Convergence of 234U/238U activity ratios toward equality after failure to achieve secular equilibrium across the Uox4+- and Uox6+ -bearing portions in the metamict uranopolycrase under extreme thermal conditions: geochemical archives of actinide fate in thermally stressed waste matrices

Effective immobilization of nuclear waste actinides necessitates a precise understanding of their behavior within stable host matrices, particularly under conditions where the actinide matrix is subjected to thermal stress arising from both the significant thermal energy input during synthesis and the radiogenic heat generated by radionuclide decay. This work explores the thermochemical evolution of 234 U and 238 U pertaining to the U ox 4 + - and U ox 6 + -bearing portions in a metamict uranopolycrase characterized by SEM, EMP, and XRD analyses, a natural analogue for engineered Synroc matrices, after annealing under extreme thermal conditions at 1000 °C. Comprehensive mineralogical characterization confirmed the presence of structurally robust uranopolycrase. The 234 U/ 238 U activity ratios at 1000 °C exhibit values of 0.962 ± 0.007 and 0.963 ± 0.005 in the U ox 4 + - and U ox 6 + -bearing portions, respectively, with initial U ox 4 + ratios reflecting secular equilibrium and an initial U ox 6 + ratio of 0.944, indicating strong isotopic homogenization between the two redox states at 1000 °C. Due to the overall 234 U/ 238 U activity ratio of 0.963 ± 0.004 remaining below unity, the two redox fractions converged toward equality after failing to attain secular equilibrium. The corresponding values reported in our earlier study at 800 °C were likewise centered at approximately 0.963. Furthermore, the relative quantities of uranium in the oxidation-state forms remained largely unchanged at these temperatures, demonstrating stability in uranium redox distribution under extreme thermal conditions. The origin of the contrasting redox responses exhibited by the parent and daughter uranium isotopes remains unclear, particularly regarding the atomic-scale processes responsible for their redistribution between the two isotopic populations. Activation energy calculations demonstrated higher kinetic barriers for uranium redox transitions compared to metamict analogues, supporting the mineral's superior stability. These findings advance understanding of uranium geochemistry in complex oxides and highlight uranopolycrase as a natural analogue for actinide immobilization studies. Further investigations are needed to clarify the underlying mechanisms.