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Detection of alpha particle emitters originating from nuclear fuel inside reactor building of Fukushima Daiichi Nuclear Power Station

福島第一原子力発電所原子炉建屋内の核燃料起因$$alpha$$核種の検知

森下 祐樹  ; 鳥居 建男 ; 宇佐美 博士; 菊地 弘幸*; 宇津木 弥*; 高平 史郎*

Morishita, Yuki; Torii, Tatsuo; Usami, Hiroshi; Kikuchi, Hiroyuki*; Utsugi, Wataru*; Takahira, Shiro*

福島第一原子力発電所の原子炉建屋で採取されたスミヤ試料上の$$alpha$$核種を$$alpha$$線イメージング検出器で測定した。極薄のGAGGシンチレータとSiPMアレーで検出器を構成した。$$alpha$$スペクトルは5-6MeV領域に確認され$$^{238}$$Puのエネルギーと一致した。$$gamma$$線スペクトロメトリで$$^{241}$$Amのピークも確認できた。これらの測定結果から福島第一原子力発電所の$$alpha$$核種として$$^{238}$$Puや$$^{241}$$Amが存在することを現場での測定により明らかにした。

We measured alpha emitters obtained from a reactor building in the Fukushima Daiichi Nuclear Power Plant (FDNPP) by using an alpha particle imaging detector. For developing the detector, we used a very thin (0.05-mm-thick) a cerium-doped Gd$$_{3}$$(Ga,Al) $$_{5}$$O$$_{12}$$ (Ce GAGG) scintillator and silicon photomultiplier (SiPM) arrays as the photodetector. The floor of the reactor building in FDNPP was wiped off by using smear papers, and the radioactivity of these papers was measured by the alpha particle imaging detector. In addition, we measured a Plutonium (Pu) sample (mainly 5.5 MeV alpha particles from $$^{238}$$Pu) obtained from a nuclear fuel facility by using of the same detector for comparison with the smear papers. The alpha spectrum was in the energy range of 5-6 MeV, which corresponds to the alpha particle energy of $$^{238}$$Pu (5.5 MeV). The correlation coefficient of the alpha spectra of the smear papers and the Pu sample had a strong positive linear relation. Moreover, the peak of $$^{241}$$Am was identified by gamma spectrum measurement. Based on these results, it is highly likely that the alpha emitters in the reactor building of FDNPP are $$^{238}$$Pu and $$^{241}$$Am, which originated from the nuclear fuel. The surface contamination level of alpha emitters exceeded 4 Bq/cm$$^{2}$$.

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