原子能蒸汽供应系统翻译课程设计用.docx

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原子能蒸汽供应系统翻译课程设计用

Chapter10NuclearSteamsupplySystems

10.1Introduction

Nuclearenergyisoneoftheimportantfuelsfortoday’selectricpowergeneration.Nuclearenergyresultsfromchangesinthenucleusofatoms.Asanucleussplits,itreleasesatremendousamountofheat.Thisnucleussplittingprocessiscompletelyfissioned,itwillcreateasmuchheatastheburningof1500shorttonsofcoal.In1982,approximately12%ofelectricpowerproducedintheUnitedStateswasgeneratedfromnuclearpowerplants.

Thedevelopmentofnuclearpowerprogressedslowlyinthenineteenthandearlytwentiethcenturies.In1911thephysicistErnestRutherfordfirstdiscoveredtheexistenceofasubatomicparticle,laterreferredtoasthenucleus.AlthoughRutherforddidnotsucceedinsplittinganucleus,helatershowedthepossibilityofafissionprocess.In1932,thephysicistJamesChadwickdiscoveredtheexistenceofaneutron.In1938,twoGermanchemists,OttoHahnandFritzStrassmannreportedtheyhadproducedtheelementbariumbybombardinguraniumwithneutrons.Thisexperimentwaslaterreferredtoasthefirstmanmadefissionreaction.Thisreactionhadinfactsplitanuraniumnucleusintotwonearlyequalfragments,oneofwhichwasabariumnucleusandanotherwasakryptonnucleus.Inthisfissionprocesstwoneutronswerealsoemitted.Themassofthetwonucleiandtwoneutronsproducedwassomewhatlessthanthatofanuraniumnucleusandaneutroncombined.Thereactionhadthereforeproducedasignificantamountofenergy.

InthesameperiodAlbertEinsteindevelopedhisfamousrelativitytheoryandrelatedthemattertoenergybytheequationE=mc2.Theequationstatesthattheenergy（E）inasubstanceequalsthemass（m）ofthatsubstancemultipliedbythespeedoflightsquared（c2）.Theequationhadbeenusedbyscientiststoestimatetheamountofenergyreleasedinafissionprocess.

ThefirstmanmadechainfissionprocesswasnotproduceduntilDecember2,1942,whenthephysicistEnricoFermiandhisassociatesconstructedanatomicpile,using50shorttonsofnaturaluraniumembeddedin500shorttonsofgraphite.Cadmiumrodswereusedtocontrolthechainreaction.Withthesuccessfuldevelopmentofatomicbombsin1945andthefirstnuclear-poweredvessel,thesubmarineNautilusin1954,thefirstfull-scalenuclearpowerplantbeganoperationsin1956atCalderHallinnorthwesternEngland.Nextyear,thefirstlargenuclearpowerstationintheUnitedStateswascompletedinShippingsport,Pennsylvania.Thepressurizedwaterreactoratthispowerstationproduced60MWofelectricity.

By1960,nuclearpowergeneratingsystemsintherangeof150to200MWwereincommercialoperation.Atthattime,thesereactorswerestillinthedemonstrationphase.Bythemiddle1960snuclearreactorsystemswerebeingorderedbyutilitycompaniesonthebasisoffavorableeconomiccomparisonswithfossil-fuelpowerplants.Thesenuclearreactorsystemswereintherangeof600MWandwereincommercialoperationby1970.By1975,systemsinthe1000MWrangewereinoperation.Themostrecentnuclearreactorsystemsproduce1300MW,whichisthecurrenttechnicallimitonthesizeofasinglereactorsystem.

Present-daycommercialnuclearreactorsareofthe“fission”type.Fissiontakesplacewhenafissionablenucleus（suchasthoseofuranium）capturesafreeneutron.Captureupsetstheinternalforce,whichholdstogetherthetinyparticlescalledprotonsandneutronsinthenucleus.Thenucleussplitsintotwofissionfragments.Besidestheheatenergyproduced,fissionreleasesanaverageoftwoorthreeneutronsandsuchnuclearradiationasgammarays.Thefissionfragmentsgiveoffbetarays.Ifoneoftheneutronsemittediscapturedbyanotherfissionablenucleus,asecondfissiontakesplaceinthemannersimilartothefirst.Anotherneutronmayproduceathirdfission.Whenthefissionbecomesself-sustaining,theprocessiscalledachainreaction.Thedeviceinwhichthischainreactionisgeneratedtoproducenuclearenergyiscalledanuclearreactor.

10.2Nuclearreactorsandtheirclassifications

Nuclearreactorsusedforelectricpowergenerationconsistoffourmainparts.Theyare

（1）thefuelcore,

（2）themoderatorandcoolant,（3）thecontrolrods,and（4）thereactorvessel.

Thefuelcorecontainsthenuclearfuelandisthepartofthereactorinwhichthefissiontakesplace.Thenuclearfuelmaybeeithernaturaluraniumorenricheduranium.Thenaturaluraniumcontains0.71%fissileU-235and99.28%fertileU-238andfertilethoriumTh-232.TheenricheduraniumisproducedinagaseousdiffusionprocessandisexpectedtohaveaU-235contentupto2or3%.Thisisthreeorfourtimestheconcentrationinnaturaluranium.

Infissionprocessthefertilematerialsareconvertedtofissile.Forinstance,theU-238becomesPu-239andPu-241andTh-232becomesU-233.Theseconvertedfissilefuelsarenuclearfuelsthemselves,whicharethenutilizedinreactors.

Thenuclearfuelisgenerallycontainedincylindricalrodssurroundedbycladdingmaterials.Thefuel-rodcladdingmaterialsmustbeablenotonlytomaintainthefuelrodinshape,butalsotoholdunderthereactorconditions.Thesematerialsincludealuminum,magnesium,zirconium,stainlesssteel,andgraphite.

Themoderatoristhesubstanceusedinnuclearreactortoreducetheenergyoffastneutronstothermalneutrons.Liquidandsolidmaterialsofsmallmassnumberandlowneutroncaptureshouldbesuitable.Theseincludelightwater,heavywater,carbon,andberyllium.

Thereactorcoolantisusedtoremoveheatfromthereactorfuelcore.Theconditionsforagoodcoolantincludehighspecificheat,highthermalconductivity,andhighboilingpointatlowpressure.Thecoolantshouldalsohavelowpowerdemandforpumping,lowcost,andahighdegreeofstabilityinthereactorenvironment.Forthispurposethecoolantsincludelightwater,heavywater,air,carbondioxide,helium,sodium,potassium,andsomeorganicliquids.

Controlrodsarelongmetalrodsthatcontainsuchelementsasboron,cadmium,orhafnium.Theseelementsabsorbfastneutronsandthereforehelpcontrolachainreaction.Thecontrolrodsareattachedtoanelevator-likemechanismjustoutsidethenuclearreactor.Themechanisminsertstherodsintothefuelcoreorwithdrawsthemtoslowdownorspeedupachainreaction.Threetypesofcontrolrodsareused.Theseinclude

（1）shimrods,

（2）regulatingrods,and（3）safetyrods.Shimrodsareusedformakingoccasionalcoarseadjustmentinneutrondensity,whileregulatingrodsareusedforfineadjustment.Safetyrodsaredesignedforuseinemergency.Thesafetyrodsaremadeofboronsteelandarecapableofcomingintothereactorcoreveryrapidlyandstoppingthechainreaction.

Thereactorvesselisatanklikestructurethatholdsthereactorcoreandotherinternals.Thewallsofthevesselaredesignedforthehighpressureandradiationenvironment.Inmostcasedthevesselwallsarelinedwiththicksteelslabstoreducetheflowofradiationfromthecore.Asindicatedinthelastsection,nuclearfissiongenerateslargeamountsofneutronsandgammarays.Bothofthemareveryharmful.Becauseofthese,biologicalshieldingisrequiredaroundthereactorvessel.Thisshieldconsistsofconcreteblocks,whichmaybeupto6ftthick.

Accordingtotheircoolantandmoderator,nuclearreactorscanbeclassifiedinto:

pressurizedwaterreactor（PWR）,boilingwaterreactor（BWR）,canadiandeuterium-uraniumreactor（CANDU）,steam-generatingheavywaterreactor（SGHWR）,high-temperaturegas-cooledreactor（HTGR）,advancedgas-cooledreactor（AGR）andgascooledreactor（GCR）.

ThetwoprincipaltypesarethePWRandBWR.Bothreactorsuseenricheduraniumandlightwaterascoolantaswellasmoderator.Thedifferencebetweenthesetworeactorsisinthemannerinwhichsteamisbeingproduced.TheBWRgeneratessteamwithinthereactorvessel.Thesteamisdirectlypipedtoasteamturbineandreturnedtothevesselaftercompletingvariousprocesses.InthePWR,however,onlyhotwaterisproduced.Thehotwateristhentransferredtoaseparateheatexchangerinwhichthethermalenergyofhotwaterisutilizedtogeneratesteam.

TheprominentreactorsusingtheheavywaterasmoderatoraretheCANDUandSGHWR.Bothreactorsuseenricheduranium（UO2）andhaveaseparateheatexchangerforsteamproduction.IntheCANDU,theheavywaterreceivesheatinthenuclearreactorandreleasesheatintheseparateheatexchangerforsteamproduction.Sincethecoolantheavywaterflowsthroughthetubesinsidethereactor,thereisnoneedforapressurevessellikethatinthePWRsystem.TheSGHWRhasasimilararrangementexceptthatitusedthelightwaterascoolantinthereactor.Whiletheheavy-waterreactordoesnothaveandefficiencyashighasthePWRandBWR,itgenerallyhasconsiderablybetteroverallutilizationofthefissionenergyavailableinthenuclearfuel.

Thegas-cooledreactorsfrequentlyusegraphiteasmoderator,CO2ascoolantandnaturaluraniumasfuel.Sincethefuelcladdingismagnesiumalloy（referredtoasMagnoxinEngland）,thegas-cooledreactors（GCR）areoftencalledMagnoxreactors.Theadvancedgas-cooledreactor（AGR）isaresultofcontinueddevelopmentoftheMagnoxsystem.Again,themoderatorisgraphiteandthecoolantisCO2.TheAGRisdesignedtoraisethesystemconditionstothelevelcomparabletothoseinthefossil-fuelpowerplant.

Anotherkindofgas-cooledreactoristhehigh-temperaturegas-cooledreactor（HTGR）.UnliketheGCRandAGR,theHTCGusesheliumascoolantwhichreceivesheatinthereactorandbecomesagasofhighpressureandtemperature.Next,thegasis