Comparison of Dose Rates from Four Surveys around the Fukushima Daiichi Nuclear Power Plant for Location Factor Evaluation

Abstract

Background: The radionuclides released by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident 9 years ago are still being monitored by various research teams and the Japanese government. Comparison of different surveys’ results could help evaluate the exposure doses and the mechanism of radiocesium behavior in the urban environment in the area. In this study, we clarified the relationship between land use and temporal changes in the ambient dose rates (air dose rates) using big data.Materials and Methods: We set a series of 1 × 1 km2 meshes within the 80 km zone of the FDNPP to compare the different survey results. We then prepared an analysis dataset from all survey meshes to analyze the temporal change in the air dose rate. The selected meshes included data from all survey types (airborne, fixed point, backpack, and carborne) obtained through the all-time survey campaigns.Results and Discussion: The characteristics of each survey’s results were then evaluated using this dataset, as they depended on the measurement object. The dataset analysis revealed that, for example, the results of the carborne survey were smaller than those of the other surveys because the field of view of the carborne survey was limited to paved roads. The location factor of different land uses was also evaluated considering the characteristics of the four survey methods. Nine years after the FDNPP accident, the location factor ranged from 0.26 to 0.49, while the half-life of the air dose rate ranged from 1.2 to 1.6.Conclusion: We found that the decreasing trend in the air dose rate of the FDNPP accident was similar to the results obtained after the Chernobyl accident. These parameters will be useful for the prediction of the future exposure dose at the post-accident.

Measurement of individual external doses of Tokyo Electric Power Company Holdings employees working in Fukushima Prefecture and the relationship between individual external doses and air dose rates in areas including difficult-to-return zones

“…For example, TEPCO employees were not only working in nondecontaminated residential areas, but also on paved roads and house demolition sites. On the paved roads, radioactive cesium may have been washed away by weathering effects [37,38]. The ground under the houses may have been a little contaminated [35].…”

Statistical evaluation of individual external exposure dose of outdoor worker and ambient dose rate at evacuation ordered zones after the Fukushima Daiichi Nuclear Power Station accident

“…In other words, if there is a change in the distribution of radioactive cesium within 30 m, a dose gradient will occur. Previous studies [44][45][46][47]49] have reported that the radiation source distribution was different in each land use situations. So, it is considered that a dose gradient occurs under conditions where different land use conditions were close.…”

“…The previous study and this study results are notably lower than the expected conversion coefficient 0.7. Because approximately 90% of measurements based on airborne radiological monitoring contributed to radiation sources within a 470 m radius range [43], airborne radiological monitoring values tend to have difficulty reflecting decontamination [14,16,37] and weathering effects [44][45][46][47] on the ground. On the other hand, about 90% of ground-based measurements were contributed by radiation sources with a diameter of 100 m [43], so these measurements reflect gradient dose Measurements by airborne radiological monitoring might have been higher than actual due to the topographical effects at mountainous regions [37,48].…”