Experimental and computational assessment of U–Si–N ternary phases

Uranium nitride-silicide composites are being considered as a high-density and high thermal conductivity fuel option for light water reactors. During development, chemical interactions were observed near the silicide melting point which resulted in formation of an unknown U–Si–N ternary phase. In the present work, U–Si–N composite samples were produced by arc-melting U ₃ Si ₂ under an argon-nitrogen atmosphere to form the ternary phase. The resulting samples were characterized by SEM/EDS-EPMA and XRD, and demonstrated an equilibrium between U ₃ Si ₂ , UN, USi and a U–Si–N phase with a distinct crystallographic structure. Rietveld refinement of the ternary structure was performed, considering the ternary structures existent in the analogue U–Si–C system, and a good fit was obtained for the hexagonal U ₂₀ Si ₁₆ N ₃ phase. DFT + U calculations were performed in parallel to evaluate the thermodynamic and dynamic stability of the ternaries U ₂₀ Si ₁₆ N ₃ and U ₃ Si ₂ N ₂ . The calculated enthalpy of formation and phonon dispersion support the existence of stable U ₂₀ Si ₁₆ N ₃ and U ₃ Si ₂ N ₂ , although some soft modes in the U ₂₀ Si ₁₆ N ₃ phase phonons are observed. The results presented here thus demonstrate the occurrence of at least one ternary phase in the U–Si–N system.