Unit III Nuclear Power Plants

Embed Size (px)

Citation preview

  • 7/28/2019 Unit III Nuclear Power Plants

    1/22

    1

    Unit III Course material

    N.KARTHIKEYANUNIT III

    EE1252-POWER PLANT ENGINEERING

    UNIT III NUCLEAR POWER PLANTS

    Principles of nuclear energy Fission reactions Nuclear reactor Nuclear power plants

  • 7/28/2019 Unit III Nuclear Power Plants

    2/22

    2

  • 7/28/2019 Unit III Nuclear Power Plants

    3/22

    3

  • 7/28/2019 Unit III Nuclear Power Plants

    4/22

    4

  • 7/28/2019 Unit III Nuclear Power Plants

    5/22

    5

    PRINCIPLES OF NUCLEAR ENERGY

    Changes can occur in the structure of the nuclei of atoms. These changes are callednuclear

    reactions. Energy created in a nuclear reaction is callednuclear energy, oratomic energy.

    Nuclear energy is produced naturally and in man-made operations under human control.

    Naturally: Some nuclear energy is produced naturally. For example, the Sun and other starsmake heat and light by nuclear reactions.

    Man-Made: Nuclear energy can be man-made too. Machines callednuclear reactors, partsofnuclear power plants, provide electricity for many cities. Man-made nuclear reactionsalso occur in the explosion of atomic and hydrogen bombs.

    Nuclear energy is produced in two different ways, in one; large nuclei are split to release energy. Inthe other method, small nuclei are combined to release energy.

    Nuclear Fission: In nuclear fission, the nuclei of atoms are split, causing energy to bereleased. The atomic bomb and nuclear reactors work by fission. The element uranium is themain fuel used to undergo nuclear fission to produce energy since it has many favourableproperties. Uranium nuclei can be easily split by shooting neutrons at them. Also, once auranium nucleus is split, multiple neutrons are released which are used to split other uraniumnuclei. This phenomenon is known as a chain reaction.

    Nuclear Fusion: In nuclear fusion, the nuclei of atoms are joined together, or fused. This happensonly under very hot conditions. The Sun, like all other stars, creates heat and light through nuclearfusion. In the Sun, hydrogen nuclei fuse to make helium. The hydrogen bomb, humanity's mostpowerful and destructive weapon, also works by fusion. The heat required to start the fusion reaction

    is so great that an atomic bomb is used to provide it. Hydrogen nuclei fuse to form helium and in theproduces release huge amounts of energy.

    Radioactive decay

    In observing elements which were radioactive, physicists had discovered three types of radioactivedecay. The alpha ()particle is made up of two protons and two neutrons identical to the nucleus of thesecond-lightest element, helium. An alpha emission leaves behind a nucleus with an atomic weightdecreased by four and an atomic number decreased by two. Uranium-238, for example, emits an alpha

    particle to form Thorium-234.

  • 7/28/2019 Unit III Nuclear Power Plants

    6/22

    6

    The beta particle () is a high-energy electron. Physicists later concluded that it is produced when aneutron splits, emitting an electron and leaving behind a positively charged proton and an atomicnucleus with an atomic number that has increased by one. Carbon-14 (with six protons and eightneutrons), for example, emits a beta particle to form nitrogen-14, with seven protons and sevenneutrons. Gamma radiation ( ) is an electromagnetic wave that is often emitted along with an alpha or betaparticle.

    In nuclear fission the uranium nucleus first absorbs a neutron and then splits. As it does so it emitsneutrons that can initiate the process in another nucleus.

    Fission process

    1. The uranium fission process was slightly more complicated than it had first appeared.2. First, it was already known that naturally occurring uranium is a mixture of at least three isotopes.3. By far the largest proportion around 99.27 per cent is uranium- 238, which has 146 neutrons,

    along with its 92 protons.4. Some 0.72 per cent of the naturally occurring mineral is 235U, which has just 143 neutrons.5. There is an even smaller proportion (0.0055 per cent) of 234U, with one neutron fewer.6. Both 235Uand 238Ucan undergo fission, but it is more likely in 235Uand the results of the split

    are somewhat different because the energy required to cause fission varies.7. When a uranium nucleus absorbs a neutron and splits into barium and krypton, some of the

    neutrons are not captured by the new elements but instead are released as free neutrons.8. These free neutrons do not remain at large: instead they are in turn captured by the nuclei of

    nearby uranium atoms, where they can initiate another fission.9. When 238U is subject to fission it may or may not produce a free neutron, so on average there is

    less than one free neutron for each fission, and the process gradually dies away.

  • 7/28/2019 Unit III Nuclear Power Plants

    7/22

    7

    10.In contrast, 235U releases several free neutrons when it undergoes fission 2.5 on average. Thismeans that if there is enough 235U in the uranium mix the fission reaction can be self-sustainingso that at least one of the neutrons from each fission finds another 235U nucleus and is absorbedto initiate another fission in a chain reaction.

    The chain reaction

    How many of the neutrons initiate fission and whether a chain reaction can be started depends partly on the proportion of235U in the mix and partly on the total volume of material. Theamount of fissile material needed for a sustained nuclear chain reaction is called thecritical mass.It varies depending upon the nuclear and physical properties of the material. The important physical

    properties include the density and shape of the mass, as well as its purity.

    The nuclear properties include the nucleuss ability to capture a neutron (known as thenuclear fission cross section and generally fixed for each nucleus) and whether the process is aided bya neutron reflector (which would send the neutron back into the mass) or moderator (which wouldslow the neutron to a speed that makes it easily captured) or is interrupted by an absorber (a material

    that removes neutrons from the process). An assembly in which a chain reaction is just possible iscalled critical, and when the reaction becomes self-sustaining it is said to have reached criticality.

    In such an assembly, without a new input of free neutrons, for example from spontaneousfissions, the reaction will on average be just sustained. At 23 per cent uranium-235 the reaction canbe simply self-sustaining. This is believed to have happened in nature, where a high uraniumconcentration was combined with other conditions to allow a self-sustaining reaction to take placeover many years. The reaction is now over, but evidence remains in the proportions of elementswithin the rock deposit.

    If an assembly is less than critical, the fission reaction will reach a steady state only with asteady input of new free neutrons, and the assembly is said to be subcritical. A more than critical

    assembly is said to be supercritical. An assembly that is capable of sustaining a chain reaction withoutneeding the contribution of defined neutrons is calledprompt critical (and is therefore alsosupercritical). Even larger masses are called super prompt critical. If235U makes up most, or all, of thesample, the chain reaction may be explosive.

    Making plutoniumPlutonium, one of the two fissile elements used to fuel nuclear explosives, is not found in

    significant quantities in nature. It has an atomic number of 94, meaning its nucleus contains two moreprotons than uranium and it must be produced from uranium-238.

    The importance of plutonium is that when it captures a neutron, plutonium-239 undergoesfission more readily than uranium, and in the process it produces more excess neutrons to continue the

    reaction. This means that a smaller mass can reach criticality, with a self-sustaining or explosive chainreaction. This can make chemical handling of plutonium difficult, as volumes of the solid and itsvarious compounds, in solution or liquid form have to be kept small so that criticality does notoccur.

    Radio logically, it is relatively safe to handle because it emits alpha particles (although 241Puis principally a beta emitter) which can be blocked by simple shielding (see Panel 1.2), although theyare hazardous at very short distances or inside the body.

    Controlling the reaction

    1. The proportion of235U is not the only factor that determines whether and how fast the chainreaction takes place.

  • 7/28/2019 Unit III Nuclear Power Plants

    8/22

    8

    2. The high-energy neutrons emitted during fission can cause more conventional radioactive decay,knocking out a radioactive particle and leaving a heavy element but not initiating fission.

    3. The speed of the neutron was an important factor in the fission process, the theory of amoderator a substance that could slow down high-energy neutrons in a series of collisionsuntil they were moving slowly enough to be captured by another uranium nucleus.

    4. Moderators could include water or graphite. It was clear that fission energy could potentially beused for power generation.

    5. The techniques and technologies developed to fuel, manage and exploit the nuclear reaction arean important part of the story of nuclear power.

    Pile

    The pile consisted of uranium pellets as a neutron-producing core, separated from one another bygraphite blocks to slow the neutrons. Fermi himself described the apparatus as a crude pile of blackbricks and wooden timbers.

    Controlling the reaction or control rod

    Acontrol rodis a rod made ofchemical elementscapable of absorbingmanyneutronswithout fissioning themselves. They are used innuclear reactorsto control the rate

    of fission ofuranium andplutonium. Because these elements have differentcapture cross

    sectionsfor neutrons of varying energies, the compositions of the control rods must be designed for

    the neutron spectrum of the reactor it is supposed to control. Light water reactors (BWR, PWR) and

    heavy water reactors (HWR) operate with"thermal" neutrons, whereasbreeder reactorsoperate

    with "fast" neutronsThe key to controlling the speed of the reaction was including a material that would absorb

    neutrons in the pile but organising it in such a way that the absorber could be inserted or withdrawnat will. When the absorber was gradually withdrawn, the number of free neutrons would rise to a levelthat would initiate and maintain fission and promote the reaction, and when reinserted it would reduce

    the reaction again.Fission

    When a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron,it may undergo nuclear fission. The heavy nucleus splits into two or more lighter nuclei,releasing kinetic energy, gamma radiation and free neutrons; collectively known as fission.[1] Aportion of these neutrons may later be absorbed by other fissile atoms and trigger further fissionevents, which release more neutrons, and so on. This is known as a nuclear.

    The reaction can be controlled by using neutron poisons, which absorb excess neutrons, andneutron, which reduce the velocity of fast neutrons, thereby turning them into thermal, which aremore likely to be absorbed by other nuclei. Increasing or decreasing the rate of fission has acorresponding effect on the energy output of the reactor.

    Commonly used moderators include regular (light) water (75% of the world's reactors), solidgraphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in someexperimental types, and hydrocarbons have been suggested as another possibility.[2]

    Heat generation

    The reactor core generates heat in a number of ways:

    The kinetic energy of fission products is converted to thermal energy when these nucleicollide with nearby atoms.

  • 7/28/2019 Unit III Nuclear Power Plants

    9/22

    9

    Some of the gamma rays produced during fission are absorbed by the reactor, their energybeing converted to heat.

    Heat produced by the radioactive decay of fission products and materials that have beenactivated by neutron absorption. This decay heat source will remain for some time even after thereactor is shut down.

    A kilogram of uranium-235 (U-235) converted via nuclear processes releases approximately threemillion times more energy than a kilogram of coal burned conventionally (7.2 1013 joules perkilogram of uranium-235 versus 2.4 107 joules per kilogram of coal).

    Cooling

    A nuclear reactor coolant usually water but sometimes a gas or a liquid metal or molten saltis circulated past the reactor core to absorb the heat that it generates. The heat is carried away fromthe reactor and is then used to generate steam. Most reactor systems employ a cooling system that isphysically separated from the water that will be boiled to produce pressurized steam for the turbines,like the pressurized. But in some reactors the water for the steam turbines is boiled directly bythe reactor core, for example the boiling water reactor.

    Moderator

    In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons,

    thereby turning them into thermal neutrons capable of sustaining a nuclear chain

    reaction involving uranium-235. Commonly used moderators include regular (light) water (roughly

    75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of

    reactors).Beryllium has also been used in some experimental types, and hydrocarbons have been

    suggested as another possibility

    Nuclear Fusion

    Nuclear energy can also be released by fusion of two light elements (elements with low

    atomic numbers). The power that fuels the sun and the stars is nuclear fusion. In a hydrogenbomb, two isotopes of hydrogen, deuterium and tritium are fused to form a nucleus of heliumand a neutron. This fusion releases 17.6 MeV of energy. Unlike nuclear fission, there is nolimit on the amount of the fusion that can occur.

  • 7/28/2019 Unit III Nuclear Power Plants

    10/22

    10

    The heat produced in these reactions maintains temperatures of the order of several milliondegrees in their cores and serves to trigger and sustain succeeding reactions. The 4-hydrogen reactionrequires, on an average, billions of years for completion, whereas the deuterium-deuterium reactionrequires a fraction of a second. To cause fusion, it is necessary to accelerate the positively chargednuclei to high kinetic energies, in order to overcome electrical repulsive forces, by raising theirtemperature to hundreds of millions of degrees resulting in plasma. The plasma must be preventedfrom contacting the walls of the container, and must be confined for a period of time (of the order of asecond) at a minimum density. Fusion reactions are called thermonuclearbecause very hightemperatures are required to trigger and sustain them. Table 10.2 lists the possible fusion reactionsand the energies produced by them.

    Many problems have to be solved before an artificially made fusion reactor becomes a reality.The most important of these are the difficulty in generating and maintaining high temperatures and theinstabilities in the medium (plasma), the conversion of fusion energy to electricity, and many otherproblems of an operational nature. Fusion power plants will not be covered in this text.

    The nuclear reactors can be classified as follows:

    1. Neutron Energy. Depending upon the energy of the neutrons at the time they are captured by thefuel to induce fissions, the reactors can be named as follows:

    (a) Fast Reactors. In such reactors fission is brought about by fast (non moderated) neutrons.

    (b)Thermal Reactors or Slow Reactors. In these reactors the fast moving neutrons are sloweddown by passing them through the moderator. These slow moving neutrons are then capturedby the fuel material to bring about the fission of fundamental research.

    A NUCLEAR REACTOR

    A nuclear reactor is an apparatus in which heat is produced due to nuclear fission chain reaction.Fig. shows the various parts of reactor, which are as follows:

    1. Nuclear Fuel2. Moderator3. Control Rods4. Reflector5. Reactors Vessel6. Biological Shielding

    7.

    Coolant.

  • 7/28/2019 Unit III Nuclear Power Plants

    11/22

    11

    A NUCLEAR REACTOR

    1. NUCLEAR FUEL

    Fuel of a nuclear reactor should be fissionable material which can be defined as an element orisotope whose nuclei can be caused to undergo nuclear fission by nuclear bombardment and toproduce a fission chain reaction. It can be one or all of the following

    U233, U235 and Pu239.

    Natural uranium found in earth crust contains three isotopes namely U234, U235 and U238 and theiraverage percentage is as follows :

    U238 99.3%U235 0.7%

    U234 TraceOut of these U235 is most unstable and is capable of sustaining chain reaction and has been given thename as primary fuel. U233 arid Pu239 are artificially produced from Th232 and U238 respectivelyand are called secondary fuel.

    In order to prevent the contamination of the coolant by fission products, a protective coatingor cladding must separate the fuel from the coolant stream. Fuel element cladding should possess thefollowing properties:

    1. It should be able to withstand high temperature within the reactor.2. It should have high corrosion resistance.

    3. It should have high thermal conductivity.

  • 7/28/2019 Unit III Nuclear Power Plants

    12/22

    12

    4. It should not have a tendency to absorb neutrons.5. It should have sufficient strength to withstand the effect of radiations to which it is subjected.Uranium oxide (UO2) is another important fuel element.

    Uranium oxide has the following advantages

    Over natural uranium:1. It is more stable than natural uranium.2. There is no problem or phase change in case of uranium oxide and therefore it can be used forhigher temperatures.3. It does not corrode as easily as natural uranium.4. It is more compatible with most of the coolants and is not attacked by H2, Nz.5. There is greater dimensional stability during use.

    Uranium oxide possesses following disadvantages:1. It has low thermal conductivity.2. It is more brittle than natural uranium and therefore it can break due to thermal stresses.

    3. Its enrichment is essential.Uranium oxide is a brittle ceramic produced as a powder and then sintered to form fuel pellets.Another fuel used in the nuclear reactor is uranium carbide (UC). It is a black ceramic used in theform of pellets.

    2. MODERATOR

    In the chain reaction the neutrons produced are fast moving neutrons. These fast moving neutronsare far less effective in causing the fission of U235 and try to escape from the reactor. To improve theutilization of these neutrons their speed is reduced. It is done by colliding them with the nuclei ofother material which is lighter, does not capture the neutrons but scatters them. Each such collisioncauses loss of energy, and the speed of the fast moving neutrons is reduced. Such material is calledModerator. The slow neutrons (Thermal Neutrons) so produced are easily captured by the nuclear fueland the chain reaction proceeds smoothly. Graphite, heavy water and beryllium are generally used asmoderator.Reactors using enriched uranium do not require moderator. But enriched uranium is costly due toprocessing needed.

    A moderator should process the following properties:1. It should have high thermal conductivity.2. It should be available in large quantities in pure form.3. It should have high melting point in case of solid moderators and low melting point in case of

    liquid moderators. Solid moderators should also possess good strength and mach inability.4. It should provide good resistance to corrosion.5. It should be stable under heat and radiation.6. It should be able to slow down neutrons.

    3. Control rods

    Control rods in the cylindrical or sheet form are made of boron or cadmium. These rods canbe moved in and out of the holes in the reactor core assembly. Their insertion absorbs more neutronsand damps down the reaction and their withdrawal absorbs less neutrons. Thus power of reaction iscontrolled by shifting control rods which may be done manually or automatically.

  • 7/28/2019 Unit III Nuclear Power Plants

    13/22

  • 7/28/2019 Unit III Nuclear Power Plants

    14/22

    14

    have high heat transfer coefficient. The various fluids used as coolant are water (light water or heavywater), gas (Air, CO2, Hydrogen, Helium) and liquid metals such as sodium or mixture of sodium andpotassium and inorganic and organic fluids.

    COOLANT CYCLES

    The coolant while circulating through the reactor passages take up heat produced due to chain reactionand transfer this heat to the feed water in three ways as follows :

    (a) Direct Cycle.In this system coolant which is water leaves the reactor in the form of steam.Boiling water reactor uses this system.(b) Single Circuit System. In this system the coolant transfers the heat to the feed water in the steamgenerator. This system is used in pressurized reactor.(c) Double Circuit System.In this system two coolants are used. Primary coolant after circulatingthrough the reactor flows through the intermediate heat exchanger (IHX) and passes on its hest to thesecondary coolant which transfers its heat in the feed water in the steam generator. This system isused in sodium graphite reactor and fast breeder reactor.

    8. REACTOR CORE

    Reactor core consists of fuel rods, moderator and space through which the coolant flows.

    Electrical Generator

    In electricity generation, an electric generator is a device that converts mechanical energy toelectrical energy. The reverse conversion of electrical energy into mechanical energy is done by amotor; motors and generators have many similarities. A generator forces electrons in the windings toflow through the external electrical. It is somewhat analogous to a water pump, which creates a flowof water but does not create the water inside. The source of mechanical energy may be a reciprocating

    or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine,a wind turbine, a hand crank, compressed air or any other source of mechanical energy.

    SITE SELECTION

    1. Availability of water. At the power plant site an ample quantity of water should be available forcondenser cooling and made up water required for steam generation. Therefore the site should benearer to a river, reservoir or sea.2. Distance from load center. The plant should be located near the load center. This will minimisethe power losses in transmission lines.

    3. Distance from populated area. The power plant should be located far away from populated areato avoid the radioactive hazard.4. Accessibility to site. The power plant should have rail and road transportation facilities.5. Waste disposal. The wastes of a nuclear power plant are radioactive and there should be sufficientspace near the plant site for the disposal of wastes.

    Safeguard against earthquakes. The site is classified into its respective seismic zone 1, 2, 3, 4, or 6.The zone 5 being the most seismic (earth vibration), which is unsuitable for nuclear power plants.About 300 km of radius area around the proposed site is studied for its past history of tremors, andearthquakes to assess the severest earthquake that could occur for which the foundation building andequipment supports are designed accordingly. This ensures that the plant will retain integrity ofstructure, piping and equipments should an earthquake occur. The site selected should also take into

  • 7/28/2019 Unit III Nuclear Power Plants

    15/22

    15

    account the external natural events such as floods, including those by up-stream dam failures andtropical cyclones.

    COMPARISON OF NUCLEAR POWER PLANT AND STEAM POWER PLANT

    The cost of electricity generation is nearly equal in both these power plants. The other advantages anddisadvantages are as follows:

    (i) The number of workman required for the operation of nuclear power plant is much less than asteam power plant. This reduces the cost of operation.(ii) The capital cost of nuclear power plant falls sharply if the size of plant is increased. The capitalcost as structural materials, piping, storage mechanism etc. much less in nuclear power plant thansimilar expenditure of steam power plant. However, the expenditure of nuclear reactor and buildingcomplex is much higher.(iii) The cost of power generation by nuclear power plant becomes competitive with cost of steampower plant above the unit size of about 500 mW.

    URANIUM ENRICHMENT

    In some cases the reaction does not take place with natural uranium containing only 0.71% of U235.In such cases it becomes essential to use uranium containing higher content of U335. This is called U235concentration of uranium enrichment. The various methods of uranium enrichment are as

    1. The gaseous diffusion method.

    2. Thermal diffusion method.3. Electromagnetic Method.4. Centrifugation Method.

    SAFETY MEASURES FOR NUCLEAR POWER PLANTS

    Nuclear power plants should be located far away from the populated area to avoid the radioactivehazard. A nuclear reactor produces and ( particles, neutrons and -quanta which can disturb thenormal functioning of living organisms. Nuclear power plants involve radiation leaks, health hazard toworkers and community, and negative effect on surrounding forests.

    At nuclear power plants there are three main sources of radioactive contamination of air.

    1. Fission of nuclei of nuclear fuels.2. The second source is due to the effect of neutron fluxes on the heat carrier in the primary cooling

    system and on the ambient air.

    3. Third source of air contamination is damage of shells of fuel elements.

    This calls for special safety measures for a nuclear power plant. Some of the safety measures are asfollows.

    1. Nuclear power plant should be located away from human habitation.2. Quality of construction should be of required standards.3. Waste water from nuclear power plant should be purified. The water purification plants must

    have a high efficiency of water purification and satisfy rigid requirements as regards thevolume of radioactive wastes disposed to burial.

    4. An atomic power plant should have an extensive ventilation system. The main purpose of thisventilation system is to maintain the concentration of all radioactive impurities in the air

    below the permissible concentrations.

  • 7/28/2019 Unit III Nuclear Power Plants

    16/22

    16

    5. An exclusion zone of 1.6 km radius around the plant should be provided where no publichabitation is permitted.

    6. The safety system of the plant should be such as to enable safe shut down of the reactorwhenever required.

    MAJOR NUCLEAR POWER DISASTERS

    Chernobyl is near Kiev, Ukraine, in the former Soviet Union. Destroyed by steam and hydrogenexplosions followed by fire, it caused many deaths on site, increased cancer rates in the thousands ofsquare miles it contaminated.Three Mile Island Located 10 miles southeast of Harrisburg PA on the Susquehanna River.The accident, and radiation release, caused no immediate deaths. The cleanup cost more than $1.5Billion.

    Boiling Water Reactor (BWR)

    Boiling Water Reactor(BWR)

    This design has many similarities to the PWR, except that there is only a singlecircuit in which the water is at lower pressure (about 75 times atmospheric pressure) sothat it boils in the core at about 285C. The reactor is designed to operate with 12-15%of the water in the top part of the core as steam, and hence with less moderating effectand thus efficiency there. BWR units can operate in load-following mode more readilythen PWRs.

  • 7/28/2019 Unit III Nuclear Power Plants

    17/22

    17

    The steam passes through drier plates (steam separators) above the core andthen directly to the turbines, which are thus part of the reactor circuit. Since the wateraround the core of a reactor is always contaminated with traces of radionuclides, itmeans that the turbine must be shielded and radiological protection provided duringmaintenance. The cost of this tends to balance the savings due to the simpler design.

    Most of the radioactivity in the water is very short-lived*, so the turbine hall can beentered soon after the reactor is shut down.A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750

    assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondarycontrol system involves restricting water flow through the core so that more steam in thetop part reduces moderation.

    Pressurised Water Reactor (PWR)

    This is the most common type, with over 230 in use for power generation and

    several hundred more employed for naval propulsion. The design of PWRs originated asa submarine power plant. PWRs use ordinary water as both coolant and moderator. Thedesign is distinguished by having a primary cooling circuit which flows through the coreof the reactor under very high pressure, and a secondary circuit in which steam isgenerated to drive the turbine.

    A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core,and a large reactor would have about 150-250 fuel assemblies with 80-100 tonnes ofuranium.

    Water in the reactor core reaches about 325C, hence it must be kept underabout 150 times atmospheric pressure to prevent it boiling.

    Pressure is maintained by steam in a pressuriser (see diagram). In the primarycooling circuit the water is also the moderator, and if any of it turned to steam the fissionreaction would slow down. This negative feedback effect is one of the safety features ofthe type. The secondary shutdown system involves adding boron to the primary circuit.

    The secondary circuit is under less pressure and the water here boils in the heatexchangers which are thus steam generators. The steam drives the turbine to produce

  • 7/28/2019 Unit III Nuclear Power Plants

    18/22

    18

    electricity, and is then condensed and returned to the heat exchangers in contact withthe primary circuit.

    Pressurised Heavy Water Reactor (PHWR or CANDU)

    The CANDU reactor functions in a manner similar to a pressurized water reactor (PWR).

    Pressurized coolant is passed through the fuel bundles to cool them. This hot, pressurized cooling

    water is carried to a steam generator where the heat energy is transferred to light water and converts it

    into steam. This steam is then used to turn the steam turbines which turn the generator, creating

    electricity.

    One of the unique features of a CANDU reactor is that it allowson-line fuelling. The fuel bundles are

    placed in horizontal tubes (called pressure tubes). These tubes can be loaded remotely from either end

    while the reactor is running (on-line). This avoids scheduled shutdowns to replace the fuel. The

    CANDU design requires significantly more plumbing than a PWR reactor, as each pressure tubehas high pressure heavy water passing through it.

    The typical lifespan of a fuel bundle in the reactor is one to two years. As a fuel bundle is loaded in

    one end of the pressure tube, a spent fuel bundle is pushed out of the other end. The picture to the

    right taken during a shutdown shows the machine that loads the fuel bundles.

  • 7/28/2019 Unit III Nuclear Power Plants

    19/22

    19

    Advantages:

    Use 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.

    Advanced Gas-cooled Reactor (AGR)

    These are the second generation of British gas-cooled reactors, using graphitemoderator and carbon dioxide as coolant. The fuel is uranium oxide pellets, enriched to2.5-3.5%, in stainless steel tubes. The carbon dioxide circulates through the core,reaching 650C and then past steam generator tubes outside it, but still inside theconcrete and steel pressure vessel.

    Control rods penetrate the moderator and a secondary shutdown system involvesinjecting nitrogen to the coolant. The AGR was developed from the Magnox reactor, also

  • 7/28/2019 Unit III Nuclear Power Plants

    20/22

    20

    graphite moderated and CO2 cooled, and two of these are still operating in UK. They usenatural uranium fuel in metal form. Secondary coolant is water.

    Molten Salt Reactor (MSR)

    Molten Salt Reactors (MSRs) are liquid-fueled reactors that can be used for production of

    electricity, actinide burning, production of hydrogen, and production of fissile fuels. Electricity

    production and waste burndown are envisioned as the primary missions for the MSR. Fissile, fertile,

    and fission isotopes are dissolved in a high temperature molten fluoride salt with a very high boiling

    point (1,400 C) that is both the reactor fuel and the coolant. The near-atmospheric-pressure molten

    fuel salt flows through the reactor core.

    The traditional MSR designs have a graphite core that results in a thermal to epithermal

    neutron spectrum. In the core, fission occurs within the flowing fuel salt that is heated to ~700 C,

    which then flows into a primary heat exchanger where the heat is transferred to a secondary molten

    salt coolant. The fuel salt then flows back to the reactor core.

    The clean salt in the secondary heat transport system transfers the heat from the primary heat

    exchanger to a high-temperature Brayton cycle that converts the heat to electricity. The Brayton cycle

    (with or without a steam bottoming cycle) may use either nitrogen or helium as a working gas.

    Supercritical-Water-Cooled Reactor (SCWR)The Supercritical-Water-Cooled Reactor (SCWR) system is a high-temperature, high-

    pressure water cooled reactor that operates above the thermodynamic critical point of water(374C, 22 MPa, or 705F, 3208 psia) SCWRs are built upon two proven technologies:Light Water Reactors (LWRs), which are the most commonly deployed power-generatingreactors in the world, and supercritical fossil-fired boilers, a large number of which arealso in use around the world.

  • 7/28/2019 Unit III Nuclear Power Plants

    21/22

    21

    SCWRs are promising advanced nuclear systems because of their high thermalefficiency (i.e., about 45% versus about 33% efficiency for current LWRs) and considerableplant simplification. Operation above the critical pressure eliminates coolant boiling, so thecoolant remains single-phase throughout the system. Thus, the need for recirculation and jetpumps, pressurizers, steam generators, and steam separators and dryers in current LWRs iseliminated.

    The SCWR system is primarily designed for efficient electricity production, with an

    option for actinide management based on two options in the core design: the first option isan SCWR with a thermal or fast-spectrum reactor; the second option is a closed cycle with afast-spectrum reactor and full actinide recycle based on advanced aqueous processing at acentral location.

    Lifetime of nuclear reactors.

    Most of today's nuclear plants which were originally designed for 30 or 40-yearoperating lives. However, with major investments in systems, structures and

    components lives can be extended, and in several countries there are active programs

  • 7/28/2019 Unit III Nuclear Power Plants

    22/22

    22

    to extend operating lives. In the USA most of the more than one hundred reactors areexpected to be granted licence extensions from 40 to 60 years. This justifies significantcapital expenditure in upgrading systems and components, including building in extraperformance margins Some components simply wear out, corrode or degrade to a lowlevel of efficiency. These need to be replaced. Steam generators are the most prominent

    and expensive of these, and many have been replaced after about 30 years where thereactor otherwise has the prospect of running for 60 years.

    Load-following capacity

    Nuclear power plants are essentially base-load generators, running continuously.This is because their power output cannot readily be ramped up and down on a dailyand weekly basis, and in this respect they are similar to most coal-fired plants. (It is alsouneconomic to run them at less than full capacity, since they are expensive to build butcheap to run.) However, in some situations it is necessary to vary the output accordingto daily and weekly load cycles on a regular basis, for instance in France, where there isa very high reliance on nuclear power