Dose Estimates from Protracted External Exposure of Inhabitants Living in Contaminated Area of Russia after The Chernobyl Accident

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1 Dose Estimates from Protracted External Exposure of Inhabitants Living in Contaminated Area of Russia after The Chernobyl Accident H. Yonehara 1, S. K. Sahoo 1, K. Kurotaki 1, M. Uchiyama 1, V.P. Ramzaev, N. Barishkov, A. Mishin, A. Barkovski 1 National. Institute of Radiological Sciences, Chiba , Japan Institute of Radiation Hygiene, St.-Petersburg, Russia INTRODUCTION Significant contamination with long-lived nuclides can be still found in living environments more than 1 years after the Chernobyl power plant accident. The evidences of excess incidence of the thyroid cancer among the young people in the contaminated areas have been reported(1,). The risks from the solid cancer and hereditary effects might be revealed after a long latent period. With respect to the radiation risk assessment from these effects, it is important to estimate the doses to inhabitants due to the protracted exposure as well as the doses with high dose rate just after the accident. The information of temporal change of the dose to the population is also significant in the risk analysis. Although there are many reports for investigations of the status of the exposure in early stage, the information in the remote stage is not enough to analyze the exposure. From the point of view, measurement of dose rate and soil contamination in the living environments and personal dose measurement were carried out in heavily contaminated areas in Bryansk region, Russia in the period of to investigate the present status of external exposure for the inhabitants. The protracted external dose to the inhabitants with a long-term temporal change was estimated using a model with the information presented from this study and previous ones. MATERIAL AND METHODS Heavily contaminated area in south-west districts of Bryansk region in Russia was investigated in this study. Personal monitoring and dose rate measurements in the living environments were carried out in the period from 1996 to 1998 to investigate the current status of external dose to inhabitants in this area. Glass dosimeter, model SC-1, manufactured by Toshiba glass Co. Ltd., Japan was employed for measurements of personal dose and long-term averaged dose in the environments(). Glass dosimeters were read using a measuring system, model FGD-0 Toshiba glass Co. Ltd. A personal dosimeter (model PDM-101) manufactured by Aloka Co. Ltd. and NaI scintillation spectrum surveymeter commercially called as SS-γ manufactured by Hamamatsu Photonics Co. Ltd. were used for measurement of the dose rate in air in the environments. These dosimeters were calibrated with an ion chamber calibrated in Electrotechnical Laboratory of Agency of Industrial Science and Technology, Tsukuba, Japan. The soil samples were collected at 7 points within an area of 80 m for each location. The core samples were divided into the 7 layers which was 0-, -4, 4-6, 6-8, 8-10, and 15-0 cm in depth. Each layer was mixed with corresponding layer of samples at 7 points. The activity concentration of the soil samples were measured with a germanium detector, model GC019, made by Canberra Co. Ltd., US. Depth distribution of 17 Cs contamination in soil in the environments was also analyzed to establish the model for individual dose estimation. RESULT S and DISCUSSION The typical depth profiles of 17 Cs concentration in soil samples collected in different living environments which are yard, kitchen garden, collective farm, pasture and forest are shown in Figure 1., Figure., Figure, Figure 4, and Figure 5, respectively. From these results, while large portion of 17 Cs contamination still remained in the surface of the ground in forest, the distributions were found to be flat within the depth of 0cm in the most living environment except yards. Different distributions were found in samples for yard. The reason for a distribution with large gradient in the yard might be that grass would overgrown on the ground or any plowing has not carried out there. These profiles of flat distribution were due to human activities which might be mostly plowing or countermeasures. 1

2 Figure Cs depth profile in soil collected in yard. Figure. 17 Cs depth profile in soil collected in kitchen garden. $%%&' ' Figure. 17 Cs depth profile in soil collected in collective farm.

3 ) Figure 4. ' 17 Cs depth profile in soil collected in pasture. & *#+ ' Figure Cs depth profile in soil collected in forest. The relationship between 17 Cs contamination in soil and absorbed dose rate in air measured with the dosimeter is shown in Figure 6. The activity concentrations of 17 Cs contamination in soil were normalized to those in May The air absorbed dose rate due to the contamination was calculated from 1cm depth dose equivalent measured with the dosimeter (PDM-101) as below. ADR = 0.8 x (DER NDR), where ADR: Air Absorbed Dose Rate (µgy h -1 ) due to the contamination DER: 1 cm Dose Equivalent Rate (µsv h -1 ) measured with the dosimeter NDR: 1 cm Dose Equivalent Rate (µsv h -1 ) due to natural sources. (0.0µSv h -1 ) C(t) [µgy h -1 /(MBq m - )] is defined here as dose rate in air corresponding to the initial deposition of 17 Cs on the ground due to the accident as a function of time (year) after the accident. The value of C(t) in different places was analyzed using the results of the measurements of dose rate in air and activity in soil samples in the contaminated area of Bryansk region in Russia. From the analysis, the value, C(1) (at 1 years after the accident) can be predicted by categorizing usage of the land. The typical values obtained from the results of the actual measurement are shown in Table 1.

4 , 1#% & ),,- $$'%*./' 0 + Figure 6. Relationship between 17 Cs contamination in soil and dose rate measured with the dosimeter. Table 1. Typical values of C(1) for different places. Type F Type P Type Y Type K C(1) Places categorized Forest Pasture Virgin land Yard Kitchen garden Farmland Temporal change of C(t) estimated with a vertical migration model of activity in soil developed by Golikov et al.(4) was shown in Figure 7. Only contribution of dose due to 17 Cs and 14 Cs contamination was taken into account in the estimation. It was assumed that every agricultural activities in collective farms and kitchen gardens started at one year after the accident. Annual Effective doses due to 17 Cs and 14 Cs contamination in the period from 1987 to 1999 in farmers, and forest workers were estimated by the model using a model with estimated values of C(t) in Figure 7. Accumulated effective dose = 1. xσσα i K i D 0 C(t) i t i α i : Occupancy factor for place i K i : Shielding factor in place i D 0 : 17 Cs surface contamination just after the accident (MBq m - ) The result of the estimation with the results of personal dose measurements was shown in Figure 8.. The personal dose measurements were carried out with the glass dosimeter and with TLD by Institute of Radiation Hygiene in previous study. The results of the estimation are in a good agreement with those obtained by using the personal dose measurements. The cumulative doses for the farmer in Novozybkov, that in Stary Vyshkov and forest worker in Novozybkov were estimated to be msv, 5 msv and 4 msv, respectively. The doses for the general inhabitants estimated by the model ranged from 10 to 60 msv. 4

5 %# % 56/%, 6/% &'' ) '4*+ &7'# '% ) Figure 7. Temporal change of C(t) estimated with a vertical migration model of activity in soil Novozybkov (farmer) IRH P.M. NIRS P.M. St. Vyshkov (farmer) IRH P.M. NIRS P.M. Novozybkov (Forest worker) Calender year NIRS P.M. Figure 8. Comparison between results of personal monitoring and model estimates P.M.: Personal dose measurement CONCLUSION Measurement of dose rate and soil contamination in the living environments and personal dose measurement were carried out in heavily contaminated areas in Bryansk region, Russia in the period of to investigate the present status of external exposure for the inhabitants. The depth profile of contamination in soil in most living environments was found to be influenced by human activities. Using a model with the information, protracted external dose to the inhabitants with a long-term temporal change was estimated. 5

6 REFERENCES 1. United Nations, Sources and effects of ionizing radiation, UNSCEAR 199 Report, United Nations (199). P. Jacob, et al., Thyroid cancer risk to children calculated, Nature Vol 9, 1-(1998). E. Piesch and B. Burgkhart, Photoluminescence dosimetry: the alternative in personal monitoring. Radioprotection, (1994) 4. V.Yu. Golikov, M.I. Balonov and A.V. Ponomarev, Estimation of External Gamma Radiation Doses to The Population After The Chermobyl Accident, The Chernobyl Papers Vol.I, S. E. Merwin and M.I. Balonov eds., Richland, Research Enterprises Publishing Segment, (199) 6