionallowed efficient power production in low head applications that was notpossible 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 theworking fluid changes pressure as it moves through the turbine and gives upits energy. The design combines radial and axial features.
The above figures shows flow in a Kaplan turbine. In the picture, pressure onrunner 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 turbines wicket gate.Water is directed tangentially, through the wicket gate, and spirals on to apropeller shaped runner, causing it to spin.The outlet is a specially shaped draft tube that helps decelerate the waterand recover kinetic energy.The turbine does not need to be at the lowest point of water flow, as long asthe draft tube remains full of water. A higher turbine location, however,increases the suction that is imparted on the turbine blades by the drafttube. The resulting pressure drop may lead to cavitation.Variable geometry of the wicket gate and turbine blades allow efficientoperation for a range of flow conditions. Kaplan turbine efficiencies aretypically over 90%, but may be lower in very low head applications.ApplicationsKaplan turbines are widely used throughout the world for electrical powerproduction. They cover the lowest head hydro sites and are especially suitedfor high flow conditions.Inexpensive micro turbines are manufactured for individual power productionwith as little as two feet of head.Large Kaplan turbines are individually designed for each site to operate atthe highest possible efficiency, typically over 90%. They are very expensiveto design, manufacture and install, but operate for decades.VariationsThe Kaplan turbine is the most widely used of the propeller-type turbines,but several other variations exist:
Propeller turbines have non-adjustable propeller vanes. They are used in lowcost, small installations. Commercial products exist for producing severalhundredwatts from only a few feet of head.Bulb or Tubular turbines are designed into the water delivery tube. A largebulb is centered in the water pipe which holds the generator, wicket gateand runner. Tubular turbines are a fully axial design, whereas Kaplanturbines have a radial wicket gate. Pit turbines are bulb turbines with a gearbox. This allows for a smaller generator and bulb.Straflo turbines are axial turbines with the generator outside of the waterchannel, connected to the periphery of the runner.S- turbines eliminate the need for a bulb housing by placing the generatoroutside of the water channel. This is accomplished with a jog in the waterchannel and a shaft connecting the runner and generator.Tyson turbines are a fixed propeller turbine designed to be immersed in afast flowing river, either permanently anchored in the river bed, or attachedto a boat or barge.Francis TurbineFrancis TurbineThe Francis turbine is a type of water turbine that was developed by JamesB. Francis. It is an inward flow reaction turbine that combines radial andaxial flow concepts.Francis turbines are the most common water turbine in use today. Theyoperate in a head range of ten meters to several hundred meters and areprimarily used for electrical power production.
The Francis turbine is a reactionturbine, which means that the working fluid changes pressure as it movesthrough the turbine, giving up its energy. A casement is needed to containthe water flow. The turbine is located between the high pressure watersource 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 therunner. 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 turbineoperation 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 acircle. If the string is pulled short, the ball spins faster. This property, inaddition to the waters pressure, helps inward flow turbines harness waterenergy.At the exit, water acts on cup shaped runner features, leaving withno swirl and very little kinetic or potential energy. The turbines exit tube isshaped to help decelerate the water flow and recover the pressure.ApplicationLarge Francis turbines are individually designed for each site to operate atthe highest possible efficiency, typically over 90%.Francis type units cover a wide head range, from 20 meters to 700 metersand their output varies from a few kilowatt to 1000 megawatt. Their sizevaries from a few hundred millimeters to about 10 meters.In addition to electrical production, they may also be used for pumpedstorage; where a reservoir is filled by the turbine (acting as a pump) duringlow power demand, and then reversed and used to generate power duringpeak 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 usedturbine in the world.
Water Turbines and its ClassificationWater TurbineWater turbine is a device that convert the energy in a stream of fluid intomechanical energy by passing the stream through a system of fixed andmoving fan like blades and causing the latter to rotate. A turbine looks like alarge wheel with many small radiating blades around its rim.Classification of Water turbinesAccording to the type of flow of water : The water turbines used as primemovers in hydro electric power stations are of four types.They are • axial flow : having flow along shaft axis • inward radial flow : having flow along the radius • tangential or peripheral : having flow along tangential direction • mixed flow : having radial inlet axial outletIf the runner blades of axial flow turbines are fixed,those are called propellerturbines.According to the action of water on moving blades water turbines are of 2types namely impulse ad reaction type turbines.Impulse Turbines :These turbines change the direction of flow of a highvelocity fluid jet. The resulting impulse spins the turbine and leaves the fluidflow with diminished kinetic energy. There is no pressure change of the fluidin the turbine rotor blades. Before reaching the turbine the fluids Pressurehead is changed to velocity head by accelerating the fluid with a nozzle.Pelton wheels and de Laval turbines use this process exclusively. Impulseturbines do not require a pressure casement around the runner since thefluid jet is prepared by a nozzle prior to reaching turbine. Newtons secondlaw describes the transfer of energy for impulse turbines.Reaction Turbines : These turbines develop torque by reacting to thefluids pressure or weight. The pressure of the fluid changes as it passesthrough the turbine rotor blades. A pressure casement is needed to contain
the working fluid as it acts on the turbine stage(s) or the turbine must befully immersed in the fluid flow (wind turbines). The casing contains anddirects the working fluid and, for water turbines, maintains the suction waterimparted by the draft tube. Francis turbines and most steam turbines usethis concept. For compressible working fluids, multiple turbine stages maybe used to harness the expanding gas efficiently. Newtons third lawdescribes the transfer of energy for reaction turbines. ribesAccording to the Head and quantity of water available the water turbines areof 2 types.Those are high head - low flow and low to medium head and highto medium discharge turbines.According to the name of the originator water turbines are of 3 types namelyPelton Wheel,Francis tubine and Kaplan turbine.Hydro Power Plant WorkingHow HydroPower Plant worksA hydroelectric power plant harnesses the energy found in moving or stillwater and converts it into electricity.
Moving water, such as a river or a waterfall, has mechanical energy.‘Mechanical energy is the energy that is possessed by an object due to itsmotion or stored energy of position.’ This means that an object hasmechanical energy if it’s in motion or has the potential to do work (themovement of matter from one location to another,) based on its position.The energy of motion is called kinetic energy and the stored energy ofposition is called potential energy. Water has both the ability and thepotential to do work. Therefore, water contains mechanical energy (theability to do work), kinetic energy (in moving water, the energy based onmovement), and potential energy (the potential to do work.)The potential and kinetic/mechanical energy in water is harnessed bycreating a system to efficiently process the water and create electricity fromit. A hydroelectric power plant has eleven main components. The firstcomponent is a dam.The dam is usually built on a large river that has a drop in elevation, so as touse the forces of gravity to aid in the process of creating electricity. A dam isbuilt to trap water, usually in a valley where there is an existing lake. Anartificial storage reservoir is formed by constructing a dam across ariver.Notice that the dam is much thicker at the bottom than at the top,because the pressure of the water increases with depth.The area behind the dam where water is stored is called the reservoir. Thewater there is called gravitational potential energy. The water is in a storedposition above the rest of the dam facility so as to allow gravity to carry thewater down to the turbines. Because this higher altitude is different thanwhere the water would naturally be, the water is considered to be at analtered equilibrium. This results in gravitational potential energy, or, “thestored energy of position possessed by an object.” The water has thepotential to do work because of the position it is in (above the turbines, inthis case.)Gravity will force the water to fall to a lower position through the intake andthe control gate. They are built on the inside of the dam. When the gate isopened, the water from the reservoir goes through the intake and becomes
translational kinetic energy as it falls through the next main part of thesystem: the penstock. Translational kinetic energy is the energy due tomotion from one location to another. The water is falling (moving) from thereservoir towards the turbines through the penstock.The intake shown in figure includes the head works which are the structuresat the intake of conduits,tunnels or flumes.These structures includeblooms,screens or trash - racks, sluices to divert and prevent entry of debrisand ice in to the turbines.Booms prevent the ice and floating logs from goingin to the intake by diverting them to a bypass chute.Screens or trash-racks(shown in fig) are fitted irectly at the intake to prevent the debris fromgoing in to the take.Debris cleaning devices should also be fitted on thetrash-racks.Intake structures can be classified in to high pressure intakesused in case of large storage reservoirs and low pressure intakes used incase of small ponds.The use of providing these structures at the intakeis,water only enters and flows through the penstock which strikes theturbine.Control gates arrangement is provided with Spillways.Spillway is constructedto act as a safety valve.It dischargs the overflow water to the down streamside when the reservoir is full.These are generally constructed of concreteand provided with water discharge opening,shut off by metal controlgates.By changing the degree to which the gates are opened,the dischargeof the head water to the tail race can be regulated inorder to maintain waterlevel in reservoir.The penstock is a long shaft that carries the water towards the turbineswhere the kinetic energy becomes mechanical energy. The force of the wateris used to turn the turbines that turn the generator shaft. The turning of thisshaft is known as rotational kinetic energy because the energy of the movingwater is used to rotate the generator shaft. The work that is done by thewater to turn the turbines is mechanical energy. This energy powers thegenerators, which are very important parts of the hydroelectric power plant;they convert the energy of water into electricity. Most plants contain severalgenerators to maximize electricity production.
The generators are comprised of four basic components: the shaft, theexcitor, the rotor, and the stator. The turning of the turbines powers theexcitor to send an electrical current to the rotor. The rotor is a series of largeelectromagnets that spins inside a tightly wound coil of copper wire, calledthe stator. “A voltage is induced in the moving conductors by an effect calledelectromagnetic induction.” The electromagnetic induction caused by thespinning electromagnets inside the wires causes electrons to move, creatingelectricity. The kinetic/mechanical energy in the spinning turbines turns intoelectrical energy as the generators function.The transformer, another component, takes the alternating current andconverts it into higher-voltage current. The electrical current generated inthe generators is sent to a wire coil in the transformer. This is electricalenergy. Another coil is located very close to first one and the fluctuatingmagnetic field in the first coil will cut through the air to the second coilwithout the current. The amount of turns in the second coil is proportional tothe amount of voltage that is created. If there are twice as many turns onthe second coil as there are on the first one, the voltage produced will betwice as much as that on the first coil. This transference of electrical currentis electrical energy. It goes from the generators to one coil, and then istransferred through an electromagnetic field onto the second coil. Thatcurrent is then sent by means of power lines to the public as electricityNow, the water that turned the turbines flows through the pipelines(translational kinetic energy, because the energy in the water is beingmoved,) called tailraces and enters the river through the outflow. The wateris back to being kinetic/mechanical/potential energy as it is in the river andhas to potential to have the energy harnessed for use as it flows along(movement.)
Pumped Storage PlantsPumped Storage Plants"Pumped Storage" is another form of hydro-electric power. Pumped storage hydro electricfacilities use excess electrical system capacity, generally available at night,to pump water from one reservoir to another reservoir at a higher elevation.During periods of peak electrical demand, water from the higher reservoir is eakreleased through turbines to the lower reservoir, and electricity is produced .Although pumped storage sites are not net producers of electricity - itactually takes more electricity to pump the water up than is recovered when waterit is released - they are a valuable addition to electricity supply systems.Their value is in their ability to store electricity for use at a later time whenpeak demands are occurring. Storage is even more valuable if intermittent intermsources of electricity such as solar or wind are hooked into a system.Pumped storage plant is a unique design of peak load plant in which theplant pumps back all or portion of its water supply during lo load period.Theusual construction is a lowand high elevation reservoirs connected through a owandpenstock.The generating pumping plant is at the lower end.The plant utilisessome of the surplus energy generated by the base load plant to pump waterfrom low elevation to highelevation reservoir during off peak hours.Duringpeak load period this water is used to generate power by allowing it to flowfrom high elevation reservoir through reversible hydraulic turbine of thisplan to low elevation reservoir.Thus same water is used again and again andextra water is required only to take care of evaporation and seepage.The main important point in this plant is reversible turbine/generator
assemblies act as pump and turbine (usually a Francis turbinedesign).During low load periods it acts as pump and pumps water from lowto high elevation reservoir.During peak load periods it acts as turbine whenwater flows from high to low elevation reservoir.To see the flash animation of pumped storage plant working Click hereAdvantages • Without some means of storing energy for quick release, wed be in trouble. • Little effect on the landscape. • No pollution or wasteDisadvantages • Expensive to build. • Once its used, you cant use it again until youve pumped the water back up. Good planning can get around this problem.Hydro Power - Introduction and TypesHydro Power :Hydro power has played an important historical role in the industrializationof society from grinding flour to powering industry. Hydro energy originatesfrom the sun, and hence, is renewable and its fuel is free.“Hydro” means “water” in Latin – so “hydro power” is made fromwater.Hydropower is the capture of the energy of moving water for someuseful purpose.The analysis of hydroelectric generation begins with thepotential energy of the water. The gravitational potential energy (PE) isdefined based on a material’s mass (m) and height (H) from a referencepoint.PE = m.g.Hwhere g is gravitational constant. The power generation (P) depends uponthe period (T) over which the water is discharged through that height, often
times referred to as the head.The water mass may be expressed in terms of its density (ρ) and volume(V), i.e., m=ρV.Often,the volume of water is measured in acre-feet which is the volume acre feetoccupied by a foot of water covering an acre of area; one acre-foot is acreequivalent to 43,560 ft³. The standard density of water is 1,000 kg/m³ or62.4 lbm/ft³. The power can then be represented in terms of the mass flow inrate or volumetric flow rate The electric power output is reduced by the hydraulicturbine-generator efficiency. generatorThere are many forms of water power: • Waterwheels , used for hundreds of years to power mills and machinery • Hydroelectric energy, a term usually reserved for hydroelectric dams. tric energy, • Tidal power, which captures energy from the tides in horizontal , direction • Tidal stream power, which does the same vertically power • Wave power, which uses the energy in waves ,Hydroelectric powerHydroelectric power now supplies about 715,000 MW or 19% of worldelectricity (16% in 2003). Large dams are still being designed. Apart from afew countries with an abundance of it, hydro power is normally applied topeak load demand because it is readily stopped and started. Nevertheless,hydroelectric power is probably not a major option for the future of energyproduction in the developed nations because most major sites within thesenations are either already being exploited or are unavailable for otherreasons, such as environmental considerations.Hydropower produces essentially no carbon dioxide or other harmfulemissions, in contrast to burning fossil fuels, and is not a significantcontributor to global warming through CO2.Hydroelectric power can be far less expensive than electricity generated be
from fossil fuel or nuclear energy. Areas with abundant hydroelectric powerattract industry. Environmental concerns about the effects of reservoirs mayprohibit development of economic hydropower sources.The chief advantage of hydroelectric dams is their ability to handle seasonal(as well as daily) high peak loads. When the electricity demands drop, thedam simply stores more water (which provides more flow when it releases).Some electricity generators use water dams to store excess energy (oftenduring the night), by using the electricity to pump water up into a basin.Electricity can be generated when demand increases. In practice theutilization of stored water in river dams is sometimes complicated bydemands for irrigation which may occur out of phase with peak electricaldemands.Tidal powerHarnessing the tides in a bay or estuary has been achieved in France (since1966), Canada and Russia, and could be achieved in other areas with a largetidal range. The trapped water turns turbines as it is released through thetidal barrage in either direction. Another possible fault is that the systemwould generate electricity most efficiently in bursts every six hours (onceevery tide). This limits the applications of tidal energy.Tidal stream powerA relatively new technology, tidal stream generators draw energy fromcurrents in much the same way that wind generators do. The higher densityof water means that a single generator can provide significant power. Thistechnology is at the early stages of development and will require moreresearch before it becomes a significant contributor.Several prototypes have shown promise. In the UK in 2003, a 300 kWPeriodflow marine current propeller type turbine was tested off the coast ofDevon, and a 150 kW oscillating hydroplane device, the Stingray, was testedoff the Scottish coast. Another British device, the Hydro Venturi, is to betested in San Francisco Bay.The Canadian company Blue Energy has plans for installing very large arraystidal current devices mounted in what they call a tidal fence in variouslocations around the world, based on a vertical axis turbine design.
Wave powerHarnessing power from ocean surface wave motion might yield much moreenergy than tides. The feasibility of this has been investigated, particularly inScotland in the UK. Generators either coupled to floating devices or turnedby air displaced by waves in a hollow concrete structure would produceelectricity. Numerous technical problems have frustrated progress.A prototype shore based wave power generator is being constructed at PortKembla in Australia and is expected to generate up to 500 MWh annually.The Wave Energy Converter has been constructed (as of July 2005) andinitial results have exceeded expectations of energy production during timesof low wave energy. Wave energy is captured by an air driven generator andconverted to electricity. For countries with large coastlines and rough seaconditions, the energy of waves offers the possibility of generating electricityin utility volumes. Excess power during rough seas could be used to producehydrogen.Hydro Electric Plants - Classification, Advantages and DisadvantagesClassification of Hydro Electric PlantsThe classification of hydro electric plants based upon :(a) Quantity of water available (b) Available head (c) Nature of loadThe classification acording to Quantity of water available is(i) Run-off river plants with out pondage : These plants does not storewater; the plant uses water as it comes.The plant can use water as andwhen available.Since these plants depend for their generting capacityprimarly on the rate of flow of water, during rainy season high flow rate maymean some quantity of water to go as waste while during low run-offperiods, due to low flow rates,the generating capacity will be low.(ii) Run-off river plants with pondage : In these plants pondage permitsstorage of water during off peak periods and use of this water during peakperiods.Depending on the size of pondage provided it may be possible tocope with hour to hour fluctuations.This type of plant can be used on partsof the load curve as required,and is more useful than a plant with out
storage or pondage.When providing pondage tail race conditions should be such that floods donot raise tail-race water level,thus reducing the head on the plant andimpairing its effectiveness.This type of plant is comparitively more reliableand its generating capacity is less dependent on avilable rate of flow ofwater.(iii) Reservoir Plants :A reservoir plant is that which has a reservoir of suchsize as to permit carrying over storage from wet season to the next dryseason.Water is stored behind the dam and is available to the plant withcontrol as required.Such a plant has better capacity and can be usedefficiently through out the year.Its firm capacity can be increased and can beused either as a base load plant or as a peak load plant as required.It canalso be used on any portion of the load curve as required.Majority of thehydroelectric plants are of this type.The classification according to availability of water head is(i) Low-Head (less than 30 meters) Hydro electric plants :"Low head" hydro-electric plants are power plants which generally utilize heads of only a fewmeters or less. Power plants of this type may utilize a low dam or weir tochannel water, or no dam and simply use the "run of the river". Run of theriver generating stations cannot store water, thus their electric output varieswith seasonal flows of water in a river. A large volume of water must passthrough a low head hydro plants turbines in order to produce a usefulamount of power. Hydro-electric facilities with a capacity of less than about25 MW (1 MW = 1,000,000 Watts) are generally referred to as "smallhydro", although hydro-electric technology is basically the same regardlessof generating capacity.(ii) Medum-head(30 meters - 300 meters) hydro electric plants :Theseplants consist of a large dam in a mountainous area which creates a hugereservoir. The Grand Coulee Dam on the Columbia River in Washington (108meters high, 1270 meters wide, 9450 MW) and the Hoover Dam on theColorado River in Arizona/Nevada (220 meters high, 380 meters wide, 2000MW) are good examples. These dams are true engineering marvels. In fact,the American Society of Civil Engineers as designated Hoover Dam as one ofthe seven civil engineering wonders of the modern world, but the massive
lakes created by these dams are a graphic example of our ability tomanipulate the environment - for better or worse. Dams are also used forflood control, irrigation, recreation, and often are the main source of potablewater for many communities. Hydroelectric development is also possible inareas such as Niagra Falls where natural elevation changes can be used.(iii) High-head hydro electric plants :"High head" power plants are the mostcommon and generally utilize a dam to store water at an increasedelevation. The use of a dam to impound water also provides the capability ofstoring water during rainy periods and releasing it during dry periods. Thisresults in the consistent and reliable production of electricity, able to meetdemand. Heads for this type of power plant may be greater than 1000 m.Most large hydro-electric facilities are of the high head variety. High headplants with storage are very valuable to electric utilities because they can bequickly adjusted to meet the electrical demand on a distribution system.The classification according to nature of load is(i) Base load plants :A base load power plant is one that provides a steadyflow of power regardless of total power demand by the grid. These plantsrun at all times through the year except in the case of repairs or scheduledmaintenance.Power plants are designated base load based on their low cost generation,efficiency and safety at set outputs. Baseload power plants do not changeproduction to match power consumption demands since it is always cheaperto run them rather than running high cost combined cycle plants orcombustion turbines. Typically these plants are large enough to provide amajority of the power used by a grid, making them slow to fire up and cooldown. Thus, they are more effective when used continuously to cover thepower baseload required by the grid.Each base load power plant on a grid is allotted a specific amount of thebaseload power demand to handle. The base load power is determined bythe load duration curve of the system. For a typical power system, rule ofthumb states that the base load power is usually 35-40% of the maximumload during the year.Load factor of such plants is high.
Fluctuations, peaks or spikes in customer power demand are handled bysmaller and more responsive types of power plants.(ii) Peak load plants :Power plants for electricity generation which, due totheir operational and economic properties, are used to cover the peak load.Gas turbines and storage and pumped storage power plants are used aspeak load power plants.The efficiency of such plants is around 60 -70%.Advantages of hydroelectric plants • operation , running and maintenance costs are low. • Once the dam is built, the energy is virtually free. • No fuel is burnt and the plant is quite neat & clean. • No waste or pollution produced. • generating plants have a long lifetime. • Much more reliable than wind, solar or wave power. • Water can be stored above the dam ready to cope with peaks in demand. • unscheduled breakdowns are relatively infrequent and short in duration since the equipment is relatively simple. • hydroelectric turbine-generators can be started and put "on-line" very rapidly. • Electricity can be generated constantlyDisadvantages of hydroelectric plants • very land-use oriented and may flood large regions. • The dams are very expensive to build.However, many dams are also used for flood control or irrigation, so building costs can be shared. • Capital cost of generators, civil engineering works and cost of transmission lines is very high. • Water quality and quantity downstream can be affected, which can have an impact on plant life. • Finding a suitable site can be difficult - the impact on residents and the environment may be unacceptable.
• fish migration is restricted. • fish health affected by water temperature change and insertion of excess nitrogen into water at spillways • available water and its temperature may be affected • reservoirs alter silt-flow patternsTop 10 Rules for Saving EnergyTop 10 Rules for Saving Energy 1. DO shut off the lights when you’re done using them,and turn off the TV, computer, video games and other electrical stuff whenyou leave the room.2. DO lower the thermostat during the winter. To keep warm withoutwasting energy, put on a sweatshirt or snuggle under a blanket.
3. DONT leave the refrigerator door open. Every time youopen the door, up to one-third of the cold air can escape. 4. DO replace a burnt-out light bulb with a new compactfluorescent bulb. Fluorescent bulbs use 75 percent less energy, and theylast 10 times longer. 5. DO remind grown-ups to use cold water in the washingmachine. Hot water won’t get the clothes any cleaner, and it wastes a lot ofenergy.
6. DO turn off dripping faucets. One drop per second canadd up to 165 gallons of hot water a month - thats more than one personuses in two weeks! 7. DON’T take a long bath – take a short shower instead.It might take 25 gallons of hot water to fill the bathtub, compared to onlyseven gallons for a quick shower. 8. DO close the curtains during hot summer days to blockthe sun. During the winter, keep the curtains open.
9. Help a grown-up put plastic sheeting on windows.Blocking cold drafts is called “weatherizing” and it can save a lot of energy. 10. DO help your mom or dad plant a tree to help shadeyour house on hot summer days.What Is Renewable Energy?What Is Renewable Energy?All the energy we use comes from the earth. The electricity we use everyday doesnt come directly from the earth, but we make electricity using theearths resources, like coal or natural gas.Both coal and natural gas are called “fossil fuels” because they were formeddeep under the earth during dinosaur times.The problem is that fossil fuels cant be replaced - once we use them up,theyre gone forever. Another problem is that fossil fuels can cause pollution.Renewable energy is made from resources that Mother Nature will replace,like wind, water and sunshine.Renewable energy is also called “clean energy” or “green power” because itdoesn’t pollute the air or the water.Why don’t we use renewable energy all the time?Unlike natural gas and coal, we can’t store up wind and sunshine to use
whenever we need to make more electricity. If the wind doesn’t blow or thesun hides behind clouds, there wouldn’t be enough power for everyone.Another reason we use fossil fuels like coal and natural gas is becausethey’re cheaper. It costs more money to make electricity from wind, andmost people aren’t willing to pay more on their monthly utility bills.How can we use renewable energy?You might be using renewable energy today without knowing it! Iowa ishome to more than 600 wind turbines, creating enough electricity to power140,000 homes. Wisconsin and Minnesota also have lots of wind farms – andthe number is growing every day.Nuclear Power Plant OperationNuclear Power Plant OperationThe below diagram shows the schematic of nuclear power plant.Nuclearpower generation is much similar to that of conventional steam powergeneration.The difference lies only in the steam generation part i.e coal oroil boiling furnance and boiler are replaced by nuclear reactor.Thus a nuclear power plant consists of a nuclear reactor,steamgenerator,turbine, generator, condenser etc. as shown in the above
figure.As in a conventional steam plant, water for raising steam forms aclosed feed system.However, the reactor and the cooling circuit have to beheavily shielded to eliminate radiation hazards.A nuclear power plant uses the heat generated by a nuclear fission processto drive a steam turbine which generates usable electricity.Fission is thesplitting of atoms into smaller parts. Some atoms, themselves tiny, splitwhen they are struck by even smaller particles, called neutrons. Each timethis happens more neutrons come out of the split atom and strike otheratoms. This process of energy release is called a chain reaction. The plantcontrols the chain reaction to keep it from releasing too much energy toofast. In this way, the chain reaction can go on for a long time.Few natural elements have atoms that will split in a chain reaction. Iron,copper, silver and many other common metals will not split, or fission. Thereare isotopes of iron, copper, etc. that are radioactive. This means that theyhave an unstable nucleus and they emit radioactivity. However, just beingradioactive does not mean that they will fission, or split. But uranium will. Souranium is suitable to fuel a nuclear power plant.As atoms split and collide, they heat up. The plant uses this heat to createsteam.The heat is transfered to the water through heat exchanging tubes insteam generator in the primary loop.After extractig this heat, water isconverted in to steam and collected at the top of steam generator.Thepressure of the expanding steam turns a turbine which is connected to agenerator in the secondary loop.After rotating turbine - generator set steampasses to the condenser.After that the function of condenser and colingtowers is same as that of thermal plant.After the steam is made, a nuclear plant operates much like a fossil fuel firedplant: the turbine spins a generator. The whirling magnetic field of thegenerator produces electricity. The electricity then goes through wires strungon tall towers you might see along a highway to an electrical substation inyour neighborhood where the power is regulated to the proper strength.Then it goes to your home.
In the case of nuclear power plant operation the following factors must beconsidered • Control -- Keeping the nuclear reaction from dying out or exploding. • Safety -- If something goes wrong it can be contained. • Refueling -- Adding more nuclear fuel without stoping the reactor. • Waste production -- The byproducts of the reaction must be manageable. • Efficiency -- Capture as much of the heat as possible.Control is the most important aspect to a design. When an atom of nuclearfuel (uranium) absorbs a neutron, the uranium will fission into two smalleratoms (waste) and release one to three neutrons. The kinetic energy of thewaste is used to heat the water for the steam turbine. The neutrons areused to fission the next lot of uranium atoms and the process continues. Ifnone of these neutrons are absorbed by another uranium atom then thereaction dies out. If too many neutrons are absorbed then the reactiongrows extremely quickly and could explode. Current reactor designs aremost usefully classified by how they ensure this nuclear reaction is kept at alevel which produces power without getting out of hand.The Nuclear Regulatory Commission (NRC), part of our government, makessure nuclear power plants in the United States protect public health andsafety and the environment. The NRC licenses the use of nuclear materialand inspects users to make sure they follow the rules for safety.Since radioactive materials are potentially harmful, nuclear power plantshave many safety systems to protect workers, the public, and the
environment. These safety systems include shutting the reactor downquickly and stopping the fission process, systems to cool the reactor downand carry heat away from it and barriers to contain any radioactivity andprevent it from escaping into the environment.One of the greatest benefits of nuclear plants is that they have no smokestacks! The big towers many people associate with nuclear plants areactually for cooling water used to make steam. (Some other kinds of plantshave these towers, too.) The towers spread the water out so as much air aspossible can reach it and cool it down. Most water is then recycled into theplant.Nuclear power plants are very clean and efficient to operate. However,nuclear power plants have some major environmental risks. Nuclear powerplants produce radioactive gases. These gases are to be contained in theoperation of the plant. If these gases are released into the air, major healthrisks can occur. Nuclear plants use uranium as a fuel to produce power. Themining and handling of uranium is very risky and radiation leaks can occur.The third concern of nuclear power is the permanent storage of spentradioactive fuel. This fuel is toxic for centuries, handling and disposal is anongoing environmental issue.CANDU ReactorCANDU ReactorThe CANDU reactor is a Pressurized Heavy Water Reactor developed initiallyin the late 1950s and 1960s by a partnership between Atomic Energy ofCanada Limited (AECL), the Hydro-Electric Power Commission of Ontario(now known as Ontario Power Generation), Canadian General Electric (nowknown as GE Canada), as well as several private industry participants. Theacronym "CANDU", a registered trademark of Atomic Energy of CanadaLimited, stands for "CANada Deuterium Uranium". This is a reference to itsdeuterium-oxide (heavy water) moderator and its use of natural uraniumfuel. This type of reactor is meant for those countries which do not prodceenriched uranium.Enrichment of uranium is costly and this reactor uses
natural uranium as fuel and heavy water as moderator.In heavy water reactors both the modeartor and coolant are heavy water(D2O). A great disadvantage of this type comes from this fact: heavy wateris one of the most expensive liquids. However, it is worth its price: this isthe best moderator. Therefore, the fuel of HWRs can be slightly (1% to 2%)enriched or even natural uranium. Heavy water is not allowed to boil, so inthe primary circuit very high pressure, similar to that of PWRs, exists.CANDU fuel is made from uranium that is naturally radioactive. Smallamounts of uranium can generate large amounts of energy in the form ofheat. The uranium is mined, refined and made into solid ceramic pellets (twopellets are the size of an AA battery). The pellets are put in metal tubes,which are welded together to form a fuel bundle that weighs around 23kg.The bundle is about the size of a fireplace log and can provide enoughenergy for an average home for 100 years. The figure below shows theCANDU reactor and its main parts.In CANDU reactors, the moderator and coolant are spatially separated: themoderator is in a large tank (calandria), in which there are pressure tubessurrounding the fuel assemblies. The coolant flows in these tubes only.The advantage of this construction is that the whole tank need not be keptunder high pressure, it is sufficient to pressurize the coolant flowing in thetubes. This arrangement is called pressurized tube reactor. Warming up of
the moderator is much less than that of the coolant; its is simply lost forheat generation or steam production. The high temperature and highpressure coolant, similarly to PWRs, goes to the steam generator where itboils the secondary side light water. Another advantage of this type is thatfuel can be replaced during operation and thus there is no need for outages.Fission reactions in the reactor core heat a fluid, in this case heavy water(see below), which is kept under high pressure to raise its boiling point andavoid significant steam formation in the core. The hot heavy watergenerated in this primary cooling loop is passed into a heat exchangerheating light (ordinary) water in the less-pressurized secondary cooling loop.This water turns to steam and powers a conventional turbine with agenerator attached to it. Any excess heat energy in the steam after flowingthrough the turbine is rejected into the environment in a variety of ways,most typically into a large body of cool water (lake, river, or ocean). Morerecently-built CANDU plants (such as the Darlington station near Toronto,Ontario) use a discharge-diffuser system that limits the thermal effects inthe environment to within natural variations.CANDU reactors employ two independent, fast-acting safety shutdownsystems. Control rods penetrate the calandria vertically and lower into thecore in the case of a safety-system trip.A second shutdown system is viagadolinium nitrate liquid "neutron poison" injection directly in to the lowpressure moderator. Both systems operate via separate and independenttrip logic.CANDU-specific features and advantagesUse of natural uranium as a fuel • CANDU is the most efficient of all reactors in using uranium: it uses about 15% less uranium than a pressurized water reactor for each megawatt of electricity produced. • Use of natural uranium widens the source of supply and makes fuel fabrication easier. Most countries can manufacture the relatively inexpensive fuel . • There is no need for uranium enrichment facility.
• Fuel reprocessing is not needed, so costs, facilities and waste disposal associated with reprocessing are avoided. • CANDU reactors can be fuelled with a number of other low-fissile content fuels, including spent fuel from light water reactors. This reduces dependency on uranium in the event of future supply shortages and price increases .Use of heavy water as a moderator • Heavy water (deuterium oxide) is highly efficient because of its low neutron absorption and affords the highest neutron economy of all commercial reactor systems. As a result chain reaction in the reactor is possible with natural uranium fuel. • Heavy water used in CANDU reactors is readily available. It can be produced locally, using proven technology. Heavy water lasts beyond the life of the plant and can be re-used .CANDU reactor core design • Reactor core comprising small diameter fuel channels rather that one large pressure vessel • Allows on-power refueling - extremely high capability factors are possible . • The moveable fuel bundles in the pressure tubes allow maximum burn- up of all the fuel in the reactor core. • Extends life expectancy of the reactor because major core components like fuel channels are accessible for repairs when needed.Pressurized Water Reactor (PWR)Pressurized Water Reactor (PWR)Pressurized water reactors (PWRs) (also VVER if of Russian design) aregeneration II nuclear power reactors that use ordinary water under highpressure as coolant and neutron moderator. The primary coolant loop is kept
under high pressure to prevent the water from boiling, hence the name.PWRs are one of the most common types of reactors and are widely used allover the world. More than 230 of them are in use to generate electric power,and several hundred more for naval propulsion. They were originallydesigned by the Bettis Atomic Power Laboratory as a nuclear submarinepower plant.The below diagram shows the PWR and its main parts. 1.Reactor vessel 2.Fuelelements 3.Control rods 4.Control rod drive 5.Pressurizer 6.Steam generator7.Main circulating pump 8.Fresh steam 9.Feedwater 10.High pressureturbine 11.Low pressure turbine 12.Generator 13.Exciter 14.Condenser15.Cooling water 16.Feedwater pump 17.Feedwater pre-heater 18.Concreteshield 19.Cooling water pumpThe pressurized water reactor belongs to the light water type: the moderatorand coolant are both light water (H2O). It can be seen in the figure that thecooling water circulates in two loops, which are fully seperated from oneanother.The primary circuit water (dark blue) is continuously kept at a very highpressure and therefore it does not boil even at the high operatingtemperature. (Hence the name of the type.) Constant pressure is ensuredwith the aid of the pressurizer (expansion tank). (If pressure falls in theprimary circuit, water in the pressurizers is heated up by electric heaters,thus raising the pressure. If pressure increases, colder cooling water isinjected to the pressurizer. Since the upper part is steam, pressure willdrop.) The primary circuit water transferes its heat to the secondary circuitwater in the small tubes of the steam generator, it cooles down and returnsto the reactor vessel at a lower temperature.
Since the secondary circuit pressure is much lower than that of the primarycircuit, the secondary circuit water in the steam generator starts to boil(red). The steam goes from here to the turbine, which has high and lowpressure stages. When steam leaves the turbine, it becomes liquid again inthe condenser, from where it is pumped back to the steam generator afterpre-heating.Normally, primary and secondary circuit waters cannot mix. In this way itcan be achieved that any potentially radioactive material that gets into theprimary water should stay in the primary loop and cannot get into theturbine and condenser. This is a barrier to prevent radioactive contamination .from getting out.In pressurized water reactors the fuel is usually low (3 to 4 percent)enriched uranium oxide, sometimes uranium and plutonium oxide mixture(MOX). In todays PWRs the primary pressure is usually 120 to 160 bars, pressurewhile the outlet temperature of coolant is 300 to 320 °C. PWR is the mostwidespread reactor type in the world: they give about 64% of the totalpower of the presently operating nuclear power plants.Two things are characteristic for the pressurized water reactor (PWR) when teristiccompared with other reactor types:
• In a PWR, there are two separate coolant loops (primary and secondary), which are both filled with ordinary water (also called light water). A boiling water reactor, by contrast, has only one coolant loop, while more exotic designs such as breeder reactors use substances other than water (i.e., liquid metal as sodium) for the task. • The pressure in the primary coolant loop is at typically 15-16 Megapascal, notably higher than in other nuclear reactors. As an effect of this, the gas laws guarantee that only sub-cooled boiling will occur in the primary loop. By contrast, in a boiling water reactor the primary coolant is allowed to boil and it feeds the turbine directly without the use of a secondary loop.CoolantOrdinary water is used as primary coolant in a PWR and flows through thereactor at a temperature of roughly 315 °C (600 °F). The water remainsliquid despite the high temperature due to the high pressure in the primarycoolant loop (usually around 2200 psig [15 MPa, 150 atm]). The primarycoolant loop is used to heat water in a secondary circuit that becomessaturated steam (in most designs 900 psia [6.2 MPa, 60 atm], 275 °C [530°F]) for use in the steam turbine.ModeratorPressurized water reactors, like thermal reactor designs, require the fastfission neutrons in the reactor to be slowed down (a process calledmoderation) in order to sustain its chain reaction. In PWRs the coolant wateris used as a moderator by letting the neutrons undergo multiple collisionswith light hydrogen atoms in the water, losing speed in the process. This"moderating" of neutrons will happen more often when the water is moredense (more collisions will occur). The use of water as a moderator is animportant safety feature of PWRs, as any increase in temperature causes thewater to expand and become less dense; thereby reducing the extent towhich neutrons are slowed down and hence reducing the reactivity in thereactor. Therefore, if reactor activity increases beyond normal, the reducedmoderation of neutrons will cause the chain reaction to slow down,
producing less heat. This property, known as the negative temperaturecoefficient of reactivity, makes PWR reactors very stable.FuelThe uranium used in PWR fuel is usually enriched several percent in 235U.After enrichment the uranium dioxide (UO2) powder is fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enricheduranium dioxide. The cylindrical pellets are then put into tubes of acorrosion-resistant zirconium metal alloy (Zircaloy) which are backfilled withhelium to aid heat conduction and detect leakages. The finished fuel rods aregrouped in fuel assemblies, called fuel bundles, that are then used to buildthe core of the reactor. As a safety measure PWR designs do not containenough fissile uranium to sustain a prompt critical chain reaction (i.e,substained only by prompt neutron). Avoiding prompt criticality is importantas a prompt critical chain reaction could very rapidly produce enough energyto damage or even melt the reactor (as is suspected to have occurred duringthe accident at the Chernobyl plant). A typical PWR has fuel assemblies of200 to 300 rods each, and a large reactor would have about 150-250 suchassemblies with 80-100 tonnes of uranium in all. Generally, the fuel bundlesconsist of fuel rods bundled 14x14 to 17x17. A PWR produces on the orderof 900 to 1500 MWe. PWR fuel bundles are about 4 meters inlength.Refuelings for most commercial PWRs is on an 18-24 month cycle.Approximately one third of the core is replaced each refueling.ControlGenerally, reactor power can be viewed as following steam (turbine) demanddue to the reactivity feedback of the temperature change caused byincreased or decreased steam flow. Boron and control rods are used tomaintain primary system temperature at the desired point. In order todecrease power, the operator throttles shut turbine inlet valves. This wouldresult in less steam being drawn from the steam generators. This results inthe primary loop increasing in temperature. The higher temperature causesthe reactor to fission less and decrease in power. The operator could thenadd boric acid and/or insert control rods to decrease temperature to thedesired point.
Reactivity adjustments to maintain 100% power as the fuel is burned up inmost commercial PWRs is normally controlled by varying the concentrationof boric acid dissolved in the primary reactor coolant. The boron readilyabsorbs neutrons and increasing or decreasing its concentration in thereactor coolant will therefore affect the neutron activity correspondingly. Anentire control system involving high pressure pumps (usually called thecharging and letdown system) is required to remove water from the highpressure primary loop and re-inject the water back in with differingconcentrations of boric acid. The reactor control rods, inserted through thetop directly into the fuel bundles, are normally only used for power changes.In contrast, BWRs have no boron in the reactor coolant and control thereactor power by adjusting the reactor coolant flow rate.Due to design andfuel enrichment differences, naval nuclear reactors do not use boric acid.Advantages • PWR reactors are very stable due to their tendency to produce less power as temperatures increase, this makes the reactor easier to operate from a stability standpoint. • PWR reactors can be operated with a core containing less fissile material than is required for them to go prompt critical. This significantly reduces the chance that the reactor will run out of control and makes PWR designs relatively safe from criticality accidents. • Because PWR reactors use enriched uranium as fuel they can use ordinary water as a moderator rather than the much more expensive heavy water. • PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials. • The reactor has high power density. • The reactor responds to supply more power when the load increases.Disadvantages • The coolant water must be heavily pressurized to remain liquid at high temperatures. This requires high strength piping and a heavy pressure
vessel and hence increases construction costs. The higher pressure can increase the consequences of a Loss of Coolant Accident.• Most pressurized water reactors cannot be refueled while operating. This decreases the availability of the reactor- it has to go offline for comparably long periods of time (some weeks).• The high temperature water coolant with boric acid dissolved in it is corrosive to carbon steel (but not stainless steel), this can result in radioactive corrosion products to circulate in the primary coolant loop. This not only limits the lifetime of the reactor, but the systems that filter out the corrosion products and adjust the boric acid concentration add significantly to the overall cost of the reactor and radiation exposure.• Water absorbs neutrons making it necessary to enrich the uranium fuel, which increases the costs of fuel production. If heavy water is used it is possible to operate the reactor with natural uranium, but the production of heavy water requires large amounts of energy and is hence expensive.• Because water acts as a neutron moderator it is not possible to build a fast neutron reactor with a PWR design. For this reason it is not possible to build a fast breeder reactor with water coolant.• Because the reactor produces energy more slowly at higher temperatures, a sudden cooling of the reactor coolant could increase power production until safety systems shut down the reactor.