Abstract:
Uranium is a critical nuclear material in both energy and defense sectors. Its high chemical reactivity facilitates rapid oxidation by environmental gases such as oxygen (O
2) and water vapor (H
2O), forming a uranium dioxide (UO
2) oxide layer, further causes mechanical properties and hydriding corrosion. Unraveling the adsorption and reaction mechanisms of O
2 and H
2O on UO
2 surfaces is fundamental to understand uranium oxidation kinetics and establish corrosion models. However, the physical mechanisms governing complex reaction sequences under mixed water-oxygen conditions remain unclear. This study developed and validated 1k antiferromagnetic (AFM) model for large-scale UO
2 surface calculations, investigating O
2 and H
2O adsorption and dissociation mechanisms. We theoretically confirmed experimental observations of chemisorption states for both O
2 and H
2O on UO
2(111), and systematically revealed adsorption and dissociation pathways on UO
2(111) and (110) surfaces. Employing ab-initio atomistic thermodynamics method, the adsorption thermodynamic phase diagrams under environmental conditions are constructed, demonstrating that O
2 monolayer adsorption originates from intermolecular electrostatic repulsion while H
2O multilayer adsorption arises from hydrogen bonding, leading to preferential H
2O adsorption and dissociation. H
2O dissociation introduces excess charges, promote subsequent O
2 adsorption in mixed environments. The adsorbed O
2 then facilitates OH dissociation, further reacting with H to form OH and O, ultimately establishing a water-oxygen cycle reaction mechanism: OH+O
2@UO
2 -> OH@UO
2+x. This work explains the reaction sequences observed in water-oxygen environments and provides key mechanistic insights into uranium oxidation corrosion.
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