Potential accident tolerant fuel candidate: Investigation of physical properties of the ternary phase U₂CrN₃

In the present study, physical properties of the ternary phase U₂CrN₃ are evaluated experimentally and by modeling methods. High density pellets containing the ternary phase were prepared by spark plasma sintering (SPS). The microstructural and crystallographic analyses of the composite pellets were performed using scanning electron microscopy (SEM), standardised energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). Evaluation of the mechanical properties was performed by nanoindentation test. The impact of temperature on lattice properties was evaluated using high temperature X-ray diffraction (XRD) coupled with modeling. Progressive change in the lattice parameters was obtained from room temperature (RT) to 673 K, and the result was used to calculate average linear thermal expansion coefficients, as well as an input for the density functional theory (DFT) modeling to reassess the degradation of the mechanical properties. The ab-initio calculation provides an initial assessment of electronic configuration of this ternary phase in a direct comparison with one of UN phase. For this goal, modeling was also employed to evaluate point defect formation energies and electronic charge distribution in the ternary phase. Results indicate that the U₂CrN₃ phase has similar mechanical properties to UN (Young's, bulk, shear moduli, hardness). No preferential crystallographic orientation was observed in the composite pellet. However, charge electron density distribution highlights the significant directionality of chemical bonds, which is in agreement with the anisotropy and non-linear behaviour of the obtained thermal expansion (α¯(a_a) = 9.12 × 10⁻⁶/K, α¯(a_b) = 5.81 × 10⁻⁶/K and α¯(a_c) = 6.08 × 10⁻⁶/K). As a consequence, uranium was found to be more strongly bound in the ternary structure which may delay diffusion and vacancy formation, promising an acceptable performance as nuclear fuel.