Research Article: Whole-Body Counter(WBC) and food radiocesium contamination surveys in Namie, Fukushima Prefecture

Date Published: March 23, 2017

Publisher: Public Library of Science

Author(s): Yoichiro Hosokawa, Kazuki Nomura, Eiki Tsushima, Kohsei Kudo, Yuka Noto, Yoshiko Nishizawa, Gayle E. Woloschak.

http://doi.org/10.1371/journal.pone.0174549

Abstract

This study examined the internal Cs exposure of residents and the Cs present in food products produced in Namie. Whole-body counter (WBC) was used for the measurement of internal exposure per each whole body of examinees.

The food products which appeared to be used for consumption, were brought by residents and commercially available food items were excluded. Most of them were wild plants or food items produced by residents. Four years of data from April 2012 to March 2013 (fiscal 2012) and April 2015 to March 2016 (Fiscal 2015) were analyzed and studied.

The average radioactivity measured by WBC was approximately 5 Bq for Cs-134, and 20 Bq for Cs-137 and the average committed effective dose was approximately 1 μSv. The average for the residents with detectable radioactivity was 25 μSv, and the human health effects are considered to be extremely low risk. However, the radioactivity of the affected individuals showed a higher value than the theoretical attenuation rate. The majority (83.2%) of individuals exhibiting radioactivity were over 50 years old. The number of food products brought in for detection decreased as the study period progressed, but the number of food products with radioactivity had increased. While the items with a higher detection rate of radioactivity included fruits such as citron and persimmon, shiitake mushrooms exhibited the highest radioactivity. Moreover, the radioactivity of seven items in these 10 items decreased from fiscal 2012 to fiscal 2015. Mushrooms had high radioactivity and were produced over a wide area.

We suggest that the elderly try to enjoy life and eat wild plants in moderation while inspecting food products. Therefore, we will continue to work in raising awareness of radiation and its potential presence in food products and thus the continuing necessity of monitoring radioactivity in food in the future.

Partial Text

Large amounts of artificial radionuclides such as radioiodine and radiocesium were released by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011 [1]. Namie is located in the northern part of Fukushima Prefecture in Japan, and its population was around 21,000 when the Great East Japan earthquake struck. Namie is located about 4 km away from the power plant and the residents were evacuated considering the impact of the accident [2], (Fig 1). The exposure of the thyroid to I-131 was considered problematic at the initial stages of the accident based on the lessons learned from the Chernobyl nuclear power plant [3,4]. There have been extensive discussions on the exposure dose of the thyroid and the possibility of increased thyroid cancer rates due to the Fukushima nuclear power plant disaster. Therefore, even now thyroid screening tests are being conducted on the children of Fukushima [5,6].

Whole-body counter (WBC) tests were conducted on the residents of Namie town at the makeshift Tsushima clinic in the town of Nihonmatsu. All examinees wanted to be tested for internal exposure. Measurements were performed for 2 minutes after the initial identification of Cs using FASTSCAN (FASTSCAN™, Canberra Inc., USA). Minimum detectable amount (MDA) values of Cs-134 and Cs-137 were defined 300 Bq and 300 Bq in a 2 min measurement for the stand-up WBC, and 340 Bq and 370 Bq in a 3 min measurement for chair type WBC [10]. WBC was calibrated with the adult male-sized bottle mannequin absorption (BOMAB) phantom (American National Standards Institute, 1999) as a standard source in the USA and Canada once every three years. It was calibrated with the Canberra RMC-II (MODEL 2257) Transfer phantom (Canberra Inc., USA) and the Standard Radionuclide source (Eckert & Ziegler, Berliner, Germany), which includes Cd-109, Co-57,Ce-139, Hg-203, Sn-113, Cs-137, Y-88, Co-60 once a year. Traceability of these methods is assured by the National Institute of Standards and Technology. The difference in WBC counting efficiencies between the calibration used by the BOMAB phantom and the transfer phantom is under 10% as confimed by Momose et al. [10].

The number of persons tested for WBC, radioactivity, and committed effective dose from fiscal 2012 to fiscal 2015 are shown in Table 2. The overall number of persons tested, and the number of individuals detected with radioactivity both decreased with age. The average radioactivity was approximately 5 Bq for Cs-134, and 20 Bq for Cs-137 and the average committed effective dose was approximately 1 μSv. The majority (83.2%) of individuals exhibiting radioactivity were over 50 years old with men accounting for 68.2%. The average committed effective dose was 24.65 μSv for individuals detected with radioactivity. The mean value of radioactivity for individuals exhibiting radioactivity is shown in Fig 2. The trend of increasing Cs-137 radioactivity was observed for individuals detected with radioactivity, and a statistically significant difference was observed for the mean values in 2012 and 2015 with the one-way analysis of variance (p < 0.05). Cs-134 and Cs-137 attenuation rates of individuals detected with radioactivity tested two or more times was compared with the theoretical attenuation rate based on the effective half-life (100 days) to examine the cause [9]. The results are shown in Fig 3. The radioactivity of the affected individuals showed a higher value than the theoretical attenuation rate. Residents of Namie continue to live in shelters; approximately 70% were evacuated within Fukushima and the rest outside of Fukushima. Temporary housing was set up at 30 locations in Fukushima, and as of November 2016, there were about 2,900 residents living in this temporary housing. However, the dose rate (less than 20mSv/y) has decreased with the progress in decontamination of the Namie town region, and the restrictions based on “Evacuation Directive Lift Prepared Area” and “Restricted Habitation Area” are scheduled to be lifted in March 2017. Short-term accommodations not recognized earlier were specially acknowledged as special accommodations in September 2016. We analyzed the Cs WBC measurement results of residents who were evacuated to the Nihonmatsu area as well as the radioactivity of food brought with them, which was not distributed as products.   Source: http://doi.org/10.1371/journal.pone.0174549

 

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