Ab initio energetics for modeling phase stability of the Np-U system

The Np-U system has been rarely studied despite this alloy being a key binary subsystem of metallic nuclear fuels. This study aims to further the understanding of the phase stability of the Np-U system through ab initio calculations. We first examined an existing CALPHAD model based on available experimental phase boundary data. We then focused on phases of which the CALPHAD model reproduces reasonably the experimental data and examined if values of the effective Hubbard U (i.e., Ueff) parameter for Np and U fitted previously in the Np-Zr and U-Zr systems are also applicable to the Np-U system, using CALPHAD predicted enthalpies as references. We found that in general DFT + U predicts improved or at least similarly accurate enthalpies compared to standard DFT when the Ueff's used are within the ranges of single-structure optimized Ueff's (0.65-0.9 eV for Np and 1-1.5 eV for U)-for example, the multi-structure optimized Ueff's (0.9 eV for Np and 1.24 eV for U)-determined in our previous U-Zr and Np-Zr studies. Finally, we focused on the intermediate phase ζ(Np,U), which at present is poorly characterized by experiments and unsatisfactorily described in the CALPHAD model. Based on ab initio calculated formation enthalpies for ordered end members and the compound energy formalism, we predicted its site occupations as functions of composition and temperature. Our study demonstrated that Ueff's fitted for U and Np determined in the U-Zr and Np-Zr systems may be transferable to the Np-U system and also provided new prediction of the site occupations for ζ(Np,U), which may serve as reference for future experimental and modeling study of the Np-U system.