Unveiling the hidden risk: Cryo-TRLFS identifies critically elusive UO2 alteration phases under chemically complex conditions

solutions, but failed for the identification of the complex seawater and cement leachate systems. Cryo-TRLFS succeeded in detecting well-resolved spectral fingerprints for all these samples. Parallel factor analysis further decomposed two distinct U(VI)-bearing fluorescent components with different fluorescence lifetimes (56 ± 5 and 242 ± 60 μsec), both attributable to metaschoepite, revealing speciation details inaccessible to XRD. This study demonstrates the critical advantage of Cryo-TRLFS in identifying the elusive, U(VI) phases of interest for risk assessment. The findings directly inform the safety protocols for Fukushima fuel debris retrieval by clarifying initial alteration products, and provide essential mechanistic data to advance predictive models for U(VI) speciation evolution in repository environments, ultimately contributing to more robust long-term safety assessments.