Synchrotron radiation analysis of microparticles from the Fukushima Daiichi nuclear power station

In March 2011, the Fukushima Daiichi Nuclear Power Station (FDNPS) suffered reactor core overheating and fuel melting following the Great East Japan Earthquake and tsunami, producing complex microparticles from vaporized and rapidly solidified nuclear and structural materials. The chemical states and local structures of key elements in these particles, particularly uranium and plutonium, remain poorly constrained. Here, we present the synchrotron-based micro-focused X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) study of microparticles recovered from inside Unit 2 of FDNPS. The particles contain uranium, zirconium, and trace plutonium uniformly incorporated into chemically homogeneous oxide matrices. Two types were identified: uranium-rich particles with cubic UO₂ and mixed U-Zr oxides with tetragonal ZrO₂, the latter persisting at room temperature, indicating rapid cooling from a high-temperature metastable phase above 1650 °C. Both uranium and plutonium are mainly in the +4 state, with localized valence increases in zirconium-rich regions, suggesting redox-driven charge compensation during crystallization. These results provide direct evidence of melt evolution, actinide mixing, and oxidation-state preservation during severe reactor accidents, informing models of core degradation and strategies for safe decommissioning at FDNPS.