Carbon and oxygen segregation at the UN(010)/U ₃ Si ₂ (001) interface: insights from first-principles calculations

The atomic structure and chemistry of hetero-phase interfaces in the UN-U₃Si₂ composite, influenced by impurity segregation, are critical to its macroscopic performance in advanced nuclear fuel applications. Using first-principles calculations, we systematically investigate the effects of C and O segregation-both individually and sequentially-on the energetic properties and embrittlement mechanisms of the UN/U₃Si₂ interface. A total of 24 interface models, encompassing variations in surface terminations and atomic configurations, reveal prevalent atomic rearrangements upon relaxation. The UN(010)/U₂-terminated U₃Si₂(001) interface with the bridge configuration is found to be the most energetically favorable. Detailed examination of the crystal structure and charge density profile highlights how impurity segregation both stabilizes and destabilizes, as well as strengthens and weakens, the interface. Both C and O atoms display strong segregation tendencies. While C enhances interfacial cohesion, O does so only in the interstitial positions and weakens the interface at the substitutional sites. In subsequent segregation, the presence of pre-existing C reduces the likelihood of O segregation due to a repulsive interaction between the two. However, when C occupies the substitutional site and O resides in the interstitial positions, the interface experiences increased embrittlement compared to the configuration with both interstitial C and O. This embrittlement arises from charge density depletion between C and U₂ atoms in the interface core, offsetting the energetic benefits of subsequent segregation. These findings provide the first quantitative theoretical framework linking atomic-scale impurity behavior to the interfacial properties of the UN-U₃Si₂ composite, offering critical insights for improving the performance and reliability of uranium-based composite interfaces.