Ab initio Enhanced CALPHAD Modeling of Actinide Rich Metallic Nuclear Fuels

Metallic fuels are promising fuel candidates for Generation IV nuclear reactors currently under active research and development. The purpose of the researches in this thesis is to increase the understanding on the phase stability of U-Pu-Zr-MA (MA = Np, Am and Cm) alloy, which is the current basis for fast spectrum metallic fuel in a fully recycled closed fuel cycle. We focused on the Np-U-Zr system and its U-Zr, Np-Zr and Np-U binary and U, Np, Zr unary sub-systems and address two problems.;The first problem is to obtain accurate ab initio energetics for actinide systems due to challenges in modeling the f-electron many-body correlation and relativistic effects. We assessed density functional theory (DFT) in both its standard form and the so-called DFT plus Hubbard U (DFT + U) modification based on the generalized gradient approximation and established a consistent set of empirical Ueff parameter ranges for Np and U that can improve the calculated energetics for Np-U-Zr alloy and its sub-systems. We also determined quantitatively how much the calculated energetics are affected by spin-orbit coupling (SOC), a relativistic effect often neglected for lighter metals. The second problem is the lack of accurate thermodynamic models for Np-U-Zr due to limited experimental data. We mitigate the problem using ab initio predicted energetics to supplement existing experimental data and assist the thermodynamic modeling using the so-called ab initio enhanced CALculation of PHase Diagrams (CALPHAD) approach. Our work developed thermodynamic models for the U-Zr and Np-Zr systems that should be of good accuracy. For the Np-U and Np-U-Zr systems, we developed models that were restricted by limited experimental data available for these systems but should also be acceptably accurate at the high temperatures at which metallic fuels are deployed in reactors.;Overall, understanding in the phase stability of Np-U-Zr and its subsystems acquired in the current thesis researches can help improve the design and use of metallic fuels. The ab initio approach and CALPHAD models established in this thesis should be applicable for studying additional properties and other related systems of metallic nuclear fuels.