Mechanisms of U and Si diffusion in Fe–X (X = Cr, Al) dilute alloys from first-principles

Abstract

Fuel–cladding chemical interaction (FCCI) is a key factor driving cladding degradation in the promising U3Si2–FeCrAl accident-tolerant fuel system, yet predictive insights into fuel element diffusion in cladding alloys remains limited. To elucidate the fundamental diffusion mechanisms, this work develops computational methodology that integrates the residence-time algorithm with the nine-frequency and oversized solute-atom models based on first-principles calculations. We apply this integrated approach to model the diffusion of U and Si in Fe–Cr and Fe–Al alloys, which serve as simplified model systems for the complex FeCrAl cladding. Our results reveal that U diffusion is markedly slower than Si due to the significant lattice strain and highly directional bonding of U. Moreover, alloying elements Cr and Al both suppress the diffusion of U and Si, with Cr exhibiting a stronger inhibitory effect than Al. This integrated strategy enables a rigorous description of atomic transport, providing the fundamental understanding of FCCI mechanisms and guiding the design of advanced accident-tolerant nuclear fuels.