9 Août 2017
Radioactively-hot particles detected in dusts and soils from Northern Japan by combination of gamma spectrometry, autoradiography, and SEM/EDS analysis and implications in radiation risk assessment
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Radioactive particles from Fukushima are tracked via dusts, soils, and sediments.
Radioactive dust impacts are tracked in both Japan and the United States/Canada.
Atypically-radioactive particles from reactor cores are identified in house dusts.
Scanning electron microscopy with X-ray analysis is used for forensic examinations.
After the March 11, 2011, nuclear reactor meltdowns at Fukushima Dai-ichi, 180 samples of Japanese particulate matter (dusts and surface soils) and 235 similar U.S. and Canadian samples were collected and analyzed sequentially by gamma spectrometry, autoradiography, and scanning electron microscopy with energy dispersive X-ray analysis. Samples were collected and analyzed over a five-year period, from 2011 to 2016. Detectable levels of 134Cs and 137Cs were found in 142 of 180 (80%) Japanese particulate matter samples. The median radio-cesium specific activity of Japanese particulate samples was 3.2 kBq kg− 1 ± 1.8 kBq kg− 1, and the mean was 25.7 kBq kg− 1 (σ = 72 kBq kg− 1). The U.S. and Canadian mean and median radio‑cesium activity levels were < 0.03 kBq kg− 1. U.S. and Canadian samples had detectable 134Cs and 137Cs in one dust sample out of 32 collected, and four soils out of 74. The maximum US/Canada radio-cesium particulate matter activity was 0.30 ± 0.10 kBq kg− 1. The mean in Japan was skewed upward due to nine of the 180 (5%) samples with activities > 250 kBq kg− 1. This skewness was present in both the 2011 and 2016 sample sets.
> 300 individual radioactively-hot particles were identified in samples from Japan; composed of 1% or more of the elements cesium, americium, radium, polonium, thorium, tellurium, or strontium. Some particles reached specific activities in the MBq μg− 1 level and higher. No cesium-containing hot particles were found in the U.S. sample set. Only naturally-occurring radionuclides were found in particles from the U.S. background samples. Some of the hot particles detected in this study could cause significant radiation exposures to individuals if inhaled. Exposure models ignoring these isolated hot particles would potentially understate human radiation dose.
The combination of gamma spectroscopy, autoradiography and SEM/EDS analysis was effective in isolating and analyzing hot particles. Many of these particles would have gone unidentified if only one of these techniques has been employed.
Samples have provided evidence that local hot spots of contamination existed at the time of the Fukushima Dai-ichi meltdowns in 2011. Local hot spots still persisted in 2016, five years after the containment failures in 2011.
Radioactively-hot dust and soil particles were routinely detected in samples from Northern Japan in both the 2011 and 2016 sample sets, with autoradiographic and SEM/EDS data showing that isolated particles could have substantially-higher specific activities than the bulk samples from which they were isolated.
Radioactive 131I (April 2011 samples only), 134Cs and 137Cs were the most commonly encountered nuclides in Japan. Primordial nuclides unrelated to events in Japan were the dominant source of radioactivity in U.S. and Canadian environmental samples collected at the same times. Thorium was detected in radioactive particles from Japan, but this radioactive element has both natural and Fukushima Dai-ichi-related sources.
Fukushima Dai-chi-related radioactive contamination was found in 80% of the particulate matter samples from Fukushima Prefecture and surrounding areas in Northern Japan. This was true for both the 2011 and 2016 sample sets. Relative variability among dust samples from Japan was very high. The highest activity levels (3σ or more above the mean) are representative of the specific collection locations at the time of sampling, and are not representative of average Japanese radiation exposures. Most of the activity detected in Japanese samples came from 137Cs and 134Cs (and in the first post-accident weeks, from 131I), although there were isolated detections of 60Co and 241Am.
Individuals in the contaminated zone, and potentially well outside of the mapped contaminated zone, may receive a dose that is higher than the mean dose calculated from average environmental data, due to inhalation or ingestion of radioactively-hot dust and soil particles. Accurate radiation risk assessments therefore require data for hot particle exposure as well as for exposure to more uniform environmental radioactivity levels.