Yukihisa Sanada

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, IDs recorded by personal dosimeters worn by outdoor workers in SRRBAs where the evacuation orders had been lifted, and their relationship with ADs derived from airborne monitoring were statistically evaluated. The Brunner–Munzel test confirmed that the median hourly-average 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 coefficient of 0.6 implicitly 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 are expected to contribute to improving external exposure assessment for outdoor workers in SRRBAs after the lifting of evacuation orders.

We developed and evaluated a real-time gamma-ray directional imaging framework using the Fractal Radiation Imaging Element (FRIE), an omnidirectional detector based on a Sierpinski tetrahedral structure. The system consists of sixteen scintillator crystals, each acquiring spectral data every second, and estimates the incident gamma-ray directions at 1-s intervals by analyzing variations in detector response. The estimated directional distributions are superimposed on images captured by a 360-degree camera, enabling intuitive visual confirmation. In this study, real-time imaging performance was comparatively evaluated using multiple analysis algorithms, including the conventional Maximum Likelihood Expectation Maximization (MLEM) method and similarity-based analyses employing pre-acquired reference response patterns with correlation coefficients, Euclidean distance, and Wasserstein distance. Although all methods were able to estimate the incident directions in real time, the similarity-based analyses-particularly the approach combining Euclidean and Wasserstein distances-demonstrated higher stability and reproducibility, with angular errors constrained within approximately 20 degrees. These results indicate that similarity-based analysis is effective for robust and real-time localization of radiation sources using the FRIE detector.

This study analysed Japan's implementation of a regulatory framework that enables limited, non-residential land use in areas where radiation levels remain too high for repopulation following a nuclear accident, focusing on a wind power project in Katsurao Village, Fukushima Prefecture. Although international initiatives have advanced protective actions for the emergency phase of radiological events, comparatively little guidance exists for the recovery phase, particularly regarding procedures for lifting evacuation orders in persistently high-dose areas. Japan's experience is unique: while evacuation orders have been lifted across multiple municipalities, some zones continue to exceed dose levels suitable for habitation, necessitating new policy approaches. To address this gap, Katsurao Village employed the framework to authorise land use in a 19.9-hectare district designated for wind power generation. A multi-stakeholder committee-comprising residents, experts, government authorities, and industry representatives-was established to evaluate dose-reduction measures, access protocols, environmental maintenance, and operational safeguards. The committee identified several unresolved issues, including the absence of standardised monitoring procedures, challenges in managing worker exposure, the need for clear demarcation and communication to prevent unauthorised entry, and mechanisms for transparent reporting in the event of abnormal findings. This case demonstrates both the feasibility and complexity of enabling controlled land use in high-dose areas, underscoring the importance of governance, technical standardisation, and sustained risk communication.

This study presents the development and field validation of a UAV-based airborne gamma-ray spectrometry system (MARK-M1), designed by the Korea Atomic Energy Research Institute for rapid radiological assessment in post-accident scenarios. The MARK-M1 integrates two LaBr₃(Ce) detectors, global positioning system, and a laser altimeter, with data transmission ensured via Bluetooth communication. Field surveys were conducted in collaboration with the Japan Atomic Energy Agency around the Fukushima Daiichi Nuclear Power Plant. An attenuation correction factor was derived from hovering flights at multiple altitudes and applied to convert aerial measurements to dose rates at 1 m above ground level. The results showed that flight speed had little effect on dose rate estimation when appropriate detector sensitivity and flight line spacing were ensured. However, in low-lying areas with sloped terrain, significant discrepancies were observed between airborne and ground-based measurements, indicating the limitations of flat-ground assumptions in attenuation models. Incorporating terrain data such as digital elevation models is suggested to improve estimation accuracy in complex topographies. These findings demonstrate both the practical utility and necessary refinements for UAV-based gamma-ray surveys in real-world emergency response applications.

An airborne survey system named the MARK-A1 was previously developed to be mounted on an unmanned aerial vehicle for the purpose of application in contaminated areas with high dose rate levels. The MARK-A1 system consists of a CZT detector, signal processing unit, and positioning and interface units linked to a PC on the ground. The weight of the system is below 1 kg for loading on a commercial drone. In the current work, for experimental verification, field testing was conducted in a high dose rate environment near the Fukushima Daiichi Nuclear Power Plant. With the cooperation of the Japan Atomic Energy Agency, the MARK-A1 was installed on an unmanned aerial vehicle to conduct airborne surveys using two flight methods, namely a flat flight and a step flight, at various speeds. The airborne survey data were converted to gamma dose rates at a height of 1 m above the ground using a flat source model to create contamination maps. For a comparative evaluation of the airborne survey results, an in situ survey was also conducted in the survey area, and it was confirmed that the step flight method better matched the surface survey results.

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.

The distribution of the ambient dose equivalent rate (i.e., air dose rate) after a nuclear disaster is crucial for zoning contaminated areas to facilitate authorities’ effective decision making. Several countries are considering a gradual characterization strategy where airborne measurement is performed first followed by ground measurement (i.e., via manborne or carborne surveys). Nonetheless, potential differences might emerge in country-specific air dose rate assessment methods. Explaining these discrepancies can improve and converge existing methodologies. The Japan Atomic Energy Agency (JAEA) and the French Institute for Radiological Protection and Nuclear Safety (IRSN), which are organizations involved in post-nuclear accident crisis management, jointly performed air dose rate measurements in 2019 at contaminated sites around the Fukushima Daiichi Nuclear Power Station. The similarities and differences between the two organizations’ methods and results were quantitatively assessed by comparing the average air dose rates obtained within a grid created with a geographic information system, and the reasons for the differences between the organizations’ results were investigated. The air dose rates obtained by the manborne measurements varied depending on the calibration method. Comparing the air dose rate assessment methods and mapping techniques used in different countries will contribute to developing international guidelines for recommending the best method for determining air dose rates.