An in-depth investigation into U and U isotopes systematics in U(IV) and U(VI) phases of betafite for enhanced understanding of actinide retention.

The superior efficacy of synroc as an immobilization matrix for actinides in spent nuclear fuel has been extensively validated, positioning it as a leading candidate for long-term nuclear waste management. In this context, isotopic investigations of natural analogues are indispensable for optimizing synroc formulations, particularly regarding their capacity to incorporate and retain actinides and their decay products over geological timescales. In this study, a naturally occurring member of the pyrochlore supergroup was identified as betafite through integrated SEM, EMPA, and XRD analyses. The isotopic behavior of uranium was investigated by comparing the activity ratios of parent (U) and daughter (U) nuclides within U(IV) and U(VI) enriched forms of the mineral, formed and preserved over approximately 2 billion years. The U/U activity ratios, 1.089 ± 0.016 in U(IV) forms and 0.956 ± 0.007 in U(VI) forms, demonstrate a preferential accumulation of radiogenic U in the tetravalent uranium sites. Based on the obtained findings, this may indicate a structural affinity and retention capacity of U(IV) for daughter isotopes, which might contribute to its stability under reducing conditions; however, further investigation is required to evaluate its long-term implications. Moreover, dissolution features and redox partitioning indicate that approximately 33% of U(IV) in the system results from the disproportionation of intermediate U(V), highlighting a critical redox transformation pathway relevant to actinide immobilization. These findings underscore the importance of redox-sensitive behavior and uranium isotope fractionation in pyrochlore-group minerals and reinforce the relevance of natural analogues in tailoring synroc compositions. Such analogues serve as empirical benchmarks for predicting the performance of actinide-bearing phases in nuclear waste repositories over geologic timescales.