三里岛核事故分析英文DOC.docx

三里岛核事故分析英文DOC.docx

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三里岛核事故分析英文DOC

HowIsJapan'sNuclearDisasterDifferent?

FukushimaDaiichimaybenoChernobyl,butithasovershadowedThreeMileIsland.

ThecontrolroomatChernobyl'sReactorNo.4isshownhere.Reactordesign,windpatterns,communicationandotherfactorscancausedifferencesintheseverityofnuclearaccidents.

PhotographbyGerdLudwig,NationalGeographic

JosieGarthwaite

For NationalGeographicNews

PublishedMarch16,2011

Thisstoryispartofa specialseries thatexploresenergyissues.Formore,visitTheGreatEnergyChallenge.

Fordecades,ThreeMileIslandandChernobylhaveservedasshorthandforthenightmareofnuclearpowergenerationgoneawry.InthewakeofJapan'sdeadlyearthquakeandtsunamilastweek,thestill-unfoldingdisasterofFukushimaDaiichihascomecloserthananynuclearcrisisinhistorytomakingitafearsometrio.

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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.

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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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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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"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