Publication: Modelling the Thermal Conductivity of UB2 and ZrB2

There is a global effort within the nuclear industry to develop an alternate fission fuel to UO2 that could be safer, more efficient and cheaper to produce.  One such alternative is uranium diboride (UB2) owing to its increased uranium density and thermal conductivity over UO2.

Before an alternative fuel can be considered as a viable option, extensive research has to be conducted on the thermal and physical properties of the fuel. and how they vary over the lifetime of the fuel rod.  The properties may then be fed into fuel performance codes to study how the fuel will behave in a reactor, to confirm that it is safe to use.

One question surrounding UB2 is regarding the optimal boron ratio in the material and how this can affect the thermal properties of the fuel.  This research was accomplished through the use of DFT calculations using VASP, and QuantumESPRESSO to model the electron and phonon transport through the material.

As UB2 shares the same crystal structure as ZrB2 (which itself is used in the nuclear industry, for example, in cladding materials) the research was conducted for both materials.

It was found that using isotopically pure boron can increase the thermal conductivity of ZrB2 by as much as 12% at low temperatures (shown in the Figure opposite).  However, in UB2 it was found that the electron-phonon scattering is a significant contribution to the thermal conductivity and is therefore not influenced by the boron ratio.

Further details of the calculations and results may be found in the full publication available online here.

Thermal Conductivity Graph
The percentage change, P, in thermal conductivity of ZrB2 as a function of temperature and boron ratio. The natural abundance of boron is shown by the red dashed line.
L. J. Evitts, S. C. Middleburgh, E. Kardoulaki, I. Ipatova, M. J. D. Rushton & W. E. Lee, ‘Influence of boron isotope ratio on the thermal conductivity of uranium diboride (UB2) and zirconium diboride (ZrB2)‘, J. Nucl Mater 528 (2020)