Measurement of individual doses of radiation by personal dosimeter is important for the return of residents from evacuation order areas after nuclear disaster

External dose measurements to investigate the body-size dependence of personal dosemeter responses in areas affected by the Fukushima Daiichi nuclear accident

To formulate necessary protective measures after a large-scale nuclear accident, it is crucial to understand the levels of radiation to which persons living in radionuclide-contaminated areas are exposed. Individual monitoring using personal dosemeters (PDs) plays a role in this, although PDs were not originally intended to be used by members of the public. The present study investigated PD responses in areas highly affected by the Fukushima Daiichi nuclear accident, as well as the dependence of those measurements on body size. Three types of commercially available electronic PDs, including D-shuttle, which has often been used in Fukushima, were placed on the front surfaces of three age-specific anthropometric phantoms imitating a 5-y-old, a 10-y-old and an adult male, and these phantoms were then exposed to radiation in an open field in the affected area. In the case of D-shuttle, the ratios of PD readings to the ambient dose rate for the 5-y-old phantom and the adult male phantom were 0.79 and 0.72, respectively. The ratios were somewhat higher for the other PDs; any PDs showed higher readings than the effective doses estimated by simulations based on the assumed ground deposition of 134Cs and/or 137Cs over the affected areas.

Evaluation of Residents' Timing of Return to or New Settlement in Kawauchi Village, at 10 Years after the Fukushima Daiichi Nuclear Power Plant Accident

Kawauchi village in Fukushima prefecture was affected by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, and residents had to evacuate from their hometown in 2011. This study clarified the timing and related factors with regard to residents returning to or newly settling in Kawauchi. A survey was conducted using a questionnaire, from February to March 2021, with assistance from the Kawauchi village office and post office. Of the 374 residents, 170 (45.5%) had returned to or newly settled in Kawauchi within the past 2 years (group 1), 84 (22.5%) in the past 2-5 years (group 2), and 99 (26.5%) after more than 5 years (group 3) following the evacuation order. An additional 21 residents (5.5%) who had lived in Kawauchi at the time of the FDNPP had not yet returned (group 4). Compared with the other groups, residents in group 1 were more satisfied with their current lives and were coping better with stress. Even though they had experienced a serious nuclear disaster, residents of Kawauchi village who returned to their hometown in the early phase had a high sense of satisfaction with their current life one decade after the FDNPP accident.

Risk Perception of Health Risks Associated with Radiation Exposure among Residents of Okuma, Fukushima Prefecture

Ten years have passed since the Great East Japan Earthquake and the subsequent Fukushima Daiichi nuclear power plant accident on 11 March 2011. Okuma is a town hosting the Fukushima Daiichi nuclear power plant. The evacuation order for Okuma was partially lifted in April 2019. To clarify factors associated with risk perceptions of radiation among the residents of Okuma, we conducted a questionnaire survey in January 2021. Our results revealed that resident anxieties regarding the health effects of radiation exposure from living in Okuma were independently associated with positive PCL-Specific score, recognition of the consultation services with radiation experts in the municipal government of Okuma, and planned request for consultation service regarding radiation exposure by radiation experts, along with being female and living with a child. It is important for radiation experts to promote periodic communication of risks with individuals on a small scale to provide accurate information about the health effects of radiation and to provide maternal and child healthcare services and support regarding child-rearing and radiation exposure, to reduce concerns about radiation exposure and facilitate healthy living and wellbeing in Okuma.

User-Guided Design of a Digital Tool for Health Promotion and Radiation Protection: Results from an Internet Needs Survey

Background: Digital tools can be powerful and effective in connecting people with life-saving and health-promoting support when facing a health crisis. To develop a digital application for radiation protection and health promotion for evacuees returning home after the Fukushima nuclear accident, we conducted a needs assessment survey and explored the association of people’s eHealth literacy (eHL) level with their digital tool knowledge, attitudes, and practice (KAP). Methods: From 339 responses to an online survey, data from 264 lay persons were analyzed. The KAP items were those used in a prior EU project, and eHL levels were assessed with a Japanese version of the eHealth Literacy Scale. Results: Multivariable analyses showed significant associations between eHL and the digital tool KAP for radiation protection (knowledge: adjusted odds ratio (aOR) = 1.10; attitude: 1.06; practice: 1.10) and for health promotion (knowledge: aOR = 1.13; attitude: 1.06; practice: 1.16). Conclusions: People with a higher eHL had a more positive KAP. For those with a lower eHL, we are formulating easy-to-understand explanations to promote the utilization of the digital tool and enthusiasm for future community-oriented digital tools.

Induction of somatic mutations by low concentrations of tritiated water (HTO): evidence for the possible existence of a dose-rate threshold

Tritium is a low energy beta emitter and is discharged into the aquatic environment primarily in the form of tritiated water (HTO) from nuclear power plants or from nuclear fuel reprocessing plants. Although the biological effects of HTO exposures at significant doses or dose rates have been extensively studied, there are few reports concerning the biological effects of HTO exposures at very low dose rates. In the present study using a hyper-sensitive assay system, we investigated the dose rate effect of HTO on the induction of mutations. Confluent cell populations were exposed to HTO for a total dose of 0.2 Gy at dose rates between 4.9 mGy/day and 192 mGy/day by incubating cells in medium containing HTO. HTO-induced mutant frequencies and mutation spectra were then investigated. A significant inflection point for both the mutant frequency and mutation spectra was found between 11 mGy/day and 21.6 mGy/day. Mutation spectra analysis revealed that a mechanistic change in the nature of the mutation events occurred around 11 mGy/day. The present observations and published experimental results from oral administrations of HTO to mice suggest that a threshold dose-rate for HTO exposures might exist between 11 mGy/day and 21.6 mGy/day where the nature of the mutation events induced by HTO becomes similar to those seen in spontaneous events.

Radiocesium levels in contaminated forests has remained stable, even after heavy rains due to typhoons and localized downpours

In recent years, Japan has suffered serious damage due to natural disasters such as earthquakes, heavy rains due to tropical storms (typhoons) and localized downpours. To assess the chronological changes in the attenuation of external exposure doses and environmental radiation contamination due to the rainfall associated with typhoons and heavy rains during October to December 2019 in Fukushima, we measured environmental radiation levels in forest areas along the Mt Okura hiking trail in Tomioka Town, Fukushima Prefecture, near the Fukushima Daiichi Nuclear Power Station. We confirmed that (1) current ambient dose rates of 0.38-0.95 μSv/h in most forest areas were 79.9-84.7% higher than in residential areas; (2) the number of sites along the hiking trail where 137Cs was detected was limited (1.1-4.7%); and (3) individual dose rates of 0.21-0.34 μSv/h were lower than ambient dose rates. These findings suggest that radiocesium has remained stable in natural forests that have not been decontaminated even though current levels are low, despite the occurrence of heavy rainfall associated with Super Typhoon Hagibis in 2019 and localized downpours. Hiking while managing exposure to environmental contamination using a personal dosimeter may be the safest model for spending time of leisure activities.

Radiocesium concentrations and GPS-coupled dosimetry in Fukushima snakes

One of the largest releases of radioactive contamination in history occurred at Japan's Fukushima Daiichi Nuclear Power Plant (FDNPP). Although the accident happened in 2011, questions still persist regarding its ecological impacts. For example, relatively little is known about radiocesium accumulation in snakes, despite their high trophic status, limited home range sizes, and close association with soil where many radionuclides accumulate. This study presents one of the most comprehensive radioecological studies of snakes published to date using a combination of whole-body radiocesium analyses, GPS transmitters, and optically stimulated luminescence (OSL) dosimeters. The objectives were to: 1) quantify whole-body radiocesium activity concentrations and internal dose rates among several common species of snakes within and around the Fukushima Exclusion Zone (FEZ), 2) determine effects of species, sex, and body size on radiocesium activity concentrations, 3) measure external dose rates using GPS-coupled dosimeters deployed on free-ranging snakes, 4) compare field-derived empirical dose rates to those generated by computer simulation software (i.e., the ERICA tool), and 5) determine if incorporating snake behavior into computer models improve simulated estimates of external dose. Whole-body radiocesium levels for snakes were highly variable among individuals (16 to 25,000 Bq/kg, FW), but were influenced more by levels of local contamination than species, sex, or size. Doses recorded by OSL dosimeters on snakes, as well as modeling in ERICA, suggest that individual movements and behavior have a substantial influence on dose rates to snakes. However, dose estimates produced with ERICA were comparable to dose received by tracked snakes. The average external plus internal dose rate for snakes captured in the FEZ was 3.6-3.9 μGy/h, with external dose contributing 80% to the total. Further research regarding reptile-specific benchmark dose rates would improve risk assessment for reptiles in radiologically contaminated areas.

Review of reduction factors by buildings for gamma radiation from radiocaesium deposited on the ground due to fallout

In order to estimate residents' external dose due to radionuclide exposure resulting from fallout deposit on the ground, the shielding and dose reduction effects provided by structures such as houses and workplaces are taken into account as most individuals spend a large portion of their time indoors. Many works on both calculation and measurement for European and American settlements have been reported and factors such as, shielding factors, protection factors, reduction factors, and location factors have been determined. However, measurement data for Japanese settlements are lacking. Thus, the Japanese government used reduction factors given in the International Atomic Energy Agency documents from American and European settlements when Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident occurred. The United Nations Scientific Committee on the Effects of Atomic Radiation used location factors from European settlements for the same reason. Soon after the FDNPP accident, several measurements and calculations were performed to obtain specific reduction factors for Japanese settlements due to this lack of data. This research reviews previous studies that determined factors such as, shielding factors, protection factors, reduction factors, and location factors and summarizes specific results for Japan. We discuss the issues in determining these factors and in applying them to estimate indoor dose. The contribution of surface contamination to the indoor ambient dose equivalent rate is also discussed.

Evaluation of Environmental Contamination and Estimated Radiation Exposure Dose Rates among Residents Immediately after Returning Home to Tomioka Town, Fukushima Prefecture

On 1 April 2017, six years have passed since the Fukushima Daiichi Nuclear Power Station (FDNPS) accident, and the Japanese government declared that some residents who lived in Tomioka Town, Fukushima Prefecture could return to their homes. We evaluated environmental contamination and radiation exposure dose rates due to artificial radionuclides in the livelihood zone of residents (living space such as housing sites), including a restricted area located within a 10-km radius from the FDNPS, immediately after residents had returned home in Tomioka town. In areas where the evacuation orders had been lifted, the median air dose rates were 0.20 μSv/h indoors and 0.26 μSv/h outdoors, and the radiation exposure dose rate was 1.6 mSv/y. By contrast, in the “difficult-to-return zone,” the median air dose rate was 2.3 μSv/h (20 mSv/y) outdoors. Moreover, the dose-forming artificial radionuclides (radiocesium) in the surface soil were 0.018 μSv/h (0.17 mSv/y) in the evacuation order-lifted areas and 0.73 μSv/h (6.4 mSv/y) in the difficult-to-return zone. These findings indicate that current concentrations of artificial radionuclides in soil samples have been decreasing in the evacuation order-lifted areas of Tomioka town; however, a significant external exposure risk still exists in the difficult-to-return zone. The case of Tomioka town is expected to be the first reconstruction model including the difficult-to-return zone.

RECOVERY FROM NUCLEAR DISASTER IN FUKUSHIMA: COLLABORATION MODEL

Since 2011, Nagasaki University has been assisting the reconstruction efforts of Kawauchi Village in the Fukushima Prefecture, which was the first village to declare it safe for residents to return to their homes. In April 2013, Nagasaki University and the Kawauchi government office concluded an agreement concerning comprehensive cooperation toward reconstruction of the village. The present study evaluates the internal and external exposure doses of residents and conducted a risk communication based on each resident's radiation dose. Furthermore, researchers began a comprehensive support of Tomioka residents who have returned to their hometown in 2017. Based on the experiences in Kawauchi village, it is clear that the cooperation of residents, local authorities and specialists is essential for the recovery of areas affected by the nuclear disaster. Accumulated experiences and practices should be carefully evaluated and recorded to prepare for future unexpected nuclear disasters.

Review of reduction factors by buildings for gamma radiation from radiocaesium deposited on the ground due to fallout

In order to estimate residents' external dose due to radionuclide exposure resulting from fallout deposit on the ground, the shielding and dose reduction effects provided by structures such as houses and workplaces are taken into account as most individuals spend a large portion of their time indoors. Many works on both calculation and measurement for European and American settlements have been reported and factors such as, shielding factors, protection factors, reduction factors, and location factors have been determined. However, measurement data for Japanese settlements are lacking. Thus, the Japanese government used reduction factors given in the International Atomic Energy Agency documents from American and European settlements when Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident occurred. The United Nations Scientific Committee on the Effects of Atomic Radiation used location factors from European settlements for the same reason. Soon after the FDNPP accident, several measurements and calculations were performed to obtain specific reduction factors for Japanese settlements due to this lack of data. This research reviews previous studies that determined factors such as, shielding factors, protection factors, reduction factors, and location factors and summarizes specific results for Japan. We discuss the issues in determining these factors and in applying them to estimate indoor dose. The contribution of surface contamination to the indoor ambient dose equivalent rate is also discussed.

Induction of somatic mutations by low-dose X-rays: the challenge in recognizing radiation-induced events

It is difficult to distinguish radiation-induced events from spontaneous events during induction of stochastic effects, especially in the case of low-dose or low-dose-rate exposures. By using a hypersensitive system for detecting somatic mutations at the HPRT1 locus, we investigated the frequency and spectrum of mutations induced by low-dose X-rays. The mutant frequencies induced by doses of >0.15 Gy were statistically significant when compared with the spontaneous frequency, and a clear dose dependency was also observed for mutant frequencies at doses of >0.15 Gy. In contrast, mutant frequencies at doses of <0.1 Gy occurred at non-significant levels. The mutation spectrum in HPRT-deficient mutants revealed that the type of mutations induced by low-dose exposures was similar to that seen in spontaneous mutants. An apparent change in mutation type was observed for mutants induced by doses of >0.2 Gy. Our observations suggest that there could be a critical dose for mutation induction at between 0.1 Gy and 0.2 Gy, where mutagenic events are induced by multiple DNA double-strand breaks (DSBs). These observations also suggest that low-dose radiation delivered at doses of <0.1 Gy may not result in DSB-induced mutations but may enhance spontaneous mutagenesis events.

Radiocesium contamination and estimated internal exposure doses in edible wild plants in Kawauchi Village following the Fukushima nuclear disaster

Kawauchi Village, in Fukushima Prefecture, is located within a 30-km radius of the nuclear disaster site of the Fukushima Daiichi Nuclear Power Plant (FDNPP). "Sansai" (edible wild plants) in this village have been evaluated by gamma spectrometry after the residents had returned to their homes, to determine the residents' risk of internal exposure to artificial radionuclides due to consumption of these plants. The concentrations of radiocesium (cesium-134 and cesium-137) were measured in all 364 samples collected in spring 2015. Overall, 34 (9.3%) samples exceeded the regulatory limit of 100 Bq/kg established by Japanese guidelines, 80 (22.0%) samples registered between 100 Bq/kg and 20 Bq/kg, and 250 (68.7%) registered below 20 Bq/kg (the detection limit). The internal effective doses from edible wild plants were sufficiently low (less than 1 mSv/y), at 3.5±1.2 μSv/y for males and 3.2±0.9 μSv/y for females (2.7±1.5 μSv/y for children and 3.7±0.7 μSv/y for adults in 2015). Thus, the potential internal exposure doses due to consumption of these edible wild plants were below the applicable radiological standard limits for foods. However, high radiocesium levels were confirmed in specific species, such as Eleutherococcus sciadophylloides ("Koshiabura") and Osmunda japonica (Asian royal fern, "Zenmai"). Consequently, a need still might exist for long-term follow-up such as environmental monitoring, physical and mental support to avoid unnecessary radiation exposure and to remove anxiety about adverse health effects due to radiation. The customs of residents, especially the "satoyama" (countryside) culture of ingesting "sansai," also require consideration in the further reconstruction of areas such as Kawauchi Village that were affected by the nuclear disaster.

Measuring and assessing individual external doses during the rehabilitation phase in Iitate village after the Fukushima Daiichi nuclear power plant accident

After the Fukushima Daiichi nuclear power plant accident in 2011, Iitate village was placed under an evacuation order because the level of radioactive materials drifting from the nuclear plant to the village was above a government-set level for allowing residents to live in the area. The evacuation advisory for most of the village was lifted on 31 March 2017. For displaced residents deciding whether or not to return to their homes, it is important to correctly understand and estimate the realistic individual external doses they will receive after returning to the village. In this study, with the support of residents of the village, we used a personal dosimeter (D-Shuttle) coupled with a global positioning system device to measure and thus understand realistic individual external doses while the residents were in Iitate village and to project the individual external doses for different administrative districts as of 1 April 2017. The measured individual external doses measured by D-Shuttle for 38 study participants showed that the doses measured during time spent inside the village were higher and more widely distributed than the doses measured during time spent outside the village. The exposure ratio (ER) was defined as the ratio of additional individual external dose measured by D-Shuttle to the additional ambient dose based on an airborne monitoring survey. The medians of the average ERs were 0.13 (min-max 0.06-0.27) for time spent at home and 0.18 (min-max 0.08-0.36) for time spent outdoors. Projected additional annual individual external doses as of 1 April 2017 for different administrative districts in the village were calculated using ERs obtained in this study. Assuming that individuals spent 8 h per day on outdoor activities and 16 h on indoor activities, additional annual individual external doses were estimated to be below 3 mSv using the mean of the average ERs for most districts in the village, and these values were well below the individual external doses estimated using the approach taken by the central government. The results of this study provide valuable information both for understanding realistic radiological situations in the village and for those who want to know their future individual external dose in order to make a decision on whether or not to live in the village.

Communicating With Residents About Risks Following the Fukushima Nuclear Accident

The Fukushima nuclear accident in March 2011 posed major threats to public health. In response, medical professionals have tried to communicate the risks to residents. To investigate forms of risk communication and to share lessons learned, we reviewed medical professionals’ activities in Fukushima Prefecture from the prefectural level to the individual level: public communication through Fukushima Health Management Surveys, a Yorozu (“general”) health consultation project, communications of radiological conditions and health promotion in Iitate and Kawauchi villages, dialogues based on whole-body counter, and science communications through online media. The activities generally started with radiation risks, mainly through group-based discussions, but gradually shifted to face-to-face communications to address comprehensive health risks to individuals and well-being. The activities were intended to support residents’ decisions and to promote public health in a participatory manner. This article highlights the need for a systematic evaluation of ongoing risk communication practices, and a wider application of successful approaches for Fukushima recovery and for better preparedness for future disasters.

Radiation Doses and Associated Risk From the Fukushima Nuclear Accident

The magnitude of dose due to the Fukushima Daiichi Accident was estimated by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2013 report published in April 2014. Following this, the UNSCEAR white paper, which comprises a digest of new information for the 2013 Fukushima report, was published in October 2015. Another comprehensive report on radiation dose due to the accident is the International Atomic Energy Agency (IAEA) report on the Fukushima Daiichi Accident published in August 2015. Although the UNSCEAR and IAEA publications well summarize doses received by residents, they review only literature published before the end of December 2014 and the end of March 2015, respectively. However, some studies on dose estimation have been published since then. In addition, the UNSCEAR 2013 report states it was likely that some overestimation had been introduced generally by the methodology used by the Committee. For example, effects of decontamination were not considered in the lifetime external dose estimated. Decontamination is in progress for most living areas in Fukushima Prefecture, which could reduce long-term external dose to residents. This article mainly reviews recent English language articles that may add new information to the UNSCEAR and IAEA publications. Generally, recent articles suggest lower doses than those presented by the UNSCEAR 2013 report.

Concentrations of Radiocesium in Local Foods Collected in Kawauchi Village after the Accident at the Fukushima Dai-ichi Nuclear Power Station

We evaluated the current concentrations of radiocesium in local foods collected in Kawauchi Village, which is located less than 30 km from Fukushima Daiichi Nuclear Power Station, to minimize public anxiety regarding internal radiation exposure through the consumption of locally produced foods after the 2011 Fukushima accident. The number of samples exceeding the regulatory radiocesium limit (100 Bq/kg for general foods) was five out of 4,080 vegetables (0.1%), 652 of 1,986 (32.8%) among edible wild plants and fungi, and eight of 647 (1.2%) in fruits. Our study confirmed that the internal radiation doses of ingesting these foods are acceptably low compared to the public dose limit, ranging from 24.4 to 42.7 μSv for males and from 21.7 to 43.4 μSv for females, although the potential for radiation exposure still exists. Long-term comprehensive follow-up should take place to clarify trends in radiocesium concentrations in local foods and the committed effective doses found in Fukushima-area residents. By constructing a system that allows residents to access information on radiocesium concentration in foods, a risk communication model between specialists and residents could be developed in the recovery phase after the Fukushima accident.

Comprehensive Health Risk Management after the Fukushima Nuclear Power Plant Accident

Five years have passed since the Great East Japan Earthquake and the subsequent Fukushima Daiichi Nuclear Power Plant accident on 11 March 2011. Countermeasures aimed at human protection during the emergency period, including evacuation, sheltering and control of the food chain were implemented in a timely manner by the Japanese Government. However, there is an apparent need for improvement, especially in the areas of nuclear safety and protection, and also in the management of radiation health risk during and even after the accident. Continuous monitoring and characterisation of the levels of radioactivity in the environment and foods in Fukushima are now essential for obtaining informed consent to the decisions on living in the radio-contaminated areas and also on returning back to the evacuated areas once re-entry is allowed; it is also important to carry out a realistic assessment of the radiation doses on the basis of measurements. Until now, various types of radiation health risk management projects and research have been implemented in Fukushima, among which the Fukushima Health Management Survey is the largest health monitoring project. It includes the Basic Survey for the estimation of external radiation doses received during the first 4 months after the accident and four detailed surveys: thyroid ultrasound examination, comprehensive health check-up, mental health and lifestyle survey, and survey on pregnant women and nursing mothers, with the aim to prospectively take care of the health of all the residents of Fukushima Prefecture for a long time. In particular, among evacuees of the Fukushima Nuclear Power Plant accident, concern about radiation risk is associated with psychological stresses. Here, ongoing health risk management will be reviewed, focusing on the difficult challenge of post-disaster recovery and resilience in Fukushima.

Compliance with the proper use of an individual radiation dosimeter among children and the effects of improper use on the measured dose: a retrospective study 18-20 months following Japan's 2011 Fukushima nuclear incident

OBJECTIVES

To identify profiles of children who did not properly use individual radiation dosimeters following Japan's 2011 Fukushima nuclear incident, and to assess how much error is generated by improper dosimeter use.

PARTICIPANTS

The participants in this study comprised 1637 school children who participated in the external radiation exposure screening programme administrated by Minamisoma City (located 20-30 km from the Fukushima nuclear plant) between 18 and 20 months after the Fukushima incident.

METHODS

We assessed the factors associated with improper use (non-use) of the dosimeters at specific time periods during the day (school commuting hours, at school, at home, outdoors and at bedtime) using logistic regression analyses. Ratios of the measured dose to regression estimates of the 'expected' dose (referred to as an error due to non-use) were also examined.

RESULTS

Only 119 children (7.3%) used the dosimeters properly in all time periods. This low rate was attributed primarily to non-use when children were in the home and outdoors, rather than at school. School level, air dose rate at home, gender, membership in outdoor sports clubs and time spent outdoors on weekends, were significantly associated with improper use, after adjustment for covariates. Data from children who did not wear the dosimeters to school and outdoors had statistically significant (but clinically insignificant) errors (ratio: 1.13, p<0.01; and 0.97, p<0.05, respectively), whereas improper use of the dosimeters at school, at home and at bedtime did not generate significant errors.

CONCLUSIONS

Well-targeted rigorous instructions on the use of the dosimeter are required, with particular focus on time periods other than school hours. However, given the small dose error due to the improper use of the dosimeters, even if the dosimeters are improperly used, solid evaluation of the radiation exposure may be possible with some accuracy.