METHOD FOR ESTIMATING THE MEASURED EXPOSURE RATE OVER THE THYROID CORRESPONDING TO CERTAIN LEVELS OF THE ABSORBED DOSE IN THE THYROID FOR THE PUBLIC IN THE EARLY PERIOD AFTER A LARGE RADIATION ACCIDENT

Abstract

In case of a large radiation accident at the nuclear power station the most important radiation hazard for the public is internal exposure to the thyroid from radioiodine. The IAEA standard published in 2013 and accounting the experience of the Chernobyl and Fukushima accidents, recommends that, in the first 1-6 days after an inhalation intake of radioiodine the exposure rate over the thyroid be measured for the public and be guided by the following operational criteria: (a) 0.5 μSv · h-1 for children under the age of 7 years, and (b) 2 μSv ·h-1 for children over 7 years and adults. These operational criteria are indicative estimates corresponding to the dose range 100–200 mGy in the thyroid for all age groups. However, a wider range of thyroid doses and a more detailed breakdown of children into five age groups according to the recommendations of the ICRP are of interest. The purpose of this paper is to develop a method for estimating the values of the measured exposure rate over the thyroid corresponding to certain levels of the absorbed dose in the thyroid for members of the public of different ages for various conditions of inhalation of radioiodine intake and times of measurement in the early period after a radiation accident. According to the method developed in the paper for the committed absorbed dose 100-200 mGy in the thyroid the exposure rates over the thyroid were calculated depending upon the time of inhalation intake after the accident and the time span between the moment of intake and the moment of measurement, which were equal to: 0.6– 4 μSv ·h-1 - for children of 1 year and 4–27 μSv · h-1 – for adults. These values are significantly higher than those recommended by the IAEA Standard 0.5 μSv · h-1 and 2 μSv · h-1, respectively. It is important to stress that the IAEA recommendations provide conservative estimates of the measured exposure rate over the thyroid in order to maximize the coverage of critical group of the public for further dosimetric and medical examination. For example, for some options of the time of radioiodine inhalation intake and the time of measurement, the IAEA recommended the values of the exposure rate over the thyroid that can lead to an overestimation of the absorbed dose in the thyroid gland by a factor of up to 10.