Vancho Kocevski1 Emily Moore1 Denise Adorno Lopes1 Simon Middleburgh2 Theodore Besmann1

1, University of South Carolina, Columbia, South Carolina, United States
2, Westinghouse Electric Sweden AB, Västerås, , Sweden

In recent years, U3Si2 is being considered to replace UO2 as an advanced technology fuel (ATF) candidate, with the Fe-Cr-Al-Y alloys being used as an advanced cladding material. To date there is no assessed CALPHAD (CALculation of PHAse Diagrams) model of the U-Fe-Si ternary space in the literature. While limited experimental data exists, more realistic physical relations for the U-Fe-Si ternary space can be established by complementing existing information with density functional theory (DFT) calculated thermodynamic data, eventually resulting in a self-consistent CALPHAD model, which is the purpose of our study.

First, we expanded the current DFT database by determining the most stable structure of the experimentally observed ternary phases. For the experimentally observed phases with partial occupancies (occupancy less than 1) at a specific Wyckoff position, we used special quasi-random structures to determine the most stable configuration at a specific composition. Subsequently, using DFT, we calculated the formation energies and vibrational entropies of the constituent binary and ternary U-Fe-Si phases to estimate their Gibbs energies. This combined DFT-CALPHAD approach will aid in understanding complex phase formation across the compositional and temperature ranges for prospective ATFs to allow assessment of potential interactions between fuel and cladding systems.

This research is being performed using funding received from the DOE Office of Nuclear Energy's Nuclear Energy University Programs.