Irradiation of members of the general public from radioactive caesium following the Chernobyl reactor accident. Field studies in a highly contaminated area in the Bryansk region, Russia

Sammanfattning: From 1990 to 1998, estimations of the effective dose from external as well as internal irradiation from 137Cs and 134Cs were carried out for inhabitants in rural villages in the Bryansk region, Russia, highly contaminated due to the Chernobyl accident in 1986. The villages were situated about 180 km from the Chernobyl power plant and the deposition of 137Cs was in the range 0.9-2.7 MBq m-2. Yearly expeditions were conducted in autumn by members of the Departments of Radiation Physics in Malmö and Göteborg, Institute of Radiation Hygiene, St. Petersburg, and the first years also the Norwegian Radiation Protection Authority. The dose levels and their change in time were investigated for various groups of the general public. The body burden of 137,134Cs, and hence, the effective dose, was estimated from measurements of the urinary concentration of caesium radionuclides, together with direct measurements of the body content using a portable detector. The effective dose from external irradiation was estimated from measurements with thermoluminescent (TL)-dosemeters worn by the participants during one month each year. In a special case study, the changes in biokinetics of 137Cs during pregnancy was investigated in a woman with an unintended intake of 137Cs via mushrooms from a highly contaminated forest in the area. During pregnancy there is an increased excretion of caesium resulting in a biological half-time of caesium which was 54% of the half-time before pregnancy. The ratio of the 137Cs concentration in breast milk (Bq L-1) to that in the mother’s body (Bq kg-1) was 15% one month after the child was born. The body burden of 137Cs in the Russian individuals calculated from the concentration of 137Cs in urine showed a good agreement with the body burden estimated from in vivo measurements in the same individuals. Normalisation of the caesium concentration in the urine samples by the use of potassium or creatinine excretion was found to introduce systematic differences as well as a larger spread in the calculated values of the 137Cs body burden as compared with calculations without normalisation, using the urinary concentration of 137Cs only. The yearly effective dose from external and internal irradiation to inhabitants in the Russian villages varied between 1.2 and 2.5 mSv as a mean for all villages studied between 1991 and 1998 and the internal effective dose was, on average, 30-50% of the total effective dose during that period. The effective dose from external irradiation decreased on average 15% per year, while the effective dose from internal irradiation varied, depending to a great extent on dietary habits and especially the availability of mushrooms. The cumulated effective dose for a 70-year period after the accident was calculated to be around 100 mSv with the assumption that the effective dose will decrease by only the physical decay of 137Cs (2% per year) after 1998. Individuals may receive considerably higher effective doses, up to 0.5 Sv during a life-time considering the large spread in dose values among individuals.