External and internal irradiation of a rural Bryansk (Russia) population from 1990 to 2000, following high deposition of radioactive caesium from the Chernobyl accident.

In 1990, a joint Nordic-Russian project was initiated in order to make independent estimations of the effective dose to selected groups of inhabitants in a highly contaminated area around the city of Novozybkov in the western Bryansk region of Russia. The inhabitants were living in six villages with initial contamination levels of (137)Cs between 0.9 and 2.7 MBq m(-2). Some villages had been decontaminated, others not. Both school children and adults participated in the study. The external irradiation of 100-130 inhabitants was determined during 1 month in September-October each year from 1990 to 2000 (except 1999), using individual thermoluminescent dosemeters. The body burden of (137,134)Cs was determined by in vivo measurements in about 500 inhabitants annually from 1991 to 2000, and for a subgroup also with analysis of the (137)Cs concentration in urine. The mean effective dose (E) from external and internal irradiation due to (137,134)Cs deposition varied between 2.5 and 1.2 mSv per year between 1990 and 2000. The total mean E decreased, on average, by 9% per year, while the mean external dose decreased by 16% per year. The dose rate from internal radiation decreased more slowly than the dose rate from external radiation, and also showed an irregular time variation. The contribution from the internal dose to the total E was 30-50%, depending on the village. Predictions for the long-term changes in the effective dose to people living in the areas are presented. The cumulated E for the 70 years following the accident was estimated to be about 90 mSv with the assumption that both internal and external dose decrease by 2% per year after year 2000. The highest E during a life-time received by single individuals living in the area may amount to around 500 mSv considering the individual variations in E.