Structure, stability, and actinide leaching of simulated nuclear fuel debris synthesized from UO₂, Zr, and stainless-steel

To evaluate the chemical structure and stability of nuclear fuel debris comprising UO₂, Zr, and stainless-steel (SUS) generated during the Fukushima Daiichi Nuclear Power Plant accident in Japan in 2011, the simulated debris of a UO₂–SUS–Zr system and those of other fundamental-component-containing systems were synthesized and characterized. The simulated debris was synthesized by heat treatment at 1600 °C under inert (Ar) or oxidative (Ar + 2% O₂) atmospheres for 1–12 h. ²³⁷Np and ²⁴¹Am tracers were doped for leaching tests from the simulated debris. The simulated debris was characterized using X-ray diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, Raman spectroscopy, and Mössbauer spectroscopy, and the results revealed that the major uranium phase of the UO₂–SUS–Zr debris comprised a Zr(IV)-and-Fe(II)-containing U^IVO₂ solid-solution, regardless of the treatment conditions. The results of a long-term immersion test of the simulated debris in pure water and seawater revealed that the macroscale crystal structure of the simulated debris was chemically durable under wet conditions for at least a year. Furthermore, the leaching test results indicated that the leaching ratios of U, Np, and Am from the UO₂–SUS–Zr debris were small and lower than 0.08% for all the experiments in this study.