nuclear science

> INTRODUCTION

De la fission à la réaction en chaîne 6> The operation of a nuclear reactor

FROM RESEARCH

TO INDUSTRY

> From fission to chain reaction

NUCLEAR FISSIONCOMPONENTS OF A NUCLEAR REACTORTHE VARIOUS TYPES OF REACTOR

1 > The atom2 > Radioactivity3 > Radiation and man4 > Energy5 >Nuclear energy: fusion and fission6 >How a nuclear reactor works7 > The nuclear fuel cycle8 >Microelectronics9 > The laser: a concentrate of light10 >Medical imaging11 >Nuclear astrophysics12 >Hydrogen

© Commissariat à l’Énergie Atomique et aux Energies Alternatives, 2005Communication DivisionBâtiment Siège - 91191 Gif-sur-Yvette cedexwww.cea.fr

ISSN 1637-5408.

6 >How a nuclear reactor works

THE COLLECTION

1 > The atom2 > Radioactivity3 > Radiation and man4 > Energy5 >Nuclear energy: fusion and fission6 >How a nuclear reactor works7 > The nuclear fuel cycle8 >Microelectronics9 > The laser: a concentrate of light10 >Medical imaging11 >Nuclear astrophysics12 >Hydrogen

From fission to chain reaction 6> How a nuclear reactor works

> CONTENTS 32

introductionApower station is a factory that manufac-

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 tur-bine directly. 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,

“The first nuclear reactor was constructedin the USA in 1942. The first French reactorcame into operation six years later.”

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 15th 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 gen-erator produces the heat needed to turn waterinto steam. The other components (turbine,alternator, etc.) are similar in all types of powerstation.* Megawatt: One million watts.

NUCLEAR FISSION 4Fissile atoms 5The energy liberated during fission 5Neutrons and the chain reaction 6

COMPONENTS OF A NUCLEAR REACTOR 7The fuel 8The control rods: Neutron traps 9The coolant: Carrier of heat 9The moderator: Neutron decelerator 10The steam generator: Heat exchanger 10

THE VARIOUS TYPES OF REACTOR 11Reactor families 12The pressurised water reactor (PWR) 15The fast neutron reactor (FNR) 15The gas cooled reactor (GCR) 15

> INTRODUCTION 3How a nuclear

reactor w

orks

Overview of the core

of a reactor.

Loading the reactor and closing the reactor vessel headat the Civaux nuclear power station (Unit 1).

The builders of thefirst experimentalreactor: LewKowarski (left) andFrédéric Joliot-Curie(right).

The building housingthe first experimentalreactor, Zoé.


Designed and produced by Spécifique - Cover photo by © PhotoDisc - Illustrations by YUVANOE - Printed by Imprimerie de Montligeon - 03/2005

© M

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> NUCLEAR FISSION 54

WHEN AN ATOM UNDERGOES FISSION, IT RELEASES ENERGY IN THE FORM OF HEAT. IN A NUCLEAR POWER STATION, THIS HEATIS USED TO GENERATE ELECTRICITY.

FISSILE ATOMSThe nucleus of some heavy atoms can be splitin two by striking it with a suitable projectile.Neutrons are particularly good projectiles forsplitting atoms.Neutrons carry no electrical charge whichenables them to get close to the positivelycharged nucleus without being repelled by anelectrostatic force. A neutron can therefore penetrate right into the heart of the nucleusand cause it to split in two. The split is notdue to the mechanical shock of the impact ofthe neutron. The split is triggered internallywithin the nucleus by the arrival of the extraneutron. The additional neutron disturbs thebalance of nuclear forces within the neutron(see the Nuclear energy: Fusion and fissionbooklet). This fragmentation of the nucleus iscalled a fission reaction.Atoms capable of splitting in two following acollision are said to be fissile. The most widelyknown fissile atoms are uranium 235 and plutonium 239. The two parts of the nucleusresulting from the fission of a heavy nucleusare known as fission products. These are usuallyradioactive.


Nuclear fission

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© CERN Geneva

THE ENERGY LIBERATED DURING FISSIONWhen a nucleus undergoes a fission reactionit releases a large quantity of energy.The two fission products absorb a large partof this energy in the form of kinetic energy.They are ejectedat a very highspeed of around8,000 km persecond. Theseheavy projectilescollide with otheratoms in theirpath. These mul-tiple collisions cause them to decelerate rap-idly, losing energy in the process, and theyeventually come to a halt in the mass of uranium. The energy lost heats the surroundingmaterial. The initial energy of the fission products is eventually all converted into heat

Atoms with unstable nuclei are saidto be radioactive. These nucleitransform themselves naturally intoother nuclei with the emission ofradiation (see the Radioactivitybooklet).

Kinetic energy is the energy carriedby a moving body. It is proportionalto the mass and the velocity. A vehicle travelling at high speedhas more kinetic energy than thesame vehicle travelling at a lowerspeed. If the vehicle crashes into anobject at high speed the damage isgreater than if the crash occurred atlow speed. Similarly, a car has lessenergy than a lorry travelling at thesame speed.

“Each fission reaction liberates two or three high energy neutrons travellingat 20,000 km/s.”

and the local temperature of the uranium rises.In a nuclear power station, this heat is recoveredand converted into electricity.

NEUTRONS AND THE CHAIN REAC-TIONIn addition to the fission products, each fission reaction also produces an average oftwo or three high energy neutrons that moveat high speed (20,000 km/s) through the sur-rounding uranium atoms. These neutrons carryonly a small proportion of the energy liberatedduring fission, most of which is carried by thefission products. However, as the neutrons aremuch lighter than the fission products, theymove at very high speeds.With their small size and electrical neutrality,neutrons can travel over relatively long distances before they interact with anotheratomic nucleus. If this happens to be an atomof uranium 235, the neutron can triggeranother fission reaction.The two or three neutrons liberated during afission reaction can trigger new fission reac-tions which liberate more neutrons which

trigger more fission reactions, and so on andso on… This is called a chain reaction.In a nuclear reactor, the chain reaction is con-trolled in order to maintain a constant rate offission reactions. Of the two or three neutronsliberated during a fission reaction, only one triggers a new reaction and the others are simplycaptured. The system is in equilibrium. One fission reaction leads to one new fission reac-tion, which leads to one more, and so on. In anuncontrolled chain reaction, one fission reac-tion could lead to two, which could lead to four,then eight, etc. In a controlled chain reaction,the quantity of heat liberated per second in themass of uranium is completely under control.


6 > NUCLEAR FISSION

Controlled chain reaction in a nuclear reactor

“In a nuclear reactor,the chain reaction is controlled in orderto maintain a constantrate of fissionreactions.”

THE COMPONENTS OF A NUCLEAR REACTOR AREUSED TO RECOVER AND TRANSFORM THE HEAT.

Components of a nuclear reactorComponents of a nuclear reactor

© Framatom

e


7

Neutron.

> COMPONENTS OF A NUCLEAR REACTOR

From fission to chain reaction 6> How a nuclear reactor worksFrom fission to chain reaction 6> How a nuclear reactor works

98

THE FUELThe fuel used in a nuclear power station con-tains fissile atoms capable of giving up theirenergy through fission. The most common fissile atoms are uranium 235, plutonium 239and plutonium 241. Uranium 235 is the onlyone that occurs naturally. For this reason, ura-nium 235 is the main fissile atom used innuclear power stations.The nuclear fuel is loaded into the core of thereactor (see The nuclear fuel cycle booklet).

Reactor vessel

This is a sealed metal enclosure containing the reactorcore, the structure supporting the core, and the guidesfor the control rods.

Fuel assembly

> COMPONENTS OF A NUCLEAR REACTOR© Framatom

e/Rey Robert

The fuel rods contain uranium oxide.

© C.P

auquet/Framatom

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THE CONTROL RODS: NEUTRON TRAPSWithin the reactor, the chain reaction is continu-ally kept under control by means of “control rods”.These rods contain a material capable of absorb-ing neutrons, such as boron. The control rods canbe moved in and out of the reactor core. Theirheight can be adjusted, or they can be removedcompletely, according to the number of neutronsthat need to be absorbed. The control rods areused to control the operation of the reactor. In the event of an incident, all the control rodsare dropped into the reactor core stopping thechain reaction almost instantaneously.

THE COOLANT: CARRIER OF HEATThe energy liberated in the form of heat whenthe uranium 235 nuclei undergo fission mustbe recovered in order to be used to generateelectricity. This task is performed by thecoolant. As its name implies, the coolant isa fluid capable of carrying away heat. Thisfluid circulates around the uranium fuel rodsand provides two functions: it takes up heatfrom the fuel and carries it out of the reac-tor core. It also maintains the core tempera-ture at a level that will not damage the mate-rials in the core.The fuel is contained in a sealed metal enclosureisolating it from the coolant. This prevents anydirect contact between extremely hot fuel ele-ments and the coolant which could otherwiseresult in chemical reactions. It also stops anyfuel particles from being carried out of the

© CEA

Loading the reactor core. Simulation of the neutron flux.

“While the control rods trap neutrons, the coolant extracts heatfrom the reactor.”

Reactor vessel head

Maintenancecolumn

Coolant circuit guide tube

Coolantcircuitinlet Coolant

circuitoutlet

Reactorvessel Control

rod mechanism

Internalinstrumentation reactor core by the coolant. The uranium itself

is less dangerous than fission products, mostof which are highly radioactive.

Core of a nuclear reactor

Control rod

Moderator

Hot coolant

Uraniumfuel rod

Cold coolant


10 > COMPONENTS OF A NUCLEAR REACTOR

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 the neutrons 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 diffi-cult to obtain sufficient fission reactions. In

order to obtain a greater number of fission reac-tions easily, 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 thermal neutrons. 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 and produces steam. This steam is used to drivea turbine connected to an alternator gener-ating electricity. After it leaves the turbine,the steam is condensed back into water in acondenser cooled by river water.

“The moderator slowsthe neutrons so thatthey can be captured byatoms and trigger afission reaction.”


The various types of reactor

THE CHOICE OF THE COMBINATION OF FUEL,COOLANT AND MODERATOR DETERMINESTHE REACTOR FAMILY TO WHICH THE POWER STA-TION BELONGS.

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Transporting a steam generator.

© Framatom

e/G. L

iesse

The various types of reactor

> THE VARIOUS TYPES OF REACTOR


13

REACTOR FAMILIESA nuclear power station is designed to produceelectricity from nuclear fuel. However, althoughthey all share the same basic principle of operation, there are several different familiesof nuclear reactors.

There are four main components of a reactorcore:• The fuel assemblies, in which fission reac-tions take place.• The coolant fluid to carry the heat out of thereactor.• The moderator (except in the case of a fast neutron 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.

The Civaux nuclear power station (Vienne).

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“Although they allshare the same basicprinciple of operation,there are several different families of nuclear reactors.”


> THE VARIOUS TYPES OF REACTOR12

“Pressurised water reactors produce half of the world’s nuclear electricity.”

REACTOR FAMILIES

TYPE 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 developed in France. All the reactors of this generation 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 reactor accounts for 40% of the nuclear power stations 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 water of reactor in the world. (3% uranium 235) acts as both coolant and moderator. It was also developed in the 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 slow slowed down. There is one prototype the 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).

> THE VARIOUS TYPES OF REACTOR

From fission to chain reaction 6> How a nuclear reactor worksFrom fission to chain reaction 6> How a nuclear reactor works

> THE VARIOUS TYPES OF REACTOR 1514

Diagram showing the operation of a pressurised water reactor

Steam generator (heat exchanger) Steam

Controlrods Boiling water

Turbine Alternator

Condenser Cooler: river or sea water, or air

Reactorvessel

Pressuriser

Hot coolant (320 °C)

Reactor core

Pump PumpCold coolant (280 °C)

Nuclear reactor

Cruas nuclear power station–reactor building–fuel rod loading probe in the open reactor vessel.

The Phénix fast neutron reactor.

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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 to produce more fissile material (breeder) or toincinerate long-lived nuclei (actinides).

THE GAS COOLED REACTOR (GCR)The use of helium as the coolant makes it possible 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 turbine technology 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.

“Fast neutron reactors do not use a moderator.”

© Photothèque EDF/P. Bérenger

“Some types of reactor use helium as the coolant, carrying heat out of the reactor and stabilising the temperature.”

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