Volatilization of B
C control rods in Fukushima Daiichi nuclear reactors during meltdown; B-Li isotopic signatures in cesium-rich microparticles
笛田 和希*; 高見 龍*; 蓑毛 健太*; 諸岡 和也*; 堀江 憲路*; 竹原 真美*; 山崎 信哉*; 斉藤 拓巳*; 塩津 弘之
; 大貫 敏彦*; Law, G. T. W.*; Grambow, B.*; Ewing, R. C.*; 宇都宮 聡*
Fueda, Kazuki*; Takami, Ryu*; Minomo, Kenta*; Morooka, Kazuya*; Horie, Kenji*; Takehara, Mami*; Yamasaki, Shinya*; Saito, Takumi*; Shiotsu, Hiroyuki; Onuki, Toshihiko*; Law, G. T. W.*; Grambow, B.*; Ewing, R. C.*; Utsunomiya, Satoshi*
Boron carbide control rods remain in the fuel debris of the damaged reactors in the Fukushima Daiichi Nuclear Power Plant, potentially preventing re-criticality; however, the state and stability of the control rods remain unknown. Sensitive high-resolution ion microprobe analyses have revealed B-Li isotopic signatures in radioactive Cs-rich microparticles (CsMPs) that formed by volatilization and condensation of Si-oxides during the meltdowns. The CsMPs contain 1518-6733 mg kg
of
B and 11.99-1213 mg kg
of Li. The
B/
B (4.15-4.21) and
Li/
Li (213-406) isotopic ratios are greater than natural abundances (
4.05 and
12.5, respectively), indicating that
B(n,
)
Li reactions occurred in B
C prior to the meltdowns. The total amount of B released with CsMPs was estimated to be 0.024-62 g, suggesting that essentially all B remains in reactor Units 2 and/or 3 and is enough to prevent re-criticality; however, the heterogeneous distribution of B needs to be considered during decommissioning.