Non-stoichiometry Effects and Phase Equilibria in the Uranium-Carbon-Nitrogen Ternary System

Abstract

Uranium carbonitride—a solid solution of stoichiometric UC and UN compounds—is considered as a potential nuclear fuel, and is only one of multiple phases that occur in the ternary U-C-N system. Explanation of available observed data and successful synthesis of the ternary U(C, N) compound requires understanding of complex phase equilibria that take place in the system. A subregular solution model is introduced to account for the non-stoichiometry of two-component solid phases (α-, β-UC₂, α-U₂N₃), and an ideal solution model—for the substitutional nature of C–N interactions in the non-metal sublattice of U(C, N). The model better reproduces the phase fields compared to the previously reported approach, and is in good agreement with the available observed data. The stability diagram of U(C, N) with its composition and temperature (or pressure) as axes is used to compare the model predictions and the observed equilibrium data. While the theory is consistent with the available data, there is a certain disagreement with the observed equilibrium N₂ partial pressure.

Appendix

Thermodynamic parameters. The data are from References 27 and 28 (Tables AI, AII, AIII).

Table AI Expansion Coefficients for Energies of $g_{\text{UN(fcc)}}^{0}$, $g_{\text{UC(fcc)}}^{0}$, $g_{{{\text{UC}}_{2} ( {\text{fcc)}}}}^{0}$, $g_{{{\text{UC}}_{2} ( {\text{tet)}}}}^{0}$, $g_{\text{UVa(fcc)}}^{0}$, $g_{\text{UVa(tet)}}^{0}$, and $g_{{{\text{Va}}_{ 2} {\text{N}}_{ 3} }}^{ 0}$ End-Members

Table AII Reference Terms for the U₂N₃ End-Member of α-U₂N₃

Table AIII Expansion Coefficients for Stoichiometric Compounds