Understanding the interface interaction between U₃Si₂ fuel and SiC cladding.

Abstract

Triuranium disilicide (U₃Si₂) fuel with silicon carbide (SiC) composite cladding is being considered as an advanced concept/accident tolerant fuel for light water reactors thus, understanding their chemical compatibility under operational and accident conditions is paramount. Here we provide a comprehensive view of the interaction between U₃Si₂ and SiC by utilizing density functional theory calculations supported by diffusion couple experiments. From the calculated reaction energies, we demonstrate that triuranium pentasilicide (U₃Si₅), uranium carbide (UC), U₂₀Si₁₆C₃, and uranium silicide (USi) phases can form at the interface. A detailed study of U₃Si₂ and SiC defect formation energies of the equilibrated materials yielding the interfacial phases U₂₀Si₁₆C₃, U₃Si₅ and UC reveal a thermodynamic driving force for generating defects in both fuel and cladding. The absence of either the U₃Si₂ or SiC phase, however, causes the defect formation energies in the other phase to be positive, removing the driving force for additional interfacial reactions. The diffusion couple experiments confirm the conclusion with demonstrated restricted formation of U₃Si₅, UC, and U₂₀Si₁₆C₃/USi phases at the interface. The resulting lack of continuous interaction between the U₃Si₂ and SiC, reflects the diminishing driving force for defect formation, demonstrating the substantial stability of this fuel-cladding system.