Radiocesium concentrations in wild boars captured within 20 km of the Fukushima Daiichi Nuclear Power Plant

www.nature.com/scientificreports OPEN Radiocesium concentrations in wild boars captured within 20 km of the Fukushima Daiichi Nuclear Power Plant Limeng Cui1,2, Makiko Orita1 ✉, Yasuyuki Taira1 & Noboru Takamura1 The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011 released large amounts of artificial radioactive substances into the environment. In this study, we measured the concentration of radiocesium (134Cs + 137Cs) in 213 muscle samples from wild boars (Sus scrofa) captured in Tomioka town, which is located within 20 km of the FDNPP. The results showed that 210 (98.6%) muscle samples still exceeded the regulatory radiocesium limit (100 Bq/kg) for general foods. Radiocesium (134Cs + 137Cs) levels ranged from 87.1–8,120 Bq/kg fresh mass (FM), with a median concentration of 450 Bq/kg FM. The median committed effective dose was estimated to be 0.070–0.26 μSv/day for females and 0.062–0.30 μSv/day for males. The committed effective dose for one-time ingestion of wild boar meat could be considered extremely low for residents in Tomioka. The relatively high levels of radioactivity found in this study suggest that the high variability of food sources may have led to the large accumulation of radioactive substances. These results suggest that comprehensive long-term monitoring is needed to identify risk factors affecting recovery from a nuclear disaster. The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred in 2011 released large amounts of artificial radioactive substances into the environment, particularly cesium-137 (137Cs; 8.8 PBq; half-life: 30.2 years), cesium-134 (134Cs; 9.0 PBq; half-life: 2.1 years), and iodine-131 (131I; 120.0 PBq; half-life: 8 days)1. The introduced radionuclides were deposited over a wide area of Fukushima Prefecture and accumulated in local food2–4. From April 2012, the Japanese government set the regulatory limit for radiocesium in general foods as 100 Bq/kg5. After the Chernobyl Nuclear Power Plant accident, researchers reported that game animals were contaminated with artificial radionuclides6–8. Among all such animals, wild boars showed an especially high radiocesium concentration9,10. Gulakov et al. measured wild boars captured in a 10–35-km zone from the Chernobyl Nuclear Power Plant in 2008, and found that the average concentration of 137Cs in the muscle tissue of wild boars remained as high as 37,000 Bq/kg11, even at 22 years after the accident. Tomioka town (37° 20′43.6″N, 141°0′31″E) is located within 20 km of the FDNPP12,13. Immediately after the disaster, almost all residents of Tomioka town were forced to evacuate. The Tomioka town office led infrastructure recovery efforts and decontamination processes to remove radiocesium fallout from the town. On April 2017, the Japanese government lifted the evacuation order for Tomioka town, except for a difficult-to-return zone that comprised almost 15% of the total town area. Although the residential areas, farmland, and forests close to residential areas have been widely decontaminated, it has been reported that the forest area remains contaminated with radionuclides derived from the FDNPP, 8 years since the accident12. Highly contaminated wild boars were reported as a considerable issue that led residents to hesitate to return to their hometown. In fact, internal radiation exposure from food remains a matter of concern for the residents of Tomioka town13, who wish to know the radioactive levels of wild boar, including the possibility of consuming wild boar in the future. Therefore, the aims of this study were to determine the levels of radiocesium contamination in wild boars found in Tomioka town, Fukushima Prefecture, and, since wild boar is a traditional ingredient in Japanese cuisine, to evaluate the internal radiation exposure risk of consuming wild boar meat. 1 Department of Global Health, Medicine and Welfare, Atomic Bomb Disease Institute, Nagasaki University Graduate School of Biomedical Sciences, Nagasaki, Japan. 2Department of Radiation Protection, Beijing Research Center for Preventive Medicine, Beijing, China. ✉e-mail: Scientific Reports | (2020) 10:9272 | https://doi.org/10.1038/s41598-020-66362-6 1 www.nature.com/scientificreports www.nature.com/scientificreports/ Figure 1. Distribution of radiocesium (134Cs + 137Cs) concentrations in the muscle tissue of wild boars from January to December 2019. Figure 2. Time-dependency of radiocesium (134Cs + 137Cs) concentrations in the muscle tissue of wild boars. (Upper error bars: the largest data point excluding any outliers, Lower error bars: the lowest data point excluding any outliers, Open circles: outliers; Asterisks: extreme). N = 213 Median (Minimum– Maximum) Radiocesium (Bq/kg) 450 (87.1–8,120) 134 Cs (Bq/kg) 28.6 (n.d.*–509) 137 Cs (Bq/kg) 420 (81.1–7,610) Table 1. Radionuclide concentrations (Bq/kg FM) in the muscle tissue of wild boars. *n.d.: could not be determined. Results Radioactivity concentration. Among the 213 wild boar (Sus scrofa) samples collected, 3 (1.4%), 110 (51.6%), 55 (25.8%), and 45 (21.2%) had radiocesium (134Cs + 137Cs) levels of <100, 100–500, 501–1,000, and >1,000 Bq/kg fresh mass (FM), respectively (Fig. 1). The minimum and maximum radiocesium concentrations were 87.1 Bq/kg FM and 8,120 Bq/kg FM, respectively, with a median concentration of 450 Bq/kg FM (Table 1). No significant correlation was found between radiocesium concentration and males and females (Mann–Whitney Test, p = 0.516) or between radiocesium concentration and the weight of the wild boars (Spearman correlation coefficient, p = 0.376). The average 134Cs/137Cs activity ratios in all samples were 0.08 in January 2019 and 0.06 in December 2019. The distribution of radiocesium concentrations in the muscle tissue of wild boars for each month is shown in Fig. 2. Radioactivity concentrations varied significantly with month (Jonckheere–Terpstra test, p < 0.05). Committed effective dose. Among 213 samples collected that contained radiocesium, the median committed effective dose ranged from 0.070 to 0.26 μSv for females and from 0.062 to 0.30 μSv for males, considering one-time ingestion of wild boar meat as the meat source (Table 2). Scientific Reports | (2020) 10:9272 | https://doi.org/10.1038/s41598-020-66362-6 2 www.nature.com/scientificreports www.nature.com/scientificreports/ Age (y) Female Male Median (Minimum– Maximum) Median (Minimum– Maximum) 1–6 0.069 (0.012–1.3) 0.072 (0.012–1.4) 7–14 0.18 (0.030–3.7) 0.19 (0.031–3.8) 15–19 0.26 (0.049–4.6) 0.30 (0.058–5.4) 20–29 0.17 (0.032–3.0) 0.18 (0.035–3.3) 30–39 0.15 (0.029–2.7) 0.21 (0.040–3.7) 40–49 0.16 (0.030–2.8) 0.24 (0.046–4.3) 50–59 0.12 (0.023–2.2) 0.18 (0.035–3.3) 60–69 0.070 (0.013–1.2) 0.12 (0.024–2.2) å 70 NA* 0.062 (0.012–1.1) Table 2. Committed effective doses for one-time ingestion of wild boar meat from Tomioka town (μSv/day). NA*: not available. Median pork consumption was 0 g (...truncated)