Local Levels of Radiation Exposure Doses Due to Radiocesium for Returned Residents in Tomioka Town, Fukushima Prefecture

Abstract

Tomioka Town is located within a 20-km radius of the Fukushima Daiichi Nuclear Power Station. Radiation dose rates due to radiocesium in residents’ living spaces were evaluated from the measurements of ambient dose rates and environmental samples after returning home. The mean ambient dose rates were 0.15–0.18-μSv/h indoors and 0.23–0.26-μSv/h outdoors during 2018 and 2019, and the additional radiation dose rates were calculated to be 1.4 mSv/y in 2018 and 1.1 mSv/y in 2019. Ambient dose equivalent from surface soils within housing sites were estimated to be 0.66 mSv/y in 2018 and 0.54 mSv/y in 2019. Moreover, committed effective doses from local foods were calculated in 19–74 μSv/y for children and 39–100 μSv/y for adults during 2018 and 2019. These findings suggest that current radiation exposure doses have been controlled at the levels close to the public dose limit (1 mSv/y) in residents’ living spaces.

Assessment of localized and resuspended 137Cs due to decontamination and demolition in the difficult‐to‐return zone of Tomioka town, Fukushima Prefecture

“…The difficult‐to‐return zone in Tomioka town, where approximately 4800 people had lived before the FDNPS accident, remains quite large at 8.5 km 2 (Tomioka Town Office, 2017 ). Although the ambient dose rates ranged from 1.9 to 3.8 μSv/h before the decontamination and demolition work in 2017, the ambient dose rates (median) in the SRRB decreased dramatically, from 1.0 to 0.32 μSv/h, as a result of the decontamination work conducted in 2018–2019 (Cui et al, 2020 ; MOE, 2019 ; Yamaguchi et al, 2021 ). Although in the Yonomori district (SRRB) in Tomioka town, demolition work was conducted using standard methods, such as cleaning rooms and removing interior materials; demolishing interior materials, roof and window frames, walls, and frameworks; removing walls and roof materials; and removing concrete foundations using heavy machinery, the annual external effective dose (median) for the decontamination workers in the SRRB was estimated to be 0.66 mSv/y (less than the public dose limit, 1 mSv/y) from July 2018 to July 2019 (Cui et al, 2020 ) (Supporting Information: Appendices S1 and S2 ).…”

“…However, the internal exposure dose rates due to aspiration of airborne dust, including 137 Cs, were extremely low compared with the estimated annual effective dose of decontamination workers or the annual effective dose limits recommended by the Japanese government (Tables 1 and 2 ) (MHLW, 2018 ). Additionally, the external exposure doses due to 137 Cs in the surface soil were low because the decontamination process, which included removing surface soil and fallen leaves and stripping topsoil from gardens, was close to being fully complete in the SRRB during the recovery and reconstruction period after the FDNPS accident (Supporting Information Appendices S1 and S2 ) (Cui et al, 2020 ; Matsuo et al, 2019 ; Yamaguchi et al, 2021 ). In the difficult‐to‐return zone of Tomioka town, the mean ambient dose rates were 1.4 (12)–1.8 (16) µSv/h (mSv/y) outdoors in 2018 and 1.5 (13)–1.6 (14) µSv/h (mSv/y) outdoors in 2019 (Yamaguchi et al, 2021 ).…”

“…Additionally, the external exposure doses due to 137 Cs in the surface soil were low because the decontamination process, which included removing surface soil and fallen leaves and stripping topsoil from gardens, was close to being fully complete in the SRRB during the recovery and reconstruction period after the FDNPS accident (Supporting Information Appendices S1 and S2 ) (Cui et al, 2020 ; Matsuo et al, 2019 ; Yamaguchi et al, 2021 ). In the difficult‐to‐return zone of Tomioka town, the mean ambient dose rates were 1.4 (12)–1.8 (16) µSv/h (mSv/y) outdoors in 2018 and 1.5 (13)–1.6 (14) µSv/h (mSv/y) outdoors in 2019 (Yamaguchi et al, 2021 ). Moreover, in the difficult‐to‐return zone, the mean radiocesium concentrations in surface soils (0–5 cm) within the assembly halls were 3012 (19–5720) Bq/kg‐dry in 2018 and 1805 (20–5873) Bq/kg‐dry in 2019 for 134 Cs and 31 603 (243–58 719) Bq/kg‐dry in 2018 and 25 658 (223–83 001) Bq/kg‐dry in 2019 for 137 Cs (Yamaguchi et al, 2021 ).…”

“…In the difficult‐to‐return zone of Tomioka town, the mean ambient dose rates were 1.4 (12)–1.8 (16) µSv/h (mSv/y) outdoors in 2018 and 1.5 (13)–1.6 (14) µSv/h (mSv/y) outdoors in 2019 (Yamaguchi et al, 2021 ). Moreover, in the difficult‐to‐return zone, the mean radiocesium concentrations in surface soils (0–5 cm) within the assembly halls were 3012 (19–5720) Bq/kg‐dry in 2018 and 1805 (20–5873) Bq/kg‐dry in 2019 for 134 Cs and 31 603 (243–58 719) Bq/kg‐dry in 2018 and 25 658 (223–83 001) Bq/kg‐dry in 2019 for 137 Cs (Yamaguchi et al, 2021 ). The annual ambient dose equivalents from the surface soil were estimated to be 0.35 (3.1) µSv/h (mSv/y) in 2018 and 0.44 (3.9) µSv/h (mSv/y) in 2019 (Yamaguchi et al, 2021 ).…”

“…On April 1, 2017, the national government declared that the residents who had previously resided in approximately 88% of the town could return to their homes, as the radiation dose rates had reached acceptable levels (e.g., <20 mSv/y) (Fukushima Prefectural Government, 2014 ). Our previous studies have found that the external and internal exposure doses in residential living spaces (e.g., the evacuation order‐lifted area) of Tomioka town have decreased to levels close to the public dose limit (e.g., 1 mSv/y) in the recovery and reconstruction period after the FDNPS accident (Matsuo et al, 2019 ; Yamaguchi et al, 2021 ).…”