Integrated radiation air dose rate maps over the 80 km radius of the Fukushima Daiichi Nuclear Power Plant and the entire Fukushima Prefecture during 2011–2022

In this study, we created integrated radiation air dose rate maps in the Fukushima region during 2011-2022 using airborne, car-borne, and walk surveys and fixed-location measurements. We applied a Bayesian geostatistical method to the 80 km radius of the Fukushima Daiichi Nuclear Power Plant and the entire Fukushima Prefecture while considering the history of the evacuation zone lifting in Fukushima. The integrated maps in this study fixed the bias to underestimate the air dose rates in forest areas and created integrated maps with a wider area and time series than previous studies. Furthermore, the results improved our understanding of the spatial heterogeneity of air dose rates. The correlations between walk and airborne surveys and walk and car-borne surveys changed over time (the walk survey includes fixed-location measurements), with the decreasing rates of air dose rate being faster for car-borne surveys, walk surveys, fixed-location measurements, and airborne surveys, in that order. The temporal changes in the variograms indicate that the heterogeneity of the radiocesium distribution in the environment varies with time due to decontamination activities and weathering effects. The trends inside and outside the evacuation zone differ. The temporal changes in the air dose rates from the integrated maps show a decrease between fixed-location measurements and the airborne survey. This decreasing rate mostly reflects the overall decrease in air dose rates. The results of this study could be used to evaluate detailed exposure doses to the public.