Early Intake of Radiocesium by Residents Living Near the Tepco Fukushima Dai-ichi Nuclear Power Plant After the Accident. Part 2: Relationship Between Internal Dose and Evacuation Behavior in Individuals

Difference in the Cesium Body Contents of Affected Area Residents Depending on the Evacuation Timepoint Following the 2011 Fukushima Nuclear Disaster

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, I. In a previous dose estimation, we used the results of whole-body counter (WBC) measurements targeting Cs and Cs, based on the assumption that these radioisotopes were incorporated at the same time as 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 that the cesium detection rate in the WBC results was several times higher in the late evacuees [who evacuated outside the 20-km radius of the FDNPP at 3:00 p.m. (Japanese Local Time) on 12 March or later] compared to the prompt evacuees (who evacuated before 3:00 p.m. on 12 March). Among the adults, the cesium detection rates (and the 90th percentile values of the Cs intake) of the prompt and late evacuees were about 20% (5.4 × 10 Bq) and 60% (1.6 × 10 Bq), respectively. Approximately 20% of the individuals analyzed were categorized as late evacuees. These differences in cesium would be caused by exposure to the radioactive plume in the afternoon on 12 March, which was likely to influence the late evacuees. On the other hand, the intake on 15 March, when the largest release event occurred, was expected to be relatively small for Namie town's residents. In conclusion, the trace of the early intake remained in the WBC measurements, although this would not necessarily be true for all subjects. The results obtained from this study would provide useful information for the reconstruction of the early internal thyroid doses from radioiodine in the future.

Reassessment of Internal Thyroid Doses to 1,080 Children Examined in a Screening Survey after the 2011 Fukushima Nuclear Disaster

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 I) due to the lack of direct measurements. For the residents, only about 1,300 data points related to I 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 I 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 I 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 I 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.

A new approach for reconstructing the 131I-spreading due to the 2011 Fukushima nuclear accident by means of measuring 129I in airborne particulate matter

To retrieve the diffusion trajectory of the ¹³¹I dispersed in the environment by the nuclear power plant accident in Fukushima in 2011, airborne particulate matter (APM) samples collected in the Tokyo metropolitan area were analyzed for their ¹²⁹I contents by means of accelerator mass spectrometry. In evaluating blank levels of chemicals and filters used for collecting APM, we established the analytical procedure for determining the ¹²⁹I activity of as low as 10^-8 Bq for a small piece of filter samples (about 0.1 cm²). Coupled with ¹³¹I data determined just after the accident, activity ratios of ¹²⁹I/¹³¹I were obtained with a mean value of 2.29 × 10^-8 (±28% of a standard deviation). This value is systematically smaller than a mean value of soil samples by 16-24% and the inventory data by 27%, suggesting that ¹²⁹I was partly lost from APM. As ¹²⁹I can be a proxy of ¹³¹I for APM, it is possible to trace how ¹³¹I in the particulate phase spread in eastern Japan and, furthermore, evaluate the internal radiation exposure due to ¹³¹I by inhalation of ¹³¹I-containing airborne particulates.

RAPID MONITORING OF INTERNAL CONTAMINATION USING A MOBILE RADIOBIOASSAY LABORATORY FOLLOWING RADIATION EMERGENCIES

In any radiation emergency, it may be necessary to monitor large numbers of people for internal contamination resulting from inhalation/ingestion of radionuclides released from the accident. The National Radiation Emergency Medical Center of the Korea Institute of Radiological and Medical Sciences constructed a mobile radiobioassay laboratory for rapid field-based monitoring of internal contamination. The main features of the mobile laboratory were designed and the results of performance were tested for rapid monitoring in this paper. We found that maximum throughput for internal contamination monitoring using the whole body counter installed in the laboratory was about 200 people per day. The minimum detectable activities were estimated for the in-vivo and in-vivo radiobioassay systems in the mobile unit. This mobile unit will improve the population monitoring capabilities for internal contamination of individuals affected following nuclear or radiological emergencies.