Determining the formation enthalpies and phase diagram of high-density uranium fuels by mixing GGA and GGA plus U calculations

Density Functional Theory (DFT) calculations of formation enthalpies often exhibit significant discrepancies when compared to experimental values, particularly for compounds that transition between localized and delocalized electronic states. This is particularly true for uranium compounds. To mitigate this error, we propose adopting a mixed calculation method that combines the generalized gradient approximation (GGA) with the GGA+U method, as previously suggested, for the calculation of formation enthalpies in nuclear fuel systems. By fitting ten uranium-, silicon-, or carbon-containing binary compounds, we obtained energy corrections (EU, E S , E C ), which were then applied to the formation enthalpy calculations of the uranium- silicon-carbon ternary compounds U 20 Si 16 C 3 and U3Si2C2, with the formation enthalpy of U 20 Si 16 C 3 differing from the experimental value by only 20 meV/atom. Further leveraging these calculated formation enthalpies, we constructed the U-Si-C ternary phase diagram, which accurately reproduces all experimentally verified stable phases. Notably, our approach avoids the spurious stable phases predicted by other theoretical methods, such as Fitted Elemental-Phase Reference Energies (FERE) or pure GGA+U calculations, thus demonstrating its improved accuracy in predicting the U-Si-C phase stability.