Radiation Monitoring in the Residential Environment: Time Dependencies of Air Dose Rate and Cs-137 Inventory

Background: Residential areas have some factors on the external exposure of residents, who usually spend a long time in these areas. Although various survey has been carried out by the government or the research institutions after the Fukushima Daiichi Nuclear Power Plant accident, the mechanism of radiocesium inventory in the terrestrial zone has not been cleared. To better evaluate the radiation environment, this study investigated the temporal changes in air dose rate and Cs-137 inventories (Bq/m(2)) in residential areas and agricultural fields. Materials and Methods: Air dose rate and Cs-137 inventories were investigated in residential areas located in an evacuation zone at 5-8 km from the Fukushima Daiichi Nuclear Power Plant. From December 2014 to September 2018, the air dose rate distribution was investigated through a walking survey (backpack survey), which was conducted by operators carrying a gamma-ray detector on their backs. Additionally, from December 2014 to January 2021, the Cs-137 inventories on paved and permeable grounds were also measured using a portable gamma-ray detector. Results and Discussion: In the areas where decontamination was not performed, the air dose rate decreased faster in residential areas than in agricultural fields. Moreover, the Cs-137 inventory on paved surfaces decreased with time owing to the horizontal wash-off, while the Cs-137 inventory on permeable surfaces decreased dramatically owing to the decontamination activities. Conclusion: These findings suggest that the horizontal wash-off of Cs-137 on paved surfaces facilitated the air dose rate decrease in residential areas to a greater extent compared with agricultural fields, in which the air dose rate decreased because of the vertical migration of Cs-137. Results of this study can explain the faster environmental restoration in a residential environment reported by previous studies.