Volatilization of B 4 C control rods in Fukushima Daiichi nuclear reactors during meltdown: B-Li isotopic signatures in cesium-rich microparticles.

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⁺¹¹ B and 11.99-1213 mg kg ^-1 of ⁷ Li. The ¹¹ B/ ¹⁰ B (4.15-4.21) and ⁷ Li/ ⁶ Li (213-406) isotopic ratios are greater than natural abundances (~4.05 and ~12.5, respectively), indicating that ¹⁰ B(n,α) ⁷ Li reactions occurred in B ₄ 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.
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