"Invisible" radioactive cesium atoms revealed; Pollucite inclusion in cesium-rich microparticles (CsMPs) from the Fukushima Daiichi Nuclear Power Plant

宮崎 加奈子*; 武原 政人*; 蓑毛 健太*; 堀江 憲路*; 竹原 真美*; 山崎 信哉*; 斉藤 拓巳*; 大貫 敏彦*; 高野 公秀 ; 塩津 弘之  ; 岩田 孟 ; Vettese, G. F.*; Sarparanta, M. P.*; Law, G. T. W.*; Grambow, B.*; Ewing, R. C.*; 宇都宮 聡*

Miyazaki, Kanako*; Takehara, Masato*; Minomo, Kenta*; Horie, Kenji*; Takehara, Mami*; Yamasaki, Shinya*; Saito, Takumi*; Onuki, Toshihiko*; Takano, Masahide; Shiotsu, Hiroyuki; Iwata, Hajime; Vettese, G. F.*; Sarparanta, M. P.*; Law, G. T. W.*; Grambow, B.*; Ewing, R. C.*; Utsunomiya, Satoshi*

Radioactive Cs contamination has been one of the central issues in Fukushima and other legacy sites; however, atomic-scale characterization of radioactive Cs has never been successful. Here we report, for the first time, the direct imaging of radioactive Cs atoms using high-resolution high-angle annular dark-field scanning transmission electron microscopy. As inclusions in Cs-rich microparticles, 27-36 wt.% of Cs (as CsO) occurs in a type of zeolite called pollucite. The normalized formula for pollucite are expressed as (Cs,K,Ba)(Fe,Zn,X)SiO, (Cs,K,Ba)(FeZnX)SiO, and (Cs,K,Ba)(FeZnX)SiO after normalization with 12 oxygen atoms (X includes other trace cations; Ti, Mn, Rb, Zr, Mo, and Sn). Atomic-resolution image of radioactive Cs atoms are obtained when viewing along the [111] zone axis, a view supported by image simulations using the multi-slice method. The occurrence of pollucite indicates that locally enriched Cs reacted with siliceous substances during meltdowns, presumably through hydrothermal reactions. In case of predominant occurrence of pollucite in debris, incorporation in pollucite structure retards leaching of radioactive Cs. Still, the atomic-resolution imaging of radioactive Cs is an important advance for better understanding the fate of radioactive Cs inside and outside of damaged reactors during severe accidents like Fukushima Daiichi.