NUCLEARENERGYPROFESSIONAL(SPECIAL)ENGLISHCOURSEChapter10NeutronChainReactions

(中子鏈?zhǔn)椒磻?yīng))Chapter14NeutronChainReactions

(中子鏈?zhǔn)椒磻?yīng))Thepossibilityofachainreactioninvolvingneutronsinamassofnuclearfuelsuchasuraniumdependson:(a)nuclearproperties(特性)

suchascross

sections(截面)

andneutronsproducedperabsorption;(b)thesize,shape,andarrangementofthematerials.14.1CRITICALITYANDMULTIPLI-

CATION(臨界性和增殖性)

Toachieveaself-sustainingchainreaction(自持鏈?zhǔn)椒磻?yīng)),oneneedingnoexternalneutronsupply,a"criticalmass"(臨界質(zhì)量)

ofuraniummustbecollected.Toappreciatethisrequirementwevisualizethesimplestnuclearreactor,consistingofametalsphereofuranium-235.接上頁(yè)(P199第二段第四行中)SupposethatitconsistsofonlyoneatomofU-235.Ifitabsorbsaneutronandfissions,theresultantneutronsdonothingfurther,therebeingnomorefuel.Ifinsteadwehaveasmallchunk(大塊)

ofuranium,sayafewgrams,theintroductionofaneutronmightsetoff(導(dǎo)致)achainofseveralreactions,producingmoreneutrons,butmostofthemwouldescapethroughthesurfaceofthebody,aprocesscalledleakage(泄漏).接上頁(yè)（P200第二行中）Suchanamountoffuelissaidtobe“subcritical”(次臨界的).

Nowifwebringtogetherabout50kgofU-235metal,theneutronproductionbalancestheleakagelosses,andthesystemisself-sustainingor“critical(臨界的).”Thesizeisthecriticalvolumeandtheamountoffuelisthecriticalmass.Neutronshadtobeintroducedtostartthechainreaction,butthenumberismaintainedwithoutfurtheradditions.Theterm(術(shù)語(yǔ))

"criticalmass"hasbecomepopulartodescribeanycollectionofentitieslargeenoughtooperateindependently.P200第二段第三行末Ifweaddstillmoreuraniumtothe50kgrequiredforcriticality(臨界狀態(tài)),moreneutronsareproducedthanarelost,theneutronpopulation(數(shù)量)

increases,andthereactoris“supercritical(超臨界的).”Earlynuclearweaponsinvolvedtheuseofsuchmasses,inwhichtherapidgrowthofneutronsandresultingfissionheatcausedaviolentexplosion(爆炸).14.2MULTIPLICATIONFACTORS

(增殖因子)Thebehaviorofneutronsinanuclearreactorcanbeunderstoodthroughanalogy(類推)

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

illnessoraccident.Onaveragethelifeexpectancy(期望值)

atbirthmightbe75years,accordingtostatisticaldata(統(tǒng)計(jì)數(shù)據(jù)).接上頁(yè)(P200第三段第六行中)Anindividualmaydiewithoutanheir(后嗣),withone,orwithmany.Ifonaveragethereisexactly1,thepopulationisconstant.Fromtheotherviewpoint,iftheratesofbirthanddeatharethesameinagroupofpeople,thepopulationisagainsteady(穩(wěn)定的).Iftherearemorebirthsthandeathsby1%peryear,thepopulationwillgrowaccordingly.Thisapproach(方法)

emphasizesthecompetitionofprocessrates.P200倒數(shù)第二段Thesameideasapplytoneutronsinamultiplyingassembly.Wecanfocus(聚焦,集中)

attentiononatypicalneutronthatwasborninfission,andhasvariouschancesofdroppingoutof(不參與)

thecyclebecauseofleakageandabsorptioninothermaterialsbesidesfuel.Ontheotherhandwecancomparethereactionratesfortheprocessesofneutronabsorption,fission,andleakagetoseeifthenumberofneutronsisincreasing,steady,ordecreasing.Eachofthemethodshasitsmerits(優(yōu)點(diǎn))

forpurposesofdiscussion,analysis,andcalculation.P202第二段ThefractionsofabsorbedneutronsthatformU-236andthatcausefission,respectively,aretheratiosofthecrosssectionforcapturecandfissionftothatforabsorptiona.Theaveragenumberofneutronsproducedbyfissionis.Nowletbethecombinationf/a,andnotethatitisthenumberofneutronsperabsorptioninuranium.Thusletting

L

bethefractionnotescapingbyleakage,k=L

.P203第一行Thesystemiscriticalifk=l,orL

=l.MeasurementsshowthatLisaround2.2forfastneutrons,thusLmustbel/2.2=0.45,whichsaysthatasmanyas45%oftheneutronsmustremaininthesphere,whilenomorethan55%escapethroughitsboundary(邊界).14.3

NEUTRONFLUXANDREACTORPOWER

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

v.Supposethattherearenneutronsinthevolume.接上頁(yè)(P205第七行)TherateatwhichthefissionreactionoccursisthusRf=nvNf

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

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

w.P205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.14.4

REACTORTYPES

(反應(yīng)堆類型)Althoughtheonlyrequirementforaneutronchainreactionisasufficientamountofafissileelement,manycombinationsofmaterialsandarrangementscanbeusedtoconstructanoperablenuclearreactor.Severaldifferenttypesorconceptshavebeendevised(設(shè)計(jì))

andtestedovertheperiodsince1942,whenthefirstreactorstartedoperation,(a)PurposeThemajorityofreactorsinoperationorunderconstructionhaveaspurposethegenerationoflargeblocksofcommercial(商業(yè)的)

electricpower.Othersservetrainingorradiationresearchneeds,andmanyprovidepropulsionpower

(推動(dòng)力)

forsubmarines(潛水艇).Availablealsoaretestedreactorsforcommercialsurface

shipsandforspacecraft(宇宙飛船).接上頁(yè)Atvariousstagesofdevelopmentofanewconcept,suchasthebreederreactor(增殖堆),therewillbeconstructedbothaprototypereactor(原型堆),onewhichtestsfeasibility(可行性),andademonstrationreactor(示范堆),onethatevaluates(評(píng)定)

commercialpossibilities(可能性).(b)NeutronEnergyAfastreactor(快堆)

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

containsagoodneutronmoderating(緩和的)

material,andthebulk

of

(大多數(shù))theneutronshaveenergyinthevicinity(附近)

of0.leV.(c)ModeratorandCoolant

(慢化劑與冷卻劑)Insomereactors,onesubstanceservestwofunctions---toassist(幫助)

inneutronslowingandtoremovethefissionheat.Othersinvolveonematerialformoderatorandanotherforcoolant.Themostfrequentlyusedmaterialsarelistedbelow:

Moderators

Coolants

lightwater(輕水)lightwaterheavywater(重水)carbondioxide(CO2)graphite(石墨)heliumberyllium(鈹)liquidsodium(液體鈉)(d)FuelUraniumwithU-235contentvaryingfromnaturaluranium(=0.7%)toslightlyenriched(=3%)tohighlyenriched(=90%)isemployedinvariousreactors,withtheenrichment(富集)

dependinguponwhatotherabsorbingmaterialsarepresent.接上頁(yè)ThefissileisotopesPu-239andU-233areproducedandconsumedinreactorscontainingsignificantamountsofU-238orTh-232.Plutoniumservesasfuelforfastbreederreactorsandcanberecycled

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

UC.(e)ArrangementInmostmodernreactors,thefuelisisolatedfromthecoolantinwhatiscalledaheterogeneous(異類的,非均勻的)

arrangement.Thealternative(可供選擇的方法)isahomogeneous(同類的,均勻的)

mixtureoffuelandmoderatororfuelandmoderator-coolant.(f)StructuralMaterialsThefunctionsofsupport(支撐),retention(保存)

offissionproducts,andheatconduction(熱傳導(dǎo))

areprovidedbyvariousmetals.Themainexamplesarealuminum,stainlesssteel,andzircaloy(鋯合金),(analloyofzirconium(鋯)).接上頁(yè)Byplacingemphasisononeormoreoftheabovefeaturesofreactors,reactorconceptsareidentified.Someofthemorewidelyusedorpromisingpowerreactortypesarethefollowing:PWR(pressurizedwaterreactor)(壓水堆),athermalreactorwithlightwaterathighpressure(2200psi(poundspersquareinch))andtemperature(600oF)servingasmoderator-coolant,andaheterogeneousarrangementofslightlyenricheduraniumfuel.接上頁(yè)BWR(boilingwaterreactor)(沸水堆),similartothePWRexceptthatthepressureandtemperaturearelower(1000psiand550oF).HTGR(hightemperaturegas-cooledreactor)(高溫氣冷堆),usinggraphitemoderator,highlyenricheduraniumwiththorium,andheliumcoolant(1430oFand600psi).接上頁(yè)CANDU(Canadiandeuteriumuranium)(加拿大重水鈾反應(yīng)堆)usingheavywatermoderatorandnaturaluraniumfuelthatcanbeloadedanddischargedduringoperation.LMFBR(liquidmetalfastbreederreactor)(液金屬快增殖堆),withnomoderator,liquidsodiumcoolant,andplutoniumfuel,surroundedbynaturalordepleteduranium

(貧鈾).P208倒數(shù)第二段Toillustrate,wecanidentifythecomponentsandtheirfunctionsinamodernpressurizedwaterreactor.Figure14.5givessomeindication(跡象)

ofthesizesofthevariousparts.P208倒數(shù)第一段ThefreshfuelinstalledinatypicalPWRconsistsofcylindricalpellets(芯塊)

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

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

toformafuelrod(orpin).P209第五行Controlrods(控制棒),consistingofanalloyofcadmium(鎘),silver(銀),andindium(銦),providetheabilitytochangetheamountofneutronabsorption.Therodsareinsertedinsomevacant(空的)

fuelpinspacesandmagneticallyconnectedtodrivemechanisms.14.6THENATURALREACTOR

(天然反應(yīng)堆)Untilthel970s,ithadbeenassumedthatthefirstnuclearreactorwasputintooperationbyEnricoFermi(恩里科.費(fèi)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.接上頁(yè)(P214第三行)Thewatercontentinarichveinofore(礦脈)

wassufficienttomoderateneutronstothermalenergy.Itisbelievedthatthis“reactor”operatedoffandon(斷斷續(xù)續(xù)地)

forthousandsofyearsatpowerlevelsoftheorderofkilowatts.ThediscoveryoftheOklophenomenon(奧克勞現(xiàn)象)resultedfromtheobservations(觀測(cè))

o