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Pressurized Water Reactor 1600 MWe (EPR) Nuclear Power Plant Olkiluoto 3, Finland Functional Description with Poster
Pressurized Water Reactor1600 MWe (EPR)
Nuclear Power Plant Olkiluoto 3, Finland
Functional Description with Poster
he enclosed poster shows the Finnish nuclearpower plant Olkiluoto 3 which will start commer-cial operation in 2009. The plant is equipped withan EPR, a 1600-MW-class advanced pressurizedwater reactor*. The poster illustrates the layoutand design of the plant buildings and structuresas well as the configuration, relative size andlocations of the plant systems and components.
However, this kind of illustration can only providea limited amount of information on how the vari-ous components, systems and structures func-tion together.
And that is exactly what this brochure is for: to de-scribe how a pressurized water reactor like thisoperates so that the poster is easier to under-stand. The various descriptions given in thisbrochure are directly referenced to the numbers
shown on the poster.
By No Means a CompleteDescription
But not each and every one of the 85 itemsshown on the poster is going to be described oreven mentioned in this brochure as that wouldproduce a book of enormous proportions! In-stead, we will be focusing on those systems andcomponents that are important for understandinghow a nuclear power plant of this kind works.
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* This type of reactor is known as a pressurized water reactor because thewater inside the reactor pressure vessel and the connected reactor coolantsystem is kept at such a high pressure that it cannot boil, despite its tempera-ture of approximately 327°C.
A Few Words of Introduction
The first EPR (on the right in this photo montage) is being built for the
Finnish utility Teollisuuden Voima Oy (TVO) at Olkiluoto in Western Finland.
Commercial operation is scheduled to start in 2009.
In October 2004, the French utility Electricité de France (EDF) selected
Flamanville in Normandy as the site for the first EPR in France
(on the left in this photo montage).
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Prospects for Nuclear Power
What is the Difference between Fossil-Firedand Nuclear Power Plants?
Nuclear Energy Comes from the Nucleus ofan AtomHow water slows down neutronsHow do you control a chain reaction?Radiation and fission products
The Reactor Core: the Source of Energy at aNuclear Power PlantUranium fuel: “packed full” of energyFuel operating cycles of up to 24 monthsFuel storage
How Much Power Does a Nuclear PlantGenerate?Thermal power output Electric power output
The Reactor Building Contains the ReactorPressure Vessel…
…and All Other Components of the NuclearSteam Supply SystemSteam generators, pressurizer, reactor coolant pumps,accumulators, controlled area
The Two Cycles of a Pressurized Water Reactor PlantThe reactor coolant system carries the heat to the steamgeneratorsThe steam, condensate and feedwater cycle supplies theturbine with steamThe remaining heat is discharged via the turbine condenser to the plant environsBack to the steam generator – and it all starts over again
How is Electricity Generated?
A Brief Look at the EPR’s Safety ConceptThe “self-regulating” reactorSafety systems and equipmentControl of beyond-design-basis eventsThe nerve center of the power plant
Want to Find Out More?
Table of Contents
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World energy demand and contributions from individual energysources (source: WEC).
Chronology of generations of nuclear reactors.
Prospects for Nuclear Power
Today, nuclear power makes a decisive contributiontowards ensuring that we have a safe, clean and eco-nomical supply of electricity. Since the mid-1980s,the amount of electricity produced by nuclear powerplants has doubled, reaching a total of 2525 tera-watt-hours* in 2003, equivalent to approximately16% of the world’s power demand. At the end of2003, 439 reactors were in operation in 30 countries,including all of the major Eastern and Western in-dustrial nations as well as newly industrialized coun-tries such as Brazil, China, India and Korea. Another33 reactors were under construction in 13 countries(sources: IEA, IAEA).
In order to meet the world’s growing demand forelectric power while at the same time protecting ourclimate and environment, nuclear energy will contin-ue to be needed as one of the main sources of pow-er in the coming decades. Consequently, in thecourse of the last 10 years, nuclear power plant ven-dors all over the world have greatly enhanced thesafety and economic efficiency of their pressurizedwater reactor (PWR) and boiling water reactor (BWR)product lines to reflect current market requirementsand ensure political and social acceptance.
The world’s leading nuclear power plant vendorFramatome ANP – an AREVA and Siemens compa-ny – has committed itself to further developing nu-clear technology. It offers two new designs based on
proven light water reactor technology that are nowready for construction: the EPR pressurized water re-actor and the SWR 1000 boiling water reactor. Thesethird-generation reactors incorporate more compre-hensive safety features than existing nuclear powerplants. The first EPR is being built in Finland, and inOctober 2004 the French utility Electricité de France(EDF) selected Flamanville in Normandy as the site forthe first EPR in France.
In addition to the light water reactor designs com-mercially available today, international research anddevelopment programs are also focusing on reactortechnologies that might be able to be implementedsometime in the future. These “fourth-generation re-actors” could be ready for commercial operation in20 to 30 years’ time but still need further develop-ment and testing to verify their safety and cost-effectiveness. In the long term, fourth-generation re-actors could be a supplement to existing reactorproduct lines and could open up new applications fornuclear power such as cogeneration of process heat,production of drinking water through seawater de-salination, and hydrogen production. Furthermore, inthe second half of the 21st century, nuclear fusionwith its enormous energy potential may become aviable source of power, although its general suitabil-ity for commercial power generation has yet to bedemonstrated.
19701950 1990 2010 2030 2050
CommercialpowerreactorsPWR, BWR,CANDU,VVER/RBMK
Reactors with furtherenhanced safety andincreased competitive-ness: advanced watercooled reactors,e.g. EPR, SWR 1000,ABWR, AP1000
Futureadditionalreactorconcepts:e.g. HTR,FR
Fusion
Generation IV
Generation III, Generation III+
Generation II *
Early prototype reactors
EPR / SWR 1000 +New Types +Fusion
Generation I
*TCE = Tons of coal equivalent (1 TCE = 29.31 x 106 J)
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30Billion TCE*
19801970
7.9
1990 1998 2020 2050
10.412.6
13.7
19.4
Hydro
Gas
Oil
Coal
Nuclear
Otherrenewables
27.1
* incl. modernization, uprating and plant life extension
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* 2525 billion kilowatt-hours
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Fossil-fired power plant Nuclear power plant with pressurized water reactor
In all thermal power plants, water is made to evap-orate inside a “boiler” by applying heat. The resultingsteam drives the bladed wheels of a turbine. This tur-bine is coupled to a generator that produces theelectricity. The combined unit formed by the turbineand the generator is called a “turbine generator set”.
Basically a nuclear power plant operates in just thesame way as a fossil-fired thermal power plant, ex-cept that the heat is generated differently: not by
Every year in Germany, the use of nuclear energy preventsthe same amount of carbon dioxide from being emittedinto the atmosphere as is produced each year by carsand trucks (around 165 million tons). In exchange for this,small quantities of radioactive waste have to be accom-modated. Each year, a modern 1300-MW-class nuclearpower plant – such as the Konvoi-series unit Isar 2 – pro-duces approximately 50 cubic meters (m3) of radioactiveprocess waste with negligible heat generation (NHGW),conditioned and packaged ready for storage in a finalrepository. The plant's spent fuel assemblies, if directlysent to a repository without prior reprocessing, yieldaround 45 m3 of heat-generating waste (HGW). Thisamount can be substantially reduced by reprocessing,however, to just 10 m3 of NHGW plus 3 m3 of HGW, thelatter comprising the vitrified, highly radioactive fissionproducts (source: Kernenergie Basiswissen, January 2004).
What is the Difference between
Fossil-Fired and Nuclear Power Plants?
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0.75
1
kg CO2pro kWh*
AnthraciteLignite
0.95
Oil Naturalgas
Solar(photo-voltaics)
Wind
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Hydro Uranium
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Fossil Renewables Nuclear
Emissions resulting from
operation
fuel supply
construction
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Carbon dioxide (CO2) emissions from different types of power plants(source: Siemens Power Generation, status: 2002).
Reactorpressurevessel
TurbineTurbine Generator
Cooling water(sea, river, cooling tower)
Cooling water(sea, river, cooling tower)
Reactorcoolantpump
Pressurizer
Steam generator
Steam generator
burning coal, oil or natural gas (these are called “fossilfuels” because they were formed during early geo-logical eras) but by nuclear fission inside the reactor.All over the world the energy source used in a nuclearpower plant is nevertheless also called a “fuel”, eventhough this term is not strictly correct. At a pressur-ized water reactor plant, water circulating in thereactor coolant system conveys the heat from thereactor to the steam generators which produce thesteam to drive the turbine.
Nuclear Energy Helps Protect Our Climate
Generator
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Reactor coolant system comprising four loops connected to the reactor at the center.
The nuclear fission process
The nucleus of an atom consists of two types ofelementary particles: protons and neutrons. The pro-ton carries a unit of positive electrical charge, whilethe neutron is electrically neutral. The number of pro-tons determines the properties of the chemical ele-ments. The neutrons act like a “glue”, binding theprotons together as they would otherwise fly apart asa result of electrostatic repulsion. In order for anatomic nucleus to be stable, the ratio of the numberof protons to the number of neutrons must lie withina narrow range; if the nucleus contains too many ofone or other type of elementary particle, it will be-come instable and sooner or later disintegrate, or“decay”, while emitting radiation. This process iscalled “radioactivity”. Very heavy nuclei are capableof splitting either spontaneously or by being given a“hefty knock”. One such heavy nucleus is the heavymetal uranium with its 92 protons.
The fuel used in a pressurized water reactor mainlyconsists of the non-fissile isotope uranium-238 (it contains 146 neutrons, giving it a total of 238 par-ticles). Only the fissile isotope uranium-235 (with 143 neutrons), which makes up no more than 5 per-cent of the fuel, is actually responsible for generatingpower. The fissile uranium nuclei are bombarded withneutrons. When a neutron strikes a uranium-235nucleus, the nucleus splits into two or three fissionfragments, turning the binding energy that is releasedon fission into kinetic energy. The fission fragmentsfly apart at high speed. Since they are embedded ina crystal lattice, however, they are not able to movefreely but are very quickly brought to a stop. Whenthis happens, the kinetic energy is converted to heat.
Each time a uranium nucleus splits, two or three neu-trons are also emitted. Traveling at an average speedof around 10,000 kilometers per second, these so-called “fast neutrons” are able to move freely withinthe fuel and the surrounding materials. To ensure thatat least one of these fast neutrons strikes anotheruranium-235 nucleus, thereby causing it to split (andinitiating the desired chain reaction), they have to beslowed down before they are lost from the reactorcore through leakage or are “captured” (absorbed) byother nuclei without having contributed anything tothe fission process.
How water slows down neutrons
The heat produced by fission is conveyed out of thereactor core through the four reactor coolant loopsconnected to the reactor at the center. The water in
Moderator (e.g. water) slows down fast neutrons.
Fast neutrons are not slowed down if moderatortemperature is too high or there is not enoughmoderator (loss-of-coolant accident at a light waterreactor), meaning no further fission reactions.
Fission of a
nucleusproduces
fastneutrons.
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Nuclear Energy Comes from
the Nucleus of an Atom
2. The number of fissile uranium-235 nuclei in thenuclear fuel is much too low and the fissile nucleiare too widely distributed among the non-fissileuranium-238 nuclei for an uncontrolled chain re-action to take place.
Radiation and fission products
The fragments produced when a nucleus is split areknown as fission products. A large number of differ-ent atoms are produced in this way, some of themradioactive (e.g. xenon-133, krypton-85 and iodine-131). These atoms continue to give off radiation for aperiod of time that depends on their respective “half-lives” – a term denoting the time needed for half ofthe nuclei to decay. All fission products decay via anumber of intermediate stages into stable chemicalelements that are no longer radioactive. In the caseof many of these, this only lasts a few seconds; withothers it may take many years or even decades.
The “decay heat” (or “decay heat power”) that con-tinues to be produced after the reactor has been shutdown (bringing the chain reaction to a stop) is gen-erated by the radioactive decay of the fission pro-ducts. This heat is removed by the residual heatremoval systems until the decay process comesto a natural end.
In addition to fission products, other radioactive sub-stances known as “activation products” are also gen-erated during reactor operation. These are elementsthat are not normally radioactive (e.g., alloying ele-ments in fuel cladding and structural materials) butbecome radioactive as a result of absorbing neu-trons.
The radioactive materials in a nuclear power plant arethe real source of danger. For that reason all of aplant’s safety systems and equipment are designedto prevent the release of such materials into the en-vironment, or to limit their release to allowable levels.
The most important tasks involved inoperating a nuclear power plant are:
1. Production of thermal energy by sustaining acontrolled chain reaction of nuclear fission,
2. Removal of this thermal energy and conversion toelectrical energy,
3. Prevention of fission product release,
4. Safe and reliable removal of decay heat from thereactor after it has been shut down.
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the loops circulates continuously at a pressure ofapproximately 160 bar and a temperature of around327°C. Each loop contains a heat exchanger (steamgenerator) and a pump.
The reactor water is also called “reactor coolant” asit serves to remove the heat from the reactor and thuscool it. It is ordinary water (H2O), also known as “lightwater” to distinguish it from the “heavy water” usedin a different, less common type of reactor.
Apart from acting as a coolant, the water also per-forms another important function inside the reactor:it serves as a “moderator”, which means that it slowsdown the neutrons. The initially fast neutrons losetheir kinetic energy as a result of repeated collisionswith the moderator’s atoms, particularly the water’shydrogen atoms.
These collisions and the resulting friction slow theneutrons down to a velocity of around 2000 metersper second. At this velocity they are much more likelyto cause further fissions of uranium-235 nuclei. Theslowed-down neutrons are called “thermal neutrons”because they are in thermal equilibrium with their sur-roundings (i.e. moving at approximately the samespeed as gas molecules at a temperature of around300°C).
How do you control a chain reaction?
In order for the thermal output of a reactor to stay ata constant level, an equal number of fission eventsmust occur over a given period of time. For this tohappen, exactly one of the two or three neutronsemitted with each fission must go on to split anoth-er uranium-235 nucleus. The excess neutrons arecaptured, for example, by neutron-absorbing mate-rials provided for reactor control (control rods and/orboric acid dissolved in the coolant) or by uranium-238 nuclei. This process of neutron capture results innon-fissile uranium-238 becoming fissile plutonium-239 and plutonium-241, which can also be splitthrough neutron capture, just like uranium-235. Theparticipation by the freshly created plutonium in thefission process increases the energy yield of the fuelby around 50%.
There are two main factors that have to be remem-bered in this connection:
1. A self-sustaining chain reaction can only occur inthe presence of a moderator.
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Slow neutron collides with another uranium-235nucleus, causing a new fission reaction.
The reactor core, comprising 241 fuel assemblies, islocated inside the reactor pressure vessel . Thefuel assemblies serve to generate the heat inside thereactor core.
Each fuel assembly contains 265 fuel rods arrangedin a square-shaped bundle. Each fuel rod comprisesa thin-walled tube, 9.5 mm in diameter, made from aspecial alloy of the metal zirconium and containing acolumn of sintered uranium dioxide fuel pellets. Thelength of this fuel column – 4.2 meters – is also calledthe “active length” of the fuel.
Each fuel bundle is equipped with a number of con-trol rod guide thimbles. The thimbles are empty tubesinto which the control rods of the control assembliesare inserted. The material contained inside thesecontrol rods absorbs neutrons, making them un-available for further fissions. The number of neutronsthat are absorbed varies, depending on how far thecontrol rods are inserted into the reactor core. If therods are fully inserted into the core, the chain reac-tion stops entirely. The EPR has a total of 89 controlassemblies, each with 24 control rods.
During normal operation the control assemblies areused to provide a uniform power distribution acrossthe core. To quickly shut down the reactor, they arereleased to drop under the force of gravity into thecore (i.e. solely utilizing laws of nature, without anyform of propulsion).
Uranium fuel: “packed full” of energy
The energy content of uranium is far greater than thatof oil, natural gas or coal because here use is madeof the binding energy of the elementary particles inthe atomic nucleus rather than the heat generated byburning carbon. This means that only small stocks ofuranium are required to operate a nuclear reactorover many years. In the case of an oil-fired plant,however, even the most gigantic oil storage tankswould run dry after only a few months. Apart fromthis, oil and coal are not only suitable for generatingelectricity but also serve as raw materials for otherbranches of industry (e.g. for making pharmaceuticaldrugs and plastic).
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Uranium dioxide fuel pellets – each fuel rodcontains a 4.2-meter-longcolumn of these pellets
265 fuel rods make up one fuel assembly.
There are 241 fuelassemblies in the reactor core.
The Reactor Core: the Source of Energy
at a Nuclear Power Plant
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One kilogram of nuclear fuel enriched to 4.3% inuranium-235 (in the form of a cube having sides thatare 45 millimeters long) produces the same amountof electricity as 102 tons of oil (six trucks carrying 17 tons each) or 140 tons of hard coal (five railroadcars of 28 tons each).
Fuel operating cycles of up to 24 months
Depending on the length of the fuel cycle on whichthe nuclear power plant is operating, one-quarter toone-third of the “spent” fuel assemblies have to bereplaced every 12 to 24 months with fresh fuel usinga specially designed refueling machine . The freshfuel assemblies as well as the fuel assemblies re-maining in the reactor are positioned according to acarefully calculated “core loading plan” in order to en-sure optimum utilization of the nuclear fuel within thespecified design limits.
This periodic refueling outage is also used for in-spections and maintenance work.
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Fuel storage
The spent fuel assemblies removed from the reactorduring refueling as well as the fresh fuel assembliesare stored in a separate “fuel building” .
The fresh fuel assemblies to be loaded into the reac-tor are stored in a special dry storage facility calledthe “new fuel store”. Before they are placed in thecore, the fuel assemblies are subjected to a detailedexamination using special inspection equipment.
The spent fuel assemblies are stored in the spent fuelpool . Here they are kept in storage racks underwater (for shielding and cooling purposes) until theirradioactivity and heat generation levels have droppedsufficiently to allow them to be placed in special ship-ping casks for transfer to a reprocessing plant or aninterim storage facility. The fuel pool can accommo-date six to seven years’ worth of spent fuel from thereactor.
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View inside the reactor pressure vessel of a nuclear powerplant (German Konvoi-series Isar 2 with rated electric output of1300 megawatts) during core loading.
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A large city like Frankfurt am Main (Germany) has an average electricitydemand of 365 megawatts (MW). Thus an advanced nuclear power plantwith an EPR-type pressurized water reactor rated for an electric output ofaround 1600 MW can continuously supply all of the power needed byfour such cities. (Photo: Tourismus+Congress GmbH Frankfurt am Main)
Power is described using a standard unit of mea-surement called the watt (abbreviated to “W”). Poweris the ability to do a certain amount of work in a givenunit of time. When these values are very large or verysmall, other more suitable units of measurement areused instead: 1000 W are called a kilowatt (kW),while 1000 kW are a megawatt (MW). By way ofcomparison: a domestic fan heater consumesaround 2 kW and a washing machine around 1 kW,while a medium-sized automobile engine has a power rating of approximately 100 kW. The energythat a washing machine needs to wash one load oflaundry is calculated from the electricity it consumesand the duration of the washing cycle: if the latter is1 hour (h), then the washing machine needs 1 kWtimes 1 h = 1 kWh (kilowatt-hour). An EPR with anelectric output of 1600 MW produces each hour1,600,000 kWh (1600 MW times 1 h); that is enoughto run 1.6 million washing machines at the sametime!
Thermal power output
The EPR shown in the poster has a “thermal” outputof 4300 MW. This represents the total heat output ofthe reactor; in other words, all of the thermal energyproduced by the nuclear fission process occurring inthe fuel.
Electric power output
Just like in any thermal power plant, only about a thirdof the reactor’s thermal output can actually be con-verted to electric power due to the laws of physics.The other two-thirds are unavoidably “lost” to the en-vironment via the power plant’s cooling systems (e.g.cooling tower and river water).
Around 100 MW of the gross electric output pro-duced by the generator is directly tapped off to cov-er the “auxiliary power” requirements of the powerplant itself (e.g. to operate pump motors, electronicequipment and lighting).
The remaining amount is termed the “net electricoutput” and is what is actually fed into the high-volt-age offsite power grid. In the case of Finland’s nu-clear power plant unit Olkiluoto 3 shown in the posterthe net electric output is approximately 1600 MW.
How Much Power Does
a Nuclear Plant Generate?
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1 Control rod drive mechanism2 Liquid level probe3 RPV closure head4 Control rod guide assembly5 Coolant inlet nozzle6 Core barrel7 Fuel assembly8 Lower core support grid9 Flow distribution plate
10 Coolant outlet nozzle11 Fuel assembly with inserted control rod
Section through reactor pressure vessel of EPR showing RPV internals
The Reactor Building
Contains the Reactor Pressure Vessel…
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The reactor building of the EPR – in the shape ofa cylinder with ellipsoidal dome – is a double-shellstructure with internal steel liner. The outer concreteshell protects the entire building against damagefrom natural and external man-made hazards. It isdesigned, for example, to withstand the impact of anairplane (military jet or large civilian aircraft) on the re-actor building dome, as well as the pressure wave re-sulting from the explosion of a liquefied gas tanker ona nearby river. The inner concrete shell can withstanda buildup in pressure such as that occurring in the(highly unlikely) event of a double-ended reactorcoolant pipe break. The gastight steel liner on the in-side prevents the release of radioactive materials tothe outside environment.
The reactor building is also designed to withstandearthquakes so that none of the equipment insidecan be damaged, let alone destroyed, by such vi-brations. In the highly improbable event of a core meltaccident, the molten core material would be collect-ed and cooled in a specially designed “coriumspreading area” situated at the lowest point in thereactor building but still inside the containment. Theextremely robust double-walled containment would reliably keep any radioactivity confined insidethe building.
Reactor pressure vessel
The reactor pressure vessel (RPV)can rightly be called the “heart”
of the nuclear plant. It contains thereactor core with its 241 fuel as-semblies as well as various inter-nals such as control rod guide as-
semblies, measuring instruments and devices forcoolant flow distribution. Above the reactor coolantinlet and outlet nozzles, the RPV is closed off by acover – the RPV closure head – on which the controlrod drive mechanisms are mounted. The RPV de-signed for the EPR, complete with its closure head,will weigh 526 tons. It will be 12.7 meters (m) high,will have an inside diameter of around 4.9 m and shellwalls that are 0.25 m thick.
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Steam generators
The four steam generators arelarge heat exchangers which serveas the interface between the four re-actor coolant loops (the primarysystem) and the non-radioactivesteam, condensate and feedwater
cycle (secondary system). They produce the steamused to drive the turbine , which in turn drivesthe generator .
Inside each steam generator are thousands of U-shaped tubes. The hot reactor coolant flows at atemperature of around 327°C through these tubes.It gives up its heat to the colder feedwater from thesecondary system that is passing over the outside ofthe tubes. This causes the feedwater to graduallyheat up and eventually evaporate. The resultingsteam exits the steam generator and is conveyed tothe turbine via the main steam piping .7
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On the right: section through steam generator. Each of the steam generatorsdesigned for the EPR has a heat-transfer surface of 7960 square meters and aheight of around 23 meters.
1 Secondary manway 7 Steam dryer2 Emergency feedwater nozzle 8 Steam separator3 Horizontal supports 9 Emergency feedwater sparger4 Divider plate 10 Feedwater sparger5 Coolant inlet nozzle 11 Feedwater nozzle6 Steam outlet nozzle 12 Tube bundle shroud
13 Tube bundle14 Tubesheet15 Vertical supports16 Coolant outlet nozzle
Steam generator on its way to the Chinese nuclear power plant unit Ling Ao 2
…and All Other Components of the
Nuclear Steam Supply System
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Pressurizer being rigged into place at the French nuclearpower plant unit Golfech 2. The pressurizer for the EPR will be14.4 meters high and will weigh around 150 tons when empty.
Pressurizer
Just like the expansion tank of acentral heating system, the task ofthe pressurizer is to compensatefor fluctuations in reactor coolantvolume caused by temperature
changes, and thus keep the operating pressure ofthe coolant circulating through the reactor coolantsystem at a relatively constant level. This is neces-sary, for example, to prevent steam bubbles fromforming inside the system which would impedecoolant flow as well as heat transfer from the fuel rodsto the water.
The pressurizer is a vertical pressure vessel filled withwater in its lower section and steam above this. It isconnected to the reactor coolant system by a pipeknown as the surge line. Pressure is increased usingelectrical heaters installed in the lower (water-filled)section to evaporate some of the water (to produceadditional steam), and reduced by spraying waterthrough nozzles into the upper (steam-filled) section(to condense steam). Valves directly mounted on thetop of the pressurizer provide overpressure protec-tion by blowing down steam to a separate relief tank.
Reactor coolant pumps
The four reactor coolant pumpskeep the coolant in continuous cir-culation by pumping it back to thereactor pressure vessel after it hasgiven up some of its heat in thesteam generators. They are vertical
single-stage centrifugal pumps. Each of the fourpumps requires approximately 9 megawatts of elec-tricity to operate (this roughly corresponds to thepower needed to drive 100 mid-size automobiles)and delivers a volumetric flow of around 28,000 cu-bic meters per hour.
Accumulators
The four accumulators are engineered safety fea-tures for emergency core cooling (one is provided foreach reactor coolant loop) that are located inside thecontainment in the immediate vicinity of the coolantloops. They contain borated water which is kept un-der pressure by means of a nitrogen cushion so thatthe water inventory can be automatically injected in-to the coolant loops for emergency core cooling inthe (highly unlikely) event of a double-ended reactor
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coolant pipe break resulting in a large drop in reactorpressure. The boric acid contained in the water ab-sorbs neutrons and thus reliably prevents the chainreaction from resuming, even at low temperatures.
Controlled area
The four safeguard buildings , the fuelbuilding and the nuclear auxiliary building aredirectly adjacent to the reactor building . Togetherthey form the “controlled area” that houses all of theplant components and piping systems containingradioactivity. The reactor building, safeguard build-ings 2 and 3, and the fuel building are protected bydouble concrete walls against the effects of thenatural and external man-made hazards describedon Page 11. Physical separation of safeguard build-ings 1 and 4 ensures that if, for example, an aircraftshould crash onto one of the buildings, the other willnot be damaged. Access to this area is strictlycontrolled and is only possible through the accessbuilding .J
A
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EDCB
One of the distinguishing features of a pressurizedwater reactor is its dual-cycle configuration consist-ing of the radioactive reactor coolant system (prima-ry system) and the non-radioactive steam, conden-sate and feedwater cycle (secondary system). Theseare supplemented by a cooling system that dis-charges the remaining heat to the sea or river.
The reactor coolant system carries theheat to the steam generators
Four symmetrically arranged coolant loops, eachconsisting of a steam generator , a reactor coolantpump and the connecting reactor coolant piping
, are joined to nozzles on the reactor pressure ves-sel . Together with the pressurizer , connectedvia its surge line, they form the reactor coolant sys-tem.
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The Two Cycles of a
Pressurized Water Reactor Plant
The two cycles of a pressurized water reactor nuclear power plant
Reactor-pressurevessel
TurbineGenerator
Cooling water (sea, river,cooling tower)
Reactorcoolantpump
Pressurizer
Steam generator
From the reactor pressure vessel the hot reactorcoolant is conveyed through the reactor coolant pip-ing to the steam generators where it flows throughthe U-tubes, giving up part of its heat (energy) to thefeedwater from the steam, condensate and feedwa-ter cycle flowing along the outside of the tubes. Thereactor coolant pumps then pump the coolant – nowaround 30 degrees cooler after passing through thesteam generators – back to the reactor pressure ves-sel and into the reactor core.
Fluctuations in reactor coolant volume caused bytemperature changes are compensated for in theshort term by the pressurizer and in the long term bythe chemical and volume control system . 13
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The steam, condensate and feedwatercycle supplies the turbine with steam
The steam, condensate and feedwater cycle is alsocalled the “conventional” part of the nuclear powerplant since its components are also found in tradi-tional coal-, oil- and gas-fired power plants.
The main components of the steam, condensateand feedwater cycle, all housed inside the turbinebuilding , are the turbine , generator ,condenser , main condensate pumps (not shownin the poster), feedwater pumps , feedwaterheaters and feedwater tank .
The “main steam” produced in the steam generatorsat a pressure of 78 bar and temperature of 293°C
exits the reactor building through the four main steamlines and is first conveyed to the main steam valvecompartments . These house the isolation valvesprovided to prevent steam generator dryout throughuncontrolled steam blowdown in the event of a pipebreak downstream of the valve compartments. Safe-ty and relief valves, also installed here, reliably protectthe secondary system against overpressure.
From the valve compartments the main steam linesenter the turbine building where they are first rout-ed to the high-pressure (HP) section of the steam tur-bine . The steam is admitted at the center of theturbine casing and then flows outwards to bothsides*, striking the rows of turbine blades that in-
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crease in size from one stage to the next and arerigidly attached to the turbine shaft. The high-pressure, high-temperature steam expands and theenergy released in this process causes the turbineshaft to rotate. Approximately one-third of theturbine’s total output is produced in this HP section.
The steam – now much cooler (178°C) and wetterand at a considerably lower pressure (10 bar) than on admission – flows out of the two ends of the“double-flow” HP section of the turbine into thecrossover lines on its way to three (or two) double-flow low-pressure (LP) turbine sections . Beforeentering the LP sections, however, the steam isrouted through a component called a moistureseparator/reheater (MSR) . Here the steam com-ing from the HP turbine section is first dried and thenreheated (to increase its energy content again) usingsteam extracted from the main steam line. This pre-vents the last-stage blading of the LP turbinesections from being damaged by the impingement of condensate water droplets in the steam.
Once around one-third of the energy contained in thesteam has been converted to work (mechanical en-ergy) at the turbine blades, causing the turbine shaft
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A low-pressure turbine rotor at Isar 2. At Olkiluoto 3 the lastturbine stage will have a diameter of around 7 meters (m) anda blade length of 1.8 m.
* Hence the term “double-flow” for this type of turbine
to rotate, the steam – which has now expandeddown to a pressure of around 0.03 bar* – enters thecondensers installed underneath the turbine. Thecondensers are heat exchangers in which circulatingwater (from the cooling tower, river or sea) flowsthrough thousands of titanium tubes approximately2 centimeters in diameter. The task of the con-densers is to condense the steam exiting the turbine,thereby turning it back into water (known as “con-densate”). When it comes into contact with the coldcondenser tubes, the steam – now at a temperatureof only around 30°C – condenses to form waterdroplets.
The remaining heat is discharged via the turbine condenser to the plantenvirons
Before we look at the next stage in the steam, con-densate and feedwater cycle, we should first explainthat, because of the steam condensation processdescribed above, a considerable proportion (over60%) of the heat contained in the steam is lost. Thisis the latent heat of condensation, which is dis-charged to the cooling tower, river water or sea-water via the condenser. This happens at all thermalpower plants; i.e. also at those firing coal, oil or gas.The cooler the circulating water, the lower the steampressure which, at temperatures below 100°C, is inthe vacuum range. And the lower the steam pressurein the condenser, the higher the energy gradient thatcan be utilized to produce electric power – and thusthe higher the efficiency.
Due to the low seawater temperatures in Finland thethermal efficiency of Olkiluoto 3 will be around 37%.
Back to the steam generator – and it all starts over again
The condensate collecting in the condenser ispumped through LP feedwater heaters to thefeedwater tank . This is basically a large buffer tankcapable of absorbing short-term inventory fluctua-tions in the steam, condensate and feedwater cycle.
The feedwater pumps then pump the water fromthe feedwater tank via HP feedwater heaters andthe feedwater valves back into the steam gener-ators where it is heated and evaporated all overagain.
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The Two Cycles of a Pressurized Water Reactor Plant
Turbine generator set at Isar 2: the exciter and the generator are in the foreground
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* Corresponds to the pressure of air at an altitude of around 24 kilometers
The steam turbine drives the generator which finallyconverts the rotational energy of the turbine shaft toelectrical energy.
The principle underlying this is that of “electromag-netic induction”: when a magnetic field moves pasta conducting coil, an electrical voltage is produced inthe coil, causing a current to flow in a connected con-ductor circuit.
The three-phase alternating current generator usedto generate electricity in power plants has the gener-ator rotor at the center with its rotating magnetic fieldand three coils spaced 120° apart in the generatorstator. The rotor, caused to rotate by the turbineshaft, induces an alternating voltage in each coil, oneafter the other.
However, in order that a magnetic field can build upin the generator rotor, an electrical current has to flowthrough its windings. The direct current needed forthis is supplied by the exciter (a kind of auxiliarygenerator) connected to the main generator.
The electricity generated by the turbine generator setis stepped up from 27 kilovolts (kV) to 400 kV in threegenerator transformers and fed into the offsitepower grid via high-voltage switchgear .6768
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Three-phase alternating current generator withgenerator rotor (1) and generator stator (2)
(1)
(2)
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How is Electricity Generated?
T
– Loss of reactor control systems (control rods fail todrop into the reactor core, or boric acid injectionfails to operate)
– Natural phenomena or external man-made haz-ards (e.g. earthquake, aircraft crash, explosionpressure wave, etc.).
To ensure that radioactivity remains safely and reliablyconfined inside the reactor containment under nor-mal and accident conditions, numerous engineeredsafeguards are incorporated into the plant design.
The passive safety features do not require an oper-ating signal or electric power to perform their safetyfunctions. They function solely by virtue of their pres-ence, like the many protective barriers made of con-crete and steel.
Several passive barriers (see diagram on Page 19)reliably prevent unacceptable releases of radioactivesubstances and direct radiation from the nuclearpower plant. The active safety systems maintain theoperating parameters of the plant within the designlimits so as to protect these barriers. The safety sys-tems are controlled by an electronic “brain”, the re-actor protection system . It performs all of theswitching operations required to guarantee reactorsafety automatically, without human intervention.This means that safety does not depend on promptand correct actions taken by plant operating per-sonnel.
The reactor protection system monitors all key plantoperating parameters and initiates certain actions toactivate safety systems, if required, as soon as theplant approaches certain specified safety limits.These active safety systems include:
– The reactor scram system, which drops neutron-absorbing control rods into the core by gravity.
– The containment isolation system: if there is a riskthat radioactivity might be released into the reactorbuilding during an accident, this system en-ables the reactor building to be isolated from theoutside atmosphere by automatically closing isola-tion valves installed in all of the pipes passingthrough the reactor building wall.
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A Brief Look
A large number of safety features are incorporat-ed into the design of a nuclear power plant to protectpeople living in the vicinity of the plant and the plantpersonnel against the hazards of radiation. Thesesafety features ensure that there will be no unac-ceptable releases of radioactivity into the reactorcontainment or the plant environs either during nor-mal plant operation or in the event of an accident.
Safety systems are also deployed to ensure that thereactor can be safely and reliably shut down at anytime, and subsequently maintained in a safe condi-tion (i.e. with adequate cooling) by continuously re-moving the decay heat generated in the core aftershutdown, thus preventing the reactor from over-heating.
The “self-regulating” reactor
By “self-regulating” we mean the inherent (built-in)safety of a nuclear reactor deriving from the physicalproperties of water and uranium: when the temper-ature inside the reactor core rises, more fission neu-trons are captured by uranium-238 or are able to “es-cape” through the moderator without being sloweddown.
As a result, the number of fissions per unit time, andhence the level of heat generation, drops again. Thisself-regulating capability ensures that the reactorcannot get out of control even in the event of failureof the reactor control systems (neutron-absorbingcontrol rods and boric acid injection).
A sudden, uncontrolled increase in fissions – result-ing in a “runaway” reactor – is physically impossible.For economic reasons and to avoid unnecessarystressing of plant components and their materials,however, this self-regulating capability is backed upby fast-acting control equipment.
Safety systems and equipment
Despite the vast number of safety precautions takento prevent abnormal conditions from arising duringplant operation, the nuclear power plant systems arenevertheless designed to withstand the effects ofhighly improbable accidents. These accidents couldinclude the following:
– Large breaks in system piping (reactor coolantpipe, main steam line, feedwater line, etc.)
at the EPR’s Safety Concept
– The emergency core cooling system: this systemcools the reactor core in the event of pipe breaksin the reactor coolant system leading to a loss ofcoolant. The four safety injection pumps areable to inject enough coolant to make up for smallpipe breaks. The four low head safety injectionpumps can make up for larger losses of coolantand are also used for long-term removal of decayheat from the reactor following shutdown. Further-more, the accumulators already mentionedabove are also there to automatically dischargetheir inventory of borated (neutron-absorbing) wa-ter into the core in the event of a large break in thereactor coolant system resulting in a large drop inreactor pressure.
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– In the event of failures in the steam, condensateand feedwater cycle, heat removal from the reac-tor coolant loops via the steam generators can bemaintained by the four main steam relief valves andby the emergency feedwater system .
– The emergency power supply system takesover supplying power to the safety-related sys-tems using diesel generators that are automatical-ly started up if, in the event of an accident, the gen-erator should no longer be able to meet the plant’sauxiliary power requirements and offsite powersupplies have also been lost.
– As a backup for the emergency diesel generators,supplementary diesels of diverse design as well asbatteries are available to supply power to electricalequipment needed for safe plant shutdown.
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1. Except for a few percent, the vast majority ofthe fission products arising from nuclear fis-sion are retained in the crystal lattice of theuranium pellets.
2. Cladding tubes made of Zircaloy that areseal-welded to make them gas- and pres-sure-tight enclose the fuel pellets and holdback the fission products.
3. The reactor pressure vessel is like a strongsuit of armor designed to withstand all loadsinduced by pressure, temperature and radi-ation.
4. The concrete shield, a thick-walled cylindermade of reinforced concrete, surrounds thereactor pressure vessel and is nearly impen-etrable for the radiation emanating from thereactor pressure vessel.
5. The inner reinforced-concrete shell of the re-actor building that surrounds the Nuclear Is-land of the plant is completely leak-tightthanks to its steel liner, the only means ofaccess to the interior being via airlocks.
6. The outer reinforced-concrete shell of thereactor building is primarily there to provideprotection against natural phenomena andexternal man-made hazards. Together withthe inner concrete shell it acts as a finalshield, reducing the radiation leaving theplant to levels far below the permitted limits.
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Barriers to prevent releaseof radioactive substancesand direct radiation
1
A Brief Look at the EPR’s Safety Concept
Most of the active safety systems are configured withquadruple redundancy, with no more than two ofthese redundant subsystems being needed to con-trol an accident. If an accident should occur, the plantoperating personnel have plenty of time to think andreact calmly because, within the first 30 minutes, thesafety systems automatically carry out all switchingoperations necessary to bring the plant into a stableand safe condition.
Control of beyond-design-basis events
The probability of incidents or accidents developinginto beyond-design-basis events has been mini-mized thanks to the safety concept implemented inthe plant design.
Plant systems are always designed with a certainsafety margin so that even an event that exceedsdesign limits can be controlled either by virtue ofthese margins or by means of additional “accidentmanagement” measures.
In addition, further precautions have been incorpo-rated into the design of the EPR to ensure that therecan be no impact on the environment even if beyond-design-basis events should occur. These include, forexample, special design features to ensure that, inthe event of a postulated core melt accident, themolten core material (corium) can be kept inside thecontainment by the corium spreading area andreliably cooled.
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Emergency diesel generator at Konvoi-series plant Isar 2
If a core melt accident were to occur despite all the accidentprevention measures deployed, the molten core material(corium) would be collected in a specially designed coriumspreading area underneath the reactor pressure vesselinside the containment and would be cooled by the water inthe in-containment refueling water storage tank . 15
14
As there is no technology in which the failure of individualcomponents or systems can be completely ruled out, allsafety systems in a nuclear power plant such as the EPRare designed with multiple redundancy; i.e. with multipletrains and subsystems of identical design. Depending onthe specific design, at least two more trains or subsystemsare provided than would normally be needed to perform adesignated (safety) function (this is called the “n+2” princi-ple); i.e. one of the trains/subsystems may be under repairand another one may be inoperable due to a single failure.Most of the safety systems of the EPR are designed withquadruple redundancy; i.e. each has four identical trains orsubsystems. In order to prevent more than one of thesetrains/subsystems from being damaged by one and the
Maximum Safety through Redundancy and
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Computer study ofstate-of-the-artnuclear power plantcontrol room
The nerve center of the power plant
The nerve center of the nuclear power plant is thecontrol room that contains all control and moni-toring equipment needed for plant operation. Fromhere the entire power plant can be monitored and op-erated by just five people. Three other people are al-so on duty, standing by to perform any maintenancework that might be necessary outside the controlroom.
All major plant parameters are displayed on large wallscreens and there are a number of control desks atwhich all active components of the nuclear powerplant (control rods, pumps and valves, etc.) can bemanually operated.
If, although this is very unlikely, the control roomshould not be available for use, there is also a remoteshutdown station from which the power plant can besafely shut down at any time.
22
same event, they are physically separated from one an-other; e.g. installed in the four safeguard buildings.
However, as redundant safety systems of identical designcould also fail due to a common cause (e.g. a componentdesign deficiency or manufacturing defect), componentsof different designs are additionally provided for perform-ing certain safety functions as an added safety precau-tion (e.g. equipment made by different manufacturers, orhydraulic vs. electric actuators). This is known as theprinciple of diversity.
Application of all of these design principles together – re-dundancy, physical separation and diversity – makes thelikelihood that a safety system could fail very, very small.
Diversity
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As was said at the start, in a brief description likethis we have only been able to touch on the most im-portant plant systems and components and howthey interrelate. If you would like to receive more in-formation, just send an e-mail to:
Want to
Find Out More?
23 |
Want to build your own reactor? Then just purchase our EPR cardmodel kit. Withthe kit’s 32 size-A3 sheets containing more than 3000 parts you can put togetheran extremely detailed model of the EPR (the one being built at Olkiluoto 3) in around320 hours.
The kit can be obtained from the following addresses:
Canada:
Lighthouse Model Art2 Owl Ridge Drive, Richmond HillOntario L4S 1P6, Canada www.lighthousemodelart.com
Finland:
RisteysasemaHämentie 1700500 Helsinkiwww.risteysasema.fi
Germany:
INFORUM Verlags- und VerwaltungsgesellschaftRobert-Koch-Platz 410115 Berlinwww.kernenergie.de (inforum -> Shop -> EPR-Modell)
Scheuer & Strüver GmbHSorbenstr. 2220537 Hamburgwww.moduni.deOrder No. 1000501
Great Britain:
Marcle ModelsTurnagain, Finch Lane, AMERSHAM, Bucks. HP7 9NEwww.marcle.co.uk
USA:
H&B Precision Card Models P.O. Box 8786 Reston, VA 20195 Tel/Fax: (703) 620-9720 email: [email protected]
Paper Models International9910 S.W. Bonnie Brae DriveBeavertonOR 87008-6045www.papermodels.net
Please visit the suppliers’ websites fordetails on prices and shipping charges.However, if you wish to purchase a largenumber of kits (20 or more) then pleasecontact us directly at:
EPR Cardboard Model Kit
With manufacturing facilities in over 40 countries and a sales network in over 100, AREVA offerscustomers technological solutions for highly reliable nuclear power generation and electricitytransmission and distribution.
The group also provides interconnect systems to the telecommunications, computer and auto-motive markets.
These businesses engage AREVA's 70,000 employees in the 21st century's greatestchallenges: making energy available to all, protecting the planet, and acting responsibly towardsfuture generations.
www.areva.com
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