Development of calculation tool for iodine 131 biodistribution depending on the aerosol particle distribution.

Radiation damage to tissues depends on radiation exposure levels. Therefore, we have studied accurate estimations of radiation exposure levels so far. Recently, we developed a tool that can calculate the respiratory tract deposition of radionuclides based on polydisperse particle size distribution. As a next step, there is a need for a new calculation tool for the biodistribution of radionuclides because a fraction of the radionuclides deposited in the respiratory tract is absorbed into the body. In this study, a calculation tool for iodine 131 biodistribution depending on aerosol size distribution was developed. The developed tool in this study is used with the former tool.

Long-Term Analysis of Internal Exposure Dose-Reduction Effects by Food Regulation and Food Item Contribution to Dose after the Fukushima Daiichi Nuclear Power Plant Accident.

Over 10 years have passed since the Fukushima Daiichi Nuclear Power Plant accident. This study verifies the efficacy of longitudinal regulation on internal exposure doses and analyzes food group contributions to radiation doses using accumulated monitoring test results. The committed effective doses in 10,000 virtual persons from fiscal year (FY) 2012 to 2021, with and without regulation, were estimated as products of radioactivity concentrations randomly sampled from the test results, food intake, and dose coefficient. The distributed values of food intake rather than a mean value in dose estimation were assumed to reflect food intake variations and avoid underestimation of internal exposure doses for high-intake consumers. Furthermore, the ingestion of radioactive cesium from the calculation was analyzed per food group. The 95th percentile of the internal exposure dose (the dose of a "representative person") was less than 1 mSv/year in both FYs. The regulation effect was substantial in FY 2012, and no noticeable difference in radiation doses was found between the regulation and no regulation conditions after FY 2016. Internal exposure doses decreased until approximately FY 2016 and then remained constant. It was also shown that not only radioactivity concentration but also food intake is a major factor affecting cesium intake. In summary, it was confirmed that Japan had ensured food safety regarding radioactive materials.

Longitudinal Verification of Post-Nuclear Accident Food Regulations in Japan Focusing on Wild Vegetables.

Focusing on the importance of wild vegetables for local residents, this study aims to validate the effects of food regulations under the current criteria (e.g., 100 Bq/kg for general foods) established approximately a year after the Fukushima Dai-ichi Nuclear Power Plant accident. Over 2,500,000 monitoring tests were performed under the criteria until fiscal year (FY) 2020. We estimated changes in internal exposure dose using test results. The effective dose was estimated using the radioactive concentration randomly sampled from the results, food intake, and dose conversion factor. As a new attempt, dose estimation reflecting the intake of wild vegetables that may have irreplaceable value for local residents was conducted. The median, 95th, and 99th percentile of the estimated dose without reflecting the wild vegetables' intake were 0.0485, 0.183, and 10.6 mSv/year, respectively, in the estimation with all test results (no regulation) and 0.0431, 0.0786, and 0.236 mSv/year, respectively, in the estimation with results within the standard limits (regulated) in FY2012. These doses decreased with time. Although estimated doses with or without the reflection of wild vegetables' intake were similar, estimation that is more plausible is possible, particularly for a high percentile, by reflecting the wild vegetables' intake. Radiation doses (regulated) were significantly less than 1 mSv/year in different FYs. In Japan, food regulation measures benefit food safety.

222Rn and 226Ra Concentrations in Spring Water and Their Dose Assessment Due to Ingestion Intake.

222Rn and 226Ra concentrations of less than a few to several thousands of Bq L-1 have been observed in several underground bodies of water around the world. Although regulations for these concentrations in water have been implemented internationally, there are currently no regulations in place in Japan. However, concentrations that exceed these internationally recognized regulatory values have also been observed in Japan. In this study, concentrations in spring water in the northern part of Japan were measured and the effective dose from intake of the water was evaluated. 222Rn concentrations were measured using a liquid scintillation counter, and 226Ra concentrations were measured using a high purity germanium detector after chemical preparation. The measured 222Rn concentrations (=12.7 ± 6.1 Bq L-1) and 226Ra concentrations (<0.019-0.022 Bq L-1) did not exceed the reference values set by international and European/American organizations. A conservative estimate of the annual effective ingestion dose of 8 µSv for 222Rn and 226Ra obtained in this study is much smaller than the estimated overall annual effective dose of 2.2 mSv from natural radiation to the Japanese population. However, this dosage accounts for 8% of the WHO individual dosing criteria of 0.1 mSv/year for drinking water.

Estimation of Effect of Radiation Dose Reduction for Internal Exposure by Food Regulations under the Current Criteria for Radionuclides in Foodstuff in Japan Using Monitoring Results.

This study examined the effect of food regulations under the current criteria (e.g., 100 Bq/kg for general foods) established approximately a year after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Foods are monitored to ensure that foods exceeding the standard limit are not distributed; ~300,000 examinations per year have been performed especially since FY2014. This study comprehensively estimated the internal exposure dose resulting from the ingestion of foods containing radioactive cesium using the accumulated monitoring results. Committed effective dose was conservatively calculated as the product of the radioactive concentration randomly sampled from test results, food intake, and dose coefficient. The median, 95th, and 99th percentile of the dose were 0.0479, 0.207, and 10.6 mSv/y, respectively, in the estimation with all test results (without regulation), and 0.0430, 0.0790, and 0.233 mSv/y, respectively, in the estimation with results within the standard limits (with regulation) in FY2012. In FY2016, the dose with and without regulation were similar, except for high percentile, and those doses were significantly smaller than 1 mSv/y, which was adopted as the basis for the current criteria. The food regulation measures implemented in Japan after the FDNPP accident have been beneficial, and food safety against radionuclides has been ensured.

Long-Term Measurements of Radon and Thoron Exhalation Rates from the Ground Using the Vertical Distributions of Their Activity Concentrations.

A long-term measurement technique of radon exhalation rate was previously developed using a passive type radon and thoron discriminative monitor and a ventilated type accumulation chamber. In the present study, this technique was applied to evaluate the thoron exhalation rate as well, and long-term measurements of radon and thoron exhalation rates were conducted for four years in Gifu Prefecture. The ventilated type accumulation chamber (0.8 × 0.8 × 1.0 m3) with an open bottom was embedded 15 cm into the ground. The vertical distributions of radon and thoron activity concentrations from the ground were obtained using passive type radon-thoron discriminative monitors (RADUETs). The RADUETs were placed at 1, 3, 10, 30, and 80 cm above the ground inside the accumulation chamber. The measurements were conducted from autumn 2014 to autumn 2018. These long-term results were found to be in good agreement with the values obtained by another methodology. The radon exhalation rates from the ground showed a clearly seasonal variation. Similar to findings of previous studies, radon exhalation rates from summer to autumn were relatively higher than those from winter to spring. In contrast, thoron exhalation rates were not found to show seasonal variation.

Discriminative Measurement of Absorbed Dose Rates in Air from Natural and Artificial Radionuclides in Namie Town, Fukushima Prefecture.

Ten years have elapsed since the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, and the relative contribution of natural radiation is increasing in Fukushima Prefecture due to the reduced dose of artificial radiation. In order to accurately determine the effective dose of exposure to artificial radiation, it is necessary to evaluate the effective dose of natural as well as artificial components. In this study, we measured the gamma-ray pulse-height distribution over the accessible area of Namie Town, Fukushima Prefecture, and evaluated the annual effective dose of external exposure by distinguishing between natural and artificial radionuclides. The estimated median (range) of absorbed dose rates in air from artificial radionuclides as of 1 April 2020, is 133 (67-511) nGy h-1 in the evacuation order cancellation zone, and 1306 (892-2081) nGy h-1 in the difficult-to-return zone. The median annual effective doses of external exposures from natural and artificial radionuclides were found to be 0.19 and 0.40 mSv in the evacuation order cancellation zone, and 0.25 and 3.9 mSv in the difficult-to-return zone. The latest annual effective dose of external exposure discriminated into natural and artificial radionuclides is expected to be utilized for radiation risk communication.

A unique high natural background radiation area - Dose assessment and perspectives.

The biological effects of low dose-rate radiation exposures on humans remains unknown. In fact, the Japanese nation still struggles with this issue after the Fukushima Dai-ichi Nuclear Power Plant accident. Recently, we have found a unique area in Indonesia where naturally high radiation levels are present, resulting in chronic low dose-rate radiation exposures. We aimed to estimate the comprehensive dose due to internal and external exposures at the particularly high natural radiation area, and to discuss the enhancement mechanism of radon. A car-borne survey was conducted to estimate the external doses from terrestrial radiation. Indoor radon measurements were made in 47 dwellings over three to five months, covering the two typical seasons, to estimate the internal doses. Atmospheric radon gases were simultaneously collected at several heights to evaluate the vertical distribution. The absorbed dose rates in air in the study area vary widely between 50 nGy h-1 and 1109 nGy h-1. Indoor radon concentrations ranged from 124 Bq m-3 to 1015 Bq m-3. That is, the indoor radon concentrations measured exceed the reference levels of 100 Bq m-3 recommended by the World Health Organization. Furthermore, the outdoor radon concentrations measured were comparable to the high indoor radon concentrations. The annual effective dose due to external and internal exposures in the study area was estimated to be 27 mSv using the median values. It was found that many residents are receiving radiation exposure from natural radionuclides over the dose limit for occupational exposure to radiation workers. This enhanced outdoor radon concentration might be as a result of the stable atmospheric conditions generated at an exceptionally low altitude. Our findings suggest that this area provides a unique opportunity to conduct an epidemiological study related to health effects due to chronic low dose-rate radiation exposure.

An Improved Passive CR-39-Based Direct 222Rn/220Rn Progeny Detector.

An improved passive CR-39-based direct 222Rn/220Rn progeny detector with 3 detection channels was designed and tested in this study to measure and calculate equilibrium equivalent concentration (EEC) of both 222Rn and 220Rn without the equilibrium factor. A theoretical model was established to calculate the EEC with optimization. Subsequently, an exposure experiment was carried out to test the performance of this detector, and we compared the chamber experiment and the theoretical model by estimating and measuring various parameters. The deposition flux of progeny derived from the prediction agreed well with the value measured in the exposure chamber. The energy-weighted net track density (NTD) measured by this detector is much more reliable to reflect the linear relation between NTD and time-integrated EEC. Since the detector is sensitive to the exposure environmental condition, it is recommended to apply the detector to measure the EEC after its calibration in a typical indoor environment.

Passive-Type Radon Monitor Constructed Using a Small Container for Personal Dosimetry.

The International Commission on Radiological Protection (ICRP) recently recommended a new dose conversion factor for radon based on the latest epidemiological studies and dosimetric model. It is important to evaluate an inhalation dose from radon and its progeny. In the present study, a passive radon personal monitor was designed using a small container for storing contact lenses and its performance was evaluated. The conversion factor for radon (222Rn), the effect of thoron (220Rn) concentration and the air exchange rate were evaluated using the calibration chamber at Hirosaki University. The minimum and maximum detectable radon concentrations were calculated. The conversion factor was evaluated as 2.0 ± 0.3 tracks cm-2 per kBq h m-3; statistical analyses of results showed no significant effect from thoron concentration. The minimum and maximum detectable radon concentrations were 92 Bq m-3 and 231 kBq m-3 for a measurement period of three months, respectively. The air exchange rate was estimated to be 0.26 ± 0.16 h-1, whose effect on the measured time-integrated radon concentration was small. These results indicate that the monitor could be used as a wearable monitor for radon measurements, especially in places where radon concentrations may be relatively high, such as mines and caves.

Measurement of ambient gamma dose rate in Metro Manila, Philippines, using a portable NaI(TI) scintillation survey meter.

Radiological data such as ambient dose equivalent rate obtained from radiation monitoring in Metro Manila are useful for the detection of any anomalous increase of radiation dose rate levels due to nuclear or radiological emergencies. In this study, ambient dose equivalent rates were measured in different locations in Metro Manila using a portable NaI(Tl) scintillation survey meter to determine the background radiation levels within the capital. Ambient dose equivalent rates measured range from 32.7 ± 2.2 to 59.3 ± 8.7 nanosieverts per hour (nSv/h) with computed mean and median values of 41.7 and 39.6 nSv/h, respectively. These values were lower than the Philippines' average dose rate which is 52 nanograys per hour (nGy/h). No significant trend was also observed in the monthly variation of ambient dose equivalent rate for most locations, with the dose rates being relatively consistent throughout a year. No significant trend was further observed in the monthly variation of ambient dose equivalent rate for the whole Metro Manila. Data obtained in this study were used to develop a dose rate distribution map of Metro Manila which could be used as a baseline reference of emergency responders for environmental radioactivity monitoring during nuclear or radiological emergencies that may affect Metro Manila.

Comparison of Radon and Thoron Concentration Measuring Systems Among Asian Countries.

Comparison is an important role in the quality control and quality assurance for any measuring system. Due to the future legal regulations regarding radon levels in the air, maintaining the system quality and harmonization of results as well as validation of radon and thoron measuring systems is important. The aim of this work is to validate the degrees of equivalence and measurement precisions of the existing five radon and four thoron measuring systems located in four Asian countries (China, India, Japan and Thailand) through comparison experiment. In this project, comparison experiment was performed in order to derive the ratio between assigned value obtained from one transfer measurement device for radon and one transfer measurement device for thoron belongs to National Institutes for Quantum and Radiological Science and Technology and participants' value from their measuring instrument. As a result, the ratio value associated with measurement uncertainty was derived for each activity concentration. Finally, measurement bias and degrees of equivalence between the assigned values and values of measurement quantity from participants' measuring instruments were statistically analysed and presented.

DEVELOPMENT OF CALCULATION TOOL FOR RESPIRATORY TRACT DEPOSITION DEPENDING ON AEROSOLS PARTICLE DISTRIBUTION.

Inhalation exposures occur by inhaled radioactive nuclides depositing in the various locations in the respiratory tract (International Commission on Radiological Protection Publication 66). Respiratory tract deposition depends on particle size. The sensitivity to ionising radiation is different among respiratory regions. Under actual atmospheric environments, the radionuclides attach to aerosols of various size in the atmosphere, so the particle size of radionuclides changes differently. Therefore, it is important for the estimation of health impact to calculate the respiratory tract deposition under atmospheric environment wherein the various sizes of radioactive nuclides (i.e. polydisperse particles) exists. In this study, a tool which can calculate the respiratory tract deposition on the basis of polydisperse particle size distribution was developed to estimate dose depending on variable aerosol particle sizes.

MEASUREMENT OF AMBIENT GAMMA DOSE RATES ALONG TWO INDUSTRIAL FACILITIES IN LEYTE ISLAND, PHILIPPINES.

Natural background radiation to which humans are continuously exposed to come from the primordial radionuclides on the surface of the Earth. Industrial activities which concentrate natural radionuclides usually in residues and waste materials may enhance natural radioactivity on the ground via airborne contamination. In this study, ambient gamma dose rates were measured inside and outside two industrial facilities in Leyte, Philippines, to assess possible contribution of NORM materials in enhancement of natural radiation background in these areas. Ambient gamma dose rates measured at selected sites from Tacloban City to Isabel, Leyte and in nearby areas of Kananga and Ormoc City, Leyte were within the range of the background ambient gamma dose rates measured in the Philippines, ranging from 21 to 124 nSv/h. The workers in the phosphate rock storage and phosphogypsum pond areas in the phosphate fertiliser production plant received the highest annual effective external dose of 0.76 mSv.

EVALUATION OF A RADON AIR MONITOR IN THE MEASUREMENT OF RADON CONCENTRATION IN WATER IN COMPARISON WITH A LIQUID SCINTILLATION COUNTER.

The World Health Organisation (WHO) recommends that the concentration of radon in water should be no more than 100 kBq m-3 (100 BqL-1) and the Codex Alimentarius Commission states that the limit of quantification (LOQ) of a method should be no more than one-fifth of this value. In this study, a degassing method with an RAD7 device was used to measure radon concentrations in water, compared to a liquid scintillation counter (LSC) method used as the reference, to investigate whether the numerical value of the LOQ of this method was more than 1/5 (20 kBq m-3) of 100 kBq m-3. The degassing method with leak prevention was shown to reach a target value of 20 kBq m-3 or less under a relative humidity of 6% or lower in the chamber of the RAD7 device. Accordingly, the RAD7 degassing method with leak prevention can be used to accurately measure radon concentrations in water within the guidance level set out by the WHO.

Comparative Study of Performance using Five Different Gamma-ray Spectrometers for Thyroid Monitoring under Nuclear Emergency Situations.

A performance test was carried out using five different gamma-ray spectrometers applicable to thyroid monitoring. The energy resolution and efficiency for 7.6-cm × 7.6-cm NaI(Tl), 5.1-cm × 5.1-cm CeBr3, 3.8-cm × 3.8-cm SrI2(Eu), and 2.5-cm × 2.5-cm SrI2(Eu) gamma-ray scintillation spectrometers and a CdTe gamma-ray semiconductor spectrometer were evaluated using the Oak Ridge Institute for Nuclear Studies thyroid-neck phantom with a mock I source for prompt thyroid monitoring after a nuclear emergency. The respective energy resolutions of the full-energy peak for ~0.360 MeV of 3.8-cm × 3.8-cm SrI2(Eu) and 2.5-cm × 2.5-cm SrI2(Eu) scintillation spectrometers were 4.2% and 4.3%, and these values were very close to the value obtained by the CdTe semiconductor spectrometer. The efficiencies of four of the gamma-ray spectrometers were compared based on the efficiency of the 7.6-cm × 7.6-cm NaI(Tl) scintillation spectrometer at 0 cm from the phantom surface, and these values were ~70% for 5.1-cm × 5.1-cm CeBr3, ~30% for 3.8-cm × 3.8-cm SrI2(Eu), 10% for 2.5-cm × 2.5-cm SrI2(Eu) and 2% for 1-cm × 0.1-cm CdTe. Furthermore, the detection limits at various dose rates for the four gamma-ray scintillation spectrometers were evaluated using the method based on International Organization for Standardization publication ISO 11929:2010.

Numerical modeling of the sources and behaviors of 222Rn, 220Rn and their progenies in the indoor environment-A review.

222Rn, 220Rn and their short-lived progenies are well known radioactive indoor pollutants, identified as the leading environmental cause of lung cancer next to smoking. Apart from the conventional measurement methods, numerical modeling methods are developed to simulate their physical and decay processes in 222Rn and 220Rn's life cycle, estimate their levels, concentration distributions, as well as effects of control strategies in the indoor environment. In this article, we summarized the numerical models used to illustrate the physical processes of each source of 222Rn and 220Rn entry into the indoor environment, and the application of Jacobi room models and CFD (Computational Fluid Dynamic) models used to present the behaviors of indoor 222Rn, 220Rn and their progenies. Furthermore, we consider that the development of numerical modeling of 222Rn and 220Rn would have a bright prospect in the directions of stochastic methods based on a steady-state model, the fine simulation of the time-dependent model as well as the multi-dimension model.

Low 134Cs/137Cs ratio anomaly in the north-northwest direction from the Fukushima Dai-ichi Nuclear Power Station.

A low 134Cs/137Cs ratio anomaly in the north-northwest (NNW) direction from the Fukushima Dai-ichi Nuclear Power Station (FDNPS) is identified by a new analysis of the 134Cs/137Cs ratio dataset which we had obtained in 2011-2015 by a series of car-borne surveys that employed a germanium gamma-ray spectrometer. We found that the 134Cs/137Cs ratio is slightly lower (0.95, decay-corrected to March 11, 2011) in an area with a length of about 15 km and a width of about 3 km in the NNW direction from the FDNPS than in other directions from the station. Furthermore, the area of this lower 134Cs/137Cs ratio anomaly corresponds to a narrow contamination band that runs NNW from the FDNPS and it is nearly parallel with the major and heaviest contamination band in the west-northwest. The plume trace with a low 134Cs/137Cs ratio previously found by other researchers within the 3-km radius of the FDNPS is in a part of the area with the lower 134Cs/137Cs ratio anomaly that we found. Our result suggests that this lower 134Cs/137Cs ratio anomaly is the area which was contaminated before March 13, 2011 (UTC) in association with the hydrogen explosion of Unit 1 on March 12, 2011 at 06:36 (UTC) and it was less influenced by later subsequent plumes.

Investigation of Natural Radioactivity in a Monazite Processing Plant in Japan.

Monazite is a naturally occurring radioactive material that is processed for use in a variety of domestic applications. At present, there is little information available on potential radiation doses experienced by people working with monazite. The ambient dose rate and activity concentration of natural radionuclides in raw materials, products, and dust in work sites as well as the Rn and Rn concentrations in work sites were measured in a monazite processing plant in Japan. Dose estimations for plant workers were also conducted. The activity concentration of the U series in raw materials and products for the monazite processing plant was found to be higher than the relevant values described in the International Atomic Energy Agency Safety Standards. The ambient dose rates in the raw material yard were higher than those in other work sites. Moreover, the activity concentrations of dust in the milling site were higher than those in other work sites. The Rn concentrations in all work sites were almost the same as those in regular indoor environments in Japan. The Rn concentrations in all work sites were much higher than those in regular indoor environments in Japan. The maximum value of the effective dose for workers was 0.62 mSv y, which is lower than the reference level range (1-20 mSv y) for abnormally high levels of natural background radiation published in the International Commission of Radiological Protection Publication 103.

Measurements of radon exhalation rate in NORM used as consumer products in Japan.

Twenty-five beauty products known to contain natural radionuclides were collected, and their 222Rn mass exhalation rates were measured. The effective doses to workers due to 222Rn exhaled from these products were estimated. The 222Rn mass exhalation rates of these products were below 177 µBq kg-1 s-1 and were almost identical to those of natural rocks in Japan. The maximum effective dose of 222Rn exhaled from these products was 71 µSv y-1.