三里岛事故文档格式.docx

三里岛事故文档格式.docx

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∙In1979acoolingmalfunctioncausedpartofthecoretomeltinthe#2reactoratThreeMileIslandinUSA.Thereactorwasdestroyed.

∙Someradioactivegaswasreleasedacoupleofdaysaftertheaccident,butnotenoughtocauseanydoseabovebackgroundlevelstolocalresidents.

∙Therewerenoinjuriesoradversehealtheffectsfromtheaccident.

TheThreeMileIslandpowerstationisnearHarrisburg,PennsylvaniainUSA.Ithadtwopressurizedwaterreactors.OnePWRwasof800MWeandenteredservicein1974.Itremainsoneofthebest-performingunitsinUSA.Unit2wasof900MWeandalmostbrandnew.

Theaccidenttounit2happenedat4amon28March1979whenthereactorwasoperatingat97%power.Itinvolvedarelativelyminormalfunctioninthesecondarycoolingcircuitwhichcausedthetemperatureintheprimarycoolanttorise.Thisinturncausedthereactortoshutdownautomatically.Shutdowntookaboutonesecond.Atthispointareliefvalvefailedtoclose,butinstrumentationdidnotrevealthefact,andsomuchoftheprimarycoolantdrainedawaythattheresidualdecayheatinthereactorcorewasnotremoved.Thecoresufferedseveredamageasaresult.

Theoperatorswereunabletodiagnoseorrespondproperlytotheunplannedautomaticshutdownofthereactor.Deficientcontrolroominstrumentationandinadequateemergencyresponsetrainingprovedtoberootcausesoftheaccident.

Thechainofevents

Withinsecondsoftheshutdown,thepilot-operatedreliefvalve（PORV）onthereactorcoolingsystemopened,asitwassupposedto.About10secondslateritshouldhaveclosed.Butitremainedopen,leakingvitalreactorcoolantwatertothereactorcoolantdraintank.TheoperatorsbelievedthePORVhadshutbecauseinstrumentsshowedthemthata"

close"

signalwassenttothevalve.However,theydidnothaveaninstrumentindicatingthevalve'

sactualposition.

Respondingtothelossofcoolingwater,high-pressureinjectionpumpsautomaticallypushedreplacementwaterintothereactorsystem.Aswaterandsteamescapedthroughthereliefvalve,coolingwatersurgedintothepressuriser,raisingthewaterlevelinit.（Thepressuriserisatankwhichispartofthereactorcoolantsystem,maintainingproperpressureinthesystem.Thereliefvalveislocatedonthepressuriser.InapressurizedwaterreactorlikeTMI-2,waterintheprimarycoolingsystemaroundthecoreiskeptunderveryhighpressuretokeepitfromboiling.）

Operatorsrespondedbyreducingtheflowofreplacementwater.Theirtrainingtoldthemthatthepressurizerwaterlevelwastheonlydependableindicationoftheamountofcoolingwaterinthesystem.Becausethepressuriserlevelwasincreasing,theythoughtthereactorsystemwastoofullofwater.Theirtrainingtoldthemtodoalltheycouldtokeepthepressuriserfromfillingwithwater.Ifitfilled,theycouldnotcontrolpressureinthecoolingsystemanditmightrupture.

Steamthenformedinthereactorcoolingsystem.Pumpingamixtureofsteamandwatercausedthereactorcoolingpumpstovibrate.Becausetheseverevibrationscouldhavedamagedthepumpsandmadethemunusable,operatorsshutdownthepumps.Thisendedforcedcoolingofthereactor.（Theoperatorsstillbelievedthesystemwasnearlyfullofwaterbecausethepressuriserlevelremainedhigh.）However,asreactorcoolantwaterboiledaway,thereactor'

sfuelcorewasuncoveredandbecameevenhotter.Thefuelrodsweredamagedandreleasedradioactivematerialintothecoolingwater.

At6:

22amoperatorsclosedablockvalvebetweenthereliefvalveandthepressuriser.Thisactionstoppedthelossofcoolantwaterthroughthereliefvalve.However,superheatedsteamandgasesblockedtheflowofwaterthroughthecorecoolingsystem.

Throughoutthemorning,operatorsattemptedtoforcemorewaterintothereactorsystemtocondensesteambubblesthattheybelievedwereblockingtheflowofcoolingwater.Duringtheafternoon,operatorsattemptedtodecreasethepressureinthereactorsystemtoallowalowpressurecoolingsystemtobeusedandemergencywatersuppliestobeputintothesystem.

CoolingRestored

Bylateafternoon,operatorsbeganhigh-pressureinjectionofwaterintothereactorcoolingsystemtoincreasepressureandtocollapsesteambubbles.By7:

50pmon28March,theyrestoredforcedcoolingofthereactorwhentheywereabletorestartonereactorcoolantpump.Theyhadcondensedsteamsothatthepumpcouldrunwithoutseverevibrations.

Radioactivegasesfromthereactorcoolingsystembuiltupinthemakeuptankintheauxiliarybuilding.DuringMarch29and30,operatorsusedasystemofpipesandcompressorstomovethegastowastegasdecaytanks.Thecompressorsleaked,andsomeradioactivegaswasreleasedtotheenvironment.

TheHydrogenBubble

Whenthereactor'

scorewasuncovered,onthemorningof28March,ahigh-temperaturechemicalreactionbetweenwaterandthezircaloymetaltubesholdingthenuclearfuelpelletshadcreatedhydrogengas.Intheafternoonof28March,asuddenriseinreactorbuildingpressureshownbythecontrolroominstrumentsindicatedahydrogenburnhadoccurred.Hydrogengasalsogatheredatthetopofthereactorvessel.

From30Marchthrough1Apriloperatorsremovedthishydrogengas"

bubble"

byperiodicallyopeningtheventvalveonthereactorcoolingsystempressurizer.Foratime,regulatory（NRC）officialsbelievedthehydrogenbubblecouldexplode.However,suchanexplosionwasneverpossiblesincetherewasnotenoughoxygeninthesystem.

ColdShutdown

Afterananxiousmonth,on27Apriloperatorsestablishednaturalconvectioncirculationofcoolant.Thereactorcorewasbeingcooledbythenaturalmovementofwaterratherthanbymechanicalpumping.Theplantwasin"

coldshutdown"

.

Publicconcernandconfusion

WhentheTMI-2accidentisrecalled,itisofteninthecontextofwhathappenedonFridayandSaturday,March30-31.TheheightoftheTMI-2accident-inducedfear,stressandconfusioncameonthosetwodays.TheatmospherethenandthereasonsforitaredescribedwellinthebookCrisisContained,TheDepartmentofEnergyatThreeMileIsland,byPhilipLCantelonandRobertC.Williams,1982.ThisisanofficialhistoryoftheDepartmentofEnergy'

sroleduringtheaccident.

"

Fridayappearstohavebecomeaturningpointinthehistoryoftheaccidentbecauseoftwoevents:

thesuddenriseinreactorpressureshownbycontrolroominstrumentsonWednesdayafternoon（the"

hydrogenburn"

）whichsuggestedahydrogenexplosion-becameknowntotheNuclearRegulatoryCommission[thatday];

andthedeliberateventingofradioactivegasesfromtheplantFridaymorningwhichproducedareadingof1,200millirems（12mSv）directlyabovethestackoftheauxiliarybuilding.

Whatmadethesesignificantwasaseriesofmisunderstandingscaused,inpart,byproblemsofcommunicationwithinvariousstateandfederalagencies.Becauseofconfusedtelephoneconversationsbetweenpeopleuninformedabouttheplant'

sstatus,officialsconcludedthatthe1,200milliremsreadingwasanoff-sitereading.Theyalsobelievedthatanotherhydrogenexplosionwaspossible,thattheNuclearRegulatoryCommissionhadorderedevacuationandthatameltdownwasconceivable.Garbledcommunicationsreportedbythemediageneratedadebateoverevacuation.Whetherornottherewereevacuationplanssoonbecameacademic.WhathappenedonFridaywasnotaplannedevacuationbutaweekendexodusbasednotonwhatwasactuallyhappeningatThreeMileIslandbutonwhatgovernmentofficialsandthemediaimaginedmighthappen.OnFridayconfusedcommunicationscreatedthepoliticsoffear."

（Page50）

Throughoutthebook,CantelonandWilliamsnotethathundredsofenvironmentalsamplesweretakenaroundTMIduringtheaccidentperiodbytheDepartmentofEnergy（whichhadtheleadsamplingrole）orthethen-PennsylvaniaDepartmentofEnvironmentalResources.Buttherewerenounusuallyhighreadings,exceptfornoblegases,andvirtuallynoiodine.Readingswerefarbelowhealthlimits.Yetapoliticalstormwasragingbasedonconfusionandmisinformation.

NoRadiologicalHealthEffects

TheTMI-2accidentcausedconcernsaboutthepossibilityofradiation-inducedhealtheffects,principallycancer,intheareasurroundingtheplant.Becauseofthoseconcerns,thePennsylvaniaDepartmentofHealthfor18yearsmaintainedaregistryofmorethan30,000peoplewholivedwithinfivemilesofThreeMileIslandatthetimeoftheaccident.Thestate'

sregistrywasdiscontinuedinJune,1997,withoutanyevidenceofunusualhealthtrendsinthearea.

Indeed,morethanadozenmajor,independenthealthstudiesoftheaccidentshowednoevidenceofanyabnormalnumberofcancersaroundTMIyearsaftertheaccident.Theonlydetectableeffectwaspsychologicalstressduringandshortlyaftertheaccident.

Thestudiesfoundthattheradiationreleasesduringtheaccidentwereminimal,wellbelowanylevelsthathavebeenassociatedwithhealtheffectsfromradiationexposure.Theaverageradiationdosetopeoplelivingwithin16kilometresoftheplantwas0.08millisieverts,withnomorethan1millisieverttoanysingleindividual.Thelevelof0.08mSvisaboutequaltoachestX-ray,and1mSvisaboutathirdoftheaveragebackgroundlevelofradiationreceivedbyU.S.residentsinayear.

InJune1996,17yearsaftertheTMI-2accident,HarrisburgU.S.DistrictCourtJudgeSylviaRambodismissedaclassactionlawsuitallegingthattheaccidentcausedhealtheffects.TheplaintiffshaveappealedJudgeRambo'

sruling.TheappealisbeforetheU.S.ThirdCircuitCourtofAppeals.However,inmakingherde