Relativistic density-functional study of the solid solubility of transition metal/γ-uranium alloys: The role of d-d orbital interactions

The alloying behavior of transition metals (TMs) in solid γ-phase uranium (γ-U), which is expected to be used as fuel for next-generation nuclear reactors, was investigated using the discrete-variational Dirac-Fock-Slater molecular orbital method. Using a model cluster, U₈/TM, as the minimum unit of γ-U/TM alloys, we evaluated the d-orbital energy of the TM (Md), the bond order between the TM and U atoms, and the orbital overlap population (OOP) between the atomic orbitals of the TM and U atoms. We subsequently examined the effect of these quantities on the maximum solid solubility (MSS) of the γ-U/TM alloys. The interaction between the U-6d and TM-d atomic orbitals was found to play a key role in determining the MSS of the γ-U/TM alloys. The magnitude of the MSS can be explained in terms of the stabilization energy, which is affected by the Md and the OOP, formed by d-d orbital interactions. We also mapped the MSS of γ-U/TM alloys using the Md and the OOP as the alloying parameters. These results will assist the quantum design of nuclear fuel materials.