CEA - How a nuclear reactor work

2> CONTENTS > INTRODUCTION 3The builders of thefirst experimentalreactor: LewKowarski (left) andFrédéric Joliot-Curie(right).The building housingthe first experimentalreactor, Zoé.How a nuclearreactor worksNUCLEAR FISSION 4Fissile atoms 5The energy liberatedduring fission 5Neutrons andthe chain reaction 6COMPONENTSOF A NUCLEAR REACTOR 7The fuel 8The control rods:Neutron traps 9The coolant: Carrier of heat 9The moderator:Neutron decelerator 10The steam generator:Heat exchanger 10THE VARIOUS TYPESOF REACTOR 11Reactor families 12The pressurisedwater reactor (PWR) 15The fast neutronreactor (FNR) 15The gas cooledreactor (GCR) 15© Médiathèque EDFLoading the reactor and closing the reactor vessel headat the Civaux nuclear power station (Unit 1).Overviewof the coreof a reactor.© Médiathèque EDF“The first nuclear reactor was constructedin the USA in 1942. The first French reactorcame into operation six years later.”introductionstation is a factory that manufac-Apower tures electricity. Some power stations burnfossil fuels, some are driven by water… andsome use nuclear energy. All generate electricityin the same way by using a turbine to drive analternator. The electricity is generated in thealternator. The differences lie in the methodused to drive the turbine. In a hydro-electricpower station, water from a dam drives the turbinedirectly. In traditional power stations, afossil fuel (coal, natural gas or oil) is burnedto convert water into steam. The steam thendrives the turbine. In a nuclear power station,uranium nuclei are used in place of the fossilfuel. The heavy uranium nuclei break down,releasing the energy needed to boil the water.The steam is then used to drive a turbine inthe same way as in a traditional power station.The first nuclear reactor was constructed byEnrico Fermi in the USA in 1942. It consistedof a stack of 6 tonnes of metallic uranium,34 tonnes of uranium oxide and 400 tonnesof graphite. The ‘Fermi pile’ (so-called becauseof its stacked construction) generated a poweroutput of just half a watt. In France, the firstexperimental reactor, Zoé, was built by the CEAat their research centre in Fontenay-aux-Roses.This reactor became operational for the firsttime on the 15 th of December 1948. Thepower output was increased to 150 kW in 1953and the reactor was shut down in 1976. TheZoé reactor building is now used as a museumdevoted to the atom.Modern French electricity generating reactorsdeliver between 900 and 1,450 megawatts*of electricity. Within the reactor, the steam generatorproduces the heat needed to turn waterinto steam. The other components (turbine,alternator, etc.) are similar in all types of powerstation.* Megawatt: One million watts.From fission to chain reactionDesigned and produced by Spécifique - Cover photo by © PhotoDisc - Illustrations by YUVANOE - Printed by Imprimerie de Montligeon - 03/20056 > How a nuclear reactor worksFrom fission to chain reaction6 > How a nuclear reactor works

10 > COMPONENTS OF A NUCLEAR REACTOR11© Framatome/G. LiesseTransporting a steam generator.THE MODERATOR:NEUTRON DECELERATORIn addition to the enclosed fuel rods, thecoolant and the control rods, most reactorsalso contain a moderator (see the diagram onpage 9). The moderator slows down theneutrons as they are often too energetic totrigger new fission reactions effectively.Because of their high energy, these neutronsmove at high speed (20,000 km/s). They areknown as fast neutrons.If a neutron is travelling at a very high speed,it can pass very close to a uranium atomwithout being captured. This makes it difficultto obtain sufficient fission reactions. In“The moderator slowsthe neutrons so thatthey can be captured byatoms and trigger afission reaction.”order to obtain a greater number of fission reactionseasily, the neutrons must be slowed downconsiderably, from 20,000 km/s to a speed ofaround 2 km/s. Neutrons travelling at suchspeed are called slow neutrons or thermalneutrons. Neutrons are slowed when they passthrough a material made up of atoms with nucleithat do not absorb neutrons. Just like a billiardball, the neutrons loose energy each time theycollide and bounce off one of these nuclei.The neutrons are slowed more rapidly if theycollide with light nuclei with a mass similar tothat of a neutron, such as hydrogen.The material containing these atoms is calledthe moderator.In an efficient reactor design, the fuel and themoderator are positioned alternately; fuel,moderator, fuel, moderator, etc.THE STEAM GENERATOR:HEAT EXCHANGERThe coolant is heated by contact with the fuel.It leaves the reactor core at a temperature ofbetween 300 and 550°C.The coolant is then used to heat water in the“steam generator”. The water boils andproduces steam. This steam is used to drivea turbine connected to an alternator generatingelectricity. After it leaves the turbine,the steam is condensed back into water in acondenser cooled by river water.© Médiathèque EDFTHE CHOICE OF THE COMBINATION OF FUEL,COOLANT AND MODERATOR DETERMINESTHE REACTOR FAMILY TO WHICH THE POWER STA-TION BELONGS.The varioustypes of reactorFrom fission to chain reaction6 > How a nuclear reactor worksFrom fission to chain reaction6 > How a nuclear reactor works

12> THE VARIOUS TYPES OF REACTOR> THE VARIOUS TYPES OF REACTOR“Pressurised water reactors produce halfof the world’s nuclear electricity.”13© Médiathèque EDFREACTOR FAMILIESA nuclear power station is designed to produceelectricity from nuclear fuel. However, althoughthey all share the same basic principle ofoperation, there are several different familiesof nuclear reactors.The Civaux nuclearpower station (Vienne).There are four main components of a reactorcore:• The fuel assemblies, in which fission reactionstake place.• The coolant fluid to carry the heat out of thereactor.• The moderator (except in the case of a fastneutron reactor) used to slow down the neutrons.• The control rods to control the chain reaction.Each of these components, and especially thefirst three, can exist in a number of differentforms. For example, the coolant may be a gas(e.g. carbon dioxide) or a liquid (e.g. water).However, of the many possible combinations offuel, coolant and moderator, only a few havebeen used to manufacture actual reactors. Themain types of reactors are shown in the tableon the next page.“Although they allshare the same basicprinciple of operation,there are severaldifferent familiesof nuclear reactors.”REACTOR FAMILIESTYPE OF REACTOR FUEL MODERATOR COOLANTUNGG reactor(Natural Uranium Graphite Gas Natural uranium Solid carbon Carbon dioxidein English) (0.7% uranium 235) (graphite)First type of reactor to be developedin France. All the reactors of thisgeneration are shut down,the last one in 1994.CANDU reactorType of reactor developed in Canada. Natural uranium Heavy water* Pressurised heavy waterRBMK reactor“Reactor Bolshoi Moschnosti Kanalynyi” Enriched uranium Carbon (graphite) Boiling wateror “Channelised Large Power Reactor” (1.8% uranium 235)in English. This type of reactoraccounts for 40% of the nuclear powerstations in states of the former USSR,(for example, Chernobyl).Boiling water reactor (BWR)Reactor family developed in the USA, Enriched uranium Boiling water in the coreJapan and Sweden (3% uranium 235)Pressurised water reactor (PWR)The most common type Enriched uranium Pressurised liquid water. The pressurised waterof reactor in the world. (3% uranium 235) acts as both coolant and moderator.It was also developed inthe ex-USSR as the “VVER”.Fast neutron reactor (FNR)This type of reactor does not use a Enriched uranium None Liquid sodium.moderator. The neutrons are not or plutonium Does not slowslowed down. There is one prototypethe neutronsreactor of this type in France,the Phénix reactor (250 MWe).*Heavy water: Water in which the hydrogen atom has been replaced by deuterium, aheavy isotope of hydrogen (see The atom booklet).From fission to chain reaction6 > How a nuclear reactor worksFrom fission to chain reaction6 > How a nuclear reactor works

14> THE VARIOUS TYPES OF REACTOR > THE VARIOUS TYPES OF REACTOR 15ControlrodsDiagram showing the operation of a pressurised water reactorSteam generator(heat exchanger)PressuriserSteamBoiling waterTurbineAlternator“Some types of reactor use heliumas the coolant, carrying heat outof the reactor and stabilising thetemperature.”ReactorvesselThe Phénix fast neutron reactor.© Médiathèque EDFNuclear reactorHot coolant(320 °C)Reactor corePumpCold coolant (280 °C)PumpCondenserCooler:river or sea water,or airCruas nuclear power station–reactor building–fuel rod loading probe in the open reactor vessel.THE PRESSURISED WATERREACTOR (PWR)This is the most widely used reactor type inthe world. Pressurised water reactors producearound half of the world’s nuclear electricity.In France, all the nuclear reactors except Phénixreactor are PWR types. Thirty-four deliver 900MWe (megawatts of electricity), 20 deliver1,300 MWe and 4 deliver 1,450 MWe.THE FAST NEUTRON REACTOR(FNR)The fast neutron reactor was designed to makea more complete use of fissile material in thefuel (uranium and plutonium) than in reactorsusing thermal neutrons.The coolant may be a liquid metal, such assodium (Phénix) or a gas such as helium.They have the advantages of being able toproduce more fissile material (breeder) or toincinerate long-lived nuclei (actinides).“Fast neutronreactors do notuse a moderator.”© Photothèque EDF/P. BérengerTHE GAS COOLED REACTOR (GCR)The use of helium as the coolant makes itpossible to design reactors with a direct cycle.In this type of reactor, the very hot helium drivesa turbo-alternator directly without the need foran intermediate heat exchanger, therebyincreasing the thermodynamic efficiency. Thistype of reactor has been considered in the past,but recent developments in gas turbinetechnology have led to a revival in interest.They are suitable for use in economic and safesmall-scale installations (100 to 300 MWe).This type of reactor is also capable of operatingwith fast neutrons giving it all the additionaladvantages of the FNR.From fission to chain reaction6 > How a nuclear reactor worksFrom fission to chain reaction6 > How a nuclear reactor works