Unveiling the hidden risk: Cryo-TRLFS identifies critically elusive UO alteration phases under chemically complex conditions

Mei, H.*; Aoyagi, Noboru   ; Takaki, Seiya ; Saito, Takumi*

The study examines how uranium dioxide, a key component of spent nuclear fuel and Fukushima fuel debris, alters under oxidizing conditions relevant to storage and accident environments. Factors such as boric acid, hydrogen peroxide, seawater, and cement leachates were included to simulate realistic chemical conditions. After five months, the alteration products were analyzed using X ray diffraction and cryogenic time resolved laser fluorescence spectroscopy. X-ray diffraction detected only small amounts of metaschoepite in simple systems and failed to identify products in more complex environments. In contrast, cryogenic fluorescence spectroscopy successfully identified uranium six phases in all samples and revealed detailed chemical forms that X ray methods could not detect. Two distinct fluorescent components of uranium were identified, both linked to metaschoepite. The results highlight the importance of advanced spectroscopy methods for detecting subtle uranium phases that control early reactivity and release. These findings improve understanding of uranium behavior in nuclear waste systems and contribute to safer management strategies for Fukushima debris and long term storage of spent nuclear fuel.