Evaluation of environmental contamination and estimated exposure doses after residents return home in Kawauchi Village, Fukushima Prefecture.

The external and internal exposure doses due to artificial radionuclides after the return of residents to their homes in Kawauchi Village, Fukushima Prefecture, including the restricted area within a 20-km radius from the Fukushima Dai-ichi Nuclear Power Plant (FNPP), were evaluated by gamma spectrometry of the soils and local agricultural products. The prevalent dose-forming artificial radionuclides from all samples were determined to be (134)Cs and (137)Cs (radiocesium). The estimated external effective doses from soils sampled on December 24 and 25, 2012 were 0.0017-9.2 mSv/y in the areas within a 20- to 30-km radius from the FNPP and 0.39-1.3 mSv/y in the areas within a 20-km radius from the FNPP. These levels appeared to be decreasing, despite the distance from the FNPP (median: 0.21 (0.012-0.56) mSv/y), compared to the levels just before the return of the residents to their homes on December 19 and 20, 2011 (median: 0.85 (0.40-1.4) mSv/y). The committed effective doses from the local agricultural samples in Kawauchi Village from May 1, 2012 to March 31, 2013 were sufficiently low, in the range of 18-44 µSv/y for males and 20-48 µSv/y for females (the range was 18-48 µSv/y for children and 25-43 µSv/y for adults), compared to the public dose limit (1 mSv/y, ICRP, 1991), although the potential for radiation exposure still exists. Residents who have not yet returned may be able to return to their homes with a long-term follow-up of environmental monitoring and countermeasures to reduce unnecessary radiation exposure.

Evaluation of Environmental Contamination and Estimated Radiation Exposure Dose Rates among Residents Immediately after Returning Home to Tomioka Town, Fukushima Prefecture.

On 1 April 2017, six years have passed since the Fukushima Daiichi Nuclear Power Station (FDNPS) accident, and the Japanese government declared that some residents who lived in Tomioka Town, Fukushima Prefecture could return to their homes. We evaluated environmental contamination and radiation exposure dose rates due to artificial radionuclides in the livelihood zone of residents (living space such as housing sites), including a restricted area located within a 10-km radius from the FDNPS, immediately after residents had returned home in Tomioka town. In areas where the evacuation orders had been lifted, the median air dose rates were 0.20 µSv/h indoors and 0.26 µSv/h outdoors, and the radiation exposure dose rate was 1.6 mSv/y. By contrast, in the "difficult-to-return zone," the median air dose rate was 2.3 µSv/h (20 mSv/y) outdoors. Moreover, the dose-forming artificial radionuclides (radiocesium) in the surface soil were 0.018 µSv/h (0.17 mSv/y) in the evacuation order-lifted areas and 0.73 µSv/h (6.4 mSv/y) in the difficult-to-return zone. These findings indicate that current concentrations of artificial radionuclides in soil samples have been decreasing in the evacuation order-lifted areas of Tomioka town; however, a significant external exposure risk still exists in the difficult-to-return zone. The case of Tomioka town is expected to be the first reconstruction model including the difficult-to-return zone.

Measurement of individual doses of radiation by personal dosimeter is important for the return of residents from evacuation order areas after nuclear disaster.

To confirm the availability of individual dose evaluation for the return of residents after the accident at the Fukushima Dai-ichi Nuclear Power Plant (FNPP), we evaluated individual doses of radiation as measured by personal dosimeters in residents who temporarily stayed in Evacuation Order Areas in Kawauchi village, which is partially located within a 20 km radius of the FNPP. We also compared individual doses with the external radiation doses estimated from the ambient dose rates and with doses estimated from the concentrations of radionuclides in the soil around each individual's house. Individual doses were significantly correlated with the ambient doses in front of the entrances to the houses (r = 0.90, p<0.01), in the backyards (r = 0.41, p<0.01) and in the nearby fields (r = 0.80, p<0.01). The maximum cumulative ambient doses in the backyards and fields around the houses were 6.38 and 9.27 mSv/y, respectively. The maximum cumulative individual dose was 3.28 mSv/y, and the median and minimum doses were 1.35 and 0.71 mSv/y. The estimated external effective doses from concentrations of artificial radionuclides in soil samples ranged from 0.03 to 23.42 mSv/y. The individual doses were moderately correlated with external effective doses in the backyards (r = 0.38, p<0.01) and in the fields (r = 0.36, p<0.01); however, the individual doses were not significantly correlated with the external effective doses in front of the entrances (r = 0.01, p = 0.92). Our study confirmed that individual doses are low levels even in the evacuation order area in Kawauchi village, and external effective dose levels are certainly decreasing due to the decay of artificial radionuclides and the decontamination of contaminated soil. Long-term follow-up of individual doses as well as internal-exposure doses, environmental monitoring and reconstruction of infrastructure are needed so that residents may return to their hometowns after a nuclear disaster.