Internal doses to the public in the Fukushima prefecture from the Fukushima Daiichi NPP accident

Abstract

The article is devoted to the analysis of internal dosimetry in residents of Fukushima Prefecture due to the accident at the Fukushima-Daiichi NPP. On March 24-30, 2011, screening studies were conducted on measurements of 131I content in thyroid gland of children in those municipalities where the Japanese program for assessing the radiological consequences of the radiation accident “SPEEDI” predicted doses in the thyroid of one-year children from 100 to 500 mSv. Among 1080 children measured, 55% had results below the background, and the maximum estimate of the equivalent dose in the thyroid was 43 mSv. Measurements of several dozen inhabitants with other instruments and methods confirmed low levels of 131I in the thyroid. In total, the 131I thyroid direct measurement database of Fukushima residents contains approximately 1300 individuals. 75% quartile of individual thyroid dose distributions was below 10 mSv, the median was from 0 to 3.7 mSv among children aged from 0 to 15 years in the five examined municipalities. The contribution of short-lived iodine radionuclides was estimated as 15% of the dose from 131I for the main radioactive release on March 15. The effective dose of internal exposure to cesium radionuclides (134Cs + 137Cs), estimated from 10 thousand whole body measurements for the first year after the accident, was below 0.1 mSv in 90% of the examined people. Whole body measurements of children 6-15 years old in the second year after the accident confirmed the values of the effective dose of internal exposure less than 0.1 mSv, while the estimates of the effective dose of external exposure in the same children with using individual dosimeters determined the median distribution as 0.66 mSv with a maximum value of 3.45 mSv . The measurement results do not provide any basis to expect a future increase in the incidence of thyroid cancer and other radiogenic diseases among the Fukushima residents. Preventive evacuation of the population before the radioactive release, stay in enclosed dwellings, strict radiation control of food products have effectively reduced the expected doses of internal exposure to residents. In the predictions of the radiological consequences of the accident, it is necessary to take into account the actually implemented measures to protect the population, and local characteristics of the style of life and living.