Consideration of the conversion factor from air dose rates to individual external dose per hour after the Fukushima accident - Individual external dose measurements of a TEPCO employee in Fukushima prefecture over a 9 year Period.

The accident at Fukushima Daiichi Nuclear Power Plant (FDNPP) in 2011 resulted in the dispersion of radioactive materials throughout the surrounding area and an increase in the air dose rate was even confirmed in Fukushima City, which is located approximately 60 km northeast of FDNPP. A Tokyo Electric Power Company (TEPCO) Holdings employee, who has lived and worked in Fukushima City since the FDNPP accident, measured individual external doses, GPS data, and his activities in Fukushima Prefecture over a 9 year period beginning in 2014. This data provides valuable information about the area. The data show the following results.  Comparison of the air dose rate at the home location to the individual external dose per hour at the home shows that the average conversion factor has increased over the 9 years and exceeded 0.3 since 2019, indicating an overall relatively good correlation.  Individual external doses measured in the office on the fourth floor of a concrete structure in Fukushima City have not changed significantly from 2014 to 2022, when air dose rates showed a decrease.  Outdoor individual external doses, such as those measured when commuting on foot, have a relatively strong correlation with air dose rates from airborne monitoring with the conversion factor of about 0.6. The conversion factor do not differ significantly from 2014 to 2022. In this study, the individual external dose data is applicable to the TEPCO measurer and is not necessarily representative of all residents of Fukushima City. However individual external dose data for 9 consecutive years will be useful for estimating individual external doses from air dose rates, and estimating annual additional exposure doses, if even some of them are applicable to similar life patterns and lifestyles.

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.

Following the accident at the Fukushima Daiichi Nuclear Power Station, evacuation orders were issued for the surrounding communities. In order to lift the evacuation order, it is necessary to determine individual external doses in the evacuated areas. The purpose of this study was to determine the quantitative relationship between individual external doses and ambient dose rates per hour as conversion coefficients. More specifically, individual external doses of Tokyo Electric Power Company Holdings employees in difficult-to-return zone were measured broadly over a long period (fiscal year 2020 to fiscal year 2022). To obtain highly accurate estimates, we used not only ambient dose rates based on airborne radiological monitoring data, but also Integrated dose rate map data that had been statistically corrected to correspond to local ambient dose rate gradients on the ground. As a result, the conversion coefficients based on the ambient dose rate map measured by airborne radiological monitoring were 0.42 for the Evacuation-Order Lifted Zones (ELZs), 0.37 for the Special Zones for Reconstruction and Rehabilitation (SZRRs), and 0.47 for the Difficult-to-Return Zones without SZRRs (DRZs). On the other hand, the conversion coefficients based on the Integrated dose rate map which is a highly accurate dose rate map based on statistical analysis of various types of monitoring that have been studied in government projects in recent years, were 0.78 for the ELZs, 0.72 for the SZRRs and 0.82 for the DRZs. Using these conversion coefficients, the individual external dose can be estimated from two representative ambient dose rate maps provided by the government.

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 0.16µSv h-1(0.05-0.63µSv h-1), 0.57µSv h-1(0.15-3.92µSv h-1), and 1.36µSv h-1(0.14-11.91µSv 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 1.5µSv h-1differed 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 (0.24-20.89µSv h-1). The conversion coefficient is confirmed to be 0.7, similar to previous studies.