ARCHIVESHISTORIQUES
DELACOMMISSION
COLLECTION RELIEEDES
DOCUMENTS "COM"
COM(79)238
Vol.1979/0096

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COMMISSION OPTHEEUROPEAN COMMUNITIES
COM(79)238 final
Brussels ,4thMay 1979
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REPORT ONTHE ACCIDENT ATTHE "THREE MILE ISLAND "
NUCLEAR POWER STATION
?
C0M(79 >238 final

COM(79)238
INTRODUCTION
Following theaccident which occurred ,atthe "Three Mile Island "nuolear
power plant intheUS,theCommission oftheEuropean Communities decided ,
asaninitial measure ,tosend twoofficials totheUSinorder toobtain
information onthe spot *
Several Member States andother countries had also thought itnecessary
tohave representatives atthescene oftheaocident .Asaresult ,
Belgium ,Prance ,Italy ,theNetherlands ,theFederal Republic ofGermany
and theUnited Kingdom were represented .
TheNuclear Regulatory Commission ,theFederal organization responsible
fornuolear safety ,wasanxious tomake public assoon aspossible
information concerning thecourse oftheaccident *TheOffice of
International Programmes ,inparticular ,endeavoured toprovide the
foreign visitors with preliminary information asthis became available *
Briefing sessions arranged with NRCexperts onspeoifio .aspeots ofthe
accident made it -possible toexchange opinions andclarify anumber of
points *
On5April 1979 »"theNRCarranged avisit totheaotual site during which
Mr*Harold Denton ,principal NRCofficial atthepower station ,gave an
account inMiddletown ,thelocality closest tothepower plant ,ofthe
situation intheplant atthat moment *
The report ontheaccident and theassociated events isbased onthe
preliminary written andoral information supplied bytheNRCandonthe
information contained inthe "NUREG "reports published bytheNRCin
respect -ofthepower plant inquestion *Itwaspossible tooonsult these
reports atthePublio Document Room inWashington .DC.
•/••t

-2- COM(79 )238
Inview cf "thepreliminary nature oftheinformation ,the contents of
this report should likewise beregarded asprovisional .Inparticular ,
itsconclusions should bereviewed inthelight oftheresults ofthe
detailed examination concerning thedata ofthe aocident .
Independently oftheanalysis oftheacoident data bytheNRGitself ,
aneleven-man commission hasbeen setupbyPresident Carter andwill
produce areport within sixmonths .

COM(79 )238
THETHREE MILE ISLAND NUCLEAR POWER STATION
General Plant Description
TheThree Mile Island power station issituated inthesouth –eastern
part ofthestate ofPennsylvania .Thestation isonanelongated
island intheSusquehanna river ,ontheeastside .Theclosest community
isMiddletown atadistance ofabout 3miles (5km).Thedistance to
Harrisburg isjust over 10miles (16km).
Thestation isowned "byMetropolitan Edison Company ,Jersey Central
Power andLight Company andPennsylvania Eleotric Company .Allthree
owner companies aresubsidiaries ofGeneral Publio Utilities Corporation .
TheThree Mile Island power station consists oftwogenerating units ,
both equipped with pressurized water reactors ofBabcock andWiloox
design .Unit 1hasanetcapaoity of792MWe ,Unit 2anetoapaoity
of880MWe. '
Theacoident occurred inUnit 2,on28March 1979 »while Unit 1wasshut­
down foryearly maintenance .
Theconstruction ofTMIUnit 2started more than 10years ago.The
reaotor wasmade oritical on28March 1978 andtheunit started to
deliver electricity tothenetwork on30December 1978 .

-4- COM(79)238
DESCRIPTION OFTHEACCIDENT
Atabout4»00a«nicon28March1979»thesecondary(nonnuclear)
coolingsystemoftheThreeMileIslandpowerstationsufferedamalfunction.
Thissystemnormallypumpswaterthroughtheplant'ssteamgenerators
wherethewaterturnstosteamwhichthenflowstoturnaturbine
generator.Thesteamisthencondensedbacktowater,whichispumped
byacondenstate pumpthroughacleanupsystem,throughafeedwater
pump,andfinallybacktothesteamgenerator,andflowcontinues
aroundthisloop.
Amalfunctioninthemainfeedwatersystemcausedthefeedwaterpump
tocutout,whichinturncausedtheturbinegeneratortoshutdownand
stopgeneratingelectricity.Sincethesteamgeneratorswerenot
removingheatduetothestoppageoffeedwaterflow,thereactorcoolant
systempressureincreasedand.thepressurizerreliefvalveopenedto
reducereactorpressure.Thereactorshutdownautomatically bytherapid
insertionoftheplant'scontrolrodsasdesigned,andthenuclearchain
reactionstoppedleavingbehindprincipallyresidual,ordecay,heat.
Theseeventsalloocurredwithinthefirst30secondsfollowingthe
initialevent.
Uptothispoint,thissequenceisnormalgiventheinitiatingeventand
plantresponsewasasexpected,andtheauxiliaryfeedwatersystemshould
start–upanddeliversecondarycoolanttotheplant'stwosteamgenerators
toremoveheat.Inaddition,thepressurizerreliefvalveshouldclose
asreactorpressuredecreases.
Allthreeoftheauxiliaryfeedwaterpumpsarereportedtohavestarted
butwereunabletodeliverflowbecausetheirflowpathswereblocked
byclosedvalves.Auxiliaryfeedwaterflowwasestablished throughthe
openedvalvedabouteightminuteslater.Inaddition,thepressurizer
reliefvalvefailedtocloseandthereforeallowedthereactorcoolant
systempressuretocontinuetodecrease.

-5- COM(79)238
2As-thereactorpressurereaohedapresetvalue(112kg/cm),the
plant'sEnergencyCoreCoolingSystem(ECCS)startedasdesignedand
begantoinjectcoldwaterintothereaotor.Atthispointanindication
ofarapidlyrisingpressurizerlevelapparentlyledtheplantoperators
toterminatetheECCSflow.TheThreeMileIslandincidenthadbeen
underwayfor11–12minutes.
Betweenabout1and2hoursintothetransient,theoperatorsturned
offthefourlargepumpswhichcirculatethereactorcoolantthrough
thereactor.Itisfollowingthisaotionthattheseveredamagetothe
nuclearfuelbegan.Forthenextseveralhourstherewasaverylarge
temperaturedifferenceacrossthereactorooreindicatinglittleflow
ofcoolantthroughthecore.
Itisthoughtthatthehightemperatureinthereactorcoreledto
swellingandburstingofanumberoffueltubes,causingtherelease
offissionproductstotheprimarycoolant.Thisisalsotheperiod
whenthezirconium-water reactionproducinghydrogen,musthavetaken
place.Thehydrogencollectedinthetopofthereactorvesseland
thisgasbubblecausedproblemsinthecirculationoftheprimarycoolant.
Duringthisseveralhourperiod,whenfueldamagewasoccurring,
primarycoolantfromthereactorprimarycoolantsystemwasbeing
dischargedtothereactorcontainmentfloorfromflowoutofthe
pressurizer reliefvalveandthroughthedraintank.Partofthis
coolantwasautomatically pumpedfromthereactorcontainmentbuilding
floortotanksintheauxiliarybuilding.Thetanksoverflowed
permittingradioactivity tobeventedfromtheauxiliarybuilding.
Thisdischargewassecuredinabout40minutes.Thereaotorcontainment
wassealed(isolated)atabout9*00a.m.

-6- COM(79)238
Through theafternoon andearly evening of28March 1979 »"thelicensee
isolated the stuck open relief valve and tried todepressurize the
reactor coolant system sufficiently to "beable toturn ontheresidual
heat removal system .Since hisattempt failed ,itwasdecided to
repressurize thesystem .
After repressurization (about 8.00 p.m.)oneofthemain reactor
coolant pumps inloop Awasrestarted andflow through thereactor core
was established .Heat wasbeing transferred outofthereactor through
thesteam generator while using thecondenser .Theprimary system was
maintained atapressure ofapproximately 70kg/cmandatemperature
of138°C.
Reactor cooling hasessentially been inthis form since that time ,
while hydrogen from theprimary coolant system wasevacuated by
degassing .This wasbrought about bytransferring ,gradually ,the
hydrogen tothepressurizer andventing itinacontrolled manner to
thecontainment .Theother efforts have mainly been devoted tomain­
taining this condition while aseries ofanalyses have been conducted
andwhile measurements have been taken toconfirm avariety of
parameters .These efforts have been directed towards preparing for the
next steps intheoooldown prooess .

-7- COM(79 )238
ACTIVITY RELEASED AND ENVIRONMENTAL EXPOSURE
Discharges totheatmosphere during theaooident consisted essentially
ofradioactive noble gases although small amounts ofiodine –131and
caesium –137were also released *
Liquid radioactive effluents were released ,deliberately ,into the
Susquehanna River *These releases were probably within thelimits
cited intheTechnical Specifications fortheplant *
USofficials estimate theaverage individual dose ina50mile radius
around theplant ,received during thefirst week after theaccident ,to
be1*1mrem .Themaximum external whole body dose toahypothetical
individual staying permanently intheopen airatthemost exposed
point accessible tothepublic would have been less than 100mrem .For
comparison ,thedose limit foramember ofthepopulation is500mrem
peryear andnatural background isoftheorder of100mrem peryear .
Analysis ofairandmilk samples showed iodine-131 tobepresent ina
fewcases and ,inmilk ,traces ofcaesium-137 were occasionally detected
theresults forwater ,soil andvegetation were below thelimit of
detection .Inallcases thepeak concentrations present corresponded
topotential doses ofless than 5%oftheannual dose limits .
Upto4April ,3members oftheplant personnel received doses ofabout
4rem ;other personnel reoeived lower doses .Themaximum permissible
whole body dose foroccupational exposure is5rem/year .
From theabove itappears that theradiation doses received asaresult
oftheaocident bythepopulation living around theTMIplant andby
plant personnel oannot beoonsidered assignificant from thehealth
point ofview .
•••/•••

-8- COM(79)238
EMERGENCY PLANNING ANDMEASURES
TheStateofPennsylvania hastwoorganizations foremergencyplanning:
–ThePennsylvania EmergencyManagementAgency(PEMA),whichhasthe
taskofimplementing theprotectivemeasuresforthepopulationas
decidedbytheGovernor.
–TheRadiological AssessmentBranchoftheEnvironmental Resources
Department ,whichrecommendsprotectivemeasurestotheGovernor,
ThePEMAhas4countryEmergencyOperationCenters(BOC),ofwhichone
isinHarrisburg,towhichitcommunicates theordersorrecommendations
oftheGovernor.TheEDCsareresponsibleforalertingthepopulation.
Theaccidentstartedon28Marchat4.00a.m.At7°00a.m.thelicensee
declaredaninternalsiteemergency;ageneralemergencywasdeclared
at7*30a.m.whenahighlevelofradioactivity wasdetectedinthe
containment "building.However,sinceduringthefirstdaysthedose
readingsaroundtheplantremainedlow,nooff–siteintervention measures
weretaken.
OnFridaymorning,30Aprilat8.40a.m.,ventingofoneoftheradioactive
wastegasdecaytanksthroughthestackproducedaradiationdoserate
abovethestackof1.2R/hr(latercorrectedto0.6R/hr).Asaresult
ofthis,NRC,nothavingbeeninformedbythelicenseeofhisintention
toventthetanksandhencebeingunawareoftheoriginoftheactivity
release,suggestedtotheGovernorofPennsylvania thatpregnantwomen
andpre–schoolchildreninanareaoffivemilesaroundtheplantshould
beevacuated.AtnoontheGovernorgavethisrecommendation (notanorder)
tothepopulation.ThePfflAwiththeaidoftheoivildefensewarned
Middletownresidentswithsoundtrucks.(Thiswastheonlyprotective
measuretakenbytheGovernor).However,everybodytreateditasan
order.Asaresultofthis,nearlythewholepopulationofMiddletown ,
3.5milesfromtheplant,leftthearea;23schoolswereclosed.During
theweekendof31Maroh–1April,asanexplosionofthehydrogenbubble
inthereactorvesseloftheTMIplantwasfeared,plansweredrawn

-9- COM(79)238
uptoevacuatetheentireregionupto25milesaroundthesite.
Suohanorderwashowevernevergiven#Theauthorities fearedthat
anevacuationoouldcauseanimmensetrafficjam;nevertheless ,
thousandsofpeople(estimatesrangefrom80,000to200,000)
voluntarilylefttheirhomesduringthecourseoftheweekend.
Problemsencountered duringtheBnergenoy
Accordingtostatements madebyNRCofficialsduringbriefings,the
followingproblemswereencountered :
–TheStateandlooalauthorities werenotsufficiently preparedfor
suchanemergency,(in1974Metropolitan Etlisonwrotetothe
Middletownboroughoffioialsthat"...eventheworstpossibleacoident
postulatedbytheAECwouldnotrequireevacuationoftheboroughof
Middletown...itcanbeseenthatitisunnecessarytohavespecific
evacuationroutesspecified•••")•
–Therecommendation foraselectiveevacuationendorsedbythe
Governorhadunexpectedoonsequences .Otherpeoplethought,
"Whynotme?".Asaresultmorethan50$ofthepopulationin
the5–1milezone(25,000people)lefttheareaandinthe25mile
zone(650,000people)atleast10%(somesay30$).
–TheDepartmentofHealth,MucationandWelfare(H.E.W.)hasfixed
Protective ActionGuidesandhasrecommended thedistribution ofa
thyroidblockingagent,potassiumiodide(KI),tothepopulation
incaseofradioaotive iodinereleases.Atthetimeoftheaocident
noKIwasavailable(H.E.W.hadjuststartedoontaotingKImanufacturers).
Twodaysaftertheaccident5Q,QOQvialsofKI(inliquidform)were
shippedtoMiddletown.However,aslittle'iodinewasinpractice
dischargedtotheenvironment ,nousewasmadeofit.

-10- COM(79)238
Theenvironmental monitoring wascarriedoutlayseveralorganizations
–StateofPennsylvania
–DepartmentoftheEnvironment
–Environmental ProtectionAgency
-NRG
Duringthefirst3daystherewereverylongdelaysinobtaining
monitoringresults.SomeBampleswerereceivedseveraldays
aftercollection ;manywGrereoeivedwithoutreferenceto
samplinglocationortime("bothextremelyimportantfordeciding
onprotectivemeasures).
Inthe"beginningthereweremanycommunication problemsbetween
theplantoperator,NRCandtheGovernment.Laterpermanent
opentelephonelinesbetweenthevariousorganizations involved
wereinstalled,withservicemenfromthetelephonecompany
permanentlyinattendance.

-11- COM(79)238
CONCLUSIONS
Tnapreliminary analysis theNuclear Regulatory Commission has
identified thefollowing sixpotential failures whioh have led tothe
core damage andactivity releases intheTMIplant .
1.Atthetime oftheinitiating event ,loss offeedwater ,both ofthe
auxiliary feedwater trains were valved outofservice .
2.Thepressurizer eleotromatio relief valve ,which opened during
theinitial pressure surge ,failed toolose when thepressure
decreased below the actuation level .
3.Following rapid depressurization ofthepressurizer ,thepressurizer
level indication mayhave lead toerroneous inferences ofhigh level
inthereactor coolant system .Thepressurizer level indication
apparently ledtheoperators toprematurely terminate high pressure
injection flow ,even though substantial voids existed inthereactor
ooolant system .
4.Because thecontainment does notisolate onhigh pressure injection
(HPI)initiation ,thehighly radioactive water from therelief
valve discharge waspumped outofthecontainment bytheautomatic
initiation ofatransfer pump .This water entered theradioactive
waste treatment system intheauxiliary building where some ofit
overflowed tothefloor .Out–gassing from this water anddischarge
through theauxiliary building ventilation system andfilters was
theprincipal source ofthe offsite release ofradioactive noble
gases .
5.Subsequently ,thehigh pressure injection system wasintermittently
operated attempting tocontrol primary coolant inventory losses
through theelectromatic relief valve ,apparently based on
pressurizer level indication .Duetothepresence ofsteam and/or
non-condensible voids elsewhere inthereactor coolant system ,this
ledtoafurther ,reduction inprimary ooolant inventory c

-12- COM(79 )238
6.Tripping ofreactor coolant pumps during thecourse ofthetransient ,
toprotect against pump damage duetopump vibration ,ledtofuel
damage since voids inthereactor coolant system prevented natural
circulation .
Following this analysis NRChasissued alist ofactions to "betaken
"byalllioensees ofoperating light water reactors .This list ofaotions
tobetaken hasalso been transmitted tooperators oflight water
reaotors inEurope .
Ofthesixpotential failures ,one (l)isahuman error ,one (2)is
purely mechanical failure andone (4)adesign failure .Three (3)(5)(6)
ofthe failures can beseen ascombinations ofmechanioal failures and
human errors .
Themost important failure ,which isatthebasis ofthewhole sequence "
ofevents ,isthat theauxiliary feedwater system ofthesteam generators
wasvalved out .The operation ofthereactor under these circumstances
isinflagrant violation oftheNRC regulations laid down inthe
technical specifications oftheplant (NUREG 0432-app .AFeb.8/1978 )
which specify that :"Three independent steam generator emergency
feedwater pumps andassociated flow paths shall beOPERABLE ••••"
Without this violation theaccident would nothave occurred .Furthermore ,
thequestion remains whyaction toopen thevalves wastaken only 8minutes
later .
The failure ofthe electromatic relief valve toclose when thepressure
deoreased below the actuation level was amechanical failure .There
isnoexplanation yet forthe fact that action toisolate this valve
wasonly taken more than twohours after thestart oftheaccident .
Thepremature termination ofthehigh pressure core cooling injection
seems toresult from anerroneous level reading .Thequestion whether
other instrument data were available totheoperators remains open .
/■

-13- COM(79 )238
Onthe "basis ofinformation available atthepresent time the
erroneous level reading may "beconsidered amechanical failure *
There mayalso beadesign aspect related tothis failure ,asit
remains unclear whether thelevel indicator wasdesigned towork
intheconditions ofpressure variations andgasbubbles inthesystem
asexperienced during theaccident *
AtTMItheisolation ofthecontainment building isinitiated only
byoverpressure inthecontainment building (0*28 kg/cm)andthis
occurred only 5hours after thebeginning oftheaccident *Inother
designs thereactor building isalso isolated when theemergency core
cooling system starts tooperate *Ifthis hadbeen thecase atTMl t
nohighly radioactive primary water would have been pumped into the
auxiliary building *This isadesign failure which diminished the
efficiency ofthecontainment toperform itsfunction oflimiting
activity release .
Theintermittent operation oftheemergency core cooling system was
based onthereading ofthelevel inthepressurizer .This probably
erroneous reading isthe onethat lead tothepremature termination
ofthehigh pressure injection andthecomments that canbemade
concerning this failure areofthe same nature asthose related tothe
premature termination ofthe high pressure core cooling injection *
Thestopping oftheprimary coolant pumps wasbased ontheknowledge that
natural circulation would besufficient tocool the shut-down reactor *
However ,thetests which demonstrated this natural circulation cooling
didnotsimulate thelowpressure conditions during theaccident nor
didthey account forthepossible presence ofgasbubbles intheprimary
system .Theplant operators probably didnotrealize this difference
inconditions .
Inaddition ,thefollowing preliminary comments canbemade regarding
theaccident .They should bereviewed when more information beoomes
available .

-14- COM(79 )238
Human Errors
•*Thehuman errors that occurred show that more attention must "bepaid to
1)thequalifications ofplant operators ;their expertise should "be
such that decisions such astheblocking ofanimportant system
inviolation ofthetechnical specifications would never come up
for consideration®
2)theinspection ofnuclear installations ;astrict andconstant
control byanindependent body isofprime importance .NRChad
already permanent inspection onsome nuclear installations .It
isprobable that theprogramme forthedispatohing ofresident
inspectors will beaccelerated .
–Atthe outset ofthe accident and inthe crucial hours that followed
some difficult but important decisions hadtobetaken bytheplant
operator .Probably some ofthese were wrong .Therefore technical
baclMip teams which canbesent immediately toaplant indifficulty
toassist the lioensee inhis decisions could beuseful .
Design Failures
–Remote operation ofthepurge valves onthe reactor vessel isnot
possible atTMIoranyother light water rieactor .Thereason isthat
accidental opening ofthevalves could betheorigin ofaloss of
coolant accident .This point should bere-examined inthelight of
theexperience from theTMIaccident .Theremote operation ofthe
purge valves oould inthis case have permitted therelease ofthe
hydrogen from thereactor vessel .
–Effluent monitoring inthestack through which theauxiliary building
isvented seemed nottobeworking correctly .Since these detectors
must give theultimate information ontheradiological risks forthe

-15- COM(79 )238
environment andontheemergency measures tobetaken ,they should
"bereliable under all circumstances .
–Thestorage capacity oftheliquid andgaseous waste collection
tanks wasmuch too slnall ;during theaocident they hadtohe
emptied several times into theenvironment andnewtanks hadtohe
rushed tothesite forliquid waste storage# Waste production during
anaocident should therefore betaken into acoount when designing
thewaste storage capacity *
Emergency Response Planning
ThePennsylvania State andlocal authorities were notsufficiently
prepared tocope with anemergency ofthis kind .Theaocident hasshown
that particular attention must "begiven toemergency planning and
preparedness ,especially with regard to :
–reference accidents forwhich emergency plans must beworked out ,
–setting ofprotective action levels ,
–specific responsibilities ofparticipating organizations ,
–emergency equipment andpersonnel ,
–communication systems ,
–evacuation routes ,reception centres ,
–distribution ofthyroid blocking agents (Kl),
–environmental monitoring programmes andteams ,
–training ofpersonnel involved inemergenoy interventions .

-16-
Radiological Consequences ofthe AccidentCOM(79 )238
According "to "thesurvey results ,doses received bymembers ofthe
public asaconsequence oftheaccident arelowcompared with thedose
limits# The environmental contamination has also been minimal .
Occupational exposure upto4April was still within theannual limits .
Therefore from ahealth point ofview theconsequenoes oftheaocident
canbeconsidered asnot significant .
Safety Evaluation
TheTMIaccident has shown that thesafety systems have performed their
function oflimiting thereleases ofradioactivity totheenvironment to
acceptable levels .However ,itisalso clear that thesafety evaluation
studies concerning thepressurized water reactors have notyet fully
covered all thepossible major occurrences .This isinparticular the
case forhydrogen formation bythezirconium-water reaction andhydrogen
accumulation inthetopofthereactor vessel ,causing cooling problems
andrisk ofexplosion .Thesafety evaluation ofPWRreactors will need
tobereviewed and extended with respect tothese problems .
Information
TheUSpublic appears toberather well informed about nuclear energy
anditsrisks .Ingeneral thereaction ofthepublic ,even during the
more oritical phases oftheaccident cannot beoonsidered excessive .
Inaddition opinion polls have shown that theattitude towards nuclear
energy ,which wasrather positive ,hasnotchanged significantly asa
result ofthe accident .
During thecourse oftheaccident theUSauthorities ,andmore particularly
NRG ,have shown continuous concern tomake public allinformation which
became available .This isparticularly well demonstrated bythetelex
information whiohwasmade available bytheUSembassies .TheNRCoffioe
ofinternational programmes hasalso organized technical briefings and

-17- COM(79 )238
meetings forthebenefit offoreign technicians .
This attitude ofpreparedness toshare theavailable information is
tobeappreciated allthemore inthat NRCwasparticularly cautious
with regard totheevaluation ofwhat happened because ofthefar
reaching implications anypremature conolusions might have *