Cesium-137 stored on and discharged from banks of an agricultural canal in Iitate, Fukushima

This study aimed to investigate the dynamics of ¹³⁷Cs around banks along an agricultural canal for paddy fields in Iitate, Fukushima, Japan. Five plots (2.4-12.6 m²) on the banks were monitored intermittently during six time periods from May 2018 to November 2019. We directly collected runoff water samples discharged from the banks followed by partitioning it into particulate and dissolved fractions and determining ¹³⁷Cs in them. To investigate the source of ¹³⁷Cs in the runoff water, we sequentially extracted ¹³⁷Cs in various chemical forms from litter samples collected on the banks. The results showed that the discharge rates of the dissolved ¹³⁷Cs per unit area from the plots were lower than those observed at the downstream of the agricultural canal, whereas more than 50% of the ¹³⁷Cs discharged from the plots was in the dissolved fraction. Moreover, the results indicate that ¹³⁷Cs stored in the standing plants and the litter was the primary source of the dissolved ¹³⁷Cs discharged into the agricultural canal. The concentrations of the water-soluble ¹³⁷Cs in the litter per plot area may have been retained by the sufficiently higher concentrations of ¹³⁷Cs in litter in other chemical forms and those in the standing plants, which are the source of the litter.

Synthesis of the JHPS International Symposium on Tritiated Water

As the decommissioning of Fukushima Daiichi nuclear power plant (NPP) progresses, the issue of how to deal with tritiated water has been attracting attention, both domestically and internationally. This article summarises the live discussion at the International Symposium on Tritiated Water, which was held by the Japan Health Physics Society (JHPS) in June 2020. Two issues - the scientific safety of tritiated water and social consensus building - were covered in the live discussion. The importance of further disclosure and dissemination of information based on steady monitoring was highlighted. It was also pointed out that scientific knowledge and scientific research data are merely the bottom line to achieve social consensus. Through the discussions, it was recognised that the role of JHPS is not only to look at the technical issues of safety, but also to look at social issues from the point of view of radiation protection, and to support the solution of these issues.

Reassessment of early 131I inhalation doses by the Fukushima nuclear accident based on atmospheric 137Cs and 131I/137Cs observation data and multi-ensemble of atmospheric transport and deposition models

The Fukushima Dai-ichi Nuclear Power Plant accidents following the March 11, 2011 Tohoku earthquake, and subsequent tsunami released radioactive materials into the atmosphere and caused significant public health concerns, particularly thyroid cancers in children. However, the lack of measurement data for atmospheric concentrations of ¹³¹I has caused persistent and widespread uncertainty. This study estimated the maximum potential thyroid doses of inhaled ¹³¹I in the early post-accident phase between March 12 and 23, 2011 by using the hourly measured data of the ¹³⁷Cs concentrations at 101 suspended particulate matter (SPM) monitoring sites, a new multi-model ensemble (MME) method of simulating ¹³⁷Cs concentrations using two Atmospheric Transport and Deposition Models (ATDMs), the ¹³¹I/¹³⁷Cs ratio obtained from measurement data analysis, and the internal exposure model. Based on the measurements, the maximum potential thyroid doses were estimated at 3.1-160 mSv at 5 sites in the Fukushima-Hamadori area for 1-year-old children assumed to remain outdoors, whereas they were less than 4.3 mSv at the other sites in the base case of the ¹³¹I/¹³⁷Cs ratio. The spatial distribution of the maximum potential of early inhalation doses was estimated by using the MME and measurements. The inhalation thyroid doses in the evacuation scenarios were compared to the estimates reported by previous studies. The results of the present study were almost congruent with the outcomes of previous investigations except for thyroid doses contributed by highly contaminated plumes on March 12 and 15. The sensitivity analysis for the ¹³¹I/¹³⁷Cs ratio indicated that these plumes carried the potential to significantly increase the thyroid doses of residents.

Measurements of radiocesium in animals, plants and fungi in Svalbard after the Fukushima Daiichi nuclear power plant disaster

An earthquake struck the eastern part of Japan on March 11, 2011. The Fukushima Daiichi nuclear power plant was severely damaged by the earthquake and subsequent tsunami, leading to the emission of large amounts of radioactive pollutants, including ¹³⁴Cs and ¹³⁷Cs, into the environment. From August 23 to September 1 in 2011, and from August 27 to September 4 in 2013, we collected samples of animals, plants, fungi and lichens from Svalbard, Norway and measured the radioactivity of ¹³⁴Cs and ¹³⁷Cs contained in the samples. Though no radioactivity of ¹³⁴Cs, which has a half-life of approximately 2 years, was observed, radioactivity of ¹³⁷Cs, which has a half-life of approximately 30 years, was observed in some samples of lichens and fungi. We failed to detect the radioactivity of ¹³⁴Cs in any of the samples we collected, therefore, it was impossible to say clearly that the radioactivity is derived from Fukushima or not. Nevertheless, the radioactivity data documented in this report are a useful reference for the future surveys of radioactivity within the Arctic.

Radiocesium in seawater, sediments, and marine megabenthic species in coastal waters off Fukushima in 2012-2016, after the 2011 nuclear disaster

In bottom-sediment samples collected in 2012 from a coastal strip (∼30 km × 120 km) off the Fukushima Daiichi Nuclear Power Plant (FDNPP), radiocesium activity concentrations were generally higher south of the FDNPP, with high activity concentration patches in the north. In periodic surveys conducted at nearshore sites during 2012-2016, no clear temporal trends were observed in radiocesium activity concentrations in seawater or bottom sediment, and activity concentrations were higher in fish than in invertebrates. During 2012-2014, radiocesium activity concentrations tended to decrease in fish, but during 2012-2013 in the south, some increases were observed. Radiocesium activity concentrations were significantly higher in some fish (e.g., Okamejei kenojei) directly offshore and south of the FDNPP than in the north. Activity concentrations in fish stomach contents were significantly correlated with those in muscle tissue, suggesting that the consumption of contaminated prey contributed greatly to radiocesium contamination in demersal fish.

Application of topographical source model for air dose rates conversions in aerial radiation monitoring

After the Fukushima Daiichi Nuclear Power Station (FDNPS) accident in 2011, aerial radiation monitoring (ARM) using a manned helicopter was conducted to rapidly measure air dose rates and the deposition of radioactive nuclides over a large area. Typically, the air dose rate is obtained by conversion from the count rate using conventional flat source model (FSM). The converted dose rate obtained via aerial monitoring poorly matches the results of ground measurement in the mountain and forest areas because FSM does not consider topographical effects. To improve the conversion accuracy, we developed new methods to analyze aerial monitoring data using topographical source model (TSM) based on the analytical calculation of the gamma-ray flux. The ARM results converted using both FSM as well as TSM were compared with ground measurement data obtained after the FDNPS accident. By using TSM, the conversion accuracy was improved. In addition, to determine a parameter sensitive to topographical effects, we examined five parameters and it was clear that the difference between the elevation just below the helicopter and the mean elevation within the measurement area was the most influential.

Radioactive contamination of nest materials of the Eurasian Tree Sparrow Passer montanus due to the Fukushima nuclear accident: The significance in the first year

The 2011 Fukushima nuclear accident contaminated large areas of eastern and northeastern Japan, releasing vast amounts of radiation. Here we investigated radioactive contamination of the nest materials of the Eurasian Tree Sparrow Passer montanus from the breeding season of 2011 directly after the accident to the next breeding season of 2012 at two sites. In Tokyo (222 km southwest of the plant), ambient dose rates in the nestboxes were lower than those in Ibaraki (175 km southwest of the plant), where the levels of 2011 were higher than those of 2012. Further, the amount of radioactive Cs in each nest increased with the increase in nest weight, with a higher increment at Ibaraki than at Tokyo. These data suggested higher nest contamination levels in the breeding season directly after a nuclear accident than in later seasons, and an increment of nest contamination levels via nest materials of birds.

Numerical simulation on the long-term variation of radioactive cesium concentration in the North Pacific due to the Fukushima disaster

Numerical simulations on oceanic (134)Cs and (137)Cs dispersions were intensively conducted in order to assess an effect of the radioactive cesium on the North Pacific environment with a focus on the long-term variation of the radioactive cesium concentration after the Fukushima disaster that occurred in March 2011. The amounts of (134)Cs and (137)Cs released into the ocean were estimated using oceanic monitoring data, whereas the atmospheric deposition was calculated through atmospheric dispersion simulations. The highly accurate ocean current reanalyzed through a three-dimensional variational data assimilation enabled us to clarify the time series of the (134)Cs and (137)Cs concentrations in the North Pacific. It was suggested that the main radioactive cesium cloud due to the direct oceanic release reached the central part of the North Pacific, crossing 170°W one year after the Fukushima disaster. The radioactive cesium was efficiently diluted by meso-scale eddies in the Kuroshio Extension region and its concentration in the surface, intermediate, and deep layers had already been reduced to the pre-Fukushima background value in the wide area within the North Pacific 2.5 years after the Fukushima disaster.