Backward analysis of U-bearing particles from the Fukushima Daiichi Nuclear Power Station using the Scheil–Gulliver solidification model of the U–Zr–Fe–O system

• Published in: Journal of nuclear materials Vol. 614; p. 155892
• Main Authors: Itoh, Ayumi, Mizokami, Masato, Hirai, Mutsumi
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.08.2025
• Subjects: Backward estimation of fuel debris; Fukushima Daiichi Nuclear Power Station; Nearest-neighbor search; Scheil–Gulliver; U-bearing particle; Backward estimation of fuel debris; Scheil–Gulliver; Nearest-neighbor search; U-bearing particle; Fukushima Daiichi Nuclear Power Station
• ISSN: 0022-3115
• DOI: 10.1016/j.jnucmat.2025.155892

•Crystal structure and U–Zr–Fe ratio of U-bearing particle reflects debris state.•Fe-addition promoted separation of solid solution to U-rich and U-depleted fluorite.•Solid phases without Fe can be formed even from Fe-containing liquid states.•U-bearing particles could originate from liquid above 1800 K during accidents. For the decommissioning of the Fukushima Daiichi Nuclear Power Station (FDNPS), a methodology is needed for evaluating the chemical conditions where fuel debris formed during the accident, which is also a starting point for estimating current in-vessel state. The crystal structures and compositions of fuel debris provide information about their formation mechanisms and solidification paths. In this study, we performed Scheil–Gulliver solidification simulation for a wide range of U–Zr–Fe–O compositions (1829 points), obtaining theoretical phase relations in the solidified state. Then, we developed a method of backward estimating possible liquid-state compositions and demonstrated this method on six samples taken from the FDNPS (so-called as "U-bearing particles") by comparing their morphologies and compositions with their theoretical counterparts in the solidified state. Results showed that all samples were likely formed from liquids with a wide range of oxygen to metals ratio from 0.14 to 1.70, and fluorite oxide without Fe could form even from Fe-containing compositions. Introducing Cr into the analysis with the U–Zr–Fe–Cr–O system is expected to improve this method, enabling the inclusion of Cr-rich spinel formation in the search conditions in future work.