2009年12月20日大学英语四级(CET-4)模拟试卷7

12009年12月20日大学英语四级(CET-4)模拟试卷7PartⅠWritingDirections:forthispart,youareallowedthirtyminutestowriteacompositiononthetopic:AreFamiliesDemandingTooMuchfromTheirChildren?Youshouldwriteatleast120wordsfollowingtheoutlinegivenbelowinChinese:AreFamiliesDemandingTooMuchfromTheirChildren?1.有人认为知识产权保护很重要2.有人认为盗版更实际3.你的看法或：1.越来越多的人关注动物福利2.也带来一些矛盾3.你的看法PartⅡReadingComprehension(SkimmingandScanning)Directions:Inthispart,youwillhave15minutestogooverthepassagequickly.Y(forYES)ifthestatementagreeswiththeinformationgiveninthepassage;N(forNO)ifthestatementcontradictstheinformationgiveninthepassage;NG(forNOTGIVEN)iftheinformationisnotgiveninthepassage.Forquestions8-10,completethesentenceswithinformationgiveninthepassage.Nuclearpowerplantsprovideabout17percentoftheworld'selectricity.Somecountriesdependmoreonnuclearpowerforelectricitythanothers.InFrance,forinstance,about75percentoftheelectricityisgeneratedfromnuclearpower,accordingtotheInternationalAtomicEnergyAgency.IntheUnitedStates,nuclearpowersuppliesabout15percentoftheelectricityoverall,butsomestatesgetmorepowerfromnuclearplantsthanothers.Therearemorethan400nuclearpowerplantsaroundtheworld,withmorethan100intheUnitedStates.Haveyoueverwonderedhowanuclearpowerplantworksorhowsafenuclearpoweris?Inthisarticle,wewillexaminehowanuclearreactorandapowerplantwork.UraniumUraniumisafairlycommonelementonEarth,whichwastakenintotheplanetduringtheplanet'sformation.Uraniumisoriginallyformedinstars.Oldstarsexploded,andthedustfromtheseshatteredstarsaggregatedtogethertoformourplanet.Uranium-238(U-238)hasanextremelylonglife(4.5billionyears),andthereforeisstillpresentinfairlylargequantities.U-238makesup99percentoftheuraniumontheplanet.U-235makesupabout0.7percentoftheremaininguraniumfoundnaturally,whileU-234isevenrarerandisformedbythedecayofU-238.NuclearFission(裂变)Anuclearfissionhappenswhenauranium-235nucleuswithaneutroncapturesanotherneutron.Thenucleusthensplitsintotwolighteratomsandthrowsofftwoorthreenewneutrons(thenumberofejectedneutronsdependsonhowtheU-235atomhappenstosplit).Thetwonewatomsthenproducegammaradiationastheysettleintotheirnewstates.Therearethreethingsaboutthisinducedfissionprocessthatmakeitespeciallyinteresting:·TheprobabilityofaU-235atomcapturinganeutronasitpassesbyisfairlyhigh.Inareactorworkingproperly(knownasthecriticalstate),oneneutronejectedfromeachfissioncausesanotherfissiontooccur.·Theprocessofcapturingtheneutronandsplittinghappensveryquickly,ontheorderofpicoseconds(1×1012seconds).·Anincredibleamountofenergyisreleased,intheformofheatandgammaradiation,whenasingleatomsplits.Thetwoatomsthatresultfromthefissionlaterreleasebetaradiationandgammaradiationoftheirownaswell.Theenergyreleasedbyasinglefissioncomesfromthefactthatthefissionproductsandtheneutrons,together,weighlessthantheoriginalU-235atom.Thedifferenceinweightisconverted2directlytoenergyatarategovernedbytheequationE=mc2.InorderforthesepropertiesofU-235towork,asampleofuraniummustbeenrichedsothatitcontains2percentto3percentormoreofuranium-235.Three-percentenrichmentissufficientforuseinaciviliannuclearreactorusedforpowergeneration.Weapons-gradeuraniumiscomposedof90-percentormoreU-235.InsideaNuclearPowerPlantTobuildanuclearreactor,whatyouneedissomemildlyenricheduranium.Typically,theuraniumisformedintopellets(芯块)withapproximatelythesamediameterasacoinandalengthofaninchorso.Thepelletsarearrangedintolongrods,andtherodsarecollectedtogetherintobundles.Thebundlesarethentypicallysubmergedinwaterinsideapressurevessel.Thewaterisusedtolowertheheat.Inorderforthereactortowork,thebundle,submergedinwater,mustbeslightlysupercritical.Thatmeansthat,lefttoitsowndevices,theuraniumwouldeventuallyoverheatandmelt.Topreventthis,controlrods(棒)madeofamaterialthatabsorbsneutronsareinsertedintothebundle.Raisingandloweringthecontrolrodsallowoperatorstocontroltherateofthenuclearreaction.Whenanoperatorwantstheuraniumcoretoproducemoreheat,therodsareraisedoutoftheuraniumbundle.Tocreatelessheat,therodsareloweredintotheuraniumbundle.Therodscanalsobeloweredcompletelyintotheuraniumbundletoshutthereactordowninthecaseofanaccidentortochangethefuel.Theuraniumbundleactsasanextremelyhigh-energysourceofheat.Itheatsthewaterandturnsittosteam.Thesteamdrivesasteamturbine,whichspinsageneratortoproducepower.Insomereactors,thesteamfromthereactorgoesthroughasecondary,intermediateheatexchangertoconvertanotherloopofwatertosteam,whichdrivestheturbine.OutsideaNuclearPowerPlantOnceyougetpastthereactoritself,thereisverylittledifferencebetweenanuclearpowerplantandacoal-firedoroil-firedpowerplantexceptforthesourceoftheheatusedtocreatesteam.Thereactor'spressurevesselistypicallyhousedinsideaconcretelinerthatactsasaradiationshield.Thatlinerishousedwithinamuchlargersteelcontainmentvessel.Thisvesselcontainsthereactorcoreaswellthehardwarethatallowsworkersattheplanttorefuelandmaintainthereactor.Thesteelcontainmentvesselisintendedtopreventleakageofanyradioactivegasesorfluidsfromtheplant.Finally,thecontainmentvesselisprotectedbyanouterconcretebuildingthatisstrongenoughtosurvivesuchthingsascras