Divergent structural organization trends driven by oxygen speciation in uranium-bearing chloride molten salts

초록

Despite unavoidable, the effect of oxygen contamination on the thermophysical and structural properties of molten salts remains largely unexplored. To elucidate how oxygen influences chloride-based molten salts with uranium, we analyze two distinct scenarios where oxygen is present either as molecular oxygen (O2) or oxide ions (O2-). Specifically, molecular dynamics simulations based on universal machine learning interatomic potentials (MLIP) are performed on KCl-UCl3, NaCl-UCl3, and NaCl-KCl-UCl3 melts at 1,000 K with oxygen contents ranging from 0 to 2.0 wt%. The simulations show that O2 and O2- modify the polyhedral network of chloride melts in distinct ways, leading to contrasting trends in density. In the O2 gas ingress scenario, the density decreases, primarily driven by compositional dilution from the addition of light oxygen species. Structurally, strong U-O bonding induces localized densification that partially mitigates this reduction. However, the emergence of rigid, near-linear U-O-U motifs connected via corner-sharing imposes geometric constraints that lower packing efficiency, further contributing to the density decrease. Crucially, this structural reorganization displaces tightly bound chloride ions to active sites, thereby providing a thermodynamic driving force for corrosion. In contrast, O2- ion accumulation effectively replaces chloride ions, and triggers a global contraction of the anion framework. This contracted environment subsequently facilitates the formation of dense, high-connectivity uranium clusters connected via polyhedral edge-and face-sharing, synergistically driving a consistent increase in density. Our analysis indicates that O2 gas ingress and O2-ion accumulation have separable effects on density, arising from fundamentally different structural responses and packing mechanisms.

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