[1B12-14] 1F Waste

[1B14] Volatilization of B4C control rods in Fukushima Daiichi nuclear reactors during meltdown: B–Li isotopic signatures in cesium-rich microparticles

*Satoshi Utsunomiya¹, Kazuki Fueda¹, Kenta Minomo¹, Kenji Horie^3,8, Mami Takehara³, Shinya Yamasaki², Takumi Saito⁴, Hiroyuki Shiotsu⁵, Toshihiko Ohnuki⁶, Rodney C Ewing⁷ (1. Kyushu University, 2. Tsukuba Univ., 3. NIPR, 4. University of Tokyo, 5. JAEA, 6. TITECH, 7. Stanford University, 8. The Graduate University for Advanced Studies (SOKENDAI))

Keywords:Fukushima Daiichi Nuclear Power Plant, 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.