Joel mc

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

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.

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

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

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

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

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]

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

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

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.

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]

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

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

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.

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

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]

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

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

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]

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.

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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