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

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.