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.

Research highlights▶ The highlights of the present paper demonstrate that the maximum solid solubility (MSS) of transition metal (TM) atoms in solid γ-U phase, which was determined experimentally, can be explained in terms of both the TM d orbital energy (Md) and the TMd–U6d orbital overlap populations (OOP), which were obtained using first-principle calculations based on relativistic density-functional theory. Interestingly, when the correlation map between Md and the OOP is made, γ-U/TM alloys with a lower MSS appear in the lower left of the OOP–Md map, whereas those with a higher MSS appear in the upper right. Thus, the map obtained using the OOP and Md is useful for designing not only γ-U/TM alloys but also other alloy systems.