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


 Since the Fukushima Daiichi Nuclear Power Station accident, evacuation orders have been lifted except for the Difficult-to-Return Zones (DRZ). Within the DRZ, there has been designated a Special Zone for Reconstruction and Revitalization (SZRR). Decontamination and infrastructure development has been promoted in the SZRR so that evacuation orders may be lifted. Previous studies measured individual external doses in the evacuation order-lifted zone (ELZ) and other living areas where there was main an annual additional individual external dose of less than approximately 5 mSv/y. However, there have been few reports about the measurement of individual external doses in an SZRR or outside of an SZRR (O-SZRR), where were evacuation orders remained in place. In SZRR and O-SZRR, the Tokyo Electric Power Company Holdings employees work mainly outdoors. Therefore, the employees’ individual external doses and air dose rates were measured in these zones from March 2020 to August 2021. Our key results were: ・The median (min63imum to maximum) individual external doses at outdoor locations were 0.16μSv/h (0.05-0.63μSv/h), 0.57μSv/h (0.15-2.67μSv/h), and 1.36μSv/h (0.14-11.91μSv/h) for the ELZ, SZRR, and O-SZRR, respectively. ・The conversion coefficients from air dose rate by airborne monitoring to individual external dose were 0.23, 0.35, and 0.50 for the ELZ, SZRR, and O-SZRR, respectively. In all cases, the conversion coefficients were below 0.6, which was used in the national government model for estimating external exposure dose from air dose rate. ・The conversion coefficient from air dose rate at a height of 1 m above ground level to individual external dose was 0.7 within a range of air dose rates of 0.24 - 20.89μSv/h. The range of air dose rate in this study is wider and higher than the range identified in previous studies, but the conversion coefficient is confirmed to be 0.7, similar to previous studies.