Science of the Total Environment 409 (2011) 4811–4817

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Measurements of long-term external and internal radiation exposure of inhabitantsof some villages of the Bryansk region of Russia after the Chernobyl accident

C. Bernhardsson a,⁎, I. Zvonova b, C. Rääf a, S. Mattsson a

a Department of Clinical Sciences Malmö, Medical Radiation Physics, Lund University, Skåne University Hospital Malmö, 205 02, Malmö, Swedenb Research Institute of Radiation Hygiene, ul. Mira, 8, 197101, St. Petersburg, Russia

⁎ Corresponding author. Tel.: +46 40 33 67 31; fax:E-mail address: christian.bernhardsson@med.lu.se (C

0048-9697/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.scitotenv.2011.07.066

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 February 2011Received in revised form 26 July 2011Accepted 29 July 2011Available online 8 September 2011

Keywords:ChernobylLong-termEffective doseInternalExternal

A Nordic–Soviet programme was initiated in 1990 to evaluate the external and internal radiation exposure ofthe inhabitants of several villages in the Bryansk region of Russia. This area was one of the number of areasparticularly affected by the nuclear accident at the Chernobyl Nuclear Power Plant in 1986. Measurementswere carried out yearly until 1998 and after that more irregularly; in 2000, 2006 and 2008 respectively. Theeffective dose estimates were based on individual thermoluminescent dosemeters and on in vivomeasurements of the whole body content of 137Cs (and 134Cs during the first years of the programme). Thedecrease in total effective dose during the almost 2 decade follow-up was due to a continuous decrease in thedominating external exposure and a less decreasing but highly variable exposure from internal irradiation. In2008, the observed average effective dose (i.e. the sum of external and internal exposure) from Chernobyl137Cs to the residents was estimated to be 0.3 mSv y−1. This corresponds to 8% of the estimated annual dose in1990 and to 1% of the estimated annual dose in 1986. As a mean for the population group and for the period ofthe present study (2006–2008), the average yearly effective dose from Chernobyl cesium was comparable tothe absorbed dose obtained annually from external exposure to cosmic radiation plus internal exposure tonaturally occurring radionuclides in the human body. Our data indicate that the effective dose from internalexposure is becoming increasingly important as the body burdens of Chernobyl 137Cs are decreasing moreslowly than the external exposure. However, over the years there have been large individual variations inboth the external and internal effective doses, as well as differences between the villages investigated. Thesevariations and differences are presented and discussed in this paper.

+46 40 96 31 85.. Bernhardsson).

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1. Introduction

The Bryansk region of Russia was one of the most contaminatedareas after the Chernobyl Nuclear Power Plant accident in 1986 (IAEA,2006). This area is in the southwestern part of Russia, 150–250 kmnorth/northeast of Chernobyl. Atmospheric transport of radionuclidesin combination with rainout of the passing plume on April 27–30,1986, caused serious contamination leading to 137Cs levels up toseveral MBq m−2. Direct countermeasures, including evacuation andtemporary relocation of the inhabitants as well as major decontam-ination measures, were initiated to reduce the exposure of thepopulation. Specifically, the USSR Ministry of Health introducedprotective measures to reduce the radiation exposure of thepopulation by imposing restrictions on consumption of certainfoodstuffs and also renewed the annual dose limits for a limitedpart of the population (Balonov et al., 1999). To compare the exposureof the Chernobyl affected inhabitants with the dose limits (350 mSv

over 70 y, as of 1988) for proper remedial actions, large-scalemonitoring programmes were initiated. As the extent and magnitudeof the contamination levels became clear, and in response to thedecisions of many people to stay in their villages instead of resettlingto other areas with lower contamination levels, the scale of thecountermeasures was increased (EMERCOM, 1996). The exposuresituation and the dose limits have changed over time and in 1996 theRussian intervention level was defined as an additional annualeffective dose contribution from the Chernobyl accident exceeding1 mSv (as cited by Jacob et al., 2001). According to the Russian dosecatalogue (Russian Government, 2006), there were still 428 Russiansettlements with a predicted annual dose exceeding that limit in 2004(Jacob et al., 2009).

A joint Nordic–Soviet project was initiated in 1990with the purposeof providing the population of the Bryansk region with independentdose estimates from researchers outside the Soviet Union. The externaland internal exposure to radiation of the people in different villages inBryansk was measured yearly from 1990 to 2000, except in 1999. Theeffective dose estimates were first carried out by a Swedish–Norwegian–Russian group (Erkin et al., 1994; Wallström et al., 1995;Wøhni, 1995) and later on mainly by the Swedish–Russian group; the

4812 C. Bernhardsson et al. / Science of the Total Environment 409 (2011) 4811–4817

major findings were presented by Thornberg (2000; Thornberg et al.,2001, 2005). There it was shown that the external exposure levelsgradually decreased, with a rate which has also been indicated in otherstudies (UNSCEAR, 2000) and that the internal exposure decreased overlong time run, but fluctuated from year-to-year.

An important question from a radiation protection point of view ishow long it takes after a deposition of radioactive cesium to reachsteady state conditions in terms of external and internal exposure ofinhabitants in the affected area. To address this question, twoadditional studies were performed in 2006 and 2008. Their aimswere to assess the exposure of individuals in the same villages asbefore and to compare the findings with earlier results. In combina-tion with these studies, a more detailed mapping of the radiation inthe environment was carried out on a small scale, including studies ofthe variations within houses, gardens, schoolyards, etc. New dose-meters and dosemeter materials were also tested in the area, but themajor findings of these measurements will be presented elsewhere.

2. Materials and methods

Each year from 1990 to 2000 (except in 1999), in September orOctober around 4 villages in the Bryansk region of Russia were visitedwith the aim of estimating the inhabitants' annual effective doses ofradiation from external and internal exposure using TLD and whole-body measurements (Erkin et al., 1994; Wallström et al., 1995;Thornberg et al., 2001, 2005). These villages are located within a radiusof about 40 km from the town of Novozybkov, and had initial 137Cscontamination levels that ranged from 0.9 to 2.7 MBqm−2 (Ramzaev etal., 2006). The area is characterised by light sandy soils of low fertility,which fix cesium weakly (Shutov et al., 1993). Depending on themagnitude of the deposition within the villages, different measureswere taken to reduce the contamination level. In this study, the villageswere categorised as follows: i) non-decontaminated, i.e. no systematicmeasures were taken to reduce the radiation level; ii) partlydecontaminated, i.e. roads (mainly unpaved)were coveredwith gravel;and iii) fully decontaminated, i.e. the topsoil around public buildingswas removed and the roads were coveredwith asphalt, and some of thebuildings were cleaned. The main parts of these measures wereundertaken in the summer of 1989 (Balonov, 1993). In the presentpaper results from the two new surveys conducted in 2006 and 2008,have been used together with the previous ones to study the timepattern of the Chernobyl exposure in the villages (Table 1).

2.1. Effective dose from external irradiation from cesium

About twenty dosemeters were distributed to people (age range,6–79 y) in each of the villages visited in 2006 and 2008. The TL chipswere regular LiF:Mg,Ti (Harshaw TLD-100) with dimensions of3.2×3.2×0.9 mm3. All TL chips were stored in a lead container untilthe night before they were distributed. The dosemeters wereassembled in special polymethyl methacrylate (PMMA) holders thatconsisted of two identical 27×58×4 mm3 plates with two TL chips inbetween each plate. These dosemeters were then distributed to theparticipants, who were told to wear the dosemeters on a cord around

Table 1Characteristics of the 5 villages and the town (Novozybkov), surveyed in 2006 and 2008.

Demenka Starye Bobovichie

137Cs deposition in 1986 [MBq m−2] 1.2 1.1Countermeasures N.D.a P.D.b

Population in 1986 397 1173Population in 2002 294 1044Population in 2008 300 1050

a N.D., non-decontaminated.b P.D., partly decontaminated.c F.D., fully decontaminated.

the neck, like a necklace, during the measuring period. At the end ofthe measurement period, which was typically 1.5–3 months, thedosemeters were collected and the TL chips were shipped to Sweden,where a Toledo-type reader (Toledo, type 654; D.A. Pitman Ltd,Weybridge Surrey, UK) was used to assess the signals. The TL chipswere usually read within a few weeks after they were collected, butsome of the chips were read several months later. In 2008, thedosemeter holders were also prepared with two cavities filled with aregular Swedish household salt (NaCl; about 20 mg). The absorbeddose in the salt was estimated by measuring the optically stimulatedluminescence (OSL) from the salt crystals using a TL/OSL reader(TL/OSL-DA-15; Risø National Laboratory, Roskilde, Denmark). Foreach dosemeter, the mean OSL signal from 5 individual NaCl readings(5±0.5 mg each) was used to assess the absorbed dose using a pre-determined calibration factor (500 counts mg−1 mGy−1) taken fromBernhardsson et al. (2009, submitted for publication).

For the purpose of quantifying the individual effective dose fromexternal exposure, we used the same methods that were used in theprevious years (Thornberg, 2000). Accordingly, individual TL dose-meterswere carried by the participants on a string around the neck. Thedosemeters contained two LiF:Mg,Ti chips which were used todetermine the absorbed dose to the body surface, Dsurface (Gy). Theexternal effective dose, Eext (Sv), was assessed from the dosemeterreading by using different conversion factors based on phantommeasurements and Monte Carlo simulations (Jacob et al., 1986, 1988).Thedosemeter reading,Dsurface,wasfirst converted to the correspondingair kerma free in air, Kair (Gy), using the Kair/Dsurface conversion factor. Tothen assess the external effective dose a conversion factor from airkerma free in air to effective dose, Eext/Kair, was used. This factor variesonly slightly with photon energies between 0.2 and 1 MeV. Thus, theindividual external effective dose, Eext, was calculated according toEq. (1).

Eext = Dsurface·Kair

Dsurface

!·

EextKair

� �ð1Þ

For adults (N15 y), it has been estimated that Kair/Dsurface=1.11 Gy Gy−1 and Eext/Kair=0.83 Sv Gy−1 for a rotationally invariantgeometry. Experiments during the previous expeditions (Thornberg,2000) confirmed the product of the two latter terms in Eq. (1) (i.e. theEext/Dsurface ratio); hence, a value of 0.92 Sv Gy−1 has been used for alladults, which is also in agreementwith Golikov et al. (2007). For schoolchildren, the effective dose was calculated by multiplying the bodysurface dose by 0.95 Sv Gy−1 (Golikov et al., 1999).

The absorbed dose registered by the TLDs, Dsurface, has twocomponents:

Dsurface = Dperson+ Dts: ð2Þ

In Eq. (2),Dperson is the absorbed dose (Chernobyl 137Cs and naturalbackground radiation) accumulated when the dosemeters were wornon a person's body. The termDts is the absorbed dose from exposure ofthe TLDs during transportation and storage. In turn, Dts is dependent

Stary Vishkov Veprin Yalovka Novozybkov

1.3 0.9 2.7 0.7P.D.b N.D.a F.D.c P.D.b

757 256 1353 44697628 178 678 43038523 N/A 620 42178

4813C. Bernhardsson et al. / Science of the Total Environment 409 (2011) 4811–4817

on several variables that are determined by the history of each TLD.The term Dts thus represents the absorbed dose to the TLDs when thedosemeter was not worn by a person. It is determined as shown inEq. (3).

Dts =tT

DTL−Df

� �+ Df + Dtransp· T− tð Þ ð3Þ

The first term is the dose accumulated by the referencebackground TLDs inside the lead container. DTL is the total dose tothe reference background dosemeters, whereas Df is the doseaccumulated during air transport (one-way outside the lead contain-er), t is the time the TL chips were inside the lead container, and T isthe total time between calibrations. The final term in Eq. (3) is thetransportation dose, where Dtransp is the average dose rate when thedosemeters were transported and stored outside the lead container.

The two terms in Eq. (3), Df and Dtransp, were estimated by in situmeasurements in each village and during transportation. The radia-tion protection instruments SRV2000 (Rados, Finland) and GR-100(Exploranium, Canada) as well as an electronic personal dosemeter(Aloka, Mydose mini, PDM 101; Aloka Co. Ltd, Japan) were used toestimate these terms. After correcting the total dose for the additionaldose, Dts in Eq. (2), the absorbed dose to the TLDs when worn by aperson, Dperson, was reduced using a pre-Chernobyl background doserate of 45 μSv month−1 (terrestrial and cosmic radiation) (Fogh et al.,1999; Thornberg, 2000). To estimate the annual effective dose fromexternal exposure, a factor of 0.94 was used to account for theshielding effect of snow during winter (Thornberg et al., 2001).

2.2. Effective dose from internal irradiation from cesium

In each village, the inhabitantswere offered to get their body burdenof 137Cs determined. For this purpose, in vivo measurements wereconducted inside public buildings (schools and community offices)withrelatively low background radiation levels using two portable (63 mm(Ø)×63 mm) NaI(Tl) detectors coupled to a multichannel analyzer(DigiDart, EG&G Ortec, USA). During the 60 s acquisition times, eachperson satwith thedetector in his or her lapwith it pointing towards theabdomen (Palmer geometry; Palmer, 1966). The background count ratewas determined, and an individual correction for the shielding of themeasured person was carried out. The body burden of 137Cs (and inearlier years, 134Cs as well) was calculated using individual calibrationfactors that depended on the weight and waist circumference of the

Table 2Mean external dose rate (TLD) per unit deposition of 137Cs in the studied villages during thevillages located in the area. The average initial ground deposition of 137Cs (MBq m−2) is gmeasured by OSL in salt (μGy month−1 per MBq m−2) is shown on the last row in the tabl

Year Dose rate per 137Cs deposition (μSv month−1/MBq m−2)

Demenkaa Kusnetza St. Bobovichieb

(1.2) (0.95) (1.1)

1990 – – 1771991 – 180 –

1992 – 170 951993 – 134 891994 – 125 621995 – 123 –

1996 – 88 491997 – 70 51998 – 54 252000 – – –

2006 39 – 302008 33 – 22008 (OSL) – – 61

a Non-decontaminated.b Partly decontaminated.c Fully decontaminated.

measured person (Kaidanovsky and Dolgirev, 1997; Zvonova et al.,2000).

The individual body content of cesium was estimated in residentsof both sexes and ages, which ranged from 2 to 86 y, for a total of 318persons in 2006 and 292 in 2008. A few inhabitants in the town ofNovozybkov were also measured during the expeditions (57 personsin 2006 and 13 in 2008). To determine the annual internal effectivedose, Ėint (mSv y1), from 137Cs in the body, the age- and weight-dependent dose conversion factors presented in Publication 67 of theICRP (ICRP, 1993) were applied according to Eq. (4) (Zvonova et al.,1995).

Eint = r mð Þ· Am

ð4Þ

In Eq. (4), r(m) is the internal dose rate coefficient(mSv kg kBq−1 y−1) in a personwith a bodymassm (kg) and an activityA (kBq) of 137Cs in the body.

3. Results and discussion

3.1. External exposure from Chernobyl cesium

After the TLDs were collected from the inhabitants, they werestored in schools or other public buildings before being transportedback to Sweden. During storage and air transportation, the absorbeddose contributions to the dosemeters were estimated to Dtransp =0.05 μSv h−1 and Df=4.3 μSv, respectively. However, in 2008 a signif-icant background dose was added due to the prolonged storage timeafter the dosemeters were collected. No dosemeters at all werereturned from the village of Demenka that year. Nevertheless, theaverage estimate for Demenka in 2006 was extrapolated for use in2008. The extrapolation was based on more than 70 outdoor mea-surements in Demenka, at the same locations in 2006 and 2008 usingthe GR-110 instrument. Based on these measurements, the radiationlevel outdoors was on average 15% lower in 2008 than in 2006. Theexternal exposure level in Demenka in 2008 has been estimatedassuming a similar behaviour of the population in 2008 and 2006 interms of outdoor and indoor occupancy, and a parallel decrease inindoor exposure.

Table 2 summarises themeanexternal dose rate per 137Cs depositionin each of the villages visited from1990 to 2008. The data in Table 2, andas indicated in Thornberg (2000), show that the normalised effective

autumns of 2006 and 2008 as well as results from previous years in the same and otheriven in parentheses. The estimated absorbed dose rate per unit deposition of 137Cs ase.

St. Vishkovb Veprina Yalovkac Average

(1.3) (0.90) (2.7)

106 – – 142– 183 80 148– 169 81 12981 147 – 11392 112 48 88– 121 37 94182 105 41 93– 92 51 5548 99 25 60– – – 3429 – 7 2715 112 14 1664 180 39 86

4814 C. Bernhardsson et al. / Science of the Total Environment 409 (2011) 4811–4817

dose (the effective dose per unit ground deposition of 137Cs) was lowerin settlements where countermeasures have been carried out. From1990 to 2008, thenormaliseddose rates in Yalovkawere on average halfof those in the non-decontaminated villages and were generally lowerthan in the partly decontaminated villages. During the present study,thesefindingsmainly persisted over time, allowing one to conclude thatthe initial countermeasures in the form of decontamination of buildingsand their surroundings were the major reason for this difference.

As a comparison to the TLD dose estimates and as a test of the NaCldosemeters, the monthly absorbed dose rate per 137Cs deposition(μGy month−1 per MBq m−2) as registered by NaCl (read by OSL) inthe same dosemeter holders has also been determined. In the villageof Veprin no LiF chips were used (due to a lack of LiF chips) andtherefore, only an average value of the absorbed dose for NaCl is givenfor this village (Table 2). The absorbed dose to the NaCl is on averagehigher than the normalised effective dose to the TLDs, presumablydue to an underestimation of the background correction of the NaCldosemeters. Using the average conversion factor, which can bededuced from Table 2 (a factor of 0.2 μSv μGy−1), the normalisedeffective dose rate for the village of Veprin may be roughly estimated(40 μSv month−1 per MBq m−2 in 2008). The low level observed in St.Bobovichie in 2008 is difficult to explain based on the results of theTLDs. Nevertheless, although slightly varying results have beenobserved between the TLDs and salt dosemeters, NaCl showspromising properties as a tool for dosimetry in future events. Theprecision of the NaCl dose estimates may be improved by introducingindividual calibrations of each NaCl dosemeter and bymore accuratelydetermine the background signals.

When looking at the standard deviation of the external effectivedose rate to themean value (the coefficient of variation, Cv), there wasa clear difference in adults vs. children. The Cv of the monthly effectivedose was higher for adults (Cv,2006=85% and Cv,2008=140%) comparedto children (Cv,2006=50% and Cv,2008=70%). One reason for this is thatschoolchildren spendmostof their time in the sameor similar places, i.e.indoors during the school year, whereas adults have many differentoccupations and therefore spend their time at different locations.Golikov et al. (2002) determined average occupancy factors for ruralpopulations in the Bryansk area, based on a survey of 808 inhabitants.There it is shown that schoolchildren spend almost twice as much timeindoors as outdoor workers. If considering the whole population in thestudied villages the dose distribution is close to log-normal. The morehighly exposed individuals in this population, represented by theindividuals who exceed the 95th-percentile of absorbed dose, referredto as representative person by ICRP (2006), received an annual externaleffective dose that exceeds 0.9 mSv y−1 and 0.8 mSv y−1 in 2006 and2008, respectively.

3.2. Internal exposure from Chernobyl cesium

The measurements of cesium body content were usually con-ducted in schools during school hours. Thus, most of the participantswere schoolchildren. The averaged values might therefore be more

Table 3The average whole body concentration of 137Cs (±1 standard deviation of the mean) per yeais in parentheses.

Whole body concentration of 137Cs (kBq kg−1)

Demenka St. Bobovichie St. Vishkov

2006 2008 2006 2008 2006 2

Adult men 0.15±0.08 0.08±0.02 0.13 0.05±0.01 0.11±0.04 0(6) (10) (1) (3) (3) (

Adult women 0.15±0.03 0.06±0.01 0.07±0.01 0.05±0.01 0.04±0.01 0(31) (27) (15) (13) (8) (

Adolescents/children(b18 y)

0.06±0.01 0.05±0.01 0.06±0.01 0.08±0.02 0.08±0.01 0(10) (16) (86) (64) (47) (

Village average 0.15 0.06 0.06 0.08 0.07 0

representative of a young, rather than an adult, population. Some(n=13) of the measured individuals had a body burden of cesiumthat was very low, even below the minimum detectable activity(MDA) which on average was 0.5 kBq. These values were reported as“less than” a certain activity, and, for the purpose of calculating themean effective dose in the group, these individuals were assigned avalue of ½⋅MDA.

The amount of cesium in the body is related to body size andhence, the individual amount of cesium is generally highest in men,next highest in adult women, and lowest in adolescents/children. It isthusmore informative to look at thewhole body concentration (WBC)of cesium. The mean values of the 137Cs WBCs in the participatinginhabitants of the four villages and in the town of Novozybkov areshown in Table 3. Although the amount of cesium was lowest inchildren, the difference in themeanWBC between children and adultsis not very different. This is consistent with what was reported duringthe 1991–2000 period (Thornberg, 2000), except in 1998, when theaverage body concentration was significantly lower in childrencompared to the adults. The latter finding was associated with asuccessful mushroom harvest and the adults are assumed to haveeaten a larger amount of mushrooms than the children.

The data in Table 3 show that the average WBC of 137Cs wascomparable in the four villages and in Novozybkov. In Demenka,however, the average body concentration decreased by almost onethird from 2006 to 2008. Since the groups of persons investigated inDemenka were similar in 2006 and 2008, the higher levels registeredin 2006 could be explained by a higher intake of forest productsamong the measured (adult) participants that year. In Novozybkov,the inhabitants are not as dependent on forest products and otherlocally produced food as are the people living in the village; therefore,the average body concentration is slightly lower among the measuredindividuals fromNovozybkov. Supporting this argument, the standarddeviation of the mean was lower in Novozybkov compared to thevillages, which may indicate more diverse food intake among theindividuals in the villages. However, there was no systematiccorrelation between WBC and soil contamination in the villages.This might have to do with the measured people and their attitudestowards following the food recommendations. People living in themost contaminated villages, for example, might be more aware of therisk associated with the contamination and thus more careful aboutwhat they eat and how they prepare their food.

The log-normal distributions of the 137Cs body concentrations in2006 and 2008 had similar widths (b0.80 kBq kg−1) and 95th-percentile values (N0.20 kBq/kg), corresponding to an annual internaleffective dose of the representative person at 0.5 mSv y−1. However,the values of the WBC were slightly shifted to lower values in 2008:WBC2006=0.09 kBq kg−1 and WBC2008=0.07 kBq kg−1. In Fig. 1, theinhabitants are divided into three age groups: pre-school children,school children, and adults. A comparison with urban adults waspossible because the participants from Novozybkov were all adults.From 2006 to 2008, the WBC of 137Cs decreased in all of theinvestigated groups. The average WBC of the adults in Novozybkov

r in each village, stratified by age and sex (for adults). The number of persons measured

Yalovka Veprin Novozybkov

008 2006 2008 2006 2008 2006 2008

.08±0.01 0.12±0.01 0.18±0.04 – 0.14±0.03 0.05±0.01 0.05±0.017) (14) (14) (16) (33) (12).04±0.01 0.05±0.01 0.06±0.02 – 0.08±0.01 0.04±0.01 0.0517) (21) (15) (15) (24) (1).03±0.004 0.07±0.01 0.05±0.01 – 0.07±0.02 – –

49) (76) (57) (5).03 0.07 0.07 – 0.11 0.05 0.05

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

1-7 yrs. 8-17 yrs. Adults Novozybkov

Bod

y co

ncen

trat

ion

137 C

s (k

Bq

kg-1)

Age group

2006

2008

Fig. 1. Body concentration of 137Cs in 2006 and 2008. Uncertainty bars refer to 1standard deviation of the mean. In Novozybkov, the studied group consisted of adults(N17 y) only.

4815C. Bernhardsson et al. / Science of the Total Environment 409 (2011) 4811–4817

remained almost unchanged from 2006 to 2008, whereas the adults ofthe villages showed an average decrease of 30%. The changes in theWBC values can be large from year to year for several reasons, such asthe amount of available forest food or participants with differentattitudes in terms of following the food recommendations. In addition,despite that the group of measured persons was composed of similarinhabitants (in terms of age, sex, occupation) in each village and year,it was not necessarily the same individuals from year to year. Thismaypartly explain some of the year to year variability in the observedWBCin the villages.

3.3. Effective dose from external and internal exposure from Chernobylcesium

The annual effective doses of the inhabitants in the studied villagesfrom external and internal exposure, Ėext and Ėint, are presented asarithmetic means in Fig. 2. The estimates of Ėext and Ėint for 1986 to1990 (early period) are from the study by Ramzaev et al. (1996) andinserted is a magnified graph with the results from the present work1991–2008 (late period). Ramzaev et al. (1996) studied 12 villages inthe Bryansk area in 1986–1994, three of these were the same villagesas in the present study. Comparing the average values for these threevillages during the years the studies overlapped, i.e. 1991–1994, there

Fig. 2. Average annual effective dose from external and internal exposure in villages inthe Bryansk region from 1986 to 2008. Inset: average annual effective dose fromexternal and internal exposure in villages in the Bryansk region during the “late phase”.N.B. the effective doses were averaged over all age groups in all villages visited within agiven year.

is fairly good agreement (ĖRamzaev/ĖBernhardsson) for Ėint {1.1} and Ėext{0.7}. If the comparison is made using the data for the 12 villagesmeasured by Ramzaev et al. (1996) and all the five villages in thiswork the same ratios were Ėint {0.9} and Ėext {0.7}. This indicates thatthe average values presented here are representative of other villagesin the Bryansk area as well. An intercomparison study between thelaboratories within the project (St. Petersburg institute of radiationhygiene, Norwegian radiation protection authority and Department ofradiation physics at the University of Göteborg) was carried out in1991 (Wøhni, 1995) which showed a maximal difference of 4%between the laboratories TLD dose estimates. Hence, the under esti-mation indicated here, compared to Ramzaev et al. (1996), is mainlyrelated to variations in the sample groups of the investigated persons(i.e. sex, age, occupation, house construction).

During the early period after the Chernobyl release, therewere rapidchanges in both the external and internal effective doses. These changeswere mainly related to countermeasures including changes in thebehaviour of the inhabitants, such as their awareness of what to eat andhowmuch time they spent in the forest, aswell as downwardmigrationand fixation of cesium in the soil matrix. In the first years after theaccident (1986–1990) the average total effective dose was reduced byabout 80%. Thereafter, from1990 to1991, therewas a noticeable declinein the total effective dose. This is partly due to the lower Ėext, in thepresent study compared to the study of Ramzaev et al. (1996) whichwas related to different groups of inhabitants. But, it is probably also aneffect of themajor countermeasures initiated in 1989when village areaswere cleaned and the upper soil layerswere shifted to deeper layers andasphalting on certain roads etc. After 1990, hereafter referred to as thelate period, the average effective dosewas reduced from2.5 mSv y−1 to0.3 mSv y−1. There is a tendency to an increasing importance of Ėint after1986 and in some years during the late period it is dominating the totaleffective dose. Based on the measurements in Fig. 2, it is possible todescribe the decrease of the exposure during the late period as a sum ofthree exponentials. In the beginning of the late period (1991–1993) theaverage effective dose (sum of Ėext and Ėint) decreased at a rate of 20%y−1. After that period, there was a decrease of about 12%y−1 between1994 and 1998, and from 1998 to 2008, the estimated rate of decreasewas similar, on average about 9%y−1. Incorporated into these factors isthephysical decay of 137Cs,whichalone accounts for a yearly decrease of2.3%. To understand the other factors that explain the decrease, Ėext andĖint need to be considered separately. Though the decrease is slow, theexternal exposure decreases mainly due to redistribution of cesiumwithin the soil and nearby surroundings. The biggest reduction in theexternal dose rate, which was due to vertical migration, was during thefirst year after the accident. Golikov et al. (1993) investigated a virginsoil plot in the Bryansk area and found that the decrease in the externaldose rate, due to vertical migration, varied from 10% in 1987–1989 to30% between 1989 and 1991. Furthermore, recalculated values by Hilleet al. (2000) show an estimated dose rate reduction of 63% in 1986–1992 and 7% in the time period following 1992. A similar value is inagreement with the magnitude of the steady deceleration of theexternal dose in our measurements (assuming it is related to verticalmigration and physical decay). Amore recent study from the same area,carried out in 1996–2003, by Ramzaev et al. (2006) indicated anexternal dose rate reduction corresponding to 0.5-2%y−1 depending onlocation. The behaviour averaged values estimated by the TLDs show adecrease of the external effective dose of 2–3%y−1 in the period 2000–2008. Theannual decrease is also related to the time spentoutdoors or inforests by the specific participants, but this variation becomes relativelyconstant when the values are averaged. For radionuclides inside thebody, the effective dose decreases with a slower rate. In addition themagnitude of Ėint varies in a more complex and unpredictable way anddoes not follow the same temporal trend as Ėext. Individually, the mostimportant factors are the attitude towards and economic means forfollowing the food recommendations. Collectively, the major factor isthe abundance and intake of forest food (mushrooms, game, berries):

4816 C. Bernhardsson et al. / Science of the Total Environment 409 (2011) 4811–4817

these foodstuffs can have up to 100 times higher concentrations ofcesium than local milk andmeat and hence contribute considerably (10to 70%) to the internal effective dose (Balonov et al., 1999; Fesenko et al.,2000; Jesko et al., 2000). By 1992, supplies of imported food werelimited due to economic restrictions. Therefore people consumed moreforest products and more locally produced food in spite of recom-mended restrictions (Zvonova et al., 1995). This is shown in Fig. 2. Inyearswith a rich harvest ofmushrooms and berries, the average internaleffective dose can increase considerably, as in 1998 when Ėint was 4times as high as in the preceding year. In addition, due to seasonalvariations in the abundance of forest products, a significantly higher137Cs body concentration is expected during the autumn compared toother seasons of the year (Sekitani et al., 2010). The results presentedhere may thus slightly overestimate the annual average Ėint in someyears.

In 2006 and in 2008, the average effective dose to the inhabitants ofthe villages was 0.55 mSv y−1 and 0.34 mSv y−1, respectively, whichcorresponds to a reduction in the average effective dose by a factor of 40over 20 y (Fig. 2). The ratio of Ėext to Ėint shows how the exposurecomponents have changed during the survey period. In 2006 and 2008,the value of Ėint was about 50% that of Ėext. As the external exposuredecreases the dose contribution from internal exposure to the totaleffectivedose is becomingmore important (Fig. 2). In certain years,withrich mushroom harvest or during years of economic depression, theinternal exposure of the majority of the rural inhabitants, i.e. personsthat consume forest products, completely dominates the total exposure.Atmost during themeasuringperiod, the average internal effectivedosewas twice as high as the average effective dose from external exposure.

4. Conclusions

Over nearly two decades (1990–2008) since the Chernobyl PowerPlant accident, the average effective external and internal radiationdoses to the inhabitants of the villages of the Bryansk area havedecreased significantly. In the first years after the accident (1986–1990) the average annual effective dose decreased rapidly, down toalmost 1/7 over the 4 y. Ten to fifteen years after the accident, theyearly decline in the total exposure from Chernobyl 137Cs levelled outto an almost constant rate of decline of only a few percent per year.The slow decline in dose rate demonstrated in these particularsettlements more than two decades after the Chernobyl accident is aclear indication that the decline functions applied in preparednessmodels should be considered more carefully when predicting longterm dose contributions.

In 2008, the total annual effective dose to a representative personof the rural villages was determined to be 1.3 mSv y−1, justifyingoptimised countermeasures according to the Russian interventionlimit. The external radiation exposure is related to the contaminationlevel in each village, to the countermeasures taken and the time spentoutdoors vs. indoors, and to the shielding capacity of the houses. Theinternal exposure varies from year to year and depends on counter-measures, socioeconomic factors and on the intake of forest products.In the future, the external exposure component of the effective dosewill decrease further, while the contribution of the internal exposurecomponent will increase relative to the total effective dose. Thehighest doses of radiation will most likely be found in people whoconsume the greatest amounts of locally produced foodstuffs, andespecially forest products. These general conclusions should applyalso to inhabitants in other villages of the Bryansk region with similarsoil characteristics.

The long-termmeasurements described in the present paper showthat the patterns of time variations are different for external andinternal exposure, between nearby villages and between differentpopulation groups in the same area. This demonstrates the impor-tance of measuring programmes even in remote periods after anevent, to accurately estimate population doses both for remedial

actions and for comparison to model calculations for predicting futuredoses.

Acknowledgements

The authors appreciate the cooperation and participation of thepeople in the studied villages and the help of the local authoritiesin the Bryansk area. The project was financially supported by theSwedish Radiation Safety Authority.

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