Airborne radioactivity measurements are necessary to know the contamination level and internal doses for residents after a nuclear accident. In addition, measurements of radon progenies in air, which are the risk factor of lung cancer, are also important to evaluate lung dose. In these measurements, a filter sampling is used to collect radioactive aerosols. However, it is well known that results of the measurement using a filter are strongly dependent on characteristics of the used filter. Selection of a suitable filter is important to achieve the high-resolution and long-term measurement. “Surface collection efficiency (SCE)” and “stability of air flow rate” were examined for six types of filter that are commercially available in Japan. In Japan, cellulose-glass fiber filter paper (HE-40T) is used for an environmental monitoring in Japan. In this study, it was found that the SCE of HE-40T was lower than that of mixed cellulose ester type membrane filter by Merck Millipore (DAWP02500). Attenuation ratio of flow rate for DAWP02500 was evaluated to be 2.9% which was lowest in six filters. The results suggest that the DAWP02500 is the most suitable for collecting radioactive aerosols for a long term.
Radiation protection of patients in medicine is based on two major radiation protection principles, that is justification and optimisation. For medical exposures, the International Commission on Radiological Protection (ICRP) recommends optimisation according to the ALARA (as low as reasonably achievable) principle. ICRP recommended the use of Diagnostic Reference Level (DRL) for patients in Publication 73 to promote optimisation of protection in radiological Diagnostic. In ICRP 2007 Recommendation and International Atomic Energy Agency (IAEA) Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards (BSS), instead of the guidance level that was the concept of optimisation so far, DRL is adopted as a tool for dose optimisation. In Japan, National DRL was released in June 2015 from the Japan Network for Research and Information on Medical Exposures (J-RIME), but understanding and dissemination have not progressed sufficiently. In such circumstances, J-RIME decided to revise the DRL targeting 2020. Various activities are gradually beginning toward revision. With reference to ICRP Publication 135 “Diagnostic Reference Levels in Medical Imaging”, J-RIME aims to create a new DRL that can withstand international evaluations.
The International Commission on Radiological Protection (ICRP) issued in February 2018 the Publication 138, titled “Ethical foundation of radiological protection system.” Although the ICRP has longtime recognized that ethics is an essential component of their radiological protection system, they have rarely described explicitly about the ethical foundation of the system. For this reason, the Commission established in 2012 the task group 94 to clarify and describe the ethical foundation of the system. As the result, the publication shows the process of evolution of the radiological protection system through 20th century until now, which leads to the identification of four core ethical values underpinning the system: beneficence and non-maleficence; prudence; justice; and dignity, along with related procedural values: accountability; transparency; and inclusiveness (stakeholder participation). As one of the authors of this publication, I will briefly introduce the developing process and the outlines of it, and additionally will provide a short summary of my experience of discussion through the workshops worldwide, concerning the nuclear accident of the Fukushima Daiichi Nuclear Power Station, as well as commonalities and diversities of Western and Asian perspectives of ethical core values.
In August 2018, the latest analysis of the UK National Registry for Radiation Workers (NRRW 3rd update) has been published. The NRRW studies have been published almost every ten years since the first analysis (1992). The series of NRRW aimed to analyse cancer risk from low dose occupational radiation exposure. This latest analysis is the study using third analysis data and an additional ten years of follow-up information, but did not include additionally dosimetry information. As the set of ten years lag period, only the risks of cancer were analysed, but excluding leukaemia risks owing to its lag period as two years. The same statistical methods were used in the series of NRRW study. This review provides an outline and summary of the key points of NRRW 3rd update. We denote introduction in chapter 1, summary in chapter 2, comparison with previous studies and other studies in chapter 3, discussion about results in chapter 4, meaning and limitation in chapter 5 and conclusion in chapter 6.
Rainwater containing radioactive materials originating from the USA’s nuclear weapon test conducted at Bikini Atoll was observed throughout Japan in 1954. It has been reported that the maximum gross beta activity observed at that time in Kyoto was 523 pCi/mL (19,000 Bq/L). This measurement, however, focused on the gross beta activity contained in a small amount of rain sampled at the beginning of rainfall, which is different from present observations that are based on the average gross beta activity contained in rain collected during a 24-h period. As a result of reviewing and converting the 1954 data to be equivalent to current measurement, the maximum value was reduced to 50 pCi/mL (1,800 Bq/L), with a resultant surface deposition density of 310 mCi/km2 (11,000 MBq/km2). These values are well below 1/10 of the past maximum observed a few days after China’s fifth nuclear weapon test in 1966.
Large quantities of radionuclides were released as a result of Fukushima Daiichi Nuclear Power Station accident. It is known that these radionuclides contaminated inside houses as well as outdoor environment. Considering the radiation protection of residents after a nuclear power station accident, it is important to know the influence of radionuclides inside houses to radiation dose to residents. In this study, we investigated removal factors and fractions of fixed contamination of various materials inside houses in Okuma Town, Futaba Town, and Namie Town to assess the contamination level inside house appropriately. Nine kinds of materials: fibers, woods (smooth), woods (rough), glasses, concretes (smooth), concretes (rough), plastics, PVCs and metals, were used in examinations. The lowest and the highest removal factors were 23% ± 16% of woods (rough) and 79% ± 7.7% of glasses, respectively. Removal factors of all materials were higher than 10% which is recommended by Japanese Industrial Standard. The negative correlation was found between removal factors and fractions of fixed contamination. Using this correlation, the decontamination efficiency, which means the ratio of the activity removed from the surface by one smear sample to the activity of the total surface activity, was proposed. The air dose rate from the contamination inside house was calculated using obtained decontamination efficiencies and removal factor of 10%. In the case using the removal factor of 10%, the air dose rate derived by indoor contamination was approximately 2 times higher than the case using obtained decontamination efficiencies. We found that the air dose rate derived by indoor contamination was much lower than the air dose rate outside house, and the influence of indoor contamination on the external exposure was small.
The International Commission on Radiological Protection (ICRP) internal dose assessment model, currently adopted in Japanese regulation, assumes uniform distribution of radionuclides in bone marrow blood (ICRP Publication 60). Recent studies have revealed a localization of hematopoietic stem cells (HSCs) and immune cells in the perivascular region of the bone marrow sinusoids, suggesting a need to consider nonuniform distributions of the blood source and HSCs. To evaluate energy transfer to HSCs, a simplified model of cervical vertebrae with bone tissues and blood vessels was built using data from the adult Japanese male phantom. Doses absorbed by HSCs from blood and hard bone sources were calculated using a Monte Carlo simulation, and absorbed fractions (AFs) and specific absorbed fractions (SAFs) from electrons were compared with those in the ICRP 1990 model. In the cervical vascular model, electron SAFs from sinusoidal blood in the red bone marrow (RBM) to the target perivascular region were 1.2 to 6.9 times higher than the SAF in the ICRP 1990 model, suggesting an underestimation of the RBM dose. Electrons from the cortical bone source to the perisinusoidal target exhibited energy transfer. The ICRP 1990 model underestimates electron SAFs from radionuclides in sinusoidal blood and cortical bones. A more elaborate model is needed to examine doses for the RBM and effects on hematopoietic and immune functions.
The whole-body retention of radiocesium derived from soil was investigated in Wistar strain rats using three types of environmentally contaminated soil containing 134Cs and 137Cs collected in Fukushima Prefecture, garden soil doped with 134CsCl solution, and ionic solutions of 134CsCl. Approximately 44–67% of the administered activity of the 134Cs and 137Cs in the three types of soil was cleared exponentially in 12 h, with a half-life of 2.9–7.9 h, and the remaining activity decreased below the detection limit by 30 h after initial administration. In the rats administered with the artificially contaminated soil, a biphasic retention curve (fast and slow) was observed, because the radioactivity administered was high enough to trace over the long-term. Approximately 84% and 16% of the administered activity was cleared with half-lives of 4.5 and 173 h, respectively. In contrast, approximately 23% and 77% was cleared with half-lives of 17 and 166 h, respectively, in the rats administered with the 134Cs in ionic solution. By analyzing the retention curve, we estimated that only 21% of orally administered radiocesium in artificially doped soil was absorbed into the gastrointestinal tract. This suggests that if the dose coefficient indicated by systemic models for Cs of the International Commission on Radiological Protection, in which gastro-intestinal absorption rate is assumed to be 1.0 for public, is used for the radiocesium in soil, the effective dose may be significantly larger than the actual dose.