Car-borne survey and dose assessment from external radiation exposure in Bangka Island.

With a history of more than 200 years of tin mining, Bangka Island has brought along a byproduct of heavy minerals containing radionuclide elements. There are some concerns about this byproduct material contributing to natural radiation in the environment. In this study, a car-borne survey was conducted to accurately assess natural background radiation in Bangka Island. Indoor and outdoor ambient dose rates in 146 houses were also measured to assess the radiation dose from external exposure received by the public. Soil samples were collected and measured using a gamma spectroscopy system to evaluate the contributions of specific radionuclides to external terrestrial exposure. From 3790 measurement points during the car-borne survey, the highest ambient dose equivalent rate was 596 nSv h-1 measured in Muntok area, with a mean value of 101 nSv h-1 and a median value of 95 nSv h-1. The ambient dose equivalent rate distribution map showed a relatively higher value in the northern coastal area of the island, where the Pemali tin deposit is located. The annual effective dose received from external radiation in the 146 houses in Bangka Island ranged from 0.44 to 1.30 mSv year-1, with a median value of 0.66 mSv year-1. The soil contained a relatively high amount of thorium (232Th), which contributed 69% to external radiation exposure in Bangka Island.

Effects of irradiation on cumulative mortality in mice: shifting toward a younger age of death.

Recently, the question of whether cancer risk is only accelerated but not increased by radiation exposure has been raised. To explore this matter, we analyzed whether the cumulative mortality of irradiated mice could be explained by x-axis (age) shifted cumulative mortality of nonirradiated mice. We reanalyzed publicly available data on observed cumulative mortality or prevalence in irradiated female B6C3F1 mice that lived their entire lifespan. The results showed that the irradiated curve was well matched to uniformly shifted nonirradiated curve for the cumulative mortality of all causes of death but not for the cumulative mortality of all solid tumors and prevalence of ovarian tumors as is. After adjusting lifetime mortalities, it was also well matched for all solid and ovarian tumors. The shifted days by irradiation were 71-116 days for all causes of death, 56-135 days for all solid tumors, and 41-140 days for ovarian tumors in the 1.9 Gy-irradiated group. The response was switched between irradiation at 35 and 105 days consistently for all the above indexes, supporting the hypothesis that radiation sensitivity differs between juvenile and adults. The shifted days of all causes of death showed a tendency of linear response to dose. This concept of shifting the age of death can be applied not only for all cause of death but also for mortality of all solid tumors after adjusting the magnitude. These findings contribute to the discussion on the application of the 'shifting age of death' concept to radiation protection.

QUANTITATIVE EVALUATION OF CONSERVATISM INCLUDED IN PARAMETER SETTINGS FOR CLEARANCE LEVEL CALCULATIONS: A CASE STUDY IN JAPAN.

In the evaluation of radiation safety standards, representative, average or conservative parameter values are set for dose calculation, and it is expected that excessive conservatism may be occasionally included in the standards already established. In this study, we attempted to quantitatively evaluate the 'factor of conservatism' for parameters used in the dose calculations for an actual case of clearance level calculations in Japan. The 'factor of conservatism' was calculated using the selected value for the parameter actually applied in the clearance level calculation and the expected values evaluated from the probability distribution. As a result, it was estimated that the 'factor of conservatism' for the parameters used in the clearance level calculation for the main nuclear facilities in Japan is approximately 1 to 7.

Estimation of the mortality rate of workers in Japan.

BACKGROUND: Risk-based decision-making is used to identify risk factors for which threshold points have not been identified. The occupational mortality rate was referred to as a reference risk. This study aimed to analyze recent trends in worker mortality using three data sources. METHODS: The Reports of Worker Casualties (RWC), the Annual Statistics Report of the Industrial Accident Compensation Insurance Council (ASR), and the Annual Business Report of the Industrial Accident Compensation Insurance Council (ABR) were used. Mortality rates were estimated by age group and industry category (overall, and manufacturing and construction industries) from 1991 to 2019. The mortality rates were compared with those estimated in Western countries. RESULTS: The mortality rates for the three industry categories in the RWC and ASR decreased, whereas those for the manufacturing and construction industries in the ABR did not. In 2019, the mortality rates in the ABR were 3.1, 4.8, and 3.1 times higher than those in the RWC overall and in the manufacturing and construction industries, respectively. The differences decreased when deaths after long-term medical treatment were considered. The mortality rate trends in Japan were similar to those in Western countries. The upper mortality rate among Japan and Western countries was at least five to 15 times higher than the lowest. CONCLUSION: When occupational mortality rates are used as a reference risk, it is important to consider any changes with time, the data sources used, and the differences between countries.

Comprehensive exposure assessments from the viewpoint of health in a unique high natural background radiation area, Mamuju, Indonesia.

Mamuju is one of the regions in Indonesia which retains natural conditions but has relatively high exposure to natural radiation. The goals of the present study were to characterize exposure of the entire Mamuju region as a high natural background radiation area (HNBRA) and to assess the existing exposure as a means for radiation protection of the public and the environment. A cross-sectional study method was used with cluster sampling areas by measuring all parameters that contribute to external and internal radiation exposures. It was determined that Mamuju was a unique HNBRA with the annual effective dose between 17 and 115 mSv, with an average of 32 mSv. The lifetime cumulative dose calculation suggested that Mamuju residents could receive as much as 2.2 Sv on average which is much higher than the average dose of atomic bomb survivors for which risks of cancer and non-cancer diseases are demonstrated. The study results are new scientific data allowing better understanding of health effects related to chronic low-dose-rate radiation exposure and they can be used as the main input in a future epidemiology study.

Quantitative evaluation of conservatism in the concept of committed dose from internal exposure for radiation workers.

For compliance with dose limits, the International Commission on Radiological Protection (ICRP) recommends that the committed dose be assigned to the year in which radionuclide intake occurred in the case of internal exposure. For radiation workers, the committed dose is evaluated over the 50 year period following the intake, which is a rounded value for the working-life expectancy of a young person entering the workforce. In this study, we develop an approach to the quantitative evaluation of the conservatism in the concept of the committed dose from internal exposure for radiation workers from the viewpoint of radiological risk. Actual annual doses due to an intake of radionuclides for strontium-90 (90Sr), caesium-137 (137Cs), and plutonium-239 (239Pu) were simulated. Risks of fatal cancer, i.e. unconditional death probability rates, were calculated in accordance with the risk estimation method in ICRP Publication 60. It was found that the conservatism ranged from 1.1 to 1.6 for90Sr, 1.0 to 1.6 for137Cs, and 1.6 to 2.2 for239Pu. The importance of understanding the extent of this conservatism and the uncertainty for practical radiological protection are also discussed.

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.

Effect of Radiation Dose Rate on Circulatory Disease Mortality among Nuclear Workers: Reanalysis of Hanford Data.

The excess relative risk (ERR) of mortality for circulatory disease among nuclear workers was reanalyzed by taking into consideration the annual dose as the dose rate using publicly available epidemiological data of the Hanford site dedicated to the cohort study of nuclear workers in the US, the UK, and Canada. Values of the dose rate (cut-points) were chosen at 2 mSv y intervals from 2 to 40 mSv y, and risk estimates were made for 32,988 workers, considering the doses accumulated below and above each cut-point to have different effects. Similarly to that in the previous study for cancer by Sasaki et al., examinations of sensitivity analysis were also carried out for different risk models, lag periods, and impacts of adjusting the monitoring period to find the effect of the dose rate. As a result, emergence of a statistically significant difference between ßL1, which is the ERR for the doses accumulated below the specified cut-point, and ßH1, which is the ERR for that above the specified cut-point, was observed for cut-point of the dose rate of 4, 6, 34, 36, and 38 mSv y. While statistically negative values were estimated for ßL1 (4 and 6 mSv y) and for ßH1 (34, 36, and 38 mSv y), the overall relationship between the ERR and the cut-point of the dose rate was found to be similar to that obtained by the analysis of the mortality for all cancers excluding leukemia.

Report of the Japan Health Physics Society ad hoc working group for the Plutonium intake accident.

This paper describes an overview of the radiation protection response to the Plutonium intake accident that occurred at the Plutonium Fuel Facility of the Oarai Research and Development Center of the Japan Atomic Energy Agency on 6 June 2017. In the hood of the analyzing room at the Plutonium Fuel Facility five workers were checking a storage container of fast reactor nuclear fuel material. Around 11:15 a.m., vinyl bags inside the fuel material container containing Plutonium and enriched uranium burst during the inspection work. All the workers heard the bang, which caused misty dust leakage from the container. This event caused significant both skin and nasal α-contamination for three workers and just skin α-contamination for one worker. Decontamination was conducted in the shower room. Then the five workers were transferred to the Nuclear Fuel Cycle Engineering Laboratory to evaluate inhalation intake of Plutonium etc in the lungs. The maximum values of 2.2 × 104 Bq for 239Pu and 2.2 × 102 Bq for 241Am were estimated by the lung monitor. Based on these results, injection of a chelate agent was conducted for prompt excretion of Plutonium etc. The next morning, the five workers were transferred to the National Institute of Radiological Sciences for treatment including decontamination of their skin and measurement by a lung monitor. At that time no obvious energy peak was confirmed for Plutonium. The Japan Health Physics Society launched an ad-hoc working group for Plutonium intake accident around the middle of June to survey issues and to extract lessons for radiological protection. The authors, who are the members of the ad-hoc working group, here report the activity of the working group.

Effect of Radiation Dose Rate on Cancer Mortality among Nuclear Workers: Reanalysis of Hanford Data.

The excess relative risk of mortality for all cancers excluding leukemia among nuclear workers was reanalyzed by taking the annual dose as the dose rate into consideration using publicly available epidemiological data from the Hanford site dedicated to the cohort study of nuclear workers in the United States, the United Kingdom, and Canada (Three Countries Study). Values of the dose rate (cut points) were chosen at 2 mSv y intervals from 2 to 40 mSv y, and risk estimates were made for 32,988 workers, considering doses accumulated below the cut point and above the cut point to have different effects. Although the procedure to extract the study population and the methodology used for analysis basically followed those in the Three Countries Study, additional examinations were also carried out for different risk models, lag periods, and impacts of adjusting the monitoring period to find the effect of the dose rate. As a result, no statistically significant difference in dose rate was found among the excess relative risks under different calculation conditions.

Case Study of the Minimum Provable Risk Considering the Variation in Background Risk: Effect of Residual Risk on Epidemiological Studies and a Comparative Assessment of Fatal Disease Risk Due to Radiation Exposure.

In order to prove a small increment in a risk of concern in an epidemiological study, a large sample of a population is generally required. Since the background risk of an end point of interest, such as cancer mortality, is affected by various factors, such as lifestyle (diet, smoking, etc.), adjustment for such factors is necessary. However, it is impossible to inclusively and completely adjust for such factors; therefore, uncertainty in the background risk remains for control and exposed populations, indicating that there is a minimum limit to the lower bound for the provable risk regardless of the sample size. In this case study, we developed and discussed the minimum provable risk considering the uncertainty in background risk for hypothetical populations by referring to recent Japanese statistical information to grasp the extent of the minimum provable risk. Risk of fatal diseases due to radiation exposure, which has recently been the focus of radiological protection, was also examined by comparative assessment of the minimum provable risk for cancer and circulatory diseases. It was estimated that the minimum provable risk for circulatory disease mortality was much greater than that for cancer mortality, approximately five to seven times larger; circulatory disease mortality is more difficult to prove as a radiation risk than cancer mortality under the conditions used in this case study.

Photon-Fluence-Weighted let for Radiation Fields Subjected to Epidemiological Studies.

In order to estimate the uncertainty of the radiation risk associated with the photon energy in epidemiological studies, photon-fluence-weighted LET values were quantified for photon radiation fields with the target organs and irradiation conditions taken into consideration. The photon fluences giving a unit absorbed dose to the target organ were estimated by using photon energy spectra together with the dose conversion coefficients given in ICRP Publication 116 for the target organs of the colon, bone marrow, stomach, lung, skin and breast with three irradiation geometries. As a result, it was demonstrated that the weighted LET values did not show a clear difference among the photon radiation fields subjected to epidemiological studies, regardless of the target organ and the irradiation geometry.

Neutron field produced by 25 MeV deuteron on thick beryllium for radiobiological study; energy spectrum.

Biological data is necessary for estimation of protection from neutrons, but there is a lack of data on biological effects of neutrons for radiation protection. Radiological study on fast neutrons has been done at the National Institute of Radiological Sciences. An intense neutron source has been produced by 25 MeV deuterons on a thick beryllium target. The neutron energy spectrum, which is essential for neutron energy deposition calculation, was measured from thermal to maximum energy range by using an organic liquid scintillator and multi-sphere moderated 3He proportional counters. The spectrum of the gamma rays accompanying the neutron beam was measured simultaneously with the neutron spectrum using the organic liquid scintillator. The transmission by the shield of the spurious neutrons originating from the target was measured to be less than 1% by using the organic liquid scintillator placed behind the collimator. The measured neutron energy spectrum is useful in dose calculations for radiobiology studies.