First-principles study of solute diffusion in zirconium: Vacancy-mediated and interstitial mechanisms of N, O, Si, and U

摘要: The fuel-cladding chemical interaction (FCCI) between uranium-based fuels (UO2 , UN, U3Si2 ) and Zr cladding poses a critical challenge for accident-tolerant fuels; however, the atomic-scale diffusion behaviors of key elements (N, O, Si, U) in the hexagonal close-packed (hcp) Zr matrix remain insufficiently elucidated. Herein, we systematically compare the diffusion mechanisms of these four solutes in hcp-Zr using first-principles calculations combined with the eight-frequency model for vacancy-mediated diffusion and a mean jump frequency method for interstitial diffusion.We find that atomic size mismatch governs site preference: small N and O atoms spontaneously occupy octahedral interstitial sites and diffuse via an interstitial mechanism, whereas larger Si and U favor substitutional incorporation. Although Si and U both prefer substitutional sites, their vacancy-mediated diffusion mechanisms differ qualitatively: Si exhibits near-random diffusion, whereas U experiences significant solute-vacancy binding that produces frequent back-jumps and significantly reduces its net diffusion efficiency. Furthermore, when thermal expansion and the entropy change associated with interstitial formation are taken into account, interstitial diffusion becomes increasingly favorable at elevated temperatures, suggesting a transition for U from vacancy-mediated to interstitial-dominated diffusion at high temperatures, in contrast to N, O, and Si which are interstitial diffusers. These findings demonstrate that FCCI kinetics is collectively governed by intrinsic mobility and solubility, both of which are modulated by atomic size and temperature, thereby offering a quantitative framework for the rational design of diffusion-resistant