R4: Mineral sciences of the Earth surface

[R4-09] Volatilization of B₄C control rods in Fukushima Daiichi nuclear reactors during meltdown: B–Li isotopic signatures in cesium-rich microparticles

Kazuki Fueda¹, Toshihiko Ohnuki⁷, Shinya Yamasaki², Kenji Horie³, Mami Takehara³, Gareth Law⁶, Bernd Grambow⁵, Rodney C. Ewing⁴, *Satoshi UTSUNOMIYA¹ (1. Kyushu University, 2. Tsukuba Univ., 3. NIPR, 4. Stanford Univ., 5. Univ. Nantes, 6. Univ. Helsinki, 7. TITECH)

Keywords:Fukushima Daiichi, Cesium-rich microparticles (CsMPs), Boron isotope, Control rod, Secondary ion mass spectrometry

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^–1 of ¹⁰⁺¹¹B and 11.99–1213 mg kg^–1 of ⁷Li. The ¹¹B/¹⁰B (4.15–4.21) and ⁷Li/⁶Li (213–406) isotopic ratios are greater than natural abundances (~4.05 and ~12.5, respectively), indicating that ¹⁰B(n,α)⁷Li reactions occurred in B₄C 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.