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 (DRZs). Within the DRZs, there has been designated a special zone for reconstruction and revitalisation (SZRR). Decontamination of the SZRR has been promoted so that evacuation orders may be lifted. Previous studies measured individual external doses in the evacuation order-lifted zones (ELZs) and other living areas where the annual additional individual external dose was overall less than approximately 5 mSv y -1 . However, there have been few reports about the measurement of individual external doses in a SZRR or outside of an SZRR (O-SZRR). In SZRRs and O-SZRRs, 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 through January 2021. Our key results were:The median (minimum to maximum) individual external doses at outdoor locations were 0.16 μ Sv h -1 (0.05-0.63 μ Sv h -1 ), 0.57 μ Sv h -1 (0.15-3.92 μ Sv h -1 ), and 1.36 μ Sv h -1 (0.14-11.91 μ Sv h -1 ) for the ELZ, SZRR, and O-SZRR, respectively.The conversion coefficients for the air dose rate measured by airborne monitoring to individual external dose were 0.23, 0.38, and 0.50 for the ELZ, SZRR, and O-SZRR, respectively. The conversion coefficients were below 0.6, which was used in the national government model for estimating external exposure dose from air dose rate. In addition, the conversion coefficients for the SZRR and O-SZRR in air dose rates of less than 1.5 μ Sv h -1 differed from those obtained for the entire measurement range of this study.The conversion coefficient from air dose rate at a height of 1 m above ground level to individual external dose was researched across a broader and higher range of air dose rates than in the previous study (0.24-20.89 μ Sv h -1 ). The conversion coefficient is confirmed to be 0.7, similar to previous studies.