Influence of dynamics of I fallout due to the ChNPP ... · iodine radionuclides in the summer of 1986 and individual diet. The present work describes a method-ology for calculating

"Radiation & Risk", 1996, issue 7 Scientific Articles

40

Influence of dynamics of 131I fallout due to the ChNPP accidenton value of absorbed doses in thyroid for population

of Bryansk and Kaluga regions of Russia

Pitkevich V.A., Khvostunov I.K., Shishkanov N.G.

Medical Radiological Research Center of RAMS, Obninsk

Proposed is a model for estimating absorbed doses in thethyroid using the time-dependencies between the volume concen-tration in the surface air layer and density of 131I fallout onsoil obtained by modelling atmospheric transport of 131I. Usingof Bryansk and Kaluga regions of the Russian Federation as anexample it has been shown that the dynamics of 131I fallout onthe territories contaminated after the ChNPP accident has asignificant effect on estimation of absorbed doses in the thy-roid in inhabitants of these regions.

Individual absorbed doses in the thyroid of inhabitants ofsome settlements in Bryansk and Kaluga regions contaminated dueto the ChNPP accident have been estimated from the results ofdirect radiometry of thyroid carried out in May-June 1986.Verification and comparison of data of thyroid radiometry forcontaminated territories of Bryansk and Kaluga regions has alsobeen conducted. The relation between estimated 131I activitiesand absorbed doses in the thyroid of inhabitants of these re-gions reflects differences in character and intensity of radio-active contamination of the territories under consideration.The results indicate that adjustment of the values is requiredand this, as a rule, lowers estimated activities of incorpo-rated 131I. The average value of this correction is about 0.1�Ci and it varies depending on the age of a person under exami-nation.

The average absorbed doses in the thyroid for different agegroups of people living in the contaminated points is higher bya factor of 5 to 15 in the Bryansk region than in the Kalugaregion. The paper presents an analysis of the relationship be-tween the estimated absorbed doses in the thyroid and the 137Cscontamination density. It has been shown that the statisticalhypothesis concerning the linear relationship between the aver-age thyroid dose and 137Cs contamination density seems to be un-justified for the residents of the contaminated areas of theKaluga and Bryansk regions.

Introduction

The most significant of possiblenegative consequences for health of thepopulation living on the Chernobyl con-taminated areas is internal irradiationof the thyroid gland with iodine radio-nuclides. Russian National Medical andDosimetric Registry (RNMDR) contains fewdata on individual internal absorbeddoses for thyroid. That is why it is soimportant to estimate and reconstructindividualized absorbed thyroid doses

for persons included in RNMDR based onall available data about environmentalcontamination and also data of individ-ual radiometry of thyroid.

As a result of the Chernobyl accidenta large amount of iodine radionuclideswas released into the atmosphere whichled to the contamination of extended ar-eas in May-June 1986. The characteristicfeature of the Chernobyl accident wasthat the contamination was prolonged be-cause of a series of sequential releasesof radioactive materials up to the end


41

of May 1986 [1]. This was, among otherthings, supported by measurements of dy-namics of 131I and 132Te deposited on col-lectors in the locations of meteosta-tions on the territory of the formerUSSR [2]. However, there is still noclear and comprehensive understanding ofhow the contamination with the aerosoland gaseous forms of iodine was beingdeveloped over the whole territory ofCIS. Using different approaches andbased on the results of gamma spectrome-try of soil samples, maps of 131I deposi-tions on soil in CIS and Russia havebeen generated (see [3] and [4]). Thesedata, however, do not account for thedynamics of contamination. Works [5, 6]contain results of modelling atmospherictransport of radionuclides over differ-ent distances, but they can not be useddirectly for estimating absorbed thyroiddoses.

In work [7] using a statistical model[8] for turbulence diffusion of radioac-tive material in the atmosphere an at-tempt was made to reconstruct dynamicsof 131I, 132I and 133I depositions on theterritory of Russia. This work does notclaim to give a definitive solution forreconstruction of the dynamics of con-tamination of CIS territory with iodineradionuclides. Nevertheless, the resultsobtained in this work can become a basisfor estimating absorbed thyroid doses,taking by way of an example two regionsof Russia - Bryansk and Kaluga in whichradiological measurements of thyroidwere conducted on a mass scale in May-June 1986. In this case, there is noneed for assumptions concerning thefunction of 131I release to the environ-ment, as is usually done in all pub-lished works on estimation of absorbedthyroid doses.

A large-scale dosimetric examinationof the population of Russia was under-taken in Bryansk [9] and Kaluga [10] re-gions in May-June 1986. In [11] absorbedthyroid doses were estimated in the as-sumption of instantaneous release of 131Ion the territory of Kaluga region (con-ventionally 1 May 1986). In work [9] itis assumed that the function of 131I in-take by man is constant during 15 daysafter contamination and later it de-creases exponentially. The guidelines[12] include methods for estimating ab-sorbed thyroid doses under different as-sumptions concerning the character ofiodine intake by thyroid.

In this work we use the function of131I inflow on soil and near surface airlayer estimated from the atmospherictransport model with allowance for “ lo-cal effective precipitation” [7], whichcorrelate the model function with themeasurements of 131I and reconstructiondata.

The absorbed thyroid dose from expo-sure to iodine radionuclides is primar-ily determined by contamination of theenvironment and foods. The contaminationof foods and diet govern the rate of in-take of radionuclides by man. The ab-sorbed dose is also influenced by pa-rameters of iodine metabolism in the or-ganism. These parameters are well under-stood, but they mostly apply to a sta-tistically averaged “ standard” person.Reduction in the error in dose estima-tion can be achieved by using as manyindividual indicators as possible, inparticular, direct measurements of in-corporated activity, characteristics ofcontamination of the area, migration ofman during the most intense intake ofiodine radionuclides in the summer of1986 and individual diet.

The present work describes a method-ology for calculating individual thyroiddoses in part of the population in Bry-ansk and Kaluga regions, for whom meas-urements of incorporated thyroid 131I ac-tivity were made in 1986. For Kaluga re-gion we used verified data of individualthyroid radiometry for persons includedin RNMDR and living at the time of theaccident in Zhisdra, Ulyanovo and Khvas-tovichi areas. The model considers expo-sure of thyroid with incorporated 131Itaken in per os or by inhalation.

1. Dosimetric model forthyroid irradiation

The content of iodine radionuclidesin the thyroid is determined by dynamicsof radionuclide intake and removal. Thebalance of these processes is describedby the following equation:

dh t

dtb t k h tout

( )( ) ( ) ( )� � �� , (1)

h(t=0) = 0 ,

where b(t) , kBq ��day -1 is the rate ofradionuclide intake by man;

h(t) , kBq is the time dependent 131Iactivity in thyroid;


42

�� is the decay constant for 131I:�� = 0.0862 day -1 ;k

out, day -1 is the rate constant of

biological of 131I from thyroid: kout

=0.693/ T

b; T

b is the half-life period,

measured in days.In this work we don’t use the formu-

lae from guidelines [12], as they aretoo awkward. Besides, our formal ap-proach is different, but seems prefer-able to us. The main task in dose esti-mation is calculation of intake rateb(t) . With the prescribed b(t) equation(1) takes the general form:

h t e b x e d xeff efft xt

( ) ( )� ��

��

0

, (2)

where ��eff = �� + k

out is effective rate

of radionuclides removal from thyroid.Two main components determine the

amount of activity entering the thyroid:per os with contaminated milk and by in-halation [9, 12, 13].

The studies of dynamics of iodine ac-cumulation in milk of milking cows [14]have shown that the iodine concentrationin milk is maximum on the 3-5th day af-ter beginning of consuming contaminatedforage. In model [15] modifying theclassical model [16] the radionuclidesconcentration in milk is approximatedwith the following relation:

c t C g t

g t Q e e

t

rt st

1( ) ( )

( ) ( )

� �

� � �� ,(3)

where t , day is the time intervalcounted from the beginning of consump-tion of contaminated fodder;

c1(t) , kBq ��l

-1 is the specific concen-tration of 131I in milk;

Ct, kBq ��day -1 is the initial rate of

activity intake by cow;Q, r, s are approximation parameters;Q = 0.818 day ��l

-1 ; r = 0.176 day -1 ;s = 0.9 day -1 .The approximation c

1(t) in formula (3)

implicitly accounts for the competingprocesses of chronic intake of iodineintake by cows with fodder and reductionin activity due to metabolic processesand radioactive decay. The approximationparameters are selected so that themodel reproduces in the best possibleway experimental results in terms oftime of reaching maximum level of activ-ity in milk and the value of integral

activity secreted by a cow in milk [14,15]. The parameters values used in ourwork were obtained for the beginning ofthe spring grazing season.

Model [15] predicts the milk contami-nation with one-time contamination ofthe environment, when at time moment t =0 the initial contamination density ��0,kBq/m 2 is formed. The initial rate ofactivity intake by a cow is proportionalto the initial contamination density[14]:

Ct � �� 0, (4)

where ��, (kBq/day)/(kBq/m 2) is the pa-rameter accounting for activity transferfrom grass and soil to cow milk.

Generalisation of the model [15] forlong-term fall-out consists in the fol-lowing. Let the contamination density beformed as a result of deposition n ofportions of the radionuclide at momentst

1, t

2, ..., t

n, each of which contrib-

uted to the total contamination density,respectively ��

1, ��

2, ..., ��

n. Using (3)

and (4) we get the dynamics of specificconcentration of iodine radionuclides inmilk at long-term depositions in thefollowing form:

c t c t t

t t

ii

n

i ii

n

( ) ( )

( ) .

� � �

� � �

��

��

�

�

11

0

1

� �

(5)

The rate of 131I activity intake by manis primarily influenced by milk consump-tion and duration of staying in the con-taminated area in May-June 1986. The in-halation intake of activity is deter-mined by the concentration of radioac-tive aerosol in the air near-surfacelayer x(t) , Bq/m 3 and lung ventilationrate w, m 3/day. The rate w is dependenton the age and can be estimated from thepublished data [13].

We have calculated the activity in-take rate b(t) with the formula:

��

b t f k c t L c t L

k x t w t s t

inm

ina

( ) ( ( ) ( ) )

( ) ( ) ( ),

� � � �

� � �

1 1 2 2

�(6)

where f = 0.3 is the part of radionu-clide activity passing from blood tothyroid;


43

kin

m = 1 is the part of radionuclideactivity passing from intestinal tractto blood;

kin

a = 0.63 is the part of radionuclideactivity passing from lung to blood[17];

c1(t) , c

2(t) , kBq/l is the 131I concen-

tration in milk produced in the publicand private sectors, respectively;

L1, L

2, l/day is the consumption of

milk produced by the state and privatesectors, respectively;

��(t) is a function accounting for mi-gration of a person in the period of in-tense transfer of radioactive iodine inmilk;

s(t) is a function accounting forprotection measures, for example admini-stration of stable iodine etc.

It is known [9] that the contamina-tion of milk from the private sector inthe first period after the accident was,on the average, by a factor of p = 2-3higher that from state farms. In ourcalculations we used the relation

L1

= L2

/ p .

It should be pointed out that formula(5) is used for estimating concentrationL

2 of 131I radionuclide in milk of the

private sector.Function ��(t) is expressed by the

following formula:

�( ),

,t

t t

t ta

a

��

�

�

1

0

�

�, (7)

where ��ta is time of staying in the a-

th populated point.The age-dependent biokinetic parame-

ters of intake and removal of radioac-tive iodine and weight of thyroid weretaken from work [9] and are included inTable 1. The consumption of milk forchildren is taken to be 0.7 l/day [18].

For the adult population of Bryansk re-gion a similar value of 0.7 l/day wasobtained based on the poll results [9].

The absorbed thyroid dose rate p(t)and accumulated by moment t dose D (t)were calculated with the formula:

p t kh t E

m

D t p d

d

t

( )( )

,

( ) ( ) ,

��

� �

��

� �0

(8)

where kd is a coefficient accounting

for dimensions of the quantities;<E��> is the mean energy of ��-decay of

131I -0.23 MeV/dec.;m is the age-dependent thyroid

weight.In literature [9, 13, 19-21] one can

find numerous estimates of biokineticparameters permitting calculation ofthyroid dose depending on age at a givenabsorbed activity.

Figure 1 shows the estimated absorbedthyroid doses from 131I activity of 37kBq transferred to man’s blood as afunction of age in comparison with theliterature data.

As can be seen from Figure 1 the ab-sorbed thyroid dose decreases approxi-mately an order of magnitude with age.The main contribution to this dependenceis made by the growth of thyroid weight(Table 1).

Therefore, for standard 131I intake ourestimates of absorbed thyroid dose arein good agreement with the literaturedata, since we used generally acceptedvalues of the parameters. Hence, it maybe concluded that future estimation ofabsorbed doses can be influenced only bymeasured activities of 131I in thyroidand dynamics of its release to the envi-ronment.

Table 1Biokinetic parameters of formation of absorbed thyroid dose used in the

present work

Age,years

Thyroid weight,g [9]

Half-life,day [9]

Volume of lung venti-lation, m 3/day [13]

0 1.6 - -0.54 1.7 16 80.75 1.8 18 81.5 2.5 20 82.3 3.8 25 8

5 4.8 30 11


44

6.5 6.5 35 118.5 8 40 1410 9 50 1411 9.7 55 1412 10.5 60 17

13.5 12 65 1715 13 70 17

>18 20 100 20

0 5 10 15 20 25 300

5

10

15 M odel of thi s w ork H einri cs et al . [20] Johnson [21] ICR P [19]

D,

cGy

T, years

Fig. 1. Absorbed thyroid dose D as a function of age T.37 kBq of 131I got into the blood.

The measured value of the incorpo-rated activity at moment t = t

i permits

calculation of the individual intakerate b

i(t) which is related to intake

rate b(t) (6) in the following fashion:

b t b b t

b tb t

b d

ii( ) ( )

( )( )

( )

~

~

�

��

�

0

0

� �

. (9)

The constant b0

i is found from condi-tion h(t

i)=A

i, where A

i is measured value

of individual activity at moment ti. Us-

ing formula (2) we get:

bA

b t d

i i

eff i

t i0

0

�

� �z~( )exp[ ( )]� � � �

.(10)

A similar method for estimating indi-vidual rate of iodine radionuclides in-take by thyroid was used in work [9,13]. In particular, in work [9] the in-take rate was approximated by the fol-lowing function:

b tb t t

b e t tk t t( ),

,( ( ))��

�

��

0 0

0 00 0

, (11)

where t0

= 15 days;k = 0.693/ T

1, T

1 = 5 days;


45

b0

is activity intake rate for thyroidin the first period.

Thus, we have discussed the main de-tails in estimation of the absorbed thy-roid dose with long term fallout of 131I.

Figures 2 and 3 show, as an example,results of reconstruction, using themethod described in work [7] of the rateof 131I release to the environment forfour towns of Kaluga region (Zhizdra,Ulyanovo, Khvastovichi) and Bryansk re-gion (Novozybkov). Figure 2 presents

rates of 131I deposition on the groundsurface and Figure 3 - correspondingvolumetric concentrations of 131I in theair near-surface layer.

The presented results indicate aqualitative difference in the dynamicsof formation of radioactive contamina-tion in Bryansk and Kaluga regions whichshows itself in the significant addi-tional contamination on the territory ofKaluga region in the period 16-22 May1986.

0 5 10 15 20 250

200

400

600

Zhizdra

0 5 10 15 20 250

100

200

300

400

Ulyanovo

0 5 10 15 20 250

200

400

600Khvast ovi chi

0 5 10 15 20 250

2000

4000

6000

8000

f 1(t)

, kB

q/m

2/d

ay

t, day

N ovozybkov

Fig. 2. Dynamics of 131I influx to the soil surface [7] for some populated points inKaluga and Bryansk regions which was used for estimating absorbed thyroid doses.

The time interval t is counted from the time of the Chernobyl accident.


46

0 5 10 15 20 25101

102

103

104

105

Zhizdra

0 5 10 15 20 25101

102

103

104

105

f 2(t)

, B

q/m

3

t, day

Ulyanovo

0 5 10 15 20 25101

102

103

104

105

Khvast ovi chi

0 5 10 15 20 25101

102

103

104

105

106

N ovozybkov

Fig. 3. Dynamics of the 131I concentration in the air near-surface layer for somepopulated points of Kaluga and Bryansk regions reconstructed from data of [7] and

used for estimating absorbed thyroid doses.

Figure 4 (the upper part) shows anexample of a calculation of the individ-ual rate of 131I intake by thyroid of a3-year old child. It was assumed thatthe child lived all the time in one ofthe specified populated points: Zhizdra,Ulyanovo, Khvastovichi of Kaluga regionor Novozybkov of Bryansk region. Themeasurement of incorporated activity waspresumably made on 25 May 1986, that is29 days after the Chernobyl accident andit was 37 kBq. The intake rate calcu-lated based on our model is also pre-sented here [9].

The difference in our estimates ofintake rate and those by model [9] forBryansk region primarily applies to theperiod t < 15 days. The intake rate de-pendence for Kaluga region is qualita-tively distinct from model [9] by havinga second peak 20-21 days after the acci-

dent. It should be said that the pres-ence of the second peak on the 20-21stday is not strictly proven by now. Weare currently investigating in detailthe dynamics of iodine depositions onthe territory of the former USSR.

The same figure (lower part of Figure4) shows the dynamics of 131I incorpo-rated activity in thyroid of a 3-yearold child calculated with considerationof the estimated intake rate. All func-tions intersect in the point ( t = 29day; A = 37 kBq) when the measurementwas presumably made.

Data of Figure 4 demonstrate the im-portance of consideration of contamina-tion dynamics when calculating absorbedthyroid dose even with direct measure-ments of incorporated 131I activity. Onthe territory of Bryansk region the con-tamination dynamics is well described by


47

the approximation of one-time deposi-tion. For this reason, the calculationsof this work and model [9] are in quali-tative agreement for Bryansk region. Theopposite is true for Kaluga region. Thedose calculated by model [9] differs

from that estimated by the model de-scribed in the present work approxi-mately by a factor of 2 for Zhizdra ofKaluga region.

0 10 20 30 40 50 60

10

100

Zhi zdra - 88 cG y

K hastovi chi - 132 cG y

M odel [9] - 168 cG y

Zhi zdra U lyanovo K hastovi chi N ovozybkov M odel [9]

Fig. 4. Intake rate b(t) (upper part) and 131I activity incorporated in the thyroidof a 3 year old child A(t) (lower part) calculated by our model and model [9].

The starting data: 131I incorporated activity for 25 May 1986 - 37 kBq;the child lives in one of the indicated populated points all the time.

2. Verification of results of thyroidradiometry in the population of the con-taminated areas in Bryansk region

The measurements of the activity ofiodine radionuclides in thyroid for thepopulation of Bryansk region used inthis work were performed in May-June1986 in the laboratory of clinic dosime-try of the Bryansk oncological dispen-sary with the methodological help of

MNIRRI (Moscow) and NIIRG (St.-Petersburg). These data include 1619measurements of incorporated activity inresidents of the region and residents ofother regions of CIS, mostly thoseevacuated from the zones of heavy radio-active contamination.

In the large-scale examinations of1986 a radiometric unit “ Gamma” wasused. It was employed in the oncologicaldispensary for measuring incorporated


48

activity of 131I administered for diag-nostic and medical purposes. This unitis based on a scintillation detector NaI(TI) with a cone collimator of 1 cmlead; the minor diameter is 4.5 cm andthe major one - 13 cm, the length - 19.4cm. During measurements the detector wasplaced at a large distance from a thy-roid, which is the reason for using anapproximation of a point source for ir-radiation of radionuclides incorporatedin the thyroid.

The unit was calibrated in 1986 by asource of 131I with a known activitywhich was placed in a flask for injec-tions. During the calibration of the“ Gamma” unit radiometer the source wasin a lead container. For measurementsthe container lid was opened and thecontainer was connected with a collima-tor with a detector and then the count-ing rate N

s above the flask was meas-

ured. For assessment of incorporated ac-tivity of 131I the following relation wasused

AA

NN Ni

s

s

� � �( )1 2� ,

(12)

where N1, pulse/s is the counting rate

of the unit when the collimator edge isnear the chin;

N2, pulse/s is the counting rate of

the unit when the collimator edge is onthe thigh of the patient;

�� is a coefficient accounting for“ glow” of the detector by �-radiationof cesium radionuclides incorporated inthe body of the patient (in this workthe value �� is taken to be = 0.9 [22]);

As is activity of the 131I in the cali-

bration source;N

s, pulse/s is the counting rate of

the unit from a calibration source;A

i is the estimated activity in thy-

roid.The time of measurement of incorpo-

rated activity and activity of the cali-bration source was not taken down duringthe studies of 1986, but as reported in[23] it was constant and was equal to120 s.

During the time period when the in-take of 131I by thyroid was most signifi-cant, and this lasted for about twomonths after the accident, several cali-bration sources of 131I were used in

“ Gamma” . We have not found any recordsin the logs, in which measurements wereregistered, what sources of 131I and whattime interval were used. To answer thisquestion we considered the time depend-ence of their activity (the logs containfor each patient an activity of the con-trol source and a number of pulses re-corded by the unit). The activities ofcontrol sources (in logarithmic scale)depending on time interval since the ac-cident are shown in Figure 5 (the upperpart) with estimated decrease in activ-ity due to radioactive decay of 131I.When three control (calibration) sourcesare used in measurements, the plotshould show three series of points fit-ting the straight lines in keeping with131I decay.

As is seen from Figure 5 it can beassumed that 4 calibration sources wereused in the “ Gamma” unit. Neverthe-less, for the first series (to 30 daysafter the accident) the activities ofthe control sources were close to thetheoretical curve except the interval17-19 May 1986 (21-23 days). Consideringthat the later measurements of the con-trol sources in this series are close tothe theoretical curve (i.e. this curvecan be assumed to originate from one ofthe control sources) we introduce ad-justments for the mentioned time inter-val. Thus, for the period 17-19 May 1986we have reduced the activities of thecontrol sources according to the recordsin the logs so that they correspond todecay of the same control source of 131I.This procedure, of course, does notguarantee obtaining reliable data frommeasurements of incorporated activity inthe considered period, but we have notfound any better way to explain andeliminate the detected contradictions.

An important technical characteristicof the unit is stability of sensitivityduring measurements. In 1994 we experi-mentally determined the sensitivity ofthe unit using calibration sources of131I. These works were conducted togetherwith the laboratory of clinical dosime-try of the Bryansk oncological dispen-sary. For calibration of the unit, whoseoperation during counting was controlledby a spectrometer, we used two 131Isources with the activity of 6.9 �Ci(0.255 MBq) (01.11.1994) and 10.68 �Ci(0.395 MBq).

The radioactive solution of 131I wasplaced in two sealed polyethylene cylin-


49

drical capsules of 1.25 diameter and 4.5cm length. Each vial contained 5 ml ofthe solution with radionuclides of 131I.The resulting sources were calibrated in4�-geometry in the laboratory of radioi-sotopic methods of MRRC of RAMS.

During the calibration of the“ Gamma” unit using the indicatedsources of 131I in 1994 the energy reso-lution of the detector used in May-June1986 measurements was determined Theresolution on the main line of 131I was18%. The measurements were performedwith the following geometry:

1) two sources of 131I were arrangedalong the horizontal surface in the formof the letter V at an aperture angle of45�;

2) a detector with the collimator wasplaced vertically above the sources atdifferent distances from the horizontalsurface.

During the measurements in May-June1986 the edge of the detector collimatorwas aligned horizontally near the chinof the examined person. For estimatingthe effect of screening of iodine radio-nuclides irradiation in thyroid by theadjacent tissues we conducted measure-ments of irradiation from a source of V-shape with allowance for screening ofthe source by organic glass of differentthickness. Depending on the age of thepatient the distance between the frontedge of the collimator and the skin sur-face in the area of thyroid was variedfrom 3 to 7 cm. The geometry of measure-ments and parameters were selected sothat the 1986 measurement conditionscould be simulated.

The current monitoring of measure-ments with use of a multichannel spec-trometer with a personal computer hasmade it possible to establish the lowerand upper thresholds for discriminatinga detector signal in such a way that thephotopeak of absorption of 131I radiationcould be recorded in full. The number ofpulses recorded by the unit by not morethan 0.5% differed from the photopeakarea estimated from a computer spectrum.

The results of measurements are pre-sented in Table 2.

As can be seen from data of Table 2,with the change in distance between thecollimator edge and source plane of 4 to6 cm the recording rate decreases by 17%in measurements without screening. Ascreening of 5 mm of organic glass leadsto reduction in the fluence of unscat-tered radiation by another 5%. So, onlydue to the uncertainty in the geometryof measurements in 1986 the error in de-termination of incorporated activity of131I could be 20-25%.

In Table 2 results from 8 to 10 wereobtained in measurements of sources at adistance of 5 cm with the screening oforganic glass, and result 11 - measure-ments in a half-opened container. Theanalysis has shown that measurementscompare well within the error. Hence,results of the calibration of the“ Gamma” unit in 1986 using the method-ology accepted at that time based onmeasurement of sources in a half-openedlead container can be considered as jus-tified. A possible methodological errordue to the uncertainty in measurementgeometry could reach 25%. A greater er-ror in measurements could have beenbrought in by measurement of activity ofthe calibration source in 1986 and meas-urements of discrimination thresholds.

Figure 5 (lower part) presents thesensitivity of “ Gamma” unit estimatedbased on the log records of 1986 as afunction of time for the performedlarge-scale measurements. In this figurethe solid line shows the sensitivitymeasured by us in 1994. By data of 1994(Table 2) the sensitivity of “ Gamma”was about 0.15 pulse/s/kBq. But accord-ing to the records of measurements inthe logs it changed from 0.00067 to 0.46pulse/s/kBq. A sharp change in the unitsensitivity 30-45 days after the acci-dent was the result of constructionmodifications of the operating unit[22].


50

10 20 30 40 50 600,0

0,2

0,4

0,6

t, day

U nit sensi ti vi ty in 1986; and m easured in 1994

I, p

uls

e/s/k

Bq

10 20 30 40 50 60

100

Ak,

kBq

A cti vi ty from logsU sed acti vi ty

Fig. 5. Measurement of characteristics of the radiometric unit “ Gamma” in 1986estimated from records in logs and from our calibration data of 1994.

Ak is the activity of 131I calibration source; I is the sensitivity of “ Gamma” unit;

t is the time after the accident.

Table 2Results of calibration measurements of 131I sources with the radiometric unit

“ Gamma” of the Bryansk oncological dispensary in 1994

d is the distance between the collimator edge and the surface on whichthe sources were placed, cm;��l is the thickness of organic glass screening, mm;N is the number of pulses registered in the area of 131I photopeak over the

time interval �T = 60 s (minus background);n is the counting rate, pulse/s;The measurement background was, on the average, 25 pulses per 60 s.

¹ d, cm ��l , mm N, pulse n, pulse/s

1 4 0 6521 108.72 6 0 5533 92.23 10 0 4225 70.44 14 0 3370 56.25 4 0 6699 111.76 4 2.15 6430 107.27 5 0 6085 101.4


51

8 5 2.15 5898 98.39 5 4.9 5820 97.0

10 5 8.4 5611 93.511* 6.5 0 5742 95.712* 9 0 4740 79.013* 12 0 3606 60.1

* - The measurements were conducted in the same way as in 1986. The detector wasplaced above the lead container with the 131I calibration source.

A significant change in the unit sen-sitivity during the period of large-scale examinations could have led towrong estimates of activity in thyroidif measurements of the calibrationsource and of the patient had been con-ducted at different time. But since themeasurement procedure was based on com-parison of activity in thyroid and ac-tivity of calibration source estimatedat the same time, i.e. at the same sen-sitivity of the unit [23] it may be con-sidered that the measurement results areacceptable for further estimation of ab-sorbed thyroid doses.

3. Analysis of results of thyroid radi-ometry in the population of the contami-nated areas in Kaluga region

Measurements of 131I activity incorpo-rated in thyroid in residents of popu-lated points in Kaluga region were per-formed in May-June 1986 by teams of MRRCof RAMS. In the large-scale examinationthyroid doses were estimated by one-timemeasurement of �-radiation dose ratenear a throat with a radiometer of FSR-68-01 (Field Scintillation Radiometer)type [26]. According to this method 131Iactivity in thyroid at time moment t

0

can be estimated by the formula:

A k G P t P t� � � �[ ( ) ( )],1 0 3 0 (13)

where k is the adjustment coefficient ofa specific device varying from 1 to 1.6;

G is a calibration constant relatingthe unit readings with 131I activity inthyroid and equal to 1/180 �Ci( �R/hour);

P1(t

0) is the dose rate near thyroid;

P3(t

0) is the dose rate in the room

where the measurements are made withouta patient.

It is relevant to say at this pointthat the methodological guidelines [12]recommend that as P

3(t

0) one should use a

result of dose rate measurement with thedetector placed right up to the humeralpart of the arm of a patient. It is alsosuggested that the following values ofthe calibration coefficient G be taken:

1/710 - for children of less than 3years old;

1/540 - for children of 3 to 10years;

1/360 - for all other patients.In work [24] the values recommended

for the coefficient G are:1/290 - for children of 1 to 8 years;1/250 - for children of 8 to 16

years;1/220 - for all other patients.The methodological guidelines [25]

recommend the value of 1/165 for the co-efficient G.

As can be seen, different works rec-ommend the values of the calibration co-efficient which are significantly dif-ferent from each other, particularly inwork [12], and this may significantlyinfluence the estimate of 131I incorpo-rated activity.

Formula (13) implicitly assumes thatP

1 is contributed only by irradiation of

131I accumulated in thyroid and that thepatient body does not screen signifi-cantly the detector of radiometer-dosimeter FSR-68-01. In reality the so-called ‘physiological” distribution ofiodine leads to its occurrence in blood,salivary glands, IT, bladder and otherorgans. One should also expect radionu-clides of cesium in the body of a pa-tient living on the contaminated area.These factors can be taken into accountif another measurement P

2 is made, for

example, near a thigh or liver and thevalue of screening of the background ra-diation near the measured parts of bodyis known.

Let us designate time dependenciesq

s(t) as 131I activity in thyroid ( s=1),

131I activity outside thyroid ( s=2) and137Cs activity in the whole body ( s=3)


52

for a given person. Let ks

x,y,t be the con-version coefficient from activity todose rate measured with a detector onthe body surface in the point with co-ordinates ( x,y ) at time moment t ; ��x,y,t -the coefficient of screening of the de-tector by the body in point ( x,y ) at mo-ment t .

The general equations determiningdose rate at time moment t

i in points 1

(near larynx - P1) and 2 (near liver -

P2) can be written as:

P t P k q Pr i ri sri

ssi ri

i( ) � � � � ��

�1

3

3� ,

r=1,2 . (14)

For solution of this system with re-spect to q

1

1

some assumptions and addi-

tional measurements should be made.Table 3 presents results of our model

studies of screening of the detectorFSR-68-01 (irradiation variants 1-8) anddetector of “ Photon” dosimeter (irra-diation variant 9). A polyethylene tank(phantom) of 34 �30�10 cm 3 and 10 l vol-ume filled with water was placed at aheight of 40 cm vertically or horizon-tally, thus simulating different partsof the body for a sitting person(child).

The �-radiation in the room where theradiometric study was performed in 1986was simulated with �-irradiation fromnatural sources in brick houses withconcrete ceilings and �-irradiation ofsimulated ring sources of 2 m radius:131I; 131I in a protected lead container of20 mm thick; 137Cs. The ring source wassimulated by rotating the phantom by 45 �

around the detector at the same positionof the point source and detector. Thescreening coefficient �� was calculatedas a ratio of mean values of dose rateswith and without a phantom and for thering source by integration of �� for allpositions of the phantom.

As follows from data of Table 3, whenusing the radiometer-dosimeter FSR-68-01the effect of detector screening by thephantom was not observed, which seems tobe explained by a better efficiency ofthe scintillation detector FSR-68-01 toregister the scattered and reflected ra-diation which is softer than the inci-dent one. Measurements with a gas-discharge detector of dosimeter “ Pho-

ton” , in which working with hardness iscompensated with the accuracy of 30% inthe energy interval 0.1-4 MeV, supportthis assumption (variant 9 of Table 3).

Table 4 shows results of our estima-tion of the screening coefficients of �-irradiation of natural sources when thedetector FSR-68-01 is placed near larynx( ��12) and liver ( ��

22) of a sitting person.These coefficients do not practicallydiffer for a given person, they areclose to unity, but are decreasing withheight and weight. In further calcula-tions it was assumed that ��

1i = ��2I = ��I = ��

and equals 1.0 for children to 14 years;0.95 for adolescents from 14 to 18 and0.90 for persons older 18 years.

For estimation of 131I thyroid incorpo-rated activity the system (14) should besolved with respect to q

1

1. Let us intro-duce additional quantities:

��

�

��

�

Cs

I

P P

P P

P P

P P

��

� �

��

� �

122

32

222

32

133

33

233

33

;

,

which can be estimated from measurementsof dose rate near larynx and liver,given only Cs or I radionuclides occurin the body of a patient, respectively.Then, considering (14) and equality ofscreening coefficients ��

1 = ��

2 = ��

131I ac-tivity in thyroid at the moment of meas-urement of dose rate near larynx andliver will be determined by the formula:

A qk G

P P

q

qP P

P

I

Cs I

I

� ��

� � �

� � � � � �

� � � �

11

11 21

31

32 22

232

1311

�

� � �

� �

( ) ( )

( ) . (15)

Where the adjustment coefficient isdetermined by

� � �1 121

111

211

221

k

k

k

k

and primarily accounts for the contribu-tion of �-irradiation of thyroid in doserate measurements near liver. Our meas-urements simulating conditions of indi-


53

vidual radiometry in 1986 show that thiscoefficient can be taken to be 1 evenfor newly-born with accuracy not worsethan 10%. For older persons the approxi-mation error =1 is much less.

The value of ��Cs (we will refer to itas a coefficient of illuminating of thedetector FSR-68-01 by �-irradiation of137Cs used in thyroid radiometry) was es-timated from results of our measurementsof dose rate near larynx and liver withthe detector FSR-68-01 among 73 resi-

dents of Ulyanovo district of Kaluga re-gion in July 1986, when radioactive io-dine had practically decayed. For fur-ther analysis 21 persons were selectedfor whom results of dose rate measure-ments were considerably (not less than1.5 times) higher the dose rate in theroom. Figure 6 presents values of coef-ficient ��Cs as a function of age. Theanalysis has not revealed any signifi-cant dependence of ��

Cs on age. The meanvalue of ��

Cs was 0.875 �0.026.

Table 3Screening coefficient �� of detector of radiometer-dosimeter FSR-68-01 (variants 1-8)

and dosimeter “ Photon” (variant 9) using ��-irradiation of natural and man-made( 131I, 137Cs) sources of phantom irradiation

Irradiationgeometryvariant

Phantomand detector

position

Sources of�-irradiation

Dose rate,�R/hour

�

1 Natural 11.39 � 0.06 1.041 � 0.010

2 Natural 11.05 � 0.10 1.039 � 0.019

3 Natural 11.49 � 0.10 1.063 � 0.020

4 Natural 11.25 � 0.10 1.001 � 0.010

5 Natural 11.46 � 0.10 1.092 � 0.01

6Natural + ringsource of 137Cs

23 � 1 1 1.00 � 0.10

7

Natural + ringsource of 131I 377 � 15 1 1.00 � 0.08

8 Natural + ringsource of 131I in

screened container

132 � 6 1 1.05 � 0.09

9 Natural 12.16 � 0.14 2 0.962 � 0.025

1 - the natural background in the room was 12.65 � 0.11 �R/hour;


54

2 - in measurements with the dosimeter FSR-68-01 the dose rate was 8.09 � 0.04�R/hour.

Table 4Coefficients of screening of ��-radiation of natural sources with the detector of

FSR-68-01 dosimeter placed near larynx ��1 and liver ��

2 depending on antropometric in-dicators of a patient

Weight,

Height,

Larynx Liver

¹ kg cm Dose rate*,�R/hour

��1

Dose rate*,�R/hour

��2

1 115 172 8.41 � 0.18 0.95 � 0.03 8.54 � 0.17 0.90 � 0.03

2 80 180 11.22 � 0.11 0.91 � 0.02 11.48 � 0.11 0.89 � 0.02

3 66 172 8.29 � 0.17 0.91 � 0.03 8.20 � 0.19 0.93 � 0.04

4 56 165 11.22 � 0.11 0.92 � 0.02 11.48 � 0.11 0.95 � 0.02

5 55 165 11.22 � 0.93 0.93 � 0.02 11.48 � 0.11 0.95 � 0.02

6 15 95 8.07 � 0.13 1.00 � 0.03 8.07 � 0.13 1.01 �0.04

* - a mean dose rate for a series of measurements in the point of detector with-out a patient.

0 10 20 300,1

1

< � Cs > = 0.875 �� 0.026

� C

s

T, years

Fig. 6. Coefficient ��Cs estimated from measurements of dose rate near larynx and

liver performed in July 1986 depending on age ( Ò) of a patient.

The mean value ��I was calculated by

the results of dose rate measurementsnear larynx and liver in 3 patients withremoved thyroid, approximately a week


55

after oral administration of sodium io-dide with 131I in the department oftreatment with open radionuclides ofMRRC of RAMS and it appeared to be1.13 �0.09. During computer scintigraphyof these patients 131I was found to befixed in salivary glands and IT. Theseexaminations were made with patientswith an empty bladder in which signifi-cant amount of iodine to be excretedfrom the body can accumulate. Unfortu-nately, this requirement was not ob-served during the large-scale radiomet-ric measurements in Kaluga region in1986. The presence of 131I in the bladderincreases its contribution to dose ratenear liver in estimation of ��

I whichleads to reduction in ��I . With this inmind we used the approximation ��

Cs = ��I =

�� = 0.875. Considering all the assump-tions made the formula (15) can be writ-ten as:

A k G P P

P

� � � � � �

� � � �

[

( ) ].11 21

311

�

� � (16)

Comparing formulae (13) (the activitycalculated by it will be designated asA

1) and (16) (the activity calculated by

it will be designated as A2) it can be

shown that accounting for illuminationand screening of detector FSR-68-01leads to a change in individual esti-mated activity by value ��:

A A

k G P P

P

1 2

21 31

311

� � �

� � � � � � �

� � �

�

[ ( )

( ) ].

� �

�

(17)

As is seen from formula (17), thedisplacement value depends on incorpo-rated activity of cesium radionuclides(through dose rate in the area of the

liver - P21

), external dose rate in theroom where the measurements of incorpo-rated 131I activity ( P

31) were made and

patient age (through the screening coef-ficient ��). Parameter �� can have differ-ent values because of two competing fac-tors: illumination and detector screen-ing. From fully verified data of indi-vidual radiometry in 1986 (measurementsnear larynx and liver) for 747 residentsof Kaluga region we selected those pairsfor which the unit readings near larynxand liver are not less than 1.5 timeshigher the dose rate in the room wherethe individual radiometry of thyroid wasperformed. By this group of 82 persons,we estimated mean value of adjustment ��:3.41 �0.41 kBq (0.092 �0.012 �Ci). Resultsof comparison of estimates of incorpo-rated 131I activity by formulae (13) and(16) are presented in Figure 7.

As can be seen from the figure indi-vidual �� are mostly close to the abovementioned mean value. This gives groundfor using a mean value of �� for a groupof patients in Kaluga region with nodata of dose rate measurements nearliver.

Thus, the performed analysis of partof data of individual radiometry of thy-roid in some areas of Kaluga regions inMay-June 1986 has shown that they can beapplied to estimate absorbed thyroiddoses after introducing correspondingadjustments.


56

0 10 20 300

10

20

30

A1 = A2 + 3.41 �� 0.43

A1

,kB

A 2, kBq

Fig. 7. The significance of taking into account cesium and iodine radionuclides dis-tributed in the whole body (“ illumination” ) and background screening in measure-ments of 131I incorporated activity in thyroid. A

1 is estimated 131I activity without

considering of “ illumination” and screening of FSR-68-01 detector; A2 is esti-

mated 131I activity with consideration of “ illumination” and detector screening.The solid line and formula is a regression ratio of A

1 and A

2.

4. Absorbed thyroid dosesfrom incorporated 131I

Results of individual measurements ofincorporated activity in residents oftwo populated points in Bryansk andKaluga regions are shown in Figures 8and 9 for different age groups. The fig-ures indicate the mean incorporated ac-tivity of 131I and standard errors inmean values (given sufficient number ofmeasurements) depending on time. Thesolid line shows a calculated theoreti-cal time dependence of incorporated 131Iactivity in thyroid.

The calculations were made for themean age for the group and mean transferrate estimated from individual measure-ments and characterised by a radioe-cological parameter �� in the model (see(4)).

The comparison of instrumental meas-urements and calculations with our modelindicates that the large-scale examina-tion of the population in Bryansk regionwas started several days after the pre-dicted time of reaching the maximum con-centration of 131I incorporated activityin thyroid. The measurements continuedtill complete radioactive decay of 131I.The analysis of data in Figures 8 and 9shows that the large-scale examinationin Kaluga region was started soon afterreaching the second predicted maximum.The qualitative difference from Bryanskregion was that the examination inKaluga region was carried out for ashort time, but it covered practicallyall children population of the contami-nated territories [10].


57

0 10 20 30 40 50 60

1

10

Estimate for 2ears old131I

measurement in0 4 years old

0 10 20 30 40 50 60

1

10

Estimate for 8years old131I

meas rement in4 12 ears old

0 10 20 30 40 50 60

1

10

A,kBq

t,d


measurement in12 18 years old

0 10 20 30 40 50 60

1

10

Estimate for 25ears old131I

measurement in morethan 18 years old

Fig. 8. Mean measured activities of 131I in thyroid A for different age groups ofresidents of Novozybkov, Bryansk region as a function of time; t is time after theaccident. The solid line is calculation by our model for mean intake rate of 131I ac-tivity in the thyroid determined by the coefficient �� = 0.16, see (4).


58

0 10 20 30 40 50 600,1

1

10


measurements in 0-4years old

0 10 20 30 40 50 600,1

1

10



0 10 20 30 40 50 600,1

1

10

A,kBq

t,day



0 10 20 30 40 50 600,1

1

10


measurements in more than18 years old

Fig. 9. Mean measured activities of 131I in thyroid A for different age groups ofresidents of the Ulyanovo, Kaluga region as a function of time t after the acci-dent, �� = 0.55.

Figures 10-13 present distributionsof residents of Bryansk and Kaluga re-gions (by the results of individual ra-diometry available to the author, it isthese groups that are implied later on,when reference is made to residents ofBryansk and Kaluga regions ) with respectto absorbed thyroid doses for differentage groups.

The analysis of the results in Fig-ures 10-13 leads us to make the follow-ing conclusions. Mean absorbed thyroiddoses decrease by a factor of 5 to 10when passing from a younger age group toadults in agreement with the known de-pendence (Figure 1). The indicated trendis observed for residents of both Bry-ansk and Kaluga regions. The distribu-tions show that a significant part ofdoses exceeding both probable and meanvalues by a factor of 5 to 10. This isbecause of individual features in diet,time of staying in the contaminated ar-eas and other factors determining indi-vidual absorbed dose.

The mean absorbed thyroid doses inBryansk region, according to our calcu-

lations, are higher those in Kaluga re-gion by a factor of 5 to 15 for differ-ent age groups (see text in italics be-low Figure 8). The ratio of absorbedthyroid doses for the population of thetwo regions reflects features of radioe-cological situation in May-June 1986.The essential contamination of the ter-ritory of Bryansk region occurred on 28-30 April 1986 and was characterised bythe high density of surface contamina-tion with iodine radionuclides as com-pared with the territory of Kaluga re-gion [4, 7].

The mentioned radioecological fea-tures were confirmed quantitatively bythe total of individual doses derivedbased on reconstruction of 131I contami-nation dynamics and measurements of thy-roid incorporated activity of 131I.

It should be noted that the meandoses for Bryansk region calculated inthis work for separate populated pointscannot be used for the whole region andthey may differ from those used in [9]for estimating collective dose and pre-


59

dicting incidence of radiation in-duced thyroid neoplasms.

Work [9] contains doses calculated intwo ways: without consideration of pro-tection measures and considering large-scale protection measures during 8-15day after the accident. With identical

starting data on incorporated activityour estimates of absorbed doses aresomewhat higher the variant presented in[9] without consideration of protectionmeasures, but lower calculations in [9]assuming that protection measures wereimplemented.

0 100 200 300 400 500 6000

10

20

M ean 86. 7 cGyStand. dev. 198 cGyM axim um 1250 cGySam ple 699 persons

All age groups

N,

%

D , cGy

Fig. 10. Distribution of residents of contaminated populated points of the Bryanskregion by absorbed thyroid dose D for all age groups; N is the percentage of exam-ined patients .


60

0 300 600 900 12000

10

20

30

40

Mean 210 cGyStand. dev. 211 cGyMaximum 1200 cGySample 154 persons

N,%

0 150 300 450 6000

10

20

30

Mean 79.6 cGyStand. dev. 128 cGyMaximum 1250 cGySample 283 persons

Agegroup4 to 12

years

0 100 200 3000

10

20

Mean 48.4 cGyStand. dev. 87.6 cGyMaximum 500 cGySample 37 persons

Agegroup12 to 18

years

D, cGy0 20 40 60 80 100

0

20

Mean 17.7 cGyStand. dev. 33 cGyMaximum 300 cGySample 225 persons

Agegroupmore than 18

years

Agegroup0 to 4

years

Fig. 11. Distribution of residents of contaminated populated points of the Bryanskregion by absorbed thyroid dose D for different age groups; N is the percentage ofexamined patients.

0 20 40 600

5

10

M ean 7.1 cGyStand. dev. 10. 1 cGyM axim um 193 cGySam ple 644 persons

All age groups

N,

%

D , cGy

Fig. 12. Distribution of residents of contaminated populated points of the Kalugaregion by absorbed thyroid dose D for all age groups; N is the percentage of exam-ined patients.


61

0 15 30 45 600

5

10

15

Age group

M ean 3.5 cG y

M axim um 4.6 cG y

Stand. dev. 19. 3 cG y

Sam ple 130 persons

Age group

Fig. 13. Distribution of residents of contaminated populated points of the Kalugaregion by absorbed thyroid dose D for different age groups; N is the percentage ofexamined patients.

In our calculation we did not usedata of work [9] on implemented large-scale protection measures restrictingthe oral intake of radioactive iodinewith contaminated milk. This aspect ofdose estimation is planned to be ana-lysed in detail in next publication,which will include data of individualpolling of the population in the con-taminated areas of Bryansk and Kalugaregions.

Data of individual thyroid radiometryneeded for correct estimation of ab-sorbed thyroid dose for residents ofcontaminated areas are much more scarcethan the number population itself.Therefore, estimation of population doserequires using some other methods. Themost wide-spread method is estimation ofthe absorbed thyroid dose by mean den-sity of 137Cs depositions in a populated

point [9, 11] using the statistical hy-pothesis:

D a b� � � � , (18)

where D is the mean absorbed thyroiddose for a group of persons;

a, b are parameters;�� is the mean density of soil con-

tamination with 137Cs.In work [9] parameter a in equation

(18) is taken to be 0; in work [11] itis not 0 and dependent (as is parameterb) on geographical location of a givenpopulated point (belonging to a certainadministrative area). A strict statisti-cal analysis of hypothesis (18) shouldtake into account the following consid-eration which is of principal impor-tance.


62

Individual absorbed thyroid dose D isa random value with an unknown, in ageneral case, distribution law. The pre-sented distributions of absorbed doses(Figures 10-13) show that the D distri-bution law is rather complex. It can beassumed that it is close to alog normaldistribution (for all distributions inFigures 10-13 the mean square deviationof absorbed dose of the order of magni-tude of the mean value).

Hence, for a strict analysis of hy-pothesis (18), in fact, one should in-troduce an additional quantity - alognormally distributed random valuewith a zero mathematical expectancy andan unknown variance s��

2 (which should beestimated as are parameters a and b).The unknown variance s��

2, in a generalcase is dependent on �� and its distribu-tion law (which is lognormal too).

After finding the parameters of equa-tion (18) by the method of likelihoodmaximum, using a given sample one shouldverify the proposed hypothesis about arandom value (zero mathematical ex-pectancy and belonging to the class oflognormal distributions). Only aftersuch a procedure, providing statisticalsignificance of parameter estimates, canthe frequently used formula (18) be con-sidered to be well justified. In works[9, 11] such estimates have not been de-rived, but the standard formulae of theleast square method are used (for randomvalues with the normal distributionlaw).

Therefore, hypothesis (18), in thestrict sense of the word, can not beconsidered justified. In the followingarticle we will discuss this issue basedon a better knowledge of the dynamics ofdepositions of radionuclides consideringdata of collectors [2]. We are also go-ing to address the rest data of individ-ual radiometry after their verification.In the present work only starting datafor such analysis will be presented.

Figure 14 compares the mean absorbedthyroid doses for different age groupsof residents in Bryansk region with meandensity of 137Cs contamination of popu-lated points. The figure also shows es-timated correlation coefficient r be-

tween individual values of absorbed thy-roid doses and mean density of 137Cs con-tamination of populated points. As canbe seen from the figures the low valueof the correlation coefficient does notallow us to propose, at his stage, a hy-pothesis on linear relation of mean ab-sorbed thyroid dose and mean density of137Cs contamination of the populatedpoint.

The ratio of absorbed thyroid dosesand mean 137Cs contamination density canbe estimated wrong if residents of cit-ies and town are included in the analy-sis. People living in such populatedpoints consumed milk which was mixedfrom produce from different places. Atypical example of this is the situationin the settlement Krasnaya Gora of Kras-nogorsky district of Bryansk region. Aswas first noted in [9], the thyroid in-corporated 131I activity in the popula-tion of the indicated populated pointsis considerably above the activity pre-dicted based on 137Cs contamination den-sity of 218 kBq/m 2. The reason was thatthe settlement Krasnaya Gora receivedmilk from neighbouring heavily contami-nated areas (to 2960 kBq/m 2 by 137Cs)which was consumed by residents and thisinfluenced the 131I incorporated activityand hence absorbed thyroid dose.

The totality of presented data do notlet us accept the hypothesis on relationof absorbed thyroid doses and 137Cs con-tamination density in the populatedpoints as a function (18), even withoutthe strict approach described above. Itshould also be borne in mind that basedon analysis of gamma-spectrometry ofsoil samples from Kaluga region [4], thecorrelation coefficient between 131I and137Cs deposition density is about 0.7,which (taking into account what is saidabove) is also an argument for abandon-ing the hypothesis in question. We be-lieve that methodologically it would bemore appropriate to continue the workusing all available data of 131I measure-ments in 1986 to determine more


63

0 1500 3000 45000

200

400

Agegroup0 to 4

years

r = 0.19

0 1500 3000 45000

100

200

300Agegroup4 to 12

years

r = 0.35

0 1500 3000 45000

50

100

Agegroup12 to 18

years

<D>,cG

r = 0.65

0 1500 3000 45000

50

100

150

�, 137Cs, kBq/m 2

Agegroupmore than 18

years

r = 0.40

Fig. 14. A comparison of age absorbed thyroid doses < D > in residents of the con-taminated areas of Bryansk region as a function of the 137Cs contamination density �.r is correlation coefficient between individual doses and the mean contaminationdensity of 137Cs.

accurately dynamics and density of 131Idepositions on the contaminated areasof CIS.

Conclusions

The space-time characteristics offormation of radioactive contaminationis a factor of significance which shouldbe taken into account in estimation ofabsorbed thyroid doses. The need for anew methodological approach to dose cal-culation based on retrospective recon-struction of dynamics of radioactivecontamination has been confirmed quanti-tatively.

The verification of data of individ-ual radiometry of residents of Bryanskregion obtained in Bryansk oncologicaldispensary in May-June 1986 has shownthat these data can be used for furtherestimation of absorbed thyroid dose.

The verification of data of individ-ual thyroid dosimetry in residents ofKaluga region obtained by specialists of

MRRC of RAMS in May-June 1986 usingradiometers-dosimeters FSR-68-01 hasdemonstrated the necessity of introduc-ing an adjustment which tends to reducethe earlier estimated activities of in-corporated 131I. The mean value of ad-justment is about 0.1 �Ci and it doesnot depend on age of a patient.

The mean absorbed thyroid doses inresidents of some contaminated points ofBryansk region are higher those inKaluga region by a factor of 5 to 15 indifferent age groups.

Based on the body of data of individ-ual thyroid radiometry in residents of


64

Bryansk region we have failed to findquantitative confirmation to the hy-pothesis [9, 11] concerning a dependencelike D = a + b ��, where D is mean thyroiddose; �� is 137Cs soil contamination den-sity; and a and b are constants.

In conclusion the authors expresstheir sincere thanks to Prof.E.G.Matveenko, G.A.Davydov andB.Ya.Drozdovsky, heads of departments ofMRRC of RAMS, G.E.Baleva, Bryansk on-cological dispensary, Director of infor-mation-computer center of Health Depart-ment of Administation of Bryansk regionB.G.Kvitko and head of laboratory ofclinical dosimetry of Bryansk oncologi-cal dispensary V.N.Antropov for supportin work, assistance in data collectionand processing and members of Laboratoryof radiation ecological informatics ofMRRC of RAMS O.M.Klokova andT.I.Khvostunova for thorough work withthe initial data.

References

1. Buzulukov Yu.P. and Dobrynin Yu.L.Release of radionuclides during theChernobyl accident. The Chernobyl pa-pers, Vol. 1 - Doses to the Sovietpopulation and early health effectsstudies. Ed. Steven E. Merwin andMichail Balonov. - Washington: REPS,1993. - P. 3-22.

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Documents

Influence of dynamics of I fallout due to the ChNPP ... · iodine radionuclides in the summer of 1986 and individual diet. The present work describes a method-ology for calculating