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Joel M. Cagulangan Jr. IV-Sun
Mrs. Maria Fe M. Jadumas
Chernobyl disaster
Chernobyl disaster
The nuclear reactor after the...
International AtomicEnergyAgencyexaminesthe environmentalconsequencesof the
accident.[8]
AnotherUN agency, UNSCEAR,hasesti...
Steam turbine tests
An inactive nuclearreactorcontinuestogenerateasignificantamountof residualdecayheat.Inan
initial shutd...
out,includingthe disablingof the emergencycore coolingsystemorECCS,a passive/active systemof
core coolingintendedtoprovide...
the existingsteamvoids inthe core andthe steamseparators.[38]
Since waterweaklyabsorbsneutrons
(andthe higherdensityof liq...
The subsequentcourse of eventswasnotregisteredbyinstruments;itisknownonlyasa resultof
mathematical simulation.Apparently,t...
that the radiationlevelswere somewhere above0.001 R/s (3.6 R/h, or 0.3 µA/kg),whilethe true levels
were muchhigherinsome a...
The NorwegianAgricultural Authorityreportedthatin2009 a total of 18,000 livestockinNorwayneeded
to be givenuncontaminatedf...
riskof thyroidcancerassociatedwiththe Chernobyl accidentisstill highaccordingtopublished
studies.[142][143]
The German aff...
those publicationsandpaperswere writtenbyleadingEasternEuropeanauthoritiesandhave largely
beendownplayedorignoredbythe IAE...
The Chernobyl ShelterFund,setupin1997, hasreceived €810 millionfrominternational donorsand
projectsto coverthisprojectandp...
Fukushima Daiichi nuclear disaster
Fukushima Daiichi nuclear disaster
The Fukushima Daiichi nuclear disaster FukushimaDaii...
was the slightlyolderBWR-3design.[25]
Atthe time of the earthquake,Reactor4had beende-fueledand
Reactors5 and 6 were in co...
nuclearpowerreactorsinJapan were systematicallydownplayedandmismanagedsoastocompromise
operational safety.[46]
Many report...
Unit 1 was equippedwithadifferentcoolingsystem, the "IsolationCondenser"or"IC",whichisentirely
passive.Thisconsistsof aser...
temperature control valveswasnotexaminedfor11years.Inspectionsdidnotcovercoolingsystems
devicessuchaswater pumpmotorsand d...
Units1, 2 and 3[edit]
In Reactors1, 2 and 3, overheatingcausedareactionbetweenthe waterandthezircaloy,creating
hydrogengas...
longperiods.Visual inspectionof the spentfuelpool on30 April revealednosignificantdamage tothe
rods.A radiochemical examin...
caesium-137,whichisabout11% of Chernobyl emissions.Earlierestimationswere 150 PBq and
12 PBq.[147][148]
In 2011 scientists...
On 26 August,the governmenttookcharge of emergencymeasurestopreventfurtherradioactivewater
leaks,reflectingtheirlackof con...
These percentagesrepresentestimatedrelativeincreasesoverthe baseline ratesandare not absolute
risksfor developingsuchcance...
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Joel mc

  1. 1. Joel M. Cagulangan Jr. IV-Sun Mrs. Maria Fe M. Jadumas Chernobyl disaster Chernobyl disaster The nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left). Reactor 3 (center right). Time 01:23 (Moscow Time UTC+3) Date 26 April 1986 Location Pripyat, (former Ukrainian SSR,Soviet Union) The Chernobyl disaster(Ukrainian:Чорнобильськакатастрофа,Chornobylska Katastrofa – ChornobylCatastrophe;alsoreferredtoasChernobyl orthe Chornobyl accident) was a catastrophicnuclearaccidentthat occurredon 26 April 1986 at the Chernobyl NuclearPowerPlant in Ukraine (thenofficiallythe UkrainianSSR),whichwasunderthe directjurisdictionof the central authoritiesof the SovietUnion.Anexplosionandfire releasedlarge quantitiesof radioactive particles intothe atmosphere,whichspreadovermuchof the westernUSSRand Europe. The Chernobyl disasterwasthe worst nuclearpowerplant accidentinhistoryintermsof costand casualties,[1] andisone of onlytwoclassifiedasa level 7event(the maximumclassification) on the International NuclearEventScale (the otherbeingthe FukushimaDaiichi nucleardisaster in 2011).[2] The battle tocontainthe contaminationandavertagreatercatastrophe ultimately involved over500,000 workers and cost an estimated18billionrubles.[3] Duringthe accidentitself, 31 people died,andlong-termeffectssuchascancers are still beinginvestigated. Overview The disasterbeganduringasystemstestonSaturday,26 April 1986 at reactor numberfourof the Chernobyl plant,whichisnearthe cityof Pripyatand inproximitytothe administrative border withBelarusandthe DnieperRiver.There wasasuddenandunexpectedpowersurge,andwhenan emergencyshutdownwasattempted,anexponentiallylargerspike inpoweroutputoccurred,whichled to a reactor vessel rupture andaseriesof steamexplosions.Theseeventsexposed the graphite moderatorof the reactor to air,causingit to ignite.[4] The resultingfiresentaplume of highlyradioactive falloutintothe atmosphereandoveranextensive geographical area,includingPripyat. The plume driftedoverlarge partsof the western SovietUnion andEurope.From1986 to 2000, 350,400 people were evacuatedandresettledfromthe mostseverelycontaminatedareasof Belarus, Russia, and Ukraine.[5][6] Accordingto officialpost-Sovietdata,[7][8] about60% of the falloutlandedinBelarus. Russia,Ukraine,andBelarushave beenburdenedwiththe continuingand substantial decontamination andhealthcare costsof the Chernobyl accident.A reportbythe
  2. 2. International AtomicEnergyAgencyexaminesthe environmentalconsequencesof the accident.[8] AnotherUN agency, UNSCEAR,hasestimatedaglobal collective dose of radiationexposure fromthe accident"equivalentonaverage to21 additional daysof worldexposuretonaturalbackground radiation";individualdoseswerefarhigherthanthe global meanamongthose mostexposed,including 530,000 local recoveryworkerswhoaveragedan effectivedose equivalenttoan extra50 yearsof typical natural backgroundradiationexposure each.[9][10][11] Estimatesof the numberof deathsthatwill eventuallyresultfromthe accidentvaryenormously;disparitiesreflectboththe lackof solidscientific data and the differentmethodologiesusedtoquantifymortality –whetherthe discussionisconfinedto specificgeographical areasorextendsworldwide,andwhetherthe deathsare immediate,shortterm,or longterm. Thirtyone deathsare directlyattributedtothe accident,all amongthe reactorstaff and emergency workers.[12] AnUNSCEARreportplacesthe total confirmeddeathsfrom radiationat64 as of 2008. The Chernobyl Forumpredictsthe eventual deathtoll couldreach4000 among those exposedtothe highestlevelsof radiation(200,000 emergencyworkers,116,000 evacueesand270,000 residentsof the mostcontaminatedareas);thisfigure isatotalcausal deathtoll prediction,combiningthe deathsof approximately50 emergencyworkerswho diedsoonafterthe accidentfrom acute radiationsyndrome, nine childrenwhohave diedof thyroidcanceranda future predictedtotal of 3940 deathsfrom radiation-inducedcancerandleukemia.[13] In a peer-reviewedpublicationinthe InternationalJournalof Cancerin2006 the authors(followinga differentconclusionmethodologytothe Chernobyl forumstudy,whicharrivedatthe total predicted deathtoll of 4000 aftercancer survival rateswere factoredin) stated,withoutenteringintoadiscussion on deaths,thatinterms of total excesscancersattributedtothe accident:[14] The risk projectionssuggestthatbynowChernobyl mayhave causedabout1000 casesof thyroidcancer and 4000 casesof othercancersin Europe,representingabout0.01% of all incidentcancers since the accident.Modelspredictthatby2065 about 16,000 casesof thyroidcancerand 25,000 cases of other cancers maybe expecteddue toradiationfromthe accident,whereasseveral hundredmillioncancer casesare expectedfromothercauses. Alsobaseduponextrapolationsfromthe linearno-thresholdmodel of radiationinduceddamage,down to zero,the Unionof ConcernedScientists estimatesthat,amongthe hundredsof millionsof people livinginbroadergeographical areas,there will be 50,000 excesscancercasesresultingin25,000 excess cancer deaths.[15] For thisbroadergroup,the 2006 TORCH report,commissionedbythe EuropeanGreens political party, predicts30,000 to 60,000 excesscancerdeaths.[16] Intermsof non-scientificpublications,twoaffiliated withthe anti-nuclearadvocacygroupGreenpeace,have beenreleased,one of whichreportsthe figure at 200,000 or more.[17] The Russianfounderof that region'schapterof Greenpeace alsoauthoredabooktitled Chernobyl: Consequencesof theCatastropheforPeopleand theEnvironment,whichconcludes thatamongthe billionsof people worldwidewhowere exposedtoradioactive contaminationfromthe disaster,nearlya millionpremature cancerdeathsoccurredbetween1986 and 2004.[18] The book,however,hasfailed the peerreview process.[19][20] Of the five reviewspublishedinthe academicpress,fourconsideredthe bookseverelyflawedandcontradictory,andone praiseditwhile notingsome shortcomings. The review by M. I. Balonovpublishedbythe NewYorkAcademyof Sciences concludesthatthe reportisof negative value becauseithasverylittle scientificmeritwhilebeinghighlymisleadingtothe layreader.It characterizedthe estimate of nearlyamilliondeathsasmore inthe realmof science fiction than science.[21] The accidentraisedconcernsaboutnuclearpowerworldwideandslowedor reversed the expansionof nuclearpowerstations.[22] Alsothe accidentraisedconcernsaboutthe safety of the Sovietnuclear powerindustry,slowingitsexpansionfora numberof yearsand forcingthe Sovietgovernmentto become lesssecretive aboutitsprocedures.[23][notes 1] The governmentcoverupof the Chernobyl disaster was a "catalyst"forglasnost,which"pavedthe wayforreformsleadingtothe Sovietcollapse".[24] Accident On 26 April 1986, at 01:23 (UTC+3), reactor foursufferedacatastrophicpowerincrease,leadingto explosionsinitscore.Thisdispersedlarge quantitiesof radioactive fuel and core materialsintothe atmosphere[25]:73 andignitedthe combustiblegraphitemoderator.The burninggraphite moderator increasedthe emissionof radioactiveparticles,carriedbythe smoke,asthe reactorhad not been encasedbyany kindof hard containmentvessel.The accidentoccurredduringanexperimentscheduled to testa potential safety emergencycore cooling feature,whichtookplace duringanormal shutdown procedure.
  3. 3. Steam turbine tests An inactive nuclearreactorcontinuestogenerateasignificantamountof residualdecayheat.Inan initial shutdownstate (forexample,followinganemergency SCRAM) the reactorproducesaround 7 percentof itstotal thermal outputandrequirescoolingtoavoid core damage.RBMK reactors,like those at Chernobyl,use waterasa coolant.[26][27] Reactor4at Chernobyl consistedof about1,600 individual fuel channels;eachrequiredacoolantflow of 28 metrictons(28,000 litersor7,400 U.S. gallons) per hour.[25] Since coolingpumpsrequire electricity tocool a reactoraftera SCRAM,in the eventof a powergrid failure,Chernobyl'sreactorshadthree backup diesel generators;these couldstartupin15 seconds,but took60–75 seconds[25]:15 toattainfull speedandreachthe 5.5-megawatt(MW) outputrequiredtorun one mainpump.[25]:30 To solve thisone-minute gap,consideredanunacceptable safetyrisk,ithadbeentheorised that rotational energy fromthesteamturbine (asitwounddownunderresidual steampressure) could be usedto generate the requiredelectrical power.Analysisindicatedthatthisresidualmomentumand steampressure mightbe sufficienttorunthe coolantpumpsfor 45 seconds,[25]:16 bridgingthe gap betweenanexternal powerfailure andthe full availabilityof the emergencygenerators.[28] Thiscapabilitystill neededtobe confirmedexperimentally,andprevioustestshadendedunsuccessfully. An initial testcarriedoutin1982 showedthatthe excitation voltage of the turbine-generatorwas insufficient;itdidnotmaintainthe desired magneticfield afterthe turbine trip.The systemwas modified,andthe testwasrepeatedin1984 but againprovedunsuccessful.In1985, the testswere attemptedathirdtime but alsoyieldednegative results.The testprocedure wastobe repeatedagainin 1986, andit was scheduledtotake place duringthe maintenance shutdownof ReactorFour.[28] The test focusedonthe switchingsequencesof the electrical suppliesforthe reactor.The test procedure wasto beginwithanautomaticemergencyshutdown.Nodetrimentaleffectonthe safetyof the reactor was anticipated,sothe testprogramwasnot formallycoordinatedwitheitherthe chief designerof the reactor(NIKIET) orthe scientificmanager.Instead,itwasapprovedonlybythe director of the plant(andeventhisapproval wasnotconsistentwithestablishedprocedures).[29] Accordingto the testparameters,the thermal outputof the reactorshouldhave been no lowerthan 700 MW at the start of the experiment.If testconditionshadbeenasplanned,the procedure would almostcertainlyhave beencarriedoutsafely;the eventualdisasterresultedfromattemptstoboostthe reactor outputonce the experimenthadbeenstarted,whichwasinconsistentwithapproved procedure.[29] The Chernobyl powerplanthadbeeninoperationfortwoyearswithoutthe capabilitytoride through the first60–75 secondsof a total loss of electricpower,andthuslackedanimportantsafetyfeature.The stationmanagerspresumablywishedtocorrectthisat the firstopportunity,whichmayexplainwhy theycontinuedthe testevenwhenseriousproblemsarose,andwhythe requisiteapproval forthe test had notbeensoughtfromthe Sovietnuclearoversightregulator(eventhoughthere wasa representative atthe complex of 4reactors).[notes 2]:18–20 The experimental procedure wasintendedtorunas follows: The reactor was to be runningat a lowpowerlevel,between700MW and 800 MW. The steam-turbine generatorwastobe run up to full speed. Whenthese conditionswereachieved,the steamsupplyfor the turbine generatorwastobe closedoff. Turbine generatorperformance wastobe recordedtodetermine whetheritcouldprovide the bridging powerforcoolantpumpsuntil the emergencydiesel generatorswere sequencedtostartand provide powerto the coolingpumpsautomatically. Afterthe emergencygeneratorsreachednormal operatingspeedandvoltage,the turbinegenerator wouldbe allowedtofreewheel down. Conditionsbefore the accident The conditionstorun the testwere establishedbefore the dayshiftof 25 April 1986. The dayshift workershadbeeninstructedinadvance andwere familiarwiththe establishedprocedures.A special teamof electrical engineers waspresenttotestthe new voltage regulatingsystem.[30] Asplanned,a gradual reductioninthe outputof the powerunitwasbegunat 01:06 on 25 April,andthe powerlevel had reached50% of itsnominal 3200 MW thermal level bythe beginningof the dayshift. Atthis point,anotherregional powerstationunexpectedlywentoffline,andthe Kiev electrical gridcontrollerrequestedthatthe furtherreductionof Chernobyl'soutputbe postponed,aspowerwas neededtosatisfythe peakeveningdemand.The Chernobyl plantdirectoragreed,andpostponedthe test.Despite thispostponement,preparationsforthe testnotaffectingthe reactor'spowerwere carried
  4. 4. out,includingthe disablingof the emergencycore coolingsystemorECCS,a passive/active systemof core coolingintendedtoprovide watertothe core ina loss-of-coolantaccident.Giventhe otherevents that unfolded,the systemwouldhave beenof limiteduse,butitsdisablingasa "routine"stepof the testis an illustrationof the inherentlackof attentiontosafetyforthistest.[31] Inaddition,hadthe reactor beenshutdownforthe day as planned,itispossible thatmore preparationwouldhave been takeninadvance of the test. At 23:04, the Kievgridcontrollerallowedthe reactorshutdowntoresume.Thisdelayhadsome serious consequences:the dayshifthadlongsince departed,the eveningshiftwasalsopreparingtoleave,and the nightshiftwouldnottake overuntil midnight,well intothe job.Accordingtoplan,the testshould have beenfinishedduringthe dayshift,andthe nightshiftwouldonlyhave hadtomaintaindecayheat coolingsystemsinanotherwise shut-downplant.[25]:36–38 The nightshifthad verylimitedtimetoprepare forandcarry outthe experiment.A furtherrapid reductioninthe powerlevel from50%wasexecutedduringthe shiftchange-over. Alexander Akimov waschief of the nightshift,andLeonidToptunovwasthe operatorresponsibleforthe reactor's operational regimen,includingthe movementof the control rods.Toptunovwasa youngengineerwho had workedindependentlyasaseniorengineerforapproximatelythreemonths.[25]:36–38 The test plancalledfora gradual reductioninpoweroutputfromreactor 4 to a thermal levelof 700– 1000 MW.[32] Anoutputof 700 MW was reachedat 00:05 on 26 April.However,due tothe natural productionof xenon-135,a neutronabsorber,core powercontinuedtodecrease withoutfurther operatoraction—aprocessknownas reactor poisoning.Asthe reactorpoweroutputdroppedfurther, to approximately500 MW, Toptunovmistakenlyinsertedthe control rodstoofar—the exact circumstancesleadingtothisare unknownbecause AkimovandToptunovdiedinthe hospitalonMay 10 and 14, respectively.Thiscombinationof factorsrenderedthe reactorinanunintendednear- shutdown state,withapoweroutputof 30 MW thermal or less. Xenon-135in a reactor acts exactlyasif it were extracontrol rodsinserted.Undernormal operationitis burnedoff (thatis,absorbsa neutronand convertsintoxenon-136,whichismuchlessabsorptive,much lessa poison).Atlowerpowerthe creationof xenon-135exceedsthe abilityof the reducedneutronsto burn it.A reactorpoisoned bythe xenon-135fissionproductcanonlybe treatedinone of two ways:by shuttingdownthe reactorand allowingthe xenontodecayaway,whichtakesabout24 hours,or to remove control rodsuntil the poisoningisfirstovercomebycreatingfarmore neutronsthanusual and overwhelmingthe lossescausedbythe poisoning,andthensecondbythe xenon-135absorbing neutronsandbeingconvertedintoxenon-136.Thislatteriswhatwas done;the dangeristhat the large amountof xenon-135,asit isconverted,effectivelyremovescontrol rodsveryquicklyfromthe reactor, whichcan be drivenintoa "promptsupercritical"state inseconds.Standardoperatingprocedure was for 28 rodsto alwaysbe inserted,makingitimpossible forthe reactorto gosupercritical.AsGrigoriy Medvedevwrites,"...the reactor'scapacityfor excursionnow exceededthe abilityof the available safetysystemstoshutitdown."[33] The reactor was nowproducing5 percentof the minimuminitial powerlevel establishedassafe forthe test.[29]:73 Control-roompersonnel decidedtorestore powerbydisabling the automaticsystem governingthe control rodsandmanuallyextractingthe majorityof the reactorcontrol rodsto their upperlimits.[34] Severalminuteselapsedbetweentheirextractionandthe pointthatthe poweroutput beganto increase andsubsequentlystabilize at160–200 MW (thermal),amuchsmallervalue thanthe planned700 MW. The rapidreductioninthe powerduringthe initial shutdown,andthe subsequent operationata level of lessthan200 MW ledto increased poisoningof the reactorcore by the accumulationof xenon-135.[35][36] Thisrestrictedanyfurtherrise of reactorpower,andmade it necessarytoextractadditional control rodsfromthe reactor core in orderto counteractthe poisoning. The operationof the reactor at the lowpowerlevel andhighpoisoninglevel wasaccompaniedby unstable core temperature andcoolantflow,andpossiblybyinstabilityof neutronflux,whichtriggered alarms.The control roomreceivedrepeatedemergencysignalsregardingthe levelsinthe steam/water separatordrums,and large excursionsorvariationsinthe flow rate of feedwater,aswell asfrom relief valvesopenedtorelieveexcesssteamintoa turbine condenser,andfromthe neutronpowercontroller. In the periodbetween00:35 and 00:45, emergencyalarmsignalsconcerning thermal- hydraulicparameterswere ignored,apparentlytopreserve the reactorpowerlevel.[37] Whenthe powerlevel of 200 MW was eventuallyachieved,preparationforthe experimentcontinued. As part of the test plan,extrawaterpumpswere activatedat01:05 on26 April,increasingthe water flow.The increasedcoolantflow rate throughthe reactorproducedanincrease inthe inletcoolant temperature of the reactorcore (the coolantnolongerhavingsufficienttimetorelease itsheatinthe turbine andcoolingtowers),whichnowmore closelyapproachedthe nucleate boilingtemperature of water,reducingthe safetymargin. The flowexceededthe allowedlimitat01:19, triggeringanalarmof low steampressure inthe steam separators.Atthe same time,the extrawaterflow lowered the overall core temperature andreduced
  5. 5. the existingsteamvoids inthe core andthe steamseparators.[38] Since waterweaklyabsorbsneutrons (andthe higherdensityof liquidwatermakesita betterabsorberthansteam),turningonadditional pumpsdecreasedthe reactorpowerfurtherstill.The crew respondedbyturningoff twoof the circulationpumpstoreduce feedwaterflow,inanefforttoincrease steampressure,andalsotoremove more manual control rods to maintainpower.[31][39] All these actionsledtoan extremelyunstable reactorconfiguration.Nearlyall of the control rodswere removedmanually,includingall but18 of the "fail-safe"manuallyoperatedrodsof the minimal 28 whichwere intendedtoremainfullyinsertedtocontrol the reactoreveninthe eventof a lossof coolant,outof a total 211 control rods.[33] While the emergencySCRAMsystemthatwouldinsertall control rods to shutdownthe reactor couldstill be activatedmanually(throughthe "AZ-5"button),the automatedsystemthatcoulddothe same had beendisabledtomaintainthe powerlevel,andmany otherautomatedandevenpassive safetyfeaturesof the reactorhadbeenbypassed.Further,the reactor coolantpumpinghadbeenreduced,whichhadlimitedmarginsoanypowerexcursionwould produce boiling,therebyreducing neutronabsorptionbythe water.The reactorwasin an unstable configurationthatwasclearlyoutside the safe operatingenvelope establishedbythe designers.If anythingpusheditintosupercriticality,itwasunable torecoverautomatically. Experimentandexplosion At 1:23:04 a.m. the experimentbegan.Fourof the MainCirculatingPumps(MCP) were active;of the eighttotal,six are normallyactive duringregularoperation.The steamtothe turbineswasshutoff, beginningarun-downof the turbine generator.The diesel generatorstartedandsequentiallypickedup loads;the generatorswere tohave completelypickedupthe MCPs'powerneedsby01:23:43. Inthe interim,the powerforthe MCPswas to be suppliedbythe turbine generatorasitcoasteddown.As the momentumof the turbine generatordecreased,however,sodidthe poweritproducedforthe pumps.The waterflowrate decreased,leadingtoincreasedformationof steamvoids(bubbles) inthe core. Because of the positive voidcoefficientof the RBMK reactor at low reactorpowerlevels,itwasnow primedtoembarkon a positive feedback loop,inwhichthe formationof steamvoidsreducedthe ability of the liquidwatercoolanttoabsorbneutrons,whichinturnincreasedthe reactor'spoweroutput.This causedyetmore water to flashintosteam, givingyetafurtherpowerincrease.Duringalmostthe entire periodof the experimentthe automaticcontrol systemsuccessfullycounteractedthispositive feedback, continuouslyinserting control rods intothe reactorcore to limitthe powerrise.However,thissystem had control of only12 rods, andnearlyall othershad beenmanuallyretracted. At 1:23:40, as recordedbythe SKALA centralizedcontrol system, anemergencyshutdownof the reactor,whichinadvertentlytriggeredthe explosion,wasinitiated.The SCRAMwasstartedwhenthe EPS-5 button(alsoknownasthe AZ-5 button) of the reactor emergencyprotectionsystemwaspressed: thisengagedthe drive mechanismonall control rodstofullyinsertthem, includingthe manual control rods that hadbeenincautiouslywithdrawnearlier.The reasonwhythe EPS-5buttonwaspressedisnot known,whetheritwasdone asan emergencymeasure inresponse torisingtemperatures,orsimplyas a routine methodof shuttingdownthe reactoruponcompletionof the experiment. There isa viewthatthe SCRAMmayhave beenorderedasa response tothe unexpectedrapidpower increase,althoughthere isnorecordeddataconclusivelyprovingthis.Some have suggestedthatthe buttonwas notpressed,andinsteadthe signal wasautomaticallyproducedbythe emergency protectionsystem;however,the SKALA clearlyregisteredamanual SCRAMsignal.Inspite of this,the questionasto whenorevenwhetherthe EPS-5buttonwaspressedhasbeenthe subjectof debate. There are assertionsthatthe pressure wascausedbythe rapidpoweraccelerationatthe start, and allegationsthatthe buttonwasnotpresseduntil the reactorbegantoself-destructbutothersassert that ithappenedearlierandincalm conditions.[40]:578[41] Afterthe EPS-5 buttonwaspressed,the insertionof control rodsintothe reactor core began.The control rod insertionmechanismmovedthe rodsat 0.4 m/s,sothat the rodstook 18 to 20 secondsto travel the full heightof the core,about7 meters.A biggerproblemwasaflawedgraphite-tipcontrol rod design,whichinitiallydisplacedneutron-absorbingcoolantwithmoderatinggraphitebeforeintroducing replacementneutron-absorbingboronmaterial toslow the reaction.Asaresult,the SCRAMactually increasedthe reactionrate inthe upperhalf of the core as the tipsdisplacedwater.Thisbehaviorwas knownaftera shutdownof anotherRBMK reactor induced aninitial powerspike,butasthe SCRAMof that reactor wassuccessful,the informationwasnotwidelydisseminated. A fewsecondsafterthe startof the SCRAM, the graphite rodtipsenteredthe fuel pile.A massive powerspike occurred,andthe core overheated,causingsome of the fuel rods tofracture,blockingthe control rod columnsandjammingthe control rods at one-thirdinsertion,withthe graphite tipsinthe middle of the core.Withinthree secondsthe reactoroutputrose above 530 MW.[25]:31
  6. 6. The subsequentcourse of eventswasnotregisteredbyinstruments;itisknownonlyasa resultof mathematical simulation.Apparently,the powerspikecausedanincrease infuel temperatureand massive steambuildup,leadingtoarapidincrease insteampressure.Thiscausedthe fuel claddingto fail,releasingthe fuel elementsintothe coolant,andrupturingthe channelsinwhichthese elements were located.[42] Then,accordingto some estimations,the reactorjumpedtoaround30,000 MW thermal,tentimes the normal operational output.The lastreadingonthe control panel was33,000 MW. Itwas not possible toreconstructthe precise sequence of the processesthatledtothe destructionof the reactor and the powerunitbuilding,buta steamexplosion,like the explosionof asteamboilerfromexcess vapor pressure,appearstohave beenthe nextevent.There isageneral understandingthatitwassteam fromthe wreckedfuel channelsescapingintothe reactor'sexteriorcoolingstructure thatcausedthe destructionof the reactorcasing,tearingoff and liftingthe 2000-tonupperplate,towhichthe entire reactor assemblyisfastened,sendingitthroughthe roof of the reactor building.Apparently,thiswas the firstexplosionthatmanyheard.[43]:366 Thisexplosionrupturedfurtherfuel channels,aswell as severingmostof the coolantlinesfeedingthe reactorchamber,andas a resultthe remainingcoolant flashedtosteamandescapedthe reactor core.The total waterlossincombinationwithahighpositive voidcoefficientfurtherincreasedthe reactor'sthermal power. A second,more powerful explosionoccurredabouttwoor three secondsafterthe first;this explosiondispersedthe damagedcore andeffectivelyterminatedthe nuclearchainreaction.However, thisexplosionalsocompromisedmore of the reactorcontainmentvessel andejectedsuperheated lumpsof graphite moderator.The ejectedgraphiteandthe demolished channelsstillinthe remainsof the reactor vessel caughtfire onexposure toair,greatlycontributingtothe spreadofradioactive falloutandthe contamination of outlyingareas.[44] Accordingto observersoutsideUnit4,burning lumpsof material andsparksshot intothe air above the reactor.Some of themfell ontothe roof of the machine hall andstarteda fire.About25 percentof the red-hotgraphite blocksandoverheatedmaterialfromthe fuel channelswasejected.Partsof the graphite blocksandfuel channelswere outof the reactorbuilding.Asaresultof the damage to the buildinganairflowthroughthe core wasestablishedbythe hightemperatureof the core.The air ignited the hot graphite andstarteda graphite fire.[25]:32 There were initiallyseveral hypothesesaboutthe nature of the secondexplosion.One view wasthatthe secondexplosionwascausedby hydrogen,whichhadbeenproducedeitherbythe overheatedsteam- zirconiumreactionorby the reactionof red-hotgraphite withsteam thatproducedhydrogen and carbon monoxide.Anotherhypothesiswasthatthe secondexplosionwasathermal explosion of the reactor as a resultof the uncontrollable escape of fastneutrons causedbythe completewaterloss inthe reactorcore.[45] A thirdhypothesiswasthatthe explosionwasasecondsteamexplosion. Accordingto thisversion,the firstexplosionwasamore minorsteamexplosioninthe circulatingloop, causinga lossof coolantflowandpressure, thatinturn causedthe waterstill inthe core toflashto steam.Thissecondexplosionthendidthe majorityof the damage tothe reactorand containment building. However,the sheerforce of the secondexplosion,andthe ratioof xenonradioisotopes releasedduring the event,indicate thatthe secondexplosioncouldhave beenanuclearpowertransient;the resultof the meltingcore material,inthe absence of itscladding,watercoolantandmoderator, undergoingrunawaypromptcriticality similartothe explosionof a fizzlednuclearweapon.[46] This nuclearexcursionreleased40 billion joulesof energy,the equivalentof abouttentonsof TNT.The analysisindicatesthatthe nuclearexcursionwaslimitedtoasmall portionof the core.[46] Contraryto safetyregulations, bitumen,acombustiblematerial,hadbeenusedinthe constructionof the roof of the reactor buildingandthe turbine hall.Ejectedmaterial ignitedatleastfive firesonthe roof of the adjacentreactor 3, whichwasstill operating.Itwasimperative toputthose firesoutand protectthe coolingsystemsof reactor3.[25]:42 Inside reactor3, the chief of the nightshift,Yuri Bagdasarov,wantedtoshut downthe reactor immediately,butchief engineerNikolaiFominwouldnot allowthis.The operatorswere given respirators andpotassiumiodidetabletsandtoldtocontinue working.At05:00, however,Bagdasarovmade hisowndecisiontoshutdownthe reactor,leavingonly those operatorsthere whohadto workthe emergencycoolingsystems.[25]:44 Radiation levels The radiationlevelsinthe worst-hitareasof the reactorbuildinghave beenestimatedtobe 5.6 roentgens persecond(R/s) (1.4milliamperes perkilogram),equivalenttomore than 20,000 roentgensperhour.A lethal dose isaround500 roentgens(5Gy,0.13 coulombs perkilogram) over5 hours,so insome areas,unprotectedworkersreceivedfatal dosesinlessthanaminute. However,adosimetercapable of measuringupto1000 R/s (0.3 A/kg) wasburiedinthe rubble of a collapsedpartof the building,andanotherone failedwhenturnedon.All remainingdosimetershad limitsof 0.001 R/s (0.3 µA/kg) andtherefore read"off scale".Thus,the reactorcrew couldascertainonly
  7. 7. that the radiationlevelswere somewhere above0.001 R/s (3.6 R/h, or 0.3 µA/kg),whilethe true levels were muchhigherinsome areas.[25]:42–50 Because of the inaccurate lowreadings,the reactorcrew chief AlexanderAkimovassumedthatthe reactor wasintact.The evidence of piecesof graphite andreactorfuel lyingaroundthe buildingwas ignored,andthe readingsof anotherdosimeterbroughtinby04:30 were dismissedunderthe assumptionthatthe newdosimetermusthave beendefective.[25]:42–50 Akimovstayedwithhiscrewinthe reactor buildinguntilmorning,sendingmembersof hiscrew totry to pumpwater intothe reactor. None of themwore any protective gear.Most,includingAkimov,diedfromradiationexposure within three weeks.[33]:247–48 Fire containment Shortlyafterthe accident,firefightersarrivedtotryto extinguishthe fires.Firstonthe scene wasa Chernobyl PowerStationfirefighterbrigade underthe commandof Lieutenant VolodymyrPravik,who diedon9 May 1986 of acute radiationsickness.Theywere nottoldhow dangerouslyradioactive the smoke andthe debriswere,andmaynot evenhave knownthatthe accidentwasanythingmore than a regularelectrical fire:"We didn'tknowitwasthe reactor.No one had toldus."[49] Grigorii Khmel,the driverof one of the fire engines,laterdescribedwhathappened: We arrivedthere at10 or 15 minutestotwointhe morning....We saw graphite scatteredabout.Misha asked:"Isthat graphite?"Ikickeditaway.But one of the fightersonthe othertruck pickeditup."It's hot,"he said.The piecesof graphite were of differentsizes,some big,some small,enoughtopickthem up... We didn'tknowmuchabout radiation.Eventhose whoworkedthere hadnoidea.There wasnowater leftinthe trucks.Misha filleda cisternandwe aimedthe waterat the top. Thenthose boyswhodied wentupto the roof – Vashchik,Kolyaandothers,andVolodyaPravik....Theywentupthe ladder...andI neversawthemagain.[50]:54 Anatoli Zakharov,afiremanstationedinChernobyl since 1980, offersa differentdescriptionin2008: I rememberjokingtothe others,"There mustbe anincredible amountof radiationhere.We'll be lucky if we're all still alive inthe morning."[51] He alsosaid: Of course we knew!If we'dfollowedregulations,we wouldneverhave gone nearthe reactor.But itwas a moral obligation –ourduty.We were like kamikaze.[51] The immediate prioritywastoextinguishfiresonthe roof of the stationandthe areaaround the buildingcontainingReactorNo. 4 to protectNo. 3 and keepitscore coolingsystemsintact.The fires were extinguishedby5:00, butmany firefightersreceivedhighdosesof radiation.The fire inside reactor 4 continuedtoburnuntil 10 May 1986; it ispossible thatwell overhalf of the graphite burnedout.[25]:73 The fire was extinguishedbyacombinedeffortof helicoptersdroppingover5000 metrictonsof sand, lead,clay,andneutron-absorbingboronontothe burningreactorandinjection of liquidnitrogen.The UkrainianfilmmakerVladimirShevchenko capturedfilmfootage of an Mi-8 helicopterasitsmainrotor collidedwithanearby constructioncrane cable,causingthe helicoptertofall nearthe damagedreactor buildingandkillingitsfour-mancrew.[52] Itisnow knownthatvirtuallynone of the neutronabsorbers reachedthe core.[53] From eyewitnessaccountsof the firefightersinvolvedbefore theydied(asreportedon the CBC televisionseries Witness),one describedhis experience of the radiationas"tastinglike metal", and feelingasensationsimilartothatof pinsandneedles all overhisface.(Thisissimilartothe descriptiongivenby LouisSlotin,aManhattanProjectphysicistwhodieddaysafterafatal radiation overdose fromacriticalityaccident.)[54] The explosionandfire threwhotparticlesof the nuclearfuelandalsofarmore dangerous fission products,radioactive isotopessuchas caesium-137,iodine-131, strontium-90andotherradionuclides, intothe air: the residentsof the surroundingareaobservedthe radioactivecloudonthe nightof the explosion. Equipmentassembledincludedremote-controlledbulldozersand robot-cartsthatcoulddetect radioactivityandcarry hotdebris.ValeryLegasov(firstdeputydirectorof the KurchatovInstitute of AtomicEnergyinMoscow) said,in1987, "But we learnedthatrobotsare not the great remedyfor everything.Wherethere wasveryhighradiation,the robotceasedtobe a robot—the electronicsqu A robot sentintothe reactor itself hasreturnedwithsamplesof black, melanin-richradiotrophic fungi thatare growingonthe reactor'swalls.[125] Of the 440,350 wildboar killedin the 2010 huntingseasoninGermany,over1000 were foundtobe contaminatedwithlevelsof radiationabove the permittedlimitof 600 becquerelsperkilogram, due to residual radioactivityfromChernobyl.[126]
  8. 8. The NorwegianAgricultural Authorityreportedthatin2009 a total of 18,000 livestockinNorwayneeded to be givenuncontaminatedfeedforaperiodof time before slaughterinordertoensure thattheir meatwas safe forhuman consumption.Thiswasdue toresidual radioactivityfromChernobyl inthe plantstheygraze on inthe wildduringthe summer.1,914 sheepneededtobe givenuncontaminated feedfora periodof time before slaughterduring2012, and these sheepwere locatedinjust18 of Norway'smunicipalities,adecrease of 17 fromthe 35 municipalitiesaffectedanimalswerelocatedin during2011 (117 municipalitieswere affectedduring1986).[127] The after-effectsof Chernobyl were expectedtobe seenfora further100 years,althoughthe severityof the effectswoulddecline overthatperiod.[128] Scientistsreportthisisdue toradioactive caesium- 137 isotopesbeingtakenupbyfungi suchas Cortinariuscaperatus whichisinturneatenby sheepwhilst grazing.[127] The UnitedKingdomwasforcedto restrictthe movementof sheepfrom uplandareaswhen radioactive caesium-137fell acrosspartsof NorthernIreland,Wales,ScotlandandnorthernEngland.In the immediate aftermathof the disasterin1986, a total of 4,225,000 sheephadtheirmovement restrictedacrossa total of 9,700 farms,inorder to preventcontaminatedmeatenteringthe humanfood chain.[129] The numberof sheepandthe numberof farmsaffectedhasdecreasedsince 1986, Northern Irelandwasreleasedfromall restrictionsin2000 and by 2009 369 farmscontainingaround190,000 sheepremainedunderthe restrictionsinWales,CumbriaandnorthernScotland.[129] The restrictions applyinginScotlandwere liftedin2010, whilstthose applyingtoWalesandCumbriawere liftedduring 2012, meaningnofarmsinthe UK remainrestrictedbecause of Chernobyl fallout.[130][131] The legislationusedtocontrol sheepmovementandcompensatefarmers(farmerswere latterly compensatedperanimal tocoveradditional costsinholdinganimalspriortoradiationmonitoring) was revokedduringOctoberandNovember2012 bythe relevantauthoritiesinthe UK.[132] Human impact Main article:Chernobyl disastereffects The Chernobyl Forumfirstmeton3 February2003 fora three-daymeeting.Itconsistedof the International AtomicEnergyAgency(IAEA),otherUnitedNationsorganizations(FAO,UN-OCHA,UNDP, UNEP, UNSCEAR,WHO, andthe WorldBank),andthe governmentsof Belarus,Russia,andUkraine.A secondmeetingwasheldon10–11 March 2004, and a thirdon 18–20 April 2005. The aimof the Forum was to "scientificallyclarifythe radiological environmental andhealthconsequencesof the Chernobyl accident,toprovide advice onandto contribute toa scientificallysoundremediationandhealthcare programmes,andto considerthe necessityof,andopportunitiesforcontinuedresearch/learning lessons."[133] A reportwaspublishedbyChernobyl Forumin2005. On the deathtoll of the accident,the reportstatesthat twenty-eightemergencyworkers("liquidators") diedfromacute radiationsyndrome includingbetaburns and15 patientsdiedfromthyroidcancerin the followingyears,anditroughlyestimatedthatcancerdeathscausedbyChernobyl mayreacha total of about4000 amongthe 5 millionpersonsresidinginthe contaminatedareas,the reportprojected cancer mortality"increasesof lessthanone percent"(~0.3%) on a time spanof 80 years,cautioning that thisestimate was"speculative"since atthistime onlyafew cancer deathsare linkedtothe Chernobyl disaster.[134] The reportsaysitisimpossible toreliablypredictthe numberof fatal cancers arisingfromthe incidentassmall differencesinassumptionscanresultinlarge differencesinthe estimatedhealthcosts.The reportsaysitrepresentsthe consensusview of the eightUN organisations. Thyroid cancer The 2005 Chernobyl Forumreportrevealedthyroidcanceramongchildrentobe one of the main health impactsfromthe Chernobyl accident.Inthatpublicationmore than4000 cases were reported,andthat there wasno evidence of anincrease insolidcancersorleukemia.Itsaidthatthere was an increase in psychological problemsamongthe affectedpopulation.[134] DrMichael Repacholi,managerof WHO's RadiationProgramreportedthatthe 4000 casesof thyroidcancerresultedinnine deaths.[135] Accordingto UNSCEAR,up tothe year2005, an excessof over6000 casesof thyroidcancer have been reported.Thatis,overthe estimatedpre-accidentbaseline thyroidcancerrate,more than 6000 casual cases of thyroidcancer have beenreportedinchildrenandadolescentsexposedatthe time of the accident,a numberthatisexpectedtoincrease.Theyconcludedthatthere isnootherevidence of majorhealthimpactsfromthe radiationexposure.[136] Well-differentiated thyroidcancers are generallytreatable,[137] andwhentreatedthe five-yearsurvival rate of thyroidcancer is96%, and 92% after30 years.[138] UNSCEARhadreported15 deathsfrom thyroid cancer in2011.[139] The International AtomicEnergyAgency (IAEA) alsostatesthatthere hasbeenno increase inthe rate of birthdefects orabnormalities,orsolidcancers (suchas lungcancer) corroborating UNSCEAR'sassessments.[140] UNSCEARdoesraise the possibilityof longtermgeneticdefects,pointingto a doublingof radiation-inducedminisatellite mutations amongchildrenbornin1994.[141] However,the
  9. 9. riskof thyroidcancerassociatedwiththe Chernobyl accidentisstill highaccordingtopublished studies.[142][143] The German affiliate of the InternationalPhysiciansforthe Preventionof NuclearWar (IPPNW) argued that more than 10,000 people are todayaffectedbythyroidcancerand50,000 cases are expectedinthe future.[144] Otherhealth disorders FredMettler,a radiationexpertatthe Universityof New Mexico,putsthe numberof worldwidecancer deathsoutside the highlycontaminatedzone at"perhaps"5000, fora total of 9000 Chernobyl- associatedfatal cancers,saying"the numberissmall (representingafew percent) relativetothe normal spontaneousriskof cancer,butthe numbersare large inabsolute terms".[145] The same reportoutlined studiesbasedindatafoundinthe RussianRegistryfrom1991 to 1998 that suggestedthat"of 61,000 Russianworkersexposedtoanaverage dose of 107 mSvabout 5% of all fatalitiesthatoccurredmay have beendue toradiationexposure."[134] The report wentintodepthaboutthe risksto mental health of exaggeratedfearsaboutthe effectsof radiation.[134] Accordingtothe IAEA the "designationof the affectedpopulationas"victims"ratherthan "survivors"hasledthemtoperceive themselvesashelpless,weakandlackingcontrol overtheirfuture". The IAEA saysthat thismay have ledtobehaviourthathas causedfurtherhealtheffects.[146] FredMettlercommentedthat20 yearslater"The populationremainslargelyunsureof whatthe effects of radiationactuallyare andretaina sense of foreboding.A numberof adolescentsandyoungadults whohave been exposedtomodestorsmall amountsof radiationfeelthattheyare somehow fatally flawedandthere isnodownside tousingillicitdrugsorhavingunprotectedsex.Toreverse such attitudesandbehaviorswilllikelytake yearsalthoughsome youthgroupshave begunprogramsthat have promise."[147] Inaddition,disadvantagedchildrenaroundChernobylsufferfromhealthproblems that are attributable notonlytothe Chernobyl accident,butalsotothe poorstate of post-Soviethealth systems.[140] The UnitedNationsScientificCommittee onthe Effectsof AtomicRadiation (UNSCEAR),partof the Chernobyl Forum,have producedtheirownassessmentsof the radiationeffects.[148] UNSCEARwasset up as a collaborationbetweenvariousUnitedNationbodies,includingthe WorldHealthOrganisation, afterthe atomic bombattacks onHiroshimaandNagasaki,to assessthe long-termeffectsof radiation on humanhealth.[149] Deaths due to radiation exposure The numberof potential deathsarisingfromthe Chernobyldisasterisheavilydebated.The WHO's predictionof 4000 future cancerdeathsin surroundingcountries[150] isbasedonthe Linearno-threshold model (LNT),whichassumesthatthe damage inflictedbyradiationatlow dosesisdirectlyproportional to the dose.[151] Radiationepidemiologist RoyShore contendsthatestimatinghealtheffectsina populationfromthe LNTmodel "isnotwise because of the uncertainties".[152] Accordingto the Unionof ConcernedScientiststhe numberof excesscancerdeathsworldwide (includingall contaminatedareas) isapproximately27,000 basedon the same LNT.[153] Anotherstudycritical of the Chernobyl ForumreportwascommissionedbyGreenpeace,whichasserted that the most recentlypublishedfiguresindicate thatinBelarus,RussiaandUkraine the accidentcould have resultedin10,000–200,000 additional deathsinthe periodbetween1990 and2004.[154] The ScientificSecretaryof the Chernobyl Forumcriticizedthe report'sreliance onnon-peerreviewed locally producedstudies.Althoughmostof the study'ssourceswere frompeer-reviewedjournals,including manyWesternmedical journals,the highermortalityestimateswerefromnon-peer-reviewed sources,[154] while GregoryHärtl (spokesmanforthe WHO) suggestedthatthe conclusionswere motivatedbyideology.[155] Chernobyl:Consequencesof theCatastropheforPeopleand theEnvironment isan Englishtranslationof the 2007 Russianpublication Chernobyl.Itwaspublishedin2009 by the New York Academyof Sciences intheirAnnalsof theNewYork Academy of Sciences.It presentsananalysisof scientific literature andconcludesthatmedical recordsbetween1986, the year of the accident,and2004 reflect 985,000 premature deathsasa resultof the radioactivityreleased.[156] Though,itwasimpossible to preciselydetermine whatdose the affectedpeople received,knowingthe factthatthe receiveddoses variedstronglyfromone individual tothe otherinthe populationabove whichthe radioactivecloud travelled,andalsoknowingthe factthat one cannottell forsure if a cancer inan individual fromthe formerUSSR isproducedby radiationfromChernobyl accidentorbyothersocial or behavioral factors, such as smokingoralcohol drinking.[157] The authors suggestthatmost of the deathswere inRussia,BelarusandUkraine,thoughothers occurredworldwide throughoutthe manycountriesthatwere struckbyradioactive falloutfrom Chernobyl.The literature analysisdrawsonover 1000 publishedtitlesandover5000 internetand printedpublicationsdiscussingthe consequencesof the Chernobyl disaster.The authorscontendthat
  10. 10. those publicationsandpaperswere writtenbyleadingEasternEuropeanauthoritiesandhave largely beendownplayedorignoredbythe IAEA andUNSCEAR.[156] Thisestimate hashoweverbeencriticizedas exaggerated,lackingaproperscientificbase.[21] Other conditions Accordingto KennethMossman,aProfessorof HealthPhysics andmemberof the U.S.Nuclear RegulatoryCommissionadvisorycommittee,[163] the "LNTphilosophyisoverlyconservative,andlow- level radiationmaybe lessdangerousthancommonlybelieved".[164] YoshihisaMatsumoto,aradiation biologistatthe TokyoInstitute of Technology,citeslaboratoryexperimentsonanimalstosuggestthere mustbe a thresholddose belowwhichDNA repairmechanismscancompletelyrepairanyradiation damage.[152] Mossmansuggeststhatthe proponentsof the currentmodel believe thatbeing conservative isjustifieddue tothe uncertaintiessurroundinglow level dosesanditisbetterto have a "prudentpublichealthpolicy".[163] Anothersignificantissueisestablishingconsistentdataonwhichto base the analysisof the impact of the Chernobyl accident.Since 1991 large social and political changeshave occurredwithinthe affectedregionsandthese changeshave hadsignificantimpactonthe administrationof healthcare,on socio-economicstability,andthe mannerinwhichstatistical dataiscollected.[165] RonaldChesser,a radiationbiologistat TexasTechUniversity,saysthat"the subsequentSovietcollapse,scarce funding, imprecise dosimetry,anddifficultiestrackingpeopleoverthe yearshave limitedthe numberof studies and theirreliability."[152] Economic and political consequences It is difficulttoestablishthe total economiccostof the disaster.AccordingtoMikhail Gorbachev, the SovietUnion spent18 billionrubles(theequivalentof US$18 billionatthattime) oncontainment and decontamination,virtuallybankruptingitself.[3] InBelarusthe total costover30 yearsisestimated at US$235 billion(in2005 dollars).[140] On-goingcostsare well known;intheir2003–2005 report, The Chernobyl Forum statedthatbetween5% and7% of governmentspendinginUkraine isstill relatedto Chernobyl,while inBelarusover$13 billionisthoughttohave beenspentbetween 1991 and2003, with 22% of national budgethavingbeenChernobyl-relatedin1991, fallingto6% by2002.[140] Much of the currentcost relatestothe paymentof Chernobyl-relatedsocial benefitstosome 7millionpeople across the 3 countries.[140] A significanteconomicimpactatthe time wasthe removal of 784,320 ha (1,938,100 acres) of agricultural landand694,200 ha (1,715,000 acres) of forestfromproduction.While muchof thishas beenreturnedtouse,agricultural productioncostshave risendue tothe needforspecial cultivation techniques,fertilizersandadditives.[140] Politically,the accidentgave greatsignificancetothe new Sovietpolicyof glasnost,[166][167] andhelped forge closerSoviet-USrelationsatthe endof the ColdWar, throughbioscientificcooperation.[168]:44– 48 But the disasteralsobecame akeyfactor inthe Union'seventual 1991 dissolution,andamajor influenceinshapingthe new EasternEurope.[168]:20–21 Aftermath Followingthe accident,questionsarose aboutthe future of the plantanditseventual fate.All workon the unfinishedreactors5 and6 was haltedthree yearslater.However,the trouble atthe Chernobyl plantdidnot endwiththe disasterinreactor4.The damagedreactorwas sealedoff and200 cubic meters(260 cu yd) of concrete was placedbetweenthe disastersite andthe operational buildings.[citation needed] The work wasmanagedby GrigoriyMihaylovichNaginskiy,the DeputyChief Engineerof InstallationandConstructionDirectorate –90. The Ukrainiangovernmentcontinuedtoletthe three remainingreactorsoperate because of anenergyshortage inthe country. Decommissioning Main article:Chernobyl NuclearPowerPlant§Decommissioning In 1991, a fire broke outinthe turbine buildingof reactor2;[169] the authoritiessubsequentlydeclared the reactor damagedbeyondrepairandwastaken offline.Reactor1wasdecommissionedinNovember 1996 as part of a deal betweenthe Ukrainiangovernmentandinternational organizationssuchasthe IAEA to endoperationsatthe plant.On 15 December2000, then-PresidentLeonidKuchmapersonally turnedoff Reactor3 in an official ceremony,shuttingdownthe entire site.[170] Radioactive waste management Containmentof the reactor The Chernobyl reactorisnowenclosedinalarge concrete sarcophagus,whichwasbuiltquicklytoallow continuingoperationof the otherreactorsatthe plant.[171] A NewSafe Confinement wastohave beenbuiltbythe endof 2005; however,ithassufferedongoing delaysandas of 2010, when constructionfinallybegan,wasexpectedtobe completedin2013. This was delayedagainto2016, the endof the 30-year lifespanof the sarcophagus.The structure isbeingbuilt adjacentto the existingshelterandwill be slidintoplace onrails.Itis to be a metal arch 105 metres (344 ft) highand spanning257 metres(843 ft),to coverboth unit4 andthe hastilybuilt1986 structure.
  11. 11. The Chernobyl ShelterFund,setupin1997, hasreceived €810 millionfrominternational donorsand projectsto coverthisprojectandpreviouswork.Itand the NuclearSafetyAccount,alsoappliedto Chernobyl decommissioning,are managedbythe EuropeanBankfor Reconstructionand Development(EBRD).[citation needed] By 2002, roughly15,000 Ukrainianworkerswere still workingwithinthe Zone of Exclusion,maintaining the plantand performingothercontainment- andresearch-relatedtasks,oftenindangerous conditions.[168]:2 A handful of Ukrainianscientistsworkinside the sarcophagus,butoutsidersare rarely grantedaccess.In 2006 an Australian 60 Minutesteamledby reporterRichardCarletonandproducer StephenRice were allowedtoenterthe sarcophagusfor15 minutesandfilminside the control room.[172] On 12 February2013 a 600 m2 (6,500 sqft) sectionof the roof of the turbine-building,adjacenttothe sarcophagus,collapsed.Atfirstitwasassumedthatthe roof collapsedbecause of the weightof snowon it.Howeverthe amountof snowwas not exceptional,andthe reportof a Ukrainianfact-findingpanel concludedthatthe part collapse of the turbine-buildingwasthe result of sloppyrepairworkandaging of the structure.The reportmentionedthe possibilitythatthe repairedpartof the turbine-building addeda largerstrainon the total structure thanexpected,andthe bracesinthe roof were damagedby corrosionand sloppywelding.ExpertssuchasValentinKupny,formerdeputydirectorof the nuclear plant,didwarnthat the complex wasonthe verge of a collapse,leavingthe buildinginanextremely dangerouscondition.A proposedreinforcementin2005 was cancelledbya superiorofficial.Afterthe 12 Februaryincident,radiationlevelswere upto19 becquerelspercubicmeterof air:12 timesnormal. The report assumedradioactive materialsfrominsidethe structure spreadtothe surroundingsafterthe roof collapsed.All 225 workersemployedbythe Chernobylcomplex andthe Frenchcompanythatis buildingthe newshelterwere evacuatedshortlyafterthe collapse.Accordingtothe managersof the complex,radiationlevelsaroundthe plantwere atnormal levels(between5 and6 mS/h) andshouldnot affectworkers'health.AccordingtoKupnythe situationwasunderestimatedbythe Chernobyl nuclear complex managers,andinformationwaskeptsecret.[173][174] Radioactive materials and waste management As of 2006, some fuel remainedinthe reactorsat units1 through3, mostof itin eachunit's spentfuel pool,as well assome material inasmall spentfuel interimstorage facilitypond(ISF-1). In 1999 a contract was signedforconstructionof a radioactive waste managementfacilitytostore 25,000 usedfuel assembliesfromunits1–3 and otheroperational wastes,aswell asmaterial from decommissioningunits1–3 (whichwill be the firstRBMKunitsdecommissionedanywhere).The contract includedaprocessingfacilityable tocutthe RBMK fuel assembliesandtoput the material incanisters, whichwere tobe filledwith inertgasandwelded shut. The canisterswere tobe transportedto dry storage vaults,where the fuel containerswouldbe enclosed for upto 100 years.Thisfacility,treating2500 fuel assembliesperyear,wouldbe the firstof itskindfor RBMK fuel.However,afterasignificantpartof the storage structureshadbeenbuilt,technical deficienciesinthe concept emerged,andthe contract wasterminatedin2007. The interimspentfuel storage facility(ISF-2) will nowbe completedbyothersbymid-2013.[citation needed] Anothercontracthas beenletfora liquidradioactive waste treatmentplant,tohandle some 35,000 cubic metersof low- andintermediate-level liquidwastesatthe site.Thiswill needtobe solidifiedand eventuallyburiedalongwithsolidwastes onsite.[citation needed] In January2008, the Ukrainiangovernmentannounceda4-stage decommissioningplanthat incorporatesthe above waste activities andprogressestowardsaclearedsite .[97] Lava-like fuel-containingmaterials(FCMs) Main article:Corium(nuclearreactor) Accordingto official estimates,about95% of the fuel inReactor4 at the time of the accident(about 180 metrictons) remainsinsidethe shelter,withatotal radioactivityof nearly18 millioncuries (670PBq).The radioactive material consistsof core fragments,dust,andlava-like "fuel containingmaterials"(FCM) –alsocalled"corium"– that flowedthroughthe wreckedreactor building before hardeningintoa ceramicform. Three differentlavasare presentinthe basementof the reactorbuilding:black,brown,and a porous ceramic.The lavamaterialsare silicate glasses withinclusionsof othermaterialswithinthem. The porous lavaisbrownlava that droppedintowaterandthuscooledrapidly. It isunclearhowlongthe ceramicformwill retardthe release of radioactivity.From1997 to 2002 a seriesof publishedpaperssuggestedthat the self-irradiationof the lavawouldconvertall 1,200 metric tonsintoa submicrometerandmobile powderwithinafew weeks.[175] Butithasbeenreportedthatthe degradationof the lavaislikelytobe a slow and gradual processratherthan suddenandrapid.[176] The same paperstatesthat the lossof uraniumfromthe wreckedreactorisonly10 kg (22 lb) peryear;this lowrate of uraniumleachingsuggeststhatthe lavaisresistingitsenvironment.[176] The paperalsostates that whenthe shelterisimproved,the leaching rate of the lavawill decrease.[176]
  12. 12. Fukushima Daiichi nuclear disaster Fukushima Daiichi nuclear disaster The Fukushima Daiichi nuclear disaster FukushimaDaiichi ( pronunciation) genshiryoku hatsudenshojiko? ) wasanucleardisasteratthe FukushimaINuclearPowerPlant thatbeganon11 March 2011, resultinginameltdown of three of the plant'ssix nuclearreactors.[6] The failureoccurred whenthe plantwashit bya tsunami triggeredby the magnitude 9.0Tōhokuearthquake.[7] The plant beganreleasingsubstantial amountsof radioactive material on12 March,[8] becomingthe largest nuclearincidentsince the Chernobyl disasterinApril 1986 and the largest(afterChernobyl) to measure Level 7on the International NuclearEventScale,[9] initiallyreleasinganestimated10–30%of the earlierincident'sradiation.[10] InAugust2013, it wasstated[by whom?] thatthe significantamountof radioactive waterwasamongthe most pressingproblemsaffectingthe cleanupprocess,whichis expectedtotake decades.There have beencontinuedspillsof contaminatedwateratthe plant,and some intothe sea.Plantworkersare tryingto lowerthe leaksusingmeasuressuchasbuildingchemical undergroundwalls,buttheyhave notyetimprovedthe situationsignificantly.[11] Nonetheless,tokeep the matter inperspective,the entirerelease of radioactivityintothe seawilladdlessthan0.01% to the backgroundradiation.[12][13] Althoughno shortterm radiationexposure fatalities werereported,[14] some 300,000 people evacuated the area,15,884 (as of 10 February2014[15] ) people dieddue tothe earthquake and tsunami,andas of August 2013 approximately1,600deathswere relatedtothe evacuationconditions, such as livingin temporaryhousingandhospital closures.[16] The exactcause of the majorityof these evacuation-relateddeathswereunspecifiedbecause thatwouldhinderthe deceasedrelatives' applicationforfinancial compensation.[17][18] The WorldHealthOrganization indicatedthatevacuees were exposedtosolittle radiationthat radiation-inducedhealthimpactsare likelytobe below detectable levels,[19] andthatanyadditional cancerriskfromradiationwassmall—extremelysmall,for the most part—andchieflylimitedtothose livingclosesttothe nuclearpowerplant.[20] A 2013 WHO reportpredictsthatfor populationsthatwouldhave stayedandlivedinthe mostaffectedareas,and accordingto the (disputed) LNThypothesis,there wouldhave beena70% higherriskof developing thyroidcancerfor girlsexposedasinfants(butexpertssaidthe overall riskwassmall:the radiation exposure meansabout1.25 out of every100 girlsinthe area coulddevelopthyroidcancerovertheir lifetime,insteadof the natural rate of about0.75 percent),a7% higherriskof leukemiainmales exposedasinfants,a6% higherriskof breastcancer infemalesexposedasinfantsanda 4% higherrisk, overall,of developingsolidcancersforfemales.[21] The WorldHealthOrganizationstatedthata2013 thyroidultrasoundscreeningprogramwas,due tothe screeningeffect,likelytoleadtoanincrease in recordedthyroidcasesdue toearlydetectionof non-symptomaticdiseasecases.[22] The FukushimaNuclearAccidentIndependentInvestigationCommission foundthe nucleardisasterwas "manmade"andthat itsdirectcauseswere all foreseeable.The reportalsofoundthatthe plantwas incapable of withstandingthe earthquakeandtsunami.TEPCO,regulatorsNuclearandIndustrial Safety Agency (NISA) andNSCandthe governmentbodypromotingthe nuclearpowerindustry(METI),all failedtomeetthe mostbasicsafetyrequirements,suchasassessingthe probabilityof damage, preparingforcontainingcollateral damage fromsuchadisaster,anddevelopingevacuation plans.[23][24] A separate studyfoundthatJapanese plantsoperatedbythe largestutilitycompanieswere particularlyunprotectedagainstpotential tsunamis.[7] Overview of incident The plant comprisedsix separate boilingwaterreactors originallydesignedby General Electric(GE) and maintainedbytheTokyoElectricPowerCompany (TEPCO).Units2through6 were BWR-4,while Unit1
  13. 13. was the slightlyolderBWR-3design.[25] Atthe time of the earthquake,Reactor4had beende-fueledand Reactors5 and 6 were in coldshutdownforplannedmaintenance.[26] Immediatelyafterthe earthquake,followinggovernmentregulations,the remainingreactors,1–3, automatically SCRAMmed;control rods shutdownsustained fissionreactions.Althoughfissionstops almostimmediatelywithaSCRAM,fissionproductsinthe fuel continue torelease decayheat,initially about6.5% of full reactorpower.Thisisstill enoughtorequire active reactorcoolingforseveral daysto keepthe fuel rods belowtheirmeltingpoints.In GenerationIIreactors like the GEMark I, cooling systemfailure mayleadtoa meltdown eveninaSCRAMmedreactor.[27] Coincidentwiththe SCRAM,emergencygeneratorswere automaticallyactivatedtopowerelectronics and coolingsystems.The tsunami arrivedsome 50 minutesafterthe initial earthquake.The 14metres (46 ft) hightsunami overwhelmedthe plant's seawall,whichwasonly10 metres(33 ft) high,[7] withthe momentof the tsunami strikingbeingcaughtoncamera.[28] The tsunami waterquicklyfloodedthe low- lyingroomsinwhichthe emergencygeneratorswerehoused.[29] The diesel generators were floodedand beganto fail soonafter,theirjobbeingtakenoverbyemergencybattery-poweredsystems.Whenthe batteriesranout the nextdayon 12 March, active coolingsystemsstopped,andthe reactorsbeganto heatup. The powerfailure alsomeantthatmanyof the reactor control instrumentsalsofailed.[27] As workersstruggledtosupplypowertothe reactors' coolantsystemsand control rooms, multiple hydrogen-airchemical explosionsoccurredfrom12 March to 15 March.[27][30][31] Itis estimated that the hot zirconiumfuel cladding-waterreaction inReactors1-3 produced800 kilograms(1,800 lb) to 1,000 kilograms(2,200 lb) of hydrogengaseach, whichwasventedoutof the reactor pressure vessel andmixedwiththe ambientair.The gaseventuallyreached explosive concentrationlimits in Units1 and 3. EitherpipingconnectionsbetweenUnits3and 4 or fromthe zirconiumreactioninUnit4 itself,[32] Unit4 alsofilledwithhydrogen.Explosionsoccurredinthe uppersecondary containment buildinginall three reactors.[33] TEPCO admittedforthe firsttime on12 October2012 that it hadfailedtotake strongermeasuresto preventdisastersforfearof invitinglawsuitsorprotestsagainstitsnuclearplants.[34][35][36][37] There are no clearplansfor decommissioningthe plant,butthe plantmanagementestimateisthirtyorforty years.[38] On 22 July2013, more thantwo yearsafterthe incident,TEPCOrevealedthatthe plantisleakinghighly radioactive waterintothe PacificOcean.ThishadpreviouslybeendeniedbyTEPCO.[39] The report promptedJapanese PrimeMinisterShinzōAbe toorderthe governmenttostepin.[40] On20 August,ina furtherincident,TEPCOannouncedthat300 metrictons of radioisotope-contaminatedwaterhadleaked froma storage tank.[41] On26 August,the governmenttookcharge of emergencymeasurestoprevent furtherradioactive waterleaks. Background[edit] Followingthe 1999 Tokaimuracriticalityaccident,there wasinterestinJapanfordevelopingradiation- resistantrobotsforuse in the eventof nuclearaccidents- othercountries(e.g.Germany andFrance) alreadyhadthemavailable.The Japanesegovernmentbudgeted3billionyen(US$38 million) for researchanddevelopment.Severalcompaniesproducedstate of the artprototypesin2001, which were testedanddeemedtechnical successes.InDecember2002, a task force (whichincludedTEPCO executives) furtherconcludedthatthe robotswere unnecessary:the possibilityof Chernobyl-scale disasterswascompletelydiscountedanditwasthusassumedthathumanemployees- comparedto whomthe robotshad limitedspeedandrange- wouldstillbe able tooperate inthe eventof an accident.The program halted,andthe prototypesremainedinstorage until March2006; some were subsequentlydonatedto TohokuUniversity.The terminationof the programleftJapanwithout functional radiation-resistantrobotstosendintoFukushimawhenthe crisisbegan.[42] As the crisisunfolded,the Japanesegovernmentsentarequestforrobotsdevelopedbythe U.S. military.The robotswentintothe plants,andtookpicturestohelpassessthe situation,butthey couldn'tperformthe full range of tasksusuallycarriedoutby humanworkers.FollowingFukushima, effortstodevelop humanoidrobots thatcouldsupplementrelief effortshave accelerated dramatically.[43] Similarly,pre-Fukushima,Japan'sNuclearSafetyCommissionsaidinitssafetyguidelinesforlight-water nuclearfacilitiesthat"the potential forextendedlossof powerneednotbe considered."[44] Regulation[edit] Three investigationsintothe Fukushimadisastershowedthe man-madenature of the catastrophe and itsroots inregulatorycapture associatedwitha"networkof corruption,collusion,and nepotism."[45][46] Regulatorycapture referstothe "situation whereregulatorschargedwithpromoting the publicinterestdefertothe wishesandadvance the agendaof the industryor sectortheyostensibly regulate."Those withavestedinterestinspecificpolicyorregulatoryoutcomeslobbyregulatorsand influencetheirchoicesandactions.Regulatorycapture explainswhysome of the risksof operating
  14. 14. nuclearpowerreactorsinJapan were systematicallydownplayedandmismanagedsoastocompromise operational safety.[46] Many reportssay that the governmentsharesblame withthe regulatoryagencyfornotheeding warningsandfor notensuringthe independence of the oversightfunction.[47] The New YorkTimessaid that the Japanese nuclearregulatorysystemsidedwithandpromotedthe nuclearindustrybecause of amakudari ('descentfromheaven') inwhichseniorregulatorsacceptedhighpayingjobsatcompanies theyonce oversaw.Toprotect theirpotential futurepositioninthe industry,regulatorssoughttoavoid takingpositionsthatupsetorembarrassthe companies.TEPCO'spositionasthe largestelectricalutility inJapan made it the mostdesirable positionforretiringregulators.Typicallythe "mostseniorofficials wentto workat Tepco,while those of lowerranksendedupatsmallerutilities."[48] In August2011, several topenergyofficialswere firedbythe Japanese government;affectedpositions includedthe Vice-ministerforEconomy,Trade and Industry;the headof the Nuclearand Industrial SafetyAgency,andthe headof the AgencyforNatural ResourcesandEnergy.[49] Simplifiedcross-sectionsketchof a typical BWRMark I containmentasusedinunits1 to 5. Key: RPV: reactor pressure vessel. DW: dry well enclosingreactorpressure vessel. WW:wetwell - torus-shapedall aroundthe base enclosingsteamsuppressionpool.Excesssteamfrom the dry well entersthe wetwellwaterpool viadowncomerpipes. SFP: spentfuel pool area. SCSW:secondaryconcrete shieldwall. The FukushimaI(Daiichi) NuclearPowerPlantconsistsof six GE lightwater,boilingwater reactors(BWR) witha combinedpowerof 4.7 gigawatts,makingFukushimaDaiichi one of the world's25 largestnuclearpowerstations.FukushimaDaiichiwasthe firstGE-designednuclearplanttobe constructedandrun entirelybythe TokyoElectricPowerCompany (TEPCO). Reactor 1 is a 439 MWe type (BWR-3) reactor constructedinJuly1967. It commencedoperationon26 March 1971.[50] It wasdesignedtowithstandanearthquake witha peakgroundacceleration of 0.18 g (1.74 m/s2 ) and a response spectrumbasedonthe 1952 KernCountyearthquake.[51] Reactors2 and 3 are both 784 MWe type BWR-4.Reactor 2 commencedoperatinginJuly1974, andReactor 3 in March 1976. The earthquake designbasisforall unitsrangedfrom0.42 g (4.12 m/s2 ) to 0.46 g (4.52 m/s2 ).[52][53] All unitswere inspectedafterthe 1978 Miyagi earthquake whenthe groundacceleration reached 0.125 g (1.22 m/s2 ) for30 seconds,butno damage to the critical parts of the reactorwas discovered.[51] Units1–5 have a Mark 1 type (lightbulb torus) containmentstructure;unit6has Mark 2 type (over/under) containmentstructure.[51] InSeptember2010, Reactor 3 waspartiallyfueledby mixed- oxides(MOX).[54] At the time of the accident, the unitsand central storage facilitycontainedthe followingnumbersof fuel assemblies:[55] There isno MOX fuel inany of the coolingponds.The onlyMOX fuel isloadedinthe Unit3 reactor. Coolingrequirements These reactorsgenerate electricitybyusingthe heatof the fissionreactiontocreate steam.Whenthe reactor stopsoperating,the radioactive decay of unstableisotopescontinuestogenerate heatfora time.Thisdecayand the decayheatthat resultsrequirescontinuedcooling.[59][60] Initiallythisdecayheat amountsto approximately6%of the amountproducedby fission,[59] decreasingoverseveraldaysbefore reachingcoldshutdown levels.[61] Exhaustedfuel rodsthathave reachedcoldshutdowntemperaturestypicallyrequireseveral yearsin a spentfuel pool before theycan be safelytransferredto drycask storagevessels.[62] The decay heatinthe Unit4 spentfuel pool hadthe capacityto boil about70 tonnesof waterper day (12 gallonsperminute).[63] On16 April 2011, TEPCOdeclaredthatcoolingsystemsforUnits1-4 were beyondrepairandwouldhave tobe replaced.[64] Coolingsystems[edit] In the reactor core,circulationisaccomplishedviahighpressure systemsthatcycle waterbetweenthe reactor pressure vessel and heatexchangers.Thesesystemsthentransferheattoa secondaryheat exchangerviathe essentialservice watersystem,usingwaterthatispumpedoutto seaor an onsite coolingtower.[65] Whenthe reactor isnot producingelectricity,coolingpumpscanbe poweredbyotherreactorunits,the gridor bydiesel generatorsorbatteries.[66][67] Units2 and 3 were equippedwithsteam-turbinedriven emergencycore coolingsystems thatcanbe directlyoperatedbysteam producedbydecayheatandwhichcan injectwaterdirectlyintothe reactor.[68] Some electrical powerisneededtooperate valvesandmonitoringsystems.
  15. 15. Unit 1 was equippedwithadifferentcoolingsystem, the "IsolationCondenser"or"IC",whichisentirely passive.Thisconsistsof aseriesof pipesrunfromthe reactor core to the inside of alarge tank of water. Whenthe valvesare opened,steamflowsupwardtothe ICwhere the cool waterinthe tank condenses the steamback to water,and itruns undergravityback to the reactor core.For reasonsthatare unclear, at the beginning,Unit1's ICwas operatedonlyintermittentlyduringthe emergency.However,duringa 25 March 2014 presentationtothe TVA,Dr Takeyuki Inagaki explainedthatthe ICwas beingoperated intermittentlytomaintainreactorvessel level andtopreventthe core fromcoolingtooquicklywhich can increase reactorpower.Unfortunately,asthe tsunami engulfedthe station,the ICvalveswere closedandcouldnot be reopeneddue tothe lossof power. Backup generators[edit] Two emergencydieselgeneratorswere availableforeachof units1–5 and three forunit6.[69] In the late 1990s, three additional backupgeneratorsforUnits2 and4 were placedinnew buildings locatedhigheronthe hillside,tocomplywithnew regulatoryrequirements.All six unitswere given access to these generators,butthe switchingstationsthatsentpowerfromthese backupgeneratorsto the reactors' coolingsystemsforUnits1 through5 were still inthe poorlyprotectedturbinebuildings. All three of the generatorsaddedinthe late 1990s were operational afterthe tsunami.If the switching stationshadbeenmovedtoinside the reactorbuildingsortoother flood-proof locations,powerwould have beenprovidedbythese generatorstothe reactors'coolingsystems.Because the generatorshadto workat full powerwhenthe wave hitthe crankshaftsshatteredandthe systemcollapsed.Thesebrittle crankshaftsare also usedinBritishreactors.[70] The reactor's emergencydiesel generatorsandDCbatteries,crucial componentsinpoweringcooling systemsaftera powerloss,were locatedinthe basementsof the reactorturbine buildings,in accordance withGE's specifications.Mid-levelengineersexpressedconcernsthatthisleftthem vulnerable toflooding.[71] FukushimaIwasnot designedforsucha large tsunami,[72][73] norhadthe reactors beenmodified whenconcernswere raisedinJapan andby the IAEA.[74] FukushimaIIwasalsostruck by the tsunami.However,ithadincorporateddesignchangesthat improveditsresistance toflooding,reducingflooddamage.Generatorsandrelatedelectrical distributionequipmentwere locatedinthe watertightreactorbuilding,sothatpowerfromthe electricitygrid wasbeingusedbymidnight.[75] Seawaterpumpsforcoolingwere protectedfrom flooding,andalthough3of 4 initiallyfailed,theywererestoredtooperation.[76] Central fuel storage areas[edit] Usedfuel assembliestakenfromreactorsare initiallystoredforatleast18 monthsinthe poolsadjacent to theirreactors.Theycan thenbe transferredtothe central fuel storage pond.[3] FukushimaI'sstorage area contains6375 fuel assemblies.Afterfurthercooling,fuelcanbe transferredtodrycask storage, whichhas shownnosignsof abnormalities.[77] Zircaloy[edit] Many of the internal componentsandfuel assemblycladdingare made from zircaloy because itis relativelytransparenttoneutrons.Atnormal operatingtemperaturesof approximately300°C (572 °F), zircaloyisinert.However,above 1200 degreesCelsius,zirconiummetalcanreactexothermicallywith waterto form free hydrogen gas.[78] The reactionbetweenzirconiumandthe coolantproducesmore heat,acceleratingthe reaction.[79] Safety issues[edit] 1967: Layout of the emergency-coolingsystem[edit] On 27 February2012, NISA orderedTEPCOto reportby 12 March 2012 regardingitsreasoningin changingthe pipinglayoutforthe emergencycoolingsystem.Thesechangeswere made afterthe plans were registeredin1966 and the beginningof construction. The original plansseparatedthe pipingsystemsfortworeactorsinthe isolationcondenserfromeach other.However,the applicationforapproval of the constructionplanshowedthe twopipingsystems connectedoutside the reactor.The changeswere notnoted,inviolationof regulations.[80] Afterthe tsunami,the isolationcondensershouldhave taken overthe functionof the coolingpumps,by condensingthe steamfromthe pressure vessel intowatertobe usedforcoolingthe reactor.But the condenserdidnotfunctionproperlyandTEPCOcouldnotconfirmwhetheravalve wasopened. 1976: Falsificationofsafety records[edit] FukushimaDaiichi wascentral toa falsified-recordsscandal thatledtothe departure of seniorTEPCO executives.Italsoledtodisclosuresof previouslyunreportedproblems,[81] althoughtestimonybyDale Bridenbaugh,aleadGE designer,claimedthatGE waswarnedof majordesign flawsin1976, resultingin the resignationsof several GEdesigners whoprotestedGE'snegligence.[82][83][84] In 2002, TEPCO admittedfalsifyingsafetyrecordsforunit1.The scandal and a fuel leakatFukushima Daini forcedthe companyto shutdownall 17 of itsreactors.[85] A powerboard distributingelectricityto
  16. 16. temperature control valveswasnotexaminedfor11years.Inspectionsdidnotcovercoolingsystems devicessuchaswater pumpmotorsand diesel generators.[86] 1991: Back-up generator of reactor 1 flooded[edit] On 30 October1991, one of two backupgeneratorsof Reactor1 failed,afterfloodinginthe reactor's basement.Seawaterusedforcoolingleakedintothe turbine buildingfromacorrodedpipe at20 cubic metersperhour,as reported byformeremployeesinDecember2011. An engineerwasquotedas sayingthat he informedhissuperiorsandof the possibilitythatatsunami coulddamage the generators. TEPCO installeddoorstopreventwaterfromleakingintothe generatorrooms. The Japanese NuclearSafetyCommission commentedthatitwouldreviseitssafetyguidelinesand wouldrequire the installationof additional powersources.On29 December2011, TEPCO admittedall these facts:itsreportmentionedthatthe roomwas floodedthroughadoorand some holesforcables, but the powersupplywasnotcut off by the flooding,andthe reactorwasstoppedforone day. One of the two powersourceswascompletelysubmerged,butitsdrive mechanismhadremained unaffected.[87] 2008: Tsunami study ignored[edit] In 2007, TEPCO setup a departmenttosuperviseitsnuclearfacilities.Until June 2011 its chairmanwas Masao Yoshida,the FukushimaDaiichi chief.A 2008 in-house studyidentifiedanimmediate needto betterprotectthe facilityfromfloodingbyseawater.Thisstudymentionedthe possibilityof tsunami- wavesupto 10.2 metres(33 ft).Headquartersofficialsinsistedthatsucha riskwas unrealisticanddid not take the predictionseriously.[88][verification needed] A Mr. Okamura of the Active FaultandEarthquake ResearchCenterurgedTEPCOandNISA to review theirassumptionof possible tsunamiheightsbasedonatenthcenturyearthquake,butitwasnot seriouslyconsideredatthattime.[89] The U.S.NuclearRegulatoryCommission warnedof ariskof losing emergencypowerin1991 (NUREG-1150) and NISA referredtothe reportin2004. No actionto mitigate the riskwas taken.[90] Location[edit] The plant waslocatedinJapan,which,like the restof the PacificRim,isin an active seismiczone. The International AtomicEnergyAgency (IAEA) hadexpressedconcernaboutthe abilityof Japan's nuclearplantsto withstandseismicactivity.Ata2008 meetingof the G8's NuclearSafetyandSecurity Group inTokyo,an IAEA expertwarnedthata strong earthquake witha magnitude above 7.0couldpose a "seriousproblem"forJapan'snuclearpowerstations.[91] The regionhadexperiencedthree earthquakesof magnitude greaterthan8, includingthe 869 JoganSanrikuearthquake,the 1896 Meiji- Sanrikuearthquake,andthe 1933 Sanrikuearthquake.[citation needed] Earthquake[edit] The 9.0 MW Tōhokuearthquake occurredat14:46 on Friday,11 March 2011 with epicenternearHonshu Island.[92] Itproducedmaximumground g-forcesof 0.56, 0.52, 0.56 (5.50, 5.07 and 5.48 m/s2 ) at units2, 3 and 5 respectively.Thisexceededtheirdesigntolerancesof 0.45, 0.45 and 0.46 g (4.38, 4.41 and 4.52 m/s2 ).The shock valueswere withinthe designtolerancesatunits1,4 and 6.[53] Whenthe earthquake struck,units1, 2 and 3 were operating,butunits4,5 and 6 had beenshutdown for periodicinspection.[52][93] Reactors1,2 and 3 immediatelyunderwentanautomaticshutdown (calledSCRAM).[94][95] Whenthe reactors shutdown,the plantstoppedgeneratingelectricity,cuttingoff power.[96] One of the twoconnectionstooff-site powerforunits1–3 also failed,[96] so13 on-site emergencydieselgenerators beganprovidingpower.[97] Tsunami The earthquake triggereda13-to-15-metre (43 to 49 ft) maximumheighttsunami thatarrived approximately50 minuteslater.The wavesovertoppedthe plant's10 metres (33 ft) seawall,[98][99][100] floodingthe basementsof the turbine buildingsanddisablingthe emergency diesel generators[69][101][102] atapproximately15:41.[96][103] TEPCO thennotifiedauthoritiesof a"firstlevel emergency".[94] The switchingstationsthatprovidedpowerfromthe three backupgeneratorslocatedhigheronthe hillsidefailedwhenthe buildingthathousedthemflooded.[70] Powerforcontrol systemsswitchedover to batteriesthatwere designedtolastabouteighthours.[104] Furtherbatteriesandmobilegenerators were dispatchedtothe site.Theywere delayedbypoorroadconditionsandthe firstarrivedonlyat 21:00 11 March,[97][105] almostsix hoursafterthe tsunami. Multiple unsuccessful attemptswere made toconnectportable generatingequipmenttopowerwater pumps.The failure wasattributedtofloodingatthe connectionpointinthe Turbine Hall basementand the absence of suitable cables.[101] TEPCOswitcheditseffortstoinstallingnew linesfromthe grid.[106] One generatoratunit6 resumedoperationon17 March, while external powerreturnedto units5 and6 onlyon 20 March.[107]
  17. 17. Units1, 2 and 3[edit] In Reactors1, 2 and 3, overheatingcausedareactionbetweenthe waterandthezircaloy,creating hydrogengas.[112][113][114] On 12 March, an explosioninUnit1 wascausedby the ignitionof the hydrogen,destroyingthe upper part of the building. On 14 March, a similarexplosionoccurredinthe Reactor3 building,blowingoff the roof andinjuring elevenpeople. On the 15th, an explosioninthe Reactor2 buildingdamageditandpartof the Reactor 4 building. Core meltdowns[edit] There existsconsiderableuncertaintyaboutthe amountof damage the reactor cores sustainedduring the accident– TEPCOrevisedseveral timesoverthe pastyearsthe estimatesaboutthe extentof the core meltforthe three affectedreactorunitsandthe locationof the moltennuclearfuel ("Corium") withinthe containmentbuildings.[115] Asof 2015 it can be assumedthatmostfuel hasmeltedthrough the Reactor Pressure Vessel (RPV,commonlyknownasthe "reactorcore") and isrestingonthe bottom of the PrimaryContainmentVessel (PCV),havingbeingstoppedbythe concrete of the PCV.[116][117][118][119] On 16 March 2011 TEPCO estimatedthat70% of the fuel inUnit 1 had melted,and33% inUnit 2, furthersuspectingthatUnit3's core mightalsobe damaged.[120] In the TEPCO reportof the ModularAccident AnalysisProgram(MAAP) fromNovember2011 further estimatesare made tothe state andlocationof the fuel.[121] The reportcame tothe conclusionthatthe RPV in Unit1 had beendamagedduringthe disaster,andthat"significantamounts"of moltenfuel had fallenintothe bottomof the PCV – the erosion of the concrete of the PCV by the moltenfuel afterthe core meltdownwasestimatedtohave beenstoppedinapprox.0.7metres(2 ft 4 in) depth,withthe thicknessof the containmentbeing7.6metres(25 ft).Gas samplingdone before the reportdetectedno signsof an ongoingreactionof the fuel withthe concrete of the PCV and all the fuel inUnit1 was estimatedtobe "well cooleddown,includingthe fuel droppedonthe bottomof the reactor". Furthermore the 2011 MAAP reportshowedthatfuel inUnits2 and 3 had melted,howeverlessthan Unit 1, and fuel waspresumedtobe still inthe RPV,withnosignificantamountsof fuelfallentothe bottomof the PCV.The reportfurthersuggestedthat"there isa range inthe evaluationresults"from "all fuel in the RPV (none fuel fallentothe PCV)"inUnit2 and Unit 3, to "most fuel inthe RPV (some fuel inPCV)".ForUnit 2 and Unit3 it wasestimatedthatthe "fuel iscooledsufficiently".The largerdamage inUnit 1 incomparisonwiththe othertwounitswas accordingto the reportdue to longertime thatno coolingwaterwasinjectedinUnit1, whichresultedinmuchmore decayheatto accumulate – for about 1 daythere was nowater injectionforUnit1, while Unit2 andUnit 3 had onlya quarterof a day withoutwaterinjection. In November2013 Mari Yamaguchi reportedforAssociatedPressthatthere are computersimulations whichshowthat "the meltedfuel inUnit1,whose core damage was the mostextensive,hasbreached the bottomof the primarycontainmentvessel andevenpartiallyeatenintoitsconcrete foundation, comingwithinabout30 centimeters(one foot) of leakingintothe ground" – a KyotoUniversitynuclear engineersaidwithregardstothese estimates:"We justcan'tbe sure until we actuallysee the inside of the reactors."[115] Accordingto a December2013 reportTEPCO estimatedforUnit1 that"the decayheatmusthave decreasedenough,the moltenfuelcanbe assumedtoremaininPCV (Primarycontainervessel)".[116] In August2014 TEPCOreleasedanewrevisedestimate thatreactor3 had a complete meltthroughin the initial phase of the accident.Accordingtothisnew estimate withinthe firstthree daysof the accidentthe entire core contentof reactor 3 hadmeltedthroughthe RPV andfallentothe bottomof the PCV.[118][119][122] Theseestimateswere basedonasimulation,whichindicatedthatreactor3's melted core penetratedthrough1.2metres(3 ft11 in) of the PCV'sconcrete base,and came close to 26–68 centimetres(10–27 in) of the PCV'ssteel wall.[117] In February2015 TEPCO startedthe "Muonscanning"processfor Units1, 2 and 3.[123][124] Withthis scanningsetupitwill be possibletodetermine the approximateamountandlocationof the remaining nuclearfuel withinthe RPV,butnotthe amountandrestingplace of the Coriumin the PCV. Aerial viewof the stationin1975, showingseparationbetweenunits5and 6, and 1-4. ・Unit6, notcompleteduntil 1979, isseenunderconstruction. Unit4[edit] All fuel rodsfromUnit4 had beentransferredtothe spentfuel pool onanupperfloorof the reactor buildingpriortothe tsunami.On15 March, an explosiondamagedthe fourthfloorrooftopareaof Unit 4, creatingtwo large holesina wall of the outerbuilding.Itwasreportedthatwaterinthe spentfuel pool mightbe boiling.Radiationinside the Unit4 control roompreventedworkersfromstayingtherefor
  18. 18. longperiods.Visual inspectionof the spentfuelpool on30 April revealednosignificantdamage tothe rods.A radiochemical examinationof the pondwaterconfirmedthatlittle of the fuel hadbeen damaged.[125] In October2012, the formerJapanese AmbassadortobothSwitzerlandandSenegalMitsuheiMurata saidthat groundunderFukushimaunit4 wassinking,andthe structure maycollapse.[126][127] In Novemberof 2013 TEPCOstartedthe processof movingthe 1533 fuel rodsinthe Unit 4 coolingpool to the central pool.Thisprocesswascompletedon22 Decemberof 2014.[128] Units5 and 6[edit] Reactors5 and 6 were alsonotoperatingwhenthe earthquake struck.UnlikeReactor4,theirfuel rods remainedinthe reactor.The reactors hadbeencloselymonitored,ascoolingprocesseswere not functioningwell.[citation needed] Central fuel storage areas[edit] On 21 March, temperaturesinthe fuel pondhadrisenslightly,to61 °C and waterwas sprayedoverthe pool.[3] Powerwasrestoredtocoolingsystemson24 March andby 28 March temperatureswere reporteddownto35 °C.[129] Radioactive material wasreleasedfromthe containmentvesselsforseveral reasons:deliberateventing to reduce gas pressure,deliberate discharge of coolantwaterintothe sea,anduncontrolledevents. Concernsaboutthe possibilityof alarge scale release ledtoa 20-kilometre (12mi) exclusionzone aroundthe powerplantandrecommendationsthatpeople within the surrounding20–30 km zone stay indoors.Later,the UK, France and some othercountriestoldtheirnationalstoconsiderleavingTokyo,in response tofearsof spreadingcontamination.[130] Trace amountsof radiation,including iodine- 131, caesium-134and caesium-137,were widelyobserved.[131][132][133] Between21 March and mid-Julyaround2.7× 1016 Bq of caesium-137(about8.4 kg) enteredthe ocean, about82 percenthavingflowedintothe seabefore 8April.[134] Thisemissionof radioactivityintothe sea representsthe mostimportantindividual emissionof artificial radioactivityintothe seaeverobserved. However,the Fukushimacoasthassome of the world'sstrongestcurrentsandthese transportedthe contaminatedwatersfarinto the PacificOcean,thuscausinggreatdispersionof the radioactive elements.The resultsof measurementsof boththe seawaterandthe coastal sedimentsledtothe suppositionthatthe consequencesof the accident,intermsof radioactivity,wouldbe minorformarine life asof autumn 2011 (weakconcentrationof radioactivityinthe waterandlimitedaccumulationin sediments).Onthe otherhand,significantpollutionof seawateralongthe coast nearthe nuclearplant mightpersist,because of the continuingarrival of radioactive material transportedtowardsthe seaby surface waterrunningovercontaminatedsoil.Organismsthatfilterwaterandfishat the topof the food chainare, overtime,the mostsensitivetocaesiumpollution.Itisthusjustified tomaintainsurveillance of marine life thatisfishedinthe coastal watersoff Fukushima.Despite caesiumisotopicconcentration inthe watersoff of Japan being10 to 1000 timesabove concentrationpriortothe accident,radiation risksare belowwhat isgenerallyconsideredharmfultomarine animalsandhumanconsumers.[135] A monitoringsystemoperatedbythe PreparatoryCommissionforthe Comprehensive Nuclear-Test-Ban TreatyOrganization (CTBTO) trackedthe spreadof radioactivityona global scale.Radioactiveisotopes were pickedupbyover40 monitoringstations.[136] On 12 March, radioactive releasesfirstreachedaCTBTO monitoringstationinTakasaki,Japan,around 200 kmaway. The radioactive isotopesappearedineasternRussiaon14 March and the westcoast of the UnitedStatestwodays later.By day15, traces of radioactivitywere detectableall acrossthe northernhemisphere.Withinone month,radioactive particleswere notedbyCTBTOstationsinthe southernhemisphere.[137][138] Estimatesof radioactivityreleasedrangedfrom10-40%[10][139][140][141] of thatof Chernobyl's.The significantlycontaminatedareawas10[10] -12%[139] thatof Chernobyl.[10][142][143] In March 2011, Japanese officialsannouncedthat"radioactive iodine-131exceedingsafetylimitsfor infantshadbeendetectedat18 water-purificationplantsinTokyoandfive otherprefectures".[144] On21 March the firstrestrictionswere placedonthe distributionandconsumptionof contaminated items.[145] Asof July2011, the Japanese governmentwasunable tocontrol the spreadof radioactive material intothe nation'sfoodsupply.Ratedinfoodproducedin2011, includingspinach,tealeaves, milk,fishandbeef,upto320 kilometresfromthe plant.2012 cropsdid notshow signsof radioactivity contamination.Cabbage,rice[146] andbeef showedinsignificantradiationlevels.A Fukushima-produced rice marketin Tokyowas acceptedbyconsumersassafe.[146] On 24 August2011, the NuclearSafetyCommission(NSC) of Japanpublishedthe resultsof the recalculationof the total amountof radioactive materialsreleasedintothe airduringthe accidentat the FukushimaDaiichi NuclearPowerStation.The total amountsreleasedbetween11 March and5 April were reviseddownwardsto130 PBq(petabecquerels,3.5megacuries) foriodine-131and 11 PBq for
  19. 19. caesium-137,whichisabout11% of Chernobyl emissions.Earlierestimationswere 150 PBq and 12 PBq.[147][148] In 2011 scientistsworkingforthe JapanAtomicEnergyAgency,KyotoUniversityandother institutes, recalculatedthe amountof radioactive materialreleasedintothe ocean:betweenlate Marchthrough April theyfoundatotal of 15 PBq for the combinedamountof iodine-131andcaesium-137,more than triple the 4.72 PBqestimatedbyTEPCO.The companyhad calculatedonlythe directreleasesintothe sea.The newcalculationsincorporatedthe portionof airborne radioactive substancesthatenteredthe oceanas rain.[149] In the firsthalf of September2011 TEPCOestimatedradiationreleaseatsome 200 MBq (megabecquerels,5.4millicuries) perhour.Thiswasapproximatelyone four-millionththatof March.[150] Tracesof iodine-131were detectedinseveral JapaneseprefecturesinNovember[151] and December2011.[152] Accordingto the FrenchInstitute forRadiologicalProtectionandNuclearSafety,between21March and mid-Julyaround27 PBqof caesium-137enteredthe ocean,about82 percentbefore 8 April.This emissionrepresentsthe mostimportantindividual oceanicemissionsof artificialradioactivityever observed.The Fukushimacoasthasone of the world'sstrongestcurrents(KuroshioCurrent).It transportedthe contaminatedwatersfarintothe PacificOcean,dispersingthe radioactivity.Asof late 2011 measurementsof boththe seawaterandthe coastal sedimentssuggestedthatthe consequences for marine life wouldbe minor.Significantpollutionalongthe coastnearthe plantmightpersist, because of the continuingarrival of radioactive material transportedtothe seaby surface water crossingcontaminatedsoil.The possiblepresence of otherradioactivesubstances,suchas strontium- 90 or plutonium,hasnotbeensufficientlystudied.Recentmeasurementsshow persistent contaminationof some marine species(mostlyfish) caughtalongthe Fukushima coast.[153] Migratory pelagicspecies are highlyeffectiveandrapidtransportersof radiationthroughout the ocean.Elevatedlevelsof 134 Cs appearedinmigratoryspeciesoff the coastof Californiathatwere not seenpre-Fukushima.[154] As of March 2012, nocases of radiation-relatedailmentshadbeenreported.Expertscautionedthat data was insufficienttoallowconclusionsonhealthimpacts.MichiakiKai,professorof radiation protectionatOita Universityof NursingandHealthSciences,stated,"If the currentradiationdose estimatesare correct,(cancer-relateddeaths)likelywon'tincrease."[155] In May 2012, TEPCO releasedtheirestimateof cumulative radiationreleases.Anestimated538.1 PBqof iodine-131,caesium-134andcaesium-137was released.520PBq wasreleasedintothe atmosphere between12–31 March 2011 and 18.1 PBqintothe ocean from26 March – 30 September2011. A total of 511 PBq of iodine-131wasreleasedintoboththe atmosphere andthe ocean,13.5 PBq of caesium- 134 and13.6 PBqof caesium-137.[156] TEPCOreportedthatat least900 PBq hadbeenreleased"intothe atmosphere inMarch lastyear [2011] alone".[157][158] In 2012 researchersfromthe Institute of Problemsinthe Safe Developmentof NuclearEnergy,of the RussianAcademyof Sciences,andthe Hydrometeorological Centerof Russiaconcludedthat"onMarch 15, 2011, ~400PBq iodine,~100PBqcesium, and~400PBq inertgasesenteredthe atmosphere"onthat day alone.[159] In August2012, researchersfoundthat10,000 nearbyresidentshadbeenexposedtolessthan 1 millisievertof radiation,significantlylessthanChernobyl residents.[160] As of October2012 radiationwasstill leakingintothe ocean.Fishinginthe watersaroundthe site was still prohibited,andthe levelsof radioactive 134 Csand137 Csinthe fishcaughtwere notlowerthan immediatelyafterthe disaster.[161] On 26 October2012 TEPCO admittedthatitcouldnot stopradioactive material enteringthe ocean, althoughemissionrateshadstabilised.Undetectedleakscouldnotbe ruledout,because the reactor basementsremainedflooded.The companywasbuildinga2,400-foot-longsteel andconcrete wall betweenthe site andthe ocean,reaching100 feetbelow ground,butitwouldnotbe finishedbefore mid-2014. AroundAugust2012 two greenlingwere caughtclose toshore.Theycontainedmore than 25,000 becquerels(0.67millicuries) of caesium-137perkilogram, the highestmeasuredsince the disasterand250 timesthe government'ssafetylimit.[162][163] On 22 July2013 itwas revealedthatthe plantcontinuedtoleakradioactivewaterintothe ocean, somethinglongsuspectedbylocal fishermenandindependentinvestigators.[39] TEPCOhadpreviously deniedthatthiswashappening.Japanese PrimeMinister ShinzōAbe orderedthe governmenttostep in.[40] On 20 August,ina furtherincident,itwasannouncedthat300 metrictons of heavilycontaminated waterhad leakedfromastorage tank,[41] approximatelythe same amountof wateras one eighth(1/8) of thatfoundinan Olympic-sizeswimmingpool.[164][165] The 300 metrictons of waterwas radioactive enoughtobe hazardousto nearbystaff,andthe leakwasassessedasLevel 3 on the International NuclearEventScale.[166]
  20. 20. On 26 August,the governmenttookcharge of emergencymeasurestopreventfurtherradioactivewater leaks,reflectingtheirlackof confidence in TEPCO.[167] As of 2013, about400 tonnesperdayof coolingwaterwasbeingpumpedintothe reactors.Another400 tonnesof groundwaterwasseepingintothe structure.Some 800tonnesof water perday wasremoved for treatment,half of whichwasreusedforcoolingandhalf divertedtostorage tanks.[168] Ultimatelythe contaminatedwater,aftertreatmenttoremove radionuclidesotherthan tritium,mayhave tobe dumpedintoPacific.[38] TEPCOintendtocreate an undergroundice wall toreduce the rate contaminatedgroundwaterreachesthe sea.[169] In February2014, NHK reportedthatTEPCO wasreviewingitsradiationdata,afterfindingmuchhigher levelsof radiationthanwasreported earlier.TEPCOnow saysthatlevelsof 5 millionbecquerels(0.12 millicuries)of strontiumperliterwere detectedingroundwatercollectedinJuly2013 and not 900,000 becquerels(0.02millicuries),asinitiallyreported.[170][171] Contaminationin the easternPacific[edit] In March 2014, numerousnewssources,including NBC,[172] beganpredictingthatthe radioactive underwaterplumetravelingthroughthe PacificOcean wouldreachthe westernseaboardof the continental UnitedStates.The commonstorywasthatthe amountof radioactivitywouldbe harmlessandtemporaryonce itarrived.The National OceanicandAtmospheric Administration measuredcesium-134at pointsinthe PacificOceanandmodelswere citedinpredictions by several governmentagenciestoannounce thatthe radiationwouldnotbe ahealthhazardfor North Americanresidents.Groupsincluding BeyondNuclearandthe TillamookEstuariesPartnership challengedthesepredictionsonthe basisof continuedisotopereleasesafter2011, leadingtoa demand for more recentandcomprehensivemeasurementsasthe radiationmade itswaywest.These measurementswere takenbyacooperative groupof organizationsunderthe guidance of amarine chemistwiththe WoodsHole OceanographicInstitution,anditwasrevealedthattotal radiationlevels, of whichonlyafractionbore the fingerprintof Fukushima,were nothighenoughtopose anydirectrisk to humanlife andinfact were far lessthan Environmental ProtectionAgency guidelinesorseveral other sourcesof radiationexposure deemedsafe.[173] A 2012 analysisof the intermediate andlong-livedradiationreleasedfoundabout10-20% of that releasedfromthe Chernobyldisaster.[189][190] Approximately15PBq of caesium-137was released,[191] comparedwithapproximately85PBq of caesium-137at Chernobyl,[192] indicatingthe release of 24 kilograms(53 lb) of caesium-137.[193] Unlike Chernobyl,all Japanesereactorswere inconcrete containmentvessels,whichlimitedthe release of strontium-90,americium-241andplutonium,whichwere amongthe radioisotopes releasedbythe earlierincident.[189][192] Some 500 PBq of iodine-131were released,[191] comparedtoapproximately1,760 PBq at Chernobyl.[192] Iodine-131has ahalf life of 8.02 days,decayingintoa stable nuclide.Aftertenhalf lives (80.2 days),99.9% has decayedto xenon-131,a stable isotope.[194] Risks from radiation[edit] Veryfewcancerswouldbe expectedasaresultof accumulatedradiationexposures,[195][196][197] even thoughpeople inthe areaworstaffectedbyJapan'sFukushimanuclearaccidenthave aslightlyhigher riskof developingcertaincancerssuchasleukemia,solidcancers,thyroidcancerandbreastcancer.[14] Estimatedeffectivedosesfromthe accidentoutsideof Japanare consideredtobe below (orfarbelow) the dose levelsregardedasverysmall bythe international radiological protectioncommunity.[198] In 2013 WHO reportedthatarea residentswhowere evacuatedwere exposedtosolittle radiationthat radiationinducedhealthimpactswere likelytobe below detectable levels.[19][21] The healthriskswere calculatedbyapplyingconservative assumptions,includingthe conservative linearno-threshold model of radiationexposure,amodel thatassumeseventhe smallestamountof radiationexposure will cause a negative healtheffect.[199][200] The reportindicatedthatforthose infantsinthe mostaffectedareas, lifetimecancerriskwouldincrease byabout1%.[21][201] Itpredictedthatpopulationsinthe most contaminatedareasfaceda 70% higherrelative riskof developingthyroidcancerforfemalesexposedas infants,anda 7% higherrelative riskof leukemiainmalesexposedasinfantsanda6% higherrelative riskof breastcancer infemalesexposedasinfants.[20] One-thirdof involvedemergencyworkerswould have increasedcancerrisks.[202][203] Cancer risksforfetuses were similartothose in1 yearold infants.[204] The estimatedcancerriskto childrenandadultswaslowerthaninfants.[205] The statedriskswere relative andnotabsolute.The baseline riskof thyroidcancerinfemalesis0.75%,predictedtoincrease to1.25%, a "70% higherrelative risk".[203] Thisimpliesanestimatedincreaseof only15 inthe numberof female thyroidcancercases(and approximatelyfivemale cases).Asthe five-yearnon-survival rate forthyroidcanceris4.2% and falling rapidly(halvingeachdecade),[206] itismore likelythannotthatthe numberof eventual deathswill be zero.
  21. 21. These percentagesrepresentestimatedrelativeincreasesoverthe baseline ratesandare not absolute risksfor developingsuchcancers.Due to the low baseline ratesof thyroidcancer,evenalarge relative increase representsasmall absolute increaseinrisks.Forexample,the baseline lifetime riskof thyroid cancer for femalesisjust(0.75%) three-quartersof one percentandthe additionallifetimerisk estimatedinthisassessmentforafemale infantexposedinthe mostaffectedlocationis(0.5%)one-half of one percent.[203] Accordingto a linearno-thresholdmodel (LNTmodel) the accidentwouldmostlikelycause 130 cancer deaths.[207][208] RadiationepidemiologistRoyShore counteredthatestimatinghealtheffectsfromthe LNT model "isnotwise because of the uncertainties".[209] The LNTmodel greatlyoverestimatedcasualties fromChernobyl,HiroshimaorNagasaki.Evidence thatthe LNTmodel wasinvalidhasexistedsince 1946 and wassuppressedbyNobel Prizewinner HermannMuller.[210][211][212] In April 2014 studiesconfirmedthe presence of radioactive tunaoff the coastsof the pacific U.S.[213] Researcherscarriedouttestson26 albacore tuna caught priorto the 2011 powerplantdisaster and those caughtafter.Althoughlevelswere small,lessthanone wouldgetfromeatingabanana, evidence isstillpresentonthe fishfromthe Fukushimanucleardisaster.

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