[M-AG41] 福島原発事故により放出された放射性核種の環境動態

[MAG41-04] 福島県陸域におけるネプツニウム-237とプルトニウム同位体分布

*福田 美保¹、Zheng Jian¹、山崎 慎之介¹、青野 辰雄¹ (1.量子技術研究開発機構　放射線医学総合研究所)

キーワード：福島第一原発事故、ネプツニウム-237、プルトニウム同位体

After the accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS) happened in March 2011, large amounts of radionuclides released from the FDNPS into the atmosphere and marine environments.The total amounts of ¹³⁴Cs and ¹³⁷Cs, which are volatile materials, released from the accident were estimated as 18 PBq and 15 PBq, respectively. In contrast, those of ²³⁸Pu,²³⁹Pu and²⁴⁰Pu, which are non-volatile materials, amounts were estimated as 0.0019 PBq, 0.0000032 PBq and 0.0000032 PBq and these amounts were not many compared to abundance before the accident (Report of Japan government to the IAEA Ministerial Conference on Nuclear safety, 2011). ²⁴¹Pu, half-life of which is about 14 year, detected in forest litter samples at some stations collected in April and May, 2011 and it is suggested that these samples were originated from the FDNPS accident also using the results of ²⁴⁰Pu/²³⁹Pu and²⁴¹Pu/²³⁹Pu atomic ratio (Zheng et al., 2013). It is possible to identify origin to Pu and Np isotopes using not only Pu isotope atomic ratio such as ²⁴⁰Pu/²³⁹Pu and²⁴¹Pu/²³⁹Pu but also ²³⁷Np/²³⁹Pu atomic ratio. Shozugawa et al (2012) reported that many radionuclides including²³⁹Np in terrestrial environment soil and pine leaf collected in front of FDNPS and Iiidate Villege were measured. Other than Shozugawa et al (2012), reports for Np in Environment after the FDNPS accident was almost nothing. In this study, we discussed spatial distributions and origin for Neptunium (Np) and Pulutonium (Pu) based on the results of the Neptuniumand plutonium atom ratios (²³⁷Np/²³⁹Pu,²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu) in soil samples collected in Fukushima Prefecture.Collected soil samples were air-dried,and then, ashed using muffle furnace at 450°C at least 4 hours to decompose the organic matter (Wang et al., 2015). 0.57 pg of ²⁴²Pu was used as recovery tracer for²³⁷Np and Pu isotopes. Then, the anion-exchange resin AG MP-1M was used separated of ²³⁷Np and Pu isotopes from u, Th and other interfaces (details for Haung et al. prepared).²³⁷Np and Pu isotope measured using the Sector-Field ICP-MS. The ²³⁷Np activity concentrations in soil samples were 1/100-1/1000 values compared to those of ²³⁹Pu and ²⁴⁰Pu. The rough profiles for these all radionuclide decreased with increasing core depth. Both ²³⁷Np/²³⁹Pu and ²⁴⁰Pu/²³⁹Pu atomic ratios were generally within global fallout ranges. Hereafter, it is necessary to elucidate factor controlling a values of outside the range in some layer were outside value.

This work was partially supported by Research and Development to Radiological Sciences in Fukushima Prefecture.