Modelling the external radiation exposure from the Chernobyl fallout using data from the Swedish municipality measurement system

摘要

In connection with the Chernobyl fallout and the subsequent deposition of radionuclides in Sweden, Swedish municipalities launched a measurement program to monitor the external radiation exposure. This program encompasses measurements of the ambient dose equivalent rate 1 m above ground at selected locations, and repeats those measurements at the same locations at 7-month intervals. Measurement data compiled from the seven locations with the highest deposition were combined with data from aerial surveys since May 1986 of ground deposition of Cs-137, high-resolution gamma spectrometry performed at four locations in May 1986, and measurements from fixed continuous air gamma rate monitoring stations from 28 April to 15 May 1986. Based on these datasets, a model of the time pattern of the external dose rate in terms of ambient dose equivalent rate from the Chernobyl fallout was developed. The decrease in the ambient dose equivalent rate could, on average, be described by a four component exponential decay function with effective half-times of 6.8 +/- 0.3 d, 104 +/- 26 d, 1.0 +/- 0.02 y and 5.5 +/- 0.09 y, respectively. The predominant contributions to the external dose rate in the first month were from short-lived fission products superseded by Cs-134 and then Cs-137. Integrated over 70 y and using extrapolation of the curve fits, our model predicts that Cs-137 contributes about 60% and Cs-134 contributes about 30% of the external effective dose at these seven locations. The projected time integrated 70 y external effective dose to an unshielded person from all nuclides per unit total activity deposition of Cs-137 is estimated to be 0.29 +/- 0.0.08 mSv/(kBq m(-2)). These results are in agreement with those found in Chernobyl contaminated Russian forest areas, and emphasize the usefulness of maintaining a long-term and regular measurement program in contaminated areas. (C) 2017 Elsevier Ltd. All rights reserved.