Internal exposure of the staff involved in the cleanup after the accident at the Chernobyl nuclear power plant in 1906
V .A. Kutkov » I.A. Gusevb
&5. 1 . D ementiell'
Introduction
As a result of the Chern obyl nu clear power plan t (CN' PP)accident, a gre a tamou ntofaero solparti- clesofdispersed nuclear fuelan d particl esof con- densationwasreleased to the atmosphere of work- ing places.Inhalation of those aeros o ls becam e a sou rce of internal exposure for witnesse s of the Chernobyl accident.
Con densed particles were form ed by radioac- tive vapo u rs of rela tively volatil e elemen ts e.g.,
103RI~ J06~ J3JL J3'Cs or J37Cs, subse q ue n t to inte ractionwith nouradio activematerial. Thepar- ticles of con densa tio n are a well-kn own form of aerosol in the air of the working en viro n men t in the nuclearindustry.
Aero sol particles of fragme n ted nuclear fue l con tai ned fissio nproducts an d transuraniumradi - onuclid esstronglyboundwith the uranium matrix ofNuclearFuel Pa rticles (NFP) 0,2). Th e nuclear fuel particles area rareformof airborneradioactiv- ity.Similar aerosolparticleshave on ly beendetect- ed in the environ men t after nuclear tests in the atmosp here an d thei r radiological properties are not wellinvestigated.In post-accidentpublications thoseparticle s are usu ally called "Ch e r n o byl Hot Particles".
The Che rn o byl accid e n t was the first in which the co re was destroyed an d aerosols of the disp e rsed nuclear fuel became a 1113:jor sou rce of inte rn al an d ex te r na lexposu re. The evalua tion of that exp o su re pathwayis the aim of ou r work.
Nuctear tuetpart icles
TIle radiological hazard ofairbo rne rad ioac tivity depends ontherad io n uclid ecom pos itio nofaero- solpa rticl es an dchem ica l form of rad io n uclides in thoseparticles aswell as on theiraero dynamicsize. To calculate do se of inte rnalexposu re itis neces- sarytodeterminetho se characteristics of the aero- sol an d validate the rel evan t param eters of the do sim etricmo del.
aRussianResearchCen tre,KurchatovInstitu te,Moscow.
bStateScientificCentre,Institute ofBio physics,Moscow.
cMini stry of the Russian Federation for Civil Defen se Emergenciesand EliminationoftheCo n sequence s ofNatu ral Disasters,Moscow.
62
Isotopic composition
The fuel rods of the Che rnobyl type of reactor co n ta in finely-dispersed powder ofen ric hed U02 co m pac ted in the for mofpellets. The fragme nta- tionof thefuelpell etsby explosio nan d/o ranneal- ing producedthenuclearfuel particles.The crystal lattice of these particles forms a fuel matrix and strongly holds thefission and transuranium radio- nuclidestogether(1).
A strong bond between the radionuclides and the matri x permits con ta m in ation of the popula- tionor theen viro nmen t bynuclearfuelparticlesto bemeasuredbytheactivity ofany one of tilenucle- ar fuel radionuclidetracerswhich,under the Cher- nobylaccid e n t conditions,are presentonlyinNFP.
Thetrace rsin clude theisotopesof relativelyrefrac- tory elements, e.g., Zr, Ce and transuranium iso- topes , such as Pu. The activities of nuclear fuel particles soon after the accident are co m m o n ly exp resse d in terms of 144Ce activity, as a rela tively long-lived gamma-em iu in g tracer.The activities of the main NFP radionuclides in a represen tati ve nuclearfuel particle no rmalized to that of 144Ce aregivenin Table1.
Chemicalspeciationof radionuclides:thematrixbond Due to theuran ium-oxidematrix,thebehaviour of the NFP radionuclid es both in the environment an d in the body acq u ires a collective ch a racter.
Bound by the matrix, the radionuclid es do not escape fro m the fuel particles untilthe matrix is chemically destroyed.Ifany nuclear fuel radionu- elidetracer werefoundin the samp lefrom human barri erorgans, e.g.,lu ngs orskin,deposi tsofother NFP radio n uclid es would be expect ed in those or- gans , too.Theirnormalized activities mustbeclose to those in Table1 with regard to the radioactive transformation.
The stre n gth ofa rad io n uclid e-ma trix bond in the nuclearfuelparticl eswas stud ie d usingdialysis of an aerosolsample in Ringer's solu tion.Since all co m mo n ly occurring Cscom poun ds arc so lu- ble ,100%of the 137Csactivitycou ld be expec tedto passinto so lu tion ina sho rt time.However,Che r- nobylon-sitesamples collecte d in June 1986 con- tained abo ut 40%ofaerosolCsinarelativelyinsol- uble form, probably as part of fuel particl es. the rem ainderbeing soluble(3).In 1990-199I, samples fro m the "Sa rc o p hagus" (specialento mb men tof the dam aged Chernobyl reactor) showed that at least 30% of Cs was still in a nontransportable Rapp.trimest.statist.saoff.mond..49(1996)
[
f
Table1
Activitiesofthemain isotopesin thefuel of theChernobylUnit4core,normalized to that of 14'Ce,24hoursaftertheaccident Tableau1
Activitedes principaux isotopesducombustible ducrsur de l'Unite4de Tchernobylnormalisee parrapport
a
celle de '''Ce, 24heuresapresI'accidentIsotope Normalizedactivity- Isotope Normalizedactivity- Isotope Normalizedactivity-
Activitenormalrsee Activitencrmalisee Activitenormalisee
95Zr 1.410.0 131, 5.910-' 143Ce 7.210-1
95Nb 1.310.0 132/ 9.710-' '''Ce 1.0 10.0
97Nb 4.510- ' 1331 6.610-' 148Pm 6910.2
97mNb 4.110-' 1351 8.310-2 '56Eu 5.510-'
99Mo 1.210.0 13'CS 3.810-2 239Np 5.010.0
99mTc 1.2 10.0 136CS 1.910-' 238pU 2.010-'
'03Ru 1.110.0 '37CS 7.3 10.2 239pU 2.1 10"
I06Ru 3.010-' l406a 9.010-1 240pU 4.4 10-'
115mln 4.810.3 l40l a 1.610.0 241pu 5.010-2
'321e 5.0 10-' "'Ce 1.4 10.0 242Cm 8.610-3
fo rm,aspartof nuclea rfuelparticle s, and in2days did not pass into so lutio n through a mem brane witha porediameter of abou t0.5/-LID.
A special investigation oflong-term kineti cdia- lysisfor"Sarcop hagus"sam p les was carrie d out in 1992 (/). The test procedure resembled the one described by Eidso n & Criffith (4). The radio nu- clide leaching fro m the samplewasstu d ied for122 days afte r the onse t of dialysis. The tran sportability ofover 70%of J37Cswasfoun d to be weak in the sample,withaleaching consta n t01' 0.002
±
0.001dI .The remaining Cs fraction was 20 times as trans- po rtable .Forthetotalof alp haemitters, the dialy- sis constant was 0.005
±
0.002 dJ.The difference between the lea ch ing co nstantsfor137Csand total alpha emitters is no t statistically significant (p>0. 05).Th is confirms a su ppositio n that these radion uclid eshave similarche micalspccia tion,be- ing matrixbound. Using the summarized data on totalalp ha andCsleaching , theconstant for rad io- nuclide leaching from nuclear fuel particlesisesti- mated at0.003±
0.00 2 dl an d close to 0.004 dI fromCud d ihyet al. (5).The behaviou r of an aerosol panicle radi o- nuclide in the respiratorysystem dependson the tra ns po rtability of the com pou n ds inhaled wh ich determin es the rate of radi onuclide transfer through thebarri erbetwe en airan d bodyfluids.By analogy withuranium oxides,assigned byICRPto in halatio n class Y (6) for their transp ortabili ty in the respiratory system,all the radion u clidesenter- ing the respiratory system aspart ofnuclearfuel particlesshould be assigne d tothesam e inhalatio n class. This led us to develop a special dosimetry model to describe the beh aviour of the nuclear fuelparticleradio nuclides inthe human respirato- ry system (7, 8).
Wldhlth slahsl.quart.,49(1996)
Dimensionsof nuclearfuel particles
Theamount of radioactivitydep osited in different partsof the res piratorysyste m is associatedwith the particlesize distribution . In most cases,the distri- butionof theaero solactivitybyparticlediameteris described by a lo garithm ic-norm al distribution with two param eters: the activity medi an aerody- nami c diameter (AMAD) an d the stan d ard geo- metricdeviation.
To co rrectlyassess internal dosesfrom a mix- ture ofradionuclidcspresen tin the air in the fo rm ofco nde nsed particlesoncmustassign each radio- nuclideto acer tain inhalationclassbyits chemical speciationinaeroso land defin ethe airborne char- acteristics of that aerosol. Moreover,information onanyparticular radionu clide is uselessfor other co mponentssince,insuchamixture ,the radio nu- elides are genera lly indepen dent and maybel on g todifferent particles.
On the oth e r hand, all nuclear fuel particle rad io n u clides bel ong 10 the same particle. The collective behaviou r ofthe matrix-bound rad io n u- elide sin the environmentandin the human barri- er organs makesitpossibleto extend tothe aerosol of NFP any estima te s of AMAD obtained forany particular NFP rad io nuclide. This is a princip al featu re of NFP aerosol as distin guish ed from a mere mixture of aerosol particles carryi ng differ- ent radionuclid es.
There isno info rmation on the airbo rne char- acteristicsof theChe rno byl insituaerosol during the firstpost-acciden tdays.The only way to obtain it istouse indirect methodsbased on the investiga- tion of thebehaviourofNFP radionuclideswithin the bodies ofpersons who inhaled thataeroso lat the time of theaccide nt.Withthisaim two co ho rts of CNPPstaffan dfire me n tha t witnessed the acci- 63
dent were investigated in 1987-1992 and one co- hortofGomelinhabitantswasinvestigated in1991- 1993 (12).Analysis of kineticmCs in the body (Me th od I) and analysis of23lJpu distributi on in body(.Method ll)wereused.Theresulting data are pre sented in Table2.
The estimates thusobtained fortheA\L\D of nucleartile! part iclesareingood agree me nt,..-ith the resultsofRu dhard etal.(9)on investigation of the size distribu tio n of NFP obtained from soil samples collected in so u the rn Germa ny in May 1986.
Dose reconstruction
Method
Aspecial procedurebased on thebiokineticmod el fo rNFPradionuclides was used fo rretrospective evaluatio nof internal expos ure (1,2) .
Inthefirst stage the resultsofindividualin vivo measurements of 95Zr and 95Nb in accide nt wit- ness es were used inamod el for reconstructio n of the inhalation intake ofrad io n uclidesasapart of the fuelparticles. The Af,L\D offuelparticleswas takento be abou t 15urn.Tha t value resultedfro m the validatio n ofthe dosirnc try model for matrix- bo und radion uclid es with resul ts ofpo st-mor tem andin vivomeasuremen ts of contamina tio n in wit- nesses of the Che rno hyJacciden t, prese nted in TablE2.
In the second stage the reconstructed fuel in- takesandtheresultsof individualmeasurementsof 131I, I:HCs, 137Cs,103Ru and106Ruin thebodywere used forevaluating theinhalationintakeofvol atile rad ionuclides with particles formed by conde nsa- tion. The AMAD values of those particles were assu medto be abo u t I funfor1- and Ru-eon taining particles and 5 J-Lrn forCs-co ntain ing particles ae-
cord ing tomeasurem ents byPo pov etal.(3),made at the Chc mo bylnuclear power plantsitein 1986.
In the third stage the commi tted eq u ivalent do seinsome ta rg etorgans andtissuesandeffec tive do se due to two fo rms of aerosols inhaled were calculate d bymeans of the "R-MAI'\''' com puter mod el (10).
Internalexposureof deceased accidentwitnesses on CNPPstaff
The 23 (ou t of 25) member s of CNPP staff and firemen who died of acute radiation sickness in 1986formCoho rtIof accident witnesses for whom thedosesofin ternalexpos ure werereconstructed. The research invol ved invivomonitorin gofgam- ma-emitting radio nuclidc clearance with urine (11) as wellaspost-mortem studieson radionu clide distribution in the organs (3, 11). The results of recon struction ofindividual internal dos es,accu- mulated to the time ofdeath for the members of Coho rt I,are presen ted inTable3.
Internal exposureof livingaccidentwitnesses on CNPP staff
The results of in vivo measurem ents of radio- nuclidebody content ofCNPP personn el whowit- nessed theaccidentwereusedfor doserecon struc- tion.
Cohort II-A of abo ut 125NFP-earrierswas ex- amine dwitha semico nd uctorbodycoun ter byof- ficers of the Institute of Biophysics of the USSR Ministry of Public Health (!BP) during the treat- men t in the Clinica l depart ment of !BP in 1986.
Mostofthese peopl e didnot continue the irwork inCNPPafter the accide n t.
Coho rt II-Bof abo ut 250 NFP-earrierswasex- amined in I98!>-88 with a semico nductor bod y
Table2
AMAD (activity medianaerodynamicdiameter) of aerosolnuclearfuelparticles Tableau2
Olarnetre aerodynamlq ue medianen activite(AMAD) desaerosolsde particulesducombustible nucleaire
AMAD.~m
•Soufre:Ref.- RiI.(1. 2.7).
bSoun:e:Ref.-Ref.(12).
64
Casespecifications-Partitularit~sdescas
15living accident witnesses ofCNPPstall, examined withsemiconductor body counterin 1986-87 (MethodI)'- 15 temoinsvivantsdeI'accidenlparm i lepersonnel delacentrale, examines
a
I'aide d'uncom pteurcorpsentiera
semi-co nducteursen 1986-87 (rnethode I)'23accidentwitnessesofCNPPstall,dead within 90 days allertheaccident and exam inedby autopsy (Method11)'- 23temoins deI'accident parmi lespersonnels de la centrale,decedes dansles90 joursapresI'accide nt etautopsies (methode11)'
21accidentwitnesses of125 Gomeldistrict Inhabitants,dead in 1990-91and examinedby autopsy (MethodlI)b- 21 ternolnsdeI'accidentparm i les125 habltantsdu districtdeGomel, decedesen1990-91 et autopsies (metnooelI)b
Rapp.trimest.statist.SiJnit.mond.,49(1996)
coun ter by officers of the Radiation MonilOring- Departmentin the town ofC hc rn o bvl.Th e results of intern al dose evalua tion for Co h o rt II-Bof the accid e n twit nesseswerepresent ed recently(1.2).
Theresults of intern aldoseevalua ti o nfo rallor Cohort11
o n5
acciden twitnesses )arc presented inJ,liJ/I'4.
It has bee n found tha t due to the larg e
A~IAD of the fu el particle s the actual r-qniva-
IC:Ilt dosesin lun g s an d thyroid are more st ro ng- ly correlate d with the intak e of particle s con - tainin g Ru and I, rcsp c cuvely, than with the intake of nuclear fuel panides, Oil the other hau d, the actua l effecti ve- dose as well as the actualeq u ivale- n t closesill bone surfaces.upper la rge intestine an d lower large imes tinc walls, liver. an d red marrow arc stro ngly correlat e d wit h intake of nucl e ar fuel particles.
Table 3
Individualdoses accumulated bytimeof death,amongChernobylaccident victimsfram CohortI Tableau3
Doses individuelles cumulees jusqu'aumoment du decesparles victimes deI'accidentde Tchernobylfaisantpartiede lacohorte 1
Equivalentdose inorgans (mSv)-Equivalentsde Effectivedose(mSv)-
Person's dose dansresorganes(mSv) Doseeffective(mSv)
code-Code
delaoersonne Thyroid-Thymide Lungs-Poumons
0-25 21 0.26 0.69
D-18 24 2.8 1.3
0-22 54 04 7 1.7
0-5 62 0.57 2.0
0-21 77 0.68 2.5
0-8 130 1.5 4.2
0-2 130 2.2 4.6
0-19 210 3.5 7.6
0-23 310 2.3 9.8
0-1 340 8.7 12
0-15 320 27 14
0-16 470 4.1 15
0-3 540 6.8 18
0-17 600 120 21
0,4 640 34 25
0-10 890 94 28
0-11 740 29 29
0-14 950 20 32
0-20 1900 19 62
D-24 2200 21 70
0-13 4100 40 140
Table 4
InternalexposureoflivingChernobyl accident witnesses from Cohortii Tableau 4
Exposilion internedes ternoinsvivantsde I'accident de Tchernobylfaisant partiedelacohorteii
Actualequivalentdose(mSv)-
EQuivalentdedose engage(mSv) Mean-Moyenne Median- Mediane Minimum Maximum
Bone surfaces- Surface des os 280 150 5.2 3600
Lungs- Poumons 160 110 4.0 2400
Wallotlower large intestine- 220 120 8.8 2900
Paroide lapartietnterleure du gras intestin
Thyroid - Thyr6ide 96 62 0.2 1800
Wall ofupper large intestine - 90 49 3.5 1 150
Paraidela partiesuperieuredu gros inteslin
Liver- Foie 56 30 16 730
Redmarrow - Moellerouge 36 19 1.1 460
Actual effective dose (mSv)- 64 40 2.3 870
Doseefficaceenqaqee (mSv)
Wld hllhststis: auert.,49(1996) 65
Summary
This articlepresentsthe results of our work from1987 to 1994 on thereconstruction ofinternalexposure of the witnesses to the Chernobyl accident with respect to inhalation of airborne particles of nuclear fuel. Our results eliminatethe hypothesisthatinhalationintake of radioactive aerosols was a considerable pathway of exposurefor staff at the Chernobylnuclearpowerplant Itwould conceivablyleadto a nonun iformdosedistribu- tion in the body.Under thatcondition,actualequivalent doses in some organs,e.g.,in lungs andinthe wall of the lower large intestine are much higher than the actual effect ive dose,Therefore it is impossible to correctly assess the health risk for certain cohortsofthe Cher- nobyl accident witnesses ifthe inhalation pathwayis neglected. In that case not only the actual effective dose,butthe actualequivalentdoses too playa role in radiologicalmeasurement,when evaluating the hazard forless-exposed accident witnesses.
That conclusion is of great importance to radiation protection policy,theplanning of epidemiolog icalstud- ies and the carryingoutof medicalremedial actionsfor the differentcohorts of accidentwitnesses. The findings shouldbe takeninto consideration in planning protec - tive measures andin assessinghealtheffectsfollowing theChernobylaccident.
R esume
Exposition interne du personnel ayanl par/icipe
a la remise en etat du site apres I'accident survenu a Tchernobyl en 1986
Le presentarticle don ne lesresultats des etudesque nous avonseftectueesde1987
a
1994 pour reconstituer l'expositloninternedes ternoins de I'accidentdeTcher- nobyl due a I'inhalation d'asrosols de partic ules du combustiblenucleaire .Nos resultats elirninentI'hypo- these selonlaquellel'inhalationd'aerosols rad ioact ifs a constitueune voie de transmission importante lors de I'exposition du personnel de la centraledeTc hernobyl aux rayonnements.On pense donc que la repartition des doses dans le corps s'est faite de tacon non uniforme.Dans ces conditions, les equivalents de do- ses engages recus par certains organes comme les poumons et la parol de la partie inferieu re du gros intestinsontbeaucoup plus elevesque ladose efficace enqaqee.11 est par consequent impossible d'evaluer correctementles risquessanitairescourusparcertai- nes cohortesdeternoins de I'accidentde Tchernobylsi I'onnegligela voie de transmissionparinhalation.Dans cecas,nonseulementlesdoses efficacesengagees, rnais egalement lesequivalentsde dose engages ser- vent de mesures radiologiquespour evaluerles risques courus parlestemoinsde I'accid entquin'ont ete que legerementexposes.66
Cette concl usion revet une importance considerable pour cequi est dela politiquedeprotection radiologi- que,dela planificationdes etudesepidemiolopiqueset dela prisede mesurescorrectives rnedicales pour les difterentescooortesdetemoins deI'acci dent Les resul- tats devraient etre pris en consideration lors de la planificationde mesures de protectionetdel'evaluation desconsequences sanitairesde I'accidentdeTcherno- byl.
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Rapp.trimest.slatist.sanft.mond.,49(1996)
