Isotopic evidence of plutonium release into the environment from the Fukushima DNPP accident

The Fukushima Daiichi nuclear power plant (DNPP) accident caused massive releases of radioactivity into the environment. The released highly volatile fission products, such as 129mTe, 131I, 134Cs, 136Cs and 137Cs were found to be widely distributed in Fukushima and its adjacent prefectures in eastern Japan. However, the release of non-volatile actinides, in particular, Pu isotopes remains uncertain almost one year after the accident. Here we report the isotopic evidence for the release of Pu into the atmosphere and deposition on the ground in northwest and south of the Fukushima DNPP in the 20–30 km zones. The high activity ratio of 241Pu/239+240Pu (> 100) from the Fukushima DNPP accident highlights the need for long-term 241Pu dose assessment, and the ingrowth of 241Am. The results are important for the estimation of reactor damage and have significant implication in the strategy of decontamination

Effects of chronic γ-irradiation on growth and survival of the Tohoku hynobiid salamander, Hynobius lichenatus

The Tohoku hynobiid salamanders, Hynobius lichenatus, were chronically irradiated with γ-rays from embryonic to juvenile stages for 450 days. At 490 μGy h−1 or lower dose rates, growth and survival were not significantly affected by irradiation, and any morphological aberrations and histological damages were not observed. At 4600 μGy h−1, growth was severely inhibited, and all the individuals died mostly at the juvenile stage. Chronic LD50 was 42 Gy as a total dose. In the liver, the number of hematopoietic cells was significantly reduced in the living juveniles, and these cells disappeared in the dead juveniles. In the spleen, mature lymphocytes were depleted in the living larvae, and almost all the heamtopoietic cells disappeared in the dead juveniles. These results suggest that this salamander died due to acute radiation syndrome, i.e., hematopoietic damage and subsequent sepsis caused by immune depression. The death would be also attributed to skin damage inducing infection. At 18,000 μGy h−1, morphological aberrations and severe growth inhibition were observed. All the individuals died at the larval stage due to a multiple organ failure. Chronic LD50 was 28 Gy as a total dose. Assuming that chronic LD50 was 42 Gy at lower dose rates than 4600 μGy h−1, a chronic median lethal dose rate could be estimated to be 14 years). These results suggest that, among guidance dose rates, i.e., 4–400 μGy h−1, proposed by various organisations and research programmes for protection of amphibians and taxonomic groups or ecosystems including amphibians, most of them would protect this salamander but the highest value may not on the whole life scale

¹³⁵Cs/¹³⁷Cs Isotopic Ratio as a New Tracer of Radiocesium Released from the Fukushima Nuclear Accident

Since the Fukushima Daiichi nuclear power plant (FDNPP) accident in 2011, intensive studies of the distribution of released fission products, in particular ¹³⁴Cs and ¹³⁷Cs, in the environment have been conducted. However, the release sources, that is, the damaged reactors or the spent fuel pools, have not been identified, which resulted in great variation in the estimated amounts of ¹³⁷Cs released. Here, we investigated heavily contaminated environmental samples (litter, lichen, and soil) collected from Fukushima forests for the long-lived ¹³⁵Cs (half-life of 2 × 10⁶ years), which is usually difficult to measure using decay-counting techniques. Using a newly developed triple-quadrupole inductively coupled plasma tandem mass spectrometry method, we analyzed the ¹³⁵Cs/¹³⁷Cs isotopic ratio of the FDNPP-released radiocesium in environmental samples. We demonstrated that radiocesium was mainly released from the Unit 2 reactor. Considering the fact that the widely used tracer for the released Fukushima accident-sourced radiocesium in the environment, the ¹³⁴Cs/¹³⁷Cs activity ratio, will become unavailable in the near future because of the short half-life of ¹³⁴Cs (2.06 years), the ¹³⁵Cs/¹³⁷Cs isotopic ratio can be considered as a new tracer for source identification and long-term estimation of the mobility of released radiocesium in the environment

Dose rate estimation of the Tohoku hynobiid salamander, Hynobius lichenatus, in Fukushima

The radiological risks to the Tohoku hynobiid salamanders (class Amphibia), Hynobius lichenatus due to the Fukushima Dai-ichi Nuclear Power Plant accident were assessed in Fukushima Prefecture, including evacuation areas. Aquatic egg clutches (n = 1 for each sampling date and site; n = 4 in total), overwintering larvae (n = 1 - 5 for each sampling date and site; n = 17 in total), and terrestrial juveniles or adults (n = 1 or 3 for each sampling date and site; n = 12 in total) of H. lichenatus were collected from the end of April 2011 to April 2013. Environmental media such as litter (n = 1 - 5 for each sampling date and site; n = 30 in total), soil (n = 1 - 8 for each sampling date and site; n = 31 in total), water (n = 1 for each sampling date and site; n = 17 in total), and sediment (n = 1 for each sampling date and site; n = 17 in total) were also collected. Activity concentrations of 134Cs + 137Cs were 1.9 - 2800, 0.13 - 320, and 0.51 - 220 kBq (dry kg) -1 in the litter, soil, and sediment samples, respectively, and were 0.31 - 220 and <0.29-40 kBq (wet kg)-1 in the adult and larval salamanders, respectively. External and internal absorbed dose rates to H. lichenatus were calculated from these activity concentration data, using the ERICA Assessment Tool methodology. External dose rates were also measured in situ with glass dosimeters. There was agreement within a factor of 2 between the calculated and measured external dose rates. In the most severely contaminated habitat of this salamander, a northern part of Abukuma Mountains, the highest total dose rates were estimated to be 50 and 15 μGy h-1 for the adults and overwintering larvae, respectively. Growth and survival of H. lichenatus was not affected at a dose rate of up to 490 μGy h-1 in the previous laboratory chronic gamma-irradiation experiment, and thus growth and survival of this salamander would not be affected, even in the most severely contaminated habitat in Fukushima Prefecture. However, further studies of the adult salamanders may be required in order to examine whether the most severe radioactive contamination has any effects on sensitive endpoints, since the estimated highest dose rate to the adults exceeded some of the guidance dose rates proposed by various organisations and programmes for the protection of amphibians, which range from 4 to 400 μGy h-1. Conversely, at one site in Nakadori, a moderately contaminated region in Fukushima Prefecture, the dose rate to the adult salamanders in spring of 2012 was estimated to be 0.2 μGy h-1. Estimated dose rates to the overwintering larvae in spring of 2012 were 1 and 0.2 μGy h-1 at one site in Nakadori, and in Aizu, a less contaminated region in Fukushima Prefecture, respectively. These results suggest that there is a low risk that H. lichenatus will be affected by radioactive contamination in these districts, though further studies on dose rate estimation are required for definitive risk characterisation

Effects of chronic gamma-irradiation on growth and sexual maturation of the Tohoku hynobiid salamander, Hynobius lichenatus

There are still considerable gaps in knowledge regarding the biological effects of chronic ionising radiation exposure in amphibians. To fill these gaps, Tohoku hynobiid salamanders, Hynobius lichenatus (Amphibia, Caudata), were chronically irradiated with 137Cs γ-rays from embryonic to adult stages over 1954 days, and the effects on their growth and sexual maturation were examined under laboratory conditions. Irradiation at a dose rate of 33 μGy h-1 had some stimulatory effects on growth (body weight increase) of H. lichenatus, while growth was temporarily or permanently suppressed at 150 or 510 μGy h-1, respectively. On day 1802, secondary sexual characteristics (a tubercle at the anterior angle of the cloacal vent for males and ovisac development for females) were observed in 91 % of the salamanders irradiated at 33 μGy h-1, and in a similar percentage of non-irradiated controls. At 150 and 510 μGy h-1, secondary sexual characteristics were not observed in any individuals. These results suggest that the derived consideration reference level (DCRL) of the International Commission on Radiological Protection (ICRP) for Reference Frog, i.e. 40–400 μGy h-1, is applicable for the protection of H. lichenatus, and that growth and sexual maturation of this salamander may not have been adversely affected even in the most severely contaminated area in Fukushima, where the highest dose rate to salamanders was estimated to be 50 μGy h-1. However, observations in the contaminated area are required to confirm this conclusion, considering the possible confounding factors which may make this salamander more sensitive to radiation in the natural environment than under laboratory conditions

Radiocaesium contamination and dose rate estimation of terrestrial and freshwater wildlife in the exclusion zone of the Fukushima Dai-ichi Nuclear Power Plant accident

To characterise the radioactive contamination of terrestrial and freshwater wildlife caused by the Fukushima Dai-ichi Nuclear Power Plant accident, biological samples, namely, fungi, mosses, plants, amphibians, reptiles, insects, molluscs, and earthworms, were collected mainly from the forests of the exclusion zone in the Fukushima Prefecture from 2011 to 2012. Caesium-134 and 137Cs were detected by gamma spectrometry in almost all the samples. Fungi, ferns, and mosses accumulated high amounts of radiocaesium, as they did in Chernobyl, with 134Cs + 137Cs activity concentrations of 104–106 Bq kg-1 fresh mass (FM). Earthworms, amphibians, and the soft tissue of the garden snail Acusta despecta sieboldiana, also had levels as high as 104–105 Bq kg-1 FM of 134Cs + 137Cs. Most of the estimated total (internal + external) dose rates to herbaceous plants, amphibians, insects, and earthworms were below the corresponding derived consideration reference levels (DCRLs) recommended by the ICRP. This suggests that, in most cases, there was little chance of deleterious effects of ionising radiation on these organisms in the exclusion zone for the first year after the accident, though the dose rates were underestimated mainly due to the lack of consideration of short-lived radionuclides