Whole-body counting of Fukushima residents after the TEPCO Fukushima Daiichi Nuclear Power Station accident

At the request of Fukushima Prefecture, the Japan Atomic Energy Agency (JAEA) started whole-body counting of residents on July 11, 2011 to assess radiation exposure after the TEPCO Fukushima Daiichi Nuclear Power Station accident. The JAEA has examined residents in Iitate, Kawamata, Namie, and 8 other local communities. The measurement capacity of the whole-body counting device is approximately 100 persons per day and the total number of people to measure reached 9,927 by the end of January 2012. Two types of whole body counters equipped with large-size NaI(Tl) detectors were used to perform the measurements. Routinely used phantoms (Canberra RMC-II transfer phantom or water-filled block phantom developed by JAEA) were used to perform peak efficiency calibration of the counters and the results of peak efficiency calibration were verified with different-sized bottle mannequin absorber (BOMAB) phantoms imitating an adult male, a 10-year-old and a 4-year-old. As the measurement started at 4 month after the accident, short half-life radionuclides such as 131I originating from the accident were not detected in this work. Approximately 80% of the residents had levels below the minimum detectable amount (MDA). No artificial nuclides other than 134Cs and 137Cs were found in the present whole-body counting. The maximum whole-body content of radiocesium (134Cs and 137Cs together) was 2.7 kBq for children 1 mSv, and the maximum CED was 3 mSv. The extrapolated 50th percentiles (medians) of the CED for 13-17 years old and those >17 years old were 0.02 mSv and 0.025 mSv, respectively. These values represent the basic knowledge for the reconstruction of internal exposure of the entire resident population

Direct measurements of employees involved in the Fukushima Daiichi nuclear power station accident for internal dose estimates: JAEA\u27s experiences

Japan Atomic Energy Agency (JAEA) performed internal dose measurements of employees involved in the Fukushima Daiichi nuclear power station accident. Nuclear Fuel Cycle Engineering Laboratories (NFCEL), one of the JAEA\u27s core centers, examined 560 of these employees by direct (in vivo) measurements during the period from April 20 to August 5 in 2011. These measurements consisted of whole-body counting for radiocesium and thyroid counting for radioiodine. The whole-body counting was conducted with two types of whole-body counters (WBCs): a standing-type WBC with two large NaI(Tl) detectors (FastscanTM, Canberra Inc.) and a chair-type WBC with HPGe detectors (GC5021, Canberra Inc.) installed in a shielded chamber made of 20-cm-thick steel. The thyroid counting was mainly performed using one of the two HPGe detectors equipped with the chair-type WBC. The subjects examined in this work were divided into two groups: Group 1 was the first 39 subjects who were measured up to June 17, 2011 and Group 2 was the remaining 521 subjects who were measured on and after June 18, 2011. We validated the performance of our direct measurements by comparing measurement results of the Group 1 subjects using two different methods (e.g., the standing-type WBC vs. the chair-type WBC). Tentative internal dose estimates of the subjects of Group 1 were also performed based on the assumption of a single intake scenario on either March 12, when the first hydrogen explosion occurred at the station or the first day of work after the accident. It was found that the contribution of 131I to the total internal dose greatly exceeded those of 134Cs and 137Cs, the other major nuclides detected in the measurements. The maximum committed effective dose (CED) was found in a male subject whose thyroid content of 131I was 9760 Bq on May 23, 2011; the CED of this subject was estimated to be 600 mSv including a small contribution of 134Cs and 137Cs. The typical minimum detectable activity for 131I in the present thyroid counting was 10 Bq for a counting time of 10 min, making it difficult to identify a residual thyroid content corresponding to a CED of 20 mSv for the subjects of Group 2

Results of whole body counting for JAEA staff members engaged in the emergency radiological monitoring for the Fukushima nuclear disaster

A massive earthquake and tsunami on March 11, 2011, resulted in the release of an enormous amount of radioactive materials into the environment. On the day after the earthquake the Japan Atomic Energy Agency (JAEA) began emergency radiological monitoring. Measurements with a whole body counter (WBC) for the staff members who had returned from Fukushima began at the end of March because a power blackout for several days and lingering increased ambient radiation levels had rendered the WBCs inoperable. The measured activity level for I-131 due to inhalation for emergency staff varied from below detection limit to 7 kBq, which corresponds to an estimated initial intake range of <1 to 60 kBq when extrapolated back to the date the staff began the monitoring in Fukushima. The measured activity levels for Cs-134 and Cs-137 were both in the ranges from below detection limit to 3 kBq. When using the median values for each set of measurements, the ratio of the initial intake of I-131 to Cs-137 was 11. The maximum committed effective dose of 0.8 mSv was recorded for a member of the 4th monitoring team dispatched from March 15 to 20

APPLICATION OF AN IMAGING PLATE FOR EVALUATING THE UNCERTAINTY IN DIRECT ORGAN MEASUREMENTS DUE TO VARIATION IN DETECTOR LOCATION

In direct organ measurements, there can be uncertainty to the quantified amount of activity due to variations in detector locations.Here, the authors demonstrate a new use of an imaging plate (IP) for evaluating this uncertainty. The method requiresonly that an array of regions of interest (ROIs) is set on a latent image obtained from the IP; each ROI conforms to an activearea of the detector to be used. In this study, the proposed method was tested in an experiment using a realistic torso phantomcontaining an 241Am liver source. The latent image of this source was obtained by irradiating the IP (20 × 40 cm2) from theanterior surface of the phantom. A comparison of responses between the IP and a high-purity germanium detector was madefor 6 of the 144 circular ROIs arranged on the latent image, showing excellent correlation between the two sets of measures.The dispersion of the photostimulated luminescence values of the 144 ROIs was found to be 8.2% (1σ) and 1.09 as a lognormalscattering factor, which was expected to be the same as the uncertainty of concern in the present measurement withthe HPGe detector

Measurements of 131I in the thyroids of employees involved in the Fukushima Daiichi nuclear power station accident

The Great East Japan Earthquake Disaster on 11 March 2011 caused an unprecedented accident at theFukushima Daiichi nuclear power station operated by Tokyo Electric Power Company (TEPCO).Nuclear Fuel Cycle Engineering Laboratories of Japan Atomic Energy Agency performed internal dosemeasurements of 560 employees involved in the accident during the period from 20 April to 5 August in2011 at the request of TEPCO. The present paper describes our measurements of 131I in the thyroid thatis the predominant contributor to the internal dose. These measurements were carried out using anHPGe detector installed in a low-background shielded chamber made of 20-cm-thick steel and thedetector was placed adjacent to the subject\u27s neck. The typical minimum detectable activity of thistechnique was 10 Bq for a counting time of 10 min; however, this sensitivity made it difficult to identify aresidual thyroid content of 131I corresponding to a committed effective dose of 20 mSv for late subjects.This paper discussed technical issues experienced through the measurements such as the influence of 131Iin the rest of the body, the calibration phantom of use, and so on