(o)unit 3 general features of nuclear reactor

Unit3GeneralFeaturesofNuclearReactor2Words&PhrasesWordsExplanationgeneralfeatures一般特征activecore活性区moderator慢化剂slowdown慢化coolant冷却剂oridnary(light)water普通水（轻水）liquidmetal液态金属controlrod控制棒startup启动shutdown停堆graphite[5^rAfait]石墨3Words&PhrasesWordsExplanationcriticalmass临界质量neutrondensity中子密度effectivemultiplicationfactor[7mQltipli5keiFEn]有效增殖因子criticality[kriti5kAliti]临界critical临界的subcritical次临界的supercritical超临界的finitesize[5fainait]有限尺寸reactivitycoefficient[kEui5fiFEnt]反应性系数built-in(orexcess)reactivity后备（剩余）反应性temperaturedistribution温度分布efficiency[i5fiFEnsi]效率4Vocabulary&PhrasesWordsExplanationthermalreactor热堆fastreactor快堆experimentalreactor实验堆（研究堆）productionreactor生产堆dualpurposereactor两用堆nuclearheatingreactor供热堆lightwaterreactor轻水堆heavywaterreactor重水堆calandria[kE5lAndriE]排管容器pressurizedwaterreactor压水堆boilingwaterreactor沸水堆breeder增殖blanket转换区Adeviceinwhichnuclearfissionenergyisreleasedinacontrolledmanneriscalledanuclearreactor.Inotherwords,thesysteminwhichacontrolled,self-sustainingfissionchainreactiontakesplaceiscalledanuclearreactor(49)5CompositionofNuclearReactorAreactorconsistsofanactivecoreinwhichthefissionchainissustainedandinwhichmostoftheenergyoffissionisreleasedasheat.(50)6CompositionofNuclearReactorThecorecontainsthenuclearfuel,consistingofafissilenuclideandusuallyafertilematerialinaddition.(51)7CompositionofNuclearReactorThefunctionofthemoderatoristoslowdownthehigh-energyneutronsliberatedinthefissionreactions.mainlyasaresultofelasticscatteringreactions.Thebestmoderatorsarematerialsconsistingofelementsoflowmassnumberwithlittletendencytocaptureneutrons;examplesareordinarywater,heavywater(deuteriumoxide),beryllium,berylliumoxide,andcarbon(asgraphite).(52)8CompositionofNuclearReactorThepurposeofthereflectoristodecreasethelossofneutronsfromthecorebyscatteringbackmanyofthosewhichhaveescaped.Hence,theuseofareflectorresultsinadecreaseinthecriticalmassofthefissilenuclide.(53)9CompositionofNuclearReactorTheheatgeneratedinthereactorcoreisremovedbycirculationofasuitablecoolant,suchasordinary(light)water,heavywater,liquidsodium(orsodium-potassiumalloy),air,andheliumetc.(54)10CompositionofNuclearReactorTherateofheatgenerationinareactorcoreisproportionaltothenuclearfissionrateandthisisdeterminedbytheneutrondensity,i.e.,thenumberofneutronsperunitvolume.(55)11CompositionofNuclearReactorThereactorcontrol,includingstartup,poweroperation,andshutdown,isthusachievedbyvaryingtheneutrondensityinthecoreandgenerallydonebymovingcontrolrods.(56)12CompositionofNuclearReactorSincetwoorthreeneutronsareliberatedineachoffissionwhereasonlyoneisrequiredtomaintainafissionchain,itwouldseemthatoncethefissionreactionwereinitiatedinagivenmassoffissilematerial,itwouldreadilysustainitself.(57)13CriticalityandReactivityHowever,suchisnotthecasebecausenotalltheneutronsproducedinfissionareavailabletocarryonthefissionchain,thatis,someneutronsarelostinnonfissionreactions(mainlyradioactivecapture),whereasotherneutronsescapefromthesystemundergoingfission.(58)14CriticalityandReactivityThefractionofneutronslostbyescapethroughthegeometricboundariescanbereducedbyincreasingthesize(ormass)ofthefissilematerial.Theminimumquantityofsuchmaterialthatiscapableofsustainingafissionchainiscalledthecriticalmass.(59)15CriticalityandReactivityInafinitesystem,thecriticalconditionisdefinedintermsoftheeffectivemultiplicationfactor(Keff)whichdescribesallthepossibleeventsinthelifeofaneutron.(60)16CriticalityandReactivityInreactorphysics,itisconvenienttouseasmallerquantitythanKeff,calledreactivity(ρ),whendiscussingneutronmultiplication.Itisdefinedby.(61)17CriticalityandReactivity1effeffkkTheeffectivemultiplicationfactor(orreactivity)effectivelydescribesthestateofanuclearreactor.(62)18CriticalityandReactivitylifKeff=1orρ=0,thesystemiscritical,i.e.,theselfsustainingfissionchaincanbemaintained;lIfKeff1orρ0,thesystemissubcritical,theneutrondensitywoulddecreasecontinuously;lIfKeff1orρ0,thesystemissupercritical,theneutrondensitywouldincreasecontinuously.InPWR,thefuelsupplycannotbecontinuouslyreplacedasitisconsumed.Consequently,atthebeginningofeachoperationalperiod,thereactorcoremustcontainallthefuel(fissilematerial)thatwillberequiredtoproduceapredeterminedquantityofenergy.(63)19CriticalityandReactivityFurthermore,additionalfuelisnecessarytoallowforthedecreaseinneutronmultiplicationarisingfromfission-productpoisonsandfromthehighoperatingtemperature,i.e.,thenegativetemperaturecoefficient.(64)20CriticalityandReactivityHence,whenathermalreactorcoreisassembledpriortocommencingoperation,itincludesaconsiderableamountofexcessfuel-some25to30percentinwater-moderatedreactors-abovethatrequiredforcriticalityintheinitialcoldcondition.Theadditionalfissilematerialinthecoreissaidtorepresentbuilt-in(orexcess)reactivity.(65)21CriticalityandReactivityIftheenergyreleasedinthereactoristobeconvertedintoelectricpower,theheatmustbetransferredfromthecoolanttoaworkingfluidtoproducesteam.(66)22HeatRemovalThehigherthetemperatureofthesteam,thegreatertheefficiencyforconversionintou