讲稿核能材料范文.doc

讲稿核能材料范文 1In nuclearphysics,nuclear fusionis anuclear reactionin whichtwo ormore atomiuclei collideat a very highspeed andjoin toform anew type of atomiucleus.During thisprocess,matter isnot conservedbecause someof thematter of the fusingnuclei isconverted tophotons(energy).Fusion is the process that powersactive ormain sequencestars.2The mostimportant fusionprocess innature isthe ohat powersstars.The result isthe fusion of fourprotons intoone alphaparticle,with therelease of two positrons,two neutrinos(which changestwo of the protonsinto neutrons),and energy,but severalindividual reactionsare involved,depending onthe massof thestar.For starsthe size of the sun orsmaller,the protonproton chaindominates.In heavierstars,the Ocycle ismore important.Both types of processesare responsiblefor the creation ofnew elementsas partof stellarnucleosynthesis.2The fusionof twonuclei withlower massesthan iron(which,along withnickel,has thelargest bindingenergy pernucleon)generally releasesenergy,while thefusionofnuclei heavierthan ironabsorbs energy.The oppositeis truefor thereverse process,nuclear fission.This means that fusiongenerally oursfor lighterelements only,and likewise,that fissionnormally oursonly forheavier elements.There areextreme astrophysicalevents thatcan leadto shortperiods offusion withheavier nuclei.This isthe processthat givesrise tonucleosynthesis,thecreationof theheavy elements during eventssuch assupernova.3The originof the energy releasedin fusionof lightelements isdue toan interplayoftwoopposing forces,the nuclear force whichbines togetherprotons andneutrons,and theCoulomb forcewhich causesprotons torepel eachother.The protonsare positivelycharged andrepel eachother but they nohelessstick together,demonstrating theexistence of another forcereferred toas nuclearattraction.This force,called thestrong nuclear force,overes electricrepulsion in a veryclose range.The effectof thisforce isnot observedoutside thenucleus,hence theforce has a strongdependence ondistance,making ita shortrange force.The sameforce alsopulls theneutrons together,or neutrons and protonstogether.4Because thenuclear forceis strongerthan theCoulomb forcefor atomiuclei smallerthan ironand nickel,building upthese nucleifrom lighternuclei by fusion releasesthe extraenergy from the attraction of these particles.For largernuclei,however,no energy is released,since thenuclearforceis shortrange andcannot continueto actacross stilllarger atomiuclei.Thus,energy isno longerreleased whensuch nucleiare madebyfusion;instead,energy is absorbed insuch processes.It takesconsiderable energyto forcenuclei tofuse,even thoseof thelightest element,hydrogen.This isbecause allnuclei havea positivecharge dueto theirprotons,and aslike chargesrepel,nuclei stronglyresist beingput closetogether.Aelerated tohigh speeds,they canovere thiselectrostatic repulsionand beforced closeenough for the attractivenuclearforceto besufficiently strongto achievefusion.The fusionof lighternuclei,which createsa heaviernucleus andoften afree neutronor proton,generally releasesmore energythan ittakes toforce thenuclei together;this is an exothermicprocessthatcan produceselfsustaining reactions.4The TokamakFusion TestReactor(TFTR)was anexperimental tokamakbuilt atPrinceton Plasma Physics Laboratory(in Princeton,New Jersey)circa1980.Following onfromthePDX(Poloidal DiverterExperiment)and PLT(Princeton LargeTorus)devices,it washoped thatTFTR wouldfinally achievefusion energybreakeven.Unfortunately,the TFTRnever achievedthis goal.However itdid producemajor advancesin confinementtime andenergy density,which ultimatelycontributed to the knowledgebase necessaryto buildITER.TFTR operatedfrom1982to1997.58Inertial confinementfusion(ICF)is a typeoffusion energyresearch that attempts toinitiate nuclear fusion reactions by heatingand pressinga fueltarget,typically in the formof apellet thatmost oftencontains amixture ofdeuterium andtritium.To press and heat the fuel,energyisdelivered to the outer layer of the targetusing high-energy beamsof laserlight,electrons orions,although fora varietyof reasons,almost allICF devicesto datehave usedlasers.The heatedouterlayerexplodes outward,producing areaction forceagainst theremainder of the target,aelerating itinwards,pressing the target.This processis designedto createshock wavesthat travelinward throughthetarget.A sufficientlypowerful set of shockwaves canpressandheat the fuel at the centerso muchthat fusionreactions our.The energyreleased bythese reactionswill thenheatthesurrounding fuel,and if the heatingis strongenough thiscould alsobegin toundergo fusion.The aimof ICFis toproduce acondition known asignition,where thisheating processcauses achain reactionthat burnsa significantportion of thefuel.Typical fuelpellets areabout thesizeof a pinheadand contain around10milligrams offuel:in practice,only asmall proportionof thisfuel willundergo fusion,but ifall thisfuel wereconsumed itwould releasetheenergyequivalent toburning abarrel ofoil.9A fusoris adevice thatuses anelectric field to heations toconditions suitablefor nuclear fusion.The machinehas avoltage betweentwo metalcages insidea vacuum.Positive ionsfall downthis voltagedrop,building upspeed.If theycollide in the center,they canfuse.This isatypeof Inertialelectrostatic confinementdevice.10A tokamak(Russian:)isadevice usinga magic fieldto confine aplasma in the shapeofatorus.Achieving astable plasmaequilibrium requiresmagic field lines thatmove aroundthe torusin ahelical shape.Such ahelical fieldcan begenerated byadding a toroidal field(traveling aroundthe torusin circles)and a poloidal field(traveling incircles orthogonaltothe toroidal field).In atokamak,the toroidal field isproduced byelectromags thatsurround thetorus,and thepoloidal field istheresult ofatoroidalelectric currentthat flowsinside the plasma.This currentis inducedinside the plasma witha secondsetofelectromags.The tokamak is one of severaltypesofmagic confinementdevices,and isone of the most-researched candidatesfor producingcontrolled thermonuclearfusion power.Magic fieldsare used for confinementsince nosolid materialcould withstand the extremelyhigh temperatureofthe plasma.An alternativetothe tokamakisthe stellarator.Tokamaks wereinvented in the1950sbySoviet physicistsIgor Tammand AndreiSakharov,inspired byan originalidea ofOleg Lavrentiev.111A stellaratorisadevice usedtoconfinea hotplasma withmagic fieldsin orderto sustaina controllednuclearfusionreaction.It isoneofthe earliestcontrolled fusiondevices,first inventedby LymanSpitzer in1950and builtthe nextyear atwhat laterbecame thePrinceton PlasmaPhysics Laboratory.The namerefers tothe possibilityof harnessingthe powersource ofthesun,a stellarobject.Stellarators werepopular in the1950sand60s,but themuch betterresults fromtokamak designsled tothem fallingfrom favorinthe1970s.More recently,inthe1990s,problems with thetokamakconcept haveled torenewed interestinthestellarator design,have beenbuilt.Some importantmodern stellaratorexperiments areWendelstein7-X,in Germany,the HelicallySymmetric Experiment(HSX)in USAand theLarge HelicalDevice,in Japan.1and anumber ofnew devices10Fieldreversed configurationHistoryeditThe FRChas beenfirst observedin laboratoryinthelate1950sduringtheta pinchexperiments witha reversedbackground magic field.first studiesabout theeffect startedattheUnited StatesNaval ResearchLaboratory(NRL)inthe1960s,and considerabledata isavailable sincethen,with over600published papers.during ProjectSherwood atLos AlamosNational Laboratory(LANL)from1975to1990,oftheUniversity ofWashington,More recentlysome researchhas beendone atthe AirForce ResearchLaboratory(AFRL),University ofWisconsin-MadisonIrvine.study FRCsin orderto usethis configurationin futurefusion powerplants theytry tobuild,like GeneralFusion,Tri-Alpha Energy,Inc.,MSNW LLCand HelionEnergy.2The3Almost allresearch wasconducted4and during18years atthe RedmondPlasmaPhysicsLaboratory5withthelarge sexperiment(LSX).67the FusionTechnology Institute(FTI)ofthe8andtheUniversity ofCalifornia,9Some privatepanies nowtheoretically andexperimentally10The FRCis alsoconsidered fordeep spaceexploration,not onlyasapossible nuclearenergy source,but asmeans ofaelerating apropellant tovery highlevels ofspecific impulse(Isp)for electricallypowered spaceshipsand fusionrockets,with interestexpressed byNASAand themedia.1112131415161718OperationeditOne approachto producingfusion poweris toconfine theplasma withmagic fields.This ismost effectiveifthefield linesdo notperate solidsurfaces butclose onthemselves intocircles ortoroidal surfaces.The mainlineconfinement conceptsof tokamak and stellaratordo thisinatoroidal chamber,which allowsa greatdeal ofcontrol overthe magionfiguration,but requiresaveryplex construction.The Field-Reversed Configurationoffers analternative inthat thefield linesare closed,providing goodconfinement,butthechamber iscylindrical,allowing easyconstruction andmaintenance.19A Field-Reversed Configurationis formedinacylindrical coilwhich producesan axialmagicfield.First,an axialbias fieldis applied,then thegas ispre-ionized,whichfreezes inthe biasfield,finally theaxial fieldis reversed.At theends,reconnection ofthe biasfield andthe mainfield ours,producing closedfieldlines.The mainfieldisraised further,pressing andheating theplasma andproviding avacuum fieldbetween theplasma andthe wall.Field-reversed configurationsand spheromaksare togetherknownaspact toroids.Unlike thespheromak,where thestrength ofthetoroidalmagicfieldis similarto thatofthepoloidal field,the FRChas littleto notoroidal fieldponent andis confinedsolely byapoloidalfield.The lackofatoroidalfieldmeansthatthe FRChas nomagic helicityand thatit hasa highbeta.The highbeta makesthe FRCattractive asa fusionreactor.Spheromaks have0.1whereas atypical FRChas1.2012In nuclear