核技术英语-核技术英语-Chapter10-Neutron-Chain-Reactions课件

NUCLEARENERGYPROFESSIONAL(SPECIAL)ENGLISHCOURSEChapter10NeutronChainReactions

(中子链式反应)NUCLEARENERGYPROFESSIONAL(SPEChapter14NeutronChainReactions

(中子链式反应)Thepossibilityofachainreactioninvolvingneutronsinamassofnuclearfuelsuchasuraniumdependson:(a)nuclearproperties(特性)

suchascross

sections(截面)

andneutronsproducedperabsorption;(b)thesize,shape,andarrangementofthematerials.Chapter14NeutronChainRea14.1CRITICALITYANDMULTIPLI-

CATION(临界性和增殖性)

Toachieveaself-sustainingchainreaction(自持链式反应),oneneedingnoexternalneutronsupply,a"criticalmass"(临界质量)

ofuraniummustbecollected.Toappreciatethisrequirementwevisualizethesimplestnuclearreactor,consistingofametalsphereofuranium-235.14.1CRITICALITYANDMULTIPLI接上页(P199第二段第四行中)SupposethatitconsistsofonlyoneatomofU-235.Ifitabsorbsaneutronandfissions,theresultantneutronsdonothingfurther,therebeingnomorefuel.Ifinsteadwehaveasmallchunk(大块)

ofuranium,sayafewgrams,theintroductionofaneutronmightsetoff(导致)achainofseveralreactions,producingmoreneutrons,butmostofthemwouldescapethroughthesurfaceofthebody,aprocesscalledleakage(泄漏).接上页(P199第二段第四行中)Supposethat接上页（P200第二行中）Suchanamountoffuelissaidtobe“subcritical”(次临界的).

Nowifwebringtogetherabout50kgofU-235metal,theneutronproductionbalancestheleakagelosses,andthesystemisself-sustainingor“critical(临界的).”Thesizeisthecriticalvolumeandtheamountoffuelisthecriticalmass.Neutronshadtobeintroducedtostartthechainreaction,butthenumberismaintainedwithoutfurtheradditions.Theterm(术语)

"criticalmass"hasbecomepopulartodescribeanycollectionofentitieslargeenoughtooperateindependently.接上页（P200第二行中）SuchanamountoP200第二段第三行末Ifweaddstillmoreuraniumtothe50kgrequiredforcriticality(临界状态),moreneutronsareproducedthanarelost,theneutronpopulation(数量)

increases,andthereactoris“supercritical(超临界的).”Earlynuclearweaponsinvolvedtheuseofsuchmasses,inwhichtherapidgrowthofneutronsandresultingfissionheatcausedaviolentexplosion(爆炸).P200第二段第三行末Ifweaddstillmor核技术英语-核技术英语-Chapter10--Neutron-Chain-Reactions课件14.2MULTIPLICATIONFACTORS

(增殖因子)Thebehaviorofneutronsinanuclearreactorcanbeunderstoodthroughanalogy(类推)

withpopulationsoflivingorganisms(生物体),forexample,ofhumanbeings.Therearetwowaystolookatchangesinnumbersofpeople:asindividualsandasagroup.Apersonisbornandthroughoutlifehasvariouschancesoffatal(致命的)

illnessoraccident.Onaveragethelifeexpectancy(期望值)

atbirthmightbe75years,accordingtostatisticaldata(统计数据).14.2MULTIPLICATIONFACTORS

接上页(P200第三段第六行中)Anindividualmaydiewithoutanheir(后嗣),withone,orwithmany.Ifonaveragethereisexactly1,thepopulationisconstant.Fromtheotherviewpoint,iftheratesofbirthanddeatharethesameinagroupofpeople,thepopulationisagainsteady(稳定的).Iftherearemorebirthsthandeathsby1%peryear,thepopulationwillgrowaccordingly.Thisapproach(方法)

emphasizesthecompetitionofprocessrates.接上页(P200第三段第六行中)AnindividualP200倒数第二段Thesameideasapplytoneutronsinamultiplyingassembly.Wecanfocus(聚焦,集中)

attentiononatypicalneutronthatwasborninfission,andhasvariouschancesofdroppingoutof(不参与)

thecyclebecauseofleakageandabsorptioninothermaterialsbesidesfuel.Ontheotherhandwecancomparethereactionratesfortheprocessesofneutronabsorption,fission,andleakagetoseeifthenumberofneutronsisincreasing,steady,ordecreasing.Eachofthemethodshasitsmerits(优点)

forpurposesofdiscussion,analysis,andcalculation.P200倒数第二段ThesameideasapplyP202第二段ThefractionsofabsorbedneutronsthatformU-236andthatcausefission,respectively,aretheratiosofthecrosssectionforcapturecandfissionftothatforabsorptiona.Theaveragenumberofneutronsproducedbyfissionis.Nowletbethecombinationf/a,andnotethatitisthenumberofneutronsperabsorptioninuranium.Thusletting

L

bethefractionnotescapingbyleakage,k=L

.P202第二段ThefractionsofabsorP203第一行Thesystemiscriticalifk=l,orL

=l.MeasurementsshowthatLisaround2.2forfastneutrons,thusLmustbel/2.2=0.45,whichsaysthatasmanyas45%oftheneutronsmustremaininthesphere,whilenomorethan55%escapethroughitsboundary(边界).P203第一行Thesystemiscritical核技术英语-核技术英语-Chapter10--Neutron-Chain-Reactions课件14.3

NEUTRONFLUXANDREACTORPOWER

(中子通量和反应堆功率)Thepowerdevelopedbyareactorisaquantityofgreatinterestforpracticalreasons.Powerisrelatedtotheneutronpopulation,andalsotothemassoffissilematerialpresent.First,letuslookatatypicalcubiccentimeterofthereactor,containingNfuelnuclei,eachwithcrosssectionforfissionfatthetypicalneutronenergyofthereactor,correspondingtoneutronspeed

v.Supposethattherearenneutronsinthevolume.14.3NEUTRONFLUXANDREACTOR接上页(P205第七行)TherateatwhichthefissionreactionoccursisthusRf=nvNf

fissionspersecond.Ifeachfissionproducesanenergyw,thenthepowerperunitvolumeisp=wRf.Forthewholereactor,ofvolumeV,

therateofproductionofthermalenergyisP=pV.Ifwehaveanaveragefluxφ=nvandatotalnumberoffuelatomsNT=NV,thetotalreactorpowerisseentobeP=φNTf

w.接上页(P205第七行)TherateatwhichP205第二段第五行Thepowerdevelopedbymostfamiliardevicesiscloselyrelatedtofuelconsumption(消耗量).…….Inareactor,itisnecessarytoaddfuelveryinfrequentlybecauseoftheverylargeenergyyieldperpound,andthefuelcontentremainsessentiallyconstant.Fromtheformularelatingpower,flux,andfuel,weseethatthepowercanbereadilyraisedorloweredbychangingtheflux.Bymanipulation(操作)

ofcontrolrods(控制棒),theneutronpopulationisallowedtoincreaseordecreasetotheproperlevel.P205第二段第五行Thepowerdeveloped14.4

REACTORTYPES

(反应堆类型)Althoughtheonlyrequirementforaneutronchainreactionisasufficientamountofafissileelement,manycombinationsofmaterialsandarrangementscanbeusedtoconstructanoperablenuclearreactor.Severaldifferenttypesorconceptshavebeendevised(设计)

andtestedovertheperiodsince1942,whenthefirstreactorstartedoperation,14.4REACTORTYPES

(反应堆类型)Al(a)PurposeThemajorityofreactorsinoperationorunderconstructionhaveaspurposethegenerationoflargeblocksofcommercial(商业的)

electricpower.Othersservetrainingorradiationresearchneeds,andmanyprovidepropulsionpower

(推动力)

forsubmarines(潜水艇).Availablealsoaretestedreactorsforcommercialsurface

shipsandforspacecraft(宇宙飞船).(a)PurposeThemajorityofrea接上页Atvariousstagesofdevelopmentofanewconcept,suchasthebreederreactor(增殖堆),therewillbeconstructedbothaprototypereactor(原型堆),onewhichtestsfeasibility(可行性),andademonstrationreactor(示范堆),onethatevaluates(评定)

commercialpossibilities(可能性).接上页Atvariousstagesofdevelo(b)NeutronEnergyAfastreactor(快堆)

isoneinwhichmostoftheneutronsareintheenergyrange0.l--lMeV,belowbutneartheenergyofneutronsreleasedinfission.Theneutronsremainathighenergybecausethereisrelativelylittlematerialpresenttocausethemtoslowdown.Incontrast,thethermalreactor(热堆)

containsagoodneutronmoderating(缓和的)

material,andthebulk

of

(大多数)theneutronshaveenergyinthevicinity(附近)

of0.leV.(b)NeutronEnergyAfastreact(c)ModeratorandCoolant

(慢化剂与冷却剂)Insomereactors,onesubstanceservestwofunctions---toassist(帮助)

inneutronslowingandtoremovethefissionheat.Othersinvolveonematerialformoderatorandanotherforcoolant.Themostfrequentlyusedmaterialsarelistedbelow:

Moderators

Coolants

lightwater(轻水)lightwaterheavywater(重水)carbondioxide(CO2)graphite(石墨)heliumberyllium(铍)liquidsodium(液体钠)(c)ModeratorandCoolant

(d)FuelUraniumwithU-235contentvaryingfromnaturaluranium(=0.7%)toslightlyenriched(=3%)tohighlyenriched(=90%)isemployedinvariousreactors,withtheenrichment(富集)

dependinguponwhatotherabsorbingmaterialsarepresent.(d)FuelUraniumwithU-235con接上页ThefissileisotopesPu-239andU-233areproducedandconsumedinreactorscontainingsignificantamountsofU-238orTh-232.Plutoniumservesasfuelforfastbreederreactorsandcanberecycled

asfuelforthermalreactors.Thefuelmayhavevariousphysicalforms---ametal,oranalloywithametalsuchasaluminum(铝),oracompoundsuchastheoxideUO2orcarbide(碳化物)

UC.接上页ThefissileisotopesPu-239(e)ArrangementInmostmodernreactors,thefuelisisolatedfromthecoolantinwhatiscalledaheterogeneous(异类的,非均匀的)

arrangement.Thealternative(可供选择的方法)isahomogeneous(同类的,均匀的)

mixtureoffuelandmoderatororfuelandmoderator-coolant.(e)ArrangementInmostmodern(f)StructuralMaterialsThefunctionsofsupport(支撑),retention(保存)

offissionproducts,andheatconduction(热传导)

areprovidedbyvariousmetals.Themainexamplesarealuminum,stainlesssteel,andzircaloy(锆合金),(analloyofzirconium(锆)).(f)StructuralMaterialsThefu接上页Byplacingemphasisononeormoreoftheabovefeaturesofreactors,reactorconceptsareidentified.Someofthemorewidelyusedorpromisingpowerreactortypesarethefollowing:PWR(pressurizedwaterreactor)(压水堆),athermalreactorwithlightwaterathighpressure(2200psi(poundspersquareinch))andtemperature(600oF)servingasmoderator-coolant,andaheterogeneousarrangementofslightlyenricheduraniumfuel.接上页Byplacingemphasisonone接上页BWR(boilingwaterreactor)(沸水堆),similartothePWRexceptthatthepressureandtemperaturearelower(1000psiand550oF).HTGR(hightemperaturegas-cooledreactor)(高温气冷堆),usinggraphitemoderator,highlyenricheduraniumwiththorium,andheliumcoolant(1430oFand600psi).接上页BWR(boilingwaterreactor)(接上页CANDU(Canadiandeuteriumuranium)(加拿大重水铀反应堆)usingheavywatermoderatorandnaturaluraniumfuelthatcanbeloadedanddischargedduringoperation.LMFBR(liquidmetalfastbreederreactor)(液金属快增殖堆),withnomoderator,liquidsodiumcoolant,andplutoniumfuel,surroundedbynaturalordepleteduranium

(贫铀).接上页CANDU(CanadiandeuteriumurP208倒数第二段Toillustrate,wecanidentifythecomponentsandtheirfunctionsinamodernpressurizedwaterreactor.Figure14.5givessomeindication(迹象)

ofthesizesofthevariousparts.P208倒数第二段Toillustrate,wecanP208倒数第一段ThefreshfuelinstalledinatypicalPWRconsistsofcylindricalpellets(芯块)

ofslightlyenriched(3%U-235)uraniumoxide(UO2)ofdiameter(直径)

about3/8in.(~lcm)andlengthabout0.6in.(~l.5cm).Azircaloytubeofwallthickness0.025in.(~0.6mm)isfilledwiththepelletstoan“activelength(工作长度)”ofl2ft(365cm)andsealed(密封的)

toformafuelrod(orpin).P208倒数第一段ThefreshfuelinstaP209第五行Controlrods(控制棒),consistingofanalloyofcadmium(镉),silver(银),andindium(铟),providetheabilitytochangetheamountofneutronabsorption.Therodsareinsertedinsomevacant(空的)

fuelpinspacesandmagneticallyconnectedtodrivemechanisms.P209第五行Controlrods(控制棒),co14.6THENATURALREACTOR

(天然反应堆)Untilthel970s,ithadbeenassumedthatthefirstnuclearreactorwasputintooperationbyEnricoFermi(恩里科.费米)

andhisassociates(同事)

inl942.Itappears,however,thatanaturalchainreactioninvolvingneutronsanduraniumtookplaceintheAfrican(非洲)

stateofGabon(加蓬),nearOklo,some2billionyearsago.Atthattime,theisotopeconcentrationofU-235innaturaluraniumwashigherthanitisnowbecauseofthedifferencesinhalf-lives:U-235,7.04l08years,U-238,4.47l09years.14.6THENATURALREACTOR

(天然接上页(P214第三行)Thewatercontentinarichveinofore(矿脉)

wassufficienttomoderateneutronstothermalenergy.Itisbelievedthatthis“reactor”operatedoffandon(断断续续地)

forthousandsofyearsatpowerlevelsoftheorderofkilowatts.ThediscoveryoftheOklophenomenon(奥克劳现象)resultedfromtheobservations(观测)

ofanunusuallylowU-235abundancein