Discrepancy between the Contamination Pattern and the Daily Radiation Survey
Data Used for External Dose Estimation of Evacuees from the 30-km Zone
IMANAKA Tetsuji and KOIDE Hiroaki
Research Reactor Institute, Kyoto University
Kumatori-cho, Sennan-gun, Osaka, 590-0494 Japan
Introduction
A series of the first explosions at the Chernobyl-4 reactor occurred at 01:23:45-50 on April 26, 1986. Thelarge amount of radioactivity release continued about 10 days due to the graphite fire at the reactor core.
Figure 1 shows daily release of radioactivity based on the data given in the report the USSR government
presented to IAEA in August 1966 (1986 USSR Report) [1] Depending on the fluctuation of wind direction,
radioactive plumes flew to various directions from the destroyed reactor. Figure 2 indicates changes of winddirections and the contamination pattern during the period of large radioactivity release [2]. On April 26, thefirst strong radioactive plume moved to the west direction. Then, on April 27-28, radioactive plumes
contaminated north-west and north areas. The basic pattern of the contamination near the Chernobyl power
plant (ChNPP) was considered to be formed in the first three days. After that, the plumes direction moved toeast and south directions.
On April 27, the next day the accident, the Pripyat city where workers of ChNPP were living wasevacuated. Meanwhile, the people living in settlements other than Pripyat were left uninformed about theaccident. It was on May 2 that the evacuation of all people within the 30-km zone was decided [3]. The
evacuation finished around May 10. That is, these evacuees had been in the strong contamination for one
week or more. Therefore, these evacuees were supposed to receive far more radiation than Pripyat citizens.
Table 1 shows estimates of average external dose for evacuees from the 30-km zone given in 1986
USSR Report. It is noted that the average dose of 26,200 evacuees from 19 settlements within 15 km fromChNPP was 450 mSv, which is 14 times larger than the value for Pripyat citizens of 33 mSv. Unfortunately,detailed information how to obtain these values was not given in the report.

④4/27-28

70③ 4/27-28

Radioactivity(MCi)
放射能放出量（ＭＣｉ）

60⑤ 4/2850① 4/2640② 4/26-27⑥ 4/293020⑧ 5/2

100

⑦ 4/29-5/1

4/26 4/27 4/28 4/29 4/30 5/1 5/2 5/3 5/4 5/5 5/6

1986

Fig.1. Daily radioactivity release, excluding
noble gases.

Fig.2. Daily direction of radioactive plumes on
the back of 137Cs contamination map.
Made from Izrael’s paper [2]- 110 -Table 1. Average external dose of evacuees reported in the 1986 USSR reportDistance from the
Number ofPopulation
Average external
Chernobyl site
Settlement(persons)dose(mSv)
3 - 6 km(Pripayt)
45,00033
3 - 7 km
57,000540
7 - 10 km
49,00046010 15 km
108,200350
15 - 20 km
1611,600
52
20 - 25 km
2014,900
60
25 - 30 km
1639,200
46
On the other hand, Chernobyl Forum, which was organized by IAEA and other international institutions
to summarize 20 years of investigation on the consequences of the Chernobyl accident, reported that theaverage dose of the evacuees from the 30-km zone was 20-30 mSv and the maximum was several hundredmSv [4]. The main source of the Chernobyl Forum report was considered to be the study by Likhtarev et.alfor Ukrainian evacuees [5], in which the average external doses of 11.5 and 18.2 mSv were estimated forevacuees from Pripyat and other-than-Pripyat, respectively, by combining daily radiation survey data insettlements within the 30-km zone with individual questionnaires on the behavior after the accident. It isnoteworthy that the dose ratio of Pripyat to other-than-Pripyat is obtained 1.6, while the corresponding ratioof the 1986 USSR report is 4.8. Although the detailed data about in the Pripyat city was shown in reference,the daily radiation survey data were not shown for other-than-Pripyat settlements in the 30-km zone [5].
Radiation survey data within the 30-km on May 1, 1986
In March, 1986, at an international conference for the 10th anniversary of the Chernobyl accident held in
Minsk, an interesting map (Fig. 3) was presented about radiation exposure survey data on May 1, 1986, fivedays after the accident in settlements within the 30-km zone was released [6]. The maximum of 3,306 µGy/h131

North-south distance from the Chernobyl NPP (km)30157
Vigrebnaya

261
Pogonnoe

104
Dernovichi
252
Orevichi

20348

Savichi

348
Radin696
Molochki

Kranosel 870
Borshchevka
122
Doblyadi-10-20435
Ulasi

1044
Kulazhin
1914
Kryuki70
Grushnoe 44
70
Suvidi
70
Pirki

96
957
Mashevo
Zalese131 Khutor Les
3306
252
Khatki
Krasnoe
Benyovka
244
Rudki
261
Zimovishche
139 148
139
1053
Nov.
Shepelichi
Chikalovichi
148
Chapaenka
165
Yanov
Krasnoe
870
3045
174 Nov. Krasnitsa
Krivaya Gora
1740 ChNPP
9174 Chistogolovka 1044 331
Starocele
653
Kopachi87 St. Krasnitsa
174
435
87
Lelev Koshovka
Lubyanka
35
Stechaika
Ilintsi
2678
61
87
174
Razezhnee
ParyshevChernobyl
Korogod12235 Bichiki
96 Zalese
Lodizhichi
3587
GlinkaCherevach78
122 Chipol
Rodinka87

10 96Denisovich 174

0157
Puchin1043045
Usov44

Rudnya Veresnya

Rossokha-30-30-20-1001020
East-west distance from the Chernobyl NPP (km)30Fig.3. Dose rate in the 30-km zone on May 1, 1986,µGy/h．

- 111 -(380 mR/h) was seen in Krasnoe village located 6 km north of ChNPP. This dose rate equals 80 mGy/day.
In the same report, the time trend of dose rate per unit 137Cs deposition measured at Khoiniki located 50
km north from ChNPP was also presented, which was137
shown as diamonds in Fig. 4. The solid curve Table 2. Relative deposition composition ( Cs=1)(Calculation 1) is calculation by Imanaka, assuming and dose conversion factor of nuclides depositedaround the Chernobyl site.
fallout composition given by Izrael et.al [7] as well asRelative
Dose factor
Nuclide Half life
dose rate conversion factors (Table 2) [8]. The dottedcomposition* (µR/h)/(Ci/km2)
91
9.7 h
1.2
20curve is obtained by reducing the deposition ratio ofSr
95
65.5
d3.3
29
Zr-Nb to half of Izrael’s values. Calculation 2 could
Zr97
17 h1.6
29
Zrreconstruct the dose rate change well at the early stage of
95
35
d3.3
15
Nb
the contamination.
99
2.75 d7.5
2.8
Mo
Assuming that all radioactive contamination in
103
39
d
5.3
9.6
Ru
Krasnoe occurred at 12:00 on April 27 and all residents
106367 d
1.3
3.7
Ruwere evacuated at 12:00 May 3, Imanaka estimated the131
8.04
d20
7.6
Iaverage dose in Karasnoe to be of 0.48 Sv, including133
21 h40
12I
internal exposure from inhalation. In the process to135
6.7h3534
I
obtain this vale, the average body shielding coefficient132
3.25 d33
46
Teof 0.8 (Sv/Gy) and the average occupation-shielding
134
2.05 y0.5
29
Cs
factor of 0.62 were used from the Likhtarev paper [5].
13613d0.339
Cs
137
Taking into consideration the distribution of individual
30 y111Cs
140
doses, it was concluded that 18 % of residents in12.8d3.6
43
Ba
140
1.67 d3.6
39
Krasnoe could receive external dose more than 1 Sv, aLa
141
32.3
d
3.5
1.8
criterion of acute radiation syndrome [8].
Ce143
1.38 d3.1
4.9
Ce
Our estimates of radiation dose for evacuees agreed
144284d2
0.55
Cewith those given in 1986 USSR report, while about 3*; Decay-adjusted at the time of the accident
times larger than those by Likhtarev et.al.

May 1Fig. 4. Dose rate change in Khoiniki normalized per unit
deposition of 137Cs.

- 112 -

Radiation survey data within the 30-km zone on other days
In 2002, Muck et.al at GSF, Germany together with Ukrainian scientists published estimates of on inhalationdose for the evacuees from the 30-km zone [9], in which daily radiation survey data for the first two weeks inthe 30-km zone were indicated. More details were found in GSF report [10]. Parts of daily radiation survey
data are plotted in Fig. 5, divided directions around ChNPP into five sectors.
It is extremely surprising that maxima of exposure rate in Fig. 5 were not seen in April 26-28 when thestrongest plumes were released, but in later days. For example, in two settlements of Chitogolovka andTolsty Les in Sector-A (West) where the first radioactive plume passed over on April 26, the maximum ofradiation exposure were recoded on May 3 and May 4, respectively, and no serious radiation increase wasrecorded in the firth three days. It is also noted that in Sector-C (North) the maximum value (3,300 µGy/h) inKrasnoe is seen on May 1, which corresponds to the maximum in Fig. 3 of 3,306 µGy/h.
Considering the plume directions and 137Cs contamination pattern shown in Fig.2, it is difficult to acceptthe exposure rate trends of Fig. 5 as real ones, in spite of the description [6] that radiation survey was carriedout every day in all settlements in the 30-km zone after the accident. Preferably we should consider that the5,000

Sector A: West
Gamma dose rate, µSv h-1

4,000

Yanov 5km
Chistagolovka 7km
Tolsty Les 25km

3,000

Pripyat city 3km

2,000

1,000

26-A
pr
27
-A
pr
28
-A
pr
29
-A
pr
30
-A
pr
1-M
ay
2-M
ay
3-M
ay
4-M
ay
5-M
ay
6-M
ay
7-M
ay
8-M
ay
9-M
ay
10-May11-M
ay12-May0

1986

5,0005,000

Sector C: North4,000-1

4,000
N Sheprlichy 7km
S Shepelichy 12km

3,000

Benyovka 12km

2,000

1,000

Gamma dose rate, µSv h

Gamma dose rate, µSv h-1

Sector B: NNW

Krasnoe 6km
Kryuki 17km

3,000

2,0001,000

026-A
pr27-A
pr28-A
pr29-A
pr30-A
pr
1May
2May
3-M
ay
4-M
ay
5-M
ay
6May
7Ma
y
8Ma
y
9-M
ay10-May11-May12-May26-A
pr27-A
pr28-A
pr29-A
pr30-Apr1-M
ay2May3-M
ay4Ma
y5-M
ay6Ma
y7-M
ay8Ma
y9-M
ay
10-M
ay11-May12-May01986

1986
5,000

5,000

Sector E: South
Krivaya Gora 7km
Zimovish 6km

3,000

Koshevka 12km

2,000

1,000

Gamma dose rate, µSv h-1

4,0004,000
Kopachi 5km
Lelev 10km

3,000

Chernobyl 15km

2,000

1,000

0026-A
pr27-A
pr28-A
pr29-A
pr30-A
pr
1-M
ay
2Ma
y3-M
ay
4-M
ay5May6-M
ay
7-M
ay
8-M
ay9-M
ay10-May
11-M
ay12-May26-A
pr27-A
pr28-A
pr29-A
pr30-A
pr
1-M
ay
2-M
ay
3-M
ay
4-M
ay
5-M
ay
6-M
ay
7-M
ay
8May
9May10-M
ay
11-M
ay12-May-1

Sector D: EastGamma dose rate, µSv hUsov 11 km

1986

1986

Fig. 5. Dose rate monitoring data supposed to be used in the previous study. A-E: direction sector.

- 113 -monitoring activities in the first week for settlements within the 30-km zone were quite insufficient.
Therefore, dose estimations that directly used the data of Fig. 5 could lead to underestimation of real values.
Our new estimation of external dose for evacuees
After detailed investigation of the radiation survey data shown in Fig. 5, we assumed that, although theplotted data in the first three days could not be accepted, the data of later period could be used to estimate
external radiation of the evacuees, by extrapolating them to the earlier period. Examples of such
extrapolation for Chistagolovka and Novaya Shepelichy villages are shown as dashed lines in Fig. 6. Then
we assumed that the deposition occurred at a time at 12:00 April 26 and 00:00 April 27 in Chistagolovka andNovaya Shepelichy, respectively. The evacuation time at 12:00 May 3 was assumed for both villages. The
results of external dose estimation based on the extrapolation method are shown in Table 3 (Model-1)
together with estimates for several other settlements in different sectors.
We also applied another method to estimate external dose for evacuees (Method-2), in which external
exposure was evaluated based on the amount of total 137Cs deposition given in ref [10], relative deposition
composition [11] and dose rate conversion factors (Table 2). The results by the Model-2 are also shown in
Table 3. A reasonable agreement can be seen between the results by Method-1 and by Method-2.
6,000

Chistagolovka: Sector-A(W) 7 km
Gamma dose rate, µSv h-1

5,000

Survey data

4,000

Extrapolation
3,000
2,000
1,000

12-May13-May12-May13-May11-May9-May

10-May

8-May

7-May

6-May

5-May

4-May

3-May

2-May

1-May

30-Apr

29-Apr

28-Apr

27-Apr

26-Apr0

1986
3,000

Gamma exposure rate, µSv h-1

Novaya Shepelichy: Sector-B(WNW) 7km
Survey data
2,000

Extrapolation

1,000

11-May10-May

9-May

8-May

7-May

6-May

5-May

4-May

3-May

2-May

1-May

30-Apr

29-Apr

28-Apr

27-Apr

26-Apr01986
Fig. 6. Extrapolation of radiation survey data to the earlier period after the accident. Survey data
between May 1 to May 7 were used for extrapolation of Chistagolovak. May 3 – May 9 was usedfor Novaya Shepelichy.
- 114 -Table3 New external dose estimation based on two different methods as well as previous
GSF/Ukraine values.
137Average external dose until
evacuation, mSv
Present study
GSF/Ukraine(2000)
Method-1 Method-2

Date ofevacuation

Csdensity,
kBq/m2

512:00April26 12:00April29

18,450

180

250

9.5

Chistogolovka

712:00April26

12:00May 3

10,00023020070(B)WNW

N.Shepelichy

700:00April27

12:00May 3

3,530

96

72

13S.Shepelichy

1200:00April27

12:00May 3830

58

1223(C)
NorthKryuki

1700:00April28

12:00May 5

15,090140

200−Usov

1100:00April28

12:00May 3

4,790

160

55

154(D)
EastKryvaya Gora

700:00April29

12:00May 4

2,150685951

Zimnovishe

600:00April29

12:00May 34,020

559542(E)SouthKopachi

500:00April29

12:00May 4

2,690

59

6553

Chernobyl

1500:00April29

12:00May 5

1,780

14146

SectorVillage

Distance,km(A)
West

Yanov

Time ofdeposition

Compared with the values given by GDF/Ukraine group, our new estimations indicate significantly
larger values in Sector-A and Sector-Bt, while values in other sectors indicate agreement each other within an
acceptable range. Considering that radioactive plumes began to contaminate Direction-D (East) and –E(South) after April 29, the agreement seen in Sector-D and –E might may reflect the situation that thesystematic radiation survey in contaminate settlements other than Pripyat became effective at this period.
Conclusion
A clear discrepancy is seen between the contamination pattern around ChNPP and the daily radiation survey
data used to evaluate external dose of evacuees. Reassessment is necessary especially west and north-west
directions from ChNPP.
Refereces
1) USSR State Commission on the Utilization of Atomic Energy, “The Accident at the Chernobyl Nuclear Power
Plant and Its Consequences” August 1986.
2) Izrael Yu. A. “Chernobyl Radionuclide Distribution and Migration” Health Physics 93 (2007)410-7.
3) Piers Paul Read, “ABLAZE: A story of Chernobyl”. Aitken Stone & Wylie Ltd. London 1993.
4) Chernobyl Forum, “Chernobyl’s Legacy: Health, Environmental and Socio-economic Impacts andRecommendations to the Governments of Belarus, the Russian Federation and Ukraine”, IAEA, 2005．
5) I.A.Likhtarev et.al., “Retrospective Reconstruction of Individual and Collective External Gamma Doses ofPopulation Evacuated after the Chernobyl Accident”, Health Physics, 66(6), (1994) 643-652.
6) I.K.Baliff and V.Stepanenko ed., “Retrospective Dosimetry and Dose Reconstruction”, ExperimentalCollaboration Project ECP-10. EUR 16540, EC, 1996.
7) Израэль Ю.А. и др. «Радиоактивное загрязнение природных сред в зоне аварии на Чернобыльскойатомной электростаныий» МЕТЕОРОЛОГИЯ И ГИЛРОЛОГИЯ 1987 №2 5-18.
8) T. Imanaka, H. Koide, “Assessment of External Dose to Inhabitants Evacuated from the 30-km Zone Soon
after the Chernobyl Accident”, Radiation Biology Radioecology, 40, No.5 (2000) 582-588.
9) K. Much et.al., “Reconstruction of the Inhalation Dose in the 30-km Zone after the Chernobyl Accident”,Health Physics, 82 (2002)157-172.
10) G. Prohl et.al., “Reconstruction of the Doses to the Population Evacuated from the 30 km-zone around theChernobyl Reactor”, GSF-Bericht 07/2000 (2000).
11) K. Much et.al., “A Consistent Radionuclide Vector after the Chernobyl Accident”, Health Physics, 82(2002)141-156.

- 115 -