Evaluation of decreasing trend in air dose rate and ecological half-life within an 80 km range from Fukushima Dai-ichi Nuclear Power Plant, using car-borne survey data measured by KURAMA systems up to 2018

Air dose rates within a range of 80 km from the Fukushima Dai-ichi Nuclear Power Plant measured via car-borne surveys using the KURAMA systems from 2011 to 2018 were analyzed. The ratio of the air dose rates relative to the first car-borne survey was 0.08 over a period of seven years, indicating a much more rapid decrease than that estimated by the physical decay of radiocesium (0.3). Differences in the decreasing trends of the air dose rates among the land-use categories and evacuation order areas were examined. The ecological half-lives were evaluated according to the land-use categories in each evacuation order area. To model the acceleration of the decrease in the air dose rates observed in the evacuation order area, we modified the bi-exponential curve formula of ecological half-life and introduced the two-group model. The ecological half-life of the fast components in the evacuation order area of ‘zone in preparation for the lifting of the evacuation order’ evaluated using the two-group model after 2013 were 0.5–1 y, which were much shorter than those before 2013 (2–3 y).