Individual external doses and conversion coefficients for outdoor workers after the lifting of evacuation orders in Specified Reconstruction and Revitalization Base Areas following the Fukushima Daiichi Nuclear Power Station accident

After the Fukushima Daiichi Nuclear Power Station accident, evacuation orders have gradually been lifted in Specified Reconstruction and Revitalization Base Areas (SRRBAs) as decontamination efforts have progressed and ambient dose rates (ADs) have decreased. By contrast, the actual state of individual external doses (IDs) during outdoor work in SRRBAs after the lifting of evacuation orders, as well as the applicability of conversion coefficients (CCs) used to estimate IDs from ADs, have not been sufficiently investigated in previous studies. In this study, individual external exposure doses using personal dosimeters carried by outdoor workers in SRRBAs where the evacuation orders had been lifted, and their relationship with ADs derived from airborne monitoring was statistically evaluated. The Brunner-Munzel test confirmed that the median hourly ID, calculated by dividing the cumulative individual dose by the corresponding measurement duration, was significantly lower after the lifting of evacuation orders than before (p < 0.01). Furthermore, the median CC was 0.39, which is lower than the value of 0.6 commonly used in the Japanese government's radiation exposure dose estimation model. The Brunner-Munzel test showed no significant difference (p > 0.01) between the median CC obtained in this study and that reported in our previous study conducted before the lifting of evacuation orders (median: 0.38). These results suggest that the CC obtained in the present study is consistent with previously reported values, despite environmental changes associated with the lifting of evacuation orders. The findings of this study are expected to contribute to the refinement of exposure dose estimation method in SRRBAs after the lifting of evacuation orders.