The dose reconstruction of populations potentially affected by the accident at the Fukushima Daiichi nuclear power plant in March 2011 is of great importance. However, it has been difficult to assess internal thyroid doses to Fukushima residents (mainly from their intake of 131I) due to the lack of direct measurements. For the residents, only about 1,300 data points related to 131I are available, and 1,080 of the data points were obtained from the screening campaign that was conducted by the Nuclear Emergency Response Local Headquarters at the end of March 2011 in Kawamata Town, Iwaki City, and Iitate Village. Here, we reassessed thyroid doses to 1,080 subjects aged ≤15 y old using new age-specific conversion factors to determine 131I thyroid contents from net signals of the devices used, with consideration for the possible uncertainty related to the measurements. The results demonstrated that thyroid equivalent doses to the subjects were <30 mSv (excluding outliers). We also demonstrate dose distributions of each age group from the above three municipalities and those of subjects from Minamisoma City and Fukushima City. One of the findings was that the 131I intake was similar among different age groups in each of the three municipalities. This was consistent with the assumption that ingestion was a dominant route of intake rather than inhalation. The range of thyroid doses to Iitate Village residents was similar to that to Iwaki City residents even though the 131I concentration in tap water was much higher in Iitate Village than Iwaki City. The range of thyroid doses to Minamisoma City residents was similar to that to Iitate Village and Iwaki City residents, and the range for Fukushima City residents was smallest among the five municipalities. Since the major route of intake has remained unclear, this paper presents the plausible upper and lower thyroid doses, between which the actual doses are thought to mostly exist, based on two intake scenarios: single inhalation and repeated ingestion. Further research is thus necessary to extract useful evidence from the individual evacuation behaviors for improving the present internal thyroid dose assessment.
Estimating the internal thyroid dose received by residents involved in the 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident has been a challenging task because of the shortage of direct human measurements related to the largest contributing radioisotope to the dose, 131 I. In a previous dose estimation, we used the results of whole-body counter (WBC) measurements targeting 134 Cs and 137 Cs, based on the assumption that these radioisotopes were incorporated at the same time as 131 I in the early phase of the accident. The main purpose of this study was to clarify whether the trace of the early intake remained in the WBC measurements that were started several months after the accident.In the present work, WBC data of 1,639 persons from Namie town, one of the heavily contaminated municipalities, were analyzed together with their evacuation behavior data. The results demonstrated 10 that the Cs detection rate in the WBC results was several times higher in the late evacuees (who 11 evacuated outside the 20-km radius of the FDNPP at 3:00 p.m. [Japanese Local Time] on 12 March or 12 later) compared to the prompt evacuees (who evacuated before 3:00 p.m. on 12 March). Among the 13 adults, the Cs detection rates (and the 90th percentile values of the 137 Cs intake) of the prompt and late 14 evacuees were about 20% (5.4 10 3 Bq) and 60% (1.6 10 4 Bq), respectively. Approximately 20% of 15 the individuals analyzed were categorized as late evacuees. These differences in Cs would be caused 16 by exposure to the radioactive plume in the afternoon on 12 March, which was likely to influence the 17 late evacuees. On the other hand, the intake on 15 March when the largest release event occurred was 18 expected to be relatively small for Namie town's residents. In conclusion, the trace of the early intake 19 remained in the WBC measurements, although this would not necessarily be true for all subjects. The 20 results obtained from this study would provide useful information for the reconstruction of the early 21
Since the International Commission on Radiological Protection recommended reducing the occupational equivalent dose limit for the lens of the eye in 2011, there have been extensive discussions in various countries. This paper reviews the current situation in radiation protection of the ocular lens and the discussions on the potential impact of the new lens dose limit in Japan. Topics include historical changes to the lens dose limit, the current situation with occupational lens exposures (e.g., in medical workers, nuclear workers, and Fukushima nuclear power plant workers) and measurements, and the current status of biological studies and epidemiological studies on radiation cataracts. Our focus is on the situation in Japan, but we believe such information sharing will be useful in many other countries.
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