三里岛核事故分析英文DOC文档格式.docx

三里岛核事故分析英文DOC文档格式.docx

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Fordecades,ThreeMileIslandandChernobylhaveservedasshorthandforthenightmareofnuclearpowergenerationgoneawry.InthewakeofJapan'

sdeadlyearthquakeandtsunamilastweek,thestill-unfoldingdisasterofFukushimaDaiichihascomecloserthananynuclearcrisisinhistorytomakingitafearsometrio.

（RelatedStory:

"

JapanTriestoAvertNuclearDisaster"

）

ItremainstobeseenhowmuchdamagewillbecausedbythecrisisattheFukushimanuclearpowercomplex,wherefourofthesixreactorshaveseenarangeofwoesincludingthreeexplosionsinfourdays,damagetotwocontainmentvessels,possibleoverheatingfromspentfuelrods,andmountingperilforthelastremaining50workersduetodangerousspikesinradiationemissions.

YetitisalreadypossibletooutlinekeydifferencesthatsetthecurrentFukushimasituationapartfromthe1979ThreeMileIslandemergencynearHarrisburg,Pennsylvania,andthedisasterinChernobyl,Ukraine,thatunfoldedsevenyearslater.

ReactorType

Japan'

sFukushimaDaiichinuclearpowercomplex,whichbeganoperatinginthe1970s,ismadeupofsixboiling-waterreactors,orBWRs—atypeof"

LightWaterReactor."

（Usingordinarywater,itisdistinguishedfrom"

heavywaterreactors,"

whichuse 

deuterium 

oxide,orD2O,insteadofH2O.）ThreeMileIslandusedanothertypeofLightWaterReactorknownasapressurized-waterreactor,orPWR.

Bothofthesereactorsusewaterfortwopurposes.Itactsasacoolant,carryingheatawayfromthenuclearfuel,andasa"

moderator,"

slowingdownthereleaseofneutronsduringfissionreactions,explainedNeilWilmshurst,vicepresidentofthenuclearsectorattheU.S.ElectricPowerResearchInstitute,theindustry'

snonprofitresearchorganization.

InaPWR,thewateriskeptunderpressure.Thismeansthetemperaturecanbehigherthantheboilingpointofwaterwithoutgeneratingasignificantamountsteam（alessefficientcoolant）,saidWilmshurst.Sothereactorcoreoperatesatahighertemperatureinthesesystems,andheatcanbetransferredmoreefficiently.Boiling-waterreactorsoperateatlowertemperatures,andtheytendtobesimpler,withfewerparts,saidWilmshurst.

Chernobyl'

sreactorswereatypecalledRBMK（fortheRussian,"

reaktorbolshoymoshchnostykanalny"

）,whichalsousedwaterforthecoolant.ButunliketheLightWaterReactors,theRMBKusedgraphiteasamoderator.AccordingtotheWorldNuclearAssociation,anindustrytradegroupbasedinLondon,nootherpowerreactorintheworldcombinesagraphitemoderatorandwatercoolantasChernobyldid,althoughRussiadoeshaveseveralRBMKreactorsinoperation.

MostnuclearreactorsintheUnitedStatestodayuseeitherBWRorPWRtechnology,whichWilmshurstandEPRIsayare"

equallysafe."

Bothtypesofreactorshaveakindofself-regulationor"

negativefeedback"

loop:

Asthereactorgetshotter,thefissionreactionslowsdown,decreasingpower,saidWilmshurst.TheRMBKdesign,ontheotherhand,"

couldgointopositivefeedback,"

wherehighertemperaturebegetsmorepower,whichinturnincreasesthetemperature,andsoon.

AccidentCause

AtthispointintheFukushimadisaster,Wilmshurstsaid,thetsunamiappearstobetheimmediateculprit,sincetheplantsshutdownastheyweredesignedtodofollowingtheearthquake.Whenthetsunamihitanhourlater,itdamagedthesiteinfrastructure,hesaid.Sowhiletheearthquakehadcutthereactors'

externalpowersupply,whichisneededtokeepcoolantpumpsdoingtheirjob,thetsunamikilledthedieselbackupgeneratorsneededtoprovidepowerforthecoolingsystem.Batteriesprovidedpowerforonlyuptoeighthours.Mobilegeneratorswerebroughtintotakeover.

（RelatedPhotos:

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20IndelibleImages"

Still,it'

stooearlytoknowforsurewhatsequenceofeventsledtowhatoutcome,saidDavidLochbaum,whodirectstheUnionofConcernedScientists'

NuclearSafetyProgramandhasworkedatthreenuclearplantsintheUnitedStatessimilartotheGeneralElectricplantsinJapan.

Accordingtothe1979 

KemenyCommissionreport 

onThreeMileIsland—thedefinitivedocumentofthatdisaster—"

equipmentfailuresinitiatedtheevent,"

but"

operatorerror"

wasthe"

fundamentalcauseoftheaccident."

Emergencycoolingsystemswereshutdown,withdireconsequences.ThreeMileIslandwouldhavebeena"

relativelyinsignificantincident,"

thecommissionfound,iftheplantoperators（orthosewhosupervisedthem）hadkepttheemergencycoolingsystemsonthroughtheearlystagesoftheaccident.

AttheChernobylreactorinUkraine,an"

ill-conceived,badlyexecutedsafetytest"

initiatedthedisaster,saidWilmshurst.Asuddensurgeofpowertriggeredasteamexplosionthatrupturedthereactorvessel,accordingtoarecentreportfromtheUnitedNations.Thisallowed"

furtherviolentfuel-steaminteractionsthatdestroyedthereactorcoreandseverelydamagedthereactorbuilding."

UnderstandingtheProblem

ThelevelofaccesstoinformationaboutwhatisgoingoninsideareactorhasincreasedinthedecadessinceThreeMileIslandandChernobyl.

AsPeter 

Bradford,whoservedontheU.S.NuclearRegulatoryCommissionatthetimeofThreeMileIsland,saidthisweek,"

AtThreeMileIsland,muchofwhatwethoughtweknewonthethirddayturnedouttobeincorrect."

Theextentoffuelmelting,andeventhefactthatahydrogenexplosionhadoccurredinthecontainmentonthefirstday,hesaid,didnotbecomeclearforyears."

Therewasallkindsofinformation...wedidn'

tknow,"

hesaid.

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EyeingJapan,CountriesReassessNuclearPlans"

DuringthefirstfewminutesoftheaccidentatThreeMileIsland,morethan100alarmswentoff,andnosystemwasinplacetofilterouttheimportantsignalsfromtheinsignificantones,accordingtothe1979Kemenyreport."

Overall,littleattentionhadbeenpaidtotheinteractionbetweenhumanbeingsandmachinesundertherapidlychangingandconfusingcircumstancesofanaccident,"

thecommissionerswrote.

Bycontrast,saidBradford,thelevelofcomputerizationandinformationtransferavailabletodaycouldgiveJapaneseofficialsmuchmoreinsighttowhathappensinthefourtroubledreactorsatFukushima—atleastintheory."

They'

vegotsomuchmoregoingonintermsoftheearthquakeandthetsunamithatwedidn'

thaveatTMI,thatI'

msurethatthesituationiseverybitasconfused,"

Bradfordsaid.

RadiationContainment

LiketheThreeMileIslandplant,theFukushimareactorshavethreebarriersdesignedtopreventradiationleakage,includingmetalcladdingsurroundingthenuclearfuel,areactorpressurevessel,andtheprimarycontainmentvessel.Chernobyllackedacontainmentvessel,saidWilmshurst.

Onceradiationisreleasedintotheenvironment,itcancontaminatevastareas."

Contaminationlevelsarenotlinear,"

saidLochbaum."

Furtherawayyoudon'

tnecessarilygetlowerdoses,"

heexplained.Amongotherfactors,prevailingwindscaninfluencewhatareasareaffected.IntheChernobylaccident,someareas100milesawayfromthefacilityhadradiationlevelshigherthanareasjust10or20milesaway.

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NuclearReactors,DamsatRiskDuetoGlobalWarming"

"

TheChernobylpatternwasquiteerratic,"

saidLyman.Radiationwasreleased"

very,veryhighbecauseofthenatureofthereactorandgraphitefire."

Weatherchangedoveraprolongedemissionperiod,asagraphitefireburnedfor10days.Soradioactivegasesandparticleswerepickedupbywindandcarriedhighintheatmosphereoverlongdistancesbeforerainingdownoncommunitiesfarfromthesource,hesaid.

Ultimately,theradiationreleasedasaresultofThreeMileIslandwasnothighenoughtopresentdetectablehealtheffectsinthegeneralpopulation.Thataccidentratedasalevel5of7onthe 

InternationalNuclearEventScale,an"

accidentwithwider[thanlocal]consequence."

AtChernobyl,alevel7"

majoraccident,"

radiationexposure 

affectedthousandsofpeople.

FukushimaDaiichihasbeenelevatedtolevel4—"

accidentwithlocalconsequences.Butitremainstobeseenhowmuchhigheronthescalethisincidentwillgo.InTokyo,180milesawayfromtheplant,peakradiationlevelswere 

recordedat23timesabovenormal 

atonepointonTuesday,buttheyreportedlydroppedtoabout10timesabovenormallaterintheday.

ExposureinPerspective

IntheUnitedStates,theaverageradiationdosefromnaturalbackgroundandman-madesources,suchasmedicalproceduresandconsumerproducts,is620millirems（mrem）peryear, 

accordingtotheNRC.

Onemillisievert（mSv）isequalto100millirems.TheJapaneseMinistryofHealthLaborandWelfareonWednesdayliftedthemaximumallowableexposurefornuclearworkersto250mSv,from100mSv,the 

AssociatedPressreported.Accordingtothe 

NuclearEnergyInstitute,radioactivityattheplanthitadoserateof1,190mremperhourTuesdayevening,butdroppedto60mremperhoursixhourslater.

TheChernobylaccidentcausedacuteradiationsicknessin134ofthe600workerswhowereatthesiteonthemorningoftheinitialexplosionandreceivedhighdosesofradiation—80,000to1.6millionmrem,accordingtothe 

UNreportandthe 

U.S.NuclearRegulatoryCommission.Ofthisgroup28