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—It has been challenging to obtain reliable estimates of thyroid equivalent doses (TEDs) to residents involved in the 11 March 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident because of the shortage of direct human measurements associated with 131I, the largest contributor to the dose of concern. The present study attempted to perform the estimation of the TEDs by inhalation of 131I to residents from Namie-town, one of the most radiologically-affected municipalities, by means of the latest atmospheric transport and dispersion model (ATDM) simulations with the Worldwide version of System for Prediction of Environmental Emergency Dose Information (WSPEEDI) ver. 2, coupled with personal behavioral data containing the history of the whereabouts of individuals shortly after the accident. We analyzed 1,637 residents who underwent direct measurements with whole-body counters several months after the accident and provided their personal behavioral data. We divided the subjects into two groups based on whether the distances between their locations and the FDNPP were >20 km as of 15:00 on 12 March in relation to the timepoint of the hydrogen explosion at the Unit 1 Reactor building. As a result, the 90th percentile TEDs of the 1,249 prompt evacuees and 388 late evacuees were 3.9 mSv (adult)–6.8 mSv (10-y-old) and 24.1 mSv (adult)–35.6 mSv (5-y-old), respectively, excluding 16 persons whose TEDs exceeded 50 mSv. The 90th percentile (median) TEDs to 1-y-old children (not included in the subjects) for the prompt and late evacuation groups were 8.1 (1.0) mSv and 36.3 (19.7) mSv, respectively. Additionally, this study provided the evidence to support the view that the explosive event at the Unit 1 Reactor building on the afternoon of 12 March 2011 could have caused the critical group among Namie-town's residents, whereas the largest release event on 15 March gave relatively small doses to the residents because their exposure took place mostly at sites that were distant

After a nuclear accident, population monitoring is necessary to determine the radiation exposure levels of the affected population. One of the monitoring components is direct thyroid measurements to assess the internal thyroid dose (mainly from 131I with a relatively short physical half-life: 8.02 days). These measurements are particularly important for young children but were not adequately performed after the 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Herein we propose a feasible and robust method to obtain as much reliable human data as possible for the dose reconstructions following a nuclear accident. The method employs three types of measurements: simplified, additional, and detailed measurements. The simplified measurements are performed using conventional NaI(Tl) survey meters to quickly triage heavily exposed individuals. The additional measurements are conducted using mobile whole-body counters to detect cesium (Cs; with longer retention in the body) as a tracer of 131I. And the detailed measurements are taken using spectrometric devices; they provide the most reliable data and are used to confirm the other measurements. We also discussed the target subjects and the monitoring period based on the experiences gained following the FDNPP accident. The early post-nuclear-accident phase is the most important period for the dose reconstruction for the affected populations; however, it is also the most challenging period in terms of allocating the necessary monitoring resources. It is thus vital that all countries that operate nuclear power plants establish and maintain a system for nuclear emergency preparedness and response.

Following a radiological or nuclear emergency, workers, responders and the public may be internally contaminated with radionuclides. Screening, monitoring and assessing any internal contamination and providing necessary medical treatment, especially when a large number of individuals are involved, is challenging. Experience gained and lessons learned from the management of previous incidents would help to identify gaps in knowledge and capabilities on preparedness for and response to radiation emergencies. In this paper, eight large-scale and five workplace radiological and nuclear incidents are reviewed cross 14 technical areas, under the broader topics of emergency preparedness, emergency response and recovery processes. The review findings suggest that 1) new strategies, algorithms and technologies are explored for rapid screening of large populations; 2) exposure assessment and dose estimation in emergency response and dose reconstruction in recovery process are supported by complementary sources of information, including 'citizen science'; 3) surge capacity for monitoring and dose assessment is coordinated through national and international laboratory networks; 4) evidence-based guidelines for medical management and follow-up of internal contamination are urgently needed; 5) mechanisms for international and regional access to medical countermeasures are investigated and implemented; 6) long-term health and medical follow up programs are designed and justified; and 7) capabilities and capacity developed for emergency response are sustained through adequate resource allocation, routine non-emergency use of technical skills in regular exercises, training, and continuous improvement.

The national system for radiation emergency medicine in Japan was revised based on the lessons learned from the Fukushima Daiichi Nuclear Power Plant accident. This system was further revised in 2019; specifically, the Nuclear Regulation Authority of Japan designated the National Institutes for Quantum Science and Technology (QST) as the national core center for coordinating and guiding four other Advanced Radiation Emergency Medical Support Centers. These centers will be responsible for providing medical treatments of severely exposed or contaminated patients as well as maintaining human resources to respond to nuclear disaster through a wide variety of training courses. Diagnostic dose assessment for patients is also one of the important missions of the centers. To enhance the capability for dose assessment of patients internally contaminated with actinides, the QST started operation of a new facility named the Dose Assessment Building for Advanced Radiation Emergency Medicine. A new bioassay laboratory was installed at this facility as well as an integrated in-vivo counter (Tamakuma et al., this issue). This laboratory has two rooms for pretreatment of excreta samples equipped with ashing, digestion and purification devices, and one analytical room with various measurement devices. Our current main interest is to establish a rapid and labor-saving bioassay method for actinides that are likely to cause significant internal exposure. In 2017, five workers from a Japanese nuclear facility accidentally inhaled plutonium compounds; this accident was the first Japanese case associated with medical intervention using decorporation agents (Ca/Zn-DTPA) [1]. QST was involved in the medical treatment and dose assessment, and our bioassay laboratory presented the 239+240Pu and 241Am data by collecting and analyzing ca. 200 urinary samples. To cope with similar situations in the future, we are attempting both the optimization of a conventional method based on alpha spectromet, International Conference on Individual Monitoring of Ionising Radiation (IM2022) and Neutron and Ion Dosimetry Symposium (NEUDOS-14)

In 2017, five workers were involved in the internal contamination accident at a nuclear facility of the Japan Atomic Energy Agency in Oarai-town, Ibaraki Prefecture, Japan. They accidentally inhaled plutonium compounds (containing Pu and Am isotopes) and the maximum committed effective dose was estimated as a range of 100−200 mSv [1]. Internal contamination with alpha-particle emitting actinides, such as 239Pu, 240Pu, is likely to bring a large amount of dose to the tissues of persons even if the intake amount is small. Therefore, after this accident, medical intervention using decorporation agents (Ca/Zn-DTPA) to patients was performed for the first time in Japan. This chelation therapy was continued for several weeks or months and ca. 200 urinary samples were collected from the five workers before and after each DTPA administration. To cope with similar situations in the future and provide timely information for prompt decision-making in radiation emergency therapy, we are attempting both the optimization of a conventional method based on alpha spectrometry and the development of a screening method based on mass spectrometry [2]. In terms of mass spectrometric method, to eliminate matrix, polyatomic interferences and tailing effects from U, after LaF 3 /CaF 3 coprecipitation, Pu isotopes were collected after removing U effectively (exhibiting a high decontamination factor of 108 for 238U) using a single chromatographic column packed with 2 mL AG MP-1M anion exchange resin. The Pu isotopes were measured using 242 Pu as a yield tracer with yields of 76±5%. Using ICP-MS/MS with low background, very low method detection limits for 239 Pu, 240 Pu, and 241 Pu of 0.057, 0.126, and 76.8 mBq L -1 , respectively, were obtained for 20 mL of urine sample. These detection limits were comparable to those of SF-ICP-MS with higher sensitivity [2]. In terms of alpha spectrometric method, after decomposing organic matter completely with HNO3 and H2O2 , in order to eliminate matrix, International Conference on Individual Monitoring of Ionising Radiation (IM2022) and Neutron and Ion Dosimetry Symposium (NEUDOS-14)

In case of nuclear accidents, it is important to determine the magnitude of radiation doses received by people living in contaminated territories. Direct measurement of individual external doses using personal dosimeters (PDs) worn by subjects is the most reliable solution for this necessity. However, it is not practical to expect that all returnees to the affected areas wear PDs. Thus, conversion factors from the ambient dose equivalent (ADEs) to the effective dose for various aged individuals must be determined appropriately. Previous studies demonstrated that the ratio of PDs to ADEs at various areas of Fukushima Prefecture mostly existed in the range of 0.6-0.7 [e.g.,1-3]. These studies provided evidence that PD readings would be reasonable estimators for the effective doses of adult subjects at locations where radioactive cesium is widely and uniformly deposited on the ground. In the present study, we experimentally obtained the response of PDs (D-shuttle, Chiyoda Technol., Tokyo, Japan; Dose i, Fuji Electric, Tokyo, Japan) attached to the front of five age-specific anthropometric phantoms (newborn, 1 y, 5y, 10y and adult male) for lateral radiations of 137Cs with various rotational angles. The PD/ADE ratio for the D-shuttle on the five phantoms with rotational irradiation simulating the exposure in Fukushima Prefecture were 0.87, 0.85, 0.83, 0.83 and 0.77, respectively. The ratio for newborns was 11 % higher than that for adult males. This body size dependency can be explained by the PD response to exposure direction. We discussed the results of this study and our field experiments to construct conversion coefficients from the ADE to the effective doses for age-group residents living in affected areas, which will be presented at the conference., International Conference on Individual Monitoring of Ionising Radiation (IM2022) and Neutron and Ion Dosimetry Symposium (NEUDOS-14)

In case of nuclear accidents, it is important to determine the magnitude of radiation doses received by people living in contaminated territories. Direct measurement of individual external doses using personal dosimeters (PDs) worn by subjects is the most reliable solution for this necessity. However, it is not practical to expect that all returnees to the affected areas wear PDs. Thus, conversion factors from the ambient dose equivalent (ADEs) to the effective dose for various aged individuals must be determined appropriately. Previous studies demonstrated that the ratio of PDs to ADEs at various areas of Fukushima Prefecture mostly existed in the range of 0.6-0.7 [e.g.,1-3]. These studies provided evidence that PD readings would be reasonable estimators for the effective doses of adult subjects at locations where radioactive cesium is widely and uniformly deposited on the ground. In the present study, we experimentally obtained the response of PDs (D-shuttle, Chiyoda Technol., Tokyo, Japan; Dose i, Fuji Electric, Tokyo, Japan) attached to the front of five age-specific anthropometric phantoms (newborn, 1 y, 5y, 10y and adult male) for lateral radiations of 137Cs with various rotational angles. The PD/ADE ratio for the D-shuttle on the five phantoms with rotational irradiation simulating the exposure in Fukushima Prefecture were 0.87, 0.85, 0.83, 0.83 and 0.77, respectively. The ratio for newborns was 11 % higher than that for adult males. This body size dependency can be explained by the PD response to exposure direction. We discussed the results of this study and our field experiments to construct conversion coefficients from the ADE to the effective doses for age-group residents living in affected areas, which will be presented at the conference., International Conference on Individual Monitoring of Ionising Radiation (IM2022) and Neutron and Ion Dosimetry Symposium (NEUDOS-14)

Technetium-99m-galactosyl human serum albumin (99mTc-GSA) is a molecular imaging agent used for evaluating hepatic function. 99mTc-GSA hepatic scintigraphy examinations were performed in 50 patients from December 2018 to November 2019 at the QST Hospital in Japan. In order to estimate the individual variation in absorbed doses for each patient, a method of internal dose assessment for multiple organs and tissues in 99mTc-GSA scintigraphy was developed using a nine-compartment biokinetic model that took advantage of the existing models. This method is also expected to give a better understanding of metabolic changes along with the disease progress. Transfer rates related to the liver and blood compartments of the biokinetic model were optimized for each examination by comparing the time-activity curves for the liver and heart derived from the model with those clinically observed on individual dynamic images over the trunk region using the gamma camera after intravenous injection of 99mTc-GSA. Simultaneous differential equations describing the activity in each compartment were numerically analyzed using EQUATRAN-G, a commercial software, by that the total amount of disintegrations taken place in each compartment over a period of 2.5 days, which corresponds to nearly 10 physical half-lives of 99mTc, was obtained. Absorbed doses of several organs and tissues were calculated using the internal dose calculation software, IDAC-Dose 2.1. As a result, the calculated average dose per unit administered activity (mGy/MBq) for the liver was the highest (0.058 ± 0.007 mGy/MBq for females and 0.042 ± 0.009 mGy/MBq for males) compared to other organs and tissues, and the average dose for males was lower than that estimated for healthy males in a past study (0.054 mGy/MBq)., 15th International Congress of the International Radiation Protection Association (IRPA15)

Internal contamination with alpha-particle emitting actinides, such as 237Np, 239Pu, 240Pu, is likely to bring a large amount of dose to the tissues of persons even if the intake amount is small. To provide timely information for prompt decision-making in radiation emergency therapy, we developed a simple and rapid method for urinary bioassay to determine ultra-trace 237Np and Pu isotopes using SF-ICP-MS and ICP-MS/MS. To avoid polyatomic interferences and tailing effects from U, 237Np and Pu isotopes were collected after removing U effectively using a simple single chromatographic column packed with 2 mL AG MP-1M anion exchange resin, exhibiting a high decontamination factor of 108 for 238U. The overall chemical fractionation between 237Np and 242Pu for the whole analytical procedure was 0.974 ± 0.064 (k=2), allowing us to measure 237Np and Pu isotopes using 242Pu as a yield tracer with yields of 76±5%. Using ICP-MS/MS with low background provided the method detection limits for 237Np, 239Pu, 240Pu, and 241Pu of 0.025, 0.025, 0.015, and 0.020 fg mL-1, respectively, for 20 mL of urine sample. Those were comparable to detection limits of SF-ICP-MS with high sensitivity. Subsequently, three urine reference materials with Pu spike, provided by the Association for the PROmotion of Quality COntrol in RADiotoxicological Analysis (PROCORAD), France, were analyzed by the developed method and the conventional alpha spectrometry technique for validation. Finally, the developed method was successfully employed to measure the contamination level of 237Np, 239Pu, 240Pu, and 241Pu in urine samples collected during decorporation therapy using DTPA, after a Pu inhalation exposure accident in Japan. The high throughput (9 h for 12 samples), simplicity, low cost, and high sensitivity of the method will allow greater numbers of related laboratories to be involved in screening activities for unexpected actinide exposure, such as in the case of a large scale radiological disaster.

Determination of the 131I activity in the thyroid of exposed people or workers is one of the major concerns that needs to be addressed following a nuclear or radiological emergency. International intercomparison exercises on bioassay measurements are one of the effective methods to improve the capability and quality of internal dose assessment. In 2017, Working Group 1 (Internal Determination of the 131I activity in the thyroid of exposed people or workers is one of the major concerns that needs to be addressed following a nuclear or radiological emergency. International intercomparison exercises on bioassay measurements are one of the effective methods to improve the capability and quality of internal dose assessment. In 2017, Working Group 1 (Internal Dosimetry) of the Asian Radiation Dosimetry Group (ARADOS) organized an intercomparison exercise on the thyroid measurement, aiming at enhancing and coordinating the radiation dosimetry capabilities of Asian countries. This paper describes the measurement experience and results of In Vivo Monitoring Laboratory of NIRP in the 2017 intercomparison exercise, which can provide technical reference for laboratories that have not yet participated in such thyroid measurement intercomparsion. It covers technical aspects such as calibration, measurement and data processing. The results presented in this intercomparison are within an acceptable range of performance criteria on the bioassay measurements. In addition, further research work considered in the field of thyroid measurement is described in the discussion section

More than nine years have passed since the 2011 nuclear disaster in Fukushima Prefecture, Japan. During this period, much effort has been made on the dose reconstruction for Fukushima residents; however, the estimation of the internal dose due to the potential intake of the short-lived radionuclides (mainly, 131I) has been challenging because of the lack of direct human measurements at the early phase of the accident. Our previous study revealed that the residual cesium body contents observed in delayed whole-body counter (WBC) measurements of residents from Namie-town, one of the most affected municipalities), varied greatly with the timepoint of their evacuations on 12 March 2011 when the first explosive event occurred at the accident site; i.e., the late evacuees had much higher residual Cs body contents compared to the prompt evacuees. The present study thus aimed to clarify this finding by reproducing the exposure situation based on the evacuees' personal behavioral data in combination with the latest atmospheric transport and dispersion model (ATDM) simulation for 356 selected subjects in adult and 15-year (13–17 yrs) age groups. The results demonstrated that the ATDM simulation-based method could reasonably reproduce the subjects' exposure situation, supporting the previous finding. However, the residual 137Cs body contents calculated by this method were only 10%–20% of those in the subjects' WBC measurements. This large discrepancy was considered to be caused by both the present method's underestimation and the overestimation of the subjects' early intake in the WBC measurements due to a conservative intake scenario not assuming potential additional intake. Additional studies are needed to further clarify the reasons for the discrepancy and to evaluate the magnitude of the inhalation dose in the accident.

The authors’ previous study suggested that a simple standing-type whole-body counter called FASTSCAN (Canberra, Meriden, CT, USA), widely installed throughout Japan after the Fukushima Daiichi nuclear power plant (FDNPP) accident in March 2011, could be used for thyroid measurements in internal dose assessment after a future radiological incident accompanied by the release of 131I into the environment. The present study performed Monte-Carlo simulations using a computational human phantom with several patterns of body surface contamination and used the results to formulate a method for probabilistic analyses of 131I thyroid activity in persons with surface contamination. The major advantage of this method is that the upper limit of the thyroid activity can be determined from the relative frequency distribution without identifying where body surface contamination remains. Therefore, this method is especially effective for use in the early phase of a radiological incident when time and resources are limited, making it difficult to physically remove all body surface contamination for the purpose of gaining more accurate population-wide thyroid screening measurements. As a case study of the FDNPP accident, the proposed method was applied to the results of in vivo measurements for a male subject with body surface contamination. In comparing the probabilistic analyses before and after removal of the subject’s contaminated work clothes, the uncertainty of the relative frequency distribution of 131I thyroid activity was reduced by their removal. Therefore, in terms of accurate estimates and avoidance of radiation exposure from their contaminated clothes, efforts to remove body surface contamination still should be made as much as possible in the chaotic situation of the early phase of a radiological incident.

It is very important to determine the precise internal thyroid doses of Fukushima residents involved in the 2011 Fukushima nuclear disaster, particularly for small children. This has been challenging due to the lack of direct human measurements to identify 131I, the biggest contributor to the thyroid doses. We previously used a dataset of late whole-body counter (WBC) measurements targeting 134Cs and 137Cs for the thyroid dose estimation in comparison with the intake ratios of 131I to 137Cs (or 134Cs) derived from thyroid and whole-body doses individually obtained from different subject groups, assuming simultaneous acute intake via inhalation. Herein, we applied the same method to the doses of residents in Iwaki city (located south of the Fukushima Daiichi Nuclear Power Plant) with a relatively high activity ratio (131I/137Cs) for the ground deposition density. Our analyses revealed that the intake ratio (131I/137Cs) for the Iwaki residents was 4.2–4.3, which is relatively consistent with the values obtained in other studies (average 3.0–5.0). No regional difference in the intake ratios from other areas was observed, but further studies are required to determine the accurate intake ratio in the early phase of the accident, in particular focusing on the reasonable interpretation of results of the late WBC measurements to evaluate the actual Cs intake.