A new Approach to Assess the Doses to the Population in the 30 km-Zone after the Chernobyl Accident

Very few data are available to assess the exposure of the population within the 30 km zone. While gamma dose rate measurments were performed in most villages within the exclusion zone, virtually no measurements of activity concentration in air or the concentration of various radionuclides relevant to the inhalation exposure were carried out due to lack of monitoring capacity. Also practically no data on activity concentration in foodstuffs are available to evaluate the exposure by ingestion of contaminated foodstuff in the various settlements. To reconstruct the exposure in the various settlements for epidemiological purposes, in particular with regard to the internal exposure, a new approach was developed. Based on the fact that there was only dry deposition within the 30 km zone during the main fallout period, and that the deposition of 137Cs which was extensively determined in the various settlements at a later stage after the accident, the integrated activity concentration in air of 137Cs was derived from measurements of 137Cs per unit area for each settlement. The ratios of the various radioisotopes to 137Cs (radionuclide vector)were determined in a two-level approach in which in a first step an evaluation of isotopic ratios for each isotope group was performed and in a second step the ratios of one guide isotope of each element group relative to 137Cs was determined in dependence on distance of the plume. Preliminary results indicate that the effective dose due to inhalation amounted to 8 - 13 times the external exposure and the exposure caused by ingestion to about 2 - 2.5 times the external exposure in a village close to the reactor site and evacuated early or exposed early in the passage of the plume. Under those circumstances, the toal effective dose is more than 10 - 15 times greater than the external exposure. In villages at further distances from the site or predominantly exposed by a longer passage of the plume and evacuated at later stages of the accident the contributions to the total dose (in terms of effective dose) were approximately: 15 - 25 % from external exposure, 30 - 50 % inhalation and 30 - 40 % by ingestion for the adult, but 5 - 8 % from external exposure, 15 - 45 % inhalation and 50 - 80 % by ingestion for the 1 y-infant. As an example, for Janov at a distance to the reactor of 4 km and evacuation on day 3, the estimated total effective dose is 130 mSv, of which 92 mSv are due to inhalation. Of this dose 40 mSv are caused by 131I and 133I equivalent to a thyroid dose of approx. 800 mSv. For Chernobyl (distance to the reactor 15 km and evaluation on day 7) the total effective dose to adults is assessed at 38 mSv. About 40 % of this dose is due to iodine resulting in a thyroid dose of approximately 300 mSv. The influence of the various assumptions on the accuracy of the results is discussed.