Understanding the interface interaction between U(3)Si(2) fuel and SiC cladding

Triuranium disilicide (U(3)Si(2)) 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 U(3)Si(2) and SiC by utilizing density functional theory calculations supported by diffusion couple experiments. From the calculated reaction energies, we demonstrate that triuranium pentasilicide (U(3)Si(5)), uranium carbide (UC), U(20)Si(16)C(3), and uranium silicide (USi) phases can form at the interface. A detailed study of U(3)Si(2) and SiC defect formation energies of the equilibrated materials yielding the interfacial phases U(20)Si(16)C(3), U(3)Si(5) and UC reveal a thermodynamic driving force for generating defects in both fuel and cladding. The absence of either the U(3)Si(2) 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 U(3)Si(5), UC, and U(20)Si(16)C(3)/USi phases at the interface. The resulting lack of continuous interaction between the U(3)Si(2) and SiC, reflects the diminishing driving force for defect formation, demonstrating the substantial stability of this fuel-cladding system.