DFT Studies of Electronic Properties and Effect of He and Xe Incorporation in Selected Ceramics

The electronic properties of several prospective nuclear fuels are not yet well known. We used Quantum Espresso and EPW codes to evaluate the electron density of states, the electronic heat capacity coefficient, the electron-phonon coupling strength, the number of mobility electrons, and the electronic heat conductivity. The electronic properties for ThN, ThC and UN using a slightly different approach that were previously evaluated are discussed and the results are compared. We confirmed that while the electronic heat capacity coefficient is linearly dependent on the electron density of states at Fermi energy, such a simple relation could not be used to determine the difference in the electronic heat conductivity of investigated materials. The highest heat conductivity was registered in ThN. These metallic fuels also have high U/Th density, therefore are more economical since enrichment is expensive. Furthermore, it is important to examine swelling in these high-density fuels. We evaluated that UN had 42% more U atoms per unit volume than UO2 and a 55% higher volume increase when accommodating one Xe atom in one interstitial of a (2 × 2 × 2) supercell. However, for He, the volume increase was 27% lower in UN. Interestingly, even though the Th atom’s density in ThN and ThC was lower than that of U atoms in the UN compound, a similar trend of volume changes was found. We concluded, therefore, that when we consider swelling, the local structural symmetry (tetrahedral versus octahedral sites) is more important than the density of atoms. The 37 % greater of absolute value of the total energy increase due to incorporation of Xe in ThC versus ThN cannot be explained by the crystal structure since a ThC-Xe supercell has a higher lattice constant than a ThN-Xe corresponding supercell. Such results can only be explained by investigating electronic structure.

Electronic Transport of Uranium Mononitride

We investigated the electronic heat capacity, thermal conductivity, and resistivity of UN using Quantum Espresso and EPW code. GGA, PBEsol functional was used. The calculated electronic heat coefficient was found to be significantly reduced (0.0176 J⋅mol-1⋅K-2 versus 0.0006 J⋅mol-1⋅K-2) when the non-local hybrid functional (B3LYP) was used. Furthermore, we calculated electrical resistivity using a very transparent Ziman’s formula for metals with the Eliashberg transport coupling function as implemented in EPW code for non-spin-polarized calculations. The number of mobile electrons in UN, as a function of temperature, was derived from the ratio of the calculated resistivity and available experimental data. The electronic thermal conductivity was evaluated from the calculated electronic resistivity via Wiedemann-Franz law with the number of mobility electrons (nav) incorporated (averaged over the temperature range 300 K - 1000 K). Both the electronic thermal conductivity and resistivity, as calculated using newly evaluated nav, compare well with experimental data at ~700 K, but to reproduce the observed trend as a function of temperature, the number of mobile electrons must decrease with the temperature as evaluated.