Investigating uranium’s high-temperature thermodynamic properties

Uranium metal is a recognized nuclear fuel for sodium fast reactors due to its significant thermal conductivity and high burnup capability, among other beneficial properties. However, metallic uranium-based nuclear fuels undergo physical phenomena that are poorly understood on a fundamental level. These phenomena include gaseous swelling, redistribution of constituents, or phase separation as consequences of the fabrication procedures.

The different forms of uranium, known as allotropic forms, are of great interest to scientists because these states are involved in the technological processes of nuclear fuel operation. Uranium has three solid phases (crystal structures) at ambient pressure—represented as alpha (α-U), beta (β-U), and gamma (γ-U). As temperatures increase, each phase transforms into the subsequent phase, eventually transforming into liquid uranium and then vapor at extreme temperatures (4432.08 Kelvin).

Understanding the thermophysical properties of γ-uranium is not only important for the operation of nuclear reactors but also for the relevant fuel-fabrication mechanics. However, most of the experimental work to date has been performed on the α phase and not on the γ phase due to the mechanical and dynamical instability of the latter at room temperature. Performing high-temperature experiments on nuclear materials pose significant challenges that ultimately limit experimental investigations of the γ phase.

To shed light on uranium’s thermal heat capacity at high temperatures, a team of LLNL researchers computed the high-temperature thermodynamic properties of uranium, combining several established theories and methodologies (density-functional theory, orbital-polarization, and Self-Consistent Ab Initio Lattice Dynamics). Their research appears in a special issue of Applied Sciences, “Feature Paper Collection in Section Materials,” and was edited by Dr. Leonid Burakovsky (LANL) and Dr. Alexander Landa (LLNL/PHYS).

This team’s combined approach was found to be computationally efficient, very robust (insignificant noise/ability to cope with errors), and accounts for strong exchanges of energy in the form of molecular vibrations. While uranium metal is nonmagnetic at ambient conditions, the team’s results indicate that upon thermal expansion, γ-U develops non-negligible magnetic moments that are included for the first time in thermodynamic theory. With this discovery, the researchers hope for more accurate modeling predictions—as magnetic interactions help improve the accuracy of the model—which can increase efficiency and lower production costs associated with fuel fabrication processes.

[P. Söderlind, A. Landa, E.E. Moore, A. Perron, J. Roehling, and J.T. McKeown, High-Temperature Thermodynamics of Uranium from Ab Initio Modeling, Applied Sciences (2023), doi: 10.3390/app13042123.]