Environmental Remediation of the difficult-to-return zone in Tomioka Town, Fukushima Prefecture

Abstract

Temporal variations in ambient dose rates in a restricted area designated as "difficult-to-return" for residents of Tomioka Town, Fukushima Prefecture were evaluated in a car-borne survey during 2018-2019. The median dose rates in the "Decontaminated area" in the difficult-to-return zone decreased rapidly from 1.0 μSv/h to 0.32 μSv/h; however, the median dose rates in the "Non-decontaminated area" and "Radioactive waste storage area" fluctuated between 1.1-1.4 μSv/h and 0.46-0.61 μSv/h, respectively. The detected rate of the cesium-137 (137 Cs) (137 Cs-detected points per all measuring points) in the "Decontaminated area" also decreased rapidly from 64% to 6.7%, accompany with decreasing in ambient dose rates. On the other hand, the detection of 137 Cs in the "Radioactive waste storage area" and "Non-decontaminated area" decreased from 53% to 17% and 93% to 88%, respectively. We confirmed that the dose rates in the Decontaminated area dramatically decreased due to decontamination work aiming to help residents return home. Moreover, the estimated external exposure dose of workers during the present survey was 0.66 mSv/y in the Decontaminated area and 0.55 mSv/y in the Radioactive waste storage area, respectively. This case of Tomioka Town within the "difficult-to-return zone" may be the first reconstruction model for evaluating environmental contamination and radiation exposure dose rates due to artificial radionuclides derived from the nuclear disaster. The Great East Japan Earthquake (magnitude 9.0) and subsequent tsunami on March 11, 2011 caused an accident at the Fukushima Daiichi Nuclear Power Station (FDNPS) that resulted in various radionuclides including cesium-134 (134 Cs), cesium-137 (137 Cs) and iodine-131(131 I) being released into the atmosphere and eventually depositing on land and at sea in the surrounding areas 1. The United Nations Scientific Committee on the Effects of Atomic Radiation estimated the total release of 134 Cs (half-life: 2.1 y), 137 Cs (half-life: 30 y) and 131 I (half-life: 8 d) to be 9.0, 8.8 and 120.0 petabecquerels (PBq), respectively 1. The Japanese government, municipalities and private companies have carried out environmental and individual radiation monitoring programs to confirm the radiation levels in the affected areas 2,3. More than 8 years have passed since the accident and it has been confirmed that artificial radionuclides with a relatively long half-life such as 134 Cs and 137 Cs still exist in soil and plant samples collected around the FDNPS 1-3. Environmental monitoring in Fukushima Prefecture have been carried out by many methods (the airborne survey by monitor stations and personnel, vehicle-borne survey, aerial-vehicle survey and radionuclide analysis of the environmental samples such as soils, sediments and foodstuffs) 3-9. These surveys and the collected data are extremely important to precise evaluation of environmental remediation in the affected areas. Following the FDNPS accident, residential areas, farmlands, forests close to residential areas, and roads...

Radiocesium levels in contaminated forests has remained stable, even after heavy rains due to typhoons and localized downpours

“…However, radiocesium was only detected at 2.4% of sites along the hiking trail and at 12.9% sites off the hiking trail 24 . When temporal variations in ambient dose rates in the difficult-to-return to areas of Tomioka Town were evaluated using the RADI-PROBE (car-based survey) during 2018-2019, we confirmed that the dose rates in the area had decreased dramatically due to decontamination efforts 25 . 137 Cs was detected at 3.8% to 93% of sites using the RADI-PROBE 25 , while the dose-forming artificial radionuclides 134 Cs and 137 Cs were only prevalent at some measuring sites (Supplementary S1) 24 .…”

“…In addition, we did not characterize the soils along the Mt Okura hiking trail, although the findings of our previous study showed that dose-forming artificial radiocesium was prevalent in all surface soil samples (0-10 cm) collected in Tomioka Town in 2017 20 . However, in the present study, we used the RADI-PROBE for high and sensitive gamma-ray detection (typically 1400 cps per µSv/h for 137 Cs) 24,25,42 . Further investigations on individual doses and ambient doses under several weather conditions are needed.…”

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

“…To evaluate 137 Cs radioactivity levels in airborne dust, including resuspended soil particles that are due to 137 Cs contamination of the surface soil, the L‐RF values used in the present study were calculated using the following equation: where 137 Cs invent is the 137 Cs radioactivity level of airborne dust (Bq/m 3 ) and 137 Cs cont is the 137 Cs contamination in the surface soil layer (0–0.5 cm) (Bq/m 2 ) collected in the present study (Abe et al, 2021; Ochiai et al, 2016). Because the decontamination in the Yonomori district of the SRRB used the same schedule and technical methods, the surface soil contamination levels that were closest to the airborne dust sampling points (in time) were adopted as the reference values (Cui et al, 2020). Actually, there are some parks, agricultural lands, forests (the boundary with the SRRB), and deposited grounds around the “Building G” and “Building Y” sites at the SRRB in the Yonomori district (Supporting Information: Appendix ) (MOE, 2019).…”

“…In particular, the national government and Tomioka town office pay attention to their internal radiation exposures due to the 137 Cs that is present around residents' houses, although they may not be directly exposed to airborne dust. Radiocesium has a relatively long half‐life, and still exists in soil and plant samples that were collected around the FDNPS, including Tomioka town (Cui et al, 2020 ; MHLW, 2018 ; Tomioka Town Office, 2021 ). Additionally, 134 Cs and 137 Cs were the dominant radionuclides comprising the indoor surface contamination of wooden houses located within the evacuation areas after the FDNPS accident, and there was a distance dependence from the FDNPS (Yoshida et al, 2016 ).…”

A mobile simulation and ARIMA modeling for prediction of air radiation dose rates

“…The main objective of the current study is to predict future air dose rates by using ARIMA model in a return zone , where the inhabitants entry is forbidden [29] and the annual cumulative dose is greater than 20 mSv [30]. Evaluating the radiological hazard indices relying on the output data.…”