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 (137Cs) (137Cs-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 137Cs 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.
Highlights
The Great East Japan Earthquake 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 (134Cs), cesium-137 (137Cs) and iodine-131(131I) being released into the atmosphere and eventually depositing on land and at sea in the surrounding areas[1]
We evaluated the effects of decontamination efforts, such as reductions in ambient and radiocesium dose rates, in three areas (“Decontaminated area”, “Radioactive waste storage area” and “Non-decontaminated area”) with markedly different characteristics in the difficult-to-return zone in Tomioka Town
On the basis of the slope of the regression line, ambient dose rates in the last surveys decreased to 28.1%, 78.9% and 72.1% of those in the first surveys in the Decontaminated area, Radioactive waste storage area and Non-decontaminated area, respectively (Table 1)
Summary
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 (134Cs), cesium-137 (137Cs) and iodine-131(131I) being released into the atmosphere and eventually depositing on land and at sea in the surrounding areas[1]. 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)[3,4,5,6,7,8,9] These surveys and the collected data are extremely important to precise evaluation of environmental remediation in the affected areas. The external exposure level and the decontamination effects on landscape within the difficult-to-return zone have not been evaluated concretely, data from the literature, databases and websites have been reported by the national and local governments[4,5,6,7,8,9,12,13]. Recent reports on the decontamination effect on landscape are not sufficiently published[14]
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