Experimental and computational assessment of U Si N ternary phases

Abstract

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-SiN ternary phase. In the present work, U-SiN 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-SiN 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

From arc-melted ingot to MTR fuel plate: A SEM/EBSD microstructural study of U3Si2

“…This secondary phase is often reported in literature; it seems clear that this phase is stabilized by impurities, even if it is sometimes labelled USi [36,37]. As the presence of oxygen in this minor secondary phase was not definitely assessed neither in the present work nor in other publications [36,37], and knowing that this phase was also observed in the U-Si-N ternary system [38] this phase will be referred to as U 8 Si 8 X in the following of the present study, with X corresponding to light impurities (mainly O and/or other light elements).…”